We propose the hetero-core fiber optic refractive index sensor using ITO nano-particles. The hetero-core optical fiber has a structure that leaks transmitted light while maintaining mechanical strength. The ITO is well-known to have an absorption band in the infrared region. The ITO nano-particles were formed on the fiber by the wet process. We have experimentally confirmed that the ITO nano-particles hetero-core fiber sensor causes a change in transmission loss at refractive index in the near-infrared region. Therefore, we demonstrated the fiber optic refractive index sensor using the ITO nano-particles film only in the infrared region.
In this paper, we propose a hetero-core optical fiber with a high refractive index material, TiO2, deposited on the inner layer of the Au island films, which is fabricated by sputtering method and annealing. It was confirmed that this structure of TiO2 deposited on the inner of the Au island films strongly excites the absorption peak in the visible light region and detects the refractive index change in the near-infrared region compared to the conventional hetero-core optical fiber sensor using Au island structure.
Laser-induced periodic surface structures (LIPSS) formation was observed inside a microhole produced in an optical fiber, when the femtosecond laser pulse number and the laser energy were optimized to be 150 and 30μJ, respectively. We have also found that LIPSS were formed only at the bottom of the microhole, which may be responsible for the strong evanescent field produced by the femtosecond laser propagating inside the microhole.
We have evaluated inner surface roughness of inline/picoliter fiber optic spectrometer fabricated by an NUV femtosecond laser drilling. A microhole fabricated by the femtosecond laser without breaking off works as inline/picoliter fiber optic spectrometer. The attractive feature of the spectrometer is very small sensing volume which has several tens of picoliter. A second harmonic 400 nm femtosecond laser with 350 fs pulse duration launched onto the glass fiber optic. A high aspect ratio of the microhole was fabricated after 1000 pulse shots, but there was inner surface roughness. Although the repetition rate was changed 10 to 1000 Hz in order to control the inner surface roughness, the inner surface roughness was occurred in each case. It was confirmed that ablated fused silica particles deposited on the inner surface of microhole. The depth of microhole was deepened with 1 kHz of repetition rate and number of 1000 shots. In comparison to 10 Hz, the depth of microhole was increased by approximately 80%. It was assumed that heat accumulation effect enlarged the length of drilling. In order to minimize inner surface roughness, the best method is to use low number laser shots. After 100 pulse shots with 30 μJ of pulse energy, an optical inner surface quality of microhole was acquired. The optical inner surface quality of microhole was verified by measuring the transmittance of 94% of infrared light emission launched from superluminescent diode in the case of 100 pulse shots with 20 μJ. The transmittance decreased to 52% changing the microhole fabricated by 30 μJ with 100 laser shots because of increasing interaction area between the microhole and propagating light.
Inner walls of microhole in a thin fused silica plate were observed after changing ablating laser pulse shots of a focused femtosecond laser at the wavelength of 400 nm with an energy of 20 μJ in a pulse width of 350 fs. Using an objective lens with an NA of 0.28, it was revealed that the inner surface of the microhole was melted with 10 laser pulse shots. By increasing the pulse numbers to 100, however, deposition of fused silica particles on the melted inner surface was observed. In order to minimize the inner surface roughness, the objective lens was changed. After 50 laser pulse shots, the inner surface structure was brought close to optical quality using an objective lens with NA of 0.65.
We have observed an optical gain at the wavelength of 126 nm in an Ar excimer (Ar2*) amplifier by utilizing a femtosecond vacuum ultraviolet (VUV) seed beam tuned at 126 nm. The maximum optical gain value of 1.1 cm-1 with a spatial distribution in the optical-field-induced ionization (OFI) Ar plasma was observed. The plasma diagnosis revealed that the plasma contraction near the plasma amplifier axis together with the plasma expansion was a key issue to observe such a high optical gain value inside the Ar plasma filament. The center axis of the contracted plasma amplifier showed the high electron density more than 1018 cm-3 even after 100 ns from the plasma production of Ar at 1 MPa. Our OFI plasma/excimer kinetics code reproduced the temporal progress of the optical gain distribution as well as the maximum gain value.
