Efficient Terahertz Photoconductive Emitters with Improved Electrode Structures

Ying-Xin Wang, Yi-Jie Niu, Wei Cheng, Zhi-Qiang Li, and Zi-Ran Zhao

1.Introduction

Terahertz time-domain spectroscopy (THz-TDS) has received much attention recently for its great application potentials in fields of medical, biological, pharmaceutical,chemical and material sciences, quality control, and security inspection[1].Pulsed terahertz sources play an important role in the development of this technique.Broadband and intense terahertz radiation is highly desired for the real-world applications of THz-TDS.Various methods have been developed for the generation of broadband terahertz pulses, such as photoconductive antennas (PCAs)[2], optical rectification in electro-optic crystals[3], and laser-Induced air plasmas[4].PCAs are one of the most promising and widely used emitters due to their good performance in terms of both emission power and spectral bandwidth.

The radiation characteristics of a PCA mainly depend on the excitation laser, the bias voltage, the substrate material, and the electrodes.Shorter incident laser pulses may lead to the generation of ultrabroadband terahertz radiation[5].Increasing the pump laser intensity or the bias voltage would enhance the emission power of the PCA as long as no saturation effects occur[6].In addition to these extrinsic factors, the intrinsic properties of PCAs are expected to be more significant.It has been shown that low-temperature grown gallium arsenide (LT-GaAs)possesses a short photocarrier lifetime and high mobility and is commonly used for fabrication of the substrate at present[7],[8].The electrode material will determine the contact type of the antenna (ohmic or Schottky) which affects its radiation efficiency[9].So far, many studies have concentrated on the improvement of the antenna performance by designing more appropriate electrode structures.The most conventional structures involve the coplanar strip line[2], dipole[10], and bow-tie[11]antennas,which exhibit slightly different emission characteristics[12].Log-periodic antennas[13]could achieve higher radiation power.However, they usually have resonant behaviors at certain frequencies and are generally employed in terahertz detectors[14].In recent years, new types of antenna structures have been proposed, including four-contact[15],fillet[16], square spiral[17], interdigital finger[18], plasmonic electrodes[19], etc., and their superior performance has been demonstrated especially for the emission efficiency.For a specific structure, the electrode geometrical parameters also have an influence on the antenna characteristics[20].Nevertheless, design and optimization of the electrode structure is of great concern for practical use of PCAs.

In this work, we design and implement two new types of antenna structures, asymmetric four-contact and arc-shaped, for the efficient generation of pulsed terahertz radiation.They are simply modified from the traditional strip line antenna.Numerical simulation and experimental measurement results show that our proposed antennas have enhanced optical-to-terahertz conversion efficiency.We experimentally demonstrate approximately 40% amplitude increase of terahertz signals emitted from the new structures in comparison to a strip line antenna with similar geometrical parameters.

2.PCA Design and Fabrication

For photoconductive terahertz emitters, the strip line is one of the most popular structures and is preferable owing to its relatively good stability with sufficient power and bandwidth.Therefore, based on this kind of structure, we explore the improvement of the antenna efficiency by simply modifying the electrode pattern.We designed two new structures, as shown in Fig.1.The first one is defined as an asymmetric four-contact antenna.At the center, it has an additional gap perpendicular to the strip line.The bias voltage was applied to the two left adjacent contacts and the other two were grounded.The electrode width is 100 μm,and the widths of the vertical and horizontal gaps are all 100 μm.It should be pointed out that Hirota et al.[15]have reported a symmetrical four-contact antenna with two crossed and identical gaps.Different from our work, they used such an emitter to investigate the polarization modulation of terahertz pulses and did not involve the problem of emission efficiency.The second one is an arc-shaped antenna where there exist four semicircular arcs on each side of the strip line.The electrode widths of the arcs and strip line are all 100 μm, and the gap size is 100 μm.The outer radius of the arc is 400 μm.For the strip line in Fig.1 (a), the electrode width and the gap size are also all 100 μm.

A layer of LT-GaAs with a thickness of 2 μm grown on a 600-μm-thick semi-insulating GaAs wafer served as the substrate material of the PCA.The grown and annealed temperatures of LT-GaAs were 300°C and 600°C,respectively.By Hall-effect measurements, the resistivity and the carrier concentration and mobility of the substrate were determined to be about 2.36×105Ω·cm, 7.27×109cm-3,and 3.65×103cm2/(V·s), respectively.We fabricated different electrode patterns on the antenna substrate by using the lithography and electron-beam evaporation techniques.The metal electrodes were made of Au-Ge-Ni alloy which had a thickness of 350 nm and forms an ohmic contact between the electrode and the substrate.Each antenna had a lateral size of 5 mm×5 mm and was mounted on a printed circuit board to facilitate applying the bias voltage.

Fig.1.Schematic of the electrode structures of PCAs: (a) strip line PCA, (b) asymmetric four-contact PCA, and (c) arc-shaped PCA.

