We present a two-well terahertz (THz) quantum cascade laser designed for high-temperature operation based on the InGaAs/AlInAs material system. The lighter effective mass and higher energy barriers increase the gain at high temperatures (T > 150 K). When processed in copper-based double metal waveguides, the devices show laser action up to a maximum operating temperature of 188 K with a maximum current density of 1.4 kA/cm(2). The low Joule heating due to reduced active region thickness and low electrical bias allows operation at 10% duty cycle up to a temperature of 170 K.

We present InP-based two-well THz QCLs lasing up to a heatsink temperature of 188K, leveraging the smaller effective mass of this material system. To address increased interface roughness scattering resulting from higher conduction band discontinuity, the same active region was grown on (411)A oriented substrates and show lasing up to 164K.

We miniaturize a vertically coupled in-plane whispering gallery mode cavity incorporating a quantum cascade gain medium, aiming to realize the mid-infrared counterpart to the vertical cavity surface emitting laser. Building on previous work with linear microcavities, we introduce a new type of quantum cascade surface emitting laser (QCSEL) by miniaturizing a buried heterostructure ring cavity. At wavelengths of 4.5 and 8 μm, we investigate the optical losses for decreasing ring diameters while benchmarking the device performance against linear microcavities. We achieve an equivalent mirror reflectivity of 0.95 and demonstrate lasing with ring diameters as small as 50 μm. Finally, we report a continuous-wave threshold power dissipation of 274 mW for a 100 μm diameter ring QCSEL, characterized on wafer level at 20 °C.

Mid-infrared laser sources operating with low power dissipation on small device footprints are essential for compact and portable spectroscopic applications [1]. Quantum Cascade Lasers (QCLs) with tunable emission wavelengths in the molecular fingerprint region [2] offer a platform for developing such sources. In particular, QCL active regions embedded in ring-shaped cavities are promising candidates. The ring geometry supports low optical losses due to the formation of whispering gallery modes (WGMs) in the cavity, providing a route toward device miniaturization and reduced power consumption [3].

A low-cost single frequency laser emitting in the mid-infrared spectral region and dissipating minimal electrical power is a key ingredient for the next generation of portable gas sensors for high-volume applications involving chemical sensing of important greenhouse and pollutant gases. We propose here a Quantum Cascade Surface Emitting Laser (QCSEL), which we implement as a short linear cavity with high reflectivity coated end-mirrors to suppress any edge emission and use a buried semiconductor diffraction grating to extract the light from the surface. By wafer-level testing we investigate the cavity length scaling, extract mirror reflectivities larger than 0.9, and achieve a pulsed threshold power dissipation of 237 mW for an emission wavelength near 7.5 $\mu$m. Finally, we demonstrate single mode emission with a side-mode suppression ratio larger than 33 dB of a 248 $\mu$m short cavity mounted with the epitaxial layer up and operated in continuous wave at 20 $^\circ$C.

