Emerging Trends, Applications, and Fabrication Techniques in Photonic Crystal Technology

Abstract

Photonic crystals have emerged as a fascinating field of research and development, offering unprecedented control over the propagation and manipulation of light. These artificial structures are engineered to have periodic variations in refractive index, enabling them to control the behavior of photons in a manner analogous to how crystals manipulate electrons. Recent advancements in photonic crystals have focused on expanding their capabilities and exploring new applications. These advancements and trends in photonic crystals demonstrate their potential to revolutionize various technological domains. From integrated photonics to sensing, quantum information processing to solar energy harvesting, photonic crystals offer unprecedented control over light and pave the way for innovative applications and devices.

1. Introduction

Photonic crystals are periodic structures that exhibit photonic bandgaps, which prevent certain frequencies of light from propagating in specific directions. These structures can be used as optical filters, waveguides, cavities, and sensors, among other applications. Photonic crystals are structures that manipulate the flow of light by creating periodic variations in the refractive index of a material. They have been an active area of research, and there have been several recent advances in the field. These advances and trends in photonic crystal technology are leading to the development of more advanced optical devices with improved performance, efficiency, and functionality [1, 2].

Photonic crystals can now be engineered to exhibit tunable optical properties, meaning that their optical behavior can be adjusted in real-time using external stimuli such as temperature, pressure, or electric fields. Advances in computational methods have allowed for more precise design of photonic crystals. Fabrication techniques have also improved, enabling the production of larger and more complex structures with greater accuracy. Photonic crystals are being integrated with other technologies, including microfluidics, optoelectronics, and sensors, to create more advanced devices with multiple functionalities [3, 4].

Advancements in the design and fabrication techniques of photonic crystals have significantly expanded their capabilities. Researchers have achieved precise control over the photonic bandgap, enabling the manipulation of light at the nanoscale. Novel materials with tailored optical properties have been integrated into photonic crystals, leading to enhanced functionality and device performance. Moreover, advancements in the integration of photonic crystals with other technologies, such as microfluidics and electronics, hold promise for multifunctional integrated systems [5, 6, 7].

Furthermore, the development of tunable and reconfigurable photonic crystals has gained considerable attention. By incorporating dynamic elements, such as liquid crystals, phase-change materials, or electro-optic polymers, researchers can actively control the properties of photonic crystals. This tunability opens doors for versatile devices, including switchable filters, reconfigurable waveguides, and all-optical logic circuits [8].

While most research has focused on two-dimensional photonic crystals, there is growing interest in three-dimensional structures. Three-dimensional photonic crystals offer enhanced control over light propagation in all three dimensions and can exhibit unique optical properties. Researchers are exploring fabrication techniques and design strategies for creating complex 3D photonic crystal structures, with potential applications in high-density integrated photonics, light-emitting devices, and advanced optical materials [9, 10].

Lastly, advancements in nanofabrication techniques, such as electron-beam lithography and focused ion beam milling, have facilitated the realization of complex and three-dimensional photonic crystal structures. By exploiting the unique optical properties arising from intricate geometries, researchers aim to create unconventional devices, including topological photonic circuits and exotic light-matter interaction platforms.

2. Photonic crystal advancement

Photonic crystals are materials that have periodic optical properties, allowing them to control and manipulate the flow of light. Periodic structures in photonic crystals exhibit unique optical properties, including photonic band gaps and waveguides. Recent advances and trends in photonic crystal technology include (Figure 1):

• Nonlinear optics: Photonic crystals are being used to manipulate nonlinear optical processes such as second-harmonic generation and four-wave mixing, enabling applications such as optical parametric amplifiers and frequency conversion.

• Topological Photonic Crystals: Inspired by the field of topological insulators in condensed matter physics, scientists have started investigating topological properties in photonic crystals. These structures exhibit protected light propagation along the edges or surfaces, making them robust against defects and imperfections. Topological photonic crystals hold promise for applications in low-loss waveguides and on-chip optical circuits [11].

• Biology: Photonic crystals have a number of exciting applications in the field of biology. One such application is in biosensors, where the specific optical properties of the photonic crystal can be used to detect biological molecules with great accuracy. For example, a photonic crystal biosensor could be used to detect the presence of a specific protein in a patient’s blood sample. Another application of photonic crystals in biology is in drug delivery. By using a photonic crystal as a carrier, drugs can be targeted to specific cells or tissues in the body. These photonic crystal drug carriers can be designed to release the drug in response to specific stimuli, such as changes in pH or temperature, allowing for targeted drug delivery with minimal side effects. Photonic crystals are also being investigated for their potential use in enhancing the efficiency of photosynthesis in plants. By using photonic crystals to channel light more efficiently into the plant cells, researchers are hoping to develop crops that can produce more food with less energy. Overall, the unique optical properties of photonic crystals make them a promising field of study for a wide range of biological applications.

• Metamaterials and Meta-surfaces: Photonic crystals can be combined with metamaterials and meta-surfaces to create unique optical properties. Metamaterials are artificially engineered materials with properties not found in nature, while meta-surfaces are two-dimensional metamaterials. The combination of photonic crystals with these structures enables the control of light in unconventional ways, leading to applications such as cloaking devices, perfect absorbers, and polarization control.

• Nano-photonics: Researchers have been exploring the integration of photonic crystals with nanoscale devices and materials. This has led to the development of novel functionalities such as nano-lasers, nanocavities, and nanoscale sensors. These advancements open up possibilities for miniaturized and highly efficient photonic devices.

