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We developed a high power storage ring oscillator using a Yb:YAG thin-disk gain medium pumped with 969nm absorption line. The intra-cavity average and peak power of 1kW and 200MW were attained, respectively at 14MHz operation. © 2020 The Author(s)

We present an energy-scaling experiment on a femtosecond infrared pulse using dual-chirped optical parametric amplification (DC-OPA). A total output energy exceeding 100 mJ with 36-fs pulse duration was achieved in the infrared region, which is the highest energy ever reported for an ultrafast optical parametric amplifier scheme. We also discuss the generation of high-energy 15–30–THz pulses through difference frequency generation between chirped signal and idler pulses based on DC-OPA. By manipulating the phase of a seed pulse in the DC-OPA system, the spectral phase of a THz pulse can be indirectly controlled precisely, which is very helpful for compressing THz pulses.

Employing a dual-chirped difference frequency generation (DC-DFG) method, we generate a source tunable from mid-infrared (3 μm) to THz (15 THz). DC-DFG, which is an energy scalable approach, enables us to generate few-mJ THz pulses.

In 2011, Dual-chirped optical parametric amplification (DC-OPA) [1] had been proposed theoretically as one of the OPA method. Y. Fu et al. experimentally demonstrated DC-OPA in 2015 and generated 10-mJ IR pulses energy in 1–2 um region [2, 3]. At present, DC-OPA method is ready to extend up to THz region [4].

We generate ultrabroadband pulses with spectrum ranging from 1-2.2 μm by optical parametric amplification in aBBO crystal pumped by a 712-nm tuned Ti:sapphire laser pulse. Monocycle pulses can be realized at the Fourier limit.

Recent efforts on high harmonic generation by using new pump laser technology, including double-chirped optical parametric amplification and high energy three-color laser synthesizer, are reported.

Femtosecond lasers have unique characteristics of ultrashort pulse width and extremely high peak intensity; however, one of the most important features of femtosecond laser processing is that strong absorption can be induced only at the focus position inside transparent materials due to nonlinear multiphoton absorption. This exclusive feature makes it possible to directly fabricate three-dimensional (3D) microfluidic devices in glass microchips by two methods: 3D internal modification using direct femtosecond laser writing followed by chemical wet etching (femtosecond laser-assisted etching, FLAE) and direct ablation of glass in water (water-assisted femtosecond laser drilling, WAFLD). Direct femtosecond laser writing also enables the integration of micromechanical, microelectronic, and microoptical components into the 3D microfluidic devices without stacking or bonding substrates. This paper gives a comprehensive review on the state-of-the-art femtosecond laser 3D micromachining for the fabrication of microfluidic, optofluidic, and electrofluidic devices. A new strategy (hybrid femtosecond laser processing) is also presented, in which FLAE is combined with femtosecond laser two-photon polymerization to realize a new type of biochip termed the ship-in-a-bottle biochip.

We generated 100 mJ infrared pulses using dual-chirped optical parametric amplification (DC-OPA), which is the highest energy ever reported for an infrared femtosecond laser source. These pulses are very useful for strong-field physics research and energy scaling of sub-keV soft X-rays by high-order harmonic generation.

This paper presents a simple technique to fabricate new electrofluidic devices for the three-dimensional (3D) manipulation of microorganisms by hybrid subtractive and additive femtosecond (fs) laser microfabrication (fs laser-assisted wet etching of glass followed by water-assisted fs laser modification combined with electroless metal plating). The technique enables the formation of patterned metal electrodes in arbitrary regions in closed glass microfluidic channels, which can spatially and temporally control the direction of electric fields in 3D microfluidic environments. The fabricated electrofluidic devices were applied to nanoaquariums to demonstrate the 3D electro-orientation of Euglena gracilis (an elongated unicellular microorganism) in microfluidics with high controllability and reliability. In particular, swimming Euglena cells can be oriented along the z-direction (perpendicular to the device surface) using electrodes with square outlines formed at the top and bottom of the channel, which is quite useful for observing the motions of cells parallel to their swimming directions. Specifically, z-directional electric field control ensured efficient observation of manipulated cells on the front side (45 cells were captured in a minute in an imaging area of ~160×120 μm), resulting in a reduction of the average time required to capture the images of five Euglena cells swimming continuously along the z-direction by a factor of ~43 compared with the case of no electric field. In addition, the combination of the electrofluidic devices and dynamic imaging enabled observation of the flagella of Euglena cells, revealing that the swimming direction of each Euglena cell under the electric field application was determined by the initial body angle. Devices for manipulating microorganisms have been made using a simple method involving a femtosecond laser by researchers in Japan and China. Electrofluidic devices that permit rapid 3D control of the orientation of microorganisms are important for exploring the functions of various microorganism structures. Koji Sugioka at the RIKEN Center for Advanced Photonics, Japan, and his co-workers have demonstrated that hybrid femtosecond-laser microfabrication is promising for making such devices because it enables electrodes of designable geometries to be integrated at desired locations. The researchers used the technique to make ‘nanoaquariums’ that can rapidly orientate the elongated unicellular microorganisms Euglena gracilis, allowing them to obtain desired images of Euglena cells much faster than when no electric field is applied. The team anticipates that the devices can be applied to the 3D alignment of other elongated cells.

