Introductory Chapter: Photonic Crystal Technology – Introduction, Advantage, and Applications

1. Introduction

The substantial technological growth in the field of electronics has facilitated progress in optics or photonics. It is now possible to design and control the optical characteristics of materials to engage with light across specific frequency ranges. Through precise reflection, light can be effectively confined within a designated space, enabling its controlled propagation along a specific pathway. However, the miniaturization of optical devices is still a big challenge because of low light coupling and increasing radiation losses [1, 2, 3]. Another major problem for guided wave application is the confinement of photons at a relatively small scale. Generally, to control the photons, either metal or dielectric-based structures are used. Metals are lossy at optical wavelength, whereas dielectrics do not confine optical modes at very small scales (i.e., tightly bend waveguides) because of increased radiation losses [4, 5, 6]. Therefore, the concept of photonic crystal (PhC) technology is used to compensate for these problems.

Controlling the propagation of light on a small scale using the photonic band gap (PBG) effect has been a recent research area globally. Photonic crystal technology provides strong dispersion, which facilitates low group velocity for the guided modes in the vicinity of PBG edges, hence providing strong light-matter interaction at a very small scale. This property is further used to design various PhC-based devices. Although conventional optical devices possess high performance, stability, and real-time measurement capability, still PhC-based technique takes an edge because of its compact structure, and capabilities of slowing down and confining the light. This leads to an improvement in light-matter interaction, and hence, the performance. Further, the integration of micro-fluidic and PhC technology, with CMOS compatible fabrication process is an added advantage. The continuous and innovative works on photonic crystal-based devices would depict that they contain extraordinary properties (band gap, photon confinement, and low losses) and overcome many limitations of conventional optical devices i.e., evanescent field loss, bending losses, etc. Considering these unique properties, this book project is primarily focused on providing Recent Advances and Trends in Photonic Crystal Technology.

2. Photonic crystal technology

Photonic crystals are artificial materials designed to control and manipulate the flow of light (photons) in a manner analogous to how semiconductors control the flow of electrons [7, 8, 9, 10]. The fundamental theoretical framework for photonic crystals and their interaction with photons was independently established by Yablonovitch and John in 1987 [11, 12]. These materials are composed of periodic arrangements of dielectric (non-conductive) materials with varying refractive indices, creating a repeating structure that forms a bandgap for certain wavelengths of light. Based on the structural characteristics, photonic crystals can be categorized into three main types: one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) geometry, which are shown in Figure 1. They prevent the transmission of light with specific wavelengths. These ranges of wavelength are referred to as forbidden bands or photonic band gaps (PBGs), which is why photonic crystals are also termed PBG materials [13, 14]. The initial experimental exploration of photonic crystals and their photonic band structures was conducted by Robertson et al. in the years 1992 and 1993 [15, 16].

Figure 1.

Schematic representation of 1D, 2D, and 3D photonic crystals structures.

The one-dimensional (1D) PhC-based structure possesses the photonic band gap along the direction of periodicity [17, 18]. These structures can be formed by placing alternating dielectric stacks periodically [19] or by etching a row of holes in an otherwise perfect waveguide [20] as shown in Figure 1(a). This type of PhC-based structure is widely used in antireflection coatings, and to improve the quality of lenses, prisms, and other optical devices [21]. At present, the 1D PhC-based structure is also explored for various other advanced applications such as antifogging layers, optical sensors, omnidirectional reflectors, and passive radiative cooling devices [22, 23, 24, 25, 26, 27, 28].

The two-dimensional (2D) PhC-based structures are realized either by growing dielectric rods in a lower dielectric material or by etching holes in a higher dielectric material periodically in two dimensions [10] as shown in Figure 1(b). The periodicity is usually in square lattice, triangular lattice, or hexagonal lattice for any of the configurations: dielectric rods in air or air holes in dielectric material [29, 30]. Etching holes in dielectric material is easier to fabricate, and thus is most commonly used [31]. This is the most explored PhC-based structure and is widely used in making most of the optical devices such as filters, splitters, cavities, multiplexers/demultiplexers, sensors, etc. [32, 33, 34].