• Integrated Photonics: Photonic crystals are being integrated into on-chip photonic circuits to enhance light manipulation and integration. By combining different functionalities such as waveguides, filters, and modulators within a single photonic crystal structure, researchers are developing compact and efficient devices for optical communication, sensing, and computing applications.

• Smart Material: Smart materials in photonic crystals refer to materials that have the ability to change their optical properties in response to changes in external conditions, such as temperature, pressure, or electric field. Smart materials in photonic crystals have potential to revolutionize the field of photonics, with applications in communication, sensing, and optical computing.

Figure 1.

Schematic diagram of photonic crystal advancement.

2.1 Non-linear optics

Nonlinear optics in photonic crystals refers to the study and application of nonlinear optical phenomena within the context of photonic crystal structures. Photonic crystals, with their unique periodic arrangement of materials, offer a platform for controlling and manipulating light-matter interactions, including nonlinear optical effects. Non-linear optics is a fascinating phenomenon that occurs in photonic crystals, leading to a variety of interesting effects due to the interaction of intense light with the material. In non-linear optics, the response of a material to light is not proportional to the incident light intensity, and new frequencies can be generated through different processes.

The design of photonic crystals with tailored non-linear properties is an active area of research, aiming to harness these non-linear optical effects for practical applications in integrated photonics, telecommunications, quantum optics, and optical signal processing. Here’s an overview of the role of nonlinear optics in photonic crystals:

• Enhanced Nonlinear Effects: Photonic crystals can enhance the strength and efficiency of nonlinear optical effects compared to traditional materials. The periodic structure of photonic crystals allows for efficient phase matching, which is crucial for enhancing nonlinear processes. By engineering the photonic crystal structure and parameters, such as the lattice constant, refractive index contrast, and defect structures, it is possible to optimize the conditions for specific nonlinear optical interactions [12, 13].

• Nonlinear Dispersion Engineering: photonic crystals provide a platform for tailoring the dispersion properties of light. The periodic structure of photonic crystals introduces photonic bandgaps, where certain frequencies of light are forbidden from propagating through the crystal. By engineering the band structure of photonic crystals, it is possible to modify the dispersion relation of light, enabling control over phase-matching conditions for nonlinear processes. This allows for efficient frequency conversion and nonlinear interactions.

• Efficient Light-Matter Interaction: Photonic crystals can enhance the interaction between light and nonlinear materials by confining light within a small volume or by increasing the intensity of light within the crystal. This enhanced light-matter interaction can lead to stronger nonlinear effects, such as frequency conversion, harmonic generation, or parametric amplification [14, 15].

• Nonlinear Photonic Bandgap Engineering: Photonic crystals can be designed to exhibit nonlinear photonic bandgaps, which are wavelength ranges where specific nonlinear processes are inhibited. By tailoring the properties of the photonic crystal, such as the lattice parameters or the choice of materials, it is possible to control the transmission or reflection of specific nonlinear wavelengths. This can be useful for designing devices that selectively control or manipulate nonlinear optical signals [16, 17].

• Nonlinear Optical Devices: Photonic crystals enable the development of nonlinear optical devices with enhanced functionalities. For example, photonic crystal waveguides can be used to guide and confine nonlinear optical signals, allowing for efficient energy transfer and enhanced nonlinear effects. Photonic crystal cavities can trap and confine light, enabling strong light-matter interactions for enhanced nonlinear phenomena. These structures can be integrated into photonic circuits for on-chip nonlinear optical applications [18, 19].

• Nonlinear Optics for All-Optical Switching: One of the key applications of nonlinear optics in photonic crystals is all-optical switching. Photonic crystal structures can be designed to exhibit large nonlinear responses, allowing for the modulation and control of light signals using other light signals. By exploiting the nonlinear effects, photonic crystals can be used to realize all-optical switches and logic gates, enabling the development of ultrafast and compact photonic integrated circuits [20].

• Quantum Information Processing: Photonic crystals are being explored for their potential in quantum information processing. They offer a platform for controlling and manipulating single photons and quantum states of light. Photonic crystal waveguides and cavities can confine and guide photons, enabling efficient photon generation, manipulation, and detection. These capabilities are crucial for quantum communication, quantum cryptography, and quantum computing [21].

Figure 2 demonstrates the wide range of potential applications for 3D nonlinear photonic crystals. The added dimensionality in these structures creates numerous channels within the nonlinear material, leading to significant uses in various fields [22].

Figure 2.

Illustration of potential applications of 3D nonlinear photonic crystals [22].

In summary, nonlinear optics in photonic crystals leverages the unique properties of photonic crystal structures to enhance and control nonlinear optical effects. This field offers opportunities for developing novel devices with enhanced nonlinear functionalities, such as efficient light sources, all-optical switches, wavelength converters, and nonlinear optical signal processing devices.

2.2 Topological photonic crystals

Topological photonic crystals are a class of photonic structures that exhibit topological properties analogous to those found in electronic topological insulators. These structures manipulate light in a unique way, enabling the propagation of light along the edges or surfaces of the crystal while blocking it in the bulk [23, 24].

In traditional photonic crystals, light propagation is governed by the bandgap, a frequency range where certain wavelengths of light cannot propagate through the crystal. However, in topological photonic crystals, protected edge or surface states exist within the bandgap due to the nontrivial topology of the crystal’s structure [25, 26].

