Giles Davies

Low-threshold terahertz quantum-cascade lasers

A quantum-cascade laser operating at λ=66 μm is demonstrated. It consists of a three-quantum-well chirped-superlattice active region embedded in a waveguide based on a single interface plasmon and a buried contact. A threshold current density of 210 A/cm2 at T=12 K, a maximum peak optical power of 4 mW, and operation up to T=44 K are achieved in a 2.7 mm long device with a high reflectivity backfacet coating.

Far-infrared (λ≃87 μm) bound-to-continuum quantum-cascade lasers operating up to 90 K

We report terahertz frequency (3.5 THz, λ≃87 μm) emission from quantum-cascade lasers employing a bound-to-continuum transition in the active region. The maximum operating temperature is in excess of 90 K. Peak powers of 20 mW at 20 K and 10 mW at 77 K are achieved. The same devices show continuous-wave operation up to 55 K with measured optical powers of 15 mW at 10 K.

Terahertz range quantum well infrared photodetector

We demonstrated a GaAs/AlGaAs-based far-infrared quantum well infrared photodetector at a wavelength of λ=84 μm. The relevant intersubband transition is slightly diagonal with a dipole matrix element of 3.0 nm. At 10 K, a responsivity of 8.6 mA/W and a detectivity of 5×107 cm √Hz/W have been achieved; and successful detection up to a device temperature of 50 K has been observed. Being designed for zero bias operation, this device profits from a relatively low dark current and a good noise behavior.

High power quantum cascade lasers operating at λ≃87 and 130μm

We report high power quantum cascade lasers operating above liquid nitrogen temperature at λ≃87 and 130μm based on a bound-to-continuum transition. For λ≃87μm, 56mW peak power in pulsed operation and 50mW continuous wave operation at 10K are demonstrated. At λ≃130μm, a peak power of 50mW was achieved and devices operated in continuous wave reached a maximum temperature of 53K with an optical power of 11.5mW at T=10K. Lifetimes are extracted from the scaling of the transport and laser parameters as a function of size using a simple rate equation model.

Absorption-sensitive diffuse reflection imaging of concealed powders using a terahertz quantum cascade laser

We report diffuse reflection imaging in air of concealed powdered samples using a terahertz quantum cascade laser. The sensitivity of the detection scheme to sub-surface absorption within samples is confirmed using fully-characterized powdered admixtures of polystyrene and polymethyl methacrylate (PMMA). Measurements of the backscattering intensity from these samples are then used in conjunction with Kubelka-Munk scattering theory, as well as several models based on the quasi-crystalline approximation, to extract the absorption coefficient of PMMA. Our research demonstrates the feasibility of high-resolution frequency-domain terahertz imaging for the detection and identification of concealed powders in a reflection geometry.

Bridging the terahertz gap

OVER THE last century or so, physicists and engineers have progressively explored and conquered the electromagnetic spectrum. Starting with visible light, we have encroached outwards, developing techniques for generating and detecting radiation at both higher and lower frequencies. And as each successive region of the spectrum has been colonized, we have developed technology to exploit the radiation found there. X-rays, for example, are routinely used to image hidden objects. Near-infrared radiation is used in fibre-optic communications and in compact-disc players, while microwaves are used to transmit signals from your mobile phone.

Self-Mixing Interferometry With Terahertz Quantum Cascade Lasers

Terahertz frequency quantum cascade lasers (THz QCLs) are compact sources of coherent THz radiation with potential applications that include astronomy, trace-gas sensing, and security imaging. However, the reliance on slow and incoherent thermal detectors has limited their practical use in THz systems. We demonstrate THz sensing using self-mixing (SM) interferometry, in which radiation is reflected from an object back into the QCL cavity, causing changes in the laser properties; the THz QCL thus acts simultaneously as both a source and detector. Well-established SM theory predicts a much weaker coupling in THz QCLs than in diode lasers, yielding a near-linear relationship between the phase of SM signals and the external cavity length. We demonstrate velocimetry of an oscillating reflector by monitoring SM-induced changes in the QCL drive voltage. We show that this yields data equivalent to that obtained by sensing the emitted THz power, thus allowing phase-sensitive THz-SM sensing without any external detector. We also demonstrate high-resolution SM-imaging at a round-trip distance of 21 m in air-the longest-range interferometric sensing with a THz QCL to date.

Population inversion by resonant magnetic confinement in terahertz quantum-cascade lasers

Ultralow-threshold terahertz laser emission exploiting in-plane confinement arising from perpendicular magnetic field applied on a quantum-cascade structure is reported. A special design strategy has been adopted that takes advantage of the selective opening and closing of relaxation channels by elastic scattering between Landau levels. The key effect is a reduction of the lower state lifetime of the lasing transition that produces population inversion. The structure shows laser action only with applied magnetic field and yields threshold current densities as low as 19 A/cm2 at 4.2 K and 32 A/cm2 at 60 K at a frequency of 3.6 THz.

Application of terahertz quantum-cascade lasers to semiconductor cyclotron resonance

Quantum-cascade lasers operating at 4.7, 3.5, and 2.3 THz have been used to achieve cyclotron resonance in InAs and InSb quantum wells from liquid-helium temperatures to room temperature. This represents one of the first spectroscopic applications of terahertz quantum-cascade lasers. Results show that these compact lasers are convenient and reliable sources with adequate power and stability for this type of far-infrared magneto-optical study of solids. Their compactness promises interesting future applications in solid-state spectroscopy.

Complementary spectroscopic studies of materials of security interest

We demonstrate that, through coherent measurement of the transmitted terahertz frequency electric fields, broadband (0.3 - 8 THz) time-domain spectroscopy can be used to measure far-infrared vibrational modes of a range of drugs-of-abuse and high explosives that are of interest to the forensic and security services. Our results indicate that absorption features in these materials are highly sensitive to the structural and spatial arrangement of the molecules. Terahertz frequency spectra are also compared with high-resolution low-frequency Raman spectra to assist in understanding the low-frequency inter- and intra-molecular vibrational modes of the molecules.