Laser Exposure Time Research Articles

Accelerated ageing of wood Kraft insulating paper by means of a pulsed KrF excimer laser radiation

We report on the successful use of a KrF excimer laser as a highly promising tool to investigate the accelerated ageing of wood Kraft papers used as insulating components in high-voltage power transformers. The effects of varying both the laser repetition rate (5 to 180 Hz) and the laser energy density (0.02 to 0.11 J cm-2) on the degradation of wood Kraft paper in air were systematically investigated. During the laser irradiation experiments, the temperature of the paper samples was real-time measured by means of an IR camera. It is shown that the ageing temperature of the paper can be controlled very precisely by simply adjusting the laser parameters. Indeed, levelling-off temperatures in the 25–320 °C range can be reached after only 3 s of laser exposure, by either varying the repetition rate at a fixed energy density or changing the laser fluence at a given pulse rate. By performing both scanning electron microscopy (SEM) observations and measurements of the degree of polymerization (DPv) of the laser irradiated paper samples, we were able to demonstrate that the laser irradiation induces a very rapid degradation of the paper samples (typical laser exposure times are around 35 s to be compared with ∼10 days for standard dynamic heating based ageing tests). Both the DPv of the irradiated paper samples and the average width of the corresponding cellulose fibers as measured from SEM micrographs, are shown to reduce by about 50% following their laser heating at ∼320 °C for 35 s of exposure time. It is pointed out that the observed laser-induced degradation is predominantly a photothermal process. Finally, by using gas chromatography/mass spectrometry (GC/MS), we were able to show that low molecular weight compounds such as acetaldehyde, methyl formate, acetone, and methanol are produced not only by the laser ageing process but also in real on-field transformers that underwent a severe short-circuit.

Analysis of the carbon-related “blue” luminescence in GaN

The properties of a broad 2.86-eV photoluminescence band in carbon-doped GaN were studied as a function of C-doping level, temperature, and excitation density. For GaN:C grown by molecular-beam epitaxy (MBE) the 2.86-eV band is observed in Si codoped layers exhibiting high n-type conductivity as well as in semi-insulating material. The peak position of the “blue” luminescence is constant with temperature in MBE GaN, but in semi-insulating GaN:C grown by metal-organic vapor-phase epitaxy it shifts from 3.0to2.86eV with increasing temperature in the range of 12–150K. The 2.86-eV band undergoes thermal quenching from 200to400K with an activation energy of ∼150meV. The characteristics of the 2.86-eV band are consistent with deep donor-deep acceptor recombination originating from carbon defects, under the assumption that the concentrations of these defects are low compared to the total carbon concentration in heavily C-doped samples. For low excitation density (4W∕cm2) the 2.86-eV band intensity decreases as a function of HeCd laser exposure time over a period of many minutes. However, no transient effects are observed for 20W∕cm2 excitation density. The transient behavior can be best explained using a model based on charge-trapping-induced Coulomb barriers which impede the diffusion of carriers to the 2.86-eV luminescence centers.

Development of three-dimensional microfabrication method using thermo-sensitive resin

We have developed a three-dimensional microfabrication method using thermo-sensitive resin. This method exploits the fact that polymerization of thermo-sensitive resins does not obey the law of linear superposition. Three-dimensional (3D) objects are created inside the thermo-sensitive resin by 3D scanning of the volume with a focused laser. Fabrication with the focus of the laser inside the resin can achieve good accuracy and high resolution of the structure. A fabrication system, which consisted of a CD-R pick-up laser and an XYZ stage, was built. Nonlinear polymerizations of the thermo-sensitive resin were verified by fabrication in various conditions of laser intensity and exposure time. In order to demonstrate the present method, a simple 3D microstructure was fabricated inside the resin.

Dependence of Fiber Bragg Grating Characteristics on its Length

A fiber Bragg grating (FBG) is normally made with a length of 10 to 30 mm. Depending on the application, however, a short length may be desired, especially in sensor applications for local detection. Under the most common fabrication conditions using the phase mask method, we have fabricated FBGs with lengths ranging from 0.35 to 15 mm using different excimer laser irradiation times. Although 0.35-mm-long FBGs can be made with laser exposure times exceeding 1 h, the practical minimum FBG length appears to be around 3 mm with exposure times in the range of 3 to 5 min.

