Spatial Light Modulator Based Terahertz Wavefront Modulation And Its Applications

In recent years, the optical technology has been widely applied in the communicaiton, sensoring, imaging and machining. Hence, how to modify the wavefront of the light by altering its amplitude, phase, polarizaiton, wave vector in a desired manner, including two dimensional and three dimensional, in m acroscale and nanoscale, becomes a very important issue. In this thesis, we mainly focus on the visible light wavefront modulation and optically controlled terahertz wavefront modulation. The main works of the thesis are shown as follows.An accelerated algorithm for three dimensional computer generated hologram(CGH) based on the ray-tracing method is proposed. The complex amplitude distribution from the center point of an object is calculated in advance and the field distributions of rest points on the hologram plane can be given by doing a small transform and an aberration to the pre-calculated field. The time consumption for generating the CGHs can be dramatically reduced by using the proposed algorithm. The simulation results show that the accelerated algorithm is eight times faster than the conventional ray-tracing algorithm. A static two dimensional car, a three dimensional teapot, and a dynamic three dimensional rotating teapot are reconstructed from CGHs calculated with the accelerated algorithm to show its validity. The proposed algorithm may benefit to the realization of the 3D display based on holography.We present a novel method for image encryption based on the disturbing of the spatially incoherent wavefront. The encryption system only contains one Fourier lens and one random phase mask. The system is illuminated with incoherent light, thus, the encrypted image is an intensity distribution and it can be directly captured with a CCD camera. We find that the random phase function, the phase functi on of fresnel lens, the distance between the lens and the SLM, and the wavelength of the laser can be employed as the keys of the systems. The numerical simulation and the optical experiment demonstrate the validity and the effectiveness of the proposed method. We also analysis the factors which could affect the practical experiment, such as quantization noise and displacement tolerances. Compare with the traditional encryption system, the proposed method is cheaper, more stable. We also proposed a scheme for double image hiding based on the incoherent encryption system. The point spread function is utilized to encoding and decoding the first image. The second image is encoded as a phase mask to disturb the information of the first image. The encrypted image only contains intensity distribution, that is not enough for the decryption of the second image, so we need to record the phase part of the encrypted image through numerical simulation. In optical experiment, the two SLMs are used to load the two original images. Considering that the point spread function is too weak to capture, we carefully adjust the parameters and also use average of multiple captures to reduce the background noise. The experiment results show the validity and the feasibility of the pro posed method. The proposed method may have potential applications for the information security in the future.Terahertz(THz) technology is a developing and promising candidate for biological imaging, security inspection and communications, due to the low photon energy, the high transparency and the broad band properties of the THz radiation. However, a major encountered bottleneck is lack of efficient devices to manipulate the THz wave, especially to modulate the THz wave front. Here we propose a spatial THz modulator(STM) to dynamically control the THz wave front with photo-generated carriers. A computer generated THz hologram is projected onto a silicon wafer by a conventional spatial light modulator(SLM). The corresponding photo-generated carrier spatial distribution will be induced, which forms an amplitude hologram to modulate the wave front of the input THz beam. The key parameters of the proposed STM(including resolution, bandwidth, modulation depth) are measured. Some special intensity patterns and vortex beams are generated by using this method. This all-optical controllable STM is structure free, high resolution, broadband and high frame rate. It is expected to be widely used in future THz imaging and communication systems.A scheme for vector terahertz(THz) beam generation is proposed. A subwavelength metal grating is utilized to adjust the polarization of the THz radiation. The amplitude and phase distributions of the THz beam are dynamically regulated by a THz computer generated hologram(CGH) pattern of the photo-generated carriers. A radially polarized THz beam and a vector THz vortex beam with a topological charge of 1 are generated to demonstrate the validity and the effectiveness of the proposed scheme. Experimental results correspond to the theoretical simulations well. Moreover, the proposed method is applicative for a broadband THz radiation. These results could be applied in the THz sensing, THz imaging, and THz communication in the future.