The three-dimensional (3D) PhC-based structures have periodicity in all three dimensions (shown in Figure 1(c)) and exhibit a complete band gap. Thus, the number of possible PhC configurations is much larger than in the case of 1D or 2D PhC-based structures. Introducing defects in the PhC-based structure many other useful devices can be obtained i.e., waveguide by line-defect and cavity by point defect. Although 3D-PhC-based structures have many interesting applications, still they are not widely used. This is because of their complex fabrication process. At the same time, they also have limited applications in the area of sensing [35, 36]. While many photonic crystals are designed and engineered by humans, similar periodic structures with photonic bandgaps can also be obtained naturally, such as the blue Morpho butterfly, the opal gemstone, and the wings of the peacock as shown in Figure 2(a–c) [10].

Figure 2.

Examples of naturally occurring photonic crystals (a) butterfly, (b) opal, and (c) feather of peacock.

2.1 Major advantages of photonic crystals

1. Periodic scalable structure: Photonic crystals consist of a regular, periodic arrangement of dielectric materials, typically in the form of a lattice, or a stack of layers. This periodicity is critical for the creation of bandgaps. Further, they also possess the electromagnetics scalability properties. This describes the ability to adjust the electromagnetic properties of the PhC-structure by just scaling the dimensions [37]. This allows for an increase in the range over which sensing or detection can occur.

2. Bandgaps: One of the most important features of photonic crystals is the presence of photonic bandgaps. Similar to electronic bandgaps in semiconductors, these are ranges of wavelengths (colors) of light that cannot propagate through the crystal. Within the bandgap, light is effectively forbidden, leading to high reflectivity or complete optical isolation [38]. Introducing defects in the perfect PhC-based structure many other useful devices can be obtained i.e., waveguide by line-defect and cavity by point defect. Because of the defect, these structure shows unusual characteristics, which are not possible in conventional optical devices i.e., sharp 90-degree bend, and mode guiding in low-index material [39].

3. Selective and tunable wavelength control: Photonic crystals can be engineered to have specific bandgaps at desired wavelengths or frequency ranges. This ability allows for precise control over the transmission and manipulation of light, making them useful for various applications. By altering the structure or composition of photonic crystals, their optical properties can be tuned. This tunability makes them versatile for different applications, including optical switches, modulators, and sensors.

4. Guiding and deflecting light: Photonic crystals can be used to create waveguides and microcavities that trap and guide light, enabling the construction of compact optical devices, such as photonic integrated circuits and lasers [40].

5. Slowing down the guided light: “Slowing down the guided light” refers to a phenomenon in which the speed of light propagation within a medium or structure is reduced compared to its speed in a vacuum. This reduction in the speed of light is often accomplished through various optical techniques and materials and has several important implications and applications in photonics and optics: controlled light-matter interactions, high-Q resonators, pulse shaping, supercontinuum generation, and quantum information processing [41]. Slowing the light also enhances the light-matter interaction. This can further be utilized to develop highly sensitive sensors [42].

6. Very less device footprint: The phrase “Very less device footprint” typically means that a device takes up very little physical space or has a small physical footprint. It suggests that the device is compact and does not occupy much room.

7. CMOS compatible fabrication process: A “CMOS compatible fabrication process” refers to a manufacturing method that is suitable for complementary metal-oxide-semiconductor (CMOS) technology [43]. CMOS is a widely used semiconductor technology in the electronics industry, especially for integrated circuits and microchips. A “CMOS compatible fabrication process” means that the manufacturing process is designed to work seamlessly with CMOS technology, ensuring compatibility and integration with CMOS-based devices and circuits.

3. Applications of photonic crystals

Photonic crystals are materials with a periodic arrangement of dielectric or metallic structures that affect the propagation of electromagnetic waves or light in unique ways due to their photonic bandgap properties. These materials have a wide range of applications in science and technology. Here are various applications of photonic crystals:

1. Photonic bandgap devices: Photonic crystals can be used to create devices that control the flow of light, such as waveguides, splitters, and switches. These devices find applications in optical integrated circuits and photonic chips for signal routing and processing.

2. Optical fiber communication: Photonic crystals can be used to manipulate and control the transmission of light in optical fibers [44]. They enable the creation of high-quality optical cavities for lasers and filters, leading to more efficient data transmission and signal processing in optical communication networks.

