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 ...

The fabrication and characterization of light-emitting one-dimensional photonic quasicrystals based on excitonic resonances is reported. The structures consist of high-quality GaAs/AlGaAs quantum wells grown by molecular-beam epitaxy with wavelength-scale spacings satisfying a Fibonacci sequence. The polaritonic (resonant light-matter coupling) effects and light emission originate from the quantum well excitonic resonances. Measured reflectivity spectra as a function of detuning between emission and Bragg wavelength are in good agreement with excitonic polariton theory. Photoluminescence experiments show that active photonic quasicrystals, unlike photonic crystals, can be good light emitters: While their long-range order results in a stopband similar to that of photonic crystals, the lack of periodicity results in strong emission.

Emission linewidths of quantum dot photonic-crystal-slab nanocavities are measured as a function of temperature and fabrication parameters with low-power and high-power, cw and pulsed, nonresonant excitation. The cavity linewidth is dominated by the absorption of the ensemble of quantum dots having a density of Х400/ m 2 ; above the absorption edge, the cavity linewidth broadens considerably compared with the empty cavity linewidth. Gain and lasing are seen for high-power pumping; it is estimated that only a small number of quantum dots contributes to the lasing. DOI: 10.1103/PhysRevB.72.193303 PACS number͑s͒: 42.55.Tv, 42.50.Pq, 42.55.Sa Recently the quality factor Q ͓mode energy divided by full width at half maximum ͑FWHM͒ mode energy linewidth͔ of photonic-crystal-slab cavities has been steadily increased by improved fabrication techniques and designs, while the volume V was kept close to a cubic wavelength in the material. This has made possible not only quantum well 1 and quantum dot 2 lasers but also the observation of strong coupling 3 -vacuum Rabi splitting with a single quantum dot ͑SQD͒. The role of the quantum dots ͑QDs͒ in the lasers is to provide gain, so several layers of high density QDs are often used. In contrast, strong coupling, can best be observed with an isolated SQD, suggesting the use of a single layer of low density QDs. However, to see strong coupling one must search to find two accidental coincidences. The QD must be situated close to an intracavity field maximum. This means it must be within the mode area of 0.15 m 2 , where the intracavity field is strong. It must also have a transition frequency close to a cavity mode; our ensemble QD lowest energy transition has a FWHM of 42.5 meV at 20 K, compared with a maximum dot-nanocavity coupling strength of 0.2 meV. For a reasonable probability for both coincidences, high dot densities ͑300-400/ m 2 ͒ have been used so far. This paper addresses two questions: Is the ensemble QD absorption detrimental to the search for strong coupling? And, if the gain is sufficient for lasing, roughly how many QDs contribute?To fabricate a photonic-crystal-slab nanocavity, a sample is grown by molecular beam epitaxy on a ͑001͒ GaAs substrate starting with a GaAs buffer layer: 800 nm Al 0.94 Ga 0.06 As sacrificial layer, 40 nm GaAs, 20 nm Al 0.1 Ga 0.9 As, single layer of self-assembled InAs QDs ͑den-sity of 300-400 m 2 ͒, and on top of the dots 20 nm Al 0.1 Ga 0.9 As and 40 nm GaAs. 4 Then a two-dimensional triangular photonic-crystal-lattice with three holes missing to form a cavity spacer is fabricated to provide in-plane light confinement. The GaAs-air interfaces on the top and bottom of the 270-nm-thick slab provide vertical confinement by means of total internal reflection, but light with small inplane wave vectors still leaks out of the cavity. As shown by Noda's group using Si, vertical confinement is further enhanced by slightly shifting outward the holes at the ends of the spacer; the Q is increased by confining gently. 5 The quantum dots are e...

A detailed experimental and theoretical study of the linear and nonlinear optical properties of different Fibonacci-spaced multiple-quantum-well structures is presented. Systematic numerical studies are performed for different average spacing and geometrical arrangement of the quantum wells. Measurements of the linear and nonlinear (carrier density dependent) reflectivity are shown to be in good agreement with the computational results. As the pump pulse energy increases, the excitation-induced dephasing broadens the exciton resonances resulting in a disappearance of sharp features and reduction in peak reflectivity.

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