arXiv subjects

Jérôme Faist

Publications and source records attributed to Jérôme Faist.

At least 55 records · Page 3Linked to original sources

arXiv2023

Impact of higher-order dispersion on frequency-modulated combs

Frequency-modulated (FM) combs form spontaneously in free-running semiconductor lasers and possess a vast potential for spectroscopic applications. Despite recent progress in obtaining a conclusive theoretical description, experimental FM combs often exhibit non-ideal traits, which prevents their widespread use. Here we explain this by providing a clear theoretical and experimental study of the impact of the higher-order dispersion on FM combs. We reveal that spectrally-dependent dispersion is detrimental for comb performance and leads to a decreased comb bandwidth and the appearance of spectral holes. These undesirable traits can be mended by applying a radio-frequency modulation of the laser bias. We show that electrical injection-locking of the laser leads to a significant increase of the comb bandwidth, a uniform-like spectral amplitudes, and the rectification of the instantaneous frequency to recover a nearly linear frequency chirp of FM combs.

physics.optics↗

arXiv2023

Quantum Cascade Surface Emitting Lasers

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 $μ$m. Finally, we demonstrate single mode emission with a side-mode suppression ratio larger than 33 dB of a 248 $μ$m short cavity mounted with the epitaxial layer up and operated in continuous wave at 20 $^\circ$C.

physics.optics↗

arXiv2023

Frequency-modulated combs via on-chip field enhancement

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, we show how a spatial modulation of the laser gain medium can enhance the gain saturation dynamics and nonlinearities to generate self-starting FM combs. We demonstrate this with tapered planarized THz quantum cascade lasers (QCLs). While simple ridge THz QCLs typically generate combs which are 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, a clear linear frequency chirp and very intense beatnotes up to -30 dBm. 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 hightemperature comb operation, up to a heat sink temperature of 115 K, with comb bandwidths of 600 GHz at 90 K. The spatial inhomogeneity also leads to dynamic switching between various harmonic states in the same device.

physics.optics↗

arXiv2023

Broadband surface-emitting THz laser frequency combs with inverse-designed integrated reflectors

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 (17.0°x 18.5°), while broadband fundamental and harmonic comb states spanning up to 800 GHz are observed.

physics.optics↗

arXiv2023

Monolithic Integration of Mid-Infrared Quantum Cascade Lasers and Frequency Combs with Passive Waveguides

Mid-infrared semiconductor lasers in photonic integrated circuits are of considerable interest for a variety of industrial, environmental, and medical applications. However, photonic integration technologies in the mid-infrared lag far behind the near-infrared range. Here we present the monolithic integration of mid-infrared quantum cascade lasers with low-loss passive waveguides via butt-coupling. The passive waveguide losses are experimentally evaluated to be only 1.2 +- 0.3 dB/cm, with negligible butt-coupling losses. We demonstrate continuous-wave lasing at room temperature of these active-to-passive waveguide coupled devices. Moreover, we report a frequency comb operation paving the way toward on-chip, mid-infrared, dual-comb sensors.

physics.optics↗

arXiv2023

Coherent Control of Mid-Infrared Frequency Comb by Optical Injection of Near-Infrared Light

We demonstrate the use of a low power near-infrared laser illuminating the front facet of a quantum cascade laser (QCL) as an optical actuator for the coherent control of a mid-infrared frequency comb. We show that by appropriate current control of the QCL comb and intensity modulation of the near-infrared laser, a tight phase lock of a comb line to a distributed feedback laser is possible with 2 MHz of locking bandwidth and 200 mrad of residual phase noise. A characterization of the whole scheme is provided showing the limits of the electrical actuation which we bypassed using the optical actuation. Both comb degrees of freedom can be locked by performing electrical injection locking of the repetition rate in parallel. However, we show that the QCL acts as a fast near-infrared light detector such that injection locking can also be achieved through modulation of the near-infrared light. These results on the coherent control of a quantum cascade laser frequency comb are particularly interesting for coherent averaging in dual-comb spectroscopy and for mid-infrared frequency comb applications requiring high spectral purity.