Novel electrofluidic microdevices based on monolithic integration of 3D metal electrodes into 3D glass microchannels have been prepared by femtosecond (fs) laser hybrid microfabrication. 3D microchannels with smooth internal walls are first prepared in photosensitive glass by fs laser-assisted chemical wet etching process combined with post-annealing. Then, 3D electrode patterning in prepared glass channels is carried out by water-assisted fs-laser direct-write ablation using the same laser followed by electroless metal plating. Laser processing parameters are optimized and the roles of water during the laser irradiation are discussed. The fabricated electrofluidic devices are applied to demonstrate 3D electro-orientation of cells in microfluidic environments.

Using a high-power attosecond pulse train, we investigated attosecond dynamics in molecules. We also developed a high-energy waveform synthesizer for isolated attosecond pulses.

Microsystems and microdevices can shrink a whole laboratory into a single chip. Due to its small size, such systems are cost effective, as they require small sample sizes and analysis time. Since the silicon-based microchip cannot provide optical transparency, there is a rising interest towards glassbased microchips and their possible applications where optical transparency during the analysis procedure is of high importance. Our objective is to fabricate microdevices and sensors using laser direct writing and thin film deposition techniques. Applying these two methods to transparent materials will open new avenues in microdevice production by enabling the realization of transparent and conductive platforms.

We report, for the first time to our knowledge, on the application of a micro-optical lens fabricated by three-dimensional (3D) femtosecond laser direct writing for two-photon fluorescence imaging of biological tissues. We show that the two-photon fluorescence images of a plant leaf tissue acquired with the micro-optical lens are comparable to that of a 5× objective lens. Our result represents an important step towards the application of micro-optical components fabricated by femtosecond laser micromachining in miniaturized nonlinear fluorescence microscopy applications, such as two-photon endoscopy.

We report on fabrication of large-volume, square-shaped microfluidic chamber embedded in glass by scanning a tightly focused femtosecond laser beam inside a porous glass immersed in water. After the hollow structure is created in the porous glass substrate, the fabricated glass sample is post-annealed at 1,050°C during which it can be sintered into a compact glass. By the use of this technique, a 1 mm × 1 mm × 100 μm microchamber connected to four microfluidic channels is created inside the transparent glass substrate, showing that our technique allows for fabrication of not only thin channel structures with arbitrary lengths and configurations, but also hollow structures with infinitely large sizes.

By using a second harmonic of near infrared femtosecond (fs) laser (λ=387 nm, 150 fs) with high NA objective lens, fabrication resolution has been greatly improved in nano-fabrication of wide band-gap semiconductor gallium nitride (GaN). We have carried out a wet-chemical-assisted fs laser ablation method, in which the laser beam is focused onto a single-crystal GaN substrate immersed in a concentrated hydrochloric (HCl) acid solution. A two-step processing involving irradiation with a fs laser beam in air followed by wet chemical treatment is also performed for comparison. In the wet-chemical-assisted ablation, theoretical diameters of ablation craters are calculated as a function of pulse energy by assuming that the reaction is based on two-photon absorption. In lower energy, the calculated curve is close to the experimental value, while the actual measured diameters in the region of higher energy are larger than calculated values. In the condition of the highest fabrication resolution, we obtained ablation craters smaller than 200 nm at full width at half maximum. We have also demonstrated the fabrication of two-dimensional (2D) periodic nanostructures on surface of a GaN substrate using the second harmonic single fs-laser pulse. Uniform ablation craters with the size as small as 410 nm in diameter are arranged with a periodicity of 1 μm. Such structures are applicable to 2D photonic crystals which improve the light extraction efficiency for blue LEDs in the near future.

