Spectral control of light action on biological particles inside soft biological tissue

Gradient forces formed by speckle structures in laser-irradiated biological tissue are studied. Speckle fields and some their parameters, such as speckle sizes, are theoretically evaluated. Multi-layered human skin is taken as biotissue. The gradients of light intensity at varying depths inside the tissue are estimated. It turned out that to provide the maximal force acting on a biological particle located at a predetermined depth inside tissue requires the irradiation at a specific wavelength under constant light power density at the skin surface. The more the depth is, the more the wavelength is shifted to the infrared. This enables one to control the performances of laser tweezers and their operation to trap a biological particle inside tissue in vivo. Examples of the applications of the designed approach are given.