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 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
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.
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 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°).
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 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.
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.
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.
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 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.
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.
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