Holographic Gratings in Photosensitive Acrylic Polymers with High Refractive Index Diphenyl Sulfide

To investigate the effects of polymer matrices on the diffraction efficiency of holographic grating, a series of chiral compounds of bornyl acrylate (BA), bornyl methacrylate (BMA), and their enantiomers derived from borneol were synthesized. Instead of conventional liquid crystals, in this investigation we used a high refractive index diphenyl sulfide (DS, n20D=1.6327) mixed with initiators, bornyl acrylate, bornyl methacrylate, and multifunctional monomers to prepare the photosensitive holographic grating films. Membranes having a large refractive index difference (Δn) between the cured grating arrays and revealing high diffraction efficiency (DE) were created by using the compositions. The dynamic formation of the holographic grating and the dependence of the diffracting efficiency on laser exposure time and intensity were carried out. It was found that the intensity of the writing laser may affect the polymerization rate leading to various results of holographic grating. Bragg grating and Raman–Nath grating depending on the incident writing angles were all obtained. We also investigated the plural recording function of the photosensitive films, the morphologies of the polymer matrices, and laser light grating through the holographic grating membranes. Thermogravimetric analyses of the photosensitive membranes were also estimated.

Lithographic Microfabrication by Using Two-Photon Absorbing Phenylenevinylene Derivative

The phenylenevinylene-based two-photon absorbing chromophore(EA4BPA-VB)with a triphenyl amine as an electron donor was synthesized. The two-photon absorption(TPA)cross-section values of EA4BPA-VB measured by two-photon-induced fluorescence method with 80 fs-pulse laser were found to beσ2 = 4.7 × 10−48 cm4s/photon. By adding this chromophore into photo-reactive SCR resin, we have successfully fabricated 3-D micro-structure(a larva)with high spatial resolution at low laser power with less than 5 mW. To improve the resolution of 3-D patternings, we also investigated a voxel size and shape depending on laser exposure time and concentration of TPA chromophore in SCR resin at near-threshold exposure condition.

Self-Limited Photo-Assisted Synthesis of Silicon Nanocrystals

ABSTRACTAnomalous growth of silicon (Si) nanocrystals (NCs) was observed during Raman scattering measurements of nanosilica SiOx (x=0.9) powder with an average diameter of 40 nm. It was found that Si NCs were formed by exposure to the laser beam. This photo-assisted synthesis is similar to the thermal synthesis of Si NCs, which forms Si NCs by thermally decomposing SiOx into Si and SiO2. However, the photo-assisted synthesis is more effective in forming Si NCs than the thermal synthesis. Even one second after irradiation of the nanosilica SiOx powder with laser, Si NCs with an average size of 5 nm were formed. The Si NC size increases with increasing the laser power and exposure time. It is interesting to observe a self-limited size for higher laser power and prolonged exposure. The photo-assisted synthesis of Si NCs is proved to be a promising technique with a wide range of applications in nanotechnology.

Metastability in the excitonic luminescence of electron‐irradiated GaN

Metastability under focused 267 nm laser beam is observed at low temperature in the excitonic luminescence of GaN after irradiation with 0.42 MeV electrons. The oxygen-bound exciton intensity, the total band edge luminescence intensity, and the luminescence decay lifetime of free and bound excitons all increase with laser exposure time. The rate of reaching the stable state depends on the average laser power. The observed metastable behavior is explained by the carrier trapping or recombination-enhanced dissociation of non-radiative defect complexes related to Ni. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Reduction of Birefringence and Polarization- Dependent Loss of Long-Period Fiber Gratings Fabricated with a KrF Excimer Laser

We discuss the effect of UV laser exposure time, repetition rate, and heating by a heating coil on the resonance peak depth and polarization-dependent loss (PDL) of long-period fiber gratings (LPFGs). The LPFGs were fabricated with a KrF excimer laser and an amplitude mask. We observed an initial increase of the resonance peak depth and PDL as the UV exposure time was increased, which eventually decreased in response to over-coupling. With the total UV fluence kept constant, the peak depth continued to increase as the repetition rate was increased beyond 10 Hz, whereas the maximum PDL decreased when the repetition rate was higher than 17 Hz. This is believed to be a thermal effect caused by the rapid delivery of UV laser pulses. We observed a similar reduction of the maximum PDL from 1.35 to 0.25 dB when the fiber was heated by an adjacent heating coil.

