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
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 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.
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.
New experiments with THz QCLs reveal the harmonic frequency comb regime, in which isolated modes are separated by several free spectral ranges and the optical spectrum does not have a reflection symmetry. Here we study the mechanism of the harmonic frequency combs. Our calculations show that the harmonic comb is favored if the QCL contains two lower laser states, with different but comparable strength of the optical transitions. The asymmetry between the two optical transitions can be linked to a linewidth enhancement factor. Our work can be extended to mid-IR QCLs with an asymmetric gain spectrum.
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.
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
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