Laser oscillation in a strongly coupled single quantum dot-nanocavity system (0905.3063v2)

Abstract: Strong coupling of photons and materials in semiconductor nanocavity systems has been investigated because of its potentials in quantum information processing and related applications, and has been testbeds for cavity quantum electrodynamics (QED). Interesting phenomena such as coherent exchange of a single quantum between a single quantum dot and an optical cavity, called vacuum Rabi oscillation, and highly efficient cavity QED lasers have been reported thus far. The coexistence of vacuum Rabi oscillation and laser oscillation appears to be contradictory in nature, because the fragile reversible process may not survive in laser oscillation. However, recently, it has been theoretically predicted that the strong-coupling effect could be sustained in laser oscillation in properly designed semiconductor systems. Nevertheless, the experimental realization of this phenomenon has remained difficult since the first demonstration of the strong-coupling, because an extremely high cavity quality factor and strong light-matter coupling are both required for this purpose. Here, we demonstrate the onset of laser oscillation in the strong-coupling regime in a single quantum dot (SQD)-cavity system. A high-quality semiconductor optical nanocavity and strong SQD-field coupling enabled to the onset of lasing while maintaining the fragile coherent exchange of quanta between the SQD and the cavity. In addition to the interesting physical features, this device is seen as a prototype of an ultimate solid state light source with an SQD gain, which operates at ultra-low power, with expected applications in future nanophotonic integrated systems and monolithic quantum information devices.

• The paper presents laser oscillation in a strongly coupled SQD–nanocavity system, achieving >90% cavity photon contribution at a threshold pump power of ~90 nW.
• It employs high-Q photonic crystal nanocavities and temperature-tuning to control exciton-mode coupling, with a coupling constant of 68 μeV.
• Numerical simulations and sub-Poissonian photon statistics validate the coherent laser dynamics, advancing quantum information and nanophotonic integration.

Analysis of Laser Oscillation in a Strongly Coupled Quantum Dot-Nanocavity System

The paper presents an empirical breakthrough in the field of cavity quantum electrodynamics (QED), through the demonstration of laser oscillation within a strongly coupled single Quantum Dot (SQD) and photonic crystal nanocavity system. The integration of SQD and nanocavities posits a notable advancement in the discipline due to its potential applications in quantum information processing and monolithic quantum devices.

Experimental Design and Methodology

The research relies on high-quality factor (Q ~ 35,000) photonic crystal (PhC) nanocavities with small mode volumes to realize the strong light-matter interaction necessary for this innovative system. This strong coupling is characterized by vacuum Rabi oscillations and demonstrated through the vacuum Rabi splitting observed in the presented photoluminescence (PL) spectra.

The authors employed a temperature-tuning technique allowing precise control of exciton-mode coupling, navigating spectral positions to observe phenomena typical of strong coupling regimes, like anticrossing and energy mixing. The quantitative analysis from empirical data reveals an exciton-mode coupling constant of $g = 68 \mu eV$, a magnitude confirming operation within the strong-coupling regime, decisively larger than associated decay rates.

Results and Numerical Validation

The paper reports direct emission characteristics signifying laser oscillation onset in strong coupling conditions, a transition emerging at a threshold pump power of approximately $90 nW$. Remarkably, >90% of cavity photons at threshold are provided by the SQD—a critical insight into the high purity and near-ideal realization of a solid-state single artificial atom-cavity system.

Simulations conducted with a quantum master equation model reflects observations, supporting findings with coherent emission dynamics and photon statistics in harmony with experimental results. Sub-Poissonian photon statistics, characterized by $g^{(2)}(0) < 1$, were evident below lasing thresholds, signaling non-classical light behaviors that transform with the reaching of the lasing threshold at higher pump powers.

Theoretical and Practical Implications

The implications of laser oscillation in a strong-coupling regime are profound, striking a compelling balance between reversible coherent exchange and the irreversible emission dynamics traditionally thought incompatible. In conventional systems, irreversible dynamics dominate laser operation, rendering coherent quantum exchange elusive. However, evidence herein suggests sustaining strong-coupling criteria—even past the onset of lasing—represents fertile ground for advancing quantum optical systems.

The findings advocate for the robust capability of SQD-photonic systems to enhance quantum information technologies. They posit potential pathways for ultra-low power operational electronics, pivotal as we progress toward scalable quantum computing and nanophotonic integrated circuits.

Prospects for Future Research

Further experimentation in cleaner, more isolated environments with precision control over nanocavity characteristics could yield deeper insights into maintaining strong-coupling dynamics in practical quantum devices. Investigation into SQD systems within different materials and varying nanostructure compositions would also prove instructive in optimizing these interactions.

The paper paves the way towards the development of compact, power-efficient quantum sources, encouraging active exploration for deployable nanophotonic and monolithic quantum systems beyond laboratory constraints, which may redefine computational and data handling paradigms in the field.