Recently , quantum cascade laser proved to be an extremely interesting platform for frequency combs both in Mid-IR and THz frequency ranges. We will discuss some peculiar aspects of these devices arising from the combination of ultrafast gain, circular cavities and strong RF modulation. Despite the ultrafast nature of the gain medium, by properly engineering dispersion we demonstrate dissipative Kerr solitons both in Mid-IR and THz , with pulse durations of 3.7 ps in the Mid-IR and 10 ps in the THz. Then, by RF modulating a circular cavity, we demonstrate a quantum walk comb in synthetic frequency space. The initially ballistic quantum walk does not dissipate into low supermode states of the synthetic lattice; instead, the state stabilizes in a broad frequency comb, unlocking the full potential of the synthetic frequency lattice. Combs as broad as 100 cm-1 in the Mid-IR with flat top profile are reported.
We demonstrate that resonant phase-modulation of circular Quantum Cascade Laser cavities gives rise to a novel kind of frequency comb, that is remarkably stable, fully tunable and broadband. When the backscattering in such ring cavities is sufficiently low, unidirectional lasing in the free-running device yields single-mode emission. As soon as resonant RF injection is enabled, the spectrum continuously and predictably broadens to span up to 100 cm-1 with nearly-flatted topped spectra. The bandwidth of the resulting comb is fully governed by the depth of the modulation and reaches the fundamental limit dictated by dispersion.
We will present new developments in THz coherent photonics enabled by a recently demonstrated broadband planarized platform based on quantum cascade gain medium. The possibility to integrate onto the same chip active (lasers, detectors, amplifiers) and passive (waveguides, splitters, antennas, chirped mirrors,) photonic elements results extremely attractive, naturally bridging microwaves to THz waves. Such approach allows the adoption of advanced photonics design techniques (inverse design) to tailor facet reflectivities in double metal, subwavelength waveguides.
We will present frequency combs exceeding 1 THz bandwidth, operating above liquid nitrogen temperature, with regular far fields and vertical emission. We will as well discuss laser dynamics engineering exploiting extreme field confinement in narrow waveguides, clearly demonstrating FM comb operation in THz QCLs. By exploiting dispersion compensation in planarized double ring cavities we will finally present the achievement of dissipative Kerr solitons with pulses of 10 ps. The application of such waveguides to high-temperature active regions allows the operation of 4 THz QCLs on Peltier cooler with currents below 2.5 A
Standard Fourier Transform Infrared Spectrometers (FTIR) rely on a Michelson Interferometer scheme which uses a linear delay line to retrieve an interference pattern. Here, we demonstrate a fast FTIR based on a rotational delay line which allows us to achieve kHz acquisition rates. We perform spectrometry measurements using it in combination either with a Mid-IR Quantum Cascade Laser (QCL) frequency comb or a strongly, low-frequency, RF-modulated QCL. Regarding the latter, the modulation enables to broaden the laser emission up to 250cm^-1 (from 6.5µm to 7.5µm) and to reduce its amplitude noise compared to the free-running case. The combination of a strongly modulated QCL with a rotational FTIR opens the possibility to fast and broadband spectroscopy in the Mid-IR region, with possible applications spanning from gas detection to process control
Ultrashort signals are integral for conducting high-resolution measurements. In the mid-infrared, the generation of ultrashort pulses is notoriously difficult to achieve and usually requires large optical setups. In our work we use direct sampling to demonstrate the spontaneous generation of stable ultrashort features in the time-domain signal of a mid-infrared quantum cascade laser frequency comb. The full-width at half-maximum of these features is measured to be ~500 fs, right below the Fourier-limit derived from the corresponding optical spectrum and RF-injection can be used for stabilization and manipulation. Using Maxwell-Bloch equation-based simulations, we can reproduce the generation of such features, including the position in relation to the instantaneous frequency and show their width can be lowered even further below the Fourier limit, thus opening new possibilities for high-resolution measurements based on quantum cascade laser frequency combs.
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.
It has been shown that ring quantum cascade lasers can emit solitons when the driving current is large enough such that spontaneous symmetry breaking occurs between the clockwise and counter-clockwise modes . We show that ring devices with very low backscattering will emit an optical frequency comb when injected with RF close to the round trip frequency. This result is interpreted as a new form of active mode-locking with a fast saturable gain. The transient behavior of the device is studied by time resolved spectroscopy.
We present optical beatnote detection from a THz QCL comb operating at 80 K in a small nitrogen-cooled dewar. The 21.7 GHz comb beatnote is detected by downconversion, directly mixing free-space signals from the QCL and a microwave synthesizer onto an NbN HEB optimized for RF frequencies. The setup constitutes a very convenient platform for the study of QCL-based optical frequency combs and a building block for compact, portable frequency comb fast spectrometers.
Because their phase can be compensated by a grating-based stretcher compressor and further controlled by RF injection, quantum cascade laser based optical frequency comb generation allowed the generation of pulse as short as 630fs after compression, value confirmed using an upconversion technique with a sub-picosecond time resolution. Another possibility is the direct generation of optical solitons using ring quantum cascade lasers in which, by using a very low lateral loss waveguide, the symmetric counter-propagating modes undergo a spontaneous symmetry breaking and generate solitons.
