Broadband Terahertz Emission from Photoconductive Devices

Abstract

This chapter explores the terahertz (THz) emission from biased semiconductor photoconductive devices. The photoconductive device is an optoelectronic device that is able to emit broadband THz radiation under the optical excitation, by an ultrafast laser, in the existence of a bias field. This chapter explains the basic principle of photoconductive devices with focusing on the main device components, being the photoconductive material and the photoconductive structure. Then, various materials and structures are discussed toward improving the performance of the photoconductive THz emitters. Furthermore, the main limitations and considerations are presented with insight into the different saturation and screening effects due to the bias field and pump fluence. Ultimately, the recent advances and studies of photoconductive THz emitters are presented in terms of material and structure, including the quantum dots, the nanostructure, the use of dielectric materials, and the grating structure on the photoconductive surfaces.

1. Introduction

Since the ultrafast (femtosecond) laser was demonstrated in the 1980s, the field of terahertz (THz) technologies has emerged with an array of applications appearing in different areas, from spectroscopy and sensing to imaging and high-speed communications [1, 2]. Terahertz radiation is nonionizing radiation and has low photon energies, thus having less chance of tissues, cells, and DNA damage during the spectroscopic, sensing, and imaging applications. In addition, the terahertz radiation can be transmitted through some opaque objects in visible light, which opens an array of detection and security applications. The late development of the THz applications is due to the challenges in the generation and detection within the THz band. Its frequencies of 0.1 to 10 THz (30 μm to 3 mm), sandwiched between the electronic and optical frequencies, cannot be generated by conventional electronics or optical methods [3]. This is because the conventional electronics technologies are insufficient to produce broadband waves at these relatively high frequencies. On the other hand, conventional optical technologies cannot emit THz frequencies due to a fundamental issue; there is no material with a bandgap energy corresponding to the THz frequencies [4]. Fortunately, various ultrafast laser and semiconductors approaches have been examined and established. That leads to demonstration of the first emission of pulsed THz radiation using a dipole photoconductive antenna in 1988 by Smith et al. [5]. After that, the photoconductive devices were used widely to emit and detect broadband THz radiation; such devices have been developed with regards to their materials and structures to enhance the emission and detection of THz radiation. Nevertheless, the THz emission can be done mainly by two main methods, being optical rectification based on electro-optic (EO) crystal and photoconductive THz emitters based on semiconductors [6, 7].

The photoconductive THz emitter is an optoelectronic device with three main components, being the photoconductive materials, the photoconductive electrodes, and the lens [8] (Figure 1). The photoconductive material is a semiconductor having bandgap energy compatible with the photon energy of the ultrashort laser pulses. In addition, the photoconductive material should have optimum characteristics, including carrier lifetime, carrier mobility, breakdown voltage, and dark resistivity [9]. The carrier lifetime is preferable to be short. However, in the case of the photoconductive detector, it must be in the subpicosecond range. A higher breakdown voltage, carrier mobility, and dark resistivity are fundamental characteristics to assure a better photoconductive THz emitter performance in the form of higher radiated power, higher SNR, and broader bandwidth. The second component is the photoconductive electrodes. The photoconductive electrodes are two metal electrodes patterned on top of the device, having a gap in between, namely, a photoconductive gap. The design and dimensions of such a gap will influence the device’s performance. The last component is the lens. The lens is typically integrated with the emitter to accumulate the radiated field; the radiated field will then be focused on the targeted radiation path.

Figure 1.

Illustration of the photoconductive device, as in (a) it shows a schematic diagram of the photoconductive THz emitter, and in (b) it shows the semiconductor band structure under the applied electric field. © IOP publishing. Reproduced with permission. All rights reserved [8].

The photoconductive THz emitter can generate the THz radiation following photoexcitation of its photoconductive gap by an ultrashort laser pulse. When the laser pulse is focused into a photoconductive gap, the laser pulse generates free electrons and holes within the semiconductor, having a rate proportional to the laser pulse [10]. The free carriers will then accelerate under a bias field, controlled by the bias voltage, V, produce a transient photocurrent, and ultimately drive the emitter to emit far-field radiation with frequencies spanning into the THz spectrum [11].