3. Optical sensors: Photonic crystals can be designed to interact with specific wavelengths of light, making them useful for creating highly sensitive sensors for detecting substances or changes in the environment. These sensors are used in areas such as biosensing, environmental monitoring, and chemical analysis [45, 46, 47].

4. Photonic crystal lasers: Photonic crystal structures can be incorporated into laser cavities to create compact and highly efficient lasers with narrow linewidths [48]. These lasers have applications in telecommunications, optical sensing, and medical devices [49].

5. Nonlinear optical devices: Photonic crystals can enhance nonlinear optical effects, such as frequency conversion and second-harmonic generation, which are important in fields like laser spectroscopy, imaging, and quantum optics [50].

6. Solar cells: Photonic crystals can improve the efficiency of solar cells by trapping and directing light within the material. This increases the absorption of sunlight and enhances the conversion of light into electricity [51].

7. Thermal emission control: Photonic crystals can be designed to control the emission of thermal radiation. This property has applications in radiative cooling for energy-efficient temperature control and in thermal imaging [52, 53].

8. Optical filters: Photonic crystal filters can be tailored to transmit specific wavelengths of light while blocking others. They are used in applications such as wavelength division multiplexing (WDM) in optical communication and spectral analysis [54, 55].

9. Quantum information processing: Photonic crystals are used in the development of quantum photonic devices and quantum computing platforms, enabling the generation, manipulation, and transmission of quantum states of light [56].

10. Metamaterials: Photonic crystals are often used as building blocks in metamaterials, which exhibit exotic electromagnetic properties not found in naturally occurring materials. Metamaterials have applications in cloaking devices, super lenses, and negative refractive index materials [57, 58].

These are just a few examples of the diverse applications of photonic crystals. Their ability to manipulate and control light at the nanoscale level continues to drive innovation in optics, photonics, and a wide range of technologies. Moreover, the improvements in device performance can be achieved by considering the advanced effects such as graded index PhC or topological PhC structures [59, 60].

3.1 Graded index photonic crystals (GPhC)

The gradient profile of either thickness or refractive index can be designed for advanced PhC devices. These graded index photonic crystal (GPhC) structures are composed of stacked layers in 1D-PhC with gradual changes in refractive index and thickness. These adjustments lead to notable dispersion characteristics, allowing for customizable engineering of photonic bandgaps (PBGs) and control over group velocities [61].

The thickness grading in photonic crystals refers to a deliberate variation in the thickness of individual layers or regions within the crystal’s periodic structure. This variation in thickness is used as a design strategy to control the propagation of light and tailor the photonic band structure, enabling the creation of unique optical properties and functionalities.

Refractive index grading in photonic crystals refers to the controlled variation of the refractive index within the structure of a photonic crystal. The bandgap characteristics of index GPhC structures are mostly investigated by adjusting the refractive index through linear, exponential, and hyperbolic grading profile. This variation in refractive index is used as a design strategy to manipulate the propagation of light within the photonic crystal and tailor its optical properties. By adjusting the refractive index in specific regions or along certain directions, photonic crystal devices can be customized to achieve desired optical functionalities. The swift progress in diverse 1D-GPhC structures has ushered in a wealth of opportunities for adaptable photonic devices, including optical reflectors, sensors, and filters [62].

3.2 Topological photonic crystals

The device performance and light-matter interaction can further be improved by considering the topological aspects of the structure. This involves utilizing a heterostructure-based topological nanophotonic structure, to achieve superior device performance. The topological effect is realized by connecting two dissimilar one-dimensional photonic crystal structures (1D-PhC) having overlapped photonic bandgaps. The structural parameters can be optimized to regulate and alter the dispersion characteristics, which results in the opposite Zak phases, which offer the excitation of the topologically protected edge states (TES) [60]. The excited TES show their robustness against surrounding perturbations and exhibit propagation of low scattered edge mode.

4. Conclusion

Overall, photonic crystals are a fascinating area of research and technology with the potential to revolutionize various aspects of photonics and optical communication. Their ability to control and manipulate light at the nanoscale opens up new possibilities for developing high-performance optical devices and systems. Further research in these areas can lead to improved sensor performance, enhanced sensitivity, and wider applicability in biomedical diagnostics and other fields.