Generation of THz transients by photoexcited single-crystal GaAs meso-structures
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- Zhang, J., Mikulics, M., Adam, R. et al. Appl. Phys. B (2013) 113: 339. doi:10.1007/s00340-013-5495-1
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We report a sub-picosecond photoresponse and THz transient generation of GaAs single-crystal mesoscopic platelets excited by femtosecond optical pulses. Our structures were fabricated by a top-down technique, by patterning an epitaxial, 500-nm-thick GaAs film grown on top of an AlAs sacrificial layer and then transferring the resulting etched away 10 × 20-μm2 platelets onto an MgO substrate using a micropipette. The freestanding GaAs devices, incorporated into an Au coplanar strip line, exhibited extremely low dark currents and ~0.4 % detection efficiency at 10 V bias. The all-optical, pump–probe carrier dynamics analysis showed that, for 800-nm-wavelength excitation, the intrinsic relaxation of photocarriers featured a 310-fs-wide transient with a 290 fs fall time. We have also carried out a femtosecond, time-resolved electro-optic characterization of our devices and recorded along the transmission line the electrical transients as short as ~600 fs, when the platelet was excited by a train of 100-fs-wide, 800-nm-wavelength optical laser pulses. The platelets have been also demonstrated to be very efficient generators of free-space propagating THz transients with the spectral bandwidth exceeding 2 THz. The presented performance of the epitaxial, freestanding GaAs meso-structured photodevices makes them uniquely suitable for THz-frequency optoelectronic applications, ranging from ultrafast photodetectors to THz-bandwidth optical-to-electrical transducers and photomixers.
Terahertz (THz) radiation, commonly understood to correspond to frequencies from approximately 0.1 to 30 THz, has very desired properties for a wide range of applications, such as non-destructive evaluation and inspection, biology and medical sciences, spectroscopic studies of carrier and intermolecular dynamics, short-range information transfer and communications, and homeland security [1, 2, 3]. The generation of transient THz signals (THz bursts) can be achieved by a variety of techniques, including ultrafast switching of photoconductive antennas and optical rectification in nonlinear crystals . Technological innovation in photonics and nanoscience enable novel THz generation techniques, such as coherent excitation of acoustic waves or polar optical phonons, laser induced gas plasma, and carrier tunneling in coupled double-quantum-well structures [4, 5, 6, 7]. However, optically excited photoconductive antennas are still the most efficient devices for THz radiation generation, reliably providing high-intensity and wide-spectral bandwidth signals. Such antennas are widely applied in THz time-domain spectroscopy, with GaAs crystals used as photoconductive emitters, due to their excellent electronic transport and optoelectronic properties. Independently, during the last decade, there has been an explosive growth of interest in novel devices, based on nanometer-sized structures [8, 9]. Most recently, we have demonstrated THz response from a single-crystal GaAs meso-whisker, as well as presented its ultra-high-speed photodetector operation with a repetition rate of above 1 THz .
In this communication, we will report our time-resolved characterization of epitaxially grown, single-crystal, freestanding GaAs platelet mesoscopic structures operated as both THz-bandwidth photodetectors and THz transient generators. The next section is devoted to our top-down fabrication process of freestanding GaAs platelets, including our unique transferring method on to a predetermined position on a substrate of choice. Section 3 presents our electrical, all-optical, and time-resolved electro-optic characterization of platelet devices operated as both photodetectors and THz transient generators. Finally, Sect. 4 contains our conclusions.