This chapter presents the photoconductive devices for THz generation, with insights into their components, limitations, and considerations, and recent progress in this field. In Section 2, a number of photoconductive materials are discussed, the influence of the material and material’s characteristics are addressed. In Section 3, the photoconductive electrodes (structure) are considered. This includes different structures characteristics based on their size, being a large aperture antenna, a small aperture antenna, and the plasmonic antenna, discussing the influences on the photoconductive THz emitters’ performance. In Section 4, the limitations of the THz emission by photoconductive devices are discussed. The presented limitations are mainly related to the bias field and optical (pump) fluence, which appear in the form of radiated power saturation. In addition, the underlying physics of the space charge and near-field saturation is provided. Ultimately, in Section 5, the recent advances in photoconductive devices technology are given, including the integrated devices and the system-on-chip technologies.

2. Photoconductive materials

In general, the photoconductive THz emitter performance differs based on the photoconductive material and structure. Therefore, the photoconductive material will be the focus of this section. The optimum photoconductive materials would be crystal lattices with a direct bandgap between the valence and conduction bands. This bandgap determined the absorption wavelength of the exciting laser pulse. Other factors that play a significant role in choosing suitable materials are low carrier lifetime and high carrier mobilities. The most studied materials for photoconductive devices are gallium arsenide (GaAs), indium gallium arsenide (InGaAs), quantum well of InGaAs, indium aluminum arsenide (InAlAs), and a combination of group III-VI materials. This section will explore the photoconductive materials GaAs, ion-implantation in GaAs, InGaAs, and multi-quantum wells InGaAs/InAlAs.

Gallium Arsenide (GaAs) is a III–V semiconductor that has a bandgap of (Eg ∼ 1.42 eV at 300 K) corresponding to the emission wavelength of 880 nm [12]. GaAs is compatible with the titanium-doped sapphire (Ti: sapphire) femtosecond pulsed laser sources typically used to illuminate the photoconductive THz emitters. The GaAs has been the most common material and is typically employed in semi-insulating (SI)-GaAs, low temperature-grown (LT)-GaAs, or ion-implanted GaAs. The SI-GaAs grown by liquid-encapsulated Czochralski at 450–600°C [13] is typically a single crystal that has a high resistivity (>10⁷ Ω cm) and a high electron mobility (μ > 7000 cm²/Vs) [14]. The SI-GaAs is considered a cost-effective substrate and has become widely used material for photoconductive devices. However, the research was ongoing to shorten the carrier lifetime. The LT-GaAs grown on SI-GaAs is proved to reduce carrier lifetime two orders of magnitude to below 1 ps compared to SI-GaAs (t > 100 ps) and efficiently generate broadband THz radiations of over 1 THz with high resistivity (10⁷ Ω cm) and reasonable mobility μ (100–300 cm²/Vs) [15]. The growth is typically done by molecular beam epitaxy (MBE) on the surface of SI-GaAs substrate and growth temperature to between 200°C and 300°C in an arsenic-rich environment [16]. In such a case, it yields a high level of crystallinity, which means higher carrier mobilities and point defects due to excess As precipitants. Higher mobility leads to fast response, and point defects significantly reduce the lifetime (below 400 fs). These point defects act as recombination centers [15]. Increasing the temperature above 250°C will increase the lifetime to be greater than 50 ps. Tani et al. conducted a direct between LT-GaAs and SI-GaAs and studied the effect of growth temperature and anneal time effects where the carrier lifetime of LT-GaAs grown at 250°C and followed by post-growth annealing at 600°C for 5 min was found to be a 0.3 ps [17]. However, the process conditions for LT-GaAs are not easy to reproduce due to unreliable temperature monitoring below 400°C.

