Improved performance of GaAs-based terahertz emitters (original) (raw)
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IEEE Journal of Selected Topics in Quantum Electronics - IEEE J SEL TOP QUANTUM ELECTR, 2011
We have improved the stability and performance of terahertz (THz) photoconductive (Auston) switches using a combination of (NH 4 ) 2 S surface passivation (SP) and silicon nitride (Si 3 N 4 ) encapsulation. The influences of SP and encapsulation on the ultrafast electron dynamics in GaAs were examined using THz emission spectroscopy and optical pump-THz probe spectroscopy. The power of THz radiation from the surface of photoexcited GaAs increased by a factor of 5 after passivation and encapsulation, while the process lengthened the trapping time for photoexcited charge carriers. By fabricating and assessing the performance of photoconductive switches, we found that passivation and encapsulation increased the average THz power generated fourfold.
Characterization and modeling of a terahertz photoconductive switch
Applied Physics Letters, 2010
We examine the terahertz ͑THz͒ performance of an ErAs:GaAs photoconductive switch under varying bias conditions and optical drive power. Despite THz power up to 287 W, saturation effects were not seen. In addition, the THz power spectra were measured with a Fourier transform infrared spectrometer, and the roll-off was found to be invariant to bias voltage and consistent with a THz pulsewidth of 1.59 ps and a peak power of 3.1 W. These results are confirmed by a large-signal, high-frequency circuit model that suggests that further increase in THz power and efficiency are possible through an increase in the mode-locked laser power and reduction in its pulse width. The model is useful in designing both the laser and photoconductive switches to maximize available power and efficiency.
Applied Optics, 2005
InAs has previously been reported to be an efficient emitter of terahertz radiation at low excitation fluences by use of femtosecond laser pulses. The scaling and saturation of terahertz emission from a (100) InAs surface as a function of excitation fluence is measured and quantitatively compared with the emission from a GaAs large-aperture photoconductive switch. We find that, although the instantaneous peak radiated terahertz field from (100) InAs exceeds the peak radiated signals from a GaAs largeaperture photoconductive switch biased at 1.6 kV͞cm, the pulse duration is shorter. For the InAs source the total energy radiated is less than can be obtained from a GaAs large-aperture photoconductive switch.
Simulation and optimisation of terahertz emission from InGaAs and InP photoconductive switches
Solid State Communications, 2005
We simulate the terahertz emission from laterally biased InGaAs and InP using a three-dimensional carrier dynamics model in order to optimise the semiconductor material. Incident pump-pulse parameters of current Ti:Sapphire and Er:fibre lasers are chosen, and the simulation models the semiconductor's bandstructure using parabolic Γ, L and X valleys, and heavy holes. The emitted terahertz radiation is propagated within the semiconductor and into free space using a model based on the Drude–Lorentz dielectric function. As the InGaAs alloy approaches InAs an increase in the emitted power is observed, and this is attributed to a greater electron mobility. Additionally, low-temperature grown and ion-implanted InGaAs are modelled using a finite carrier trapping time. At sub-picosecond trapping times the terahertz bandwidth is found to increase significantly at the cost of a reduced emission power.
Efficient terahertz devices based on III–V semiconductor photoconductors
IET Optoelectronics, 2014
A series of planar aperture and dipole antenna structures fabricated on low temperature grown GaAs and InP-based photoconductors have been evaluated as terahertz (THz) emitters and detectors in a time-domain spectroscopy system under pulsed excitation. The combination of large aperture antennas as emitters and short dipole antennas as detectors results in efficient THz devices operating at 800 nm, 1 μm and 1.55 μm excitation wavelengths. The system responses of these materials are among the best ever reported and allow high-quality measurements to be made. Finally, characterisation of a structure able to be biased vertically and its evaluation as THz emitter is reported for the first time. The THz response of this material with a strong THz signal at low-voltage bias makes the development of battery-operated THz devices possible.
Electrical and Radiation Characteristics of Semilarge Photoconductive Terahertz Emitters
IEEE Transactions on Microwave Theory and Techniques, 2004
We present experimental characterization of semilarge photoconductive emitters, including their electrical/photoconductive parameters and terahertz spectra. A range of emitters were studied and fabricated on both LT-GaAs and SI-GaAs, having a variety of electrode geometries. The spatial cone of terahertz radiation was defined. The dependencies of the photocurrent and the terahertz power on the bias voltage and the laser power were determined. A Fourier-transform interferometer is used to determine the terahertz spectra and to clarify the effects of the substrate and electrode geometry.