Metallic sodium (Na) was proposed as a transparent material in the vacuum ultra-violet (VUV) spectral range in 1930s
and in 1960s. However no clear transmission has ever been demonstrated. In this paper we describe firstly the direct
measurement of actual transmittance of a sodium samples in a spectral range longer than 115 nm which corresponds to
the shortest transmission wavelength of magnesium fluoride (MgF2) windows, resulting in several tens of %
transmittance of a 3 mm-thick solid sodium sample including MgF2 windows at the wavelength of ~120 nm. We also
find very weak temperature dependency of the transmittance up to 150 degrees centigrade where the solid sample is
melted at 97 degrees. The measured transmittance pushes us to make a simple imaging experiment illuminated by the
VUV light through a 2-mm thick sodium sample, resulting in obtaining a clear image composed of 100 μm diameter
tungsten mesh recorded on a two dimensional Charge Coupled Device detector. The result also opens a way to construct
an optical imaging device for objects inside or through a solid or a liquid sodium medium. According to the present
experiment, we can make a continuous real time transmission imaging for a liquid sodium sample if we use proper
optical setup including an intense continuous VUV source or high repetition rated intense coherent source for
holographic data acquisition. Such an experiment opens up a way to perform transmission imaging through or inside a
sodium medium for characterization of hydrodynamic and material properties.
We have observed the optical amplification of femtosecond VUV seed pulses at 126 nm by using an optical-fieldinduced
ionization (OFI) Ar2* amplifier. The maximum amplification ratio of 2.57 was observed. This corresponded to
the maximum one-pass gain value of 0.94, which was consistent with that observed in previous experiments. We
measured the spatial distribution of the gain region in the OFI Ar plasma by probing the 126 nm VUV seed pulses. The
gain region of 220 μm (FWHM) was evaluated after 20 ns of the plasma production, which indicated the average plasma
expansion temperature of 1.2 eV. An Ar+ density contour measured by using laser interferometry showed consistency
with the spatial distribution of the plasma gain region.
We have developed an autocorrelator utilizing multiphoton ionization of rare gases as a nonlinear medium to evaluate
the pulse width of a femtosecond Ti:Sapphire laser at 882 nm. The autocorrelation width of 171 fs (FWHM) was
evaluated by the autocorrelator utilizing nine-photon ionization of Ar. By using the ninth-order correlation factor of 1.06,
the actual pulse width of 161 fs (FWHM) was determined, which was consistent to that of 165 fs (FWHM) measured
with a two-photon autocorrelator. The autocorrelation measurement utilizing the multiphoton ionization of Ar should be
applied to vacuum ultraviolet (VUV) ultrashort pulses at 126 nm, since neutral Ar atoms will be ionized by two-photon
absorption. This method has a potential to become a versatile autocorrelator that characterizes femtosecond laser pulse
widths in the wide spectral range between IR and VUV.
We have observed the optical amplification of the Ar2* excimer at 126 nm pumped by optical-field-induced ionization
(OFI) caused by an infrared high-intensity laser. We have evaluated similar small signal gain coefficients of
approximately 1.0 cm-1 in two different experiments, where OFI Ar plasmas as gain media were produced in free space
filled with Ar and inside an Ar-filled hollow fiber. This indicates that the function of a hollow fiber was to guide the
infrared excitation laser and VUV Ar2* emissions, and not to regulate the OFI plasma. Despite the gain coefficient value
at 126 nm, the laser oscillation has not been observed. This was limited by the optical quality of available state-of-the-art
vacuum ultraviolet optics.
We proposed and developed a novel surface analysis system using vacuum ultraviolet (VUV) photons. When the VUV
photons were irradiated on the material surface, surface desorption was stimulated. The desorbed species were analyzed
by the mass spectrometer. First, we studied the decomposition process induced by VUV photons from excimer lamps.
We found that the different photon energy resulted in the different time dependence of the fragments signals even if the
materials had similar chemical construction. It suggested that the identification of the materials should be possible by
tracing the decomposition process. We developed an analyzing system, called "Photo-Stimulated Desorption (PSD) mass
spectrometer" using a broadband VUV radiation from the Ar plasma excited by a Q-switched Nd:YAG laser. The
desorbed species were analyzed by the quadrupole mass spectrometer. This PSD system was useful surface analysis tool
not only for the semiconductor but also plastics, which is easily affected by heat.
