Single-quantum-dot nanolasers

Lasers of ultimately small size consist of an optical cavity as small as the wavelength of light and a single-photon emitter. Since advanced atom optics has realized a single-atom laser using a single, trapped gas atom,¹ realization of such a single-emitter laser using solid-state materials has been a hot topic in both physics and the field of device application. Combination of a single semiconductor quantum dot (QD)² with a semiconductor photonic-crystal (PhC) nanocavity is one of the most promising approaches to achieve this successfully. A PhC is an optical structure with a periodicity on the order of the wavelength of light (see Figure 1) that can be used to manipulate photon activity. For example, photons can be strongly confined by creating a defect in the PhC that functions as a very small optical cavity.³

Figure 1. (a) Scanning-electron micrograph of photonic-crystal nanocavity structure. GaAS: Gallium arsenide. QDs: Quantum dots. (b) Calculated electric-field intensity. (c) Atomic-force-microscope image of an operational sample.

In free space, interactions of photons with single QDs are not strong. However, the interaction rate can be drastically enhanced by confining photons to the scale of a cubic wavelength because of cavity quantum-electrodynamic effects. In such a system, the local electric-field intensity is much stronger than that of vacuum, which leads to efficient light-matter interactions. A PhC nanocavity that contains a single indium gallium arsenide (InGaAs) QD exhibits a photoluminescence (PL) spectrum—see Figure 2(a)—when the QDs and the PhC cavity do not interact with each other (in off-resonance conditions). On the other hand, when single QDs are resonantly coupled with the nanocavity, PL exhibits vacuum Rabi splitting—see Figure 2(b)—when coherent energy exchange between a single QD and the optical-cavity field creates an exciton-polariton state (which shows a double peak). Upon increasing the optical-pumping rate, the system reaches laser oscillations and the exciton-polariton doublet changes into a sharp single peak: see Figure 2(c). Vacuum Rabi splitting is observed only when single QDs play an essential role, so that this result clearly indicates that single QDs are essential gain sources for this nanolaser.

Figure 2. (a) Photoluminescence (PL) spectrum (in arbitrary units, arb. units) of the target exciton and cavity mode at sufficiently high detuning. PL spectra recorded (b) in polariton states below the threshold and (c) at lasing.

The transition from the exciton-polariton state to lasing contains interesting physics because the former is a reversible process, while the latter is irreversible. As the optical-pumping rate increases, the Rabi oscillation—see Figure 3(a)—is disturbed by pump-induced dephasing and irreversible stimulated emission. Although the coexistence of these two processes seems contradictory in nature, it was recently predicted theoretically.⁴ However, existence of this process in reality has not yet been confirmed. If the coupling of the single QD and the cavity is very strong, the system can start lasing before the coherent catch ball of quanta disappears completely: see Figure 3(b).

Figure 3. (a) Strong-coupling system. Single quanta go back and forth between a single QD (SQD) and the optical cavity. g: Coupling (gain). (b) State transition from strong coupling (SC) to lasing. Lasing can start before strong coupling is destroyed in a ‘very’-strong-coupling system.

Numerical simulations based on the quantum master equation are a powerful tool to investigate the physics in such a cavity quantum-electrodynamic system. We experimentally measured photon-correlation functions at various pump powers as well as the zero-delay time, g⁽²⁾(0): see Figure 4. Numerical simulations reproduced the general behavior of the pump-power dependence on g⁽²⁾(0) when we assume that single QDs provide 80% of the total gain at the lasing threshold. The calculated pump rate at the strong-coupling limit, where the coherent catch ball is sustained, is ~800GHz, while the laser threshold (N_ph =1) is ~590GHz. Therefore, a comparison between our experimental results and numerical simulations indicates that the coherent catch ball of quanta is still sustained at the threshold pump power in our system: see Figure 3(b). This is the first time that the observed onset of lasing in the strong-coupling regime has been reported in solid-state material.⁵

Figure 4. Calculated mean-cavity photon number (N_ph, red line) and photon-correlation function at zero-delay time, g⁽²⁾(0) (blue line), for SQD purity of 80% as a function of pump power and experimental g^{(2 )}(0) values (purple circles). Laser oscillations begin (N_ph > 1) in the strong-coupling regime (light-blue region). P_sys, P_th: System, threshold pump rates.

In summary, we have demonstrated a single-QD nanolaser using a PhC nanocavity and an InGaAs QD. The single-QD-PhC nanocavity system shows an exciton-polariton doublet below the threshold and goes into the lasing regime sustaining the coherent exchange of quanta between single QDs and the cavity around the threshold. The onset of lasing in the strong-coupling regime has been observed in solid-state material for the first time. These results show clear promise of lasing oscillations in single-artificial-atom nanolasers, which we will further explore.

This work was supported by the Special Coordination Funds for Promoting Science and Technology.