An alternative approach is using the ion-implantation technique to create point defects and reduce the lifetime in SI-GaAs by implementing arsenic, oxygen, nitrogen, carbon, and hydrogen (proton). Implanting H+ ions are shown to decrease the carrier lifetime of GaAs to sub-picosecond. Then several groups studied the effect of As⁺³ ion implantation of SI-GaAs and introduced excess As⁺³ impurities within the crystal structure similar to LT-GaAs [11]. However, the ion-implantation technique of As⁺³ (GaAs∶ As⁺³) improved the controllability of the excess As⁺³ concentration and uniformity as compared to LT-GaAs, making it more reproducible than LT growth [11]. Salem et al. [18] characterized the GaAs:As⁺ and unprocessed GaAs implanted with various other ions, including hydrogen, oxygen, and nitrogen. Their study revealed the lowest THz pulse intensity was observed in the GaAs∶N³⁻ and all devices saturated at a higher pump fluence than nonprocessed GaAs emitter except GaAs:N⁺. A study by Liu et al. [19] revealed that using multi-energy implanted As⁺ ions leads to a shorter THz pulse and a higher bandwidth response than using single-energy ions.

The InGaAs are also employed as photoconductive material. It is a great advantage of the III-V compound to engineer the bandgap by changing the composition ratio. For example, the bandgap of the ternary compound indium gallium arsenide (In_xGa_x-1As) can be potentially varied from 1.42 eV (x = 0) to 0.36 eV (x = 1). From a practical point of view, the protentional to achieve 0.8 eV (1550 nm optical excitation) was the motivation for investigating this material for THz applications. Doping InGaAs by iron has been demonstrated to provide required recombination sites for a subpicosecond carrier lifetime, higher optical pump saturation power, and higher breakdown voltage. Wood et al. [20] investigated the InGaAs∶Fe²⁺ emitter that is grown by using Metal organic chemical vapor deposition (MOCVD) across 830-nm to 1.55-μm optical excitation and found the highest THz power at the 1.2-μm excitation wavelength. This study shows precise control of the Fe-doping added during the epitaxial growth process and the strength of engineering the bandgap of III-V materials compound.

Heterostructure devices consisting of alternate InGaAs/InAlAs multilayer stacks (multiquantum wells) have been proposed [21] as potential materials for photoconductive devices and achieve high performance at 1550 nm comparable to LT-GaAs excited at 800 nm. Sartorius et al. demonstrated the first InGa(Al)As-based THz photoconductive devices operating at 1.5 μm [22]. In their device, an MQW comprised of 12 nm InGaAs∶Be²⁺/8 nm InAlAs as the photoconductive region was grown using standard low-temperature methods on an InP substrate. Moreover, the material dark resistivity increases by four orders of magnitude comparable to bulk InGaAs due to the presence of Be²⁺ during the growth and the insertion of InAlAs, which has a higher dark resistivity than the InGaAs∶Be²⁺.

In addition to the GaAs, and InGa(Al)As, many other materials of group III-V such as InAs [23], InSb [23], GaSb [24], GaAsSb [25], and doped InGaAs [26], GaInSb [25] are studied as photoconductive material. Choosing the materials highly depends on the application and operating wavelength. Although LT-GaAs is still the most used material for photoconductive devices and is the most efficient material for 800 nm. However, it exhibits poor absorption at 1.55 μm, where other materials such as InGaAs or InGaAs/InAlAs heterostructure become more attractive. Table 1 summarized some of the photoconductive materials with the advantages, disadvantages, active layer, and the operating wavelength.

Photoconductive material	Advantages	Disadvantages	Active layer	Operating wavelength (nm)
GaAS	The most used materials for THz PCAs and is well studied.; It is the most efficient material for 800 nm.	It is not suitable for 1550 nm excitation wavelength.	LT-GaAs	780
			LT-GaAs	770
			LT-GaAs	776
			LT-GaAs	800
			SI-GaAs	800
			GaAs:Er	1550
InGaAs	Suitable for 1550 nm excitation wavelength.	Low dark resistivity.	InGaAs	1550
			InGaAs	1550
Multi-QW	Higher dark resistivity.; High performance at 1550 nm comparable to LT-GaAs excited at 800 nm.	More complication.	InGaAs/InAlAs	1550
other materials of group III-V	The ability to engineer the target excitation wavelength.	More complication.; It is not well studied.	InAs	780/ 1550
			InSb	780/ 1550
			GaSb	800
			GaAsSb	800 (up to 1440)
			InGaAs	800 and 1500
			GaInSb	800

Table 1.

