Nonlinear optical properties of BaWO4 crystal

Nonlinear refraction and nonlinear absorption in barium tungstate crystal which is widely used in Raman lasers is studied using the single-beam z-scan technique with 1 ps laser pulses at 790 and 395 nm. The nonlinear refraction-index intensity-coefficients at these wavelengths and two-photon absorption coefficient at 395 nm have been determined for the crystal. The results show that barium tungstate is characterized by weaker nonlinear refraction and absorption at 395 nm in comparison with other Raman crystals.     

Linear and nonlinear optical refractions of CR39 composite with CdSe nanocrystals

Hybrid composites of CdSe nanocrystals embedded in allyl diglycol carbonate (CR39) matrices have been prepared and characterized. The measurements show that the linear refractive index of the composite decreases as the CdSe nanocrystal’s weight-percentage concentration increases at the laser wavelengths of 632.8 nm and 532 nm. The room temperature nonlinear optical properties of the hybrid composites were investigated using a single-beam Z-scan technique with femtosecond laser pulses at the wavelengths of 794 nm and 397 nm. The experimental data reveals that the Kerr nonlinear refractive index n₂ of the composite increase at these wavelengths when the CdSe nanocrystal’s weight-percentage concentration increases. Also, the nonlinear refractive index n₂ of the CdSe/CR39 hybrid composites exhibit dispersion from a positive value at 794 nm (below the band gap) to a positive value at 395 nm (above the band gap). The measured dispersion of n₂ is roughly consistent with the Sheik-Bahae’s theory for the bound electronic nonlinear refraction resulting from the two-photon resonance.     

High resolution material ablation and deposition with femtosecond lasers and applications to photomask repair

Abstract

We describe experiments using 100 femtosecond pulses of 266 nm light to ablate Cr defects from photomasks with resolution below 100 nm. In addition to the ablative removal of Cr, experiments were carried out to deposit Cr metal onto fused silica substrates using 100 fs, 400 nm light at atmospheric pressure. Multiphoton dissociation of Cr(CO)₆ adsorbed on fused silica substrates initiates Cr deposition. The mechanisms for deposition on both transparent (fused silica) and absorbing (Cr metal) substrates are discussed. Finally we describe initial experiments to ablate Cr metal at wavelengths below 200 nm using light generated by frequency mixing of ultrashort, 30 fs pulses in an Ar filled capillary.     

Femtosecond microscopy of surface plasmon polariton wave packet evolution at the silver/vacuum interface

A movie of the dispersive and dissipative propagation of surface plasmon polariton (SPP) wave packets at a silver/vacuum interface is recorded by the interferometric time-resolved photoemission electron microscopy with 60 nm spatial resolution and 330 as frame interval. The evolution of SPP wave packets is imaged through a two-path interference created by a pair of 10 fs phase correlated pump-probe light pulses at 400 nm. The wave packet evolution is simulated using the complex dielectric function of silver.     

Optical limiting studies in a carbon-black suspension for subnanosecond and subpicosecond laser pulses

This paper presents results of measurements of short (0.3 ps, 0.2 ns, and 10 ns) laser pulse transmissions through a liquid suspension of fine carbon particles (named CBS for "carbon-black suspension") at input-pulse energies up to 10 mJ. The 10-ns pulses came from a Nd:YAG laser at 1064 nm, and the shorter pulses came from a Ti:sapphire laser at 800 nm. Limiting was observed with the 10-ns and the 0.2-ns laser pulses, but the 0.3-ps pulses produced white light and underwent the same level of attenuation in the solvent and in the CBS.     

Femtosecond laser machining of single-crystal superalloys through thermal barrier coatings

Femtosecond laser machining of single-crystal superalloys coated with thermal barrier coatings (TBCs) has been investigated. The investigations were carried out in air using a titanium:sapphire laser system (λ = 780 nm) operating at a repetition rate of 1 kHz and delivering individual pulses of 150 fs in duration. The ablation threshold of 7 wt.% yttria stabilized zirconia (7YSZ) has been measured to be 1.52 ± 0.21 J/cm². Microstructural investigations indicated a complete absence of conventional processing defects such as recast layers and microcracking in the vicinity of the machining area. The absence of machining-induced melting or delamination along interfaces of the TBC system demonstrates a significant advantage in comparison with conventional laser machining.     

Reduction, aggregation and physicochemical properties of silver nanoparticles in propan-2-ol:cyclohexane mixtures induced by a high energy electron beam

Radiolytic reduction of silver and gold ions and subsequent formation of their aggregates have been studied in propan-2-ol:cyclohexane mixture using pulse radiolysis technique. The silver sol, produced on irradiation of Ag⁺ solution with a train of electron pulses, has been characterized using XRD and TEM. The size of the particles has been found to be in the range of 30-50 nm. The silver sol emit light with a maximum at 340 nm on irradiation with a high energy electron beam. The intensity of emission has been found to decrease with decrease in concentration of Ag particles. Formation of colloidal gold has also been observed on irradiation of NaAuCl₄ solution in propan-2-ol:cyclohexane by train of electron pulses. The particles so formed are oxidized on exposure to air. No light emission has been observed from Au sol.     

