Bulk heating of transparent materials using a high-repetition-rate femtosecond laser

Femtosecond laser pulses can locally induce structural and chemical changes in the bulk of transparent materials, opening the door to the three-dimensional fabrication of optical devices. We review the laser and focusing parameters that have been applied to induce these changes and discuss the different physical mechanisms that play a role in forming them. We then describe a new technique for inducing refractive-index changes in bulk material using a high-repetition-rate femtosecond oscillator. The changes are caused by a localized melting of the material, which results from an accumulation of thermal energy due to nonlinear absorption of the high-repetition-rate train of laser pulses. Received: 21 November 2001 / Accepted: 9 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +1-858/534-7697, E-mail: cschaffer@ucsd.edu RID="**" ID="**"Current address: University of California, San Diego, Department of Physics, La Jolla, CA 92 093, USA     

Coupling of focused laser pulse to surfaces of transparent materials studied by time-resolved imaging technique

Short and intense laser pulse can process the surface and the inside of transparent materials by focusing the pulse at the desired position. Here we report the interaction of fundamental radiation (1064 nm) of the Q-switched Nd:YAG laser to the surface of PMMA as observed by an imaging system with nanosecond time resolution. The system used fundamental radiation of a Q-switched Nd:YAG laser as a processing laser and second harmonic radiation (532 nm) of another Nd:YAG laser as illuminating light. We observed shock waves which propagate into the material and into the atmosphere by shadowgraph and photoelastic method. Surface roughness of a sample is expected to affect the coupling of light and transparent materials for both normal and focused laser light. Our results have revealed the effects visually. For roughness larger than 0.6 m, all energy is absorbed at the surface, while the larger part of the energy is absorbed inside the material as the surface becomes smoother. PACS 52.38.MF; 79.20.DS; 87.63.Lk     

Processing of transparent materials using visible nanosecond laser pulses

Laser micromachining of transparent materials is an intensively studied research area from the point of view of microoptical element fabrication. One of the most promising indirect processing methods is the laser-induced back-side dry etching (LIBDE). During this method, transparent targets are contacted with solid thin layers, which absorb and transform the pulse energy resulting in etching. The applicability of LIBDE technology for processing of fused silica using a visible nanosecond dye laser (λ=500 nm, FWHM=11 ns) and a 100-nm-thick aluminium absorbing layer was investigated. The applied fluence was varied in the range of 0–3050 mJ/cm²; the illuminated area was 0.1 mm². The threshold fluence of the LIBDE etching of fused silica was found to be approximately 540 mJ/cm². The chemical composition of the surface layers on and around the etched holes was investigated by field-emission scanning electron microscopy and energy-dispersive X-ray spectrometry. It was found that on average 0.4±0.3 at. % aluminium is built into the upper ∼1-μm-thick volume of the illuminated fused silica, while the aluminium content fell below the detection limit in the case of the original surface. Our experiments proved that the LIBDE procedure is suitable for microprocessing of transparent materials using visible nanosecond laser light. PACS 42.62.-b; 61.80.Ba; 81.16.Rf; 81.65.Cf     

Rapid microfabrication of transparent materials using filamented femtosecond laser pulses

Microfabrication of transparent materials using femtosecond laser pulses has showed good potential towards industrial application. Maintaining pulse energies exceeding the critical self-focusing threshold by more than 100-fold produced filaments that were used for micromachining purposes. This article demonstrates two different micromachining techniques using femtosecond filaments generated in different transparent media (water and glass). The stated micromachining techniques are cutting and welding of transparent samples. In addition, cutting and drilling experiments were backed by theoretical modelling giving a deeper insight into the whole process. We demonstrate cut-out holes in soda-lime glass having thickness up to 1 mm and aspect ratios close to 20, moreover, the fabrication time is of the order of tens of seconds, in addition, grooves and holes were fabricated in hardened 1.1 mm thick glass (Corning $^\circledR$ Gorilla $^\circledR$ glass). Glass welding was made possible and welded samples were achieved after several seconds of laser fabrication.     

