High power generation of THz from 1550-nm photoconductive emitters

Two photoconductive emitters one with a self-complementary square spiral antenna, and the other with a resonant slot antenna were fabricated on a GaAs epilayer embedded with ErAs quantum dots. Driven with 1550 nm mode-locked lasers, ~117 μW broadband THz power was generated from the device with the spiral antenna, and ~1.2 μW from the device with resonant slot antenna. The optical-to-THz conversion is through extrinsic photoconductivity. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement OCIS codes: (140.3500) Lasers, erbium; (260.5150) Photoconductivity; (320.7080) Ultrafast devices; (230.6080) Sources. References and links 1. S. Gupta, J. F. Whitaker, and G. A. Mourou, “Ultrafast carrier dynamics in III-V-semiconductors grown by molecular beam epitaxy at very low substrate temperatures,” IEEE J. Quantum Electron. 28(10), 2464–2472 (1992). 2. A. Takazato, M. Kamakura, T. Matsui, J. Kitagawa, and Y. 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Vol. 26, No. 11 | 28 May 2018 | OPTICS EXPRESS 14472 #325080 https://doi.org/10.1364/OE.26.014472 Journal © 2018 Received 12 Mar 2018; revised 28 Apr 2018; accepted 28 Apr 2018; published 23 May 2018 17. C. Lin and C. P. Lee, “Comparison of Au/Ni/Ge, Au/Pd/Ge, and Au/Pt/Ge Ohmic contacts to n-type GaAs,” J. Appl. Phys. 67(1), 260–263 (1990). 18. S.-H. Yang, M. R. Hashemi, C. W. Berry, and M. Jarrahi, “7.5% optical-to-terahertz conversion efficiency offered by photoconductive emitters with three-dimensional plasmonic contact electrodes,” IEEE Trans. THz Sci. Technol. 4(5), 575–581 (2014). 19. N. T. Yardimci, S. Cakmakyapan, S. Hemmati, and M. Jarrahi, “A High-Power Broadband Terahertz Source Enabled by Three-Dimensional Light Confinement in a Plasmonic Nanocavity,” Sci. Rep. 7(1), 4166 (2017). 20. E. Peytavit, S. Lepilliet, F. Hindle, C. Coinon, T. Akalin, G. Ducournau, G. Mouret, and J.-F. 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Semtsiv, and W. Masselink, “Iron doped InGaAs: Competitive THz emitters and detectors fabricated from the same photoconductor,” J. Appl. Phys. 121(5), 053102 (2017). The investigation continues of ultrafast photoconductive (PC) THz sources driven by fiber lasers, preferably around λ = 1550-nm to utilize commercial fiber-optic telecom technology. Several approaches have been explored, the most common being the cross-gap (intrinsic) photoconductivity in InGaAs-based materials. The deep (~mid-gap) defect levels responsible for ultrafast electron-hole recombination in these materials are produced by several different methods, such as precipitates from low-temperature (LT) MBE growth [1, 2], ErAs nanoparticles [3,4], Br-irradiated defects [5], Fe-ion-implantation [6,7], and through InGaAsInAlAs heterostructures [8]. However, the problem of InGaAs-based approaches, in general, is the low breakdown field and the large dark current, especially the former, because the THz power from PC switches and photomixers has a nearly quadratic dependence on dc bias. A possible alternative is to use GaAs with a high concentration of erbium. Studies in the early 1990s showed that Er doping of GaAs could promote ultrafast carrier relaxation of electronhole pairs generated at λ ~800 nm [9, 10]. The doping level was found having a solubility limit of ∼7 × 1017 cm−3 at 580 °C, above which erbium is incorporated into GaAs as ErAs quantum dots [11]. The heavily doped GaAs:Er material performed well in THz PC devices driven around 800 nm [12, 13]. Furthermore, it was also shown to work with 1550-nm through the mechanism of extrinsic photoconductivity (Fig. 1) [14, 15]. For example, 1550nm-driven GaAs:Er based PC devices were demonstrated to produce ~46 μW THz power, with a power conversion efficiency of 0.075% [16]. Fig. 1. The photogeneration and relaxation processes for extrinsic photoconductivity in GaAs:Er embedded with ErAs quantum dots. Vol. 26, No. 11 | 28 May 2018 | OPTICS EXPRESS 14473

