Mohammed Salih

Dual-frequency imaging using an electrically tunable terahertz quantum cascade laser.

We report dual-frequency imaging using an electrically tunable terahertz frequency quantum cascade laser (QCL). Images of powdered materials including the explosive pentaerythritol tetranitrate (PETN) were acquired at 3.05 THz and 3.24 THz in a single scan. We show that it is possible to distinguish PETN from other materials by taking the difference-transmission and difference-attenuation images. We also demonstrate difference-intensity imaging by combining amplitude modulation of the QCL bias with lock-in detection.

Electrically tunable terahertz quantum-cascade laser with a heterogeneous active region

We demonstrate experimentally a terahertz frequency quantum-cascade laser which can be tuned electronically, in a step-wise manner from typically 3.07 to 3.40 THz, by changing the applied electric field across the device. To achieve this, the gallium growth rate was successively reduced during growth of the active region from +6% to −4% of its nominal 1 μm/h operating value. This produced a heterogeneous active region, comprising 23 sections of different lasing units stacked together. With a single-metal surface-plasmon waveguide configuration, the devices operated in pulsed mode, up to a temperature of 81 K, and gave peak powers of ∼8 mW.

Terahertz time domain spectroscopy of phonon-depopulation based quantum cascade lasers

A 3.1THz phonon depopulation-based quantum-cascade-laser is investigated using terahertz time domain spectroscopy. A gain of 25cm-1 and absorption features due to the lower laser level being populated from a parasitic electronic channel are highlighted.

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.

Resonant-phonon depopulation terahertz quantum cascade lasers and their application in spectroscopic imaging

The terahertz (THz) frequency quantum cascade laser (QCL) is a semiconductor heterostructure laser that has attracted much research interest over the past decade. We report on the high performance of THz QCLs based on a three-well resonant-phonon (RP) depopulation active region (AR) and operating in the frequency range 2.7 THz to 4.0 THz. Devices, processed into surface-plasmon waveguides, lased up to 116 K in pulsed mode with threshold current densities as low as 840 Acm−2. The effects of the design frequency and laser cavity length on performance are discussed. We also report on the operation of QCLs with reduced AR thicknesses, and show, for the first time, that the AR thickness of RP QCLs processed in a surface plasmon waveguide can be reduced to as little as 5 µm. Finally, we demonstrate the use of an electrically tuneable THz QCL, based on a heterogeneous AR, for spectroscopic imaging of the high-explosive pentaerythritol tetranitrate.

Terahertz quantum cascade lasers with thin resonant-phonon depopulation active regions and surface-plasmon waveguides

We report three-well, resonant-phonon depopulation terahertz quantum cascade lasers with semi-insulating surface-plasmon waveguides and reduced active region (AR) thicknesses. Devices with thicknesses of 10, 7.5, 6, and 5 μm are compared in terms of threshold current density, maximum operating temperature, output power, and AR temperature. Thinner ARs are technologically less demanding for epitaxial growth and result in reduced electrical heating of devices. However, it is found that 7.5-μm-thick devices give the lowest electrical power densities at threshold, as they represent the optimal trade-off between low electrical resistance and low threshold gain.

Dual-frequency imaging using an electrically tunable terahertz quantum cascade laser

We report dual-frequency imaging using an electrically tunable terahertz frequency quantum cascade laser (QCL). Images of powdered materials including the explosive pentaerythritol tetranitrate (PETN) were acquired at 3.05 THz and 3.24 THz in a single scan. We show that it is possible to distinguish PETN from other materials by taking the difference-transmission and difference-attenuation images. We also demonstrate difference-intensity imaging by combining amplitude modulation of the QCL bias with lock-in detection.

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...

Terahertz Time Domain Spectroscopy of Phonon‐Depopulation Based Quantum Cascade Lasers

A 3.1 THz phonon depopulation‐based quantum‐cascade‐laser is investigated using terahertz time domain spectroscopy. A gain of 25 cm−1 and absorption features due to the lower laser level being populated from a parasitic electronic channel are highlighted.

Switching circuit to improve the frequency modulation difference-intensity THz quantum cascade laser imaging

We demonstrate new switching circuit for difference-intensity THz quantum cascade laser (QCL) imaging by amplitude modulation and lock in detection. The switching circuit is designed to improve the frequency modulation so that it can stably lock the amplitude modulation of the QCL and the detector output. The combination of a voltage divider and a buffer in switching circuit to quickly switch the amplitude of the QCL biases of 15.8 V and 17.2 V is successfully to increase the frequency modulation up to ∼100 Hz.