The key feature of topological photonic crystals is the presence of topological edge states. These states are robust against defects, disorder, and imperfections, making them highly desirable for applications where robust light propagation is essential. The protected nature of these states allows for efficient transmission of light even in the presence of obstacles or perturbations.

In Figure 3, the scanning electron microscope image presents a device comprising two distinct regions, distinguished by blue and yellow shading. These regions represent two photonic crystals with contrasting topological properties. At the interface of these photonic crystals, helical edge states are supported, exhibiting opposite circular polarizations (${\mathrm{\sigma }}^{+}$ and ${\mathrm{\sigma }}^{-}$). Grating couplers located at both ends of the device effectively scatter light in the out-of-plane direction, facilitating its collection. Additionally, a close-up image of the interface reveals the individual unit cells of each photonic crystal, highlighted by black dashed lines [27].

Figure 3.

The fabricated topological photonic crystal structure. The routing photons with a topological photonic structure is shown [27].

One of the significant advantages of topological photonic crystals is their potential for creating low-loss waveguides and on-chip optical circuits. By exploiting the topological edge states, light can be guided along specific paths without suffering significant losses due to scattering or other forms of energy dissipation [25, 28].

Moreover, the topological properties of these crystals can be tuned and controlled by adjusting the crystal’s parameters, such as lattice geometry, refractive index, or symmetry. This tunability allows for tailoring the topological properties to suit specific applications. Potential applications of topological photonic crystals include:

• Integrated Optics: Topological photonic crystals can be used to design compact and efficient photonic devices, such as waveguides, splitters, and couplers, for on-chip optical communication and computation [29].

• Robust Light Propagation: The protected edge states in topological photonic crystals enable robust and lossless light propagation, making them promising for applications in telecommunications, optical interconnects, and quantum information processing [30, 31].

• Optical Sensors: The unique properties of topological photonic crystals can be leveraged for highly sensitive and robust optical sensors, capable of detecting changes in the environment or analytes with high precision [32].

• Nonlinear Optics: The presence of topological edge states in these crystals can enhance nonlinear optical effects, enabling the development of efficient all-optical signal processing devices, frequency converters, and optical switches [33].

Apart from topological edge states, there has been a growing interest in edge states in the disordered photonic crystal structure. A disordered photonic crystal structure refers to a photonic crystal that lacks long-range order in its arrangement of structural elements. In a regular or ordered photonic crystal, the periodicity of the structural elements creates a bandgap, which is a range of wavelengths where light cannot propagate through the material. This unique property of photonic crystals allows for precise control of light propagation and manipulation. However, in a disordered photonic crystal, the periodicity is disrupted, leading to the loss of a well-defined bandgap. The lack of long-range order results in the scattering of light within the structure, causing random interference patterns and reducing the efficiency of light manipulation [34, 35].

Edge states in disordered photonic crystal structures refer to the localized states that can appear at the boundaries or interfaces of such structures. In ordered photonic crystals, edge states are well-defined and localized at the edges of the crystal due to the periodicity and symmetry of the structure. However, when disorder is introduced into the photonic crystal, whether intentionally or naturally, the periodicity is disrupted, leading to the formation of localized states at the edges and interfaces. These states can occur within the photonic bandgap or outside of it, depending on the specific type and amount of disorder [34, 35].

Topological photonic crystals are an exciting and rapidly evolving field of research with great potential for revolutionizing various areas of photonics. Ongoing studies are focused on further understanding their fundamental properties, exploring new materials and structures, and developing practical applications for these intriguing photonic systems.

2.3 Bio-photonics

Photonic crystals have found numerous applications in the field of bio-photonics, which involves the use of light for studying and manipulating biological systems. Bio-photonics is an interdisciplinary field that combines biology, physics, and photonics (the study of light) to develop and apply optical techniques for biomedical research and clinical applications. Here are some ways in which photonic crystals are utilized in bio-photonics:

• Bio-sensing: Photonic crystals can be utilized as highly sensitive platforms for biosensing. By functionalizing the surface of the photonic crystal with biological molecules such as antibodies or DNA probes, they can selectively capture and detect target analytes such as proteins, viruses, or DNA strands. The interaction between the captured analyte and the photonic crystal leads to changes in the optical properties, allowing for label-free and real-time detection of biomolecules. This approach has applications in medical diagnostics, environmental monitoring, and biological research [36, 37].

• Cell Imaging and Analysis: Photonic crystals can be used for label-free imaging and analysis of cells and tissues. By incorporating photonic crystals into microfluidic devices or biochips, researchers can achieve enhanced light-matter interaction and optical contrast. This enables the visualization of cellular structures, tracking of cellular processes, and analysis of cell morphology and dynamics. Photonic crystals can also be used to enhance fluorescence imaging by enhancing the emission intensity and modifying the emission spectra of fluorophores [38, 39].

• Drug Delivery and Therapy: Photonic crystals can be utilized for controlled and targeted drug delivery. By engineering the properties of the photonic crystal, such as the size, porosity, and surface chemistry, drugs or therapeutic agents can be encapsulated within the crystal structure. These drug-loaded photonic crystals can release the payload in a controlled manner triggered by external stimuli such as light, temperature, or pH. Additionally, photonic crystals can be used for photothermal therapy, where they absorb light and convert it into heat to selectively destroy cancer cells or target specific tissues [40, 41, 42].