Photo-catalytic conversion of methane into methanol using visible laser

The photo-catalytic conversion of methane into methanol was investigated under different experimental parameters such as catalyst concentration, laser power, laser exposure time, effects of free radical generator (H 2O 2) and electron capture agent (Fe 3+), using visible laser light. The study was carried out at room temperature with a simple set up using a laser light, water and a semiconductor photo-catalyst WO 3. The reaction products (methanol, O 2 and CO 2) were characterized using gas chromatography. The use of WO 3 as photo-catalyst replaces the UV laser light with visible laser light. This greatly simplifies reactor design and permits flexibility in the selection of a laser source in the visible region. The oxygen to tungsten ratio in WO 3 at different temperatures was studied by XPS.

Generation and Formation of Gold Nanoparticles with Spatial Control by Two-Photon Femtosecond Laser Interference

AbstractA simple strategy for generating and distributing metallic nanoparticles in polymer matrix with a spatial control in micrometer/nanometer scale has been proposed and successfully demonstrated by combination of two-photon photoreduction and femtosecond laser interference technique. The simultaneous spatially controlling generation and organization of metallic nanopartciles were controllable by designing interference patterns and varying laser intensity and exposure time. This strategy provides a simple and rapid approach for creating templates with designed spatial distribution of the nanoparticles, which is favorable for current microelectronics technology. This method also can be utilized in developing mass production processes for future nanodevices.

Optoelectronics of liquid crystals with nanoparticulate internal structure

We present the results of a detailed characterization of electro-optical and nonlinear optical properties of liquid crystals with nanoparticulate internal structures. The experiments revealed three different states of transmission of the material: diffusive, translucent, and semitranslucent. Transformations between them are induced by electric fields and laser beams. A threshold power exists for a laser beam to change the transmission state of the material. An increase in the strength of laser/material interaction with an increase of scan speed of the beam across the material was observed. One- and two-dimensional diffraction gratings were recorded using two techniques: overlap of two 1-D gratings, and modulation of the speed of laser beam scan. We show that information pixels that are recorded in such materials may possess microstructures for certain laser power and exposure times. We also report the observation of netquake, a phenomenon of long-range propagation of thermal shocks at high laser beam power levels.

Influence of total energy delivery on success rate after contact diode laser transscleral cyclophotocoagulation: a retrospective case review and meta-analysis.

To evaluate the effect of a single treatment of contact diode laser transscleral cyclophotocoagulation (TSCP) on intraocular pressure (IOP) in patients with refractory glaucoma and perform a historical meta-analysis of the relationship between successful IOP control and various laser parameters. Clinical data for 47 eyes of 43 patients who underwent a single session of contact diode laser TSCP (power of 2000 mW, exposure time of 2 seconds and 25.6 average applications) for refractory glaucoma at Pasco Eye Institute between 1996 and 2001 were reviewed. Baseline IOP was compared with the postoperative IOP at 1 month, 3 to 6 months, and 1 year. Snellen visual acuity and the number of glaucoma medications being used 3 to 6 months postoperatively were compared with baseline. Complications were also evaluated. The relationship between successful IOP control and various laser parameters (energy applied per eye, energy per laser application, power, exposure time and number of laser applications) was examined using the results of this study and previous investigations. The mean +/- standard deviation (SD) baseline IOP was 29.4 +/- 10.9 mm Hg. Diode laser TSCP decreased average IOP to 15.7 +/- 12.0 mm Hg at 1 month, 16.3 +/- 4.2 mm Hg at 3 to 6 months, and 16.2 +/- 4.8 mm Hg at 1 year, postoperatively. This represents an IOP reduction of approximately 45%. The percentage of patients maintaining a postoperative IOP of <21 mm Hg was 95.7% at 1 month, 92.1% at 3 to 6 months, and 94.4% at 1 year after TSCP. At the 3 to 6 month postoperative visit, the average +/- SD change in Snellen visual acuity (-0.5 +/- 1.1 lines) and the number of glaucoma medications used did not differ significantly from baseline. Combined analysis of the results of this study and previous studies reveals a linear, direct correlation (r = 0.91) between the percentage of patients achieving a successful outcome (defined as a final IOP < 21 or 22 mm Hg) and the total energy delivered to the ciliary body during the treatment session. Diode laser TSCP is a safe and highly effective method for lowering IOP in patients with uncontrolled glaucoma. Higher total energy levels in a given treatment session appear to be associated with increased success as defined by the percentage of patients achieving an IOP < 21 or 22 mm Hg, without an increased risk of additional complications or vision loss.

