We present the simulation, design, fabrication, and characterization of planarized double metal quantum cascade lasers based on InGaAs/GaAsSb. Intended for astrophysical heterodyne measurements and having the cavity embedded in benzocyclobutene, the devices are equipped with thermal bridges on either side of the ridge, in order to improve the heat dissipation. The lasers are shown to vertically emit a single mode around 26 μm in pulsed operation, with peak powers of ≈ 30 μW and a current density threshold of Jth = 3.7 kA/cm2. Maximum operation temperature is around 170 K, with the maximum supported duty cycle being extended from the initial 15% to about 30% with the help of the improved thermal management technique.
Integrated thermal bridges based on the planarized waveguide platform (image by Tudor Olariu)
8. Frequency-modulated combs via field-enhancing tapered waveguides Urban Senica, Alexander Dikopoltsev, Andres Forrer, Sara Cibella, Guido Torrioli, Mattias Beck, Jérôme Faist, Giacomo Scalari. Published in: Laser & Photonics Reviews, 2023, 2300472 (2023)
Frequency-modulated (FM) combs feature flat intensity spectra with a linear frequency chirp, useful for metrology and sensing applications. Generating FM combs in semiconductor lasers generally requires a fast saturable gain, usually limited by the intrinsic gain medium properties. Here, it is shown how a spatial modulation of the laser gain medium can enhance the gain saturation dynamics and nonlinearities to generate self-starting FM combs. This is demonstrated with tapered planarized terahertz (THz) quantum cascade lasers (QCLs). While simple ridge THz QCLs typically generate combs presenting a mixture of amplitude and frequency modulation, the on-chip field enhancement resulting from extreme spatial confinement leads to an ultrafast saturable gain regime, generating a pure FM comb with a flatter intensity spectrum and a clear linear frequency chirp. The observed linear frequency chirp is reproduced using a spatially inhomogeneous mean-field theory model, which confirms the crucial role of field enhancement. In addition, the modified spatial temperature distribution within the waveguide results in an improved high-temperature comb operation, up to a heat sink temperature of 115 K, with comb bandwidths of 600 GHz at 90 K. The spatial inhomogeneity leads as well to very intense radio frequency (RF) beatnotes up to -30 dBm and facilitates dynamic switching between various harmonic states in the same device.
Frequency-modulated combs via field-enhancing tapered waveguides
7. Broadband surface-emitting THz laser frequency combs with inverse-designed integrated reflectors Urban Senica, Sebastian Gloor, Paolo Micheletti, David Stark, Mattias Beck, Jérôme Faist, Giacomo Scalari. Published in: APL Photonics, 8, 9, p. 096101 (2023) [PDF] In the News: Highlight article in Nature Photonics
THz quantum cascade lasers (QCLs) based on double metal waveguides feature broadband and high-temperature devices for their 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 and 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 a peak power of up to 13.5 mW with surface emission into a narrow beam with a divergence of (17.0° × 18.5°), while broadband fundamental and harmonic comb states spanning up to 800 GHz are observed.
Broadband surface-emitting THz laser frequency combs with inverse-designed integrated reflectors
6. Terahertz optical solitons from dispersion-compensated antenna-coupled planarized ring quantum cascade lasers Paolo Micheletti, Urban Senica, Andres Forrer, Sara Cibella, Guido Torrioli, Martin Frankié, Mattias Beck, Jérôme Faist, Giacomo Scalari. Published in: Science Advances, 9, 24 (2023)
In the News: ETH Physics
Quantum cascade lasers (QCLs) constitute an intriguing opportunity for the generation of on-chip optical dissipative Kerr solitons (DKSs). Originally demonstrated in passive microresonators, DKSs were recently observed in mid-infrared ring QCL paving the way for their achievement even at longer wavelengths. To this end, we realized defect-free terahertz ring QCLs featuring anomalous dispersion leveraging on a technological platform based on waveguide planarization. A concentric coupled waveguide approach is implemented for dispersion compensation, while a passive broadband bullseye antenna improves the device power extraction and far field. Comb spectra featuring sech2 envelopes are presented for free-running operation. The presence of solitons is further supported by observing the highly hysteretic behavior, measuring the phase difference between the modes, and reconstructing the intensity time profile highlighting the presence of self-starting 12-picosecond-long pulses. These observations are in very good agreement with our numeric simulations based on a Complex Ginzburg-Landau Equation (CGLE).