• Optogenetics: Optogenetics is a technique that uses light to control and manipulate the activity of genetically modified cells or tissues. Photonic crystals can be utilized to deliver light with precise spatial and temporal control to targeted regions of the biological system. By incorporating waveguides or nanocavities within the photonic crystal structure, light can be efficiently guided and localized, enabling precise activation or inhibition of optogenetic proteins or cells. This allows for precise control of cellular behavior and neural activity, with applications in neuroscience and biomedical research [42].

• Bio-imaging: Photonic crystals can be used as contrast agents or labels for bioimaging techniques. By incorporating fluorescent dyes or quantum dots within the photonic crystal structure, they can emit or scatter light with specific wavelengths. This allows for targeted labeling and imaging of specific biological structures or cells, providing high-resolution imaging capabilities. Photonic crystal-based contrast agents can enhance the sensitivity and specificity of imaging modalities such as fluorescence microscopy and optical coherence tomography [43, 44].

• Light Localization and Manipulation: Photonic crystals can control the propagation of light in biological samples. They can confine and localize light to specific regions, enabling precise illumination of biological samples for targeted photo-stimulation or phototherapy. By engineering the bandgap properties of the photonic crystal, light can be selectively absorbed or scattered, allowing for controlled delivery of energy to specific cells or tissues [45].

• Photonic Crystal Fibers: Photonic crystal fibers (PCFs) are optical fibers that utilize a photonic crystal structure to guide and manipulate light. PCFs have been used in various bio-photonics applications, including endoscopy, fluorescence imaging, and light delivery for photodynamic therapy. The unique properties of PCFs, such as their high flexibility, low-loss transmission, and ability to guide light in specific spatial modes, make them valuable tools in bio-photonics research and medical diagnostics [46, 47].

• Bio-photonic Waveguides: Photonic crystals can be designed to act as waveguides for the transmission of light within biological systems. They can guide and direct light along desired paths, enabling the precise delivery of light for applications such as optogenetics, where light is used to control cellular activity. Photonic crystal waveguides can provide efficient light coupling and transmission within biological samples, offering a versatile platform for studying and manipulating biological systems [48, 49, 50].

• Surface wave devices: Photonic crystal-based surface wave devices for biosensing are a promising area of research in the field of bio-photonics. These devices utilize the properties of photonic crystals to guide and manipulate surface waves, such as surface plasmon polaritons (SPPs) or surface guided modes, which are sensitive to changes in the refractive index of the surrounding medium. This sensitivity makes them well-suited for detecting and analyzing biomolecules, cells, and other biological entities in real-time and label-free manners.

When biological analytes, such as proteins, DNA, or cells, bind to the device surface, they cause changes in the local refractive index. These changes can be detected as shifts in the resonance frequency or other optical properties of the guided surface waves. This change in the output signal is proportional to the concentration or presence of the target analyte [51, 52].One of the key advantages of Photonic crystal-based surface wave biosensing is its label-free detection capability. This means that there is no need to attach fluorescent or radioactive labels to the biomolecules, making the detection process simpler, faster, and less prone to interference. Moreover, due to the nature of surface waves and the high sensitivity of Photonic crystal-based devices, real-time monitoring of binding events can be achieved. This real-time monitoring allows researchers to study the kinetics of biomolecular interactions, providing valuable information about the binding strength and specificity [53, 54].

Photonic crystal-based surface wave biosensing has the potential for a wide range of applications, including medical diagnostics, environmental monitoring, food safety, and pharmaceutical research. As research in the field continues to progress, these devices are expected to become more sensitive, compact, and integrated into various lab-on-chip and point-of-care platforms, enabling rapid and efficient biosensing in diverse settings.

In Figure 4, a typical biosensor is depicted, employing angle interrogation of a photonic crystal surface mode (PC SM). The sensor utilizes a stabilized He-Ne laser that emits a circular-polarized beam. The CMOS matrix records the reflected angular profile, which is shown in the color inset. The key advantage of this setup is its one-dimensional spatial selectivity, enabling the simultaneous monitoring of biochemical reactions in four fluid channels.

Figure 4.

Schematic of label-free biosensing via photonic crystal [55].

2.4 Metamaterial and meta-surface

Metamaterials have several potential applications in photonic crystal technology. One major way these materials could be utilized is in the design of more efficient optical devices. Photonic crystals can be combined with metamaterials and meta-surfaces to create unique optical properties. Metamaterials are artificially engineered materials with properties not found in nature, while meta-surfaces are two-dimensional metamaterials. The combination of photonic crystals with these structures enables the control of light in unconventional ways, leading to applications such as cloaking devices, perfect absorbers, and polarization control. Here are some specific applications where metamaterials are used in conjunction with photonic crystals:

• Optical switches: Another potential application of metamaterial in photonic crystal technology is the development of ultra-fast and compact optical switches. By incorporating metamaterials into a photonic crystal waveguide, the waveguide can be made to exhibit nonlinear optical phenomena, which can be used to create ultra-fast optical switches [56, 57].

• Superlensing: Metamaterials with negative refractive index, also known as negative index materials (NIMs), can be combined with photonic crystals to achieve superlensing. This involves focusing light beyond the diffraction limit, allowing for the imaging of subwavelength details. The combination of metamaterials and photonic crystals enables high-resolution imaging and microscopy applications [58].

• Cloaking Devices: Metamaterials can be integrated into photonic crystals to create cloaking devices that manipulate the path of light, making objects invisible or undetectable to certain wavelengths. By controlling the refractive index distribution in the photonic crystal, combined with the unique properties of metamaterials, researchers have developed designs for cloaking devices operating in various frequency ranges, including visible light [59].

