A compact, continuous-wave terahertz source based on a quantum-cascade laser and a miniature cryocooler

Beam Profile Characterisation of an Optoelectronic Silicon Lens-Integrated PIN-PD Emitter between 100 GHz and 1 THz

Knowledge of the beam profiles of terahertz emitters is required for the design of terahertz instruments and applications, and in particular for designing terahertz communications links. We report measurements of beam profiles of an optoelectronic silicon lens-integrated PIN-PD emitter at frequencies between 100 GHz and 1 THz and observe significant deviations from a Gaussian beam profile. The beam profiles were found to differ between the H-plane and the E-plane, and to vary strongly with the emitted frequency. Skewed profiles and irregular side-lobes were observed. Metrological aspects of beam profile measurements are discussed and addressed.

Short Barriers for Lowering Current-Density in Terahertz Quantum Cascade Lasers

Scattering due to interface-roughness (IR) and longitudinal-optical (LO) phonons are primary transport mechanisms in terahertz quantum-cascade lasers (QCLs). By choosing GaAs/Al0.10Ga0.90As heterostructures with short-barriers, the effect of IR scattering is mitigated, leading to low operating current-densities. A series of resonant-phonon terahertz QCLs developed over time, achieving some of the lowest threshold and peak current-densities among published terahertz QCLs with maximum operating temperatures above 100 K. The best result is obtained for a three-well 3.1 THz QCL with threshold and peak current-densities of 134 A/cm2 and 208 A/cm2 respectively at 53 K, and a maximum lasing temperature of 135 K. Another three-well QCL designed for broadband bidirectional operation achieved lasing in a combined frequency range of 3.1–3.7 THz operating under both positive and negative polarities, with an operating current-density range of 167–322 A/cm2 at 53 K and maximum lasing temperature of 141 K or 121 K depending on the polarity of the applied bias. By showing results from QCLs developed over a period of time, here we show conclusively that short-barrier terahertz QCLs are effective in achieving low current-density operation at the cost of a reduction in peak temperature performance.

Thermoelectric-cooled terahertz quantum cascade lasers

We demonstrate the first lasing emission of a thermo-electrically cooled terahertz quantum cascade laser (THz QCL). A high temperature three-well THz QCL emitting at 3.8 THz is mounted to a novel five-stage thermoelectric cooler reaching a temperature difference of ΔT = 124 K. The temperature and time-dependent laser performance is investigated and shows a peak pulse power of 4.4 mW and a peak average output power of 100 μW for steady-state operation.

Real-time gas sensing based on optical feedback in a terahertz quantum-cascade laser

We report on real-time gas sensing with a terahertz quantum-cascade laser (QCL). The method is solely based on the modulation of the external cavity length, exploiting the intermediate optical feedback regime. While the QCL is operated in continuous-wave mode, optical feedback results in a change of the QCL frequency as well as its terminal voltage. The first effect is exploited to tune the lasing frequency across a molecular absorption line. The second effect is used for the detection of the self-mixing signal. This allows for fast measurement times on the order of 10 ms per spectrum and for real-time measurements of gas concentrations with a rate of 100 Hz. This technique is demonstrated with a mixture of D₂O and CH₃OD in an absorption cell.

Terahertz quantum-cascade lasers as high-power and wideband, gapless sources for spectroscopy

Terahertz (THz) quantum-cascade lasers (QCLs) are powerful radiation sources for high-resolution and high-sensitivity spectroscopy with a discrete spectrum between 2 and 5 THz as well as a continuous coverage of several GHz. However, for many applications, a radiation source with a continuous coverage of a substantially larger frequency range is required. We employed a multi-mode THz QCL operated with a fast ramped injection current, which leads to a collective tuning of equally-spaced Fabry-Pérot laser modes exceeding their separation. A continuous coverage over 72 GHz at about 4.7 THz was achieved. We demonstrate that the QCL is superior to conventional sources used in Fourier transform infrared spectroscopy in terms of the signal-to-noise ratio as well as the dynamic range by one to two orders of magnitude. Our results pave the way for versatile THz spectroscopic systems with unprecedented resolution and sensitivity across a wide frequency range.

Analysis of granular packing structure by scattering of THz radiation

Scattering methods are widely used to characterize the structure and constituents of matter on small length scales. This motivates this introductory text on identifying prospective approaches to scattering-based methods for granular media. A survey to light scattering by particles and particle ensembles is given. It is elaborated why the established scattering methods using X-rays and visible light cannot in general be transferred to granular media. Spectroscopic measurements using terahertz radiation are highlighted as they probe the scattering properties of granular media, which are sensitive to the packing structure. Experimental details to optimize a spectrometer for measurements on granular media are discussed. We perform transmission measurements on static and agitated granular media using Fourier transform spectroscopy at the THz beamline of the Bessy II storage ring. The measurements demonstrate the potential to evaluate degrees of order in the media and to track transient structural states in agitated bulk granular media.