We report the optical amplification characteristics of an optical-field-induced-ionization (OFI) Ar2* excimer
vacuum ultraviolet (VUV) amplifier at 126 nm by using two experimental approaches. We have observed the
amplification of OFI Ar2* excimer emission and evaluated the gain length product of 1.0 by using an optical cavity.
We also achieved the gain length product of 5.0 by measuring the one pass amplification inside a hollow fiber with
the length of 5 cm. The use of a hollow fiber was effective to guide the VUV emission and to extend a gain length.
A small signal gain coefficient of 1.0 cm-1 was evaluated in both experimental approaches.
We have demonstrated an OFI Ar2* excimer VUV amplifier at 126 nm pumped by a high-intensity laser in the table top
size. We observed the Ar2* excimer emission centered at 126 nm with the spectral bandwidth of 10 nm (FWHM), which
was produced in the OFI plasma. Significant amplification was observed inside the OFI Ar2* excimer as a result of the
optical feedback provided by a VUV reflector. The gain-length product of 5.6 was observed at the Ar pressure of 11 atm.
The population inversion density on the order of 1017 cm-3 was evaluated inside the OFI plasma, which would be
sufficient for the amplification of a subpicosecond VUV pulse at 126 nm produced by the harmonic generation.
In vacuum ultraviolet (VUV) spectral region, coherent light sources are being thus in high demand for advanced precise
and microscopic processing. A sub-picosecond VUV light source at 126 nm has been produced by the nonlinear
wavelength up-conversion of a near infrared femtosecond Ti:Sapphire laser at 882 nm in rare gases. We obtained the
maximum output of the 7th harmonic at 126 nm in Xe at the pressure of around 2 Torr. The 126 nm beam will be
amplified by an optical-field-ionization produced Ar2 medium and then high-power sub-picosecond VUV pulses will be
obtained.
Characteristics of extreme ultraviolet (EUV) and debris emissions as well as debris reduction have been investigated for
a laser-produced plasma (LPP) EUV source by using a colloidal/liquid jet target containing tin dioxide nanoparticles and
tin chloride. The amount of deposited debris on a silicon witness plate was determined by a total laser energy irradiated
onto a target. Double-pulse laser irradiation was effective for improving the EUV conversion efficiency as a result of
plasma regulation. It was, however, not effective for reducing the deposited debris from a colloidal target with
nanoparticles. In situ low-temperature heating of the witness plate was effective to reduce the amount of deposited debris.
Room-temperature photon processing using an incoherent vacuum ultraviolet excimer lamp at 126 nm deoxidized a
deposited tin oxide layer. In addition to these active debris reduction methods, the use of a tin chloride liquid target at a
certain concentration passively reduced the amount of deposited debris as a result of production of chlorine atoms that
sputtered and/or etched deposition. The EUV CE of more than 1% was observed from a tin chloride target by using
double-pulse laser irradiation.
We realized a laser-plasma EUV target, which satisfied the high EUV CE and the debris suppression simultaneously by using low-concentration liquid jet/droplet targets containing tin oxides and chlorides. Plasma regulation by double pulse irradiation improved the EUV CE. In terms of the debris emissions, we reduced the amount of the deposited tin oxide by applying in situ heat and high-energy photons onto a witness plate. These active debris suppression resulted in the decrease of the deposition rate and deoxidation of the debris, respectively. The use of tin chloride liquid target also realized a well-balanced debris behavior, where deposited debris was cleaned by chlorine atoms or ions, resulting in an approximately zero deposition rate.
We have demonstrated an argon excimer vacuum ultraviolet (VUV) amplifier at 126 nm by using the optical-field induced ionization (OFI) of argon. The gain-length product of 5.6 was achieved as a result of the optical feedback inside the amplifier with a VUV mirror. Plasma self-channeling caused by the high-intensity pump laser was simultaneously observed when the maximum gain-length product was observed. We have also optimized the output power of a subpicosecond VUV seed beam at 126 nm produced in low-pressure
rare-gases as a result of the seventh harmonic nonlinear wavelength conversion of a Ti:Sapphire laser at 882 nm.
