Strong Photon-Exciton Coupling in the Near-Field Luminescence of Semiconductor Quantum Dots
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We present low-temperature near-field scanning optical microscopy (NSOM) measurements of self-organized InP quantum dots (QD) embedded in a GaInP layer. We observed an anomalously strong increase in the emission energy (-270 meV) of a single QD by changing the tip-to-surface distance over -0oo nm in the near-field region. The effect indicates the formation of a near-field nanocavity having extremely high photonexciton mixing and can be interpreted as the observation of the coupled photon-exciton pair. INTRODUCTION
Near-field optical scanning microscopy (NSOM) allows one to extend the spatial resolution of optical experiments far beyond the light diffraction limit. The method opens new possibilities for studying photon-matter interaction and allows the manipulation of the non-radiative components of electromagnetic waves. In the present paper we report an apparent nanocavity behavior of the NSOM spectra of InP QDs embedded in a GaInP layer. The near-field nanocavity effect was manifested by anomalously strong dependence of the emission energy of a single QD on the tip-tosurface distance. EXPERIMENT
InP QDs were grown at 7000 C by low pressure MOVPE on exactly oriented GaAs (loo) substrates. Growth was started with the deposition of a 500 nm-thick Gao.51no.5P lower barrier followed by nominally 3 monolayers of InP island deposition. After 5s growth interruption the islands were overgrown by a 50 nm-thick upper Gao. 51no.5P. The plan-view transmission electron microscopy (TEM) measurements (see Fig.ia) clearly reveal strain-induced contrast characteristic of QDs and point to a typical dot base -1oo nm and density -2-109 cm- 2.A typical QD height of lo nm was determined from crosssection TEM. The thickness of the GaInP layer is equal to 5X/2 for the QD emission wavelength 720 nm. The low-temperature near-field scanning optical microscope (NSOM) was built using a cryogenic positioning system CryoSXM by Topac, Inc. The NSOM photo-luminescence (PL) spectra were taken in collection-illumination mode at lo K under excitation of 514.5 nm line with a power 5 gaW. The spectra were measured by CCD detector using a 280 mm focal length monochromator. The spectral resolution of the system is 0.2 meV. We use a -0.25 gtm diameter tapered fiber tips with an Al coating thickness of 20 (tips of type I) and 60 nm (tips of type II). The typical transmission of the fiber tips was 1O-3, which gives an excitation power density -5W/cm2. 207
Mat. Res. Soc. Symp. Proc. Vol. 618 © 2000 Materials Research Society
RESULTS AND DISCUSSION Suatially resolved near-field PL spectra of InP ODs Using tips of type I we measured the PL of our structure in a standard NSOM regime (Fig.1, b and c). In this regime the intensity of the emission spectra gradually increases as the tip-to-sample surface distance z decreases until the tip reaches the sample surface. No change in the energy position of the QDs emission lines are observed in this case (see insert in Fig.i, c). Fig.1, b shows spatially and spectrally resolved nearfield PL intensity obtained in t
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