Multiple optical tweezers for micro Raman spectroscopy

The goal of our study is to develop a setup that combines multi-trapping and manipulation with micro Raman spectroscopy of microns size particles. Multiple trapping, in 2D or 3D (two or three dimensional) configurations, is obtained in an inverted microscope scheme by shaping the trapping beam (1064 nm) with diffractive optical elements implemented on a spatial light modulator (SLM). Manipulation of multiple particles, directly trapped by the beam, can be achieved using the dynamic displaying of the SLM. Indirect trapping and manipulation of the sample can be obtained surrounding it with trapped micro beads that are manipulated by the optical tweezers. Laser light is not directly focused on the sample but is distributed on the beads and therefore the photo-induced damaging of biological samples is reduced. This technique offers also other advantages: the sample can be kept in a stable position during the spectroscopic investigation or can be moved in x-y-z to get spatial resolved information in a scanning mode measurement and the shape of a deformable sample can be changed in a controlled manner during the measurement. Sample's excitation and Raman signal collection are accomplished with a separate laser beam (514.5 nm) in a non-inverted microscope coupled with the spectrometer. Some experimental results showing multi trapping and indirect manipulation of human red blood cells are presented and discussed.