Holographic recording using two-photon-induced photopolymerization

Molecular excitation via the simultaneous absorption of two photons can lead to improved three-dimensional control of photochemical or photophysical processes due to the quadratic dependence of the absorption probability on the incident radiation intensity. This has lead to the development of improved three-dimensional fluorescence imaging, optical data storage, and microfabrication. The latter of these involves the fabrication of three-dimensional structures using a spatial variation in the incident intensity within a photopolymerizable resin. In the past, the translation of the focal plane of a tightly focused laser beam was used to induce localized photopolymerization and fabrication of three-dimensional structures. Here we report the first successful demonstration of large-area patterning via ultrafast holography-based two-photon polymerization of a commercially available optical resin and a large two-photon cross-section dye (AF380). This opens tremendous possibilities for the wide-spread use of two-photon absorption for the three-dimensional control of photoinduced processes. Received: 21 June 1999 / Accepted: 23 June 1999 / Published online: 8 September 1999     

Application of Bessel beams to 2D microfabrication

Fs laser-based two-photon polymerisation (2PP) has been widely reported as a means of directly writing three-dimensional nanoscale structures. Usually the voxel of a high numerical aperture microscope objective is scanned through the resin to build up the required model. In the case of high aspect ratio two-dimensional structures, such as cell scaffolds, repeated scanning is required to build up the height.The voxel shape can be substantially elongated by the inclusion of an axicon lens in the laser beam line. In this report we describe the use of a Bessel beam produced in the region beyond the focus of an objective lens when the beam has been modified in this way.A Ti:sapphire laser was used to write a range of 2D square cell structures in a Zr-loaded sol-gel system. The process was characterised, in terms of the dimensions of the polymerised Bessel region, for different processing conditions. Examples of the structures are also described.     

Two-photon polymerisation of novel shapes using a conically diffracted femtosecond laser beam

Sub-micron ring, pillar and wall structures were written by two-photon polymerisation of a sol-gel resin using a femtosecond laser beam which was shaped using internal conical diffraction. The ring structure was written using a demagnified image of the ring-shaped beam which arises in conical diffraction of a narrow light beam. The pillar and wall structures were produced by imaging the Bessel beam formed by conical diffraction in combination with a converging lens.     

Two modes of laser lithography fabrication of three-dimensional submicrometer structures

The modes of laser lithography fabrication of three-dimensional submicrometer structures have been studied. The method is based on the effect of threshold two-photon polymerization of a photosensitive material at the laser beam focus. To determine the lithograph workspace in the coordinates “laser power-speed of the sample displacement with respect to the laser focus,” a series of photonic crystals with the woodpile structure is prepared. Two methods for fabricating three-dimensional structures, i.e., raster scanning and vector graphics (or the vector method) are analyzed in detail. The advantages of the vector method for fabricating periodic structures are demonstrated using crystals of inverted yablonovite as an example. The prepared samples are studied by scanning electron microscopy.     

Fabrication of grating structures by simultaneous multi-spot fs laser writing

Two-photon polymerisation is an established technique for the fabrication of three-dimensional microstructures. To date structures have mostly been developed using single beam serial writing. A novel approach to simultaneous multi-spot two-photon polymerisation, that uses a SiO₂ on glass Fraunhofer diffractive optical element to generate an array of beamlets, is described. A Ti:sapphire laser, with wavelength 790 nm, 80 MHz repetition rate, 100 fs pulse duration and an average power of 25 mW, was used to initiate two-photon polymerisation. The DOE, in combination with a high power microscope objective, efficiently transforms the laser beam into a linear array of four spots of equal intensity. The fabrication of a periodic transmission grating, using parallel processing with these four spots, is shown. The grating was written in a Zr-loaded resin prepared on a glass substrate using dip coating deposition of a Zr/PMMA hybrid prepared by the sol-gel method. The operation of the diffractive element and the performance of the diffraction grating are also discussed.     

超短脉冲激光在DBASVP分子中传播时的双光子面积演化和光限幅效应

以一维不对称π共轭分子体系（DBASVP分子）为介质,在双光子共振条件下,从双光子面积定理和严格数值求解Maxwell-Bloch方程两方面出发,分别研究超短脉冲激光在该有机分子介质中的传播过程,从而探讨双光子面积的演化规律,并分析双光子面积定理的适用性.提出了一种数值模拟分子介质光限幅特性的理论方法.分子的电子结构和电偶极矩是基于密度泛函理论利用从头计算方法得到的.研究结果表明,基于慢变幅和慢变相近似以及单模场条件下的双光子面积定理不能很好地描述超短脉冲的双光子面积在该分子介质中的演化规律.基于双光子吸收的分子光限幅特性与分子介质的厚度有关. 关键词： 双光子吸收 光限幅效应 双光子面积定理 超短脉冲激光     

Two-photon excited photoluminescence of photonic quantum ring laser structures

We report on the two-photon excited photoluminescence of photonic quantum ring laser structures using Ti:Sapphire femtosecond pulsed laser. Using two-photon excited photoluminescence microscopy, we were able to image the laser structures when optically excited and compare the results with previously obtained images on electrically pumped photonic quantum ring lasers. We also propose a method to evaluate the Rayleigh band on the circumference of these structures.     

