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 ...

Time-resolved photoluminescence spectra after nonresonant excitation show a distinct 1s resonance, independent of the existence of bound excitons. A microscopic analysis identifies excitonic and electron-hole plasma contributions. For low temperatures and low densities the excitonic emission is extremely sensitive to even minute optically active exciton populations making it possible to extract a phase diagram for incoherent excitonic populations. 1 For a long time, photoluminescence (PL) at the spectral position of the 1s exciton resonance has been considered as evidence for the existence of excitons. The rise of the 1s PL after nonresonant excitation of a semiconductor was interpreted as buildup of an excitonic population [1,2,3,4,5,6], and the PL decay was used to describe exciton recombination [7,8]. However, recently a microscopic theory predicted that PL at the 1s resonance can also originate from correlated plasma emission [9]. Accordingly, PL at the spectral position of the 1s resonance would not prove the existence of excitons, and previous interpretations may be in question. Indeed, in nonresonantly excited time resolved PL measurements the 1s resonance is developed on a sub-ps timescale at 100 K [10], much faster than any expected exciton formation time.Information about exciton formation can be gained by performing THz experiments [11].However, currently the THz results are inconclusive: Kaindl et al. [12] observed the buildup of the induced absorption corresponding to the excitonic 1s to 2p transition showing excitonic populations with formation times on a rather slow timescale of 100's of ps to ns. They claim to observe a nearly completely excitonic system 1 ns after excitation, while Chari et al. [13] only plasma contributions. Also, THz absorption is sensitive to both dark and bright excitons, and cannot answer if and how excitonic populations influence the PL.Here we address the following questions. Is the 1s PL ever dominated by plasma emission?If so, is it always dominated by plasma emission, i.e. what can we learn about excitonic populations from the 1s PL and nonlinear absorption?After ps continuum excitation, time resolved PL and corresponding probe absorption measurements are performed under identical conditions on a ns timescale. The sample (DBR42) consists of 20 MBE-grown 8 nm In 0.06 Ga 0.94 As quantum wells with 130 nm GaAs barriers; both sides are anti-reflection coated. This indium concentration places the 1s exciton resonance at 1.471 eV at 4 K, avoiding absorption in the bulk GaAs substrate that leads to impurity emission at 1.492 eV from unintentional carbon in the substrate. The results, checked on several other samples including a sample grown in a different MBE system, are insensitive to exciton linewidths or to interfacial or alloy disorder. Single-quantum-well data were noisier but exhibited a similar behavior, excluding significant radiative coupling effects.We excite nonresonantly 13.2 meV above the 1s resonance, into the heavy-hole continuum but below the light-h...

We report a novel hemispherical micro-cavity that is comprised of a planar integrated semiconductor distributed Bragg reflector (DBR) mirror, and an external, concave micro-mirror having a radius of curvature 50 microm. The integrated DBR mirror containing quantum dots (QD), is designed to locate the QDs at an antinode of the field in order to maximize the interaction between the QD and cavity. The concave micro-mirror, with high-reflectivity over a large solid-angle, creates a diffraction-limited (sub-micron) mode-waist at the planar mirror, leading to a large coupling constant between the cavity mode and QD. The half-monolithic design gives more spatial and spectral tuning abilities, relatively to fully monolithic structures. This unique micro-cavity design will potentially enable us to both reach the cavity quantum electrodynamics (QED) strong coupling regime and realize the deterministic generation of single photons on demand.

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...

The dynamics of exciton and electron-hole plasma populations is studied via time-resolved photoluminescence after nonresonant excitation. By comparing the peak emission at the exciton resonance with the emission of the continuum, it is possible to experimentally identify regimes where the emission originates predominantly from exciton and/or plasma populations. The results are supported by a microscopic theory which allows one to extract the fraction of bright excitons as a function of time.

The lack of translational invariance perpendicular to the plane of a single quantum well causes equal probability for spontaneous emission to the left or right. Combining one emission path from the left and one from the right into a common detector leads to interference fringes for fundamentally indistinguishable paths corresponding to geometries where the same in-plane momentum is transferred to the quantum well. For all other paths, no interference is observed because of the entanglement between the photon and extended Bloch states of the many-body system. In multiple-quantum-well structures the interference can be controlled via the spacing between the wells.

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