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Ultra-Short Pulse laser Resonators IN the Terahertz

Periodic Reporting for period 2 - SPRINT (Ultra-Short Pulse laser Resonators IN the Terahertz)

Reporting period: 2018-03-01 to 2019-08-31

"Ultra-short light pulses with large instantaneous intensities can probe light-matter interaction phenomena, capture snapshots of molecular dynamics and drive high-speed communications. In a semiconductor laser, mode-locking is the primary way to generate ultrafast signals. Despite the intriguing perspectives, operation at Terahertz (THz) frequencies is facing fundamental limitations: engineering ""ultrafast"" THz semiconductor lasers from scratch or finding an integrated technology to shorten THz light pulses are currently two demanding routes.

SPRINT aims to combine the potential offered by the band-gap and lithographic engineering of THz frequency QCLs with the wealth of unique physical properties of graphene (or polaritonic) components to deploy new concepts for pulse generation and sensing to drive quantum cascade lasers in the ""ultrafast"" regime. By devising quasi-crystal and random electrically pumped THz semiconductor lasers, I plan to explore the technological and scientific routes toward a completely novel generation of passively mode-locked, high-power THz QCLs operating on a controllable frequency bandwidth. To address the issue, I plan to innovatively exploit the direct integration of intracavity graphene saturable absorbers, and/or the design of “cavity-coupled” graphene metamaterial saturable mirrors or polaritonic saturable filters. A new detection approach for the measurement of such “ultrafast” high-intense light pulses and for the investigation of the fascinating intracavity dynamics will be also developed: I will exploit the nonlinearity of quantum dot in a nanowire - based transistors, engineered to detect, on ultra-short time scales, the stable THz ultrafast comb generated in the QCL resonator cavity.

Society, economic and technological impact: SPRINT is a cutting-edge multidisciplinary project built on a collaborative research across a broad range of disciplines including nano- and optoelectronics, photonics, material science, quantum engineering, frequency metrology and solid-state physics. The targeted goal is to provide groundbreaking technological steps toward the development of a new ubiquitous ultrafast technology in the underexploited THz-frequency range, aiming to unprecedented compactness, sensitivity and resolution for spectroscopic, imaging, metrological and ICT applications. SPRINT will open up horizons and research opportunities on longer-term topics: taking snapshots of ultrafast dynamics; real-time pulsed imaging and time of flight tomography; time-resolved THz spectroscopy of gases, complex molecules and cold samples; coherent control of quantum systems; quantum optics, where high-power pulses can drive molecular samples out of equilibrium; metrology, where laser excitation can match the energy levels splitting of molecules, and its pulsed nature can down-convert the spectrum to the RF domain; ultra high-speed communications where THz frequency carriers will become increasingly more important in the quest for higher bandwidth data communications. Furthermore, ultra-short pulses and high-power THz micro-lasers combined with highspeed electronic detectors can become a groundbreaking and compact alternative to bulky THz time-domain systems promising extraordinary impacts on the market for biomedical imaging, security and process control."
Research activities developed during the 1st and 2nd reporting periods.

Workpackage 1.

- Task 1.1:

As a first step, we engineered the GaAs/AlGaAs THz QCL active media. The first objective was the design a long-lived upper laser state, exploiting a reduced wavefunction-overlap between the two radiative
levels. To address this issue, spatially diagonal designs and injection strategies, targeting a reduced electron temperature in the upper laser state, were implemented. Consequently, the phonon-assisted gain relaxation will be strongly suppressed and the gain relaxation lifetime increased. The second objective was the design of a wide (beyond 1 THz) gain bandwidth, providing multi-frequency emission that provides inhomogeneous broadening of the gain spectrum. To address this issue, we overlapped the spectral gain of several different subsections with a wider spectral separation. A broad spectral gain is expected to shorten the pulse duration below picoseconds, leading to higher peak optical power.
Typically, to address applications needs, continuous-wave (CW) operation, low-divergent beam profiles and fine spectral control of the emitted radiation, are required. This, however, is very difficult to achieve in practice. Lithographic patterning has been extensively used to this purpose (via distributed feedback (DFB), photonic crystals or microcavities), to optimize either the beam divergence or the emission frequency, or, both of them simultaneously, in third-order DFBs, via a demanding fabrication procedure that precisely constrains the mode index to 3. In this first reporting period we employed the designed broadband THz QCL active regions to demonstrate wire DFB THz QCLs, in which feedback is provided by a sinusoidal corrugation of the cavity, defining the frequency, while light extraction is ensured by an array of surface holes. This new architecture, extendable to a broad range of far-infrared frequencies within the engineered gain bandwidth, has led to the achievement of low-divergent beams (10°), single-mode emission, high slope efficiencies (250 mW/A), and stable CW operation.