We investigate quantum cascade lasers emitting in the THz range based on a new, planarized waveguide platform. We measure temporal intensity profile by means of Shifted Wave Interference Fourier Transform Spectroscopy (SWIFTs) of both Fabry-Pérot and ring resonators. We observe a variety of phemomena varying the amount of microwave injected in the laser cavities. In Fabry-Pèrot devices we observe transition from FM comb behaviour to AM with pulses formation as short as 4.4 ps. Additionally, we measure with SWIFTS fundamental as well as harmonic comb states under RF injection. In the case of the ring lasers, we fabricate devices displaying spectrally dependent dispersion. We observe hysterical behaviour of the lasing spectra as a function of the RF power. In the case of very weak RF injection, we observe pulses and spectral envelopes well fitted by a sech^2 profile corresponding to anomalous dispersion region, hinting at the presence of solitons.
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.
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 3 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. The same platform is employed to demonstrate dispersion compensated ring combs operating at 3 THz.
We present a high-performance planarized waveguide THz quantum cascade laser frequency comb, where an inverse-designed active waveguide front facet with a reduced reflectivity is coupled to a patch array antenna, and all the components are optimized for an octave-spanning emission spectrum (2-4 THz). Broadband frequency comb states spanning over 800 GHz with a single narrow RF beatnote up to -50 dBm are measured at 20 K. The slope efficiency is improved by around five times, with a peak output power of 13 mW in pulsed mode (10% duty cycle at 20 K). Far-field measurements of the surface emission display a narrow symmetric pattern with a beam width of (21° x 20°).
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.
We studied the performance of hot-electron bolometers (HEBs) operating at THz optical frequencies based on superconducting niobium nitride films. We report on large optical bandwidth measurement of the voltage response of the detector carried out with different THz sources. We show that the impulse response of the fully packaged HEB at 7.5 K has a 3 dB cut-off around 2 GHz, but a considerable detection capability is also observed above 30 GHz recorded in mixing mode operation by using a THz frequency comb quantum cascade laser
Terahertz (THz) quantum cascade lasers (QCLs) based on double metal waveguides are compact sources of broadband THz radiation, which can also operate as frequency combs. We present a planarized double metal waveguide THz QCL platform, where the active region is embedded in a low-loss BCB polymer and covered by an extended top metallization. The latter enables placing bonding wires on the sides above the BCB-covered area, hindering the formation of any defects on the active region and enables the fabrication of waveguides with narrow widths below the bonding wire size. This can then be employed as a fundamental mode selection mechanism for comb operation without any side absorbers, and also features improved heat dissipation properties in continuous wave operation. The extended top metallization also enhances the RF properties of the device, as it encompasses a metallic cavity with the global ground plane. Experimentally, we present results on two different device geometries. First is a simple ridge waveguide with a width of 40 μm, narrow enough to act as a mode selection filter. Free-running frequency comb states with bandwidths above 600 GHz and single beatnotes up to -60 dBm are measured. With a strong external RF signal, close to the natural repetition frequency, we can broaden the emission to over 1.4 THz. The second type of device is a tapered waveguide, where the narrow sections act as a transversal mode filter, while the wider ones have lower waveguide losses and provide more gain. Due to a field-enhancement effect in the narrow sections, there is a significant enhancement in the four wave mixing, a third order nonlinear process responsible for comb formation. Free-running devices produce beatnotes close to -30 dBm, three orders of magnitude higher than for ridge devices. Improved comb performance is maintained also for high operating temperatures. A comb bandwidth above 200 GHz and a single beatnote above -60 dBm are measured at 115 K, very close to the maximum lasing temperature of 118 K. Beyond the improved laser and comb performance, the planarized waveguide platform also enables a relatively straightforward co-integration of active and passive elements.
Ultrastrong light-matter coupling has recently been achieved in several experimental platforms at different optical frequencies, leveraging on the collective enhancement of the interaction with the number of excitations and the simultaneous sub-wavelength electromagnetic field localization obtainable employing metallic resonators. Ever shrinking resonators have allowed to approach the regime of few electrons strong coupling, in which single-dipole properties can be modified by the vacuum field. In this work we will discuss two results that are relevant with respect to the limits of achievable light-matter coupling strength and to the measurement of such coupling in single, strongly subwavelength resonators. In the first experiment we demonstrate, theoretically and experimentally, the existence of a limit to the possibility of arbitrarily increasing electromagnetic confinement in polaritonic systems. Strongly sub-wavelength fields can excite a continuum of high-momenta propagative magnetoplasmons. This leads to peculiar nonlocal polaritonic effects, as certain polaritonic features disappear and the system enters in the regime of discrete-to-continuum strong coupling.
In the second part of the 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 meta-atom. We unravel the linewidth dependence of planar metamaterials as a function of the meta-atom number indicating quenching of the Dicke superradiance. We investigate as well ultrastrongly coupled Landau polaritons at the single resonator level, measuring a normalized coupling ratio of 0.6 resulting from coupling of the fundamental mode of a single, deeply subwavelength LC resonatorto a few thousand electrons.
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.
We report frequency combs formation in THz QCL ring cavities. The double metal waveguide laser is encapsulated in benzocyclobutene (BCB) on which the top contact is deposited. This allows to explore alternative designs such as ultrathin ring cavities and coupled double ring waveguides avoiding issues with the electrical connection of the. Ring laser operating in dense comb regime with spectral bandwidth of ~500 GHz is here reported. In addition to the frequency comb operation sech2-shaped spectra are observed in RF-injected ring lasers and dispersion compensated double ring cavities, hinting at the existence of soliton regimes in the QCL.
