Periodic Reporting for period 4 - SPRINT (Ultra-Short Pulse laser Resonators IN the Terahertz)
Berichtszeitraum: 2021-03-01 bis 2022-08-31
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 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.
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.
Workpackage 1.
1. We demonstrate quasi-crystal distributed feedback lasers with 240 mW peak optical power, and the highest slope efficiency (570 mW/A at 78K, 700 mW/A at 20K) reported to date in an electrically pumped multimode, surface emitting, disordered THz laser.
2. We demonstrated the first THz random lasers operating in continuous-wave
3. We develop high power, single mode, low divergence THz wire lasers.
Workpackage 2.
- Task 2.1 and Task 2.3 : We fabricated THz saturable absorbers by transfer coating and inkjet printing single and few-layer graphene films prepared by liquid phase exfoliation of graphite. Open-aperture z-scan measurements with a 3.5 THz quantum cascade laser show a transparency modulation ∼80%, almost one order of magnitude larger than that reported to date at THz frequencies.
- Task 2.2: we demonstrate THz saturable absorption in multilayer graphene films, grown via CVD on Nickel.
- Task 2.4: We engineer miniaturized THz FCSs, comprising a heterogeneous THz QCL, integrated with a tightly coupled, on-chip, solution-processed, graphene saturable-absorber reflector that preserves phase-coherence between lasing modes, even when four-wave mixing no longer provides dispersion compensation. This enables a high-power (8 mW) FC with over 90 optical modes, through 55% of the laser operational range.
- Task 2.4: We demonstrate mode-locking in surface-emitting electrically-pumped THz random QCLs.
- Task 2.5: We develop electrically switchable graphene THz modulators with a tunable‐by‐design optical bandwidth. Electrostatic gating is achieved by a metal grating used as a gate electrode, with an HfO2/AlOx top gate dielectric. This is patterned on a polyimide layer, which acts as a quarter wave resonance cavity, coupled with an Au reflector underneath.
- Task 2.6: we demonstrate THz saturable absorber mirrors based on intersubband-polaritons.
Workpackage 3.
- Task 3.1: we demonstrate uncooled THz photodetectors (PDs) combining the low (∼2000 kB μm–2) electronic specific heat of high mobility hexagonal boron nitride-encapsulated graphene, with asymmetric field enhancement produced by a bow-tie antenna, resonating at 3 THz. High sensitivity (NEP ≤ 160 pW Hz–1/2), fast response time (hundreds ps response time), and a 4 orders of magnitude dynamic range is achieved making our devices the fastest, broad-band, low-noise, room-temperature THz PD, to date.
- Task 3.1: We develop THz detectors based on antenna-coupled field-effect transistor (FET) with an active channel of Se-doped black phosphorus. Room-temperature responsivity of 3 V W−1 is achieved , with NEPs of 7 nW Hz−1/2 at 3.4 THz.
- Task 3.2: we develop highly efficient nanowire THz detectors and by coupling a THz QCL to scattering-type scanning near-field optical microscopy (s-SNOM) and monitoring both electrical and optical readouts, we simultaneously measure transport and scattering properties. The spatially resolved electric response provides unambiguous signatures of photo-thermoelectric and bolometric currents in the nanowire.
- Task 3.3: we demonstrate a self-starting miniaturized ultra-short pulse THz laser, exploiting an original device architecture that includes the surface patterning of multilayer-graphene saturable absorbers distributed along the entire cavity of a double-metal semiconductor 2.30–3.55 THz wire laser. Self-starting pulsed emission with 4.0-ps-long pulses in a compact, all-electronic, all-passive and inexpensive configuration is demonstrated.
Workpackage 4.
- Metrology: By exploiting a metrological grade system comprising a THz frequency comb synthesizer, we measure, for the first time, the free-running emission linewidth the tuning characteristics, and the absolute center frequency of individual emission lines of different frequency generation THz QCLs with an uncertainty of 4 × 10−10.
Furthermore, we demonstrated full stabilization and control of the two key parameters - carrier offset frequency and frequency separation between optical modes - of a THz QCL-comb against the primary frequency standard.
- Spectroscopy: We develop a bow-tie resonant cavity for THz radiation, injected with a continuous-wave 2.55 THz QCL. The bow-tie cavity employs a wire-grid polarizer as input/output coupler and a pair of copper spherical mirrors coated with an unprotected 500 nm thick gold layer. The improvements with respect to previous setups have led to a measured finesse value F = 123, and a quality factor Q = 5.1105.
- Microscopy: we devised a THz s-SNOM system that provides both amplitude and phase contrast, and achieves nanoscale (60-70nm) in-plane spatial resolution. It features a QCL that simultaneously emits THz frequency light and senses the backscattered optical field through a voltage modulation induced inherently through the self-mixing technique and employed it to test THz polaritons in topological insulators.
- Ultrafast photonics: we designed a SiO2/black phosphorus/SiO2 heterostructure in which the surface phonon modes of the SiO2 layers hybridize with surface plasmon modes in black phosphorus that can be activated by photo-induced interband excitation within ∼50 fs and disappears within 5 ps, as the electron–hole pairs in black phosphorus recombine. The excellent switching contrast and switching speed, the coherence properties and the constant wavelength of this transient mode make it a promising candidate for ultrafast nanophotonic devices.