Summary of some photoconductive materials with the advantages, disadvantages, active layer, and the operating wavelength.

3. Photoconductive structure

The photoconductive devices for THz emission have been developed extensively to fulfill the demand for high-performance THz emitters—and thus be essential for spectroscopic and imaging applications. The development of the emitters’ structure is related to its design and dimensions and how that is attributed to the high performance of the THz emission. The performance of the photoconductive THz emitters is determined in the form of radiated power (or the THz spectral amplitude), SNR, and bandwidth. It is worth noting that the bandwidth here manifests itself as is the maximum frequency in the THz spectral amplitude, as a function of frequency, f, before the noise level of the measurements system. This chapter will provide an overview of such development regarding the design and structure of photoconductive THz emitters, with insights into the different emitter structures based on the dimensions, such as large-aperture and interdigitated electrodes THz emitters, small aperture THz emitters, and plasmonic THz emitters.

In the large-aperture and interdigitated electrodes photoconductive THz emitters, the gap between the two electrodes can be large as 4 mm to 130 μm [11]. Such a gap will allow a high level of optical excitation before reaching the saturation issues. Thus, the importance of such emitters stems from the need to scale up the radiated power, which is influenced by the incident optical power. A molded has been developed by Darrow et al. to study and predict the saturation in large-aperture photoconductive THz emitter, which is expected to have the saturation at higher pump fluence than the small-aperture THz emitter does [27].

In the small-aperture photoconductive THz emitter (dipole antenna), the gap between the two electrodes is smaller than in the large-aperture THz emitters, typically below 200 μm. In this case, it will be more difficult to align the laser spot within the PC gap. Although these emitters experience the saturation issues at lower pump fluence, in comparison with the large-aperture THz emitters, these emitters provide broader bandwidth over the large-aperture THz emitters. Our recent work on the design and structure of photoconductive THz emitters based on SI-GaAs examined the influence of bowtie structure characteristics on the THz spectral amplitude and bandwidth [28]. It is found that the bandwidth can be improved from 3.4 THz to 3.7 THz by changing the design of electrode structure from a sharp bowtie to an asymmetric bowtie structure at the same photoconductive gap. That could be attributed to the smaller capacitance of the sharp bowtie structure over the asymmetric bowtie structure, which results in a shorter resistance-capacitance (RC) time constant. The RC time constant, τ_RC, can be related to the gap conductance (of the photoconductive antenna), G(t), the gap capacitance, C, and the antenna/transmission line impedance, Z₀ as [28]:

$${\tau }_{\mathrm{R}\mathrm{C}}=2Z_{0}C/\left(2Z_{0}CG\left(t\right)+1\right)\text{,}$$E1

Figure 2 illustrates the biased photoconductive gap with its equivalent circuit, here the redistribution of charge on the electrodes, can be seen as incident voltage waveform, vi(t), reflected voltage waveform, vr(t), and transmitted voltage waveforms, vt(t).

Figure 2.

A biased photoconductive gap at bias voltage, V_b. The inset shows an equivalent circuit of the photoinductive gap having the electrodes with incident voltage waveform, vi(t), reflected voltage waveform, vr(t), and transmitted voltage waveforms, vt(t). The gap conductance is shown as G(t) and the gap capacitance is shown as C. this figure is reprinted from [28].

