last updated 6 February 1999

This page is dedicated to optical spectroscopy of single particles. I became interested in this type of research during my stay at New Mexico State University (1988 to 1991).

The study of microparticles in the atmosphere has been a rapidly growing topic of research since the advent of the laser. When the word microparticle is used it is understood to include particles such as aerosols, hydrosols and droplets in colloidal suspension. Dimensions for these particles are generally in the range of 0.5 µm to 100 µm. These aerosols can be produced from airborne pollutants emitted from such places as factories, forest fires, explosions, sprays and a vast number of other sources. Particles such as these have significant effects on the propagation of electromagnetic radiation such as the case of a laser beam being absorbed in a cloud of smoke. The affects of these particles have been well researched by many workers using pulsed and continuous-wave lasers operating with wavelengths from the ultraviolet to the far infrared. Much of this research has centered around the fact that in highly symmetric particles, such as spheres, sharp resonances exist in the scattered and transmitted light which signify high electric field intensities in certain areas. Although the theoretical background for this effect dates back to 1908, it has not impacted the scientific community until the use of high speed computers to calculate the exact solution. The fruits of this research have many applications from particle sizing to micron-size spherical lasers. Isolating single particles by levitation methods opens up a fertile field by which one can observe the interaction of light with these particles. Levitating or trapping microparticles can be successfully accomplished by optical or electromagnetic levitation. Electromagnetic levitation has been done in either a quasi-electrostatic levitator or an electrodynamic (time varying field) levitator. Optical and electromagnetic levitation both have their own advantages and disadvantages which arise from the way the radiation interacts with the particle to the simplicity of the experimental set up.

The experiments done in my research used the method of optical levitation to trap single microparticles. In this type of levitation, the laser not only provided the radiation pressure to hold the particle but also the excitation needed to induce various emission processes. My investigation characterized the effect the levitation laser had on the levitated particle. The theoretical model section addressed the problem of how the bulk emission spectrum differs from the droplet emission spectrum. The first hurdle I jumped over was to determine what process caused the emission. Information such as the type of molecules that were in the droplet and other optical characteristics must be known to provide a thorough analysis.

The image below was part of a droplet breakdown thresholds experiment. The particles were irradiated with 10.6 µm pulsed TEA CO₂ laser. This particular particle had a diameter of about 50 µm. Illumination was provided by producing a laser induced plasma from a Nd:YAG laser at 532 nm. The temporal evolution of the droplet breakdown was observed by varying the time delay of the CO₂ laser pulse and the illumination pulse. Droplets were produced from a Vibrating Orifice Aerosol Generator (VOAG). (photo by R.G. Pinnick and R.L. Armstrong)

Optical levitation of several particles is shown in the image below. Levitation is by a 514.5 nm Argon ion laser. Optical levitation was produced by focussing the beam of laser light using a short focal length lens (3 cm) and a levitation cell. Click here to see a schematic diagram of the forces involved to levitate a droplet in the laser beam. The levitation laser is pointing up in this image. The levitated droplets in this picture are aqueous solution glycerol droplets < 10 µm in diameter. These particles were generated by the use of a hand-pumped nebulizer. The particles appear much larger than they are due to the enoumous amount of laser light scattered. Occasionally several droplets can be levitated using this technique.

Single particle optical levitation using a levitation cell I designed. Microscopes can be seen on the front and rear view ports on the levitation cell to collect scattered light. The scattered light was fed into a photodiode and plotted on a chart recorder enabling the size to be determined using Mie theory. The other microscope imaged light onto a linear imaging array detector as part of the levitation feedback circuit. Click here to see a schematic diagram of the optical setup. Click here to see a photo of the laboratory set-up.

Links to Current Research