Observation of morphology‐dependent input and output resonances in time‐dependent Raman spectra of optically levitated microdroplets

Abstract

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.

Optical Levitation of Single Microdroplets at Temperatures Down to 180 K

“…The size and refractive index of the particle may be derived from these MDR. [44] If the incident field satisfies a resonance condition in the microdroplet (peaks in Figure 3) the electromagnetic field inside the particle becomes intensified such that the overall intensity of the Raman spectrum is increased. These resonances are called primary (or ™input∫), morphology-dependent resonances.…”

Dependence of the stimulated Raman scattering threshold in droplets on the temporal stretching of a nanosecond laser pulse

“…If both are superimposed to Raman signal wavelengths, the signal formation is enhanced and stimulated Raman scattering (SRS) occurs. 8 Unfortunately the stimulated scattered light in droplets cannot locally be separated because different from the SRS in bulk media, the stimulated radiation in droplets is scattered isotropically. 9,10 In order to discuss possibilities for the suppression of this nonlinear optical process, the dependence on different parameters is briefly summarised.…”

Chemical composition and reaction analysis of single aerosol particles

“…The additional observable sharp peaks are the so-called morphology dependent resonances (MDRs). Analyzing these MDRs one is able to determine the radius [8,9] of an evaporating droplet with an accuracy of about 0.1 nm. Additionally, we can get exact information about the chemical composition of the investigated microparticle by use of the Raman spectra.…”