Siddhant Chowdhury

Coherent three-dimensional terahertz imaging through self-mixing in a quantum cascade laser

We demonstrate coherent terahertz (THz) frequency imaging using the self-mixing effect in a quantum cascade laser (QCL). Self-mixing voltage waveforms are acquired at each pixel of a two-dimensional image of etched GaAs structures and fitted to a three-mirror laser model, enabling extraction of the amplitude and phase parameters of the reflected field. From the phase, we reconstruct the depth of the sample surface, and we show that the amplitude can be related to the sample reflectance. Our approach is experimentally simple and compact, and does not require frequency stabilization of the THz QCL.

Transient Analysis of THz-QCL Pulses Using NbN and YBCO Superconducting Detectors

We report the time-domain analysis of fast pulses emitted by a quantum cascade laser (QCL) operating at ~ 3.1 THz using superconducting THz detectors made from either NbN or YBa2Cu3O7-δ (YBCO) thin films. The ultrafast response from these detectors allows resolution of emission features occurring on a nanosecond time-scale, which is not possible with commercially available Ge or InSb bolometers owing to their much larger time constants. We demonstrate that the time-dependent emission can be strongly affected by relatively small variations in the driving pulse. The QCL output power-current relationship was determined, based on correlation of the time-dependent emission of radiation with current flow in the QCL, under different QCL bias conditions. We show that this relationship differs from that obtained using bolometric detectors that respond only to the integrated pulse energy. The linearity of the detectors, and their agreement with measurements using a Ge bolometer, was also established by studying the QCL emission as a function of bias voltage and excitation pulse length. This measurement scheme could be readily applied to the study of ultrafast modulation and mode-locking of THz-QCLs.

Generation of continuous wave terahertz frequency radiation from metal-organic chemical vapour deposition grown Fe-doped InGaAs and InGaAsP

We demonstrate the generation of continuous wave terahertz (THz) frequency radiation from photomixers fabricated on both Fe-doped InGaAs and Fe-doped InGaAsP, grown by metal-organic chemical vapor deposition. The photomixers were excited using a pair of distributed Bragg reflector lasers with emission around 1550 nm, and THz radiation was emitted over a bandwidth of greater than 2.4 THz. Two InGaAs and four InGaAsP wafers with different Fe doping concentrations were investigated, with the InGaAsmaterial found to outperform the InGaAsP in terms of emitted THz power. The dependencies of the emitted power on the photomixer applied bias, incident laser power, and materialdoping level were also studied.

Time-resolved measurement of pulse-to-pulse heating effects in a terahertz quantum cascade laser using an NbN superconducting detector

Joule heating causes significant degradation in the power emitted from terahertz-frequency quantum-cascade lasers (THz QCLs). However, to date, it has not been possible to characterize the thermal equilibration time of these devices, since THz power degradation over sub-millisecond time-scales cannot be resolved using conventional bolometric or pyroelectric detectors. In this letter, we use a superconducting antenna-coupled niobium nitride detector to measure the emission from a THz QCL with a nanosecond-scale time-resolution. The emitted THz power is shown to decay more rapidly at higher heat-sink temperatures, and in steady-state the power reduces as the repetition rate of the driving pulses increases. The pulse-to-pulse variation in active-region temperature is inferred by comparing the THz signals with those obtained from low duty-cycle measurements. A thermal resistance of 8.2 ± 0.6 K/W is determined, which is in good agreement with earlier measurements, and we calculate a 370 ± 90-μs bulk heat-stora...

Diffuse-Reflectance Spectroscopy Using a Frequency-Switchable Terahertz Quantum Cascade Laser

We demonstrate diffuse-reflectance (DR) spectroscopy of powders using a discretely-tunable terahertz-frequency quantum cascade laser (THz QCL) with a heterogeneous active region. DR signatures were obtained at frequencies of 3.06, 3.21, 3.28, and 3.35 THz, and the relative absorption coefficients were inferred at each frequency using a Kubelka-Munk (KM) scattering model. The spectral lineshapes reproduce the absolute Beer-Lambert (BL) absorption spectra of a range of materials, which were also measured using conventional transmission-mode THz time-domain spectroscopy. It is shown that the DR technique works reliably for materials that include pharmaceutical compounds and foodstuffs, with BL absorption coefficients in the range 2-10 mm-1. This method is potentially suitable for automated material identification, without any requirement for a priori knowledge of the refractive index or scattering properties of the sampled material.