3.Simulation of Terahertz Emission from PCAs

In order to evaluate the influence of electrode structure on the antenna emission properties, we first performed numerical simulations on the radiation field by using the finite-difference time-domain (FDTD) method.In the simulation, we took the photocurrent generated by the femtosecond laser incident at the electrode gap as the excitation source and its temporal evolution was approximated by a modulated Gaussian pulse.For simplicity, here we only considered the strip line and the asymmetric four-contact antennas.The substrate material was GaAs with a nearly constant refractive index of 3.6[21]and a size of 5 mm×5 mm×0.5 mm.The metal electrode was Au with 5 mm length, 100 μm width, and 100 μm in the gap size, consistent with that of the fabricated samples.We chose an observation point at a distance of 0.4 mm from the excitation source to record the information of terahertz radiation.

Fig.2.Terahertz signals emitted from the strip line and the asymmetric four-contact PCAs obtained by FDTD simulations: (a)temporal waveforms and (b) frequency spectra.

Fig.2 illustrates the temporal waveforms and frequency spectra of terahertz signals recorded at the observation point for the two simulated structures.We can see that the terahertz signal is stronger for the four-contact antenna and the increase in peak-to-peak amplitude of the electric field waveform is 20.7% than the strip line antenna.The spectral bandwidths of both structures are almost identical.However, the four-contact antenna has more highfrequency components below about 1 THz and the lowfrequency part of its spectrum below 0.4 THz is suppressed.Based on these simulation results, we can conclude that the antenna structure would indeed affect the generated terahertz intensity and the four-contact antenna has a higher radiation efficiency under the same conditions.Consequently, the design of more reasonable geometrical structure of the electrode may provide an important mean for improvement of the antenna radiation performance.Note that we adopted the same current source for these two structures in the simulation.Actually, different electrode structures could cause different bias electric field distributions, thus yielding different photocurrents.This effect would also have influence on terahertz emission and was not taken into account in the simulation.The real differences of the radiation characteristics need to be further examined by experimental measurements.

4.Experimental Characterization of the Antenna Performance

The experimental setup for generation and detection of terahertz radiation is a conventional THz-TDS system[6].Ultrafast femtosecond laser pulses with 100 fs duration,800 nm central wavelength, and 80 MHz repetition rate served as the pump and probe light.The laser was divided into two beams by a beam splitter.The pump beam was focused onto the PCA gap to excite terahertz radiation with an estimated spot size of about 20 μm.The probe beam was guided to a 1-mm-thick ZnTe crystal for electro-optical sampling detection of terahertz pulses.Using a half-wave plate and a polarizer, the pump and probe beam power could be adjusted.The probe beam power was fixed to be about 5 mW during the experiments.

Firstly, we measured the terahertz signals generated from the strip line, the asymmetric four-contact, and the arc-shaped antennas under the same conditions of bias voltage and pump power.The bias voltage was 100 V.The pump beam with a power of about 50 mW was incident onto the gap center and then scanned to find an optimal spot position corresponding to the maximal terahertz output[9],[22].Fig.3 shows the temporal waveforms and frequency spectra of terahertz pulses emitted from the three kinds of structures.It is clear that the two new designed antennas have produced more intense terahertz radiation than the traditional strip line antenna.The peak-to-peak amplitudes of the pulse waveforms for the four-contact and arc-shaped structures are 43.6% and 38.7% larger,respectively.In the frequency domain, the signals for the two new structures are almost all enhanced within the entire useful range, especially between 0.2 THz and 1.2 THz.Therefore, our experimental observations verify that the efficiency of a terahertz photoconductive emitter could be increased by improving the electrode structure.In contrast to the simulation results, the measured terahertz signal amplitude increase for the four-contact antenna is more remarkable, which might be attributed to the effect of bias electric field distribution associated with the electrode structure, as mentioned above.In the four-contact structure,the local electric field at the center of the crossed gaps has a converging behavior and thus is enhanced, leading to more efficient generation of terahertz pulses.It could be inferred that the arc-shaped antenna should also have a local field enhancement effect because of the existence of arc structures.

Fig.3.Terahertz signals emitted from the strip line, the asymmetric four-contact, and the arc-shaped PCAs measured by the THz-TDS system: (a) temporal waveforms and (b) frequency spectra.

Fig.4.Terahertz signal amplitudes under different pump and bias conditions for the asymmetric four-contact antenna: (a) pump power and (b) bias voltage.

Fig.5.Terahertz signal amplitudes under different pump and bias conditions for the arc-shaped antenna: (a) pump power and (b)bias voltage.

5.Conclusions

In conclusion, we have designed, fabricated, and characterized two new types of PCAs, including an asymmetric four-contact structure and an arc-shaped structure.FDTD simulations and THz-TDS measurements demonstrate that the proposed antenna structures have much higher terahertz radiation efficiencies.Compared with the traditional strip line antenna, the increases of the peak-to-peak amplitudes of terahertz pulses emitted from the four-contact and arc-shaped structures can reach as large as 43.6% and 38.7%, respectively.Such increases result from the special electrode structure and the local electric field enhancement occurring in the antenna gap.This work indicates that the improvement of the antenna electrode structure is an effective and feasible way for the development of highly efficient terahertz photoconductive emitters.

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