Ultra-short pulse laser interaction with transparent dielectrics

The interaction of intense, ultra-short laser pulses (USLP) with a surface of transparent dielectrics is considered. The combination of multi-photon absorption and impact ionization generates a plasma layer at the dielectric boundary. Interaction with the plasma self-consistently determines the amount of reflected, transmitted and absorbed light, and the spatial distribution of electron density. In the present paper, we model the interaction of USLP with transparent dielectrics. We calculate the evolution of electron density profiles and the variation of reflection, transmission and absorption of laser radiation during the pulse. We show that the laser-created surface plasma acts as a filter transmitting only the leading edge of the laser pulse. The transmitted energy is approximately fixed, nearly independent of input pulse energy. The transmitted energy increases with pulse duration. This increased energy is manifested in the formation of cylindrical shock waves directly applicable to recent experiments investigating absorption and shock generation in water. PACS 79.20.Ds; 81.15.Fg; 05.45.Pg     

Ultraviolet conical emission produced by high-power femtosecond laser pulse in transparent media

In this work, supercontinuum conical emission (SC CE) accompanying the filamentation of powerful ultrashort laser pulse in BK₇ glass and fused silica is studied. The SC CE is controlled by the laser power density and the sample thickness, and the minimum SC CE cut-off wavelength is about 309?nm in the BK₇ glass and 237?nm in the fused silica. The angular distributions of the SC CE in the wavelength range less than 510?nm are measured by using a new method, and it cannot be explained by the Cerenkov emission theory but the unabridged X-Waves solution theory. Meanwhile numerical simulations of the propagation of femtosecond laser pulse in sample are performed to provide theoretical support to our results.     

Determination of nonlinear optical characteristics of transparent materials using single laser pulses

A new method of express analysis of nonlinear optical properties of materials by determining their nonlinear refraction and nonlinear absorption upon irradiation by a single laser pulse is proposed. The nonlinear optical characteristics of media are determined based on an analysis of variations in the spatial distribution of radiation focused by a cylindrical lens and passed through the medium. The method proposed is tested by the example of optical nonlinearities of CS₂.     

Machining of transparent materials using an IR and UV nanosecond pulsed laser

Channels are traditionally machined in materials by drilling from the front side into the bulk. The processing rate can be increased by two orders of magnitude for transparent materials by growing the channel from the rear side. The process is demonstrated using nanosecond laser pulses to drill millimeter-sized channels through thick silica windows. Absorbing defects are introduced onto the rear surface to initiate the coupling of energy into the material. Laser drilling then takes place when the fluence exceeds a threshold. The drilling rate increases linearly with fluence above this threshold. While UV light drills about four times faster than IR light, the pulse length (in the nanosecond regime) and the pulse repetition rate (in the 0.1–10 Hz range) do not greatly influence the drilling rate per pulse. Drilling rates in excess of 100 μm per pulse are achieved by taking advantage of the propagation characteristics of the plasma created at the drilling front. The plasma during rear-side drilling generates a laser-supported detonation wave into the bulk material. The geometry also seems to increase the efficiency of the laser-induced plasma combustion and shock wave during the pulse by confining it in front of the channel tip. Received: 1 July 1999 / Accepted: 17 April 2000 / Published online: 20 September 2000     

Picosecond laser pulse correlation by using a molecular dye

Summary We present a new, simple and cost effective scheme for the measure of the second-order correlation function of Nd-YAG laser second-harmonic (532 nm) pulses. We measure the transmission of a probe beam in a cell containing 6G-Rhodamine solution bleached by a counterpropagating pump beam. Under a convenient choice of experimental parameters, we can measure also the contribution of third-order correlation function.     

透明光学材料中缺陷吸收激光能量引起的热应力与断裂

利用热弹性理论分析了在光学材料中由于缺陷吸收激光能量引起的温度和热应力分布,并且针对一个简单的裂纹模型分析了热应力产生的应力强度因子,并且给出了一些主要参数对于应力强度因子的影响的规律。     

透明光学材料中缺陷吸收激光能量引起的热应力与断裂

 利用热弹性理论分析了在光学材料中由于缺陷吸收激光能量引起的温度和热应力分布，并且针对一个简单的裂纹模型分析了热应力产生的应力强度因子，并且给出了一些主要参数对于应力强度因子的影响的规律。     

