1. « Femtosecond Broadband Frequency Switch of Terahertz Three-Dimensional Meta-Atoms« ,
P. Goulain, A.D. Koulouklidis, J-M Manceau, C. Daskalaki, B. Paulillo, K. Maussang, S. Dhillon, J.R. Freeman, L. Li, E.H. Linfield,
S. Tzortzakis and R. Colombelli,
ACS Photonics 8, 1097-1102 (2021).
We present the experimental demonstration of a subpicosecond all-optical THz switch based on three-dimensional (3D) terahertz meta-atoms. Combining a special design of 3D meta-devices and the ultrafast dynamics of low temperature grown gallium arsenide, we can modulate the reflectance of the THz microcavities within 2.2 ps. The device enables a 280 GHz switch in resonance frequency within less than 200 fs. The switch back to the original resonance takes 800 fs. Experimental results show that the speed values are strongly convoluted by the THz probing field and, thus, the real switching times are even shorter, in the few 100 fs range.
2. « Fast amplitude modulation up to 1.5 GHz of mid-IR free-space beams at room-temperature« ,
S. Pirotta, N-L. Tran, A. Jollivet, G. Biasiol, P. Crozat, J.-M. Manceau, A. Bousseksou and R. Colombelli,
Nature Communications Volume 12, Article number: 799 (2021).
Applications relying on mid-infrared radiation (λ ~ 3-30 μm) have progressed at a very rapid pace in recent years, stimulated by scientific and technological breakthroughs like mid-infrared cameras and quantum cascade lasers. On the other side, standalone and broadband devices allowing control of the beam amplitude and/or phase at ultra-fast rates (GHz or more) are still missing. Here we show a free-space amplitude modulator for mid-infrared radiation (λ ~ 10 μm) that can operate at room temperature up to at least 1.5 GHz (−3dB cutoff at ~750 MHz). The device relies on a semiconductor heterostructure enclosed in a judiciously designed metal–metal optical resonator. At zero bias, it operates in the strong light-matter coupling regime up to 300 K. By applying an appropriate bias, the device transitions towards the weak-coupling regime. The large change in reflectance is exploited to modulate the intensity of a mid-infrared continuous-wave laser up to 1.5 GHz.
3. « Nanospectroscopy of a single patch antenna strongly coupled to a mid-infrared intersubband transition in a quantum well« ,
R. Gillibert, M. Malerba, D. Spirito, V. Giliberti, L. Li, A. G. Davies, E. H. Linfield, L. Baldassarre,R. Colombelli and M. Ortolani,
Applied Physics Letters 117, 101104 (2020).
4. « Realization of harmonic oscillator arrays with graded semiconductor quantum wells »,
C. Deimert, P. Goulain, J.-M. Manceau, W. Pasek, T. Yoon, A. Bousseksou, N.Y. Kim, R. Colombelli and Z.R. Wasilewski,
Phys. Rev. Lett. 125, 097403 (2020).
The harmonic oscillator is a foundational concept in both theoretical and experimental quantum mechanics. Here, we demonstrate harmonic oscillators in a semiconductor platform by faithfully implementing continuously graded alloy semiconductor quantum wells. Unlike current technology, this technique avoids interfaces that can hamper the system and allows for the production of multiwell stacks several micrometers thick. The experimentally measured system oscillations are at 3 THz for two structures containing 18 and 54 parabolic quantum wells. Absorption at room temperature is achieved: this is as expected from a parabolic potential and is unlike square quantum wells that require cryogenic operation. Linewidths below 11% of the central frequency are obtained up to 150 K, with a 5.6% linewidth obtained at 10 K. Furthermore, we show that the system correctly displays an absence of nonlinearity despite electron-electron interactions—analogous to the Kohn theorem. These high-quality structures already open up several new experimental vistas.
5. « Excitons bound by photon exchange »,
E. Cortese, L. Tran, J-M. Manceau, A. Bousseksou, I. Carusotto, G. Biasiol, R. Colombelli and S. De Liberato,
Nature Physics 17, 31-35 (2021).
In contrast to interband excitons in undoped quantum wells, doped quantum wells do not display sharp resonances due to excitonic bound states. In these systems the effective Coulomb interaction between electrons and holes typically only leads to a depolarization shift of the single-electron intersubband transitions. Non-perturbative light-matter interaction in solid-state devices has been investigated as a pathway to tune optoelectronic properties of materials. A recent theoretical work [Cortese et al., Optica 6, 354 (2019)] predicted that, when the doped quantum wells are embedded in a photonic cavity, emission-reabsorption processes of cavity photons can generate an effective attractive interaction which binds electrons and holes together, leading to the creation of an intraband bound exciton. Spectroscopically, this bound state manifests itself as a novel discrete resonance which appears below the ionisation threshold only when the coupling between light and matter is increased above a critical value. Here we report the first experimental observation of such a bound state using doped GaAs/AlGaAs quantum wells embedded in metal-metal resonators whose confinement is high enough to permit operation in strong coupling. Our result provides the first evidence of bound states of charged particles kept together not by Coulomb interaction, but by the exchange of transverse photons. Light-matter coupling can thus be used as a novel tool in quantum material engineering, tuning electronic properties of semiconductor heterostructures beyond those permitted by mere crystal structures, with direct applications to mid-infrared optoelectronics.
We report quantum well mid-infrared photo-detectors operating in the strong light-matter coupling regime. We claim that this is an ideal system to elucidate the elusive problem in intersubband polaritonics of the injection of electrons (single-particle fermionic states) into polaritonic modes (bosonic excitations). By reversing the perspective and focusing on the electron extraction, we have obtained experimental information on the transfer function between a polaritonic system and an electronic reservoir. In addition to its interest for fundamental science, this approach also opens promising avenues in view of adding previously unavailable functionalities to quantum well detectors and improving their performance.