Non-linear properties of buried heterostructure ring quantum cascade lasers have been investigated. Clear symmetry breaking between two counter-propagating modes has been observed with a transition to a emission that has a solitonic characteristics.
In this work present high performance QCL-based THz combs operating on fundamental and harmonic comb states operating up to 110 K in the spectral region from 2 to 4 THz . We employ double-metal, Copper- based laser resonators planarized with a polymer allowing high performance with CW operation up to 118 K . Such waveguide layout allows as well optimized RF coupling facilitating injection of high RF power. We analyze the laser emission by means of SWIFTS technique employing an Hot-Electron-Bolometer based on NbN. Different regimes are observed as the RF injection power is increased, going from FM emission to a pure AM. Spectral bandwidths as large as 700 GHz are observed corresponding to a fully coherent laser operation. For specific waveguide geometries and injection conditions pulses as short as 4 ps are observed. We present as well SWIFTS measurements for THz QCL combs operating on harmonic states under RF injection at the harmonic frequency of 17.6 GHz.
THz QCLs are promising sources towards achieving octave-spanning comb operation and self-referencing in a monolithic device. We investigate the spectral mode phase relations by means of SWIFT of free running and strongly RF modulated devices. The inspected QCLs are based on an octave-spanning, heterogeneous and a 1.8 THz spanning, homogeneous active region design. The extracted neighboring mode phase differences from SWIFT of free running devices show the FM nature of the comb emission. When strongly RF modulated the spectrum is broadened and both AM and FM states are be observed.
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 present a planarized double metal waveguide THz quantum cascade laser, where the top contact metallization extends beyond the active region, above the surrounding low-loss BCB polymer. Placing wire bonds over the BCB-covered area enables the fabrication of extremely narrow waveguide dimensions with reduced power dissipation. Compared to a standard double metal waveguide, improvements in waveguide losses, dispersion, RF and thermal properties are observed. Measurement results feature frequency comb operation with free-running beatnotes as strong as -30 dBm, self-starting harmonic states, RF-driven broadband emission, comb operation up to 110 K, and laser operation up to 118 K in continuous-wave.
We present strong radio-frequency current modulation close to their repetition frequency as a means to control the emitted state of quantum cascade laser frequency combs. In particular, more than doubling of the spectral bandwidth compared to free-running can be achieved throughout the dynamical range of the device. By changing the modulation frequency, the spectral bandwidth and center-frequency can be tuned and by fast switching between modulation frequencies we can multiplex spectral regions with negligible overlap from the same device. In the time-domain, we are able to transition from quasi-continuous to long-pulse output by injecting at high power.
In recent years, quantum cascade lasers have matured to become compact, powerful sources of coherent midinfrared light. Yet, the ultrafast carrier dynamics in these sources has so far restricted the formation of highintensity ultrashort pulses. In this work, we demonstrate the formation of ∼ 630 fs QCL pulses with a peak power of ∼ 4.3 W. We break the picosecond barrier in an approach similar to chirped pulse amplification, where we externally recompress the maximally chirped output of a quantum cascade laser frequency comb. Ultrashort pulse formation is confirmed with a novel asynchronous optical sampling technique. These results emphasise the potential of quantum cascade lasers also as sources for non-linear experiments in the mid-infrared.
Frequency red tuning of GHz-level is shown in THz Quantum Cascade Lasers, by post-processing alteration of the device. It involves changing the height difference between the active region and surrounding medium, and reapplying a material of known refractive index with an appropriate thickness according to simulations. Thus the mode effective refractive index is increased, causing the device to lase at a lower frequency. With the correct material and thickness, shifts down to 1 GHz can be observed.
We present a terahertz quantum cascade laser array of coupled parallel double metal waveguides, spaced by narrow gaps. The entire array is pumped by a single electrical source, and at a relatively low bias, an equidistant mode spacing and a narrow single beatnote are observed, which are indicating frequency comb operation. The spectrum consists of subgroups of modes separated by an integer multiple of the cavity round-trip frequency. This is a sign that the individual array elements combine to form a global comb, and also suggests the possibility of a harmonic comb formation.
THz quantum cascade lasers (QCL) frequency combs are recently attracting attention both as valuable sources for THz spectroscopy as well as a model system to study non linear generation and laser dynamics. Harmonic comb state has proven to be emerging in quantum cascade lasers and promoted by an interplay between parametric gain and spatial hole burning. We report here on robust, self-starting harmonic mode locking in double-metal THz quantum cascade lasers. Different harmonic orders can be excited in the same laser cavity depending on the pumping conditions. The harmonic state can be RF injected and frequency pulled. We study the dependence of harmonic state on the laser cavity and we also report results on harmonic state in external cavities under RF injection.
Recently, on-chip quantum-cascade-laser-based frequency combs are gaining increasing attention both in the Mid-IR and in the THz spectral regions. THz devices offer the possibility of filling the gap of comb sources in a spectral region were no table-top comb is available. I will discuss direct THz comb generation from both homogeneous and heterogeneous quantum cascade lasers. Octave spanning emission spectra and comb operation on bandwidth larger than 1 THz are reported for heterogeneous cascades. I will also report on a series of new structures with homogeneous cascade design that feature a very low threshold current density (< 100 A/cm2), a bandwidth of roughly 1 THz centered a 3 THz and an extremely wide bandwidth (>1.8 THz) when driven in the NDR region. This extremely broadband emission in the NDR is studied as well with NEGF simulation and is based on an interplay between strong photon assisted transport due to the highly diagonal transition and domain formation.These structures are also showing RF injection locking with extremely reduced microwave powers. We will discuss locking experiments as well as a method to finely control the repetition rate of the laser based on controlled optical feedback.
