During the slow evaporation of an optically levitated microdroplet of a glycerol-water mixture (3:1) (approximately 12.44 µm in radius) several morphology-dependent input resonances have been observed in its Raman spectrum. These resonances yield sudden temperature jumps of approximately 10 °C in the microdroplet as evidenced by sudden shifts in the output (Raman) resonance spectra. The latter effects could be explained by a simple energy balance calculation and the dependence of droplet refractive index and density on temperature.

Using a highly efficient Raman set-up, which allows a Raman spectrum to be recorded in less than 1 s, morphology-dependent resonances in optically levitated microdroplets of water-glycerol mixtures were studied. In addition to the output resonances, where the Raman frequency coincides with an electromagnetic sphere resonance, input resonances could also be demonstrated. The latter are observed when the evaporating droplet reaches a radius which, together with the wavelength of the incident laser light, fulfils the resonance condition for the cavity modes of a sphere. Preliminary results for droplet size change during evaporation of a particular droplet in the micrometre range are presented.

An experimental setup for studies of morphology dependent resonances (MDRs) in Raman spectra of optically levitated microdroplets is described. This instrumentation is used for the study of MDRs in the Raman spectra of glycerol/water (3 : 1) droplets. Furthermore we show how the MDRs observed in the Raman spectra and their change with time can be used to determine the radius and the index of refraction of single optically levitated droplets with very high accuracy. We further show how the results of this study can be used to calculate the efficiency factor of the radiation pressure and compare it with the experimental monitored laser power.

A simple but very efficient device for micro-Raman spectroscopy of highly absorbing solid samples is described. The device allows rapid scanning of the focused laser beam on the surface of a solid sample, which avoids heating of the sample. By means of a pair of computer-controlled piezo-translators, which are fixed to the microscope objective, the laser beam can be scanned in a great variety of traces, the scattered light of which is collected and optically transformed into a very small spot at the entrance slit of the spectrometer. The system is tested on highly absorbing (partially) polymerized diacetylene single crystals at low temperatures.

Theory and measurements are presented to explain the observability of morphology-dependent resonances in Raman spectra. An algorithm for Ðtting MDRs in Raman spectra has also been developed. Results are given of investigations on several microdroplets which were obtained from three di †erent spectral regions 1997 John