固体浸没透镜用于飞秒三维光存储研究

为进一步提高光存储密度，利用固体侵没透镜（SIL）与数值孔径为0.55的长工作距离物镜对飞秒激光脉冲进行聚焦，完成了PMMA及石英介质上的存储实验,并对聚焦物镜焦点与SIL底面离焦时的介质内焦点位置和系统的数值孔径进行了模拟。实验结果表明：当聚焦物镜焦点与SIL底面适当离焦时，实际聚焦在介质内的焦点深度不断加深，且系统的有效数值孔径不断增大。利用这一结果，在距PMMA表面20μm的地方得到了点间距1μm，层间距2.5μm的6层空间点阵；在距石英介质表面15μm的地方获得了点间距为0.6μm，层间距为2.5μm的5层空间点阵，其存储密度可达1.1×1012bits/cm3。     

Experimental research on high density data storage using solid immersion lens

We have set up a solid immersion lens (SIL) near-field static recording system for demonstrating preliminarily the feasibility of SIL technology in the higher density storage. The experimental result with recording mark size of 240 nm is obtained corresponding to a potential of density of tens of gigabits per square inch. Some factors in the SIL near-field recording are discussed.     

Three-dimensional optical storage in fused silica using modulated femtosecond pulses

Three-dimensional bitwise optical recording with a density of 500 Gb/cm3 in fused silica using a Ti:sapphire femtosecond laser modulated by binary digits is demonstrated. Laser pulses modulation is realized by modulating two circuits of trigger pulses signal which are used to control laser pulses trapping and switching out from cavity, respectively. Bits are optically readout in both a parallel reading (phase-contrast) and a serial reading (confocal-type) methods. The method for modulating laser pulses can also be used in all of pulsed laser systems which operate in cavity-dumping configuration.     

Three-zone phase-only filter increasing the focal depth of optical storage systems with a solid immersion lens

The field distribution of a Weierstrass solid immersion lens (SIL) is presented, which shows that its focal depth is only in nanoscale. A novel filter design based on a three-zone binary phase pupil function is introduced for increasing the focal depth of a near-field Weierstrass SIL system and a procedure for designing a rotationally symmetric binary phase filter is presented. Numerical results show that an appropriate three-zone phase-only filter not only can evidently increase the focal depth of the SIL system, but also can effectively lower the spreading of the spot size with distance and suppress the side-lobe intensity. PACS 42.79.Ci; 42.40.Lx; 42.79.Vb; 42.25.-p     

固体浸没透镜光存储的信号探测技术

信号探测是固体浸没透镜成为实用存储技术的一个必需的组成部分。对在该项研究中获得的理论和实验成果进行了介绍。总结了针对固体浸没透镜机理的三维矢量理论 ,对影响信号探测的因素 ,例如头盘间距、盘片结构和光的偏振态等进行了讨论。介绍了基于固体浸没透镜光存储中信号探测的新技术和重要的实验结果。     

Design and fabrication of an optimum holographic optical element lens for a femtosecond laser pulse using a hologram computer-aided design tool

Design and fabrication of an optimum holographic optical element (HOE) lens for a femtosecond laser pulse using a hologram computer-aided design (CAD) tool is presented. The hologram CAD tool, which the authors have developed can design, analyze, and evaluate holograms fabricated by interferometrical technique. The function of the tool is extended to design and analyze a HOE lens illuminated with a femtosecond laser pulse. An optimum HOE lens for a laser pulse, which has 130 fs duration, 720 nm central wavelength, and 10 nm spectrum bandwidth, is designed by the tool. The optimum HOE lens gives both high diffraction efficiency and small amount of aberration. The designed HOE lens is fabricated and its optical characteristics have been experimentally evaluated. The reconstructed point images agree with the results of the numerical simulations by the tool. The tool demonstrates the capability of designing the optimum HOE lens for a femtosecond laser pulse.     

