The ultrafast photocurrents in a typical 3D topological insulator (TI) Sb2Te3 were probed with THz emission spectroscopy by tuning the polarization of excitation pulses and the sample’s azimuthal angle. The different ultrafast photocurrents driven by the linear photogalvanic effect, circular photogalvanic effect, and thermoelectric effect were distinguished from THz emission signal. Results potentially provide insights into the ultrafast all optical modulation of THz emission with non-external bias field, which could play a vital role in future TI-based high-speed THz optoelectronic and optospintronic devices.
A grating interferometer is presented based on the quasi-Littrow configuration. We mainly use a plane mirror to make the measuring light reflect and diffract between the mirror and grating scale for several times. According to the grating Doppler shift, the more times that measuring light diffracted, the higher optical subdivision can be obtained. As an example, a grating interferometer with an optical subdivision factor of 1/12 is designed. This work provides a technique to increase the resolution of the grating interferometer, which should be interesting for high precision measurement.
Femtosecond laser technology is one of the frontiers in the fields of nonlinear optics, advanced manufacturing etc. The method of femtosecond pulse compression is an important research content. To compress the femtosecond pulse, we need to use negative dispersive elements to compensate the positive dispersion of Ti: sapphire crystal and other optical elements. For this purpose, we propose a miniature double-line-density grating pair in which the line density of the second grating is twice of the first one, the output pulse propagates along the way back. The density of the gratings is high, which will have a high diffraction efficiency and can compensate a high GVD ( group velocity dispersion ) in a small distance. The first grating is transmitted and the second one is reflective, the device will not occlude the beam propagation. With the pair of the gratings, the input positive chirped 89fs pulse is neatly compressed into the Fourier transform limited 44fs pulse with no spectral spatial walkoff and dispersion. It can be used for compression in laser cavity or out of the cavity. The gratings are easy to adjust and the structure is simple and compact, which has widespread interest in practical applications.
Based on anisotropic diffraction of photorefractive LiNbO3 crystal, we theoretically discuss an optical WDM scheme implemented by a single volume grating. Using only one grating recorded by two He-Ne laser beams at a specific wavenumber, we can realize wavelength
demultiplexing around telecommunication wavelength. The possible wavelength range suitable for WDM is discussed, which is in the range from 1081nm to 1217nm. Our theoretical scheme can be used in the design of a practical WDM.
Based on the electro-optic and piezoelectric effects, principle of a 2×2 bypass-exchange switch in photorefractive
LiNbO3 crystal is discussed. In photorefractive volume grating, the Bragg condition can be deviated by applying a
specific field during readout. That is, by applying a specific field Es or zero field, the diffraction efficiency will be 0 or 1, and this property can be used to realize the exchange or bypass operation of the switch. In this paper, on considering both
the piezoelectric and the electro-optic effects caused by the applied electric field, we analyzed relations of both the
specific field and the incident intensity ratio with respect to the writing angles and polarizations of writing beams.
Based on the coupled wave theory of Kogelnik, variations of the diffracted and transmitted beams with time are theoretically studied when PR hologram is illuminated by femtosecond pulse. We find that the waveforms of the diffracted and transmitted pulses depend on the input pulse duration, grating spacing, crystal thickness and modulation of the refractive index. By changing values of these parameters, the diffracted and transmitted pulse waveforms can be controlled to satisfy our desire. We also find that the diffracted and transmitted pulses have a displacement along the t-axis comparing to that of the input pulse. Conclusions of our study can be used in many areas, such as pulse shaping and signal processing.
This paper deals with the microstructure of the generated crystals in borate glass by femtosecond laser irradiation, Raman spectroscopy was used to study the distribution of the high temperature and low temperature phases of barium metaborate crystals produced in the borate glass, and the mechanism was discussed.
This paper summarized our developments on the integration of miniaturized optical 3D systems using crystal substrates, which include a birefringence-customized stacking technique with the building blocks of double refraction and a photorefractive integration technique with the photorefractive local holograms thermally fixed by CO2 laser heating. The principles are introduced and the corresponding configurations are given, and the examples of various integrated digital optical computing systems such as logic processor, morphological image processor, and interconnection networks are shown.
The Banyan network belongs to the class of log, (N) stage networks like the perfect shuffle and the crossover networks. In our paper, an optical scheme for constructing the Banyan network in one single -block of photorefractive LiNbO, crystal is suggested. The photorefractive crystal is used to record layered holograms according to the structure of link stage of the network and repeatedly pulsed CO, laser beam is suggested to thermally fix the local layer holograms. Programmable electro -optic modulator array is selected as switches to implement the dynamic space- variant interconnect and the reflective elements as the cascading medium to implement the Banyan network in one single block LiNbO, crystal. Because the whole network can be implemented with one single block crystal and simple reflective elements, it has the properties of compactness and miniaturization.
A scheme using CO2 laser beam to thermally fix selective layers or lumps of photorefractive holograms to integrate and miniaturize an optical 3D system into a single block of LiNbO3 crystal is suggested. Theoretical and experimental investigations on laser local thermal fixing are given, and a stage of the modified gamma network is experimentally demonstrated.
A scheme of using carbon-dioxide laser beams to thermally fix layers of photorefractive holograms to implement a solid-state optical system in a single block of LiNbO3 crystal is suggested. In this paper, a modified gamma network is adapted to demonstrate, and a stage of such a network is constructed in a block of crystal. To limit the heating region with the shape similar to the heating beam, two carbon-dioxide laser beams of narrow width are used to laterally illuminate a layer of crystal, and additional absorbers are attached on the crystal surfaces outside the heating region to guide the heating flows. Analytical discussion based on the heat- transfer equations and the proof-of-principle experimental result are given. It can be thus seen that it is possible to package an optical system into a miniaturized single-block of photorefractive crystal.
This paper proposes a novel pattern recognition methodology based on morphological transforms and genetic algorithms. An entropy function is defined to demonstrate the match degree between two functions used in genetic algorithms. Based on morphological transforms and genetic algorithms, an optimal and adaptive set of structure elements as shape discrimination operators is developed by training patterns, moreover the string of variable structure elements is utilized to encode an image and construct the DNA of the image that maps arbitrary shapes into intrinsic and compact image features. Comparing the DNA string of the image with those of stored patterns, we can implement pattern recognition and classify an image. Then an optoelectronic pattern recognition architecture based on the algorithm is shown.
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