Vertical cavity surface emitting terahertz laser

The Intermode Polariton Parametric Scattering Laser in a Strong Coupled Microcavity Via Two-Photon Absorption

Exciton-polaritons, hybrid quasiparticles from the strong coupling of excitons and cavity photons in semiconductor microcavities, offer a platform for exploring quantum coherence and nonlinear optical properties. The unique polariton parametric scattering (PPS) laser is of interest for its potential in quantum technologies and nonlinear devices. However, direct resonant excitation of polaritons in strong-coupling microcavities is challenging. This study proposes an innovative two-photon absorption (TPA) pump mechanism to address this. We observe TPA-driven PPS lasing in a strongly coupled microcavity at room temperature. High K-value exciton injections promote coherent stimulated emission of polariton scattering through intermode channels. Angle-resolved spectra confirm a TPA process, showing evolution from pump-state to signal-state. Hanbury Brown-Twiss measurement of second-order correlation g2(τ) of signal state indicates a phase transition from a classical thermal state to a quantum coherent state. Theoretical modeling provides insights into the physical mechanisms of PPS. Our work advances nonlinear phenomena exploration in strongly coupled light-matter systems, contributing to quantum polaritonics and nonlinear optics.

Transient cavity-cavity strong coupling at terahertz frequency on LiNbO3 chips

Terahertz (THz) microcavities have garnered considerable attention for their ability to localize and confine THz waves, allowing for strong coupling to remarkably enhance the light-matter interaction. These properties hold great promise for advancing THz science and technology, particularly for high-speed integrated THz chips where transient interaction between THz waves and matter is critical. However, experimental study of these transient time-domain processes requires high temporal and spatial resolution since these processes, such as THz strong coupling, occur in several picoseconds and microns. Thus, most literature studies rarely cover temporal and spatial processes at the same time. In this work, we thoroughly investigate the transient cavity-cavity strong-coupling phenomena at THz frequency and find a Rabi-like oscillation in the microcavities, manifested by direct observation of a periodic energy exchange process via a phase-contrast time-resolved imaging system. Our explanation, based on the Jaynes-Cummings model, provides theoretical insight into this transient strong-coupling process. This work provides an opportunity to deeply understand the transient strong-coupling process between THz microcavities, which sheds light on the potential of THz microcavities for high-speed THz sensor and THz chip design.

Enhanced Second-Order Nonlinearity for THz Generation by Resonant Interaction of Exciton-Polariton Rabi Oscillations with Optical Phonons

Semiconductor microcavities in the strong-coupling regime exhibit an energy scale in the terahertz (THz) frequency range, which is fixed by the Rabi splitting between the upper and lower exciton-polariton states. While this range can be tuned by several orders of magnitude using different excitonic media, the transition between both polaritonic states is dipole forbidden. In this work, we show that, in cadmium telluride microcavities, the Rabi-oscillation-driven THz radiation is actually active without the need for any change in the microcavity design. This feature results from the unique resonance condition which is achieved between the Rabi splitting and the phonon-polariton states and leads to a giant enhancement of the second-order nonlinearity.

Doubly Dressed Bosons: Exciton Polaritons in a Strong Terahertz Field

We demonstrate the existence of a novel quasiparticle, an exciton in a semiconductor doubly dressed with two photons of different wavelengths: a near infrared cavity photon and terahertz (THz) photon, with the THz coupling strength approaching the ultrastrong coupling regime. This quasiparticle is composed of three different bosons, being a mixture of a matter-light quasiparticle. Our observations are confirmed by a detailed theoretical analysis, treating quantum mechanically all three bosonic fields. The doubly dressed quasiparticles retain the bosonic nature of their constituents, but their internal quantum structure strongly depends on the intensity of the applied terahertz field.

Single and double bosonic stimulation of THz emission in polaritonic systems

The influence of the surrounding cavity on the efficiency of different types of polaritonic emitters of THz radiation has been analysed. It is demonstrated that THz lasing threshold in realistic structures cannot be achieved without a THz cavity, due to destruction of polaritons via excitonic Mott transition. Even modest values of cavity quality factor (not exceeding 50) provide significant quantum efficiency.

Bistability in bosonic terahertz lasers

We study theoretically terahertz (THz) lasing in a system consisting of a quantum well placed inside an optical microcavity and a THz cavity in the regime of two-photon excitation of 2p dark exciton states. The stability of the system with varying parameters of the microcavity under coherent pumping is analyzed. We show that the nonlinearity provided by two-photon absorption can give rise to bistability and hysteresis in the THz output. This offers an opportunity for the realization of ultrafast THz switches.

Two-photon injection of polaritons in semiconductor microstructures

We experimentally demonstrate that two-photon pumping of "dark" excitons in quantum wells embedded in semiconductor microcavities can result in exciton-polariton injection and photon lasing. In the case of a semiconductor micropillar pumped at half of the exciton frequency, we observe a clear threshold behavior, characteristic of the vertical cavity surface emitting laser transition. These results are interpreted in terms of stimulated emission of terahertz photons, which allows for conversion of "dark" excitons into exciton-polaritons.

Superradiant terahertz emission by dipolaritons

Dipolaritons are mixed light-matter quasiparticles formed in double quantum wells embedded in microcavities. Because of resonant coupling between direct and indirect excitons via electronic tunneling, dipolaritons possess large dipole moments. Resonant excitation of the cavity mode by a short pulse of light induces oscillations of the indirect exciton density with a characteristic frequency of Rabi flopping. This results in oscillations of a classical Hertz dipole array which generate supperradiant emission on a terahertz (THz) frequency. The resulting THz signal may be enhanced using the supplementary THz cavity in the weak coupling regime.

The convergence of longitudinal excitons onto the Γ5 transverse exciton in GaN and the thermal activation energy of longitudinal excitons

The crystal orientation dependence of GaN excitons was investigated via the photoluminescence (PL) technique. The PL emissions at a temperature of 10 K were obtained from two experimental configurations where the emission K vector (the propagation vector) was either parallel (K ∥ c) or perpendicular (K ∥ c) to the crystal c-axis. Longitudinal, transverse and donor-bound excitons were observed in the two configurations. However, the longitudinal excitons converged onto the transverse free exciton Γ5 in the K⊥c emission. This behavior was discussed in terms of electron screening due to the scattering of electrons moving perpendicular to charged dislocation lines. Additionally, the thermal activation energy of the longitudinal excitons was calculated from the temperature dependent PL measurements collected from the K ∥ c emission, and was found to be 5 to 6 times as high as the binding energy of the free excitons. This high energy was interpreted tentatively in view of the creation of polaritons in strong exciton-photon coupling regimes. These findings present fundamental concepts for applications such as vertical cavity surface-emitting lasers (VCSELs) and polariton lasers.

Nonlinear effects in multi-photon polaritonics

We consider theoretically nonlinear effects in a semiconductor quantum well embedded inside a photonic microcavity. Two-photon absorption by a 2p exciton state is considered and investigated; the matrix element of two-photon absorption is calculated. We compute the emission spectrum of the sample and demonstrate that under coherent pumping the nonlinearity of the two photon absorption process gives rise to bistability.

Proposal for a bosonic cascade laser

We propose a concept of a quantum cascade laser based on transitions of bosonic quasiparticles (excitons) in a parabolic potential trap in a semiconductor microcavity. This laser would emit terahertz radiation due to bosonic stimulation of excitonic transitions. The dynamics of a bosonic cascade is strongly different from the dynamics of a conventional fermionic cascade laser. We show that populations of excitonic ladders are parity dependent and quantized if the laser operates without an external terahertz cavity.