Internal modification of transparent materials such as glass can be carried out using multiphoton absorption induced by a femtosecond (fs) laser. The fs-laser modification followed by thermal treatment and successive chemical wet etching in a hydrofluoric (HF) acid solution forms three-dimensional (3D) hollow microstructures embedded in photosensitive glass. This technique is a powerful method for directly fabricating 3D microfluidic structures inside a photosensitive glass microchip. We used fabricated microchips, referred to as a nanoaquarium, for dynamic observations of living microorganisms. In addition, the present technique can also be used to form microoptical components such as micromirrors and microlenses inside the photosensitive glass, since the fabricated structures have optically flat surfaces. The integration of microfluidics and microoptical components in a single glass chip yields biophotonic microchips, in other words, optofluidics, which provide high sensitivity in absorption and fluorescence measurements of small volumes of liquid samples.

We investigated micro- and nano-fabrication of wide band-gap semiconductor gallium nitride (GaN) using a femtosecond (fs) laser. Nanoscale craters were successfully formed by wet-chemical-assisted fs-laser ablation, in which the laser beam is focused onto a single-crystal GaN substrate in a hydrochloric acid (HCl) solution. This allows efficient removal of ablation debris produced by chemical reactions during ablation, resulting in high-quality ablation. However, a two-step processing method involving irradiation by a fs-laser beam in air followed by wet etching, distorts the shape of the crater because of residual debris. The threshold fluence for wet-chemical-assisted fs-laser ablation is lower than that for fs-laser ablation in air, which is advantageous for improving fabrication resolution since it reduces thermal effects. We have fabricated craters as small as 510 nm by using a high numerical aperture (NA) objective lens with an NA of 0.73. Furthermore, we have formed three-dimensional hollow microchannels in GaN by fs-laser direct-writing in HCl solution.

A microchip made of UV transparent polymer (CYTOP) that can perform selective cell culture has been fabricated by F2 laser surface modification. The refractive index of CYTOP is almost the same as that of culture medium, which is essential for three-dimensional (3D) observation of cells. The F2 laser modification of CYTOP achieves hydrophilicity only on the laser irradiated area with little deterioration of the optical properties and surface smoothness. After the laser modification, HeLa cells were successfully cultured and strongly adhered only on the modified area of CYTOP. The cells patterned on CYTOP were applied for clear 3D observation using an optical microscope in phase contrast mode.

We report the fabrication of a novel surface-enhanced Raman scattering (SERS) substrate with a controllable enhancement factor (EF) using femtosecond laser direct writing on Ag+-doped phosphate glass followed by chemical plating at similar to 40 degrees C. Silver seeds were first photoreduced using a femtosecond laser in a laser-irradiated area and then transformed into silver nanoparticles of suitable size for SERS application in the subsequent chemical plating. Rhodamine 6G was used as a probing molecule to investigate the enhancement effect of a Raman signal on the substrate. Nearly homogenous enhancement of the Raman signal over the Substrate was achieved, and the EF of the substrate was controlled to some extent by adjusting fabrication parameters. Moreover, the ability of forming a SERS platform in an embedded microfluidic chamber would be of great use for establishing a compact lab-on-a-chip device based on Raman analysis.

We report the three-dimensional (3D) integration of microoptical components such as microlenses, micromirrors and optical waveguides in a single glass chip by femtosecond (fs) laser direct writing. First, two types of microoptical lenses were fabricated inside photosensitive Foturan glass by forming hollow microstructures using fs laser direct writing followed by thermal treatment, successive wet etching and additional annealing. One type of lens is the cylindrical microlens with a curvature radius R of 1.0 mm, and the other is the plano-convex microlens with radius R of 0.75 mm. Subsequently, by the continuous procedure of hollow microstructure fabrication, a micromirror was integrated with the plano-convex microlens in the single glass chip. Further integration of waveguides was performed by internal refractive index modification using fs laser direct writing after the hollow structure fabrication of the microlens and the micromirror. A demonstration of the laser beam transmission in the integrated optical microdevice shows that the 3D integration of waveguides with a micromirror and a microoptical lens in a single glass chip is highly effective for light beam guiding and focusing.

