Baiheng Ma

Autofocusing of digital holographic microscopy based on off-axis illuminations

An auto-focusing method for digital holographic microscopy has been proposed by employing two off-axis illumination beams. When specimens are illuminated by two plane waves in different directions, it is found that the farther the reconstruction plane is from the image plane, the wider the two reconstructed images are separated from each other. Thus, the image plane can be determinated by finding the minimum of the variation between the two reconstructed object waves on both the amplitude and phase distributions. The feasibility of the proposed method is demonstrated by the corresponding simulation and experiment.

Autofocusing based on wavelength dependence of diffraction in two-wavelength digital holographic microscopy.

An autofocusing method for two-wavelength digital holographic microscopy (TWDHM) based on the wavelength dependence of the diffraction process is proposed. Red and green lights are employed for the illumination of the TWDHM, and the generated holograms are recorded simultaneously by a color CCD camera. Due to the wavelength dependency of the diffraction process, the farther the reconstruction plane is from the image plane, the larger the difference is between the red and green light distributions. Thus, the image plane can be determined by finding the minimum of the variation between the red and green lights on their amplitude distributions. The feasibility of the proposed method is demonstrated by simulation and experiment.

Dual-wavelength slightly off-axis digital holographic microscopy

We propose dual-wavelength digital holographic microscopy with a slightly off-axis configuration. The axial measurement range without phase ambiguity is extended to the micrometer range by synthesizing a beat wavelength between the two wavelengths with separation of 157 nm. Real-time measurement of the specimen is made possible by virtue of the high wavelength selectivity of the Bayer mosaic filtered color CCD camera. The principle of the method is exposed, and the practicability of the proposed configuration is demonstrated by the experimental results on a vortex phase plate and a rectangular phase step.

Prediction of optical modulation properties of twisted-nematic liquid-crystal display by improved measurement of Jones matrix

The optical modulation properties of twisted-nematic liquid-crystal display (TN-LCD) can be totally predicted and optimized when its Jones matrix is known. An improved method for determining the Jones matrix of the TN-LCD without knowing its internal parameters is presented in this paper. Only three sets of intensity measurements are needed for the complete determination of the TN-LCD’s Jones matrix for a single wavelength, much less than the procedure offered by Moreno et al. This method is easier and stable in data processing and capable of diminishing the calculation error resulting from the fluctuation of intensity measurement effectively. The validity of this method is verified experimentally, whose results coincide well with the optical modulation properties of the TN-LCD predicted by our determined Jones matrix.

Theoretical prediction of three-dimensional shifting of a spherical focal spot in a 4Pi focusing system

A spherical focal spot can be generated by focusing radially polarized laser beams in a 4Pi focusing system, which will result in an increase in axial resolution in microscopy or higher axial trapping efficiency. We demonstrate the possibility of continuous shifting of the spherical focal spot along the prescribed trajectory in three-dimensional space by suitably modulating the phase of the input fields at the pupil planes of the microscope objective lenses. This may provide an alternative to moving trapped particles or scanning a specimen without moving either laser beams or the stage.

Femtosecond laser-induced permanent anisotropy in bacteriorhodopsin films and applications in optical data storage

In polymeric films of bacteriorhodopsin (BR) a photoconversion product named F540-state, which is excited by 790 nm femtosecond laser pulses, is stable either for photochemical reaction or thermal pathway. The optical properties of the F540-state were studied, and Jones-matrix theory was adopted to analyze the photoinduced anisotropy of the F540-state. Based on the permanently photoinduced anisotropy, write-once-read-many (WORM) optical data storage was demonstrated by using two polarization states of femtosecond pulsed laser. Since the polarization information is also written on the storage media, it is impossible to copy it in a common way. This storage technique has a potential application in advanced optical security.

Simulation and optimization of spatial light modulation of twisted-nematic liquid crystal display

The approaches to obtaining desired intensity or phase modulation by twisted-nematic liquid crystal display (TN-LCD) have been extensively studied based on the knowledge of the LCDs internal structure parameters. Generally, the TN-LCD placed between two linear polarizers (P) produces coupled intensity and phase modulation. To obtain the commonly used phase-only modulation, quarter wave plates (QWPs) are often used in front of and/or behind the LCD. Here we present a method to optimize the optical modulation properties of the TN-LCD to obtain phase-only modulation in the configuration of P-QWP-LCD-QWP-P each with proper orientation. Our method is based on the macroscopical Jones matrix descriptions for the LCD, the QWPs, and the linear polarizers. Through Jones matrix calculations, the orientations of the polarizers and QWPs can be optimized to satisfy differently desired modulation demands. In contrast to the traditional method, which requires knowledge of the LCDs internal structure parameters, our method simplified the complicated theory analysis and can work in the absence of information on the LCDs internal structure parameters, which are usually not available for the commercial products.

Holographic optical trapping and manipulation based on phase-only liquid-crystal spatial light modulator

an improved and rapid three dimensional gerchberg-saxton (gs) algorithm based on the classic gs algorithm for computer generated holograms is proposed and applied to holographic optical tweezers. theoretical simulations and experimental results have demonstrated the rapidity and efficiency of the proposed algorithm. a robust holographic optical tweezers setup based on phase-only liquid-crystal spatial light modulator(slm) is built, and stable trapping and dynamic manipulation of yeast cells and silica beads with large array traps in three dimensions are demonstrated. two special traps, i.e., line trap with intensity gradient distribution and optical vortex trap, are generated to transport and rotate micro-particles respectively. the system is verified to be robust on particles manipulations, which provides a new and powerful tool for researches on biology, colloid physics and so on.

Optical trapping with cylindrical vector beams

Recent development of cylindrical vector beams prompts its application in optical trapping, which shows more effective and improved trapping efficiency in contrast to the traditional Gaussian beam of spatially homogeneous polarization. Using the T-matrix method and vectorial diffraction theory, we calculated and compared the radiation forces exerted on dielectric particles respectively by the linearly polarized, radially polarized and azimuthally polarized beams. Theoretical calculations show that the radially polarized beam can improve the axial trapping efficiency of high-refractive-index larger particles by reducing the scattering force due to the vanishing axial component of Poynting vector near the focus, while the azimuthally polarized beam can not only steadily trap low-refractive-index small particles at the focus center but also can trap multiple high-refractive-index particles around the focus center in virtue of the hollow-ring configuration. The dependences of the trapping efficiencies on the beam parameters, particle size and the numerical aperture of objective lens are discussed. The performances of optical trapping, manipulating and sorting of biological cells, organelles and various micro-particles are demonstrated.