• Waveguides and Wavefront Manipulation: Metamaterials can be used to engineer the dispersion properties of photonic crystal waveguides. By tailoring the refractive index profile of the waveguide using metamaterial structures, the propagation characteristics of light can be controlled, allowing for efficient routing and manipulation of light signals at the nanoscale [60].

• Nonlinear Optics: The combination of metamaterials and photonic crystals enables enhanced nonlinear optical effects. Metamaterials can provide enhanced local field intensities and strong light-matter interactions within the photonic crystal structure. This can be exploited for applications such as frequency conversion, optical parametric amplification, and all-optical switching [61, 62].

• Sensing and Detection: Metamaterial-enhanced photonic crystals can be utilized for highly sensitive sensors and detectors. The unique optical properties of metamaterials, such as their ability to manipulate light at the nanoscale, can be combined with the high-quality optical resonances and tailored bandgaps of photonic crystals to create sensors for various applications, including chemical and biological sensing [63, 64].

• Energy Harvesting and Solar Cells: Metamaterials integrated into photonic crystals can enhance light absorption and trapping in solar cells. By manipulating the dispersion properties of the photonic crystal and incorporating plasmonic structures within the metamaterial, the absorption of specific wavelengths can be optimized, leading to improved energy conversion efficiency [65, 66, 67].

In Figure 5, the SRR-based biosensor consisted of the sensitive biological element, transducer or the detector element, and associated electronics or signal processors. When biotin is introduced, the resonant frequency shifted. These shifts in the resonant frequency stem from the change in the capacitance due to the binding of biotin and streptavidin [68].

Figure 5.

Binding bioprocess of biotin and streptavidin: The liquid wall (red circle) shows the receptacle for liquid solution confinement [68].

These are just a few examples of how metamaterials can be applied in conjunction with photonic crystals. The combination of these two fields allows for the creation of novel optical devices with enhanced functionalities and improved performance in various applications.

2.5 Nano-photonics

Nano-photonics in photonic crystals is an exciting field of research that combines the principles of nanotechnology and photonics to manipulate and control light on a very small scale using photonic crystals as a platform. These crystals exhibit unique optical properties and interesting effects and applications. In nano-photonics, researchers engineer and manipulate photonic crystals at the nanoscale to control light in new and innovative ways.

In Figure 6 a nano-scale beam splitter is shown. Nanophotonic beam splitters are essential components in integrated optics, serving a wide range of applications from high-speed telecom receivers to biological sensors and quantum splitters. Although high-performance multiport beam splitters have been successfully demonstrated in various material platforms using multimode interference couplers, their operation bandwidth still faces fundamental limitations [69].

Figure 6.

This is an ultra-broadband multimode interference coupler. It incorporates a central multimode region that is divided at a sub-wavelength scale to manipulate the waveguide’s anisotropy and dispersion, resulting in a nearly wavelength-independent beat length [69].

Here are some key aspects and applications of nano-photonics in photonic crystals:

• Optical communication devices: One of the most promising nano-photonic applications in photonic crystals is in the development of high-performance optical communication devices. For example, photonic crystals can be used to create ultra-compact waveguides, which can guide light with very low losses and high efficiency, enabling faster and more reliable communication in optical networks. Additionally, photonic crystals can be used to design high-quality resonators, which can be used for precise frequency filtering and signal processing [70, 71, 72].

• Solar cells: Another potential application of photonic crystals is in the development of high-efficiency solar cells. By using photonic crystals to trap light within the solar cell, more photons can be absorbed, leading to a higher conversion efficiency. In addition to these applications, photonic crystals have also shown promise in areas such as sensing, imaging, and quantum information processing. For instance, photonic crystal cavities can be used as highly sensitive biosensors, while photonic crystal lasers can be used for high-resolution imaging and quantum computing [73].

• Optical filters: Photonic crystals also have been widely explored for their application in tunable optical filters. Tunable optical filters are devices that can selectively control the transmission or reflection of specific wavelengths of light, and they are essential components in various optical communication systems, spectroscopy, and sensing applications. Photonic crystals based tunable optical filters offer unique advantages due to their ability to manipulate light propagation through the bandgap engineering and the tunability of their optical properties. Tunable optical filters based on photonic crystals offer several advantages, including compact size, low power consumption, and potentially high spectral resolution. They have the potential to replace traditional bulky and power-hungry tunable filters with more efficient and integrated solutions [74].

• Quantum information processing: Quantum information processing in photonic crystals is an exciting and promising field of research that explores the use of photonic crystal structures to manipulate and process quantum information. Photonic crystals can be employed to create efficient and robust quantum communication channels. By guiding and manipulating photons within the crystal’s bandgap, researchers can develop low-loss waveguides for transmitting quantum information over long distances.

Additionally, the ability to engineer and control the dispersion properties of photonic crystals allows for the creation of single-photon sources and quantum memories, essential elements for quantum communication protocols like quantum key distribution (QKD). Moreover, photonic crystals can be engineered to create non-classical light states, such as entangled photon pairs. Entanglement is a crucial resource for various quantum information tasks, including quantum teleportation and quantum computing. Photonic crystal structures can facilitate the generation of entangled photon pairs with high fidelity and efficiency [75, 76].

Photonic crystals hold promise for implementing certain components in quantum computing architectures. They can be used to manipulate and process qubits encoded in the quantum states of photons. Photons are particularly attractive for quantum computing due to their weak interactions with the environment, which helps reduce decoherence and improves the stability of quantum computations.

