Coherent anti-Stokes Raman scattering (CARS) microscopy is an attractive technique for label-free biochemical-specific characterization of biological specimens. However, it has very low sensitivity in monitoring and imaging molecules present in extremely low concentrations or at fast speeds. To improve this sensitivity, especially for multiplex CARS, the intensity of the pump beam and broadband Stokes beam should be enhanced simultaneously. Therefore, the gold shell particle and gold surface are demonstrated to enhance the forward and backward CARS, respectively. Results show that a signal enhancement factor of ∼25,000 can be achieved for the gold surface and an even higher enhancement factor can be achieved for the gold shell particles. Thus, we can obtain an enhanced CARS signal in a broad spectral range, which will substantially improve the detection sensitivity of hyperspectral CARS spectroscopy and imaging.
Holographic display has been considered as a promising display technology. Currently, low-speed generation of holograms with big holographic data is one of crucial bottlenecks for three dimensional (3D) dynamic holographic display. To solve this problem, the acceleration method computation platform is presented based on look-up table point source method. The computer generated holograms (CGHs) acquisition is sped up by offline file loading and inline calculation optimization, where a pure phase CGH with gigabyte data is encoded to record an object with 10 MB sampling data. Both numerical simulation and optical experiment demonstrate that the CGHs with 1920×1080 resolution by the proposed method can be applied to the 3D objects reconstruction with high quality successfully. It is believed that the CGHs with huge data can be generated by the proposed method with high speed for 3D dynamic holographic display in near future.
The influence of pulse-separation (τs) between a pair of temporally separated femtosecond laser pulses (with near ablation-threshold energy) on surface ablation of SiO2 were experimentally studied. A τs range of τs≤20 ps was considered. It was shown that a τs-independent/-dependent crater ablation area can be flexibly controlled. Once the pulse energy of the pulse pair exceeds a threshold value, crater ablation area become quasi-τs-independent at τs> ~1 ps. This τs-independent phenomenon can even be observed when each pulse within the double-pulse pair has a sub-threshold energy, which leads to a further reduction in ablation size. The experimental findings have not only confirmed our previous calculation based on a modified model, but also greatly extended the results both quantitatively and qualitatively. A dominant amount of seed electron from photoionization of self-trapped excitons (STEs) is responsible for the appearance of τs-independent phenomena. For physical interest, it is inferred that destruction of STEs will tend to break the τs-independent ablation phenomena. Experiments performed on CdWO4, a material exhibiting similar electron dynamics to that in SiO2 but a faster decay in STE population, support this conjecture. A possible improvement for the relevant theoretical modeling is also suggested based on the experimental findings.
KEYWORDS: Holograms, Computer generated holography, Modulation, 3D displays, 3D image processing, Holography, Phase shift keying, 3D image reconstruction, Spatial frequencies, Signal to noise ratio
The real-time holographic display encounters heavy computational load of computer-generated holograms and precisely intensity modulation of 3D images reconstructed by phase-only holograms. In this study, we demonstrate a method for reducing memory usage and modulating the intensity in 3D holographic display. The proposed method can eliminate the redundant information of holograms by employing the non-uniform sampling technique. By combining with the novel look-up table method, 70% reduction in the storage amount can be reached. The gray-scale modulation of 3D images reconstructed by phase-only holograms can be extended either. We perform both numerical simulations and optical experiments to verify the practicability of this method, and the results match well with each other. It is believed that the proposed method can be used in 3D dynamic holographic display and design of the diffractive phase elements.
In recent years, real-time three-dimensional (3D) holographic display has attracted more and more attentions. Since a
holographic display can entirely reconstruct the wavefront of an actual 3D scene, it can provide all the depth cues for
human eye’s observation and perception, and it is believed to be the most promising technology for future 3D display.
However, there are several unsolved basic problems for realizing large-size real-time 3D holographic display with a wide
field of view. For examples, commercial pixelated spatial light modulators (SLM) always lead to zero-order intensity
distortion; 3D holographic display needs a huge number of sampling points for the actual objects or scenes, resulting in
enormous computational time; The size and the viewing zone of the reconstructed 3D optical image are limited by the
space bandwidth product of the SLM; Noise from the coherent light source as well as from the system severely degrades
the quality of the 3D image; and so on. Our work is focused on these basic problems, and some initial results are
presented, including a technique derived theoretically and verified experimentally to eliminate the zero-order beam
caused by a pixelated phase-only SLM; a method to enlarge the reconstructed 3D image and shorten the reconstruction
distance using a concave reflecting mirror; and several algorithms to speed up the calculation of computer generated
holograms (CGH) for the display.
We propose a method to enlarge the 3D image in holographic display using a concave reflecting mirror
based on the optical reversibility theorem. The holograms are computed using the look-up table (LUT)
method, and the common data of the 3D objects are compressed to reduce the memory usage of LUT.
Optical experiments are performed and the results show that 3D image can be magnified without any
distortion in a shortened image distance, and the memory usage of LUT is reduced.
Keywords: computer holography; holographic display; magnification of 3D image size; distortion of
the image; compensation of the distortion.
KEYWORDS: 3D image reconstruction, 3D image processing, Distortion, Spatial light modulators, Holography, Image quality, Mirrors, Holograms, 3D displays, 3D modeling
We propose a simple technique to enlarge the reconstructed three-dimensional (3D) optical image and shorten the reconstructed distance simultaneously in real time holographic projection using a conventional lens or concave reflecting mirror based on the optical reversibility theorem. The main factors causing the longitudinal and transversal distortions of a 3D enlarged optical image are analyzed, and the 3D optical images are enlarged where severe distortions are precompensated by constructing objects with distortions directly instead of computing the precompensated phase iteratively so that it does not increase the computing time. Numerical simulations and optical experiments are performed for magnifying a simple cubic model. The results show that a 3D enlarged optical image is achieved successfully without any distortion and the reconstructed distance is shortened simultaneously. It is believed that this proposed technique is useful for 3D real time holographic projection in the future.
A multiscale model is developed to study to the femtosecond laser single pulse and pulse train processing of the metal
films. In our model, molecular dynamics simulation combined with the improved two-temperature model is employed in
the ablation area and the improved two-temperature model is applied in heat-affected zone. This paper extends the
improved two-temperature model to describe higher laser fluences processing by introducing the phase change. The
phase change mechanisms of the non-equilibrium thermal melting and vaporization are both analyzed, which has a
strong impact on the lattice temperature evaluation. The model can simulate phase change process of gold with higher
accuracy. It is demonstrated that the pulse train could improve the fabrication accuracy, repeatability, and controllability.
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