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We present the implementation of an efficient terahertz (THz) photoconductive antenna (PCA) emitter design that utilizes high mobility carriers in the two-dimensional electron gas (2DEG) of a modulation-doped heterostructure (MDH). The PCA design is fabricated with recessed metal electrodes in direct contact with the 2DEG region of the MDH. We compare the performance of the MDH PCA having recessed contacts with a PCA fabricated on bulk semi-insulating GaAs, on low temperature-grown GaAs, and a MDH PCA with the contacts fabricated on the surface. By recessing the contacts, the applied bias can effectively accelerate the high-mobility carriers within the 2DEG, which increases the THz power emission by at least an order of magnitude compared to those with conventional structures. The dynamic range (62 dB) and bandwidth characteristics (3.2 THz) in the power spectrum are shown to be comparable with the reference samples. Drude-Lorentz simulations corroborate the results that the higher-mobility carriers in the MDH, increase the THz emission. The saturation characteristics were also measured via optical fluence dependence, revealing a lower saturation value compared to the reference samples. The high THz conversion efficiency of the MDH-PCA with recessed contacts at low optical power makes it an attractive candidate for THz-time domain spectroscopy systems powered by low power fiber lasers.

Carrier transport in semiconductors with different dimensionalities, i.e., 3D (bulk), 2D (QW), and 0D (QD), were investigated via temperature-dependent terahertz time-domain spectroscopy (THz-TDS). The optical properties and recombination dynamics in the samples were probed via photoluminescence spectroscopy. The temperature-dependence of the THz emission from the samples was explained in the context of the drift-diffusion model using the dominant THz radiation mechanism. The THz emission from diffusion-type THz emitters such as p- and n-InAs decreases as temperature increases due to mobility decrease. Conversely, the THz emission from drift-type THz emitters such as SI-GaAs, GaAs QW, and InAs QD was found to increase with temperature due to the increase in the driving electric field. In summary, THz-TDS can be utilized to gain qualitative insights on the temperature-dependent transport characteristics and establish dominant THz radiation mechanisms.

Terahertz (THz) photoconductive antennas (PCA’s) from SI-GaAs substrates having one-dimensional (1D) and two-dimensional (2D) micron-size metal line arrays (MLA's) were fabricated. Photolithography and electron beam deposition of Ni/Au were used to fabricate spiral PCA’s and 1D/2D MLA's on the transmission side of the PCA. Compared to a reference bare PCA, the THz time-domain signal enhanced ~6x for 1DMLA and ~11x for 2DMLA, with their corresponding bandwidths broadened. The origin of the enhancement is being investigated but is currently attributed to spoof surface plasmon phenomena. Integrating MLA’s with PCA’s demonstrates a more cost-effective alternative to nanostructure fabrication within the PCA gap.

We report on the continuing efforts of the National Institute of Physics, University of the Philippines, in the design, growth, and fabrication of novel structures for THz photoconductive antennas (PCA’s) [1-3]. In particular, the talk presents recent works on the molecular beam epitaxial growth of gallium arsenide-based semiconductor trilayer films and modulation-doped heterostructures (MDH) as substrates for THz PCA emitter’s [1,2]. The improvement of these novel substrates on the performance of the emitters are primarily attributed to the surface/interface electric field modification for the trilayer design; as well as an increase in the carrier mobility for the MDH design. Additionally, work is also being undertaken on the surface modification of the PCA’s [3]. Initial results on the adsorption of Si nanowires on the PCA gap to improve THz generation will also be presented; as well recent work on the incorporation of micron-size metal line arrays on the emission side of the PCA. Being relatively large, these metal line arrays do not require electron beam lithography that are commonly employed in metamaterial enhanced PCA’s. These research activities are carried out in collaboration with the FIR Center, University of Fukui, Japan.

Terahertz (THz) photoconductive antenna (PCA) fabricated on low-temperature-grown GaAs (LT-GaAs) with layer structure consisting of a doped buffer exhibited enhanced THz emission for LT-GaAs grown at lower temperature. As THz emitter, the LT-GaAs grown at 270°C with doped buffer generated THz radiation with amplitude twice as that of its undoped counterpart. Similar effect is not observed when the LT-GaAs is grown at 320°C with doped buffer, which is expected to have higher THz emission. As THz detector, both LT-GaAs with doped buffer exhibited identical and improved detection sensitivity regardless of the LT-GaAs growth temperature.