飞秒激光与透明介质的相互作用

随着飞秒激光技术的不断成熟,飞秒激光器不但在实验室能产生小于10fs的光脉冲,啁啾放大后的飞秒光脉冲的聚焦峰值功率密度可达到10的21次方W/cm2以上,而且飞秒激光系统已实现全固体、小型化结构,其稳定性和可靠性大大提高,因此在科学技术研究中的应用越来越广.文章重点介绍飞秒激光的主要特性和它与透明介质［如熔融石英、光学玻璃、对激光透明的高分子聚合物(PMMA)等］的相互作用过程,分析它们之间的非线性相互作用过程引起的材料特性或结构变化的物理机制和可能的应用,尤其在高密度大容量三维存储和微光子器件制造等方面的应用可能性.     

Three-dimensional optical data storage using a solid immersion lens to focus a femtosecond laser pulse

A solid immersion lens (SIL) has been applied to the writing and reading of three-dimensional optical datastorage in transparent materials. Using a SIL with n=1.516 to focus a 150-fs, 800-nm Ti:sapphire laser,the 5-layer reading and writing of data are achieved in fused silica and polyethylene methacrylate at adensity of 1.1× 1012b/cm3. Some advantages of the employment of SIL have been discussed.     

High-density all-optical magnetic recording using a high-NA lens illuminated by circularly polarized pulse lights

We propose a method for high-density all-optical magnetic recording. Our analyses, based on the vector diffraction theory, show that owing to the inverse Faraday effect, circularly polarized laser pulses focused by a high numerical aperture (NA) lens can induce a small magnetization domain. For an example, the FWHM of the effective magnetization domain is 0.4646λ when NA=0.85. The magnetization direction is basically perpendicular to the surface of the optic-magneto film within the effective magnetization domain and the switching direction of magnetization can be controlled by the helicity of the incident circularly polarized light. These characteristics are useful to next-generation high-density all-optical magnetic storage.     

Fabrication of surface relief gratings on transparent dielectric materials by two-beam holographic method using infrared femtosecond laser pulses

Fabrication of surface relief-type gratings in transparent dielectrics, which are hard to machine, has been achieved by a holographic technique using two infrared femtosecond (fs) pulses from a mode-locked Ti:sapphire laser. The present method can be applied for a variety of transparent dielectrics, Al₂O₃ (sapphire), TiO₂, ZrO₂, LiNbO₃, SiC, ZnO, CdF₂, MgO, CaF₂ crystals, and SiO₂ glass. It is found that the grating formation is due primarily to laser ablation processes. Planar surface relief gratings can be fabricated by colliding two fs laser pulses on the surface of substrates which move at a constant speed, synchronized with the laser repetition rate. Received: 1 March 2000 / Published online: 7 June 2000     

Systematic study of highly efficient white light generation in transparent materials using intense femtosecond laser pulses

We report the results of a systematic study of white light generation in different high band-gap optical media (BaF₂, acrylic, water and BK-7 glass) using ultrashort (45 fs) laser pulses. We have investigated the influence of different parameters, such as focal position of the incident laser light within the medium, the polarization state of the incident laser radiation and the pulse duration of the incident laser beam on the white light generation. Our results indicate that for intense, ultrashort pulses, the position of physical focus inside the media is crucial in the generation, with high efficiency, of white light spectra over the wavelength range 400–1100 nm. Linearly polarized incident laser light generates white light with higher intensity in the blue region than circularly polarized light. Ultrashort (45 fs) pulses generate a flatter spectrum with higher white light conversion efficiency than longer (300 fs) pulses of the same laser power. We believe that a flat response over a wide range of wavelengths in the continuum may be efficiently compressed for generation of sub-10 fs pulses. PACS 52.38.Hb; 42.65.Jx; 42.65.Tg; 33.80.Wz; 52.35.Mw     

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.     

Efficiency factors of scattering,absorption, and backscattering of a femtosecond laser pulse train by a transparent spherical particle

Analytical expressions are derived for integral optical characteristics (the scattering, absorption, and backscattering efficiency factors) of aerosol particles irradiated by a femtosecond laser pulse train. The behavior of these integral characteristics attendant to variations of the particle size and pulse period-to-pulse duration ratio is investigated based on the results of numerical calculations. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 33–36, March, 2008.