We have furthermore designed and demonstrated a broadband, heterogeneous terahertz frequency quantum cascade laser by exploiting an active region design based on longitudinal optical-phonon-assisted interminiband transitions.
This allowed obtaining continuous wave laser emission with a threshold current density of ∼120 A/cm2, a record dynamic range of ∼3.1 and an emission spectrum spanning from 2.4 to 3.4 THz at 15 K.
This also allowed to demonstrate record performance frequency combs at THz frequencies during the second reporting period

We indeed demonstrate that the devised broadband lasers operate as THz optical frequency comb synthesizers in continuous-wave, with a maximum optical output power of 4 mW (0.73µW in the comb regime). Measurement of the intermode beatnote map reveals a clear dispersion-compensated frequency comb regime extending over a continuous 106 mA current range (current density dynamic range of 1.24) significantly larger than the state of the art reported under similar geometries, with a corresponding emission bandwidth of ≈ 1.05 THz and a stable and narrow (4.15 KHz) beatnote detected with a signal-to-noise ratio of 34 dB. Analysis of the electrical and thermal beatnote tuning reveals a current-tuning coefficient ranging between 5 MHz/mA and 2.1 MHz/mA and a temperature-tuning coefficient of –4 MHz/K. The ability to tune the THz QCL combs over their full spectral range by temperature and current paves the way for their use as a powerful spectroscopy tool that can provide broad frequency coverage combined with high precision spectral accuracy.


- Task 1.2: We have developed a full 3D model, aiming to simulate a set of QCL resonators having defined architectures, set by the choice of the aperiodic crystal patterning. Main objective was defining the quasicrystal tiles and the computer-generated random patter
During this first and second reporting period we had a plethora of relevant progresses beyond the state of the art:

- Continuous-wave highly-efficient low-divergence terahertz wire lasers. Terahertz (THz) quantum cascade lasers (QCLs) have undergone rapid development since their demonstration, showing high power, broad-tunability, quantum-limited linewidth, and ultra-broadband gain. Typically, to address applications needs, continuous-wave (CW) operation, low-divergent beam profiles and fine spectral control of the emitted radiation, are required. This, however, is very difficult to achieve in practice. Lithographic patterning has been extensively used to this purpose (via distributed feedback (DFB), photonic crystals or microcavities), to optimize either the beam divergence or the emission frequency, or, both of them simultaneously, in third-order DFBs, via a demanding fabrication procedure that precisely constrains the mode index to 3. We have demonstrated wire DFB THz QCLs, in which feedback is provided by a sinusoidal corrugation of the cavity, defining the frequency, while light extraction is ensured by an array of surface holes. This new architecture, extendable to a broad range of far-infrared frequencies, has led to the achievement of low-divergent beams (10°), single-mode emission, high slope efficiencies (250 mW/A), and stable CW operation.

- The first frequency tunable continuous wave random THz lasers . Random lasing has long been extensively studied theoretically and experimentally reported in a number of different systems, such as optically pumped suspended microparticles in laser dye and fine powders. Quantum cascade lasers (QCLs) represent a promising platform for the integration of aperiodic photonic patterns with the aim of controlling the intra-cavity propagation of light and its extraction into the free space. Here, we conceive and devise random THz QCLs, exploiting a broadband active material and a double-metal waveguide, operating for the first time in continuous wave (CW) with remarkably high optical powers and a rich sequence of optical modes distributed over a 500 GHz bandwidth.

- The first one dimensional quasi crystal THz laser providing single mode or multimode emission over a 530 GHz bandwidth, with maximum peak optical power of 240 mW (190 mW) in multimode (single-mode) lasers with record slope efficiencies up to ≈570 mW/A at 78 K and ≈700 mW/A at 20 K, wall-plug efficiencies of η ≈ 1% and low divergent emission.

- THz QCL combs. We demonstrated broadband THz QCLs exploiting a heterogeneous active region scheme and have a current density dynamic range of 3.2 significantly larger than the state of the art, over a 1.3 THz bandwidth. These devices operate as THz optical frequency comb synthesizers in continuous-wave, with a maximum optical output power of 4 mW). Measurement of the intermode beatnote map reveals a clear dispersion-compensated frequency comb regime extending over a continuous 106 mA current range (current density dynamic range of 1.24) significantly larger than the state of the art reported under similar geometries, with a corresponding emission bandwidth of ≈ 1.05 THz and a stable and narrow (4.15 KHz) beatnote detected with a signal-to-noise ratio of 34 dB.

- Tunable, lithographically independent, control of the free-running coherence properties of THz QCL FCs. This is achieved by integrating an on-chip tightly coupled mirror with the QCL cavity, providing an external cavity and hence a tunable Gires Tournois
interferometer (GTI). By finely-adjusting the gap between the GTI and the back-facet of an ultra-broadband, high dynamic range QCL, we attain wide dispersion compensation regions, where stable and narrow (~3 kHz linewidth) single beatnotes
extend over an operation range that is significantly larger than that of dispersion dominated bare laser cavity counterparts. Significant reduction of the phase noise is registered over the whole QCL spectral bandwidth (1