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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.

An externally applied magnetic field was used to induce increased photocarrier transport along the high mobility channel in GaAs/AlGaAs modulation-doped heterostructures (MDH). The terahertz (THz) emission from GaAs/AlGaAs MDH increases with increasing magnetic field, applied parallel to the heterojunction. The THz emission enhancement factors due to the magnetic field in MDH are higher than in undoped GaAs/AlGaAs heterojunction and in bulk SI-GaAs. This demonstrates that properly utilizing the high-mobility channel for carrier transport promises to be a viable design consideration for efficient THz photoconductive antenna (PCA) devices. Moreover, it was observed that for MDH, as well as for an undoped GaAs/AlGaAs heterojunction, the enhancement for one magnetic field direction is greater than the enhancement for the opposite direction. This is in contrast to the symmetric enhancement with magnetic field direction observed in a bulk SI-GaAs. An analysis of photocarrier trajectories under an external magnetic field supports the explanation that the enhancement asymmetry with magnetic field direction in MDH is due to the cycloid motion of electrons as affected by the GaAs/AlGaAs interface.

Terahertz (THz) emission increase is observed for GaAs thin films that exhibit structural defects. The GaAs epilayers are grown by molecular beam epitaxy on exactly oriented Si (100) substrates at three different temperatures (Ts = 320ºC, 520ºC and 630ºC). The growth method involves the deposition of two low-temperature-grown (LTG)-GaAs buffers with subsequent in-situ thermal annealing at Ts = 600ºC. Reflection high energy electron diffraction confirms the layer-by-layer growth mode of the GaAs on Si. X-ray diffraction shows the improvement in crystallinity as growth temperature is increased. The THz time-domain spectroscopy is performed in reflection and transmission excitation geometries. At Ts = 320ºC, the low crystallinity of GaAs on Si makes it an inferior THz emitter in reflection geometry, over a GaAs grown at the same temperature on a semi-insulating GaAs substrate. However, in transmission geometry, the GaAs on Si exhibits less absorption losses. At higher Ts, the GaAs on Si thin films emerge as promising THz emitters despite the presence of antiphase boundaries and threading dislocations as identified from scanning electron microscopy and Raman spectroscopy. An intense THz emission in reflection and transmission excitation geometries is observed for the GaAs on Si grown at Ts = 520ºC, suggesting the existence of an optimal growth temperature for GaAs on Si at which the THz emission is most efficient in both excitation geometries. The results are significant in the growth design and fabrication of GaAs on Si material system intended for future THz photoconductive antenna emitter devices.

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.

Photocarrier dynamics in GaAs/AlGaAs modulation-doped heterostructures (MDH) having i-AlGaAs spacer layers of different thicknesses were investigated using temperature-dependent terahertz time-domain spectroscopy (THz-TDS) and photoreflectance spectroscopy. In particular, results are discussed in the framework of the temperature dependence of the heterojunction electric field and photocarrier velocity for two i-AlGaAs spacer layer thickness values. The junction electric field, THz emission intensity and bandwidth of the MDH samples all decrease as temperature decreases. In contrast, the THz emission intensity and bandwidth of a reference bulk undoped GaAs does not significantly vary with temperature. These results imply that THz emission of MDH’s originates primarily from carrier drift due to the GaAs/AlGaAs junction electric field. A general decrease in the THz emission bandwidth of the MDH’s is attributed to a decrease in carrier velocity at lower temperatures, presumably due to the weaker electric field. Moreover, the MDH sample with thinner spacer layer exhibited a higher junction electric field. This work demonstrates the study of temperature-dependent photocarrier transport and junction electric field measurements. The results may provide useful insights in the design of MDH-based devices.

We report the molecular beam epitaxy growth of high-quality p-InAs thin films evaluated in the context of 1.55 μm femtosecond laser-excited THz emission efficiency. The presence of p-InAs is confirmed via scanning electron microscopy and X-ray diffraction. Using a GaAs buffer layer, the epitaxial growth of p-InAs layers was successfully achieved. Initiating GaAs deposition by growth interruption, we find that GaAs adheres to the GaSb substrate and provides a quasi-planar surface for the subsequent layers. We also find a significant enhancement in the THz radiation intensity of p-InAs films that is approximately twice compared to that of bulk p-InAs for 1.55 μm wavelength.

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.

We investigate the effects of an externally applied magnetic field on the terahertz (THz) emission of Gallium Manganese Arsenide (GaMnAs) films grown via molecular beam epitaxy (MBE). Results show that low Mn-doping in GaMnAs resulted to increased THz emission as compared with a SI-GaAs substrate. Further increase in Mn-doping content resulted to a comparably less THz emission, which is attributed to reduced crystallinity and higher free-carrier absorption. Under an external magnetic field, the contributions of the B u p and B d o w n -related THz emission were observed to be asymmetric: possibly due to intrinsic magnetic properties of GaMnAs.

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)

Sediments consisting of different soil minerals were collected from an archaeological site located in Pingabayanan Batangas Philippines. Each of the soil samples from selected layers or sampling depth was examined and profiled. Standard characterization techniques such as XRF, XRD, and FTIR spectroscopy were then performed for elemental and crystal analysis. All three aforementioned characterization techniques are in good agreement with the archaeological descriptions of sediments profile. Moreover, terahertz (THz) transmission spectroscopy was also performed to explore the feasibility of utilizing this novel spectroscopy for characterizing soil minerals. Terahertz time-domain spectroscopy (THz-TDS) results show good agreement to the standard characterization results in terms of detection of trace metals and minerals. Hence, the THz transmission spectra can provide necessary information related to the sedimentation profile and archaeological recordings that were conducted. Results suggest that THz-TDS may prove as a quick and non-destructive method to study soil samples, complementary to standard techniques for geoarchaeological surveys.

We study the photoluminescence (PL) and terahertz (THz) emission characteristics of MBE-grown InAs/GaAs single-layered quantum dots. Results from temperature-dependent and excitation power-dependent PL spectroscopy have revealed the presence of energy peaks corresponding to two possible quantum dot size distributions. The THz emission was found to increase as temperature increases, which we attribute to the increase in the number of carriers undergoing transport.

The photocarrier dynamics in molecular beam epitaxy (MBE)-grown single- (SLQD) and multi-layered (MLQD) InAs/GaAs quantum dots were studied. Photoluminescence (PL) spectroscopy has shown that the MLQD has more uniform QD size distribution as compared to the bimodal SLQD. Correlation between PL and THz-TDS has shown that photocarrier transport is more favored in the MLQD owing to this uniform QD size distribution, resulting to higher THz emission. The THz emission from the QD samples were found to be proportional to temperature. A drift-related photocarrier transport mechanism is proposed, wherein photocarriers generated in the QDs are accelerated by an interface electric field.

Simultaneous molecular beam epitaxial growth of GaAs/AlGaAs multiple quantum wells on two different substrates (one on GaAs (100) and another on a GaAs substrate misoriented by 4° in the (111) direction) resulted in samples of similar structure, but having different defect profiles. The on-axis sample had a higher defect density and more types of electron traps than the off-axis counterpart. Temperature-dependent terahertz (THz) emission and temperature-dependent photoluminescence were measured; in both cases, an intensity quenching was observed between 75 K – 250 K for the on-axis sample, but not in the off-axis sample. We attribute the THz emission quenching to the electron traps present in the sample, which decreases the photocarriers participating in setting up the surface field.