Solid Immersion Microscopy Images Quantum Dots

Daniel S. Burgess

Researchers at Boston University and at the National Institute of Standards and Technology in Boulder, Colo., have used a lens that increases numerical aperture with solid immersion microscopy to collect photoluminescence spectra from individual quantum dots. The technique has implications for the research and development of devices based on quantum dots, such as single-photon sources.

Similar in principle to microscopy using an oil-immersion objective, solid immersion microscopy improves the diffraction-limited spatial resolution of an optical setup by increasing the refractive index of the material in the object space and, thus, the numerical aperture. A variant employs a numerical-aperture-increasing lens, a hemispherical piece of the same material as the sample under study that is held in as close proximity as is possible. This improves the resolution by a factor of n to n², where n is the refractive index of the material, and similarly increases the magnification and scan relaxation of the microscope.

Applications include the imaging of semiconductor circuits, layered dielectrics, and biological and chemical specimens, which are observed from the back surface of the sample holder.

The investigators have applied the technique to the collection of photoluminescence spectra from a sample of self-assembled InGaAs quantum dots in GaAs barriers with an areal density of approximately 100 million dots per square centimeter. They excited the sample with a CW Ti:sapphire laser through a 3.22-mm-diameter GaAs numerical-aperture-increasing lens. The lens/sample assembly was cooled to as low as 4.2 K in a helium flow cryostat, which was mounted on an X-Y-Z stage. The photoluminescence response was directed back through the lens and microscope to a fiber-coupled monochromator and a CCD camera for analysis.

Presuming that the <40-nm-across quantum dots approximated point sources, the researchers could ignore the actual width of the dots and use 350 nm, the measured full width half maximum of a spectral image of a dot, as the spatial resolution of the optical setup. This differs from the 280 nm derived from theoretical calculations as a result of an air gap between the lens and the back side of the sample, but it is sufficient to collect spectra from individual dots.

To compare the performance of the approach with that of a conventional high-numerical-aperture microscope, they collected peak intensities from 13 quantum dots in two configurations of the setup. Using a 0.6-NA, 40× objective yielded a mean value of 138 counts per second from the dots. Using the numerical-aperture-increasing lens and a 0.12-NA achromat yielded a mean value of 830 counts per second, better than the predicted fivefold improvement in collection efficiency.

Bennett B. Goldberg, a professor of physics and of electrical and computer engineering at the university, said that the difficulty and expense of fabricating the numerical-aperture-increasing lens currently limit the widespread application of the approach. The scientists are investigating the potential of solid immersion microscopy for thermal imaging, in situ lens formation in semiconductor devices and high-resolution spectroscopic studies of quantum dots.

Applied Physics Letters, Aug. 15, 2005, 071905.