Improving the resolution of a solid immersion lens optical system using a multiphase Fresnel zone plate

We propose a method to improve the resolution of near-field optical system with a solid immersion lens by using a multiphase level Fresnel zone plate. The analyses are based on scalar angular spectrum theory. The results show that the multiphase Fresnel zone plate can not only decrease the spot size but also decrease the sidelobe intensity and enhance greatly the diffractive efficiency compared with annular amplitude filter or binary 2-, 3-, 4-zone phase filter.     

Dispersion effect in optical microscopy systems with a supersphere solid immersion lens

This paper studies the dispersion effect of the supersphere solid immersion lens (SIL) on a near field optical microscopy system by using the vector diffraction theory. Results show that when a real non-monochromatic beam illuminates a supersphere SIL microscopy, the dispersion effect of the SIL has an important influence on the image quality. As the wavelength bandwidth of the non-monochromatic beam increases, the size of the focused spot increases and its intensity decreases in near-field microscopy systems with a supersphere SIL.     

Evanescent imaging with induced polarization by using a solid immersion lens

Image contrast enhancement, high lateral resolution, and height information are obtained with induced polarization evanescent imaging using a solid immersion lens. Experiments are conducted by imaging features on a patterned Si substrate. Imaging theory is used to predict optimum orientation of high-spatial-frequency samples, and a topographical image is derived from the induced polarization image through a calibration procedure. A numerical aperture of 1.5 is used in the experiment. Height accuracy of +/-2 nm is demonstrated with a known sample.     

High-order optical vortex harmonics generated by relativistic femtosecond laser pulse

Harmonics generated by an intense femtosecond Gaussian laser pulse normally incident on a spiral-shaped thin foil target were studied.By using two-dimensional(2D) particle-in-cell(PIC) simulation,we observed evident odd harmonics signals in the reflection direction and found that the reflected field has a helical structure determined by the target shape.This method provides a new way to generate an intense ultraviolet vortex with high-order topological charge by use of ultrahigh intense laser-driven harmonics.     

Controllable photoinduced optical attenuation in a single-mode optical fiber by irradiation of a femtosecond pulse laser

Novel optical attenuation fibers were fabricated by the irradiation of a focused infrared femtosecond pulsed laser onto the core of a silica glass single-mode optical fiber. Optical attenuation at a wavelength of 1.55 microm proportionally increased with increasing numbers of irradiation points and was controllable under laser irradiation conditions. The single-mode property of the waveguide and the mode-field diameter of the optical fiber were maintained after irradiation of the femtosecond laser. It is suggested that the attenuation results from optical scattering at photoinduced spots formed inside the fiber core.     

Three-dimensional optical data storage using third-harmonic generation in silver zinc phosphate glass

We demonstrate the possibility of three-dimensional optical data storage inside a specific zinc phosphate glass containing silver by using third-harmonic generation (THG) imaging. Information is stored inside the glass with femtosecond laser irradiation below the refractive index modification threshold. We use the same laser for THG readout. The capability of storage with this technique is discussed.     

All optical electron injector using an intense ultrashort pulse laser and a solid wire target

Energetic electron bunches were generated by irradiating a solid tungsten wire 13 μm wide with 50 femtosecond pulses at an intensity of ∼3×10¹⁸ W/cm². The electron yield, energy spectrum and angular distribution were measured. These energetic electron bunches are suitable for injection into a laser driven plasma accelerator. An all-optical electron injector based on this approach could simplify timing and alignment in future laser-plasma accelerator experiments. PACS 41.75.Ht; 41.75.Lx; 52.38.Kd; 52.38.Ph     

Shortening of a laser pulse with a self-modulated phase at the focus of a lens

We found that, at the focus of a chromatic lens, a laser pulse with a self-modulated phase can be shortened due to the radial dependence of the group delay imposed by the lens. Normally, this group delay stretches a short pulse into a long pulse by spreading the arrival time of the pulse at the focus. However, for a pulse with a self-modulated phase, it causes the fields with different phases to overlap, thus resulting in destructive interference that shortens the pulse.     