Photonic crystals can be used to create efficient and low-noise quantum memories. Quantum memories are devices that can store quantum information encoded in the states of photons for a certain period of time. These memories are essential for buffering quantum information in quantum networks and quantum repeaters. Photonic crystal structures can be engineered to facilitate quantum frequency conversion, where the frequency of photons is changed while preserving their quantum properties. This is crucial for interfacing different quantum systems and technologies that operate at different wavelengths [77].

Laser: Photonic crystals play a crucial role in the development and enhancement of lasers. They offer a platform for tailoring and controlling the optical properties of lasers, leading to improved performance and new functionalities. There are some key ways that photonic crystals are utilized in laser technology.

Photonic crystals can be designed to act as laser cavities, which are the core component of a laser that provides feedback to sustain coherent light amplification. The periodicity of the photonic crystal structure can create a photonic bandgap that confines light within the cavity and allows for efficient light amplification through stimulated emission. This confinement enhances the interaction of light with gain media, leading to lower threshold powers and improved laser efficiency [78].

By exploiting the photonic bandgap, photonic crystal lasers can achieve lower lasing thresholds compared to conventional lasers. The photonic crystal structure suppresses spontaneous emission and enhances the optical feedback within the cavity, requiring lower pump powers to achieve lasing action. This is especially beneficial for reducing the power consumption and increasing the overall efficiency of lasers [79].

In addition, nano-scale photonic crystal structures can be used to create nano-lasers, which are compact and efficient laser sources that operate at nanoscale dimensions. Nano-lasers are promising for integration with other nanophotonic components, such as on-chip photonic circuits, and for applications in nano-photonics and optoelectronics [80, 81].

2.6 Integrated photonics

Integrated photonics can be combined with photonic crystals to create advanced and compact optical devices with enhanced functionalities. Integrated photonics, also known as photonic integrated circuits (PICs) or optical integrated circuits, is a technology that aims to manipulate and control light on a chip-scale platform. Similar to electronic integrated circuits (ICs), which have revolutionized electronics, integrated photonics seeks to integrate multiple photonic components and functionalities onto a single semiconductor substrate, typically made of silicon, silicon nitride, or other photonic materials.

In integrated photonics, various passive and active photonic elements, such as waveguides, couplers, splitters, modulators, detectors, and lasers, are combined to form complex photonic circuits. These circuits can be designed to perform a wide range of functions, including signal processing, data transmission, sensing, and quantum information processing.

Figure 7 showcases a 1D photonic crystal cavity design utilizing a bow-tie unit cell. Photonic crystal cavities are specialized structures that enhance the temporal confinement of light in a material, leading to high-quality factors, which denote the ability to maintain light within the cavity for extended periods. On the other hand, plasmonic structures focus on spatial confinement, resulting in low mode volumes, indicating the tight spatial confinement of light in these structures. Combining these two approaches in the bow-tie unit cell offers advantages in terms of both temporal and spatial confinement of light, making it a promising configuration for various applications in photonics and optoelectronics [82].

Figure 7.

Design of photonic crystal cavities for extreme light concentration [82].

Here are a few applications where integrated photonics is used in conjunction with photonic crystals:

• Photonic crystal waveguides: Integrated photonics enables the fabrication of waveguides directly on photonic crystal platforms. These waveguides can guide and confine light within the photonic crystal structure, allowing for efficient and controlled light propagation. Photonic crystal waveguides are crucial for routing and manipulating light signals in Photonic Integrated Circuits (PICs) [83, 84].

• Photonic crystal cavities: Photonic crystal cavities are small regions within the photonic crystal structure that trap and confine light. Integrated photonics facilitates the integration of photonic crystal cavities with other optical components on a chip, enabling the creation of highly efficient and compact optical resonators. These cavities are used in applications such as lasers, filters, and sensors [85].

• Optical modulators: Integrated photonics can be used to fabricate photonic crystal-based modulators that control the intensity, phase, or polarization of light. By incorporating active materials, such as electro-optic polymers or semiconductor materials, within the photonic crystal structure, efficient modulation of light signals can be achieved. These modulators are crucial for optical communication systems and signal processing [86].

• Photonic crystal sensors: Integrated photonics enables the integration of photonic crystal structures with sensing elements, allowing for highly sensitive and compact optical sensors. Photonic crystals can be designed to interact with specific wavelengths or refractive indices, making them suitable for various sensing applications, including biosensing, environmental monitoring, and chemical detection [87].

• Nonlinear optics: Integrated photonics combined with photonic crystals can enhance nonlinear optical effects by confining and manipulating light at small length scales. This enables the generation of nonlinear optical processes, such as frequency conversion, optical parametric amplification, and second-harmonic generation, in a compact and efficient manner. These nonlinear photonic crystal devices have applications in optical frequency conversion, quantum optics, and optical signal processing [88, 89].

The major problem in integrated optics is coupling. Coupling, in the context of integrated optics, refers to the process of efficiently transferring light between different optical elements within the integrated photonic circuit. In an integrated optical circuit, various optical components, such as waveguides, couplers, modulators, filters, and detectors, are fabricated on the same chip using techniques like lithography and etching. These components work together to manipulate, guide, and process light signals on the chip [90].

The process of coupling is essential for the proper functioning of the integrated optical circuit. It involves transferring light from one waveguide or component to another with minimal loss or distortion. Efficient coupling ensures that the light propagates through the circuit with maximum power and minimum signal degradation [91].