physics.optics↗

arXiv2022

An engineered planar plasmonic reflector for polaritonic mode confinement

It was recently demonstrated that, in deep subwavelength gap resonators coupled to two-dimensional electron gases, coupling to propagating plasmons can lead to energy leakage and prevent the formation of polaritonic resonances. This process, akin to Landau damping, limits the achievable field confinement and thus the value of light-matter coupling strength. In this work, we show how plasmonic subwavelength reflectors can be used to create an artificial energy stopband in the plasmon dispersion, confining them and enabling the recovery of the polaritonic resonances. Using this approach we demonstrate a normalized light-matter coupling ratio of Ω/ω = 0.35 employing a single quantum well with a gap size of λ/2400 in vacuum.

cond-mat.mes-hall↗

arXiv2022

THz optical solitons from dispersion-compensated antenna-coupled planarized ring quantum cascade lasers

Quantum Cascade Lasers (QCL) constitute an intriguing opportunity for the production of on-chip optical Dissipative Kerr Solitons (DKS): self-organized optical waves which can travel while preserving their shape thanks to the interplay between Kerr effect and dispersion. Originally demonstrated in passive microresonators, DKS were recently observed in mid-IR ring QCL paving the way for their achievement even at longer wavelengths. To this end we realized defect-less THz ring QCLs featuring anomalous dispersion leveraging on a technological platform based on waveguide planarization. A concentric coupled-waveguide approach is implemented for dispersion compensation whilst a passive broadband bullseye antenna improves the device power extraction and far field. In these devices, comb spectra featuring sech$^2$ envelopes are presented for free-running operation. This first hint of the presence of solitons is further supported by the observation of highly hysteretic behaviour and by phase-sensitive measurements which show the presence of self-starting 12 ps-long pulses in the reconstructed time profile of the emission intensity. These observations are in very good agreement with our numeric simulations based on a Complex Ginzburg-Landau equation time-domain solver. Such devices constitute a new experimental platform for the study of soliton phenomena in the THz range, allowing as well on-chip, passive ultrashort THz pulse generation appealing for a variety of applications.

physics.optics↗

arXiv2022

Planarized THz quantum cascade lasers for broadband coherent photonics

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 results in low waveguide losses and improved 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 over 1.1 THz and RF-injected broadband incoherent states spanning over nearly 1.6 THz are observed. 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 components on the same THz photonic chip.

physics.optics↗

arXiv2022

Frequency chirped Fourier-Transform spectroscopy

Fast (sub-second) spectroscopy with high spectral resolution is of vital importance for revealing quantum chemistry kinetics of complex chemical and biological reactions. Fourier transform (FT) spectrometers can achieve high spectral resolution and operate at hundreds of ms time scales in rapid-scan mode. However, the linear translation of a scanning mirror imposes stringent time-resolution limitations to these systems, which makes simultaneous high spectral and temporal resolution impossible. Here, we demonstrate an FT spectrometer whose operational principle is based on continuous rotational, rather than linear, motion of the scanning mirror, decoupling the spectral resolution from the temporal one. This enables 0.5 cm${}^{-1}$ resolution on sub-ms time scales. Furthermore, we show that such rotational FT spectrometers can perform dual-comb spectroscopy with a single comb source, since the Doppler-shifted version of the comb serves as the second comb. In this way, we combine the advantages of dual-comb and FT spectroscopy using a single quantum cascade laser frequency comb as a light source. Our technique does not require any diffractive or dispersive optical elements and hence preserve the Jacquinot's-, Fellgett's-, and Connes'-advantages of FT spectrometers. The system supports a large optical bandwidth from visible to THz frequencies. The combination of a rotational delay line with collimated coherent or non-coherent light sources pave the way for FT spectrometers in applications where high speed, large optical bandwidth, and high spectral resolution are desired.

