1. Introduction

In the electromagnetic spectrum, THz frequencies hold an interesting band owing to numerous potential applications ranging from spectroscopy, [1–5] non-contact sensing [6–8] and high-speed communication [9–11]. However, several factors, such as the unavailability of efficient sources and detectors have hampered the exploitation of THz technology in day-to-day usage. Nonetheless, in recent years, considerable efforts have been made due to the growing importance of this field. In this direction, the advent of photo-conductive antennas (PCA) significantly advanced the terahertz research due to its compactness, low cost, and operation at room temperature [12,13]. Photo-Conductive Antennas (PCAs) are the most common of all sources for the generation of Terahertz (THz) radiation. This is mainly because these devices do not require special environmental conditions to operate and are relatively robust and cost-effective. PCA can act both as emitter as well as THz detectors [14]. Although the main focus of the community is towards enhancement of the optical-to-THz conversion efficiency of PCAs, bandwidth engineering is an important and crucial aspect which need to be addressed [15–18]. Typically, in a PCA based on GaAs, an IR pulse (800 nm) with photon energies exceeding the bandgap is used to irradiate the photo-conductive gap between the electrodes. This excitation results in the photoexcitation of charge carriers in the semiconductor substrate. Simultaneously, a bias voltage is applied across the electrodes which accelerates the photogenerated charge carriers towards the anode, resulting in an instantaneous drift current density, J(t). The acceleration of the photo-excited charges generates the emitted THz radiation. Hence, the emitted THz electric field, E_THz, is proportional to the slope of the current density in time, i.e. E_THz $\varpropto$ ∂J(t)/∂t with J(t) = n(t)qµE(t), where n(t) is photocarrier density as a function of time, q is the electron charge, µ is mobility, and E(t) is the bias field [15,19,22]. This implies that it is not the total terminal current, but the rate of change of current which determines the emitted THz amplitude and bandwidth. This factor is considered by engineering fast carrier dynamics in semiconductors by growing it in low-temperatures (ex. LT-GaAs) to introduce defect states. However, these devices still suffer from sub-linear behavior of emission with increasing applied electric field due to Joule heating from high current densities.

In this article, we demonstrate a novel and effective technique which can be implemented on any PCA design capable of enhancing the THz emission bandwidth without suffering much from the effects of Joule heating. We nanoengineer the photoconductive gap of the PCA with an embedded plasmonic metasurface structure [20–24]. The metasurface is designed to maximize the incoupling of the infrared radiation to the SI-GaAs substrate. However, this is not the only purpose it serves. The embedded plasmonic structures act as local absorbers of the drift charges. This enhances the decay rate of the drift photo-current flowing in the device due to irradiation with the infrared under applied bias. This introduces faster charge dynamics and as a result generation of THz radiation with enhanced bandwidth [25–27]. Furthermore, due to lower terminal currents, the device suffers less from Joule heating than conventional devices. This device has the potential for applications in high power THz sources and high signal bandwidth for THz communication [28–30].

2. Device and setup

2.1 The PCA description and fabrication

The schematic diagram of proposed PCA mounted on HRFZ-Si lens (for outcoupling the emitted THz) is shown in Fig. 1(a). The PCA consists of a nanopatterned photoconductive gap (25 µm by 25 µm) on 350 µm thick SI-GaAs substrate with AuGe bow-tie electrodes on both sides. The nanopatterned plasmonic metasurface array comprises of Au nano-pillars diameter 80 nm and a height of 100 nm in a hexagonal lattice with a lattice constant of 180 nm. The Au nanopillars were embedded in the GaAs substrate between the Au-Ge electrodes as demonstrated by the zoomed inset in Fig. 1(a). The SEM image of the PCA is shown in Fig. 1(b). The sample consists of 4 PCAs, two with the nano-patterned PC gap and two unpatterned PCAs which serve as references. The zoomed inset in Fig. 1(b) on the third device shows the SEM image of the plasmonic metasurface embedded in the PC gap.

 

Fig. 1. Schematic showing the fabricated PCA on SI-GaAs substrate. Inset shows the embedded metasurface in the photoconductive gap of the PCA. The TiO₂ antireflection coating has not been shown here. (b) SEM image of the fabricated devices. The First and the third devices have nanostructured PC gap, while the second and the fourth are bare PCAs for reference.

