R. Wallis

Single mode terahertz quantum cascade amplifier

A terahertz (THz) optical amplifier based on a 2.9 THz quantum cascade laser (QCL) structure has been demonstrated. By depositing an antireflective coating on the QCL facet, the laser mirror losses are enhanced to fully suppress the lasing action, creating a THz quantum cascade (QC) amplifier. Terahertz radiation amplification has been obtained, by coupling a separate multi-mode THz QCL of the same active region design to the QC amplifier. A bare cavity gain is achieved and shows excellent agreement with the lasing spectrum from the original QCL without the antireflective coating. Furthermore, a maximum optical gain of ∼30 dB with single-mode radiation output is demonstrated.

External amplitude and frequency modulation of a terahertz quantum cascade laser using metamaterial/graphene devices

Active control of the amplitude and frequency of terahertz sources is an essential prerequisite for exploiting a myriad of terahertz applications in imaging, spectroscopy, and communications. Here we present a optoelectronic, external modulation technique applied to a terahertz quantum cascade laser which holds the promise of addressing a number of important challenges in this research area. A hybrid metamaterial/graphene device is implemented into an external cavity set-up allowing for optoelectronic tuning of feedback into a quantum cascade laser. We demonstrate powerful, all-electronic, control over the amplitude and frequency of the laser output. Full laser switching is performed by electrostatic gating of the metamaterial/graphene device, demonstrating a modulation depth of 100%. External control of the emission spectrum is also achieved, highlighting the flexibility of this feedback method. By taking advantage of the frequency dispersive reflectivity of the metamaterial array, different modes of the QCL output are selectively suppressed using lithographic tuning and single mode operation of the multi-mode laser is enforced. Side mode suppression is electrically modulated from ~6 dB to ~21 dB, demonstrating active, optoelectronic modulation of the laser frequency content between multi-mode and single mode operation.

Efficient coupling of double-metal terahertz quantum cascade lasers to flexible dielectric-lined hollow metallic waveguides

The growth in terahertz frequency applications utilising the quantum cascade laser is hampered by a lack of targeted power delivery solutions over large distances (>100 mm). Here we demonstrate the efficient coupling of double-metal quantum cascade lasers into flexible polystyrene lined hollow metallic waveguides via the use of a hollow copper waveguide integrated into the laser mounting block. Our approach exhibits low divergence, Gaussian-like emission, which is robust to misalignment error, at distances > 550 mm, with a coupling efficiency from the hollow copper waveguide into the flexible waveguide > 90%. We also demonstrate the ability to nitrogen purge the flexible waveguide, increasing the power transmission by up to 20% at 2.85 THz, which paves the way for future fibre based terahertz sensing and spectroscopy applications.

Amplitude stabilization and active control of a terahertz quantum cascade laser with a graphene loaded split-ring-resonator array

We demonstrate the amplitude stabilization of a 2.85 THz quantum cascade laser with a graphene loaded split-ring-resonator array acting as an external amplitude modulator. The transmittance of the modulator can be actively changed by modifying the graphene conductivity via electrostatic back-gating. The modulator operates at room temperature and is capable of actively modulating the quantum cascade laser power level and thus stabilizing the power output via a proportional-integral-derivative feedback control loop. The stability was enhanced by more than 10 times through actively tuning the modulation. Furthermore, this approach can be used to externally control the laser power with a high level of stability.We demonstrate the amplitude stabilization of a 2.85 THz quantum cascade laser with a graphene loaded split-ring-resonator array acting as an external amplitude modulator. The transmittance of the modulator can be actively changed by modifying the graphene conductivity via electrostatic back-gating. The modulator operates at room temperature and is capable of actively modulating the quantum cascade laser power level and thus stabilizing the power output via a proportional-integral-derivative feedback control loop. The stability was enhanced by more than 10 times through actively tuning the modulation. Furthermore, this approach can be used to externally control the laser power with a high level of stability.

Investigation of hollow cylindrical metal terahertz waveguides suitable for cryogenic environments.

The field of terahertz (THz) waveguides continues to grow rapidly, with many being tailored to suit the specific demands of a particular final application. Here, we explore waveguides capable of enabling efficient and accurate power delivery within cryogenic environments (< 4 K). The performance of extruded hollow cylindrical metal waveguides made of un-annealed and annealed copper, as well as stainless steel, have been investigated for bore diameters between 1.75 - 4.6 mm, and at frequencies of 2.0, 2.85 and 3.4 THz, provided by a suitable selection of THz quantum cascade lasers. The annealed copper resulted in the lowest transmission losses, < 3 dB/m for a 4.6 mm diameter waveguide, along with 90° bending losses as low as ~2 dB for a bend radius of 15.9 mm. The observed trends in losses were subsequently analyzed and related to measured inner surface roughness parameters. These results provide a foundation for the development of a wide array of demanding low-temperature THz applications, and enabling the study of fundamental physics.

