Analyzing the effect of doping concentration in split-well resonant-phonon terahertz quantum cascade lasers.

The effect of doping concentration on the temperature performance of the novel split-well resonant-phonon (SWRP) terahertz quantum-cascade laser (THz QCL) scheme supporting a clean 4-level system design was analyzed using non-equilibrium Green's functions (NEGF) calculations. Experimental research showed that increasing the doping concentration in these designs led to better results compared to the split-well direct-phonon (SWDP) design, which has a larger overlap between its active laser states and the doping profile. However, further improvement in the temperature performance was expected, which led us to assume there was an increased gain and line broadening when increasing the doping concentration despite the reduced overlap between the doped region and the active laser states. Through simulations based on NEGF calculations we were able to study the contribution of the different scattering mechanisms on the performance of these devices. We concluded that the main mechanism affecting the lasers' temperature performance is electron-electron (e-e) scattering, which largely contributes to gain and line broadening. Interestingly, this scattering mechanism is independent of the doping location, making efforts to reduce overlap between the doped region and the active laser states less effective. Optimization of the e-e scattering thus could be reached only by fine tuning of the doping density in the devices. By uncovering the subtle relationship between doping density and e-e scattering strength, our study not only provides a comprehensive understanding of the underlying physics but also offers a strategic pathway for overcoming current limitations. This work is significant not only for its implications on specific devices but also for its potential to drive advancements in the entire THz QCL field, demonstrating the crucial role of e-e scattering in limiting temperature performance and providing essential knowledge for pushing THz QCLs to new temperature heights.

Split-well resonant-phonon terahertz quantum cascade laser.

We present a highly diagonal "split-well resonant-phonon" (SWRP) active region design for GaAs/Al0.3Ga0.7As terahertz quantum cascade lasers (THz-QCLs). Negative differential resistance is observed at room temperature, which indicates the suppression of thermally activated leakage channels. The overlap between the doped region and the active level states is reduced relative to that of the split-well direct-phonon (SWDP) design. The energy gap between the lower laser level (LLL) and the injector is kept at 36 meV, enabling a fast depopulation of the LLL. Within this work, we investigated the temperature performance and potential of this structure.

Terahertz Pulse Generation with Binary Phase Control in Nonlinear InAs Metasurface.

The effect of terahertz (THz) pulse generation has revolutionized broadband coherent spectroscopy and imaging at THz frequencies. However, THz pulses typically lack spatial structure, whereas structured beams are becoming essential for advanced spectroscopy applications. Nonlinear optical metasurfaces with nanoscale THz emitters can provide a solution by defining the beam structure at the generation stage. We develop a nonlinear InAs metasurface consisting of nanoscale optical resonators for simultaneous generation and structuring of THz beams. We find that THz pulse generation in the resonators is governed by optical rectification. It is more efficient than in ZnTe crystals, and it allows us to control the pulse polarity and amplitude, offering a platform for realizing binary-phase THz metasurfaces. To illustrate this capability, we demonstrate an InAs metalens, which simultaneously generates and focuses THz pulses. The control of spatiotemporal structure using nanoscale emitters opens doors for THz beam engineering and advanced spectroscopy and imaging applications.

Tunable quantum-cascade VECSEL operating at 1.9 THz.

We report a terahertz quantum-cascade vertical-external-cavity surface-emitting laser (QC-VECSEL) emitting around 1.9 THz with up to 10% continuous fractional frequency tuning of a single laser mode. The device shows lasing operation in pulsed mode up to 102 K in a high-quality beam, with the maximum output power of 37 mW and slope efficiency of 295 mW/A at 77 K. Challenges for up-scaling the operating wavelength in QC metasurface VECSELs are identified.

Wavelength beam-combining of terahertz quantum-cascade laser arrays.