High-spectral-resolution terahertz imaging with a quantum-cascade laser

We report on a high-spectral-resolution terahertz imaging system operating with a multi-mode quantum-cascade laser (QCL), a fast scanning mirror, and a sensitive Ge:Ga detector. By tuning the frequency of the QCL, several spectra can be recorded in 1.5 s during the scan through a gas cell filled with methanol (CH₃OH). These experiments yield information about the local absorption and the linewidth. Measurements with a faster frame rate of up to 3 Hz allow for the dynamic observation of CH₃OH gas leaking from a terahertz-transparent tube into the evacuated cell. In addition to the relative absorption, the local pressure is mapped by exploiting the effect of pressure broadening.

Cascaded stimulated polariton scattering in a Mg:LiNbO(3) terahertz laser

Cascaded Stimulated Polariton Scattering (SPS) to the fourth-Stokes order is observed experimentally in an intracavity THz polariton laser utilising Mg:LiNbO(3). The performance of the cascaded laser is presented, the origin of the cascaded fields is explained and compared to theory, and the potential consequences for using cascading to enhance the THz output from this type of device are discussed.

Evidence for frequency comb emission from a Fabry-Pérot terahertz quantum-cascade laser

We report on a broad-band terahertz quantum-cascade laser (QCL) with a long Fabry-Pérot ridge cavity, for which the tuning range of the individual laser modes exceeds the mode spacing. While a spectral range of approximately 60 GHz (2 cm(-1)) is continuously covered by current and temperature tuning, the total emission range spans more than 270 GHz (9 cm(-1)). Within certain operating ranges, we found evidence for stable frequency comb operation of the QCL. An experimental technique is presented to characterize frequency comb operation, which is based on the self-mixing effect.

Frequency modulation spectroscopy with a THz quantum-cascade laser

We report on a terahertz spectrometer for high-resolution molecular spectroscopy based on a quantum-cascade laser. High-frequency modulation (up to 50 MHz) of the laser driving current produces a simultaneous modulation of the frequency and amplitude of the laser output. The modulation generates sidebands, which are symmetrically positioned with respect to the laser carrier frequency. The molecular transition is probed by scanning the sidebands across it. In this way, the absorption and the dispersion caused by the molecular transition are measured. The signals are modeled by taking into account the simultaneous modulation of the frequency and amplitude of the laser emission. This allows for the determination of the strength of the frequency as well as amplitude modulation of the laser and of molecular parameters such as pressure broadening.

Reflective terahertz imaging with the TEM01 mode laser beam

Reflective terahertz imaging with a first-order Hermite-Gaussian laser beam was experimentally investigated. High spatial resolution targets prepared by direct laser microprocessing were used to evaluate the performance. The reflection imaging system at 2.524 THz frequency demonstrated up to diffraction limited resolution using the single focusing mirror with the numerical aperture not smaller than 0.6. The TEM(01) mode laser beam was also applied for practical samples such as silicon solar cell terahertz (THz) imaging. It is shown that usage of appropriate optics enables us to obtain high-quality THz images with the multimode laser beam.

Lateral distributed-feedback gratings for single-mode, high-power terahertz quantum-cascade lasers

We report on terahertz quantum-cascade lasers (THz QCLs) based on first-order lateral distributed-feedback (lDFB) gratings, which exhibit continuous-wave operation, high output powers (>8 mW), and single-mode emission at 3.3-3.4 THz. A general method is presented to determine the coupling coefficients of lateral gratings in terms of the coupled-mode theory, which demonstrates that large coupling strengths are obtained in the presence of corrugated metal layers. The experimental spectra are in agreement with simulations of the lDFB cavities, which take into account the reflective end facets.

Stand-alone system for high-resolution, real-time terahertz imaging

In this work we present a stand-alone, portable system for high resolution real-time THz imaging. The total weight of the apparatus is less than 15 kg and its physical dimension is of ~(65 cm)3. A quantum cascade laser emitting at 3.4 THz laser based on a third-order distributed feedback cavity is used as source. It operates in continuous-wave at 50 K with more than 1 mW output power and less than 300 mW of power consumption. High resolution real-time THz imaging is reported: resolution of 2.5 times the wavelength is demonstrated.

High-resolution terahertz reflective imaging and image restoration

We present a high-resolution terahertz (THz) reflective imaging system, operating at 2.52 THz, that employs a continuous-wave THz gas laser and a pyroelectric detector. The spatial resolution was evaluated from the system's modulation transfer function and tested by scanning a series of resolution targets. To further improve the image quality, Lucy-Richardson method was adopted to restore the scanning result. With the scanning spot profile measured using knife edge method, a satisfying restoration result can be obtained. Finally, the system's performance was observed by imaging some different test objects.