Micro-Optical Techniques

This chapter focuses on optical measurements in which spatial resolution on the order of the wavelength of light provides meaningful, distinct information from that which can be obtained through bulk measurements, such as those described in Chapter 6. These techniques include photoluminescence (PL) spectroscopy, photoluminescence excitation (PLE) spectroscopy, electroluminescence (EL), and angle-resolved reectivity measurements. There are several physical systems for which the ability to perform such measurements with wavelength-scale spatial resolution is needed. For example, it is essential when interrogating nanofabricated photonic devices (Figure 16.1a) such as microcavity lasers (McCall 1992), where the optical cavities are a few micrometers in each planar dimension and are fabricated in arrays with a device-to-device spacing of tens of micrometers. Here, the spatial resolution is needed to distinguish between cavities and between a cavity and unprocessed regions of the chip. A second example is an ensemble of solid-state emitters in or on a substrate (Figure 16.1b), which encompasses structures such as epitaxially grown self-assembled quantum dots (QDs) (Michler 2003) embedded in a semiconductor material, colloidal QDs in solution or deposited on a substrate (Murray 1993, Alivisatos 1996), uorescent molecules in a host matrix (Moerner 1999), and impurity color centers in a crystal (Gruber 1997). These materials may exhibit a density gradient across the sample, so that spatially resolved measurements can provide an understanding of optical properties as a function of the number of excited emitters, ultimately reaching the single emitter limit in very dilute (≈1 emitter per μm2) regions. Our discussion is restricted to techniques that achieve diffraction-limited spatial resolution through “conventional” methods of high numerical aperture, free space far-eld optics. For the near-infrared wavelengths that are our interest, this produces a length scale on the order of a micrometer (hence the title “Micro-optical techniques”). This chapter does not cover some of the important developments in the quest for obtaining better spatial resolution, including near-eld scanning optical microscopy (NSOM), the topic of Chapter 17, and so-called superresolution techniques like stimulated emission depletion microscopy (Hell 2009) (Chapter 15). Such tools can provide a wealth of added information, such as spatial proles of microcavity modes (Balistreri 1999), or the ability to distinguish between single uorescent centers within a dense array (Betzig 1993). Nevertheless, as we shall see throughout this chapter,

wavelength-scale spatial resolution is in many cases preferred, since the improved resolution of a technique like NSOM comes at the cost of increased complexity and sacrice in collection efciency. Space constraints also prevent us from addressing promising recent developments in improving PL collection efciencies, particularly for embedded media like quantum wells or QDs, through solid immersion lenses (Gerardot 2007) or external waveguide probes (Srinivasan 2007).