2 Fabrication of freestanding GaAs mesoscopic platelets
3 Experimental characterization of GaAs platelet photodetectors
3.1 DC measurements
3.2 All-optical pump–probe studies
To further investigate the intrinsic material photoresponse, we performed all-optical femtosecond pump–probe spectroscopy studies on the GaAs platelet. Both one-color and two-color pump–probe spectroscopy experiments were performed in reflection mode, using our MIRA oscillator, as well as a second-harmonic generator. The output from MIRA features a 100-fs-wide, linearly polarized, Gaussian pulse train with the wavelength centered at 800 nm. In the two-color version, the second-harmonic 400-nm-wavelength pulses with the width of ~150 fs were used as the pump and focused on the sample surface with a spot diameter of ~20 μm and the fluence of ~0.04 mJ/cm2 per pulse. The 800-nm-wavelength probe pulses were aimed perpendicular to the sample surface with a diameter of ~10 μm, and their fluence was much smaller (at least in the factor of 10) than that of the pump. The smaller spot size of the probe ensured that we probed a region with uniform photoexcitation. For the signal detection, the probe reflection from the sample surface was filtered by a NIR-pass filter and collected by a photodetector, whose signal was measured by a lock-in amplifier, synchronized with an acousto-optic modulator operating at a frequency of 99.8 kHz. Figure 3 shows both the one-color (800-nm/800-nm) and two-color (400-nm/800-nm) optical pump–probe time-resolved photoresponse normalized reflectivity change ΔR/R transients, denoted as the solid and dot-dashed lines, respectively. For 800-nm excitation, the ΔR/R signal has a full-width-at-half-maximum (FWHM) of 310 fs and fall time of 290 fs, which agrees very well with our characterization of the intrinsic photoresponse of a freestanding whisker meso-structure . The latter indicates that the demonstrated THz-bandwidth optoelectronic performance of our both whisker- and platelet-type devices is the material, rather than geometry limited.
For 400-nm-wavelength excitation, the ΔR/R transient presented in Fig. 3 is significantly longer than that for 800-nm excitation. The pulse in this case has ~780 fs FWHM and ~1 ps decay time. This significant broadening effect can be explained by two main factors. First, for blue light, due to its very small, ~100 nm, optical penetration depth into GaAs, highly excited carriers are generated mostly at the surface, and at the early relaxation phase, they will efficiently relax via emission of optic phonons. Thus, near the surface, we have very high excess concentrations of carriers and phonons, leading to the enhanced, both carrier–carrier and carrier–phonon scattering, and resulting a suppressed mobility μ . The 1-to-5 ratio between the 400-nm light optical penetration and the sample 500 nm thickness also means that the photogenerated carriers near the surface require an additional time for diffusion through the sample volume. Second, since the initial excess energy of photocarriers is ~1.6 eV, they are very likely to efficiently scatter into the satellite L and X valleys, as their minima in GaAs are only 0.29 and 0.48 eV above the Γ valley minimum, respectively. The intervalley scattering occurs on the 10–100 fs time scale, via emission of large wave-vector phonons , and these phonons must decay through a multi-phonon process. The observed, approximately 1-ps-wide ΔR/R transient for the 400-nm/800-nm pump–probe configuration agrees well with the above arguments.
We need to stress that under 800-nm-wavelength illumination, the FWHM of the ΔR/R photoresponse is 310 fs, which is essentially identical to the FWHM reported for the low-temperature-grown (LT-GaAs) material , widely used for ultrafast optoelectronic devices. It is well known that in LT-GaAs, structure defects, and As precipitates are responsible for carrier trapping—the key mechanism responsible for the LT-GaAs ultrafast response. Our devices, however, have been made of high-quality, single-crystal GaAs with μ as high as ~7,300 cm2/V s . More research in this aspect is clearly needed, including, possibly, extensive Monte Carlo simulations of the carrier transport in GaAs with the various levels of carrier trapping.
3.3 Electro-optic time-resolved photodetector studies
3.4 Free-space THz transient generation
We have presented DC characteristics, all-optical pump–probe spectroscopy, in-plane time-domain EO sampling, and free-space THz spectroscopy results of freestanding, epitaxially grown GaAs platelets fabricated via a top-down photolithography method, transferred onto a crystalline MgO substrate and incorporated into a CPS transmission line structure. As a photodetector, our freestanding, epitaxial GaAs element is truly unique, since it combines extremely low dark currents and the responsivity as high as 2.5 mA/W with a sub-picosecond photoresponse, and assures successful operation even in optical systems with THz laser clock-pulse rates. The platelet structure has also been demonstrated to be a very efficient generator of free-space propagating THz transients with the bandwidth extending beyond 2 THz.
The authors thank P. Song and M. Samuels for their assistance in early experiments. This work is supported in part by ARO Grant No. W911NF-12-2-0076 (Rochester). J. Z. acknowledges support from the Frank Horton Graduate Fellowship Program at the University of Rochester Laboratory for Laser Energetics, funded by the US Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302 and the New York State Energy Research and Development Authority. The support of ARO and DOE does not constitute their endorsement of the views expressed in this article.
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