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.
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.
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.
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.
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.
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.
Femtosecond lasers which have an extremely high peak power are expected to use for micro-processing, such as drilling and cutting. We have analyzed the drilling process by femtosecond laser pulses on surface of various materials including silicon with image-intensified CCD camera with a high speed gate. A femtosecond laser pulses (single and double pulses) were focused on surface of samples using a lens with a focal length of 100 mm. As a result, it was found that the smaller the pulse energy, the faster the formation of the hole. By measuring the rise time in 12 kinds of materials, it was found that the rise time strongly correlates with the thermal conductivity in these materials. Besides, it was also found that double pulsed laser suppressed the heat influence compared to the single pulse laser. These knowledges are thought to be very important and useful for developing microfabrication using a femtosecond 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.
Fabrication technique of fiber Bragg gratings (FBG) sensors, which is used the measurement of distortions of large scale architectural structures, requires a skillful technique, and consequently the costing becomes a problem. The fabrication technique without removing a polymer jacket can realize much easier fabrication and the cheaper cost. We have investigated the laser damage threshold of jackets in order to realize such a fabrication technique. The 3rd and the 4th harmonics of repetitive Nd:YAG laser pulse were used as light sources. An optical fiber coated untinged polymer jackets is used as a sample. The numbers of the shots against the same sample position was varied from 1 to 1200. As a result, damage thresholds at the shot number of 1200 were 6.9 J/cm2 and 0.9 J/cm2 for the wavelength of 355 nm and 266 nm, respectively. These values are corresponding to intensity of 1.3 MW/cm2 and 0.3 MW/cm2, respectively. It is considered that the fabrication of FBG, which need a typical intensity of ~kW/cm2, without removing the jacket should be possible by controlling the laser intensity between this level and the threshold.
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.
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 have deposited crystalline diamond films on sintered Si3N4 substrates with a conventional hot-filament method and further finished the surfaces with a diamond lapping method. The diamond mirrors fabricated are proposed and demonstrated as among the most promising mirrors for vacuum ultraviolet rare-gas excimer lasers. We have obtained pulses with 15-mJ energy, 2.7-MW peak power, and 6-ns duration at 126 nm from an argon excimer laser, and with 70 mJ, 5.5 MW, and 5 ns at 146 nm from a krypton excimer laser without any surface damage. The latter values are the highest so far obtained.
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