Time resolved spectral measurements aimed to clarify the physics of field domains in the NDR will be also presented.
A broadband, Cu-Cu THz quantum cascade laser is presented. The device shows an 50% improved dynamical range and doubled peak power in pulsed compared to Au-Au devices. The pulsed maximum lasing temperature was increased from 105 K to 133 K. In CW similar bandwidths for Cu-Cu and Au-Au devices are observed with an increased temperature performance. At 77 K the emitted power of the device is reduced by 57% compared to 30 K. At this temperature a single narrow beatnote is observed, which can be locked to an RF synthesizer. THz comb emission spans roughly 800 GHz.
WE WILL REVIEW RECENT ADVANCES IN THE REALIZATION OF WIDE BANDWIDTH, HIGH PERFORMANCE. FREQUENCY COMB SOURCES BASED ON QUANTUM CASCADE LASERS OPERATING BOTH IN THE THz AND MID-IR REGIONS OF THE E. M. Spectrum. In the Mid-IR, a grating compressor is employed to obtain pulses from a quantum cascade comb operating in CW. IN THE THz, COPPER-BASED DOUBLE METAL WAVE GUIDES ALLOW COMB OPERATION ABOVE LIQUID NITROGEN TEMPERATURE WITH RELATIVE Comb BANDWIDTHS OF 25%. Frequency COMB CONTROL BY MEANS of RF INJECTION AND COUPLED CAVITY SCHEMES WILL BE PRESENTED TOGETHER WITH NEW COMB characterization TECHNIQUES.
It has long been established that the combs emitted by quantum cascade lasers (QCLs) cannot be described as a train of short pulses, separated by the cavity roundtrip time. Instead, simulations made for typical device parameters suggest that, in steady state, these four-wave mixing driven combs have a constant temporal envelope, and undergo periodic rapid and complicated swings in frequency.
Recent work in characterising the modal phases has revealed a state which, somewhat unexpectedly, has a simple parabolic phase profile, corresponding to a linearly chirped output field. Moreover, this phase relationship was shown to be stable over time, and to be recoverable after the laser’s power had been cycled; from the perspective of a fixed external pulse compression scheme, these last two properties are critical.
In this work, we use a pair of gratings and lenses in a 4-f Martinez-type scheme to modify the phase of a high-power (~1 W) QCL comb emitted at 8.2 um with more than 100 cm-1 spectral bandwidth. By changing the position of the second grating, a parabolic phase can be added to or subtracted from the field. Employing this scheme, we demonstrate a compression of the QCL output from a 133 ps continuous wave waveform, to a train of pulses of width < 20 ps, and a peak power more than 10x that of the original. With this proof-of-principle work, we highlight the potential of the QCL system to deliver short, powerful pulses, with applications in nonlinear spectroscopy, for example.
Recently, on-chip quantum-cascade-laser-based frequency combs are gaining increasing attention both in the Mid-IR and in the THz spectral regions. THz devices offer the possibility of filling the gap of comb sources in a spectral region were no table top comb is available. We report on THz comb emission from homogeneous quantum cascade laser structure with a new active region design. It features a very low threshold current density (< 100 A/cm2), extremely wide bandwidth (>1 THz) extending from 3 THz to 4.2 THz and peculiar broadband behavior in the NDR region suggesting the presence of field domains. Time resolved spectral measurements employing an hot electron bolometer demonstrate the simultaneous lasing of all the modes in the NDR region and beatnote spectroscopy based on self-mixing proves the coherent nature of the broadband laser emission. Active control of the repetition rate is also demonstrated by using an external cavity scheme employing a piezo actuator. This active region is very promising for the future integration in an heterogeneous structure for octave spanning comb operation and also as an homogenous comb device
Quantum cascade laser-based frequency combs have attracted much attention as of late for applications in sensing and metrology, especially as sources for chip-scale spectroscopy at mid-infrared fingerprint wavelengths. A frequency comb is a light source whose lines are evenly-spaced, and only two frequencies are needed to describe the system—the offset and the repetition rate. Because chip-scale combs have large repetition rates, for many spectroscopic applications is important to be able to change both parameters independently, without substantially changing the comb spectrum or spectral structure. Although it is possible to modulate both the offset and the repetition rate of a comb by tuning the laser current and temperature, both properties affect the laser by changing its index of refraction, and both frequencies will be affected. Here, we show that by integrating a mirror onto a MEMS comb drive, the dispersion and group delay associated with a quantum cascade comb’s cavity can be modulated at kilohertz speeds. Because the MEMS mirror primarily affects the group delay of the cavity, it is able to adjust the comb’s repetition rate while leaving the offset frequency mostly unaffected. Since this adjustment is linearly independent from current adjustments and can be adjusted quickly, this provides an avenue for mutual stabilization of both parameters. In addition, we show that dynamic modulation of the comb drive is able to allow the laser to recover from comb-destroying feedback, making the resulting comb considerably more robust under realistic conditions.
