Comparison of Photoconductive Antenna Performance on LT-GaAs and SI-GaAs Substrates Mingguang Tuo1, Jitao Zhang1, Min Liang1, Wei-Ren Ng1, Michael E. Gehm1, 2 and Hao Xin1 1
Department of Electrical and Computer Engineering, University of Arizona Tucson, Arizona, USA 2 Department of Electrical and Computer Engineering, Duke University Durham, North Carolina, USA
[email protected] compared to SI-GaAs (normally 1ps), which can result in narrower pulse width (wider bandwidth) for the generated THz signal.
Abstract— In this work, butterfly shaped photoconductive antennas (PCAs) on low-temperature grown (LT) GaAs and semi-insulating (SI) GaAs substrate as terahertz (THz) emitters are experimentally characterized and compared. Dependences of the THz radiated field on the applied DC bias voltage and laser pump power are compared. Quadratic DC bias dependence is seen for SI-GaAs PCA compared to a linear dependence for LTGaAs PCA. Scaling rule can be applied to the laser power dependence to fit the measurement data and the saturation can be attributed to the screening effect. Studies of material property influence on THz radiated power allow exploration of ways to enhance PCA performance.
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EXPERIMENTAL STUDY
A. PCA Configuration The two antennas studied here are operated as the emitter in a THz-TDS setup and another PCA is used as the detector. The commercial emitter studied is the PCA-44-34-100-800-h and the detector is PCA-44-06-10-800-h (BATOP GmbH, Germany [3]), where “h” stands for the hyper-hemispherical Si lens attached behind the antenna chip which is used to collimate the generated THz beam in combination with auxiliary parabolic mirrors. Both antennas from BATOP are fabricated on a 625-μm SI-GaAs substrate with a 3-μm LTGaAs epi-layer on top. The in-house fabricated PCA is on a 360-μm thick SI-GaAs substrate with a resistivity of 9.1 107 Ω·cm. The electrodes have a layered structure of 20 nm Cr / 150 nm Au and the antenna chip size is about 2 mm 2 mm. Figure 1 shows the microscope images of the in-house fabricated PCA on SI-GaAs substrate using standard photolithography process.
INTRODUCTION
Photoconductive antenna (PCA) is commonly used as one of the important THz sources since it was proposed [1]. The operation principle of a PCA is based on optical-to-THz conversion in ultrafast photoconductive material. With a femtosecond pump laser pulse illuminating on the photoconductive material, free electron-hole pairs will be generated. Under the external DC bias voltage, the generated electrons and holes will move toward the anode and the cathode respectively, generating transient photocurrent. Incorporated with an antenna structure, THz wave will be radiated out.
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However, PCA efficiency is usually very small (i.e., less than 0.1%). With the manipulation of various parameters, including the antenna geometry, excitation laser characteristics, and photoconductive material properties [2], the generated THz power and also the efficiency may be increased. The study of photoconductive material properties is one of the important aspects to help understand how to improve the performance of the PCA, which is the focus here. In this work, the performance of a commercial PCA on low-temperature grown (LT) GaAs and an antenna with identical gap size fabricated on semi-insulating (SI) GaAs substrate are experimentally investigated in a THz-TDS system. The dependences of the THz peak field amplitude on DC bias voltage and laser power are compared. The high breakdown field of the SI-GaAs (~ 400 kV/cm) allows a higher DC bias voltage, which may lead to higher radiated THz power. Meanwhile, the mobility of SI-GaAs ( 5,000 cm2/V·s) is much larger compared to that of LT-GaAs (~ 200 cm2/V·s), which may also improve the efficiency. LT-GaAs, on the other hand, has the advantage of shorter carrier lifetime (~ 200 fs)
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Fig. 1. Microscope image of (a) the butterfly antenna structure, (b) zoomed-in image of the central antenna gap region. B. THz Peak Field Dependence on DC Bias To study the characteristics of the PCAs under different conditions, a number of measurements are carried out. To increase the signal-to-noise ratio, the 800 nm Ti: Sapphire laser is chopped at 200 Hz and the received signal is detected by a lock-in amplifier (100 MΩ current gain, 10 ms time constant). A comparison of time-domain signals between the PCA on LT-
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GaAs and SI-GaAs under an average laser excitation power of 10 mW is shown in Fig. 2. The average laser power shining on the detector is always kept at 5 mW. It can be seen that under the same condition, the THz pulse has lower magnitude for the SI-GaAs PCA compared to the LT-GaAs PCA. In addition, the pulse is wider due to longer carrier lifetime of SI-GaAs which will result in narrower bandwidth. Field Magnitude [a.u.]