Wavelength beam-combining of four terahertz (THz) distributed-feedback quantum-cascade lasers (QCLs) is demonstrated using low-cost THz components that include a lens carved out of a plastic ball and a mechanically fabricated blazed grating. Single-lobed beams from predominantly single-mode QCLs radiating peak power in the range of 50-170mW are overlapped in the far field at frequencies ranging from 3.31-3.54THz. Collinear propagation with a maximum angular deviation of 0.3∘ is realized for the four beams. The total power efficiency for the focused and beam-combined radiation is as high as 25%. This result could pave the way for future commercialization of beam-combined monolithic THz QCL arrays for multi-spectral THz sensing and spectroscopy at standoff distances.

Highly efficient terahertz photoconductive metasurface detectors operating at microwatt-level gate powers.

Despite their wide use in terahertz (THz) research and technology, the application spectra of photoconductive antenna (PCA) THz detectors are severely limited due to the relatively high optical gating power requirement. This originates from poor conversion efficiency of optical gate beam photons to photocurrent in materials with sub-picosecond carrier lifetimes. Here we show that using an ultra-thin (160 nm), perfectly absorbing low-temperature grown GaAs metasurface as the photoconductive channel drastically improves the efficiency of THz PCA detectors. This is achieved through perfect absorption of the gate beam in a significantly reduced photoconductive volume, enabled by the metasurface. This Letter demonstrates that sensitive THz PCA detection is possible using optical gate powers as low as 5 µW-three orders of magnitude lower than gating powers used for conventional PCA detectors. We show that significantly higher optical gate powers are not necessary for optimal operation, as they do not improve the sensitivity to the THz field. This class of efficient PCA THz detectors opens doors for THz applications with low gate power requirements.

Terahertz Detection with Perfectly-Absorbing Photoconductive Metasurface.

Terahertz (THz) photoconductive devices are used for generation, detection, and modulation of THz waves, and they rely on the ability to switch electrical conductivity on a subpicosecond time scale using optical pulses. However, fast and efficient conductivity switching with high contrast has been a challenge, because the majority of photoexcited charge carriers in the switch do not contribute to the photocurrent due to fast recombination. Here, we improve efficiency of electrical conductivity switching using a network of electrically connected nanoscale GaAs resonators, which form a perfectly absorbing photoconductive metasurface. We achieve perfect absorption without incorporating metallic elements, by breaking the symmetry of cubic Mie resonators. As a result, the metasurface can be switched between conductive and resistive states with extremely high contrast using an unprecedentedly low level of optical excitation. We integrate this metasurface with a THz antenna to produce an efficient photoconductive THz detector. The perfectly absorbing photoconductive metasurface opens paths for developing a wide range of efficient optoelectronic devices, where required optical and electronic properties are achieved through nanostructuring the resonator network.

High-intensity and low-divergence THz laser with 1D autofocusing symmetric Airy beams.

Single-mode THz quantum-cascade lasers (QCLs) have been realized using a wide variety of techniques to obtain a combination of large power output, good beam quality with single-lobed beams, and low far-field divergence. Beam shaping using external components has not been previously exploited due to limited commercial availability of THz optical components and also the accompanying large loss from most THz optical materials. Here, we demonstrate that excellent beam characteristics could be obtained for a THz QCL by integration of a surface-emitting distributed-feedback (DFB) QCL with a simple lens within the vacuum chamber of a cryocooler. Plano-convex lenses are made from inexpensive plastic balls and integrated in proximity with a 3.4 THz DFB QCL. With appropriately chosen lens parameters, dual-lobed Airy beams are generated that autofocus into a high-intensity single-lobed beam with large focusing efficiency. A simple and general method to generate one-dimensional autofocusing Airy beams is thus demonstrated that is applicable at any wavelength. THz laser beams with high peak intensity (57 mW/mm2 in a spot-size of 1 mm2) or low-divergence (1.4∘×1.8∘ for a beam with 118 mW peak power) are realized at 62 K in a compact electrically operated Stirling cooler. A high brightness of 5.4×106 Wsr -1m -2 is estimated for the focused beams by measuring the beam propagation ratios (M 2 parameters).

Terahertz Driven Amplification of Coherent Optical Phonons in GaAs Coupled to a Metasurface.