1. We demonstrate quasi-crystal distributed feedback lasers with 240 mW peak optical power, and the highest slope efficiency (570 mW/A at 78K, 700 mW/A at 20K) reported to date in an electrically pumped multimode, surface emitting, disordered THz laser.
2. We demonstrated the first THz random lasers operating in continuous-wave
3. We develop high power, single mode, low divergence THz wire lasers.
Workpackage 2.
- Task 2.1 and Task 2.3 : We fabricated THz saturable absorbers by transfer coating and inkjet printing single and few-layer graphene films prepared by liquid phase exfoliation of graphite. Open-aperture z-scan measurements with a 3.5 THz quantum cascade laser show a transparency modulation ∼80%, almost one order of magnitude larger than that reported to date at THz frequencies.
- Task 2.2: we demonstrate THz saturable absorption in multilayer graphene films, grown via CVD on Nickel.
- Task 2.4: We engineer miniaturized THz FCSs, comprising a heterogeneous THz QCL, integrated with a tightly coupled, on-chip, solution-processed, graphene saturable-absorber reflector that preserves phase-coherence between lasing modes, even when four-wave mixing no longer provides dispersion compensation. This enables a high-power (8 mW) FC with over 90 optical modes, through 55% of the laser operational range.
- Task 2.4: We demonstrate mode-locking in surface-emitting electrically-pumped THz random QCLs.
- Task 2.5: We develop electrically switchable graphene THz modulators with a tunable‐by‐design optical bandwidth. Electrostatic gating is achieved by a metal grating used as a gate electrode, with an HfO2/AlOx top gate dielectric. This is patterned on a polyimide layer, which acts as a quarter wave resonance cavity, coupled with an Au reflector underneath.
- Task 2.6: we demonstrate THz saturable absorber mirrors based on intersubband-polaritons.
Workpackage 3.
- Task 3.1: we demonstrate uncooled THz photodetectors (PDs) combining the low (∼2000 kB μm–2) electronic specific heat of high mobility hexagonal boron nitride-encapsulated graphene, with asymmetric field enhancement produced by a bow-tie antenna, resonating at 3 THz. High sensitivity (NEP ≤ 160 pW Hz–1/2), fast response time (hundreds ps response time), and a 4 orders of magnitude dynamic range is achieved making our devices the fastest, broad-band, low-noise, room-temperature THz PD, to date.
- Task 3.1: We develop THz detectors based on antenna-coupled field-effect transistor (FET) with an active channel of Se-doped black phosphorus. Room-temperature responsivity of 3 V W−1 is achieved , with NEPs of 7 nW Hz−1/2 at 3.4 THz.
- Task 3.2: we develop highly efficient nanowire THz detectors and by coupling a THz QCL to scattering-type scanning near-field optical microscopy (s-SNOM) and monitoring both electrical and optical readouts, we simultaneously measure transport and scattering properties. The spatially resolved electric response provides unambiguous signatures of photo-thermoelectric and bolometric currents in the nanowire.
- Task 3.3: we demonstrate a self-starting miniaturized ultra-short pulse THz laser, exploiting an original device architecture that includes the surface patterning of multilayer-graphene saturable absorbers distributed along the entire cavity of a double-metal semiconductor 2.30–3.55 THz wire laser. Self-starting pulsed emission with 4.0-ps-long pulses in a compact, all-electronic, all-passive and inexpensive configuration is demonstrated.
Workpackage 4.
- Metrology: By exploiting a metrological grade system comprising a THz frequency comb synthesizer, we measure, for the first time, the free-running emission linewidth the tuning characteristics, and the absolute center frequency of individual emission lines of different frequency generation THz QCLs with an uncertainty of 4 × 10−10.
Furthermore, we demonstrated full stabilization and control of the two key parameters - carrier offset frequency and frequency separation between optical modes - of a THz QCL-comb against the primary frequency standard.
- Spectroscopy: We develop a bow-tie resonant cavity for THz radiation, injected with a continuous-wave 2.55 THz QCL. The bow-tie cavity employs a wire-grid polarizer as input/output coupler and a pair of copper spherical mirrors coated with an unprotected 500 nm thick gold layer. The improvements with respect to previous setups have led to a measured finesse value F = 123, and a quality factor Q = 5.1105.
- Microscopy: we devised a THz s-SNOM system that provides both amplitude and phase contrast, and achieves nanoscale (60-70nm) in-plane spatial resolution. It features a QCL that simultaneously emits THz frequency light and senses the backscattered optical field through a voltage modulation induced inherently through the self-mixing technique and employed it to test THz polaritons in topological insulators.
- Ultrafast photonics: we designed a SiO2/black phosphorus/SiO2 heterostructure in which the surface phonon modes of the SiO2 layers hybridize with surface plasmon modes in black phosphorus that can be activated by photo-induced interband excitation within ∼50 fs and disappears within 5 ps, as the electron–hole pairs in black phosphorus recombine. The excellent switching contrast and switching speed, the coherence properties and the constant wavelength of this transient mode make it a promising candidate for ultrafast nanophotonic devices.
Expected results:
- Develop a new detection approach for the measurement of ultrafast high-power light pulses and for the investigation of the related unexplored intracavity dynamics. The core idea is to exploit the non-linearity of a quantum dot in a nanowire - based transistor
- Develop a new detection approach for the measurement of ultrafast high-power light pulses and for the investigation of the related unexplored intracavity dynamics. The core idea is to exploit the non-linearity of a quantum dot in a nanowire - based transistor