Self-transformation of a powerful femtosecond laser pulse into a white-light laser pulse in bulk optical media (or supercontinuum generation)

We present experimental and theoretical results on white-light generation in the filamentation of a high-power femtosecond laser pulse in water and atmospheric air. We have shown that the high spatio-temporal localization of the light field in the filament, which enables the supercontinuum generation, is sustained due to the dynamic transformation of the light field on the whole transverse scale of the beam, including its edges. We found that the sources of the supercontinuum blue wing are in the rings, surrounding the filament, as well as at the back of the pulse, where shock-wave formation enhanced by self-steepening takes place. We report on the first observation and demonstration of the interference of the supercontinuum spectral components arising in the course of multiple filamentation in a terawatt laser pulse. We demonstrate that the conversion efficiency of an initially narrow laser pulse spectrum into the supercontinuum depends on the length of the filament with high intensity gradients and can be increased by introducing an initial chirp. PACS 42.65.Jx; 42.65.Re; 42.25.Bs; 42.50.Hz     

Four-wave parametric conversion of femtosecond laser pulse in a filament induced in a solid target

Nonlinear interaction of two femtosecond laser pulses in a filament, induced by one of them inside a fused silica plate, leads to generation of the new spectral components. These spectral components reach hundreds of nanometers bandwidth. Their spatial and spectral properties can be explained by four-wave parametric coupling in the filament. The energy measurements indicate the high efficiency of this process.     

Fiber chromatic dispersion measurement based on wavelength-to-time mapping using a femtosecond pulse laser and an optical comb filter

A novel method for the measurement of chromatic dispersion of an optical fiber based on wavelength-to-time mapping using a femtosecond pulse laser (FSPL) and an optical comb filter is proposed and experimentally evaluated. In the proposed approach, the spectrum of an ultrashort optical pulse generated by an FSPL is sliced by an optical comb filter. The spectrum-sliced optical pulse is then coupled into the optical fiber under test. Thanks to the chromatic-dispersion-induced wavelength-to-time mapping in the optical fiber under test, a time-domain waveform similar to the sliced spectrum is generated at the output of the optical fiber, with different frequency components having different time delays. The time delay vs. frequency data are then recorded for the estimation of the chromatic dispersion by using least square fitting. Chromatic dispersions of two types of optical fibers with different lengths are tested. The measured dispersion values agree well with those measured by the conventional modulation phase shift (MPS) method.     

Optimizing the optical field distribution of solid immersion lens system by a continuous phase filter

In solid immersion lens (SIL) microscopy systems with high numerical aperture (NA), there always exists the aberration produced by Fresnel effects at the interface between SIL and the sample. This aberration may cause the degradation of the image of sample. We design a continuous phase filter and optimize the optical field distribution of SIL system. The numerical results show that when the continuous phase filter is used, the field distribution of SIL system can be optimized, and the focal depth and intensity of transmitted light can be increased. At the same time, the intensity of side-lobe and the resolution are kept almost unchanged.     

Laser drilling of thick material using femtosecond pulse with a focus of dual-frequency beam

Laser drilling is one of the basic, most frequently performed, material removal processes. The drilling aspect ratio is theoretically limited by the size and the focal depth of the machining laser spot. The aspect ratio can be improved by using dual focus. In this paper we describe a focus of two different frequencies based on the longitudinal chromatic aberration arisen when polychromatic collimated light is incident on a positive lens element. In the experiments, a Ti:Sapphire laser of 800 nm wavelength and 150 fs pulse duration was used as a source. Two tightly focused laser spots few hundred micrometers apart from each other were formed by focusing a combined collimated laser beam which contains the fundamental optic frequency and the second harmonic optic frequency. The focus of dual-frequency beam was used to drill a 3 mm thick PMMA plate. The drilling aspect ratio of a dual-frequency beam was compared to that of a focus of single frequency beam. Experimental results reveal that dual-frequency beam increases the aspect ratio and improves the drilling quality in terms of profile of the produced features.     

Refractive index measurement by spectrally resolved interferometry using a femtosecond pulse laser

We describe a measurement method of refractive indices by way of spectrally resolved interferometry using a femtosecond pulse laser. The method is dispersion insensitive and requires no prior precise knowledge of the geometrical thickness of the specimen. Not only the group but also the phase refractive index can be determined over the wide spectral range covered by the optical comb of the femtosecond pulse laser in use.