Xiaona Yan

Diffraction study of photorefractive hologram under ultrashort pulse readout

Summary Based on the diffraction efficiency equation of Kogelnik, the diffraction effect of a cw photorefractive hologram under ultrashort pulse illumination is theoretically analyzed. We find that the recorded grating has the function of filter to the incident pulse beam. To fully reconstruct the incident pulse, the crystal width must be smaller and grating period must be larger. In this paper we also discuss the total diffraction efficiency of the hologram, and find that the total diffraction efficiency is functions of the crystal length, pulse duration and the grating period. The conclusion of our paper can be used in ultrashort pulse shaping application.

Forced rotation of nanograting in glass by pulse-front tilted femtosecond laser direct writing.

Femtosecond pulse laser with tilted intensity front demonstrates the capability of rotating the writing of nanograting in glass in 3D space. Other than the light polarization, this phenomenon is also associated with the quill-writing effect, which depends on the correlation between the sample movement and the pulse front tilt. This is because a pondermotive force, perpendicular to the tilted intensity plane, can push the excited electron plasma forward towards the pulse front. This behavior further tilts the electrical field plane and eventually result in a forced rotation of nanograting in 3D space.

Femtosecond pulse shaping by modulating the refractive index modulation of volume holographic grating

Based on the modified Kogelniks coupled-wave theory, time- and frequency-domain diffractions of a femtosecond pulse from transmitted volume holographic gratings (VHGs) are theoretically studied. Results show that when the refractive index modulation of the VHG changes in a certain range, the number of temporal diffracted pulse will evolve from one to two, then to three, and this pulse number evolution is periodic. This particular phenomenon can be explained by diffraction intensity spectrum and the overmodulation effect of refractive index modulation of transmitted VHG. Moreover, we find centers of all temporal diffracted pulses translate along the negative time axis, and the translation is irrelevant to the refractive index modulations. We will use time delay of volume grating to give a reasonable explanation.

Quantum-interference and the concentration of Er3+ ion effect on left-handedness with zero absorption and large negative refractive index in Er3+:YAG crystal

The electromagnetically induced left-handedness with zero absorption and large negative refractive index was investigated in a solid Er3+:YAG crystal with a four-level system proposed for an atomic medium. It was found that the frequency region with simultaneous negative permittivity and negative permeability, the zero absorption intervals, and the maximum values of the negative refractive index can be adjusted by changing the signal field, the coherent field, as well as the concentration of Er3+ ion in crystal. It is shown that wider zero absorption intervals with a higher index of refraction can be easily obtained when the signal field is only off resonance. The slab fabricated by the left-handed solid medium Er3+:YAG crystal with zero absorption may be a practical candidate for designing perfect lenses.

Periodical energy oscillation and pulse splitting in sinusoidal volume holographic grating.

This paper presents dynamical diffraction properties of a femtosecond pulse in a sinusoidal volume holographic grating (VHG). By the modified coupled-wave equations of Kogelnik, we show that the diffraction of a femtosecond pulse on the VHG gives rise to periodical energy oscillation and pulse splitting. In the initial stage of diffraction, one diffracted pulse and one transmitted pulse emerge, and energy of the transmitted pulse periodically transfers to the diffracted pulse and vice versa. In the latter stage, both the diffracted and transmitted pulses split into two spatially separated pulses. One pair of transmitted and diffracted pulses propagates in the same direction and forms the output diffracted dual pulses of the VHG, and the other pair of pulses forms the output transmitted dual pulses. The pulse interval between each pair of dual pulses is in linearly proportional to the refractive index modulation and grating thickness. By the interference effect and group velocity difference we give explanations on the periodical energy oscillation and pulse splitting respectively.

Surface-enhanced Raman scattering in femtosecond laser-nanostructured Ag substrate

We demonstrate that a surface-enhanced Raman scattering (SERS) substrate could be directly fabricated on the surface of Ag film by femtosecond laser micromachining. According to the morphology observation by SEM, an amount of nanoparticles, nanoprotrusions, and nanospikes were found to form in the ablation region and the density and size distribution of these Ag nanoparticles depended possibly on the incident laser intensity. Additionally, a large area of nanostructured region was produced by fast line scanning, and an enhancement factor of ~105 was obtained in this region after the sample was soaked in the rhodamine 6G solution for 30 min.

Generation of individually modulated femtosecond pulse string by multilayer volume holographic gratings.

A scheme to generate individually modulated femtosecond pulse string by multilayer volume holographic grating (MVHG) is proposed. Based on Kogelniks coupled-wave theory and matrix optics, temporal and spectral expressions of diffracted field are given when a femtosecond pulse is diffracted by a MVHG. It is shown that the number of diffracted sub-pulses in the pulse string equals to the number of grating layers of the MVHG, peak intensity and duration of each diffracted sub-pulse depend on thickness of the corresponding grating layer, whereas pulse interval between adjacent sub-pulses is related to thickness of the corresponding buffer layer. Thus by modulating parameters of the MVHG, individually modulated femtosecond pulse string can be acquired. Based on Bragg selectivity of the volume grating and phase shift provided by the buffer layers, we give an explanation on these phenomena. The result is useful to design MVHG-based devices employed in optical communications, pulse shaping and processing.

Implementation of optical WDM by anisotropic Bragg diffraction in photorefractive LiNbO3 crystal

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.

Microstructure of the crystals generated in borate glass irradiated by femtosecond laser pulses

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 two phases of barium metaborate crystals produced.

Theory of variable-angle switch by anisotropic diffraction in photorefractive LiNbO3 crystal

Summary Based on the anisotropic diffraction of photorefractive LiNbO3 crystal, we present the theoretical result of a variable-angle switch. The deflection angle of the proposed switch can be changed by changing value of writing angles of the grating, and the result shows that the deflection angle can change in the range of −1° to +1°. The proposed theory can be used in switching design.