Ultrastrong light matter coupling has raised high interest in recent years for the predicted unusual quantum properties of its ground state, which contains photons. We have investigated such physics in a system based on the cyclotron transition of a 2D confined electrons (or holes) gas in semiconductors coupled to the modes of highly subwavelength metallic resonators in the 200-1000 GHz range. The extreme reduction of the cavity volume and surface (Seff/λ0=3 x 10-7) led to the observation of ultrastrong coupling on a small (<100) number of electrons. Such extreme conditions reveal also a previously unobserved renormalization of the cyclotron effective mass, effectively breaking Kohn’s theorem. Kohn's theorem states the independence of the cyclotron resonance frequency from many-body effects in the case of a parabolic and translationally invariant system. For our resonator the translational invariance is clearly broken since the electric field is concentrated on a circular region of around r= 350 nm for a cyclotron radius of the order of 60 nm for a free space wavelength of 1 mm (300 GHz). In our case we can reveal many body effects on the cyclotron mass because we break the translational invariance of the system with the extreme photonic confinement provided by the cavity, observing an increase of the m*/m0 of 6% with respect to the uncoupled cyclotron mass. Experiments conduced on the same 2DEG with a standard split-ring resonator at the same frequency do not show any effective mass shift.
Only recently, a novel type of intensity autocorrelator in the time-domain has been reported for the Terahertz frequency range. The technique is based on fast electro-optic sampling in a double beam configuration and its temporal resolution is ultra-fast, as short as only few hundreds of femtoseconds. In particular, the self-referencing character of the technique is suitable for any type of source, including free-running sources. These unique characteristics enable therefore the investigation of the output profile of Terhertz Quantum Cascade Laser based Frequency Combs, with typical roundtrip times of few tens of picoseconds. The output dynamics of such devices have been investigated theoretically by Maxwell-Bloch equations and experimentally using Shifted Wave Interference Fourier Transform Spectroscopy. In this work, we present the results of the direct measurement of intensity autocorrelations of a Terahertz comb around 2.5 THz, when operated in the comb and high-noise regime, with radio-frequency beatnotes of 800 Hz and few MHz, respectively. We find the laser to be both amplitude- and frequency-modulated in both regimes, with a modulation ratio of the intensity of roughly 90 percent.The technique might come to use in future for the measurement of free-running pulses at Terahertz frequencies with high temporal resolution.
Terahertz quantum cascade lasers (THz QCL) are a very promising source for efficient frequency comb generation at terahertz frequencies. They do not only provide an output power of the order of milliwatts but are also covering a large spectral bandwidth. Octave spanning devices have recently been reported by our group. They provide a very low intrinsic dispersion due to the flat gain curve and the flat losses of the resonator. This allows frequency comb operation up to more than 600 GHz bandwidth with standard broadband metal-metal waveguide Fabry-Pérot QCLs. Frequency combs at terahertz frequencies are especially interesting for spectroscopic applications employing the powerful dual-comb setup. Such a setup requires a fast detector which is difficult to get with a sufficient sensitivity at terahertz frequencies. We present here an alternative approach, which does not need a fast detector but rather uses one of the two THz QCL frequency combs as an ultrafast multiheterodyne detector integrating local oscillator (LO) and detector in one single device. Two laser ridges are fabricated on the same chip at a distance of 500 um. Part of the light from the sample laser is coupled into the LO laser via the metallic ground plane. The downconverted multiheterodyne beatnote can be measured through the laser power supply line with a bias Tee. The obtained dual-comb covers a bandwidth of 630 GHz with a central frequency of 2.5 THz. The frequency comb spacing was analysed using frequency counting techniques revealing an accuracy down to _frep=fcarrier 10^(-12) at the carrier frequency of 2.5 THz.
Recent work has been showing the possibility of generating frequency combs at terahertz frequencies using terahertz quantum cascade lasers. The main efforts so far were on getting the laser to work in a stable comb operation over an as broad as possible spectral bandwidth. Another issue is the scattered farfield of such combs due to their subwavelength facets of the used metal-metal waveguide. In contrast to single mode lasers the monolithic approaches of distributed feedback lasers or photonic crystals cannot be used. We present here a monolithic broadband extractor compatible with frequency comb operation based on the concept of an end-fire antenna. The antenna can be fabricated using standard fabrication techniques. It has been designed to support a bandwidth of up to 600 GHz at a central frequency of 2.5 THz. The fabricated devices show single lobed farfields with only minor asymmetries, increased output power along an increased dynamical range of frequency comb operation. A side-absorber schematics using a thin film of Nickel has been used to suppress any higher-order lateral modes in the laser. The reported frequency combs with monolithic extractors are ideal candidates for spectroscopic applications at terahertz frequencies using a self-detected dual-comb spectroscopy setup due to the increased dynamical range along with the improved farfield leading to more output power of the frequency combs.
Strong light-matter coupling lies at the heart of quantum optics and recently has been successfully explored also in the GHz and THz range. New, intriguing quantum optical phenomena have been predicted in the ultrastrong coupling regime, when the coupling strength Omega becomes comparable to the unperturbed frequency of the system omega_c. We recently proposed a new experimental platform where the physics of the ultrastrong coupling can be investigated at GHz-THz frequencies. We couple the inter-Landau level transition of an high-mobility 2 dimensional electron gas (2DEG) to the subwavelength photonic mode of an LC meta-atom. Our system benefits from the collective enhancement of the light-matter coupling which comes from the scaling of the coupling constant Omega with the square root of the number of electrons in the last Landau level. In our previous experiments and in literature this number varies from 10000-1000 electrons per resonator.
