Cavity quantum electrodynamics (QED) systems allow the study of a variety of fundamental quantum-optics phenomena, such as entanglement, quantum decoherence and the quantum-classical boundary. Such systems also provide test beds for quantum information science. Nearly all strongly coupled cavity QED experiments have used a single atom in a high-quality-factor (high-Q) cavity. Here we report the experimental realization of a strongly coupled system in the solid state: a single quantum dot embedded in the spacer of a nanocavity, showing vacuum-field Rabi splitting exceeding the decoherence linewidths of both the nanocavity and the quantum dot. This requires a small-volume cavity and an atomic-like two-level system. The photonic crystal slab nanocavity--which traps photons when a defect is introduced inside the two-dimensional photonic bandgap by leaving out one or more holes--has both high Q and small modal volume V, as required for strong light-matter interactions. The quantum dot has two discrete energy levels with a transition dipole moment much larger than that of an atom, and it is fixed in the nanocavity during growth.

Allowing xenon or nitrogen gas to condense onto a photonic crystal slab nanocavity maintained at 10-20 K results in shifts of the nanocavity mode wavelength by as much as 5 nm ͑Х4 meV͒. This occurs in spite of the fact that the mode defect is achieved by omitting three holes to form the spacer. This technique should be useful in changing the detuning between a single quantum dot transition and the nanocavity mode for cavity quantum electrodynamics experiments, such as mapping out a strong coupling anticrossing curve. Compared with temperature scanning, it has a much larger scan range and avoids phonon broadening. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.2076435͔Radiative coupling between a single quantum dot ͑SQD͒ and a small volume cavity alters the emission properties of the coupled system. In the weak coupling regime, the spontaneous emission has a single emission frequency, and it irreversibly escapes from the cavity. The SQD radiative emission rate is enhanced by the Purcell factor F P =3 3 Q / ͑4 2 V͒ compared with the cavityless radiative emission rate ␥ 0 ; Q and V are the quality factor and volume of the cavity, respectively, and = 0 / n is the wavelength of the light in the material with refractive index n. The several single-photon-on-demand sources that have been reported operate in the weak coupling regime. [1][2][3][4][5] In the strong coupling regime, the Rabi frequency rate of exchange of the excitation between the SQD and the cavity, g = E vac / ប, exceeds both the photon escape rate and SQD dephasing rate ␥. In the formula for g, it is assumed that the SQD has dipole moment and is in the peak of the root-mean-square intracavity field E vac satisfying 0 n 2 ͉E vac ͉ 2 V = h /2; 0 is the permittivity of vacuum, and is the frequency of the cavity mode. The strong coupling makes the spontaneous emission reversible, i.e., a photon emitted by the excited SQD has a higher probability of being reabsorbed than escaping the cavity. For zero detuning between the SQD and the cavity mode, the coupled-system spontaneous emission can occur at either of two frequencies separated by 2g. If the coupled transition of the SQD is between excited state ͉e͘ and ground state ͉g͘ and the quantized field state with n photons in the cavity mode is denoted by ͉n͘, then the two coupled-system eigenstates can be written as ͉e0͘ ± ͉g1͘. Since neither eigenstate can be written as a product of a quantum dot ͑QD͒ state and a cavity state, each has entanglement-a basic ingredient of quantum information science. Recently, there have been three experimental claims of observing this vacuum Rabi splitting using for the nanocavity a micropillar, 6 a photonic crystal slab, 7 and a microdisk. 8 The photonic crystal slab nanocavity has the smallest mode volume, ͑0.3-1͒ 3 . Therefore, it will have the largest vacuum Rabi splitting 2g for a given SQD dipole moment, since E vac ϰ 1/ ͱ V.For many years, of a photonic crystal nanocavity has been larger than ␥; therefore, since =2 / Q, it has been g / ϰ Q / ͱ V that needed to be ...

We have fabricated photonic crystal nanocavity lasers, based on a high-quality factor design that incorporates fractional edge dislocations. Lasers with InGaAsP quantum well active material emitting at 1550 nm were optically pumped with 10 ns pulses, and lased at threshold pumping powers below 220 W, the lowest reported for quantum-well based photonic crystal lasers, to our knowledge. Polarization characteristics and lithographic tuning properties were found to be in excellent agreement with theoretical predictions. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1511538͔The quest for a compact nanocavity laser, with highquality factor (Q) and small mode volume (V mode ), has been a central part of research in the field of integrated optics. Photonic crystals, 1 and planar photonic crystals in particular ͑PPC͒, 2 are promising manufacturable geometries for the realization of compact optical nanocavities and their integration with waveguides, modulators, and detectors. So far, there have been several reports on room-temperature lasing in PPC nanocavities, 3-7 and more recently, new high-Q cavity designs based on modification of two-dimensional ͑2D͒ photonic crystals have been proposed. 4,8 In this letter, we report the experimental application of one of these designs. The cavities are based on fractional edge dislocations, 8 and are used for the construction of a low-threshold laser in which the high field from the laser surrounds a void for chemical sensing or strong coupling to atomic light sources.Our laser design uses the simplest triangular lattice single-defect cavity containing a fractional edge dislocation. The cavity consists of a defect hole that is smaller than surrounding holes which define the photonic crystal mirror. The row that contains the defect hole is elongated by moving the two photonic crystal half planes a fraction of a lattice constant apart in the ⌫X direction, introducing a dislocation with width p ͑Fig. 1͒. We have shown earlier 8 that by tuning this p parameter, Q factors of single-defect cavities are significantly improved, and can reach values of over 10 000 when p/aϭ10% (a is the lattice constant͒. These high-Q values are obtained while maintaining a very small mode volume of V mode Ϸ0.1( /2). 3 The cavity used in our laser was originally designed for cavity quantum electrodynamic experiments and nanospectroscopy. Light sources or absorbing molecules can be placed into the small hole within the center of the 2D photonic crystal cavity, where the optical field intensity is the strongest. On the other hand, it is clear that the presence of a hole at the point of maximum field intensity is not desirable in low-threshold laser designs, since the overlap with the gain region ͑e.g., quantum wells͒ is decreased. Therefore, we expect even better cavity designs to yield further improvements over the performance of the lasers described here.Our structures are fabricated in InGaAsP quantum well material. Metalorganic chemical vapor deposition was used to grow the active laser structure o...

We have fabricated and characterized donor-mode nanocavities formed by a single defect cavity defined within a two-dimensional photonic crystal slab. Quantum dots emitting in the 1.1-1.3 micron range were used as luminescence sources, and a design using fractional edge dislocations was used to demonstrate well-confined dipole modes with high quality factors. By applying the fractional dislocation geometry, the measured quality factor could be increased to values as high as 2800. This compares with typical quality factors of around 1500 measured from more conventional shallow donor mode cavities with larger mode volumes.

We have fabricated photonic crystal nanocavity lasers, based on a high-quality factor design that incorporates fractional edge dislocations. Lasers with InGaAsP quantum well active material emitting at 1550 nm were optically pumped with 10 ns pulses, and lased at threshold pumping powers below 220 W, the lowest reported for quantum-well based photonic crystal lasers, to our knowledge. Polarization characteristics and lithographic tuning properties were found to be in excellent agreement with theoretical predictions.