Laser & Photonics ReviewsVolume 18, Issue 8 2470049 Back CoverFree Access Quantum Cascade Surface Emitting Lasers (Laser Photonics Rev. 18(8)/2024) David Stark, David StarkSearch for more papers by this authorFilippos Kapsalidis, Filippos KapsalidisSearch for more papers by this authorSergej Markmann, Sergej MarkmannSearch for more papers by this authorMathieu Bertrand, Mathieu BertrandSearch for more papers by this authorBahareh Marzban, Bahareh MarzbanSearch for more papers by this authorEmilio Gini, Emilio GiniSearch for more papers by this authorMattias Beck, Mattias BeckSearch for more papers by this authorJérôme Faist, Jérôme FaistSearch for more papers by this author David Stark, David StarkSearch for more papers by this authorFilippos Kapsalidis, Filippos KapsalidisSearch for more papers by this authorSergej Markmann, Sergej MarkmannSearch for more papers by this authorMathieu Bertrand, Mathieu BertrandSearch for more papers by this authorBahareh Marzban, Bahareh MarzbanSearch for more papers by this authorEmilio Gini, Emilio GiniSearch for more papers by this authorMattias Beck, Mattias BeckSearch for more papers by this authorJérôme Faist, Jérôme FaistSearch for more papers by this author First published: 12 August 2024 https://doi.org/10.1002/lpor.202470049AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat Graphical Abstract Quantum Cascade Surface Emitting Lasers In article number 2300663, David Stark, Jérôme Faist, and co-workers introduce the Quantum Cascade Surface Emitting Laser (QCSEL, pronounced "kjuxel"), a device concept targeting the mid-infrared equivalent of the Vertical Cavity Surface Emitting Laser (VCSEL). By leveraging miniaturization and wafer-level testing, the authors demonstrate the feasibility of producing low-cost and low-power consuming mid-infrared lasers in high volumes. These devices are proposed for the next generation of compact and portable gas sensing applications involving industrial process control, environmental monitoring, and medical diagnosis. Volume18, Issue8August 20242470049 RelatedInformation

A crucial element for the next generation of portable gas sensors for high-volume applications, especially involving chemical sensing of important greenhouse and pollutant gases, is the development of a low-cost, low-power consuming, single-frequency laser operating in the mid-infrared spectral range. In this regard, we propose the implementation of a Quantum Cascade Surface Emitting Laser (QCSEL). Our design involves a linear microcavity with high reflectivity coated end-mirrors and a buried semiconductor diffraction grating to extract the light from the surface.

The mid-infrared (MIR) spectral region (2-20 μm) is the molecular “fingerprint” region for many important organic and inorganic molecules [1], [2]. Miniaturized optical gas sensors based on MIR absorption spectroscopy are highly attractive for many applications such as industrial process control, environmental monitoring and medical diagnosis [3]. To enable low-cost and portable MIR gas sensors, compact and low power consuming single-mode light sources operating in the range of interest are highly desirable. $\mu \mathrm{m}$

Quantum-cascade-laser (QCL) frequency combs are compact semiconductor light sources operating in the mid-IR and terahertz frequencies. Achieving subpicosecond laser pulses with high peak power is of vital importance for performing nonlinear time-resolved spectroscopy as well for exploring nonlinear phenomena. Therefore, investigation and characterization of time-resolved free-running laser emission is a key for further improvement and optimization of these intersubband devices. In this work, we demonstrate a direct electric field measurement of a free-running terahertz QCL frequency comb using electro-optic sampling in combination with computational phase correction, where we retrieve the electric field profile and access the comb parameters. The demonstrated method is of high interest for time-resolved lowpower laser emission characterization, typical for ring terahertz QCL frequency combs and investigation of phase-compensated emission via waveguide dispersion engineering for broadband comb operation.

THz quantum cascade lasers (QCLs) based on double metal waveguides feature broadband and high-temperature devices for use in spectroscopy and sensing. However, their extreme field confinement produces poor output coupling efficiencies and divergent far-fields. Here, we present a planarized THz QCL with an inverse-designed end facet reflector coupled to a surface-emitting patch array antenna. All the components have been optimized for octave-spanning spectral bandwidths between 2-4 THz and monolithically integrated on the same photonic chip. We demonstrate this experimentally on broadband THz QCL frequency combs, with measured devices showing a seven-fold improvement in slope efficiency compared to devices with a cleaved facet. They feature peak powers of up to 13.5 mW with surface emission into a narrow beam with a divergence of (17.0x 18.5), while broadband fundamental and harmonic comb states spanning up to 800 GHz are observed.

We present low threshold quantum cascade surface emitting lasers (QCSELs) emitting at wavelengths of 4.5 micrometers or 8 micrometers. To extract the light vertically from the InP-based buried heterostructure laser a second order InGaAs/InP grating is used. Both ridge facets are formed by dry-etching followed by coating a dielectric-metal film. Due to the high reflectivity of the facets, the cavity can be shortened well below 500 micrometers reducing the threshold power to several hundred milliwatts. The proposed device concept allows large-scale fabrication and wafer-level characterization. The results are an important step towards low-cost and low-power consuming quantum cascade lasers for portable MIR gas sensors.