Here we present ultrastrong coupling between a high-mobility 2DEG (mu=2.3X 10^6 cm^2/Vs) and an extremely subwavelength hybrid-LC resonator ensemble (11 resonators) with an highly reduced effective mode volume V_eff=4 x 10^-19 m^3=4 x 10^(-10) lambda^3 at a frequency of 300 GHz. The number of optically active electrons is given by the flux quantum multiplied by the effective resonator area and is proportional to the magnetic field. At the anticrossing field of B=0.73 T we measure less than 80 electrons ultrastrongly coupled to the resonator with a normalized coupling ratio Omega/omega_c=0.35. This experiment paves the way towards the study of ultrastrong coupling physics in the regime of quantum non-linearities.
The Ultrastrong light-matter coupling regime is attained when the coupling rate Ω is comparable or larger than the resonance frequency of the light ωLC and electronic transition frequency of the matter part ωc. This regime has attained much attention due to the peculiar properties of the coupled state. Here, we present a physical system showing a high normalized coupling rate not far from unity. The LC-mode of a THz-metasurface unitcell at ωLC = 2π x 0:25 THz is coupled to the collective cyclotron transition of 4 two dimensional electron gases (2DEGs) resulting in a normalized coupling rate of Ω/ωc = 0:9, Ω being the vacuum Rabi frequency. The high coupling rate is enabled by the small mode volume provided by the LC-mode of the split ring resonator geometry which was employed. Equally important is the large collective transition dipole element of the cyclotron transition.
The quantum nature of photonic systems is reflected in the photon statistics of the light they emit. Therefore, the development of quantum optics tools with single photon sensitivity and excellent temporal resolution is paramount to the development of exotic sources, and is particularly challenging in the THz range where photon energies approach kbT at T=300 K. Here, we report on the first room temperature measurement of field g1(τ) and intensity correlations g2(τ) in the THz range with sub-cycle temporal resolution (146 fs) over the bandwidth 0.3-3 THz, based on electro-optic sampling. With this system, we are able to measure the photon statistics at threshold of a THz Quantum Cascade Laser.
We present results on the continuous tuning of the interaction strength between the cyclotron transition in a two dimensional electron gas (2DEG) and the LC mode of THz split ring resonators (SRRs) with ωLC = 2π× 0.52 THz. The interaction strength is continuously modified in a gated sample by changing the electron density in the 2DEG. We observe an asymmetric emergence of the polaritonic branches at the onset of the ultrastrong coupling. Both, the line widths and transmission amplitudes are modulated when moving from the weak coupling regime to the ultrastrong coupling regime reaching a normalized coupling rate Ω ω= 0.21.
Buried heterostructure (BH) lasers are routinely fabricated for telecom applications. Development of quantum cascade
lasers (QCL) for sensing applications has largely benefited from the technological achievements established for telecom
lasers. However, new demands are to be met with when fabricating BH-QCLs. For example, hetero-cascade and multistack
QCLs, with several different active regions stacked on top of each other, are used to obtain a broad composite gain
or increased peak output power. Such structures have thick etch ridges which puts severe demand in carrying out
regrowth of semi-insulating layer around very deeply etched (< 10 μm) ridges in short time to realize BH-QCL. For
comparison, telecom laser ridges are normally only <5 μm deep. We demonstrate here that hydride vapour phase epitaxy
(HVPE) is capable of meeting this new demand adequately through the fabrication of BH-QCLs in less than 45 minutes
for burying ridges etched down to 10-15 μm deep. This has to be compared with the normally used regrowth time of
several hours, e.g., in a metal organic vapour phase epitaxy (MOVPE) reactor. This includes also micro-stripe lasers
resembling grating-like ridges for enhanced thermal dissipation in the lateral direction. In addition, we also demonstrate
HVPE capability to realize buried heterostructure photonic crystal QCLs for the first time. These buried lasers offer
flexibility in collecting light from the surface and relatively facile device characterization feasibility of QCLs in general;
but the more important benefits of such lasers are enhanced light matter interaction leading to ultra-high cavity Q-factors,
tight optical confinement, possibility to control the emitted mode pattern and beam shape and substantial reduction in
laser threshold.
We present a THZ quantum cascade laser operating in continuous wave with an emission covering more than one octave
in frequency, and displaying homogeneous power distribution among the lasing modes. The gain medium is based on a
heterogeneous quantum cascade structure operating in the THz range. Laser emission takes place from 1.64 THz to 3.35
THz with optical powers of 3 mW and 84 modes above threshold. For narrow waveguides a collapse of the free-running
beatnote to linewidths of 980 Hz, limited by jitter, indicate frequency comb operation on a spectral bandwidth as wide as