Excimer-laser-irradiation-induced effects in C60 films for photovoltaic applications

Thin films of fullerene C60 deposited by the molecular-beam epitaxy method have been subjected to a 248 nm excimer laser for various timings. Reduction in the electrical resistance of the films and the spectral evolution of the D and G bands in the Raman spectra, due to the sharp tendency towards graphitization accompanied by an increasing level of structural disorder, are observed during laser irradiation. Based on the above results, an attempt has been carried out on these irradiated C60 films to make a device sandwiched with n-type Si, and the photovoltaic parameters are reported as a function of the laser exposure times.

Endoscopically Controlled Laser Therapy of Recurrent Epistaxis with the 940 nm Diode Laser

Summary Epistaxis is a widespread, though usually bland, symptom of highly varying etiopathogenesis. ENT specialists face the challenge of selecting the best individual treatment modality when it recurs in connection with prominent vascular convolutions and angiectases in Kiesselbach's area (Little's spot), pyogenic granulomas of the nasal mucosa, intranasal hemangiomas or disseminated manifestations of Osler-Weber-Rendu's disease in the nasal cavity. Precise dosing and excellent visualization of the entire operating area render videoendoscopy-guided laser surgery an effective approach for sealing these hemorrhage-prone areas. In our clinically controlled prospective study, we examined the suitability of the fiber-guided diode laser for managing the most frequent causes of recurrent epistaxis, as mentioned above. Despite a still limited patient population, a close follow-up ranging up to 24 months has demonstrated the high therapeutic efficiency of this novel laser system (940 nm diode laser), which is already applied with great success for turbinate hyperplasia treatment, septal crest and spur vaporization and concha bullosa resection in functional endoscopic endonasal laser surgery (FEELS) (12a). Diode laser surgery of epistaxis-generating mucosa was usually performed using a bare-fiber-guided noncontact technique with the use of special glass-spatulas for compression and coagulation of vessels in outpatients under local anesthesia; using the chopped mode, moderate, individually adjusted powers were applied over short laser exposure times and relatively long exposure pauses.The diode laser did not differ in its therapeutic efficiency from the Argon or Nd:YAG laser, the two systems we preferred to use for this treatment during the last decade. But rega rds to its lower optical penetration and coagulation depth in combination with the use of different glass-spatulas, the diode laser appeared to involve a somewhat lower potential risk of wound-healing impairment. This is particularily significant compared to the “free hand-held treatment” without the compression and coagulation method and compared to the use of the Nd:YAG laser, especially at the nasal septum.

Ultrasound imaging of Nd:YAG laser-induced tissue coagulation in porcine livers.

Absorption of laser light energy induces denaturation of proteins and thermocoagulation of irradiated tissue. Recently, MRI-guided laser coagulation in combination with MR thermometry was reported as a treatment of liver tumours. In the present study ultrasonographic imaging was evaluated for its suitability in laser induced tissue thermocoagulation. Fresh porcine livers were used for ex vivo examinations. Placement of the laser catheter and tissue coagulation during laser light emission were online monitored by ultrasonography. Nd:YAG laser-induced tissue damage was evaluated by macroscopical and microscopical examinations of histological sections. During laser light emission a marked hyperdense signal enhancement was observed by ultrasonography which strongly correlated with the extent of macroscopic tissue damage. The size of laser-induced coagulation zone depended on both the power setting and total energy delivered. Carbonization of the tissue surrounding the laser tip is a limiting factor because of laser light absorption. However our data indicate that using appropriate laser energy and exposure time prevent carbonization although carbonization can not be visualized by ultrasonography. It is concluded from the present ex vivo studies that laser coagulation can be effectively performed under ultrasonographic guidance.

