For further information, see:
J.P. Reid, H. Meresman, L. Mitchem, and R. Symes
'Spectroscopic studies of the size and composition of single aerosol droplets'
Int. Rev. Phys. Chem. 26 (2007), 139-192.
J.P. Reid and L. Mitchem
'Laser probing of single aerosol droplet dynamics'
Annu. Rev. Phys. Chem. 57 (2006), 245-271.
R. Symes, R.M. Sayer, and J.P. Reid
'Cavity enhanced droplet spectroscopy: Principles, perspectives and prospects'
Phys. Chem. Chem. Phys. 6 (2004), 474-487.
Optical Characterisation
..............................................................................................................................
Whispering gallery modes occur at particular resonant wavelengths of light for a given droplet size. At these wavelengths, the light undergoes total internal reflection at the particle surface and becomes trapped within the particle. The droplet behaves like a low-loss optical cavity with the light circulating for timescales on the order of nanoseconds.
WGMs take there name from their similarity to the acoustic waves that propagate around whispering galleries, such as that found in the dome of St. Paul's Cathedral in London.
The light is concentrated near the circumference of the particle and can be assigned a mode number and a mode order. The mode number, n, tells us the number of wavelengths around the circumference of the particle, and the mode order, l, the number of maxima in the radial dependence of the electromagnetic field within the droplet. The whispering gallery mode shown in the image above is a second order mode with mode number 60.
The inelastic scattering of light from the droplet (such as in Raman spectroscopy or fluorescence spectroscopy) may be enhanced at wavelengths that are commensurate with whispering gallery modes, providing a structured fingerprint with resonant structure.
An example of a typical Raman fingerprint acquired from a trapped water droplet is shown below. The fingerprint consists of a broad underlying band arising from spontaneous Raman scattering. In this spectrum, excitation of the O-H stretching vibrations of water molecules is accompanied by Stoke's scattering of the incident photons, providing a characteristic energy loss that is indicative of the water forming the droplet. Superimposed on the broad band, the resonant structure at whispering gallery mode wavelengths can be used to determine the size of the droplet with nanometre accuracy.
Professor Jonathan P. Reid
School of Chemistry,
University of Bristol