We thank Simone De Liberato, Alexandre Delga, Jean-Jacques Greffet, and Francois Julien for useful discussions. We acknowledge financial support from the European Union FET-Open Grant MIR-BOSE (737017) and the European Research Council (IDEASERC) (“GEM”) (306661). This work was partly supported by the French RENATECH network and Provincia Autonoma di Trento.
We report on the systematic study of two main scattering mechanisms on intersubband transitions, namely ionized impurity scattering and interface roughness scattering. The former mechanism has been investigated as a function of the dopants position within a multiple GaAs/AlGaAs quantum well structure and compared to the transition of an undoped sample. The study on the latter scattering mechanism has been conducted using the growth interruption technique. We report an improvement of the intersubband (ISB) transition linewidth up to 11% by interrupting growth at GaAs-on-AlGaAs interfaces. As a result, the lifetime of intersubband polaritons could be improved up to 9%. This leads to a reduction of 17% of the theoretical threshold intensity for polaritonic coherent emission. This work brings a useful contribution towards the realization of polariton-based devices.
In cavity quantum electrodynamics, strong light–matter coupling is normally observed between a photon mode and a discrete optically active transition. In the present work we demonstrate that strong coupling can also be achieved using ionizing, intrinsically continuum, transitions. This leads to the appearance of novel discrete polaritonic resonances, corresponding to dressed bound exciton states, kept together by the exchange of virtual cavity photons. We apply our theory to the case of intersubband transitions in doped quantum wells, where Coulomb-bound excitons are absent. In considering quantum wells with a single bound electronic subband, in which all transitions involve states in the continuum, we find that the novel bound excitons predicted by our theory are observable within present-day, realistic parameters. Our work shows how strong light–matter coupling can be used as a novel gauge to tune both optical and electronic properties of semiconductor heterostructures beyond those permitted by mere crystal properties.
We developed a technique that enables replacement of a metallic waveguide cladding with a low-index (n≈1.4) material – CaF2 or BaF2. It is transparent from the mid-IR up to the visible range: elevated confinement is preserved while introducing an optical entryway through the substrate. Replacing the metallic backplane also allows double-side patterning of the active region. Using this approach, we demonstrate strong light-matter coupling between an intersubband transition (λ∼10 μm) and a dispersive resonator at 300 K and at 78 K. Finally, we evaluate this approach’s potential as a platform for waveguiding in the mid-IR spectral range, with numerical simulations that reveal losses in the 1-10 cm−1 range.
We demonstrate ultrabroadband electro-optic detection of multi-THz transients using mechanically exfoliated flakes of gallium selenide of a thickness of less than 10 µm, contacted to a diamond substrate by van-der-Waals bonding. While the low crystal thickness allows for extremely broadband phase matching, the excellent optical contact with the index-matched substrate suppresses multiple optical reflections. The high quality of our structure makes our scheme suitable for the undistorted and artifact-free observation of electromagnetic waveforms covering the entire THz spectral range up to the near-infrared regime without the need for correction for the electro-optic response function. With the current revolution of chemically inert quasi-two-dimensional layered materials, we anticipate that exfoliated van-der-Waals materials on index-matched substrates will open new flexible ways of ultrabroadband electro-optic detection at unprecedented frequencies.
We report experimental evidence of longitudinal optical (LO) phonon-intersubband polariton scattering processes under resonant injection of light. The scattering process is resonant with both the initial (upper polariton) and final (lower polariton) states and is induced by the interaction of confined electrons with longitudinal optical phonons. The system is optically pumped with a mid-IR laser tuned between 1094 cm-1 and 1134 cm-1
(lambda=9.14 µm and lambda=8.82 µm). The demonstration is provided for both GaAs/AlGaAs and InGaAs/AlInAs doped quantum well systems whose intersubband plasmon lies at circa 10 µm wavelength. In addition to elucidating the microscopic mechanism of the polariton-phonon scattering, that is found to differ substantially from the standard single particle electron-LO phonon scattering mechanism, this work constitutes the first step towards the hopefully forthcoming demonstration of an intersubband polariton laser.
We demonstrate that intersubband (ISB) polaritons are robust to inhomogeneous effects originating from the presence of multiple quantum wells (MQWs). In a series of samples that exhibit mid-infrared ISB absorption transitions with broadenings varying by a factor of 5 (from 4 to 20 meV), we observed polariton linewidths always lying in the 4 to 7 meV range only. We experimentally verified the dominantly inhomogeneous origin of the broadening of the ISB transition, and that the linewidth reduction effect of the polariton modes persists up to room-temperature. This immunity to inhomogeneous broadening is a direct consequence of the coupling of the large number of ISB oscillators to a single photonic mode. It is a precious tool to gauge the natural linewidth of the ISB plasmon that is otherwise masked in such MQWs system, and is also beneficial in view of perspective applications such as intersubband polariton lasers.
13. « Phase Memory Preserving Harmonics from Abruptly Autofocusing Beams »,
A. D. Koulouklidis, D. G. Papazoglou, V. Y. Fedorov and S. Tzortzakis,
Phys. Rev. Lett. 119, 223901 (2017).
We demonstrate both theoretically and experimentally that the harmonics from abruptly autofocusing ring-Airy beams present a surprising property: They preserve the phase distribution of the fundamental beam. Consequently, this “phase memory” imparts to the harmonics the abrupt autofocusing behavior, while, under certain conditions, their foci coincide in space with the one of the fundamental. Experiments agree well with our theoretical estimates and detailed numerical calculations. Our findings open the way for the use of such beams and their harmonics in strong field science.