Third-harmonic generation from single gold nanoparticles

We report the first observation of third-harmonic signals from individual gold colloids down to 40 nm diameter. Excited with 1-ps pulses at 1500 nm, the colloids generate 500-nm light, close to the plasmon resonance. The third-harmonic intensity varies as the square of the colloid surface area. Although weak, the third-harmonic signals of gold labels as small as 15 nm in diameter are expected to be accessible with 100-fs pulses. They could be used in microscopy for single-biomolecule tracking.     

Optimization of dual-band continuum light source for ultrahigh-resolution optical coherence tomography

We demonstrate a dual-band continuum light source centered at 830 and 1300 nm for optical coherence tomography (OCT) generated by pumping a photonic crystal fiber having two closely spaced zero-dispersion wavelengths with a femtosecond laser at 1059 nm. By use of polarization control, sidelobe suppression can be improved up to approximately 7.7 dB. By employing compression of the pump pulses, the generated spectrum is smooth and near-Gaussian, resulting in a point-spread function with negligible sidelobes. We demonstrate ultrahigh-resolution OCT imaging of biological tissue in vivo and in vitro using this light source and compare it with conventional-resolution OCT imaging at 1300 nm.     

Preparation and characterization of gold-coated silver triangular platelets in nanometer scale

Colloids of triangular silver platelets and the platelets coated with a layer of gold have been prepared and analyzed by dynamic light scattering (DLS), transmission electron microscopy (TEM), high-resolution electron microscopy (HRTEM), ultraviolet-visible spectroscopy (UV–vis). The particle size distribution measured by DLS indicates the Ostwald ripening process in evolution of triangles is fed by small spherical particles during light illumination. The shape of most particles is a truncated equilateral triangle and the degree of truncation decreases as the size increases. Triangular platelets are of edge length 50–60 nm and thickness 6–10 nm. The edge length increases to 60–90 nm after depositing a gold layer. To preserve the triangular geometry, a gold layer of thickness less than 5 nm is preferred. The epitaxial nature of gold grown on the 1 1 1 planes of the silver crystal is clearly illustrated in HRTEM images when the thickness of gold is less than 1 or 2 nm. A moiré fringe is usually observed when the Au thickness is higher than 5 nm. The moiré fringe is simulated by rotating two overlapped lattices of the same spacing. The peak position of in-plane dipole resonance at 680 nm is sensitive to the degree of truncation of silver triangle. It shifts towards 615 nm owing to corner rounding by the heat. The gold coating broadens and shifts the peak of in-plane resonance toward shorter wavelengths, hence the cool blue color changes into a warmer color.     

Visible optical parametric generator producing nearly bandwidth-limited femtosecond light pulses at 1-kHz repetition rate

The second harmonic of a femtosecond Ti:sapphire regenerative amplifier system is used to pump parametric generator-amplifier producing visible light pulses of 160-fs duration near 570 nm and infrared pulses of < 100-fs duration between 1.23 and 1.45 μm.     

Three- and four-photon absorption of a multiphoton absorbing fluorescent probe

High-order multiphoton excitation processes are becoming a reality for fluorescence imaging and phototherapy treatment because they afford minimization of scattered light losses and a reduction of unwanted linear absorption in the living organism transparency window, making them less susceptible to photodamage, while improving the irradiation penetration depth and spatial resolution. We report the four-photon-excited fluorescence emission of (7-benzothiazol-2-yl-9,-didecylfluoren-2-yl)diphenylamine in hexane and its four-photon absorption cross section sigma4' = 8.1 x 10(-109) cm8 s3 photon(-3) for the transition S0 --> S1 when excited at 1600 nm with a tunable optical parametric generator (OPG) pumped by picosecond laser pulses. When pumped at 1200 nm, three-photon absorption was observed, corresponding to the same transition.     

Second harmonic generation in oriented purple membrane films under picosecond light excitation

Non-linear optical properties of oriented purple membrane films (OPMF) have been investigated by second harmonic generation from picosecond pulses at 860 nm. A second-order non-linear optical coefficient d_eff > 1.05×10⁻⁹ e.s.u. was observed. We propose the use of this non-linearity for measurement of ultrafast photoelectric response in OPMF after the excitation of bacteriorhodopsin by ultrashort light pulses.     

Sum-Frequency Generation of Femtosecond Pulses in CsLiB(6)O(10) Down to 175 nm

Using a 150-mum-thick CsLiB(6)O(10) crystal, we produced 100-fs, >200-nJ light pulses tunable between 175 and 180 nm by sum-frequency generation at a 1-kHz repetition rate with an all-solid-state laser system mixing the fourth harmonic of a femtosecond Ti:sapphire regenerative amplifier and the idler pulse from a traveling-wave optical parametric amplifier.     