624 GHz.
THz metamaterials have been shown to be a promising candidate for CQED experiments, i.e. Ultrastrong coupling has been demonstrated.1 Modes of split ring resonators (SRRs) have been coupled via the AC-electric field to inter landau level transitions in two dimensional electron gases (2DEGs). SRR typically exhibit two distinct electromagnetic modes.2 One mode consists of an electric field confined in the slit of the resonator, while current in the resonator ring stores the magnetic field. This mode is often called LC-mode, in analogy to the lumped circuit representation, and has a nearly Lorentzian line shape. The other mode is a dipole mode localized physically on the resonator edges. The line shape of this resonance is more sensitive to the actual resonator geometry. We studied extensively the influence of the resonator geometry on the coupling strength. The resonances studied span the frequency range from 250 GHz to 1.3 THz for the LC type and 0.8 THz to 2.5 THz for the dipole resonance. The number and position of gaps in the SRR has been varied, putting them both in series as well as in parallel. These modifications influence the mode volume of the LC-mode which changes the coupling strength in two opposite ways. The electric field magnitude increases and thus the coupling. However, the number of participating electrons in the 2DEG is reduced which reduces the √N coupling enhancement. The largest coupling we measured so far is =Ω/ω0 = 0.58.1
The interaction of large-area single-layer CVD-graphene with a metasurface constituted by THz split-ring resonators was studied via THz Time-Domain Spectroscopy in the frequency range 250 GHz÷2.75 THz. Transmission measurements showed that the presence of the graphene shifts the resonances of the THz-metasurface towards lower energies and increases the transmittance, mainly at resonance. A comparison between two possible configuration is here presented revealing a much stronger interaction for the case of split-ring resonators evaporated directly onto the CVD-graphene layer with respect to the opposite configuration. From the recent literature the presented system is a good candidate for THz modulators with possible use also in cavity-QED experiments.
A portable, stand-alone, real-time THz imaging system for high resolution is presented. The total weight of the apparatus
is less than 15 kg and its physical dimension is of approximately (65 cm)3. A quantum cascade laser emitting at 3.4 THz
based on a third-order distributed feedback cavity is used as radiation source for transmission and reflection imaging
modes. We report real-time THz imaging with a bolometric camera operating at 15 Hz producing movies with a
resolution of 120 x 160 pixels. With the help of a Stirling motor cryocooler the laser operates in continuous-wave at 40 K
with more than 1 mW output power and less than 300 mW of power consumption. We were able to image small objects
employing refractive elements that we manufactured in high density polyethylene achieving a resolution of twice the
wavelength.
We present a development of a broad gain heterogeneous quantum cascade laser for terahertz frequencies. By
placing appropriate different active-regions based on a four-quantum-well design into a double-metal
waveguide we obtained laser emitting gapless over a bandwidth of 1 THz, between 3.2 to 2.2 THz. This means
that this single-device source covers an emission range of nearly 40 % around the center frequency. In pulsed
mode operation, our devices show threshold current density as low as 285 A/cm2 and they operate up to 125 K.
We also report on continuous wave measurements.
Room temperature, continuous wave (CW) operation of distributed feedback (DFB) quantum cascade lasers with widely
spaced operation frequencies is reported. The relatively small temperature tuning range of a single device, smaller or
equal to approximately 1 % of the wavelength, usually limits their efficiency for spectroscopic investigations. By using a
bound-to-continuum active region to create a broad gain spectrum and monolithic integration of different DFB gratings,
we achieved high-performance devices with single-mode emission between 7.7 and 8.3 &mgr;m at a temperature of +30 °C.
This frequency span corresponds to 8 % of the center frequency. The maximum CW operation temperature achieved was
63 °C at the gain center and as much as 35 °C and 45 °C, respectively, at the limits of the explored wavelength range.
External cavity quantum-cascade lasers (QCLs) emitting in the long-wave infrared (LWIR) around 10 μm and in the mid-wave infrared (MWIR) around 5.2 μm have been realized. The coupling facet of the QC chips was anti-reflection coated and the optical feedback was provided by a diffraction grating arranged in a Littrow configuration. The active regions of the gain elements were based on bound-to-continuum designs having broad gain curves (Approx = to 300 cm-1 full width at half maximum). The LWIR laser was operated in pulsed mode on a thermoelectric (TE) cooler. It could be tuned over a frequency range of 150 cm-1 from 9.1 to 10.55 μm with a peak power ⩾ 30 mW. The MWIR laser was operated in continuous-wave (CW) mode on a TE cooler. It could be tuned over more than 170 cm-1 from 4.95 to 5.4 μm and was single-mode over more than 140 cm-1. Its output power was in excess of 5 mW over 130 cm-1. This broad tunability (10 times more than that of distributed-feedback QCLs) enables new applications of QCLs in high-resolution infrared spectroscopy.
The quantum cascade laser is an unipolar semiconductor laser source emitting in the mid-infrared range between 3.5 and 25 μm. During the past ten years after their invention, this technology has
reached the level of maturity required for commercialization, and QC
lasers have thus become very attractive for a large number of
applications, including gas sensing, pollution detection, atmospheric chemistry, detection of compounds, non-invasive medical
diagnostics, free-space optical data transmission or even LIDAR. Most common requirements are single-mode operation on thermoelectric cooler, high power and/or continuous-wave. Nowadays several high-power single-mode QC lasers are available at Alpes Lasers in the range from 4.3 to 16.5 μm, with a side-mode suppression ratio larger than 30 dB. We present here a specific high-average power Fabry-Perot quantum cascade laser and a distributed-feedback quantum cascade laser operating near 8 μm.
Continuous wave (CW) operation of quantum cascade lasers is reported up to a temperature of 312 K. The junction down mounted devices were designed as buried heterostructure lasers with high-reflection coatings on both facets. This resulted in CW operation at an emission wavelength of 9.1μm with an optical power ranging from 17 mW at 293 K to 3 mW at 312 K. A distributed feedback type device was fabricated and tested as well. It showed CW singlemode operation up to 260 K. These results demonstrate the potential of quantum cascade lasers as CW mid-infrared light sources for high-resolution spectroscopy and free space telecommunication systems.