Metamaterial/graphene amplitude and frequency modulators for the active control of terahertz quantum cascade lasers

Hybrid metamaterial/graphene amplitude and frequency modulators have been implemented as external optoelectronic mirrors in external cavity configurations with terahertz quantum cascade lasers (QCLs). These devices’ tunability is accomplished via the interplay between metamaterial resonant units, normally engineered in mm-size arrays, and graphene. The integration of these devices in external cavity QCLs offers unique emission features and realizes an unprecedented studied regime. The implementation of an external amplitude modulation allows the full switching of laser emission in single mode operation by electrostatically gating graphene. The introduction of more dispersive tunable architectures in frequency modulators yields additionally an all-electronic spectral laser bistability.

100 % Amplitude modulation of an external cavity terahertz QCL using an optoelectronic chopper based on metamaterials and graphene

The continuous development of terahertz (THz) sources has opened up many potential applications in spectroscopy, imaging and communications. One popular THz source is the quantum cascade laser (QCL), which has many desirable properties including compactness and high output power with a narrow emission frequency. For such a source to be successfully integrated into a THz communication system, it is necessary to have control over the amplitude, frequency and phase. For wireless communication purposes, amplitude modulators must have a reasonable modulation depth and be capable of fast modulation speeds to take full advantage of the greater bandwidth opened up by using a THz carrier wave. To this purpose, we have developed optoelectronic split ring resonator (SRR) and graphene amplitude modulators which have been combined with a THz QCL thus realising an external cavity set-up which uses the SRR/graphene devices to efficiently modulate the light feedback into the laser cavity. The SRR/graphene device is lithographically designed to have maximum reflectivity at the QCL emission frequency (2.9 THz) and the graphene acts as a variable dampener, capable of electrically modulating the reflectivity. Similar SRR/graphene device architectures have been used previously for amplitude modulation by varying the reflection from a standard CW QCL output, achieving modulation speeds >100 MHz with a modulation depth limited to around 20 % [1].

Terahertz s-SNOM with > λ/1000 resolution based on self-mixing in quantum cascade lasers

Near-field imaging techniques have great potential in many applications, ranging from the investigation of the optical properties of solid state and 2D materials to the excitation and direct retrieval of plasmonic resonant modes, to the mapping of carrier concentrations in semiconductor devices. Further to this, the capability of performing imaging with non-ionizing terahertz (THz) radiation on a subwavelength scale is of fundamental importance in biological applications and healthcare. The implementation of stable, compact solid state sources such as quantum cascade lasers (QCLs) in apertureless scanning near field optical microscopes (s-SNOM), instead of bulkier gas lasers, has been already reported with a resolution ≥ 1 μm [1] based on metallic tips. Here we report on the realization of an s-SNOM, based on tuning fork sensors [2], to maintain a constant sample/tip distance in tapping mode, and using quantum cascade lasers emitting around 3 THz as both source and detector in a self-mixing scheme [3]. The implementation of a fast and efficient feedback mechanism allowed the achievement of a spatial resolution lower than 100 nm, as shown in Fig. 1, thus achieving the record resolution with a QCL better than λ/1000. The self-mixing approach allows an extremely sensitive and fast detection scheme, which overcomes the slow response of traditional THz detectors, by monitoring the scattered signal fed back into the QCL cavity, modulating the power or the bias. In order to enhance the sensitivity of the whole apparatus, as well as the collection of the scattered light, silicon lenses have been attached to the QCLs with an antireflection parylene coating which was thick enough to strongly reduce the laser emission, but still allowed enough power for alignment. Figure 1 reports the topography a) and the THz voltage signal on the QCL b) of Au square features (top-left square corner) over a Si substrate, exhibiting an enhanced scattering. As the reference voltage used for subtraction from the QCL voltage was placed lower than the QCL voltage, the THz signal dropped on the Au square.

Mechanically robust cylindrical metal terahertz waveguides for cryogenic applications

As the ambition behind THz quantum cascade laser based applications continues to grow, abandoning free-space optics in favor of waveguided systems promises major improvements in targeted, easy to align, and robust radiation delivery. This is especially true in cryogenic environments, where illumination is traditionally challenging. Although the field of THz waveguides is rapidly developing, most designs have limitations in terms of mechanical stability at low temperatures, and are costly and complicated to fabricate to lengths > 1 m. In this work, we investigate readily available cylindrical metal waveguides which are suitable for effective power delivery in cryogenic environments, and explore the optimal dimensions and materials available. The materials chosen were extruded un-annealed and annealed copper, as well as stainless steel, with bore diameters of 1.75, 2.5, and 4.6 mm. Measurements were performed at three different frequencies, 2.0, 2.85 and 3.2 THz, with optimal transmission losses <3 dB/m demonstrated at 2.0 THz. Additionally, novel optical couplers are also presented and characterised, with the ability to change the beam path by 90° with a coupling loss of just 2.2 dB whilst maintaining mode quality, or thermally isolate sections of waveguide with a coupling loss as low as 0.5 dB. The work presented here builds on previous work1, and forms a comprehensive investigation of cryogenically compatible THz waveguides and optical couplers, paving the way for a new generation of systems to utilize THz QCLs for a host of low-temperature investigations.

Research data supporting "THz nanoscopy of plasmonic resonances with a quantum cascade laser"

The dataset contains all the experimental data required to reproduce the Figures. The profiles can be extracted from the original files.