The plasmonic THz emitter is introduced by Berry et al. by means of increasing the THz radiated power. An improvement of up to 50 times of the THz radiated power is observed using a plasmonic structure compared to a conventional photoconductive THz emitter [29]. The fabricated plasmonic THz emitter was based on LT-GaAs to provide an ultrafast photoconductor response. The distance between the two electrodes was 60 μm; the antenna structure was bowtie without fine tips; the maximum width of the antenna was 100 μm; the minimum width was 30 μm. In the plasmonic structure, the grating structure had a gap width of 100 nm, and the deposited gold had the same width of 100 nm. The optical pump focused on the gap close to the anode to maximize the THz radiation [29, 30]. Figure 3 shows a comparison between the conventional photoconductive antenna and the plasmonic photoconductive antennae as in (a) the conventional photoconductive THz emitter and (b) the plasmonic photoconductive THz emitter [32]. In addition to the plasmonic metal structure, the silver nano-islands on the photoconductive surface improve the performance of the photoconductive THz emitter [33]. Georgiou et al. have been recently demonstrated a 3-dimensional photonic-plasmonic photoconductive device. The performance of the THz emission is enhanced by employing a periodic array of nanopillars, which raise the optical absorption on the device surface and optimize the collection efficiency by converting each nanopillar into a single (nano)-photoconductive switch. As a result, nearly the overall generated current and the bandwidth are increased by 50-fold and five times, respectively. However, such a device will request high-tech technology to fabricate it [34]. On the contrary, the metal-based nano-islands structures enhance the photoconductive THz emission with a less complicated fabrication process. It is worth noting that these metal nano-islands can be produced by fine-tuning the deposition and thermal annealing process. Thus, it will offer less complexity during the fabrication in compression of the above-mentioned plasmonic structure.

Figure 3.

A schematic diagram of the photoconductive device shows in (a), the conventional photoconductive THz emitter, and in (b), the plasmonic photoconductive THz emitter. © IOP publishing. Reproduced with permission. All rights reserved [31].

Overall, the photoconductive structure plays a significant role in the performance of the photoconductive THz emitters. The large-aperture and interdigitated electrodes THz emitters mitigate the influence of saturation for scaling up the THz emission with the optical influence. The small aperture THz emitter (dipole antenna) shows a broader bandwidth, which allows discovering a more comprehensive range of THz frequencies. In addition, the recent studies on plasmonic devices present their significance to the photoconductive THz emitter performance. It also steers the future research and development of high-performance photoconductive devices for spectroscopy, sensing, and imaging applications.

4. Limitations of the THz emission by photoconductive devices

The underlying physics of the THz emission by photoconductive devices is the core of this section, which helps understand these devices’ behavior. The photoconductive THz emission scales linearly with the applied bias field and pump fluence. However, that can be precise only in the ideal case, at low levels of bias field and/or optical excitation. Higher levels of bias field influence the photoconductive THz emitters’ performance. Such influence can be seen as thermal effects, space-charge-limited current effects, etc. In addition, the photoconductive device has a limitation at a higher bias field correlated to the breakdown voltage of the photoconductive material. The pump fluence also has an impact, but that can be observed as the saturation of the THz radiation. The saturation (screening) of the THz radiation is mainly associated with two different mechanisms, being space-charge and near-field screening. This section will explore the limitation of the photoconductive THz emission with insights into the material and structure implications on photoconductive THz emitter’s performance.

The THz radiated power (or the THz field amplitude, E_THz) can be scaled with the bias field, E_b, in three different mechanisms, based on the bias filed value and the photoconductive material, being a superliner (red), a linear (blue), and a sublinear (yellow), as shown in Figure 4. The superliner behavior is seen clearly with the photoconductive THz emitters based on GaAs. This superlinearity is associated with the space-charge-limited current due to the deep EL2 traps states [29, 35]. The superliner behavior is associated with the limitations within the semiconductor. This limitation manifests itself as Joule heating within the semiconductor and is observed in the photoconductive THz emitters based on InP [32]. Collier et al. have studied the Joule heating limitation in the photoconductive THz emitters based on InP. They found a correlation between the surface quality and the carrier lifetime, which directly affects Joule heating. In the textured InP photoconductive THz emitters, the carrier lifetime is decreased, which reduces the photocurrent and ultimately diminishes the Joule heating [32].

Figure 4.

The scaling of THz radiated power as the THz field amplitude, E_THz, with the bias field, E_b, in three different methods, being a superliner (red), a linear (blue), and a sublinear (yellow).

The pump fluence impacts the radiated THz power in the form of saturation (screening). At a higher level of optical excitation, the radiated THz power will be saturated. This saturation can be classified into two mechanisms, being space-charge and near-field screening. However, each screening status differs based on the photoconductive characteristics (material and structure) and optical characteristics (pump fluence). It is worth noting that transient mobility (mobility as a function of pump fluence) plays an important role in the emitted THz power and thus in the screening of the THz field [30]. The mechanisms of these two screening effects are explained in the next paragraph.