2.7 Smart material

Smart materials can be integrated into photonic crystals to create dynamic and tunable optical devices. By incorporating materials with responsive properties, photonic crystals can be actively controlled and manipulated to change their optical characteristics. Smart materials in the context of photonic crystals refer to materials that exhibit tunable or reconfigurable optical properties in response to external stimuli. The properties of these photonic crystals can be modified or controlled using smart materials, leading to novel functionalities and applications.

For example, as shown in Figure 8, a periodic deformation of a thin liquid dielectric film driven by Surface Plasmon Polariton (SPPs) propagating on a metal– fluid interface and forming a plasmonic liquid lattice, whereas Figure 8(b) shows a periodic deformation of a pair of gas – fluid interfaces present in a symmetric liquid slab waveguide due to propagating slab waveguide modes forming a suspended photonic liquid lattice without metals [92].

Figure 8.

Schematic presentation of thin liquid dielectric film deformation forming optical liquid lattices (blue) due to surface tension effects triggered by interference of surface optical modes (red) [92].

Here are a few examples of smart materials used in photonic crystals:

• Liquid crystals: Liquid crystals are anisotropic materials that exhibit changes in their optical properties in response to external stimuli such as electric fields, temperature, or light. By incorporating liquid crystals into the structure of photonic crystals, the refractive index distribution within the crystal can be dynamically controlled. This allows for the tuning of the photonic bandgap, spectral response, or polarization properties of the crystal. Liquid crystal-based photonic crystals find applications in tunable filters, switches, and displays [93, 94, 95].

• Electroactive polymers: Electroactive polymers (EAPs) are materials that can change their shape or volume in response to electrical stimuli. They offer the ability to actively deform or modulate the structure of a photonic crystal. By integrating EAPs into the photonic crystal structure, the lattice parameters, refractive index, or photonic bandgap can be modified. This enables tunable optical devices such as deformable mirrors, waveguides, or modulators [96].

• Stimuli-responsive hydrogels: Hydrogels are three-dimensional networks of crosslinked polymer chains that can absorb and retain large amounts of water. Stimuli-responsive hydrogels, such as pH-responsive or temperature-sensitive hydrogels, undergo reversible volume changes in response to specific environmental cues. By embedding hydrogels into the structure of photonic crystals, the refractive index or photonic bandgap can be dynamically adjusted by changing the water content of the hydrogel. This enables tunable optical devices that respond to environmental changes [97, 98].

• Phase change materials: Phase change materials (PCMs) exhibit reversible transitions between different solid phases with distinct optical properties. These transitions can be triggered by changes in temperature, electric fields, or optical signals. By incorporating PCMs into photonic crystals, the crystal’s optical properties can be switched between different states, enabling reconfigurable and programmable optical devices. PCMs find applications in optical memories, modulators, and reconfigurable photonic circuits [34, 98, 99, 100].

• Optically active materials: Optically active materials, such as chiral molecules or nanostructures, have the ability to selectively interact with circularly polarized light. By incorporating these materials into the structure of photonic crystals, the crystal’s polarization properties or chiroptical response can be controlled and manipulated. This enables the design of devices for polarization control, circular dichroism, or chiral sensing [101, 102].

By integrating smart materials into photonic crystals, the optical properties of the crystals can be actively modified and controlled, leading to tunable, reconfigurable, and responsive optical devices. This opens up possibilities for dynamic photonic devices with applications in communication, sensing, displays, and optical signal processing.

2.8 Photonic spin hall effect

The Photonic Spin Hall Effect (PSHE) can also be observed in photonic crystal structures, adding another dimension to the control and manipulation of light based on its spin angular momentum. When the principles of PSHE are combined with photonic crystals, it leads to interesting spin-dependent light behaviors within these structures [103].

In a photonic crystal, light experiences different effective gauge fields due to the periodic modulation of the refractive index. This spatial variation introduces a form of spin-orbit coupling for photons within the crystal. As a result, light with different spin states acquires different effective momenta and undergoes different deflections as it travels through the photonic crystal. The coupling of the Photonic Spin Hall Effect with photonic crystals offers several significant advantages and applications:

• Spin-dependent light manipulation: Photonic crystals with PSHE can efficiently separate photons based on their spin states, allowing for selective routing and control of polarized light. This property is valuable in spin-based quantum information processing, where precise manipulation of spin states is essential [104].

• Efficient spin-photon interfaces: By engineering the photonic crystal’s properties and spin-orbit interactions, efficient interfaces between spin-polarized photons and other quantum systems, such as quantum dots or atomic ensembles, can be realized. This paves the way for enhanced quantum photonics applications [105].

• Topological photonic crystals: The combination of PSHE and photonic crystals can lead to topological photonic crystals, where nontrivial spin-textured states emerge, and protected edge modes allow robust spin-dependent light propagation [106, 107].

• Integrated photonic spin devices: The integration of PSHE with photonic crystals enables the design of compact and efficient photonic spin devices, which could find applications in polarized light detectors, modulators, and polarimeters [108].

• Photonic quantum technologies: The control of spin states in photonic crystal structures opens up opportunities for developing new protocols and functionalities in quantum communication, quantum cryptography, and quantum computation using photonic qubits [109].