Three-dimensional microfabrication by two-photon-initiated polymerization with a low-cost microlaser

Fabrication of submicrometer structures by two-photon-initiated polymerization is performed with an inexpensive and low-power microlaser. This is made possible by the design of photoinitiators with strong two-photon absorption cross sections. We analyze the influence of both material properties and irradiation conditions on the two-photon polymerization rate and show that resins based on our highly sensitive two-photon photoinitiator can be solidified with microlaser excitation, whereas commercial UV photoresins require ultrashort and intense laser pulses.     

Fabrication of three-dimensional photonic crystal structures containing an active nonlinear optical chromophore

Direct laser writing by two-photon polymerization of photosensitive materials has emerged as a very promising technique for rapid and flexible fabrication of photonic crystals. In this work, a photosensitive silica sol-gel containing the nonlinear optical chromophore Disperse Red 1 is synthesized, and the two-photon polymerization technique is employed to fabricate three-dimensional photonic crystals with stop-gaps in the near-infrared. The composite material exhibits minimal shrinkage during photopolymerization, eliminating the need for shrinkage compensation or the fabrication of support structures.     

双光子开关的自相关函数

本文推导出半导体双光子光电导开关的自相关函数,给出对照比与激光强度的关系.实验测出GaAs在1.06 μm Nd:YAG激光超窄脉冲作用下的双光子自相关函数.     

Single-shot two-photon exposure of commercial photoresist for the production of three-dimensional structures

We report the use of an amplified femtosecond laser for single-shot two-photon exposure of the commercial photoresist SU-8. By scanning of the focal volume through the interior of the resist, three-dimensional (3-D) structures are fabricated on a shot-by-shot basis. The 800-nm two-photon exposure and damage thresholds are 3.2 and 8.1TW/cm(2), respectively. The nonlinear nature of the two-photon process allows the production of features that are smaller than the diffraction limit. Preliminary results suggest that Ti:sapphire oscillators can achieve single-shot two-photon exposure with thresholds as low as 1.6TW/cm(2) at 700 nm, allowing 3-D structures to be constructed at megahertz repetition rates.     

Three-dimensional reversible laser micromachining with subnanojoule femtosecond pulses based on two-photon photochromism

Three-dimensional reversible laser micromachining of polymer materials based on two-photon photochromism is demonstrated. Unamplified 60-fs, 0.5-nJ pulses of 790-nm Ti:sapphire laser radiation are used to induce a refractive-index change in a polymethyl methacrylate (PMMA) sample doped with spiropyran molecules through a two-photon absorption process. Waveguides are written in the bulk of spiropyran-doped PMMA samples by scanning these samples with respect to a tightly focused Ti:sapphire laser beam. Laser-induced fluorescence is used for on-line monitoring of the laser-micromachining process. The structures written in photochromic samples can be erased and reconfigured due to the reversibility of the photochromic effect. Received: 23 April 2003 / Published online: 6 June 2003 RID="*" ID="*"Corresponding author. Fax: +7-095/939-51-74, E-mail: zheltikov@top.phys.msu.su     

Three-dimensional microfabrication with two-photon-absorbed photopolymerization

We propose a method for three-dimensional microfabrication with photopolymerization stimulated by two-photon absorption with a pulsed infrared laser. An experimental system for the microfabrication has been developed with a Ti:sapphire laser whose oscillating wavelength and pulse width are 790 nm and 200 fs, respectively. The usefulness of the proposed method has been verified by fabrication of several kinds of microstructure by use of a resin consisting of photoinitiators, urethane acrylate monomers, and urethane acrylate oligomers.     