We demonstrate amplification of longitudinal optical (LO) phonons by polar-optical interaction with an electron plasma in a GaAs structure coupled to a metallic metasurface using two-color two-dimensional spectroscopy. In a novel scheme, the metamaterial resonator enhances broadband terahertz fields, which generate coherent LO phonons and drive free electrons in the conduction band of GaAs. The time evolution of the LO phonon amplitude is monitored with midinfrared pulses via the LO-phonon-induced Kerr nonlinearity of the sample, showing an amplification of the LO phonon amplitude by up to a factor of 10, in agreement with a theoretical estimate.

Focusing metasurface quantum-cascade laser with a near diffraction-limited beam.

A terahertz vertical-external-cavity surface-emitting-laser (VECSEL) is demonstrated using an active focusing reflectarray metasurface based on quantum-cascade gain material. The focusing effect enables a hemispherical cavity with flat optics, which exhibits higher geometric stability than a plano-plano cavity and a directive and circular near-diffraction limited Gaussian beam with M2 beam parameter as low as 1.3 and brightness of 1.86 × 106 Wsr-1m-2. This work initiates the potential of leveraging inhomogeneous metasurface and reflectarray designs to achieve high-power and high-brightness terahertz quantum-cascade VECSELs.

2.1 THz quantum-cascade laser operating up to 144 K based on a scattering-assisted injection design.

A 2.1 THz quantum cascade laser (QCL) based on a scattering-assisted injection and resonant-phonon depopulation design scheme is demonstrated. The QCL is based on a four-well period implemented in the GaAs/Al0.15Ga0.85As material system. The QCL operates up to a heat-sink temperature of 144 K in pulsed-mode, which is considerably higher than that achieved for previously reported THz QCLs operating around the frequency of 2 THz. At 46 K, the threshold current-density was measured as ∼ 745 A/cm2 with a peak-power output of ∼10 mW. Electrically stable operation in a positive differential-resistance regime is achieved by a careful choice of design parameters. The results validate the robustness of scattering-assisted injection schemes for development of low-frequency (ν < 2.5 THz) QCLs.

Antenna coupled photonic wire lasers.

Slope efficiency (SE) is an important performance metric for lasers. In conventional semiconductor lasers, SE can be optimized by careful designs of the facet (or the modulation for DFB lasers) dimension and surface. However, photonic wire lasers intrinsically suffer low SE due to their deep sub-wavelength emitting facets. Inspired by microwave engineering techniques, we show a novel method to extract power from wire lasers using monolithically integrated antennas. These integrated antennas significantly increase the effective radiation area, and consequently enhance the power extraction efficiency. When applied to wire lasers at THz frequency, we achieved the highest single-side slope efficiency (~450 mW/A) in pulsed mode for DFB lasers at 4 THz and a ~4x increase in output power at 3 THz compared with a similar structure without antennas. This work demonstrates the versatility of incorporating microwave engineering techniques into laser designs, enabling significant performance enhancements.

Evaluating the coherence and time-domain profile of quantum cascade laser frequency combs.

Recently, much attention has been focused on the generation of optical frequency combs from quantum cascade lasers. We discuss how fast detectors can be used to demonstrate the mutual coherence of such combs, and present an inequality that can be used to quantitatively evaluate their performance. We discuss several technical issues related to shifted wave interference Fourier Transform spectroscopy (SWIFTS), and show how such measurements can be used to elucidate the time-domain properties of such combs, showing that they can possess signatures of both frequency-modulation and amplitude-modulation.

Near-field probing of Mie resonances in single TiO2 microspheres at terahertz frequencies.

We show experimentally that poly-crystalline TiO2 spheres, 20-30 µm in diameter, exhibit a magnetic dipole Mie resonance in the terahertz (THz) frequency band (1.0-1.6 THz) with a narrow line-width (<40 GHz). We detect and investigate the magnetic dipole and electric dipole resonances in single high-permittivity TiO2 microspheres, using a near-field probe with a sub-wavelength (~λ/50) size aperture and THz time-domain spectroscopy technique. The Mie resonance signatures are observed in the electric field amplitude and phase spectra, as well as in the electric field distribution near the microspheres. The narrow line-width and the sub-wavelength size (λ/10) make the TiO2 microspheres excellent candidates for realizing low-loss THz metamaterials.