The investigation continues of ultrafast photoconductive (PC) THz sources driven by fiber lasers, preferably around λ = 1550-nm to utilize commercial fiber-optic telecom technology.Several approaches have been explored, the most common being the cross-gap (intrinsic) photoconductivity in InGaAs-based materials.The deep (~mid-gap) defect levels responsible for ultrafast electron-hole recombination in these materials are produced by several different methods, such as precipitates from low-temperature (LT) MBE growth [1,2], ErAs nanoparticles [3,4], Br-irradiated defects [5], Fe-ion-implantation [6,7], and through InGaAs-InAlAs heterostructures [8].However, the problem of InGaAs-based approaches, in general, is the low breakdown field and the large dark current, especially the former, because the THz power from PC switches and photomixers has a nearly quadratic dependence on dc bias.A possible alternative is to use GaAs with a high concentration of erbium.Studies in the early 1990s showed that Er doping of GaAs could promote ultrafast carrier relaxation of electronhole pairs generated at λ ~800 nm [9,10].The doping level was found having a solubility limit of ∼7 × 10 17 cm −3 at 580 °C, above which erbium is incorporated into GaAs as ErAs quantum dots [11].The heavily doped GaAs:Er material performed well in THz PC devices driven around 800 nm [12,13].Furthermore, it was also shown to work with 1550-nm through the mechanism of extrinsic photoconductivity (Fig. 1) [14,15].For example, 1550nm-driven GaAs:Er based PC devices were demonstrated to produce ~46 µW THz power, with a power conversion efficiency of 0.075% [16].In this work we extend the GaAs:Er extrinsic-PC device performance further through the design and characterization of a spiral-antenna and a slot-antenna PC switch (Fig. 2).The spiral-antenna device yields an average THz power exceeding 100 µW, and unprecedented levels of 1500-nm-to-THz conversion efficiency, making it a competitive approach to the best intrinsic-PC THz devices.The resonant slot-antenna PC switch, producing ~1.2 µW peaked around ~290 GHz, is the first demonstration of such kind of GaAs:ErAs-based THz emitter driven by 1550-nm laser.
A 2-µm-thick epitaxial layer doped with Er at ~8.8x10 20 cm −3 was grown on a semiinsulating GaAs substrate by Molecular Beam Epitaxy (MBE).The growth was performed at 600°C, with simultaneous deposition of gallium and erbium atoms.At such high density, Er incorporates into GaAs in the form of ErAs quantum dots (with mean diameter ~1.2 nm) as shown by Transmission Electron Microscopy (TEM) [16].From the TEM images, a very low level of defects is visible other than the ErAs quantum dot formation.Of importance is that these quantum dots bring the Fermi level close to the middle of the GaAs bandgap, providing sufficient density-of-states for the 1550-nm excitation of bound electrons to the conduction band of the GaAs (Fig. 1).This was supported by infrared transmission measurements: the extracted attenuation coefficient at 1550 nm was as large as ~7435 cm −1 and peaked at ~1530 nm.It was also supported by photoconductivity and photo-Hall measurements, the latter proving that the photocarriers are indeed electrons.Finally, the photoelectron lifetime was determined to be approximately 1.7 ps by pump-probe experiments [16].Thus, the material had good merits for ultrafast photoconductive devices.
The antenna of the first device (#1) is a self-complementary square spiral shown in Fig. 2(a)) having a center region of 9 × 9-µm, and 4 turns of 4-µm wide arms with 14-µm distance between adjacent arms.The significant increase in the arms' separation compared to our previous square-spiral design reduces substantially the dark current.This makes it possible to accommodate a higher bias voltage thus allowing greater THz power generation [16].The antenna for the second device (#2) is a half-wave resonant slot (Fig. 2(b)).Based on the slot length of 200 µm and the relative permittivity εr = 13.0 of GaAs in the THz region, the center frequency for the half-wave resonance of the slot-antenna device was estimated to be ~283 GHz.
The fabrication of the PC devices consisted of standard photolithography using an imagereversal lift-off process.The metallization consisted of a stack with nominal thicknesses of ~200 Å Ni, ~1000 Å AuGe, ~200 Å Ni, and ~200 Å Au.Finally, a thermal annealing step was conducted to improve adhesion and reduce contact resistance [17].
For THz generation, the back side of each antenna chip was coupled to a high-resistivity Si hyperhemispherical lens.A 1550-nm mode-locked laser, having ≈90 fs pulses and 100 MHz repetition rate, was used to pump the devices.The maximum available power was P1550 ~83 mW.Varied laser power was obtained with a wheel attenuator.The beam was focused with an achromatic standard microscope objective (NA = 0.25, EFL = 15.2 mm) to be concentrated within the 9-μm × 9-μm center gap of the spiral antenna.The objective was found to have a transmission of T obj ~77% at 1550 nm.The efficiency of 1550 nm coupled to the device at the air-substrate interface, T substrate , is equal to 1-R substrate , and R substrate is where is n substrate = 3.69 is the refractive index of GaAs.T substrate is estimated to be ~0.67.The leaked 1550 nm through the substrate as well as the silicon lens, P leak , was measured to be ~2.8mW if the incident laser power was P 1550 ~83 mW.First both dark current and photocurrent were measured.The PC switch device #1 was biased up to 145 V.The photocurrent driven with ~64 mW