Investigation of the two-photon polymerisation of a Zr-based inorganic-organic hybrid material system

Two-photon polymerisation of photo-sensitive materials allows the fabrication of three dimensional micro- and nano-structures for photonic, electronic and micro-system applications. However the usable process window and the applicability of this fabrication technique is significantly determined by the properties of the photo-sensitive material employed. In this study investigation of a custom inorganic-organic hybrid system, cross-linked by a two-photon induced process, is described. The material was produced by sol-gel synthesis using a silicon alkoxide species that also possessed methacrylate functionality. Stabilized zirconium alkoxide precursors were added to the precursor solution in order to reduce drying times and impart enhanced mechanical stability to deposited films. This enabled dry films to be used in the polymerisation process. A structural, optical and mechanical analysis of the optimised sol-gel material is presented. A Ti:sapphire laser with 80 MHz repetition rate, 100 fs pulse duration and 795 nm is used. The influence of both material system and laser processing parameters including: laser power, photo-initiator concentration and zirconium loading, on achievable micro-structure and size is presented.     

Glassy nanostructures fabricated by the direct laser writing method

This paper reports on the synthesis of glassy nanostructures in which the framework is a face-centered cubic lattice of inverse yablonovite with a disordered glassy superstructure. The synthesis has been performed by the direct laser writing method based on two-photon polymerization of a photosensitive material. The fabricated structures have been investigated using scanning electron microscopy. A theoretical calculation of the photonic band structures of the direct yablonovite and the inverse yablonovite has been carried out.     

A comparison between degenerate paramagnetic oscillators and two-photon correlated-spontaneous-emission lasers

The degenerate parametric oscillator (DPO) is analyzed with the nonlinear material treated quantum mechanically. The source of squeezing in the DPO is the same as in the two-photon correlated-spontaneous-emission laser (CEL) — the level coherence. In this sense, they are similar. The amount of squeezing in DPO is the same as in the two-photon CEL at two-photon resonance, but the gain in DPO is usually much smaller than that in the two-photon CEL at two-photon resonance.     

Two-photon absorption and emission dynamics of bulk GaAs

Bulk n-type GaAs (dopant silicon) is investigated with picosecond pump pulses of a passively mode-locked Nd:glass laser. The two-photon absorption, the spontaneous emission, and the longitudinal amplified spontaneous emission are measured. The experimental results are compared with computer simulations and relevant material parameters are determined.     

Pattern generation using axicon lens beam shaping in two-photon polymerisation

The fabrication of three-dimensional microstructures by two-photon polymerisation has been widely reported as a viable route to the development of photonic crystals, rotors, bridges and other complex artefacts requiring nanoscale resolution. Conventionally, single point serial writing is used to write the structures but recently multipoint beam delivery using beam division optics has been reported as a method of introducing parallel processing. In this paper we present an alternative and novel approach using an axicon lens to give profiled beam delivery. This enables complete three-dimensional annular structure fabrication without the use of scanning stages. In addition, the concept of axicon delivery is developed further to investigate three-dimensional structure as a function of axicon geometry.A Ti:sapphire laser, with wavelength 795 nm, 80 MHz repetition rate, 100 fs pulse duration and an average power of 700 mW, was used to initiate two-photon polymerisation. The axicon was used, in combination with a 100× microscope objective, to form representative three-dimensional structures based on the annular cell with varying diameter. The structures were written in a Zr-loaded resin prepared on a glass substrate using dip coating deposition of a Zr/PMMA hybrid prepared by the sol-gel method. Annuli with diameters up to 50 μm were characterised in terms of topography and surface roughness using SEM and Zygo interferometer. The writing technique was also extended to fabrication of stacked structures.     

Laser polymerization-based novel lift-off technique

The fabrication of microstructures by two-photon polymerization has been widely reported as a means of directly writing three-dimensional nanoscale structures. In the majority of cases a single point serial writing technique is used to form a polymer model. Single layer writing can also be used to fabricate two-dimensional patterns and we report an extension of this capability by using two-photon polymerization to form a template that can be used as a sacrificial layer for a novel lift-off process.A Ti:sapphire laser, with wavelength 795 nm, 80 MHz repetition rate, 100 fs pulse duration and an average power of 700 mW, was used to write 2D grid patterns with pitches of 0.8 and 1.0 μm in a urethane acrylate resin that was spun on to a lift-off base layer. This was overcoated with gold and the grid lifted away to leave an array of gold islands.The optical transmission properties of the gold arrays were measured and found to be in agreement with a rigorous coupled-wave analysis simulation.     

Two-photon absorption and two-photon induced absorption of liquid toluene at 347.15 nm

Pump pulse transmission and time-delayed probe pulse transmission measurements through liquid toluene were performed with linear and circular polarized second harmonic pulses of a mode-locked ruby laser system. Two-photon absorption and two-photon induced absorption are observed. The induced absorption anisotropy is investigated. A theory of two-photon absorption in isotropic media is presented. The two-photon absorption cross-section components, the effective excited-state absorption cross-section, the absorption anisotropy relaxation time and the excited singlet-state relaxation time are determined.