Retina-sparing laser treatment of occult CNV by vitrectomy and localised detachment of the macula.

Occult choroidal neovascularisation (CNV) was treated by a novel surgical approach. In order to spare the retina during laser photocoagulation of occult CNV, the retina was detached by subretinal infusion of a ringer solution. After localised detachment of the neural retina the CNV was transretinally coagulated by an endoprobe of a diode laser. Twelve patients with occult CNV were treated in such a way. The mean duration of follow-up was 3.5 +/- 1.6 months. After 4 weeks fluorescein angiography and indocyanine green angiography were performed in all patients. Laser exposure time was 1.1 +/- 0.7 s and laser energy was 1.2 +/- 0.7 W. In 10 of 12 patients the retina could be successfully detached and performed. In all cases the retina remained unchanged during laser photocoagulation. After 5 days subtle RPE atrophy and after 1 month obvious RPE atrophy was found. After 4 weeks, in 8 of 10 cases the CNV was completely occluded and no late leakage was found. Three months after operation, retinal atrophy could be seen in all cases. Visual acuity was lower in 6 and unchanged in 4 of 10 patients. Despite acute sparing of the neural retina the long-term effects are not better than after conventional laser photocoagulation, because secondary atrophy of the retina develops.

Low energy carbonaceous and graphite phases on the surfaces of thermochemically polished chemical vapor deposited diamond films

Micro-Raman spectroscopy was used to characterize chemical vapor deposition diamond films that were thermochemically polished at elevated temperatures. A continuous wave He–Ne laser and an argon ion laser both operated at a power of 10 mW on an area of 1 μm2 were used as excitation sources of the Raman setup. New Raman peaks were observed in a spectral range of 100–700 cm−1 in addition to the common graphite related features. These peaks are assumed to be related to carbonaceous phases formed on the surfaces of the films. A peculiarity of these new features is an increase in intensity with increasing laser exposure time. However, it was observed that the intensities of the amorphous and graphite Raman lines at 1326 and the 1577 cm−1, respectively, were independent of the laser exposure time. The temperature dependency of the intensities of the diamond and of the graphite Raman lines was investigated at different polishing temperatures. Low temperature (700 °C) polishing was also done to high temperature pre-polished samples to investigate the dissolution of nondiamond carbon in the polishing plate.

Absorptance behavior of optical coatings for high-average-power laser applications

A modified photothermal deformation technique is used to measure the absorptance behaviors of optical multilayered dielectric coatings for a high-power laser system. The surface thermal-lensing modification uses an enlarged probe beam to facilitate alignment of the laser beam and data acquisition. The coatings, both reflective and transmissive types, are made by a physical vapor-deposition process. Coating absorptances are observed to depend on the laser's exposure time and power density. Time-dependent absorptance defect models are proposed. Also, micrometer-sized sites of high absorptance and an area with physical damage can be found during the spatial scans. It is proposed that absorptance values reported for coatings in high-repetition-rate or cw-laser systems include time- and power-dependent behaviors in addition to other relevant irradiation parameters.

Helium-neon laser preirradiation induces protection against UVC radiation in wild-type E. coli strain K12AB1157.

We have observed that preirradiation with a helium-neon laser (632.8 nm) induces protection against UVC radiation in wild-type E. coli strain K12AB1157. The magnitude of protection was found to depend on the helium-neon laser irradiance, exposure time, and period of incubation between helium-neon laser exposure and subsequent UVC irradiation. The optimum values for dose, irradiance and interval between the two exposures were found to be 7 kJ/m(2), 100 W/m(2) and 1 h, respectively. The possible involvement of singlet oxygen in the helium-neon laser-induced protection is also discussed.