Buried heterostructure quantum cascade lasers (BH-QCLs) operating at high temperature in mid-infrared (MIR) to THz spectral range are desired for chemical sensing and free-space optical communication (FOC). In this work, Fe doped semi-insulating InP (SI-InP) regrowth is demonstrated in a hydride vapor phase epitaxy (HVPE) reactor for advanced MIR and THz BH-QCLs grown by MBE and MOCVD. SI-InP regrowth is implemented in THz QCL pillar arrays and narrow width and reverse-taper MIR BH-QCLs for efficient heat dissipation. By exploiting SI-InP regrowth, the parasitic capacitance in MIR distributed feedback BH-QCL can be suppressed, which is exploited for high speed FOC application.

A novel class of programmable integrated photonic circuits has emerged over the past years, strongly driven by approaches to tackle unsolved computing problems in the optical domain. Photonic neuromorphic and quantum computing are examples of optical systems implemented in complex photonic circuits, which are reconfigured before and during operation. However, a key building block to enable efficient reconfigurable optical network architectures is still missing: a non-volatile optical phase shifter. Here we demonstrate such an element—compatible with silicon photonics—based on the monolithic integration of BaTiO 3 thin films with silicon waveguides. By manipulating ferroelectric domains in BaTiO 3 with electrical control signals, we achieve analogue and non-volatile optical phase tuning with no absorption changes. We demonstrate an eight-level long-term-stable photonic device with non-destructive optical readout and switching energy as low as 4.6 pJ. With our results, an analogue non-volatile photonic element is added to the integrated photonics toolbox, enabling a new generation of power-efficient programmable photonic circuits.

A terahertz intersubband emitter based on silicon is presented. The emission originates from n-type Ge/SiGe quantum cascade structures. We designed a strain-compensated single quantum active region based on a vertical optical transition and tensile-strained Si0.15Ge0.85 barriers. The 51 quantum cascade periods (corresponding to 4.2 μm) were grown on a Si1-xGex reverse graded virtual substrate on Ge/Si(001) substrates. Deeply etched diffraction gratings were processed and the surface emitting devices were characterized at 5 K with a Fourier transform infrared spectrometer. We observed two distinct peaks at 3.4 and 4.9 THz with a line broadening of 20%. This is an important step towards the realization of an Ge/SiGe THz quantum cascade laser.

We report electroluminescence at 14meV and 20meV from a n-type Ge/Si0.15Ge0.85 quantum cascade heterostructure on Si substrate grown by ultra-high vacuum chemical vapour deposition. The electroluminescence signal of the single quantum well active region design, extracted through diffraction gratings from mesa structures, is compared with its GaAs counterpart.The spectral features agree well with modeling based on Non-equilibrium Green's function calculations. The observed electroluminescence peaks show a full width at half maximum of 3meV and 4meV. These results are an important step towards the realization of an n-type THz quantum cascade laser on a non-polar material system.

The Quantum Cascade Laser (QCL) has been demonstrated in polar III-V semiconductor materials employing transitions between conduction band states [1] . Harnessing intersubband transitions allows lasing at mid-infrared and far-infrared wavelengths. Buried InGaAs/InAlAs QCLs unlocked the mid-infrared application space, because they are operational at room-temperature and in continuous wave [2] . However, THz QCLs remain limited up to 250 K in pulsed operation with a large dissipation [3] . The quenching of the laser emission is related to ther-mally activated LO phonon emission in polar materials. Exploiting intersubband transitions in non-polar group IV materials with weaker electron-phonon interaction is an exciting approach to realize a Si-based THz QCL and to eventually elevate the operation temperature [4] .