We demonstrate high-energy infrared femtosecond pulse generation by a dual-chirped optical parametric amplification (DC-OPA) scheme [Opt. Express19, 7190 (2011)OPEXFF1094-408710.1364/OE.19.007190]. By employing a 100 mJ pump laser, a signal pulse energy exceeding 20 mJ at a wavelength of 1.4 μm was achieved before dispersion compensation. A total output energy of 33 mJ was recorded. Under a further energy scaling condition, the signal pulse was compressed to an almost transform-limited duration of 27 fs using a fused silica prism compressor. Since the DC-OPA scheme is efficient and energy scalable, design parameters for obtaining 100 mJ level infrared pulses are presented, which are suitable as driver lasers for the energy scaling of high-order harmonic generation with sub-keV photon energy.

We present our recent progress on a new type of nanoaquarium based on electrofluidic devices fabricated by hybrid femtosecond (fs) laser processing. The hybrid fs laser processing involves two steps of (1) fs laser direct writing followed by thermal treatment, successive chemical wet etching and additional annealing for fabrication of three-dimensional (3D) microfluidic structures inside photosensitive glass, and then (2) water-assisted fs laser direct-write ablation followed by electroless metal plating for flexible deposition of patterned metal films on any desired locations in fabricated microfluidic structures. To show the applications of the nanoaquarium, fabricated electrofluidics are used to electrically manipulate the movement of microorganisms and worms in the microscale spaces. Flexible patterning and arrangement of electrodes to produce controllable AC electric fields in the closed microfluidic channels allows us to three-dimensionally manipulate the motions of Euglena cells due to electro-orientation. Meanwhile, 3D glass microfluidic channels monolithically integrated with vertical electrodes between which a DC voltage is applied enable us to flexibly control the movement of the nematode worm C. elegans in a closed channel based on electrotaxis.We present our recent progress on a new type of nanoaquarium based on electrofluidic devices fabricated by hybrid femtosecond (fs) laser processing. The hybrid fs laser processing involves two steps of (1) fs laser direct writing followed by thermal treatment, successive chemical wet etching and additional annealing for fabrication of three-dimensional (3D) microfluidic structures inside photosensitive glass, and then (2) water-assisted fs laser direct-write ablation followed by electroless metal plating for flexible deposition of patterned metal films on any desired locations in fabricated microfluidic structures. To show the applications of the nanoaquarium, fabricated electrofluidics are used to electrically manipulate the movement of microorganisms and worms in the microscale spaces. Flexible patterning and arrangement of electrodes to produce controllable AC electric fields in the closed microfluidic channels allows us to three-dimensionally manipulate the motions of Euglena cells due to electro-orie...

A total output energy exceeding 30 mJ has been achieved in the infrared region by dual-chirped optical parametric amplifier for generating soft x-ray harmonic pulses. Obtained spectrum supports a pulse duration shorter than 40 fs.

We experimentally demonstrate an infrared two-color polarization gating scheme for generating an intense isolated attosecond pulse using the multicycle laser. The output flux by the generalized infrared double optical gating was evaluated to be approximately 0.3 μJ/pulse, which is the highest value achieved by using the double optical gating method. The obtained submicrojoule continuum harmonic spectrum supports the generation of a pulse duration of sub-500 as.

A completely dense femtosecond optical supercontinuum is observed for the first time. It does not exhibit the oscillation-like frequency structure of the well-known optical supercontinuum generated by self-phase modulation. The newly observed optical supercontinuum is attributed to bremsstrahlung radiation from electrons that tunnel the bound potential under a strong optical field. The basic properties of the dense optical supercontinuum are studied.

We developed a robust 252 nm single-mode coherent continuous-wave light source through two-stage highly efficient frequency conversion systems using external cavities. In the first stage, we obtained the second harmonics (SH) power of 630 mW from the LBO-installed external cavity by frequency doubling of a 746 nm Ti: sapphire laser with an efficiency of almost 50% [Y. Asakawa, H. Kumagai, K. Midorikawa, M. Obara, Opt. Commun. 217 (2003) 311.]. In the second stage, 252 nm light power was obtained by doubly resonant sum-frequency mixing of 373 nm light from the first-stage conversion system and 780 nm light from another Ti:sapphire laser. In the sum-frequency external cavity, the maximum 252 nm light power of over 150 mW is obtained, which is about three times larger than that obtained in our previous experiment.