In the space-charge THz screening, the limitation of the photocurrent within the photoconductive gap is due to the high carrier densities within the photoconductive gap, affected by the high pump fluence. The charges drift in the opposite direction. Thus, the bias field screens and ultimately limits the radiated THz field [36]. In the near-field THz screening, the direction of the radiated THz field is in the opposite direction of the bias field, which limits the linear scale of the THz radiated field with the pump fluence, as increasing the pump fluence will raise the carrier densities within the semiconductor [37]. At the same pump fluence, the carrier densities in the emitter with a large photoconductive gap will be smaller than in the emitters with a small photoconductive gap. Thus, a large photoconductive gap emitter leads to scaling up the radiated THz power for higher levels, which increases the total emitter performance, before reaching the screening issues [30].

Overall, the main limitations of the THz emission by photoconductive devices can be related to the applied bias field and the exciting pump fluence. The two limitations are correlated with the photoconductive material and structure characteristics. These two limitations prevent the THz field amplitude from scaling linearly with the bias field and pump fluence. Thus, it is essential to design the photoconductive THz emitter carefully. Furthermore, the photoconductive material must be chosen judiciously to meet the demand of the high-radiated THz field for the aforementioned advanced applications.

5. Recent advances in photoconductive devices

A number of the recent advances and research in the field of photoconductive devices are discussed in this section, with insight on the development of the material and structure to enhance the photoconductive THz emission for spectroscopic, sensing, and imaging applications. The section will explore different approaches including:

• Quantum dots.

• Nanostructured electrodes (non-plasmonic) of the photoconductive device.

• Dielectric metasurfaces in photoconductive terahertz devices.

• Grating photoconductive devices.

The development of the photoconductive THz emission using such new approaches is notable. The quantum dots are mainly related to photoconductive materials. In contrast, the nanostructured electrodes, dielectric metasurfaces, and Grating photoconductive devices are associated with the photoconductive structure. Here, the main interest is to focus on improving the THz emission using these different approaches and the potential enhancement of these devices.

The quantum dots have been employed to boost the photoconductive THz emitters’ performance. Gorodetsky et al. used InAs quantum dots in bulk GaAs. The short carrier lifetime has captured with the dots in such devices and maintains high carrier mobility [38, 39]. The photo-electronic priorities of the quantum dots can be managed by controlling the characteristics of these dots during epitaxial growth. It is worth noting that quantum dots have three-dimensional effects compared with a one-dimensional effect in the quantum wells. In Gorodetsky et al. study, the active regain consists of InAs quantum dots layer (1–2 nm), InGaAs wetting (5 nm), and GaAs spacer (35 nm). At the top of these layers, an LT-GaAs layer is grown with a 30-nm thickness, the observed boost of such structure is about 5-fold at 1.0 THz [37]. In addition, GaAs with ErAs quantum dots has been demonstrated for exaction laser having a 1550-nm wavelength [40]. The observed conversion rate (from optical to THz power) is 0.18%. However, this result was obtained by using a resonant slot antenna.

Nanostructure electrodes of the photoconductive device show an improvement of the photoconductive THz generation, even without a plasmonic effect. Although the plasmonic photoconductive THz emitter is one of the breakings through in the THz generation and detection field, the nanostructure has its encasement on the performance of such devices [41]. Singh et al. examined an antenna nanostructure fabricated by utilizing an electron-beam lithography system (EBL), having a 5-nm titanium layer and a 25-nm gold layer. Hilbert-fractal design is used with different line widths up to 140 nm. An improvement of the emitted THz power by an approximate factor of two is observed using this nanostructure.