Debris characteristics and its reduction have been investigated for a laser-produced plasma (LPP) extreme ultraviolet
(EUV) source using a colloidal jet target containing tin dioxide nano-particles. Dominant deposited debris on a witness
plate was found to have a form of oxidized tin (SnOx) originated from nano-particles. Quantitative debris amounts were
determined by total laser energy irradiated onto a target, not by laser irradiation modes, such as single or double pulse
irradiation. In-situ low-temperature (100°C) heating of a plate was effective to reduce the deposited debris amount, since
colloidal debris was easily vaporized by the heat. Another approach to remove the deposited debris was roomtemperature
photon processing using incoherent vacuum ultraviolet (VUV) emission at 126 nm. X-ray photoelectron
spectroscopy (XPS) analysis has shown that the deposited SnOx debris layer was deoxidized by the 126 nm VUV photon
energy.
We have been developing an ultrashort-pulse high-intensity vacuum ultraviolet (VUV) laser. Ultrashort VUV pulses at 126 nm have been produced in rare-gases by nonlinear wavelength conversion of an infrared Ti:sapphire laser at 882 nm. This pulse will be amplified inside an Ar2* amplifier excited by optical-field-induced ionization electrons. The amplification characteristics of the Ar2* amplifier has been improved by plasma channeling induced by a high-intensity plasma-initiating laser.
We demonstrated enhancement of extreme ultraviolet (EUV) emission at 13.5 nm from a lithium plasma by use of dual laser pulses. A single laser pulse produced a lithium plasma condition for the EUV emission far beyond its optimum. Utilization of dual laser pulses, however, enhanced the EUV emission energy, and its maximum in-band EUV conversion efficiency (CE) in a measured solid angle was observed to be 2% at a delay time between 20 and 50 ns.
We demonstrated a debris-free, efficient laser-produced plasma extreme ultraviolet (EUV) source by use of a regenerative liquid microjet target containing tin-dioxide (SnO2) nano-particles. By using a low SnO2 concentration (6%) solution and dual laser pulses for the plasma control, we observed the EUV conversion efficiency of 1.2% with undetectable debris.
Bio-active hydroxyapatite (HAp) coatings were deposited by the pulsed laser deposition method using a KrF excimer laser. We changed the sintered temperature of HAp ceramics target and successfully deposited the poly-crystalline HAp coating layer at room temperature.
We observed significant amplification of the argon excimer laser emission at 126 nm initiated by femtosecond high-intensity laser-produced electrons. By introducing an optical feedback with a vacuum ultraviolet multilayer mirror, the small signal gain coefficient of 2.3 cm-1 was evaluated at 126 nm.
Characteristics of suprathermal ions and neutral particles from a laser-produced tin plasma by use of a colloidal
microjet target containing tin dioxide (SnO2) nanoparticles were investigated. Suprathermal ion emissions were
reduced by producing a low-density preplasma. Simultaneously, the maximum conversion efficiency of 1.2% at 13.5
nm with a bandwidth of 2% and a solid angle of 2&pgr; sr was observed. Neutral particles, however, were not suppressed
under the optimum laser-plasma conditions.
We have been developing the vacuum ultraviolet (VUV) light sources and novel applications using such short
wavelength emission sources. High quality amorphous Si thin films were successfully produced at room temperature as
a result of photo-dissociation of SiH4 gas by using an Ar2* excimer lamp irradiation at 126 nm. To enhance such novel
VUV processing applications, a compact VUV amplifier at 126 nm was developed by use of the optical-field-ionization
(OFI) electrons. The gain-length product around 5 was obtained as a result of the optical feedback by using a VUV
mirror. This amplifier was operated in a table-top size with a high repetition rate up to several kHz, which should be
appropriate for any process applications. We also describe the schematic concept of the ultrashort pulse high-intensity
VUV laser system at 126 nm with a pulse width of 100 fs.
We demonstrated a debris-free, efficient laser-produced plasma extreme ultraviolet (EUV) source by use of a regenerative liquid microjet target containing tin-dioxide (SnO2) nano-particles. By using a low SnO2 concentration (6%) solution and dual laser pulses for the plasma control, we observed the EUV conversion efficiency of 1.2% with undetectable debris.
The Ar2* excimer production kinetics were initiated by electrons produced in the optical-field-induced ionization (OFI) process. The use of an Ar-filled hollow fiber extended an OFI plasma length up to 30 cm. By use of a 126-nm mirror, the maximum one-pass gain coefficient at 126 nm was observed to be 0.58% cm-1 at 10 atm Ar.