Three-dimensional (3D) micromachining of photosensitive glass is demonstrated by using femtosecond (fs) laser for Lab.-on-chip, in other words, micro total analysis system (μ-TAS), application. The fs laser direct-write process followed by a thermal treatment and successive chemical etching in a HF aqueous solution produces true 3D hollow microstructures embedded in the photosensitive glass. This technique is applied for manufacturing a microfluidic structure inside the glass. Mixing of two kinds of aqueous solutions is demonstrated in the fabricated structure. A freely movable microplate is also fabricated inside glass to control a stream of reagents in the microfluidics. In the meanwhile, this technique is applied for integrating microoptics like micromirror and micro beam splitter in the glass chip for optical analysis of reactants produced in the microfluidics.

The two-photon excitation fluorescence (TPEF) process of an enhanced green fluorescent protein (EGFP) for fluorescence signals was adaptively controlled by the phase-modulation of femtosecond pulses. After the iteration of pulse shaping, a twofold increase in the ratio of the fluorescence signal to the laser peak power was achieved. Compared with conventional pulses optimized for peak power, phase-optimized laser pulses reduced the bleaching rate of EGFP by a factor of 4 while maintaining the same intensity of the fluorescence signal. Our method will provide a powerful solution to various problems confronting researchers, such as the photobleaching of dyes in two-photon excitation microscopy.

A simple and reliable “modeless” dye laser is created. Spectrally dense, tunable generation of strongly suppressed mode structure of about 6 GHz bandwidth is obtained. The laser parameters allow to detect simultaneously the entire inhomogeneously broadened spectral line for a number of practical cases.

We show that a femtosecond laser enables us to produce true three-dimensional (3-D) microstructures embedded in a photosensitive glass, which has superior properties of transparency, hardness and chemical and thermal resistances. The photosensitivity arises from the cerium in the glass. After exposure to a focused laser beam, latent images are written. Modified regions are developed by a post-baking process and then preferentially etched away in a 10% dilute solution of hydrofluoric acid at room temperature. We have measured the critical dose for modification of the photosensitive glass, and fabricated 3-D microstructures with microcells and hollow microchannels embedded in the glass based on the critical dose.

Vacuum ultraviolet (VUV)–ultraviolet (UV) multiwavelength excitation processing for precision microfabrication of hard materials, in which simultaneous irradiation of the VUV and the UV laser beams leads to high-quality ablation and highefficiency modification, is reviewed. A coaxial irradiation system of F2 and KrF excimer lasers has been developed. This system achieves well-defined micropatterning of fused silica and GaN with little thermal influence and little debris deposition. Ablation mechanism is explained as absorption of KrF excimer laser by excited-states formed by F2 laser (excited-state absorption: ESA). Additionally, this technique is applied for more efficient refractive index modification of fused silica compared with single-F2 laser irradiation, which is attributed to resonance photoionization-like process based on ESA. The discussion includes characterization of optimum experimental conditions and features of the multiwavelength excitation processing. # 2002 Elsevier Science B.V. All rights reserved.

The use of hollow-core optical fibers in ultrafast nonlinear optics opens up the way to provide favorable conditions in the interaction between optical waves and gaseous media. The fundamental properties of optical waves propagating in a hollow fiber and various nonlinear phenomena generated inside the fiber are described.

A simple and flexible technique for integrating metal micropatterns into glass microfluidic structures based on threedimensional femtosecond laser microfabrication is presented. Femtosecond laser direct writing followed by thermal treatment and successive chemical etching allows us to fabricate three-dimensional microfluidic structures such as microchannels and microreservoirs inside photosensitive glass. Then, the femtosecond laser direct-write ablation followed by electroless metal plating enables space-selective deposition of patterned metal films on desired locations of internal walls of the fabricated microfluidic structures. The developed technique is applied to integrate a metal microheater into a glass microchannel to control the temperature of liquid samples in the channel, which can be used as a microreactor for enhancement of chemical reactions.

Electrical manipulation of biological samples using glass-based electrofluidics fabricated by femtosecond laser, in which the microfluidic structures are integrated with microelectric components, is presented. Electro-orientation of movement of living cells with asymmetric shapes such as Euglena gracilis of aquatic microorganisms in microfluidic channels is demonstrated using the fabricated electrofluidics. By integrating the properly designed microelectrodes into microfluidic channels, the orientation direction of Euglena cells can be well controlled.

We propose a method to monochromatize high-order harmonics to isolate a single beam of harmonic radiation comprising femtosecond pulses. This novel method uses a multilayer-mirror approach, and has broadband tunability and high reflectivity in the soft-xray region.