Dielectric metasurfaces in photoconductive terahertz devices can be used as an alternative method to enhance the photoconductive THz emitters’ performance instead of the plasmonic structure [42]. Although the plasmonic structure delivers better THz field improvement over the dielectric structure, the dielectric structure has a substantial characteristic which is the lack of dissipation [43]. In addition, the optical absorption of the incident light (laser) onto the photoconductive device can be improved by reducing the Fresnel losses, which can be done by using thin films of dielectric materials on top of the photoconductive gap. These dielectric materials (thin films) include SiO₂, Si₃N₄, Al₂O₃, and TiO₂ [44, 45]. Figure 5 shows a bowtie antenna having a layer of TiO₂ being coated on the photoconductive gap, in (a) the schematic view of the photoconductive THz emitter, (b) the SEM image of the photoconductive THz emitter, and (c) the THz spectral amplitude obtained with using TiO₂ layer (red) and without using TiO₂ layer (black), “from [45]”.

Figure 5.

The bowtie photoconductive antenna with TiO₂ layer, coated on the photoconductive gap, in (a) the schematic view of the photoconductive THz emitter, (b) the SEM image of the photoconductive THz emitter, and (c) the THz spectral amplitude obtained with using TiO₂ layer (red) and without using TiO₂ layer (black). This figure is reprinted from [45] with the permission of AIP publishing.

The grating structure manifests itself as a periodic array of grooves, lines, slits, etc. The grating structure of the photoconductive devices for THz generation has been studied according to the effective medium approximations (or effective medium theory). The theory can be applied to describe the interaction of light with the grating structure (subwavelength) [46]. Chia et al. have modeled and simulated the influence of grating structure on the THz emission performance by COMSOL Multiphysics software with an insight into the effects of grating geometrical parameters. The author funds an improvement of about 1.63 of the photocurrents obtained by an optimized grating structure of photoconductive THz emitter over the planer emitter structure. This is due to the higher photon absorption, which leads to and leads to more carrier generation within photoconductive material, thus higher photocurrent is observed [46]. Figure 6 shows the simulated grating structure of LT-GaAs and its effects, as in a) the upper diagram shows the surface of planner photoconductive THz emitter, the lower diagram shows grating structure of the photoconductive THz emitter used in the simulation, and b) the normalized electronic concertation obtained by the two different simulated photoconductive THz emitters, “from [46]”.

Figure 6.

The simulated grating structure of LT-GaAs, as in (a) the upper diagram shows the surface of planner photoconductive THz emitter, the lower diagram shows grating structure of the photoconductive THz emitter, and (b) the normalized electronic concertation obtained by the two different simulated photoconductive THz emitters. This figure is reprinted from [46].

Nowadays, the development of photoconductive devices regarding materials and structure is a hot research topic. Several publications have discussed many schemes to achieve higher performance of THz generation by photoconductive devices to facilitate the applications in cutting-edge technologies such as THz spectroscopy, THz sensing, and THz imaging. For photoconductive materials, the research focuses on the quantum dots as well as promotes material properties such as the carrier lifetime and carrier mobility. For the photoconductive structure, the implementation of plasmonic and nanostructures shows its advantage for the aforementioned applications. However, utilizing some novel ideas such as grating structure and a precise selection of the dielectric material is demonstrated to boost the performance of photoconductive devices further.

6. Conclusion

This chapter presented the photoconductive devices for THz emission. Several materials have been employed as photoconductive materials. However, GaAs is a typical material for these applications, particularly for the sapphire femtosecond pulsed laser sources, which emit at the same range of the bandgap energy of GaAs. Furthermore, several photoconductive structures have been employed. The plasmonic structure shows the highest impact of the photoconductive THz emitters’ performance over the microstructure photoconductive THz emitters. On top of that, the screening effects of the THz field amplitude is an issue limiting the linear scaling of the THz field with the pump fluence. Such limitations can be diminished using a large-aperture photoconductive antenna. At the end of this chapter, the improvement of these devices’ performance has been considered by viewing some recent work in this area. The work has also presented the influence of the quantum dots, the nanostructured electrodes (nonplasmonic) of the photoconductive device, the dielectric materials in photoconductive terahertz devices, and the grating structure on the photoconductive surface. It is hoped that the presented work can lay a role in continuing advancements of photoconductive devices.

Conflict of interest

The authors declare no conflict of interest.