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We have adopted a standard two-stage lithography technique for the fabrication of the PCAs. In the first stage, we fabricated the AuGe pads as shown in Fig. 1(b) (deposited using a magnetron sputtering system) with electron beam lithography followed by selective etching using Ar-plasma with a positive electron beam resist N7520 as a mask. In the second stage, the nanopattern was written on electron resist N6200 with electron beam lithography with a finer aperture. Holes (diameter 80nm and depth 100nm) were etched out in GaAs by chlorine plasma. Au layer of 100 nm is then deposited on the device using DC sputtering. During this process, the holes are filled up with Au. Subsequently, lift-off was performed to remove the remaining Au on the device. An antireflection nanolayer of TiO₂ with a thickness of 220 nm was deposited on the device except the contact electrodes to ensure enhanced coupling of infrared radiation into the device. [18]

2.2 Setup description

We used a PCA characterization setup for performing the measurements. This setup is a modified THz Time Domain Spectroscopy (TDS) setup capable of measuring time domain signals emitted from PCAs, measure current-voltage (IV) characteristics studies as well as pump intensity and applied voltage-dependent THz emission studies. The setup has been extensively discussed in [15]. Optical pump-probe experiments for the measurement of carrier-recombination dynamics were performed with a conventional pump-probe setup in the reflection mode. The pump-probe measurements were performed before the deposition of TiO₂. This was done to extract enough reflection signal in the detector to ensure a fair signal-to-noise. At the same time, the TiO₂ layer doesn’t have any effect on the carrier relaxation process in the semiconductor. Thus, it is logical to measure the carrier relaxation before coating the device with an antireflection layer.

3. Results and discussion

3.1 Experimental

We have performed degenerate pump-probe measurements with 800 nm radiation on both the nano-patterned and the unpatterned bare photoconductive gap in the two PCAs on the same substrate. Figure 2(a) shows a comparison of the photo-excited carrier recombination in both the PC gaps. The black blurred circles show the carrier lifetime of un-patterned SI-GaAs substrate. The black line is an exponential fit to the data points. The carrier lifetime is defined as the time taken by the excited free electrons to recombine back from conduction band to valence band. This can be demonstrated by the formula as shown below,

 

Fig. 2. (a) Carrier recombination dynamics from pump-probe studies performed on the Au nano-patterned (blurred red circles) and the unpatterned (blurred black squares) photo-conductive gap in Photoconductive antenna (PCA) emitters. Both the signals were measured in reflection mode and have been normalized. The solid black and the solid red lines indicate the exponential fit to the data points. (b) Shows the current-voltage (I-V) characteristics of both the devices: patterned (solid curves,) and unpatterned (dotted) for different pump fluences.

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The enhancement in the bandwidth of the emitted THz radiation is shown in Fig. 3(a). The dotted black curve shows the spectrum of the emitted THz from the unpatterned PCA while the solid red curve shows the same for the patterned PCA. The black dotted and the red solid horizontal lines denote the average noise levels in the two spectra. The spectra are vertically displaced to show the enhancement of the bandwidth in case of the patterned PCA. The bandwidth of a spectrum is defined by the point in the frequency axis where the spectrum meets the noise level. For the case of the THz spectrum emitted from the patterned PCA, it can be appreciated from Fig. 3(a) that the bandwidth is ∼1.5 THz while for the unpatterned PCA, the bandwidth is ∼1 THz. Thus, the emission bandwidth for the patterned PCA is enhanced by approximately 50% compared to the emission bandwidth of the unpatterned PCA. The metasurface has a plasmonic resonance at 800 nm. Hence, the infrared light is concentrated into localized radiation hotspots in the vicinity of the Au nano-pillars throughout the lattice. This effect will be discussed in more details in the next section supported by FDTD simulations. These hotspots, due to the embedded nature of the nano-pillars, form rich sources of photo-generated carriers, resulting in high drift current densities [15]. Additionally, the incoupling efficiency of the infrared radiation into the SI-GaAs substrate is also enhanced due to the metasurface acting as an anti-reflection layer. Effectively, these factors contribute to an enhancement in the emitted THz intensity from the patterned PCA. Figure 3(b) shows the normalized THz peak amplitudes as a function of applied bias voltage for different pump powers. The enhancement can be seen as much as 100% for pump powers of 80 mW. The powers can be seen to reach some saturation value for applied voltage higher than 4 KV/cm especially for higher pump powers. This saturation most likely arises from Joule heating in the device due to high current densities.