1.3 μm InAs quantum dot resonant cavity light emitting diodes

Resonant cavity light emitting diodes (RCLEDs) containing nine sheets of self-organized InAs quantum dot (QD) active layers and operating at around 1.3 μm are demonstrated. The structure was grown directly on GaAs substrates, which includes selectively oxidized AlO_x current apertures and intracavity metal contacts. It was found that the average operating resistance is 60 Ω, while the average turn-on voltages is 1.6 V. It was also found that temperature coefficient of these RCLEDs was about 0.11 nm/°C.     

可调蓝光皮秒脉冲

利用３５１ｎｍ紫外光泵浦非临界相位匹配ＬｉＢ３Ｏ５光参量振荡器（ＬＢＯ－ＯＰＯ）,直接获得了在４７２￣４５３ｎｍ范围内波长连续可调的蓝图光皮秒脉冲序列。ＯＰＯ所用ＬＢＯ晶体的两端面切割成Ｂｒｅｗｓｔｅｒ角,避开了晶体表面增透膜的稳定性问题,保证了ＬＢＯ晶体端面低反射损耗和ＯＰＯ长期稳定工作。     

The entrapment of organic dyes into sol–gel matrix: Experimental results and modeling for photonic applications

To better understand the electronic absorption spectra in the UV–vis region of the methyl red (MR) dye in its anionic, isoelectronic and zwitterionic forms in aqueous solution, high level sequential-Monte Carlo/quantum mechanics (s-MC/QM) methodology was performed and compared to experimental measurements. The theoretical procedure consists in treating the solute–solvent system separately using classical MC simulations to build the MR aqueous solution and then, the MC structures are treated by quantum approaches. Thus, the solvent effects were investigated including on the INDO/CI-S calculations initially the micro and further the first hydration shells. As we excepted the absorption spectra are characterized by a strong band placed in the region of lowest energies. To the basic form we computed shoulders at 434.33 ± 0.09 and 436.34 ± 0.56 nm corresponding to the micro and first hydration shells, respectively. Our experimental measurements display this shoulder at 431 nm. Under acidic forms (isoelectronic and zwitterionic), we computed the red shift in relation to the basic compound. To the isoelectronic structure we found the absorption maximum located at 485.80 ± 0.34 and 480.66 ± 0.67 nm to the micro and first shells, respectively. The experimental prediction of 513 nm is in good agreement with theoretical result. Finally the zwitterionic form we did not obtain a micro layer, therefore we used only the first shell. Our theoretical results are converged to 502.13 ± 0.79 nm in good concordance with the experimental confirmation of 511 nm.

These samples were optically transparent, hard and resistant to dye leaching and to chemical attacks, being promising materials to be used in optical devices.     

Frequency upconversion of Tm and Yb codoped YLiF4 synthesized by hydrothermal method

In this paper, YLiF₄ codoped with Tm³⁺ and Yb³⁺ ions was synthesized by hydrothermal method. Yb³⁺ concentration is fixed at 1.5%, and Tm³⁺ concentration is changed from 0.1 to 0.4%. Intense upconversion luminescence is observed when the samples are excited by 980 nm. The dependence of upconversion luminescence on Tm³⁺ concentrations is presented. The results show that upconversion luminescence increases with the Tm⁺ concentration and gets its peak at 0.3 mol%. Under the excitation of 980 nm, the blue emission of 479 nm and the red emission of 647 nm are both duo to two photons process, and the UV emission of 361 nm is attributed to the three photons process. We also analyse the upconversion mechanism and process.     

A fiber-optic-based calibration system for the High Resolution Fly's Eye cosmic ray observatory

This article describes the fiber-optic-based calibration system installed at the High Resolution Fly's Eye (HiRes) astro-particle physics observatory. The HiRes detectors measure ultra violet scintillation light from distant extensive air showers. This automated calibration system delivers light from a frequency tripled 355 nm YAG laser to the 10,752 photo-multiplier tubes of the 42 HiRes-II detectors.     

Growth of thin Ag films produced by radio frequency magnetron sputtering

Thin Ag films in the thickness range D = 14–320 nm were deposited by radio frequency magnetron sputtering on glass substrates at room temperature inside a vacuum chamber with base pressure of about 5 × 10^− 6 Pa. The growth of the films was studied via X-ray diffraction and atomic force microscopy experiments. The two techniques are complementary and give us the opportunity to study the surface roughness, the statistical distribution and the average value of the grain size, as well as the texture of the samples. It is shown that the film roughness increases negligibly within the first 60 atomic layers of growth. The thicker films (D 300 nm) develop a nanocrystalline structure with a root mean square roughness of about 2.5 nm. The grain size evolves linearly with the thickness from 9.4 nm at D = 54 nm to 31.6 nm at D = 320 nm.