Terahertz optical solitons from dispersion-compensated antenna-coupled planarized ring quantum cascade lasers (image by Paolo Micheletti)
Recently, there has been a growing interest in integrated THz photonics for various applications in communications, spectroscopy and sensing. We present a new integrated photonic platform based on active and passive elements integrated in a double-metal, high-confinement waveguide layout planarized with a low-loss polymer. An extended top metallization keeps waveguide losses low while improving dispersion, thermal and RF properties, as it enables to decouple the design of THz and microwave cavities. Free-running on-chip quantum cascade laser combs spanning 800 GHz, harmonic states with over 1.1 THz bandwidth and RF-injected broadband incoherent states spanning over nearly 1.6 THz are observed using a homogeneous quantum-cascade active core. With a strong external RF drive, actively mode-locked pulses as short as 4.4 ps can be produced, as measured by SWIFTS. We demonstrate as well passive waveguides with low insertion loss, enabling the tuning of the laser cavity boundary conditions and the co-integration of active and passive elements on the same THz photonic chip.
Planarized THz quantum cascade lasers for broadband coherent photonics
Because of the ultrafast and photon-driven nature of the transport in their active region, we demonstrate that quantum cascade lasers can be operated as resonantly amplified terahertz detectors with wide RF bandwidth. Tunable responsivities up to 50 V/W and noise equivalent powers down to 100 pW/Hz1/2 are demonstrated at 4.7 THz. Constant peak responsivities with respect to the detector temperature are observed up to 80 K. Thanks to the ≈ps intersub-band lifetime, electrical bandwidths larger than 20 GHz can be obtained, allowing the detection of optical beatnotes from quantum cascade THz frequency combs.
Regenerative THz quantum detectors
3. An antipodal Vivaldi antenna for improved far-field properties and polarization manipulation of broadband terahertz quantum cascade lasers Urban Senica,Elena Mavrona, Tudor Olariu, Andres Forrer, Mattias Beck, Jérôme Faist, Giacomo Scalari. Published in: Applied Physics Letters 116.16 (2020).
We present an antipodal Vivaldi antenna for broadband double metal waveguide terahertz quantum cascade lasers and frequency combs. Its exponentially curved flare profile results in an adiabatic in-plane mode expansion, producing an improved far-field with a single-lobed beam of (23°×19°) full width half maximum with an octave-spanning bandwidth. The antenna also acts as a wave retarder, rotating the polarization from vertical toward horizontal polarization by a frequency-dependent angle. The laser's emission spectrum and current–voltage characteristics are not affected, as well as frequency comb operation. Measurements agree well with numerical simulations, and the proposed antenna covers a broad spectral range (1.5–4.5 THz).
Broadband Vivaldi Antenna for THz QCL Frequency Combs
The requisites at the Large Hadron Collider (LHC) at CERN have driven silicon tracking detectors to the fringe of the modern technology. The next upgrade of the LHC to a 10 times increased luminosity of 7.5×10^34 cm−2 s−1 will require semiconductor detectors with substantially improved radiation hardness. CERN-RD50 collaboration mandate is to provide detector technologies, which are able to operate in such an environment. Within this context, this paper describes the approaches and first results of a C++11 multi-threading software based on the Shockley–Ramo’s theorem to simulate non-irradiated and irradiated silicon micro-strips and pad detectors of complex geometries in order to understand signal formation and charge collection efficiencies of arbitrary charge distributions.
TRACS: TRansient Current Simulator
1. Unidirectional Coupling of Light from Nanowire Lasers into Silicon Waveguides Urban Senica. Master's Thesis, Technical University of Munich & University of Ljubljana.
The thesis focus is on numerical simulations of GaAs-AlGaAs core-shell nanowire lasers on silicon waveguides, which are promising in the context of chip-level optical interconnects. We employ several numerical simulation tools to investigate different properties of the chosen structures. We start by studying the propagating modes defined by the nanowire waveguide. The nanowire end facet reflectivity, an important parameter for low-threshold lasing, can be enhanced from 40% to 90% with a metal mirror. By asymmetrically placing a nanowire which supports a circularly polarized propagating wave on top of a Si waveguide, unidirectional coupling of light can be achieved via the spin-orbit coupling. The ratio between the amount of light coupled in opposite directions of the waveguide can reach values of nearly 10^4 but is extremely sensitive to the position of the nanowire. The coupling is influenced by many parameters, such as nanowire and waveguide dimensions and the thickness of the oxide, resulting in a huge parameter space which makes the task of finding an optimal structure more challenging.