There is an increasing need in many chemical sensing applications ranging from industrial process control to environmental science and medical diagnostics for fast, sensitive, and selective gas detection based on laser spectroscopy. The recent availability of novel pulsed and cw quantum cascade distributed feedback (QC-DFB) lasers as mid-infrared spectroscopic sources address this need. A number of spectroscopic techniques have been demonstrated. For example, the authors have employed QC-DFB lasers for the monitoring and quantification of several trace gases and isotopic species in ambient air at ppmv and ppbv levels by means of direct absorption, wavelength modulation, cavity enhanced and cavity ringdown spectroscopy. In this work, pulsed thermoelectrically cooled QC-DFB lasers operating at ~15.6 μm were characterized for spectroscopic gas sensing applications. A new method for wavelength scanning based on the repetition rate modulation was developed. A non-wavelength-selective pyroelectric detector was incorporated in the gas sensor giving an advantage of room-temperature operation and low cost. Absorption lines of CO2 and H2O were observed in ambient air providing information about the concentration of these species.
Continuous wave operation of quantum cascade lasers is reported up to a temperature of 311 K. Fabry-Perot and DFB devices were fabricated as buried heterostructure lasers with high-reflection facet coatings. Junction-down mounted FP-lasers emitted up to 17 mW and 3 mW of optical power per facet under continuous wave operation at 292 K and 311 K, respectively. The DFB-devices could be operated up to 253 K on a thermoelectric cooler at an emission frequency of 1114 cm-1 with a side mode suppression rate better than 30 dB. Grating-coupled external cavity quantum cascade lasers based on a bound-to-continuum active region featuring a broad gain spectrum demonstrated frequency tuning of ~10% from 1036 cm-1 to 1142 cm-1 with average output power between 0.15 and 0.85 mW under pulsed operation at room temperature.
We report the application of quantum cascade (QC) lasers to measurement of atmospheric trace gases in both closed path and open path configurations. The QC laser, a recently available commercial device, is Peltier cooled and pulsed, with emission near 965 cm-1. We use direct absorption with a rapid sweep integration and spectral fits to derive absolute concentrations from tabulated line parameters without calibration. In the closed path configuration, with absorption in a long pathlength multipass cell (210 m, 50 Torr), we examined laser line widths and sensitivity limits. We measured ammonia with a precision of 0.05 nmole/mole (0.05 ppbv) RMS at 1 Hz, limited by detector noise. The laser linewith was 0.007 cm-1 HWHM, based on measurements of ethylene absorption line shapes with a current pulse width of ~14 ns. In the open path configuration, we measured ammonia in the exhaust of automobiles driving through a probe beam. Atmospheric pressure line broadening and turbulence limit the sensitivity, giving a column density noise level at 20 Hz of 1.4 ppm-m. We observed ammonia column densities up to 40 ppm-m in the exhaust plumes. In future systems we will include a CO2 channel, allowing normalization to fuel use rate.
Physics and applications of recent quantum cascade laser active region designs are discussed. Specifically, the use of bound-to-continuum and two-phonon resonance active regions for high temperature, high duty cycle operation is reviewed. Threshold current densities as low as 3kA/cm2 at T=300K, operation with a peak power of 90mW at 425K, and single mode, high power operation up to temperatures above 330K at (lambda) approximately equals 16micrometers are demonstrated. QC lasers able to operate at high duty cycles (50%) on a Peltier cooler were used in a demonstration of a 300MHz free space optical link between two buildings separated by 350m.
Recent developments in the research in quantum cascade laser technology in university of Neuchatel are reviewed. We report operation of quantum cascade lasers at high temperature (up to 10 mW peak power at 90 degrees Celsius) and of distributed feedback quantum cascade lasers with single-mode surface emission. New results in the investigation of mid-IR emission from InGaN/GaN LED's are also discussed.
Quantum cascade lasers are coherent light sources in the mid-IR spectral region. They are based on resonant tunneling and optical transitions between discrete energy levels in the conduction band arising form size quantization in semiconductor heterostructures. QCLs have been demonstrated on GaInAs/AlInAs/InP and GaAs/AlGaAs outperforming existing semiconductor laser technologies in the mid-IR spectral range. The present paper reports the realization of a QCL based on GaAs/AlGaAs material designed with an emission wavelength of 9.3 micrometers . Specific properties inherent to this material system and their influence on laser operation are discussed in detail. The paper concludes with the presentation of a new waveguide concept, which offers considerable performance improvements.
The general status of the intersubband quantum cascade laser technology is reviewed, highlighting the major advances. Results are then reported on the electrical tuning of a QC laser close to room temperature and on a new QC laser based on a N-I-P-I superlattice.
We report on the fabrication and characterization of buried heterostructure quantum cascade (BH-QC) lasers. The buried heterostructure is formed by regrowth of InP lateral on the side walls and on top of the InAlAs/InGaAs laser structure by molecular beam epitaxy (MBE) after in situ surface cleaning. Thermal Cl2 etching is applied to the etched laser structure to remove the native oxides of the ternaries prior to regrowth of InP. Buried heterostructure QC lasers demonstrated excellent performances with lower threshold current densities (as low as 4.5kA/cm2 at T equals 300K) and higher slope efficiencies that we attribute to lower waveguide losses and a better heat dissipation.
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