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The THz peak field versus DC bias (swept from 1 V to 27 V with a step of 2 V) under fixed laser power levels for the SIGaAs antenna is shown in Fig. 3(a). The previous data measured for the LT-GaAs antenna using a home-made transimpedance amplifier at a different gain resistance is plotted in Fig. 3(b) as well. It can be observed that the antenna on LTGaAs has a linear relationship with DC bias voltage under different laser powers as described in literature [4] but a nonlinear relationship is seen for the antenna on SI-GaAs.
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D. Discussions Based on the measurement results, it can be noticed that the THz response at different DC bias voltages at fixed laser power increases linearly for the LT-GaAs PCA but quadratically for the SI-GaAs PCA. The nonlinear relationship between THz peak field and the DC bias voltage is also reported in [5] but the mechanism is still under investigation. Also, PCA antenna responses at higher bias voltage for both antennas will be studied. As for the laser pump power dependence, it is seen that the THz peak field saturates at higher pump power, which is likely attributed to the screening effect of the photo generated carriers.
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Fig. 2. A comparison of the time-domain signals of the two different photoconductive antennas at 10 V DC bias as labeled.
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Fig. 4. THz peak field dependence on laser power for PCA on LT-GaAs.
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CONCLUSION
In summary, a butterfly shaped photoconductive antenna has been fabricated on SI-GaAs substrate and experimentally compared to an antenna structure with identical gap size on LTGaAs. The peak THz field dependences on the DC bias and laser power are experimentally studied. A quadratic DC bias dependence can be observed for the SI-GaAs PCA, compared to a linear dependence for LT-GaAs PCA.
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ACKNOWLEDGEMENT
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This work is supported by NSF under the contract #1126572. The authors would also like to thank Matthew Yankowitz and Shengqiang Huang for their help on taking images of the antenna and wire-bonding.
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Fig. 3. Measured peak THz field (circles) vs. DC bias voltage under different laser power for the PCA on SI-GaAs (a) and the PCA on LT-GaAs (b). The lines are fitting curves (quadratic and linear for (a) and (b), respectively).
REFERENCES [1]
C. THz Peak Field Dependence on Laser Power Measurements at fixed DC bias voltages while sweeping the laser pump power are also done in the same setup. The dependence of the generated THz peak field on laser power at fixed DC biases for the LT-GaAs PCA is plotted in Fig. 4. The THz peak field values of the PCA on LT-GaAs increase with increasing laser power. Saturation effect can be observed at higher laser power for the LT-GaAs PCA. However, the laser power dependence of generated THz field is not monotonic as shown in Fig. 3(a). This observation is counter-intuitive and will be further investigated.
[2] [3] [4]
[5]
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D. H. Auston, K. P. Cheung, and P. R. Smith, “Picosecond photoconducting Hertzian dipoles,” Appl. Phys. Lett., vol. 45, pp. 284– 286, May 1984. Y. Huang, N. Khiabani, D. Li, and Y. Shen, “Terahertz photoconductive antenna efficiency,” iWAT 2011, pp. 152–156, March 2011. http://www.batop.com/ J. Zhang, Y. Hong, S. Braunstein, and K. Shore, “Terahertz pulse generation and detection with LT-GaAs photoconductive antenna,” IEE Proc.: Optoelectron., vol. 151, pp. 98–101, April 2004. R. Chou, T. Liu, and C. Pan, “Analysis of terahertz pulses from largeaperture biased semi-insulating and arsenicion-implanted GaAs antennas,” J. Appl. Phys., vol. 104, pp. 053121-1–7, September 2008.