We propose and demonstrate two methods for generating high-energy isolated attosecond pulse: one is an infrared two-color laser field synthesis and the other is an infrared double optical gating. These methods enable us to perform a nonlinear optics experiment.

A recent progress in a study on phase-matched high-order harmonic generation by intense femtosecond laser pulses is reviewed. We present a phase-matching model, which is established for enhancing brightness of high-order harmonics. Recent experimental results are also described, showing that, by employing 800-nm, 30-fs, 4-mJ laser pulses as fundamental wave, it is possible to produce a single-mode, >10 nJ light at 27 nm from 10-mm, 20-Torr argon, and single-mode, 1 nJ light at 9 nm to 26 nm from 70-Torr, 3-mm neon. Improvement of the coherence and the tunability of high-order harmonic light is also shown.

Femtosecond (fs) lasers have become common tools for microfabrication and nanofabrication, and a large amount of research has been carried out in this field in 2009. This paper reviews the major areas of achievement in 2009 relating to bio-microchips such as the so-called lab-on-a-chip (LOC) and optofluidics.

We have developed an amplifier chain of Ti:Sapphire laser with a spectral width of more than 200 nm, which potentially generates a terawatt sub-10 fs pulse. This laser system consists of a Ti:Sapphire oscillator followed by a pulse stretcher, dispersion compensator with an SLM and the amplifier chain. The first amplifier is a regenerative one pumped by a green laser with energy of 11.5 mJ at 1 kHz. We could successfully compensate for the gain narrowing during regenerative amplification by using high-damage-threshold chirped mirrors and specially designed partial mirrors in the cavity, resulting in pulse energy of 73 µJ. The output pulse is sent to a beam-slicer to reduce the repetition rate to 10 Hz. After 5-pass amplification in a multi-pass amplifier pumped by the second harmonic of a YAG laser with an energy of 180 mJ, we obtained an amplified pulse energy of 30 mJ.

High-quality deep etching of single crystal 6H-SiC substrate and epitaxial GaN thin film by 266-nm laser ablation coupled with a vacuum ultraviolet (VUV) Raman laser (133–184 nm), followed by chemical treatment in HF for SiC and HCl solution for GaN is demonstrated. The etch rate was as high as 35 nm pulse −1 for SiC and 55 nm pulse −1 for GaN. Scanning probe microscopy measurement indicates that the surface of the etched films was structurally well defined and cleanly patterned. Micro-Raman measurement of ablated SiC samples, and micro-photoluminescence measurement of ablated GaN samples revealed no severe damage to the optical properties or the crystal structure. The mechanism of the VUV-266 nm laser ablation of SiC and GaN is discussed based on the band structure.

OEIC (Optoelectronic integrated circuit) which interconnects various optical and electronic elements on thesame substrate, is a promising device in near future. The fabrication method ofoptical waveguides is desirableto be a vapor phase epitaxy and suitable for in situ processing. LPE (Liquid Phase Epitaxy) methods arewidely used to fabricate thin films, but recently much attention has been paid to PLD (Pulsed Laser Deposition) methods in the fabrication of optical thin films with various advantages such as adaptability to in-situ, processing, simple arrangement, stoichiometry of the films, epitaxial quality of complicated compoundmaterials as good as those by MBE (Molecular Beam Epitaxy) and MOCVD (Metal-Organic Chemical Vapor Deposition). In the present paper, it is experimentally shown that this PLD method is very effective andaffordable as fabrication techniques of these optical thin films.

Output performance of a type-II phase-matched optical parametric generator using lithium triborate pumped at 266 nm is investigated. The optical parametric generator is designed to produce a narrow linewidth signal with good spatial quality at 326.7 nm for application to injection seeding of a vacuum ultraviolet (VUV) plasma laser. A linewidth of less than 0.1 nm and a beam divergence of 2/spl times/ diffraction limit have been achieved without any diffractive optics.

We have developed the standalone, coherent scatterometry microscope (CSM) for the inspection of extreme ultraviolet (EUV) lithography mask. The low divergence, coherent high-order harmonic (HH) was generated as coherent light source for CSM at a wavelength of 13.5 nm using a commercial laser system. The HH enable us to obtain the high contrast diffraction image from the mask. The diffraction light from the 2-nm wide line-defect and tens-nm size point-defects in the mask has been observed successfully with the system.