Design and Setup of Inverted Optical Tweezer Embedded Differential Confocal Microscopy System

An inverted optical tweezer embedded differential confocal microscope system is developed, based on the principle of differential confocal microscopy and optical tweezer theory, in this thesis. The system is constructed and characterized. The main features of differential confocal microscopy include high depth resolution, large dynamic range, long working distance and high acquisition rate. The depth resolution of differential confocal microscopy is only limited by system noise. In the system build in this thesis, a depth resolution of several nanometers has been achieved. The dynamic range is as large as several micrometers, determined by wavelength of light source and numerical aperture of objective lens. Since probing the sample with far-field optical wave and without closed-loop locking the height of sample surface, differential confocal microscopy has advantage of high acquisition rate. By collaborating with lateral detection element, resolution of ten nanometers in three-dimensional positioning has also been verified in the system. The signal to noise ratio of differential confocal signal is measured as high as 80 dB. If the wavelength is 633 nm, using the objective lens of 0.85 numerical aperture, the depth resolution is two nanometers.
Optical tweezer, as known as single beam optical tarp, grabs small particles of micrometer-size even nanometer-size by the force from photo momentum transfer. The main features of optical tweezer are non-contact and non-invasive. By combining optical tweezer and optical microscopes, one can manipulate the observed object in the microscopic scale. Ray optics model and electromagnetics model describe the model of optical trap, and the mathematical theory and simulation of ray optics model are introduced in this thesis. Inverted configurations of optical tweezer embedded differential confocal microscopes are constructed. The spring constant k of optical tweezer is characterized in this system. And a quadrant photodiode is used to perform detection of lateral displacement. Combining with the depth resolution power of differential confocal microscope, three-dimensional positioning of a micro-sphere has been verified. With the spring constant k, the gradient force is estimated in the experiment. And we also point out the relationship between laser power and the gradient force.