physics.optics↗

arXiv2022

Terahertz waveform synthesis from integrated lithium niobate circuits

Bridging the "terahertz (THz) gap" relies upon synthesizing arbitrary waveforms in the THz domain enabling applications that require both narrow band sources for sensing and few-cycle drives for classical and quantum objects. However, realization of custom-tailored waveforms needed for these applications is currently hindered due to limited flexibility for optical rectification of femtosecond pulses in bulk crystals. Here, we experimentally demonstrate that thin-film lithium niobate (TFLN) circuits provide a versatile solution for such waveform synthesis through combining the merits of complex integrated architectures, low-loss distribution of pump pulses on-chip, and an efficient optical rectification. Our distributed pulse phase-matching scheme grants shaping the temporal, spectral, phase, amplitude, and farfield characteristics of the emitted THz field through designer on-chip components. This strictly circumvents prior limitations caused by the phase-delay mismatch in conventional systems and relaxes the requirement for cumbersome spectral pre-engineering of the pumping light. We provide a toolbox of basic blocks that produce broadband emission up to 680 GHz with adaptable phase and coherence properties by using near-infrared pump pulse energies below 100 pJ.

physics.optics↗

arXiv2022

Exceptional Point Singularities in Multi-Section DFB Lasers

A laser exhibits both controllable gain and loss and, under proper design conditions, is an ideal non-Hermitian system allowing the direct observation and engineering of spectral singularities such as exceptional points (EPs). A dual section distributed feedback (DFB) quantum cascade laser (QCL) is a prototype of such a system, allowing the controlled coupling of a ladder of cavity Fabry-Perot (FP) modes to a quarter wave shifted DFB mode. Tuning the coupling strength and the gain difference between these two set of modes enables probing the regimes from weak coupling to strong coupling and the robust observation of exceptional point singularities. At these exceptional points, the laser exhibits a sequence of lasing and coherent prefect absorption dynamics1,2 when pumped above transparency. Additionally, the pumping scheme allows the deliberate lifting of the exceptional point degeneracy. These results show that dual section QCL is a perfect platform to study exceptional points because the coupling parameter and system loss can be tuned in a single device.

physics.optics↗

arXiv2022

Synchronization of frequency combs by optical injection

Optical frequency combs based on semiconductor lasers are a promising technology for monolithic integration of dual-comb spectrometers. However, the stabilization of the offset frequency fceo remains a challenging feat due the lack of octave-spanning spectra. In a dual-comb configuration, the uncorrelated jitter of the offset frequencies leads to a non-periodic signal resulting in broadened beatnotes with a limited signal-to-noise ratio (SNR). Hence, expensive data acquisition schemes and complex signal processing are currently required. Here, we show that the offset frequencies of two frequency combs can be synchronized by optical injection locking, which allows full phase-stabilization when combined with electrical injection. A single comb line isolated via an optical Vernier filter serves as Master oscillator for injection locking. The resulting dual-comb signal is periodic and stable over thousands of periods. This enables coherent averaging using analog electronics, which increases the SNR and reduces the data size by one and three orders of magnitude, respectively. The presented method will enable fully phase-stabilized dual-comb spectrometers by leveraging on integrated optical filters and provides access for measuring and stabilizing fceo.

physics.optics↗

arXiv2021

An ultrastrongly coupled single THz meta-atom

Free-space coupling to strongly subwavelength individual optical elements is a central theme in quantum optics, as it allows to control and manipulate the properties of quantum systems. In this work, we show that by combining an asymmetric immersion lens setup and complementary design of metasurfaces we are able to perform THz time-domain spectroscopy of an individual, strongly subwavelength (d/λ0=1/20) meta-atom. We unravel the linewidth dependence of planar metamaterials as a function of the meta-atom number indicating quenching of the Dicke superradiance. On these grounds, we investigate ultrastrongly coupled Landau polaritons at the single resonator level, measuring a normalized coupling ratio of Ω/ω=0.60 resulting from coupling of the fundamental mode to a few thousand electrons. Similar measurements on a low loss, less doped two dimensional electron gas yield a coupling ratio Ω/ω=0.33 with a cooperativity C=4g^2/κγ= 94. Interestingly, the coupling strength of a coupled single resonator is the same as of a coupled array. Our findings pave the way towards the control of light-matter interaction in the ultrastrong coupling regime at the single electron/single resonator level. The proposed technique is way more general and can be useful to characterize the complex conductivity of micron-sized samples in the THz and sub-THz domain.