Influence of Substrate Material on Radiation Characteristics of THz Photoconductive Emitters
International Journal of Antennas and Propagation, 2015
We present in this paper spectral and spatial characteristics of terahertz emission from standard dipole antenna structures used as emitters depending on the substrate material. All antenna structures were lithographically fabricated on low-temperature (LT) grown, few-micrometers-thick gallium arsenide (GaAs) layers. To investigate the effect of the substrate material on the radiation pattern of terahertz beams, either semi-insulating gallium arsenide or high-resistivity silicon substrate wafers have been used. As detector a standard 40 µm long dipole antenna on a semi-insulating GaAs substrate with a low-temperature grown gallium arsenide layer on it has been employed; this configuration allows for broadband detection and is still efficient enough for the characterization purpose. Strong dependence of the radiation pattern on the substrate used for the terahertz source is demonstrated. The measured patterns and differences between the two cases of substrates are well explained by m...
Applied Physics Letters, 2006
The carrier dynamics of photoexcited electrons in the vicinity of the surface of (NH4)2S-passivated GaAs were studied via terahertz (THz) emission spectroscopy and optical-pump THz-probe spectroscopy. THz emission spectroscopy measurements, coupled with Monte Carlo simulations of THz emission, revealed that the surface electric field of GaAs reverses after passivation. The conductivity of photoexcited electrons was determined via optical-pump THz-probe spectroscopy, and was found to double after passivation. These experiments demonstrate that passivation significantly reduces the surface state density and surface recombination velocity of GaAs. Finally, we have demonstrated that passivation leads to an enhancement in the power radiated by photoconductive switch THz emitters, thereby showing the important influence of surface chemistry on the performance of ultrafast THz photonic devices.
Applied Physics Letters, 2014
We demonstrate here an efficient THz source with low electrical power consumption. We have increased the maximum THz radiation power emitted from SI-GaAs based photoconductive emitters by two orders of magnitude. By irradiating the SI-GaAs substrate with Carbon-ions up to 2 m deep, we have created lot of defects and decreased the life time of photo-excited carriers inside the substrate. Depending on the irradiation dose we find 1 to 2 orders of magnitude decrease in total current flowing in the substrate, resulting in subsequent decrease of heat dissipation in the antenna. This has resulted in increasing maximum cutoff of the applied voltage across Photo-Conductive Emitter (PCE) electrodes to operate the device without thermal breakdown from ~35 V to > 150 V for the 25 m electrode gaps. At optimum operating conditions, carbon irradiated (10 14 ions/cm 2) PCEs give THz pulses with power about 100 times higher in comparison to the usual PCEs on SI-GaAs and electrical to THz power conversion efficiency has improved by a factor of ~ 800. Electromagnetic radiations having frequencies in Tera-hertz (THz) range (1THz = 10 12 Hz) are not so easy to generate [1]. But due to its applications in security imaging, bio-sensing, chemical identification, material characterization etc., there is high demand of high power THz sources, particularly sources which can generate short THz pulses with broadband spectrum. Till now, photoconductive emitters (PCEs) are known to be the best sources for high power THz pulse generation. Improving the efficiency of THz pulse sources with better designs or material, is one of the major goals of ongoing research in this field. There have been several attempts to increase the THz emission from these sources by modifying the electrical and optical properties of the semiconducting substrate [2] , design of electrodes [3] and patterning the active area of PCE in between the two electrodes [4]. In THz PCEs newly photo generated charge carriers (electron-hole pairs) via laser pulse of width less than 100 fs gets accelerated under already applied electric field and this sudden jump in number of free carriers and their acceleration gives sudden rise in the current. This sudden rise in current in pico-second time domain is responsible for THz pulse emission. In the case, where the semiconductor has carrier life time of less than a pico-second like LT-GaAs, current falls down to the dark level within picoseconds as electron hole pairs recombine with each other. Such materials are useful for the generation of bipolar THz pulses. In semiconductors like SI-GaAs which has carrier life time of more than 50 ps, fall in current takes relatively much longer time. Since electric field of the emitted THz pulse , where J(t)