We present threefold enhancement of terahertz emission from silicon nanowire (SiNW)-coated gallium-arsenide photoconductive antenna over its uncoated counterpart. The enhancement is attributed to the increased photoabsorption, and possibly additional photoconductive pathways induced by the SiNWs. © 2020 The Author(s)

Spintronic THz emission from Ni/Pt bilayer grown on MgO is reported based on the novel THz emitter using metallic structures. The Ni metal was deposited first on a MgO substrate and capped with a thin Pt metal via electron beam deposition. The THz emission data was obtained using a standard terahertz time-domain spectroscopy setup using a Ti: sapphire laser excitation source. Initial measurements were done using 800nm excitation with 7 mW and 185 mW pump powers under upward and downward magnetic field orientations. Polarity reversal of the terahertz signal was observed upon changing the orientation of the magnetic field. Maximum amplitude was found at 0.5 THz with bandwidth up to ~6 THz. A saturation fluence of 85.04 mJ/cm2 was calculated from the pump fluence-dependence plot of the THz peak-to-peak signal. The results are consistent with the spintronic THz emission due to the inverse spin-Hall effect and provide insights for future development and optimizations.

We report increased radiation in the visible and terahertz (THz) regimes in silicon(Si)-based nanostructures. The nanostructures, Si nanowires (SiNWs) and porous Si (PSi), were synthesized via electroless and electrochemical surface modification, respectively. In particular, picosecond (ps) radiative lifetimes in the order of 250 ps were obtained from time-resolved photoluminescence (PL) measurements. The fast radiative lifetimes are associated with increased surface defect density in PSi. Reflectance measurements confirmed that optical absorption of the nanostructured Si samples increased relative to bulk Si. Both nanostructured Si exhibit THz emission, albeit weaker in PSi due to higher density of defects. An inverse relationship between PL and THz emission strength was therefore observed. Lastly, the wider bandwidth of the THz emission in SiNWs is attributed to the directionality of the transient photocurrent compared to the more disordered carrier transport in PSi.

Silicon nanowires (SiNWs) were etched on Si (100), (110), and (111) substrates. Slanted nanowires with respect to the surface normal were produced on the Si (110) and (111) substrates, and vertical nanowires were produced on the Si (100) substrate. Photoluminescence spectroscopy exhibited luminescence lines attributed to oxide defects from the nanowires. A comparison of the transient photocurrent as measured using terahertz time-domain spectroscopy revealed an increase in the THz emission from the SiNWs on Si (110) substrate compared to those grown on (100) substrate. Reorienting the dipole moment by applying an external 650 mT magnetic field suggested that the carrier transport was confined along the axis of the nanowires. Understanding the photocarrier and transport recombination properties in SiNWs may prove useful in the design considerations for future SiNW photovoltaic cell applications.

Porosity dependent terahertz emission of porous silicon (PSi) was studied. The PSi samples were fabricated via electrochemical etching of boron-doped (100) silicon in a solution containing 48% hydrofluoric acid, deionized water and absolute ethanol in a 1:3:4 volumetric ratio. The porosity was controlled by varying the supplied anodic current for each sample. The samples were then optically characterized via normal incidence reflectance spectroscopy to obtain values for their respective refractive indices and porosities. Absorbance of each sample was also computed using the data from its respective reflectance spectrum. Terahertz emission of each sample was acquired through terahertz - time domain spectroscopy. A decreasing trend in the THz signal power was observed as the porosity of each PSi was increased. This was caused by the decrease in the absorption strength as the silicon crystallite size in the PSi was minimized.

Carrier dynamics and photoconductivity in epitaxial-grown low-temperature GaAs on nominal and vicinal Si(1 0 0) substrates ('LT-GaAs/Si') were studied to predict their actual performance as THz photoconductive antenna (PCA) detectors. An optical-pump terahertz-probe technique was used to obtain the transmittance, carrier lifetime and photoconductivity of two LT-GaAs/Si samples, grown using different substrates and different growth protocols. The LT-GaAs grown on Si(1 0 0) substrate with a 4° tilt to 〈1 1 0〉 has better crystallinity, in agreement with other reports; while the LT-GaAs layer grown on nominal Si(1 0 0) substrate, though more structurally defective, has a much faster electron trapping time. Fabricated test PCAs with either dipole or bowtie geometries confirm the characterization results. The photoconductivity and carrier lifetime results manifest in the PCA performance, in responsivity, and in detection bandwidth. The prototypes' sensitivities, bandwidths and dynamic ranges show that with some growth optimization, LT-GaAs/Si can be tailored to create economical, broadband THz detectors.

VCSELs using high-contrast grating mirrors defined by wet oxidation of an AlGaAs spacer layer are demonstrated. The devices, operating near 960nm, have threshold current as low at 0.6mA and peak power as high as 1mW.

The carrier dynamics and recombination characteristics of vertically aligned silicon nanowires are investigated using terahertz emission and photoluminescence spectroscopy, respectively. It is observed that the presence of pores on the walls in two-step-synthesized silicon nanowires greatly affects the carrier dynamics, compared with nanowires synthesized using a one-step process. These pores become efficient carrier recombination sites wherein carriers are collected upon photoexcitation. Additionally, pores effectively diminish the surface electric field thereby inhibiting the terahertz emission. Finally, nanowire-length-dependent terahertz emission is observed only for the one-step-synthesized nanowires whereas the two-step-synthesized nanowire samples exhibited length dependence of their photoluminescence intensity.