Interferometric measurement of far infrared plasmons via resonant homodyne mixing.

We present an electrically tunable terahertz two dimensional plasmonic interferometer with an integrated detection element that down converts the terahertz fields to a DC signal. The integrated detector utilizes a resonant plasmonic homodyne mixing mechanism that measures the component of the plasma waves in-phase with an excitation field functioning as the local oscillator. Plasmonic interferometers with two independently tuned paths are studied. These devices demonstrate a means for developing a spectrometer-on-a-chip where the tuning of electrical length plays a role analogous to that of physical path length in macroscopic spectroscopic tools such as Fourier transform interferometers.

Broadband all-electronically tunable MEMS terahertz quantum cascade lasers.

In this work, we demonstrate all-electronically tunable terahertz quantum cascade lasers (THz QCLs) with MEMS tuner structures. A two-stage MEMS tuner device is fabricated by a commercial open-foundry process performed by the company MEMSCAP. This provides an inexpensive, rapid, and reliable approach for MEMS tuner fabrication for THz QCLs with a high-precision alignment scheme. In order to electronically actuate the MEMS tuner device, an open-loop cryogenic piezo nanopositioning stage is integrated with the device chip. Our experimental result shows that at least 240 GHz of single-mode continuous electronic tuning can be achieved in cryogenic environments (∼4 K) without mode hopping. This provides an important step toward realizing turn-key bench-top tunable THz coherent sources for spectroscopic and coherent tomography applications.

Superradiant decay of cyclotron resonance of two-dimensional electron gases.

We report on the observation of collective radiative decay, or superradiance, of cyclotron resonance (CR) in high-mobility two-dimensional electron gases in GaAs quantum wells using time-domain terahertz magnetospectroscopy. The decay rate of coherent CR oscillations increases linearly with the electron density in a wide range, which is a hallmark of superradiant damping. Our fully quantum mechanical theory provides a universal formula for the decay rate, which reproduces our experimental data without any adjustable parameter. These results firmly establish the many-body nature of CR decoherence in this system, despite the fact that the CR frequency is immune to electron-electron interactions due to Kohn's theorem.

Rectified diode response of a multimode quantum cascade laser integrated terahertz transceiver.

We characterized the DC transport response of a diode embedded in a THz quantum cascade laser as the laser current was changed. The overall response is described by parallel contributions from the rectification of the laser field due to the non-linearity of the diode I-V and from thermally activated transport. Sudden jumps in the diode response when the laser changes from single mode to multi-mode operation, with no corresponding jumps in output power, suggest that the coupling between the diode and laser field depends on the spatial distribution of internal fields. The results demonstrate conclusively that the internal laser field couples directly to the integrated diode.

Effective mode selector for tunable terahertz wire lasers.

We demonstrate an effective mode selector design that enables a terahertz quantum cascade wire laser to have a robust single-mode operation at frequencies much lower than the gain peak. This is achieved by selectively guiding the undesired modes into a lossy session while keeping the desired lasing mode largely unperturbed. The large mode discrimination obtained by this mode selector is necessary to further extend the tuning range to the lower half of the gain curve. Additionally, the connectors of this mode selector conveniently provide electrical bias to the wire lasers without degrading the lasing performance.

Active tuning of mid-infrared metamaterials by electrical control of carrier densities.

We demonstrate electrically-controlled active tuning of mid-infrared metamaterial resonances using depletion-type devices. The depletion width in an n-doped GaAs epilayer changes with an electric bias, inducing a change of the permittivity of the substrate and leading to frequency tuning of the resonance. We first present our detailed theoretical analysis and then explain experimental data of bias-dependent metamaterial transmission spectra. This electrical tuning is generally applicable to a variety of infrared metamaterials and plasmonic structures, which can find novel applications in chip-scale active infrared devices.