Yasutomo Ota

Laser oscillation in a strongly coupled single-quantum-dot–nanocavity system

Laser oscillations have now been observed in a solid-state system composed of an InAs quantum dot strongly coupled to the optical modes of a GaAs photonic-crystal cavity. Signs of lasing in the system are observed as the optical pump power is increased but before the strong coupling is lost. The strong coupling of photons and matter1 in semiconductor nanocavities has been a test bed for cavity quantum electrodynamics2,3 (QED). Vacuum Rabi oscillation4,5,6,7,8—the coherent exchange of a single quantum between a single quantum dot (SQD) and an optical cavity—and highly efficient cavity-QED lasers9,10,11,12,13,14,15,16,17,18,19 have both been reported. The coexistence of vacuum Rabi oscillation and laser oscillation seems to be contradictory, but it has recently been predicted theoretically that the strong-coupling effect could be sustained in laser oscillation20. Here, we demonstrate the onset of lasing in the strong-coupling regime in an SQD–cavity system. A high-quality semiconductor optical nanocavity and strong SQD–field coupling enabled the onset of lasing while maintaining the fragile coherent exchange of quanta.

Photonic crystal nanocavity laser with a single quantum dot gain

We demonstrate a photonic crystal nanocavity laser essentially driven by single quantum dot gain. A diluted quantum dot density (~0.4/cavity) resulted in clear single quantum dot feature and distinct phase transition in photon correlation measurements.

Spontaneous two-photon emission from a single quantum dot.

Spontaneous two-photon emission from a solid-state single quantum emitter is observed. We investigated photoluminescence from the neutral biexciton in a single semiconductor quantum dot coupled with a high Q photonic crystal nanocavity. When the cavity is resonant to the half energy of the biexciton, the strong vacuum field in the cavity inspires the biexciton to simultaneously emit two photons into the mode, resulting in clear emission enhancement of the mode. Meanwhile, the suppression of other single photon emission from the biexciton was observed, as the two-photon emission process becomes faster than the others at the resonance.

Strong coupling between a photonic crystal nanobeam cavity and a single quantum dot

We demonstrated the strong coupling between a one-dimensional photonic crystal nanobeam cavity and a single quantum dot (QD). Thanks to a high quality factor (∼25 000) with small mode volume [0.38×(n/λ)3] of the nanobeam cavity, an anticrossing behavior with a vacuum Rabi splitting of 226 μeV was observed. The ratio of the QD-cavity coupling strength to the cavity decay rate, which is a figure of merit of quantum optical applications, is estimated to 2.1. This is the highest value among any QD-based cavity quantum electrodynamics systems reported so far.

Vacuum Rabi splitting with a single quantum dot embedded in a H1 photonic crystal nanocavity

We report here the first observation of vacuum Rabi splitting in a single quantum dot (QD) embedded in a H1 photonic crystal nanocavity by photoluminescence measurement. The QD emission was tuned into a cavity mode by controlling the temperature. At the resonance condition, clear anticrossing with a Rabi splitting of ∼124 μeV was observed, where the cavity mode possesses the smallest mode volume V∼0.43(λ/n)3 among strongly coupled QD-cavity systems reported to date.

High Q H1 photonic crystal nanocavities with efficient vertical emission.

We report on newly-designed H1-type photonic crystal (PhC) nanocavities that simultaneously exhibit high Q factors, small mode volumes, and high external coupling efficiencies (η([perpendicular])) of light radiated above the PhC membrane. Dipole modes of the H1 PhC nanocavities, which are doubly-degenerate and orthogonally-polarized in theory, are investigated both by numerical calculations and experiments. Through modifying the sizes and positions of the air-holes near to the defect cavity, a Q factor of 62,000 is achieved, accompanied with an improved η([perpendicular]) of 0.38 (assuming an objective lens with a numerical aperture of 0.65). A further increase of η([perpendicular]) to more than 0.60 is observed at the expense of slight degradation of Q factor (down to 50,000). We further experimentally confirm the increase of both Q and η([perpendicular]), using micro-photoluminescence measurements, and demonstrate high Q factors up to 25,000: the highest value ever reported for dipole modes in H1 PhC nanocavities.

Cavity Quantum Electrodynamics and Lasing Oscillation in Single Quantum Dot-Photonic Crystal Nanocavity Coupled Systems

Our recent advances in solid-state cavity quantum electrodynamics and lasing oscillation in single quantum dot (QD)photonic crystal (PhC) nanocavity coupled systems are discussed. These include the fabrication of high-quality 2-D PhC nanocavities (Q >; 50 000), which enabled the generation of spontaneous two-photon emission from a single QD, and the realization of lasing oscillation with single QD gain in the strong coupling regime. Moreover, we have fabricated high-quality 3-D PhC nanocavities (Q ~ 38 500), which have facilitated the realization of both lasing oscillation and the Purcell effect. Lasing oscillation in a 1-D PhC nanobeam cavity with gain produced by a few QDs has also been demonstrated.

Nanocavity-based self-frequency conversion laser.

Self-frequency conversion (SFC), where both laser oscillation and nonlinear frequency conversion occurs in the same laser crystal, has been used to efficiently extend the operational wavelength of lasers. Downsizing of the cavity mode volume (V) and increasing the quality factor (Q) could lead to a more efficient conversion process, mediated by enhanced n-th order nonlinearities that generally scale as (Q/V)(n). Here, we demonstrate nanocavity-based SFC by utilizing photonic crystal nanocavity quantum dot lasers. The high Q and small V supported in semiconductor-based nanocavities facilitate efficient SFC to generate visible light, even with only a few photons present in the laser cavity. The combined broadband quantum dot gain and small device footprint enables the monolithic integration of 26 different-color nanolasers (spanning 493-627 nm) within a micro-scale region. These nanolasers provide a new platform for studying few-photon nonlinear optics, and for realizing full-color lasers on a single semiconductor chip.

Zero-cell photonic crystal nanocavity laser with quantum dot gain

We demonstrate laser oscillation in a hexagonal-lattice photonic crystal nanocavity using an InGaAs quantum dot gain material by optical pumping at 5 K. The cavity comprises a defect created by shifting several air holes in a two-dimensional photonic crystal slab structure without removing any air holes to achieve both small mode volume and a high cavity quality factor. The measured cavity quality factors and estimated mode volume for the nanocavity are ∼33 000 and 0.004 μm3 [0.23(λ0/n)3]. The laser threshold is compared between the zero-cell and L3-type nanocavity lasers, and the zero-cell nanolasers are found to have lower thresholds of about one-third of the L3-type nanolasers. This result suggests that a higher Purcell factor of the zero-cell nanolaser is reflected as a lower laser threshold.

Investigation of the Spectral Triplet in Strongly Coupled Quantum Dot–Nanocavity System

We experimentally investigated the excitation power dependence of a strongly coupled quantum dot (QD)–photonic crystal nanocavity system by photoluminescence measurements. At a low excitation power regime, we observed a vacuum Rabi doublet emission at the QD–cavity resonance condition. With increasing excitation power, in addition to the doublet, a third emission peak appeared. This observed spectral change is unexpected from conventional atomic cavity quantum electrodynamics. The observations can be attributed to featured pumping processes in the semiconductor QD–cavity system.