Extreme ultraviolet (EUV) radiation at the wavelength of around 13nm waws observed from a laser-produced plasma using continuous water-jet. Strong dependence of the conversion efficiency (CE) on the laser focal spot size and jet diameter was observed. The EUV CE at a given laser spot size and jet diameter was further enhanced using double laser pulses, where a pre-pulse was used for initial heating of the plasma.
Characteristics of extreme ultraviolet (EUV) light at the wavelength between 5 to 20 nm from rare gas cryogenic targets irradiated with a nano-second laser pulse are studied. Spatial distribution of the EUV light and the ion current was measured and found to vary as cos2θ and cos6θ, respectively, where θ is an angle to the target normal. Energetic ions were detected, which had a velocity of the order of 106 cm/s. According to the observed cos2θ spatial distribution, the spatially-integrated total EUV output energy for the xenon cryogenic target was evaluated to be 1.5 mJ/pulse, leading to the conversion efficiency of 0.2%/pulse.
We have used a pair of newly constructed electrodes to improve the discharge stability and electrical input power. The electrode shape was designed so that the discharge width became narrower, which lead to the increase of the input power density by 22%. As a result, the maximum output energy increased from 150 to 200 μJ at 147.8 nm. The pulse duration of 250 ns (FWHM) became shorter compared to the previous result (400 ns). This long pulse operation indicated the laser oscillation in an afterglow mode. The laser beam shape was circular with a beam divergence of 2.5 mrad. Because of the long pulse duration, this beam shape reflected on a cavity mode (multi-mode) as a result of the optical feedback. A small signal gain coefficient increased almost linearly with the increase of the discharge voltage. The maximum gain coefficient at 147.8 nm was 3.5%cm-1 at 31 kV.
We have observed Ar2* emission at 126 nm by use of a high intensity laser pulse as an excitation source. Kinetic analysis revealed that high-intensity laser-produced electrons via optical field induced ionization (OFI) process initiated the Ar2* production kinetics. Ar2* production kinetics initiated by OFI electrons was mainly governed by the three-body association process, which was analogous to the case of electron beam excitation. The use of a hollow fiber controlled propagation characteristics of a high intensity laser pulse in a high-pressure gas, leading to the increase of the excimer emission intensity
We have realized a stable self-sustained discharge of high-pressure rare gases (Ar and Kr) using a compact discharge device. The glow discharge was obtained up to 10 atm of pure Kr. The vacuum ultraviolet emission intensity centered at 148 nm abruptly increased when the charging voltage exceeded a certain value. This "threshold" behavior indicates the onset of the stimulated emission at the wavelength. In addition to this threshold behavior, a considerable spectral narrowing was observed when the charging voltage exceeded the threshold value. The deconvoluted spectral width was 0.5 nm (FWFIM), which was much narrower than that of 4 nm (FWHM) at the charging voltage below the threshold. This significant spectral narrowing also strongly indicates the onset of the stimulated emission at 148 nm.
A 1.5 kW high peak power and 140 ns short pulse krypton excimer lamp in VUV spectral region has been developed using a pulsed silent-discharge. In such a high peak power operation, penning ionization was a dominant destructive process as in the same case of a rare gas excimer laser operation.
Efficient VUV excimer lamps with two types of discharge configurations, expanding jet discharge and silent discharge (dielectric-barrier discharge) in a variety of rare gases and their mixtures, are presented. In the jet discharges VUV output power was 9 mW with an efficiency of 10-2% at 126 nm for argon excimers. Output powers of other excimers were 300 mW with 1.0% efficiency at 146 nm for krypton excimers and 500 mW with 1.6% efficiency at 176 nm for xenon excimers. Simultaneous emissions from hetero-nuclear rare gas excimers (ArKr*, 135 nm) as well as homo-nuclear rare gas excimers (Ar2* and Kr2*) were observed by using rare gas mixtures of argon and krypton. Output powers and efficiencies of the silent discharge excimer lamps were 500 mW and 1.6% for argon, 5 W and 13% for krypton, and 5 W and 20% for xenon excimers. In the silent discharge extremely broad band excimer emissions were observed at the center wavelengths of 145 nm for an argon/krypton mixture and of 163 nm for a krypton/xenon mixture. A PMMA plate was photo-chemically etched at the rates of 1 - 2 nm/min by the irradiation of the 172 nm radiation in air and argon gas atmospheres.
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