laser power was ~338 µA, 1.3x larger than the dark current (~260 µA) at the same bias (Fig. 3).The corresponding responsivity is calculated to be 5.28 µA/mW.The THz power was measured with a crosscalibrated Golay cell detector having an external responsivity of 7 × 10 3 V/W (Fig. 4(a)).To block the 1550-nm radiation leaking through the GaAs substrate and the silicon lens of the PC device, a thick black plastic filter was placed in front of the detector.The filter had at least 40 dB attenuation losses at the 1550 nm but passed T filter ≈80% of THz transmission as measured separately in the range of 200-1000 GHz with a frequency-domain spectrometer.Figure 4(a) shows the generated THz power vs bias voltage at a fixed laser power of 83 mW.The power at the 145 V bias is P T ~117 µW.Thus the 1550-to-THz power conversion efficiency is estimated from η ≈P T / (P 1550 T obj T substrate -P leak ) ~0.18%.Both the power and the efficiency obtained with this extrinsic PC switch are comparable to those of the best intrinsic PC switch devices (Table 1).  [18  LT-GaAs 770 720 4000 0.55% [19]   LT-GaAs 776 107 1200 1.1% [20]   LT-GaAs 800 3000 280 0.009% [21]   SI-GaAs 800 800 1500 0.18% [22]   InGaAs 1550 30 10 0.03% [23]   InGaAs 1550 20 75 0.375% [24]   GaAs:Er 1550 64 117 0.18% [this work]   Fig. 4. (b) shows the THz power vs laser power at a fixed bias voltage of 100 V.A power law curve fitting yields an exponent of ~2.15.
Next, a balanced Michelson interferometer was used to obtain the power spectrum of the generated THz signal.The interferogram was obtained and it is shown in Fig. 5(a).Because the interferogram is essentially the autocorrelation function, its FFT yields the power spectrum displayed in Fig. 5(b).The spectrum is fit in Fig. 5(b) with a shifted Lorentzian function ~A0 / {1 + [2π(f-f 0 )(2τ)] 2 }, which yields a photocarrier lifetime of ≈0.5 ps, shorter than the 1.7ps we obtained with pump-probe measurement [16].Such kind of discrepancy between the two was observed previously in semi-insulating GaAs-based photoconductors excited at 800 nm [22].
To verify the power spectrum, the radiation at three "spot" frequencies -100 GHz, 300 GHz and 600 GHz -was measured with three waveguide-mounted, horn-coupled Schottky diode detectors.The signal was modulated by the mode-lock frequency of the 1550-nm pulsed laser (i.e. 100 MHz), and then demodulated with a RF lock-in amplifier.The respective noise levels were measured using the same lock-in amplifier with the beam path to the detectors blocked.The signal-to-noise-ratios for the three frequencies were determined to be ~77 dB, ~93 dB and ~72 dB, respectively.These points fall upon the envelope of the power spectrum shown in Fig. 5(b), in a good agreement with the interferometer measurement.
The device with slot antenna (#2) was tested using the same pulsed laser and Golay cell detector with the pumped laser focused on the 9-μm × 9-μm gap placed in opposition to the slot (right side of Fig. 2(b)).At 50 V bias, the photocurrent was ~36.4 µA driven with ~64 mW of laser power, about 23x larger than the dark current of ~1.6 µA (Fig. 6).Accordingly, the responsivity is calculated to be 0.57 µA/mW.Then the THz power at 33 V bias was measured and estimated to be ~1.2 µW.The corresponding power conversion efficiency is 0.002% (Fig. 6).The Michelson-interferogram and associated power spectrum are presented in Fig. 7.The peak of the power spectrum occurs at ~290 GHz, in accordance with the half-wavelength resonance calculation.For comparison, the power spectrum from the spiral antenna is plotted in the same graph (Fig. 7(b)).Clearly, the power spectrum of the slot antenna is "narrower" with less frequency components.But, overall it shows that the slot antenna has a low Q factor.To prove the THz power was from the slot antenna and not the feed line shown in Fig. 2(b), a polarization experiment was carried out.A wire grid polarizer with ~35-dB extinction ratio was inserted between the antenna and the detector.The maximum reading from the detector was ~1.3 mV with the polarizer aligned to transmit power polarized perpendicular to the slot axis.But this reading dropped to ~0.5mV when the polarizer was rotated by 90 degrees.This polarization pattern is consistent with the antenna theory of slot antennas.In conclusion, we note that both emitters were tested under "safe" operating conditions.More power would be expected by increasing at higher bias voltages as well as increasing the 1550-nm laser power.Nevertheless, the results for the emitter with square spiral antenna already prove that THz power (>100 μW) and efficiency (~0.18%) of 1550-nm-driven extrinsic photoconductive devices based on GaAs with ErAs quantum dots are competitive with those obtained from intrinsic photoconductive devices.In addition, a 1550 nm-driven THz emitter with a resonant slot antenna was first demonstrated.

Fig. 1 .
Fig. 1.The photogeneration and relaxation processes for extrinsic photoconductivity in GaAs:Er embedded with ErAs quantum dots.

Fig. 2 .
Fig. 2. a) Photoconductive device #1 with square spiral antenna; and b) photoconductive device #2 with half-wave resonant slot antenna.The green circle shows the active region.

Fig. 5 .
Fig. 5. a) The interferogram for device #1 measured with Golay cell detector, and b) the FFT spectrum of a).Also plotted in b) are three SNR points obtained with three different Schottky diode detectors.

Fig. 7 .
Fig. 7. a) The interferogram of device #2, b) the power spectrum of a) after FFT transform and its comparison with Fig. 5(b).