In Figure 9(a), the dimensional hierarchy of a higher-order topological insulator in a dielectric photonic crystal is depicted. The different types of states, namely corner states (blue), edge states (green), and bulk states (red), are represented by distinct colors and are isolated from each other in the frequency domain. These states arise due to the topological properties of the photonic crystal. In Figure 9(b), two pseudospins are defined by the in-plane magnetic field, illustrated by purple arrows, within the unit cell of the photonic crystal. Pseudospins are a theoretical concept that mimics the behavior of electron spins in condensed matter systems. In this context, they are used to describe the polarization states of light in the photonic crystal.

Figure 9.

Higher-order quantum spin Hall effect in a photonic crystal. This figure highlights the intricate interplay between topological properties, pseudospins, and polarization states in the photonic crystal, offering opportunities for controlling light propagation and guiding it in specific directions. Such phenomena have potential applications in various areas, including optical communication, sensing, and quantum information processing [110].

Figure 9(c) demonstrates the mechanism of achieving directional localization of pseudospin-polarized corner states, which are excited by a pseudospin-dependent source. The blue and red stars represent the positions of left circular and right circular polarized light sources, respectively, acting as pseudospin-dependent sources. The blue and red spheres represent the positions of corner states with pseudospin up and down polarizations, respectively, indicated by arrows [110].

The study of Photonic Spin Hall Effect in photonic crystals is a cutting-edge research area, and it brings together the rich physics of both photonic crystals and spin-angular momentum of light. As researchers delve deeper into understanding and manipulating these phenomena, we can expect to see exciting advancements in photonic devices, quantum technologies, and other applications that rely on the precise control of light and its quantum properties.

3. New trend in photonic crystal fabrication techniques

Photonic crystal fabrication is the process of creating regular, repeating structures with periodic variations in the refractive index of materials. There are several methods that can be used for photonic crystal fabrication, including lithography techniques such as electron beam lithography and nanoimprint lithography, as well as deposition techniques such as chemical vapor deposition and atomic layer deposition. These techniques allow researchers to create photonic crystal structures, such as two-dimensional (2D) and three-dimensional (3D) photonic crystals, and can be used to fabricate photonic devices such as waveguides, filters, and sensors [9, 111].

The fabrication of photonic crystals involves the use of multiple processing steps to create the desired pattern of structures. In lithography-based processes, this typically involves creating a mask or template that guides the deposition or patterning of materials onto the substrate. Surface treatment of the substrate is then performed, followed by the application of the photosensitive material. After exposure, the desired pattern is developed through chemical processing, followed by etching or deposition to transfer the pattern onto the substrate. These techniques can be used individually or in combination to create photonic crystals with a wide range of structures and properties [41, 112].

Researches are ongoing to develop new fabrication techniques that are more cost-effective and scalable, as well as to optimize the existing techniques to increase throughput and reduce production costs. Furthermore, the use of novel materials for photonic crystal fabrication and the development of advanced computational modeling techniques can help to accelerate the design and optimization of photonic crystal structures, providing a solution to reduce the time of the design process. Here are the main advanced techniques in photonic crystal fabrication include:

• Three-dimensional (3D) direct laser writing: This technique allows for the creation of 3D photonic crystals, enabling the fabrication of complex structures that were previously impossible to produce.

• Self-assembly: By using self-assembly techniques, such as block copolymer lithography or colloidal lithography, photonic crystals can be fabricated with a level of self-organization and periodicity that is difficult to achieve with conventional lithography techniques [113].

• Reactive ion etching: Reactive ion etching can be used to create deep and highly precise patterns in a variety of materials. This technique is particularly useful for the fabrication of high-aspect-ratio structures and is often used in combination with other lithography techniques [114].

• Templated growth: Templated growth enables the fabrication of photonic crystals with a high level of precision by using a pre-patterned template to guide the growth of the crystal. This technique is particularly useful for producing inorganic photonic crystals.

• Computational design: Computational design tools, such as finite-difference time-domain simulations and inverse-design algorithms, can be used to optimize the properties of photonic crystals and guide their fabrication. By using these tools, the design and fabrication of complex photonic structures can be achieved more efficiently and accurately.

The Figure presented below illustrates the combination of 3D printing and actively switchable redox-active oligo(aniline)-based materials to create novel tunable 3D photonic materials. The process involves a direct laser writing technique to fabricate switchable functional structures with sub-micrometer features [115] (Figure 10).

Figure 10.

Direct laser writing of actively tunable 3D photonic crystals [115].

There are now some problems in photonic crystal fabrication. One of the main bottlenecks in photonic crystal fabrication is the difficulty in achieving high throughput and low-cost manufacturing. Many of the existing techniques for photonic crystal fabrication are expensive and time-consuming, requiring multiple steps and specialized equipment. This limits their scalability for large-scale production. Additionally, the materials used in photonic crystal fabrication, such as semiconductors, can be expensive and difficult to work with, further increasing the manufacturing cost.

Another bottleneck is the challenge in creating three-dimensional (3D) photonic crystals, which have more complex structures than two-dimensional (2D) photonic crystals. The fabrication of 3D photonic crystals requires precise control of the fabrication process, which can be challenging due to the need for multiple lithography steps or the use of complex templating processes.

4. Conclusion

In conclusion, the field of photonic crystals continues to evolve with exciting advancements and emerging trends. These advancements hold significant potential for revolutionizing diverse technological areas, ranging from communications and sensing to renewable energy and quantum photonics. Photonic crystals have already opened up a whole new world of possibilities for scientific research and technological development. With ongoing research and new findings, we can expect even more exciting innovations in the future.

Conflict of interest

The authors declare no conflict of interest.