physics.optics↗

arXiv2021

Femtosecond pulses from a mid-infrared quantum cascade laser

The quantum cascade laser (QCL) has evolved to be a compact, powerful source of coherent mid-infrared (mid-IR) light. However, its fast gain dynamics strongly restricts the formation of ultrashort pulses. As such, the shortest pulses reported so far were limited to a few picoseconds with some hundreds of milliwatts of peak power, strongly narrowing their applicability for time-resolved and nonlinear experiments. Here, we demonstrate an alternative approach capable of producing near-transform-limited sub-picosecond pulses with several watts of peak power. Starting from a frequency modulated phase-locked state, which most efficiently exploits the gain of the active region, ultrashort high peak power pulses are generated via external pulse compression. We assess their temporal nature by means of a novel optical sampling method, coherent beat note interferometry and interferometric autocorrelation. These results open new pathways for nonlinear physics in the mid-infrared.

physics.optics↗

arXiv2021

Breakdown of the topological protection by cavity vacuum fields in the integer quantum Hall effect

The control of the electronic properties of materials via the vacuum fields of cavity electromagnetic resonators is one of the emerging frontiers of condensed matter physics. We show here that the enhancement of vacuum field fluctuations in subwavelength split-ring resonators dramatically affects arguably one of the most paradigmatic quantum protectorates, namely the quantum Hall electron transport in high-mobility 2D electron gases. The observed breakdown of the topological protection of the integer quantum Hall effect is interpreted in terms of a long-range cavity-mediated electron hopping where the anti-resonant terms of the light-matter coupling finally result into a finite resistivity induced by the vacuum fluctuations. The present experimental platform can be used for any 2D material and provides new ways to manipulate electron phases in matter thanks to vacuum-field engineering

cond-mat.mes-hall↗

arXiv2021

Dissipative Kerr solitons in semiconductor ring lasers

Dissipative Kerr solitons are self-organized optical waves arising from the interplay between Kerr effect and dispersion. They can form spontaneously in nonlinear microresonators pumped with an external continuous-wave laser, which provides the parametric gain for the proliferation of an ultrastable frequency comb. These miniaturized and battery driven microcombs have become a disruptive technology for precision metrology, broadband telecommunication and ultrafast optical ranging. In this work, we report on the first experimental observation of dissipative Kerr solitons generated in a ring cavity with a fast semiconductor gain medium. The moderate quality factor of the ring cavity is compensated by the giant resonant Kerr nonlinearity of a quantum cascade laser, which is more than a million times larger than in Si3N4. By engineering the dispersion of the cavity, we observe the formation of bright dissipative Kerr solitons in the mid-infrared range. Soliton formation appears after an abrupt symmetry breaking between the two lasing directions of the ring cavity. The pump field of the soliton is generated by direct electrical driving and closely resembles the soliton Cherenkov radiation observed in passive microcombs. Two independent techniques shed light on the waveform and coherence of the solitons and confirm a pulse width of approximately 3 ps. Our results extend the spectral range of soliton microcombs to mid-infrared wavelengths and will lead to integrated, battery driven and turnkey spectrometers in the molecular fingerprint region.

physics.optics↗

arXiv2021

Controlling Quantum Cascade Laser Optical Frequency Combs through Microwave Injection

In this work, control over the precise state emitted by quantum cascade laser frequency combs through strong radio-frequency current modulation close to their repetition frequency is demonstrated. In particular, broadening of the spectrum from about 20 cm$^{-1}$ to 60cm$^{-1}$ can be achieved throughout most of the current dynamical range while preserving the coherence, as measured by shifted wave interference Fourier transform spectroscopy (SWIFTS). The required modulation frequency to achieve this broadening is red-shifted compared to the free-running beatnote frequency at increasing modulation powers starting from 25 dBm, whereas the range where it occurs narrows. Outside of this maximum-bandwidth range, the spectral bandwidth of the laser output is gradually reduced and the new center frequency is red- or blue-shifted, directly dependent on the detuning of the modulation frequency. By switching between two modulation frequencies detuned symmetrically with respect to the free-running beatnote, we can generate two multiplexed spectral regions with negligible overlap from the same device at rates of at least 20 kHz. In the time-domain we show with both SWIFTS and interferometric autocorrelation (IAC) measurements a transition from quasi-continuous output to pulsed ($τ_p \approx 55$ ps) output by ramping up the injection power to 35 dBm.

physics.optics↗