 

Fig. 3. (a) Spectra of emitted THz from the patterned (red solid curve) and the unpatterned (black dotted curve) PCA. The horizontal red solid and black dotted lines mark the position of the noise levels in the signals. The spectra are vertically shifted for clarity. (b) Shows the normalized emitted THz peak amplitude as a function of applied bias voltage at different infrared pump powers (black = 20 mW, red = 40 mW, green = 60 mW, blue = 80 mW and magenta = 100 mW). The solid curves represent the emitted THz amplitudes from the patterned PCA whereas the emitted THz amplitude from the unpatterned PCA is represented by the dotted curves.

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3.2 Simulations

We have performed FDTD-based numerical simulations of Au nanopillar structures embedded in SI-GaAs structures with dimensions as mentioned in the PCA description section. In the simulation, we illuminated the structure with a broadband plane wave electric field with pulse comparable to that of the femtosecond laser used in the experiments. Periodic boundary condition was used to mimic the periodicity effect of the nanostructure array. Figure 4(a) shows the reflection spectrum of the device as a function of wavelength. The structure clearly shows a reflection minimum at 800 nm. In a plane SI-GaAs substrate, the reflection is in the order of 33%. Thus, it can be concluded that the nanopatterned substrate with the anti-reflection layer acts as an anti-reflection coating, thereby increasing the in-coupling efficiency of the infrared radiation into the substrate. The direct consequence of this enhanced in-coupling is evident in the ∼100% increase in the THz emission peak as seen in the experimental results. The electric near-field intensity distribution along a transverse plane on the surface of the SI-GaAs with respect to the propagation direction, in the vicinity of the Au nanopillar at 800 nm is shown in Fig. 4(b). Local electric field intensity hotspots can be seen appearing in the extremities of the nanopillars. These hotspots are rich sites of photo-generated charge carriers which contribute to enhanced drift current densities under applied bias. Figures 4(c) and 4(d) shows the electric near-field intensity distribution of 800 nm in the two longitudinal orthogonal planes in the device along the direction of propagation. It can be appreciated from the intensity profiles (indicated by the white curves in the panels) that the near-field intensity hotspots are concentrated near the top edge of the Au-nanopillars. This distribution of near-field intensity hotspots has a profound effect on the performance of the device, especially in terms of bandwidth enhancement. The hotspots and thereby the initial photogenerated charge density are concentrated near the surface of the device, rather than in the bulk. Under applied external bias, the photogenerated charges start to accelerate towards the electrodes parallel to the surface. This restricts the drift current densities effectively to the surface of the device, making it a surface current density distribution on the semiconducting substrate. In bare semiconductor substrates, the field extends effectively up to the skin depth of the semiconductor at the irradiation frequency, which is in the order of a few microns. However, in our situation, the field extends only up to a few tens of nanometers inside the substrate as shown in Figs. 4(c) and 4(d). Since the penetration of the field and hence, the drift current in the bulk of the substrate is not significant, the Au-nanopillars acts as local recombination centers for these accelerating photogenerated charge carriers. This local recombination decreases the effective carrier lifetimes of the photoexcited drift charges as seen in Fig. 2(a). As the reduction in carrier lifetimes is directly related to the enhancement of the bandwidth of the associated electromagnetic radiation emission, we get an enhancement of bandwidth of the emitted THz as demonstrated in Fig. 3(a).

 

Fig. 4. (a) Simulated referenced reflectance spectrum of the nanostructured SI-GaAs substrate. (b) Simulated near-field intensity distribution at the surface of the SI-GaAs (X-Y plane) in the vicinity of the Au nano-pillar. The direction of polarization is indicated by the white double arrow. The translucent black circle indicates the position of the Au nano-pillar. (c) and (d) Shows the electric near-field intensity distributions along crosscut through the device along the two longitudinal planes (XZ and YZ planes) along the propagation direction. The translucent black rectangles indicate the position of the Au nanopillars. The white curve shows the intensity profile along the edge of the Au nanopillar.

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In conclusion, we have shown a technique to tweak the surface carrier recombination dynamics of semiconducting substrates to improve THz emission bandwidths from PCA emitters fabricated on them. This technique involves the fabrication of embedded plasmonic nanopatterns in the PC gap of the device. The plasmonic structures not only acts as local hotspots for photocarrier generation but also serve as local recombination centers to reduce the carrier lifetimes and hence, enhance the bandwidth of the emitted THz. This technique can be easily incorporated on all PCA based THz emitters and can be an ideal candidate in the pursuit of efficient THz sources with high emission bandwidths.