Laser resistance dependence of interface for high-reflective coatings studied by capacitance-voltage and absorption measurement.

HfO2/SiO2 bilayer coatings and multilayer high-reflection coatings without and with a modified co-evaporated interface (MCEI) have been prepared. An MCEI is designed to be evaporated at an oxygen-deficient environment to achieve higher absorption than the conventional discrete interface. Capacitance-voltage measurements and absorption measurements demonstrate that an MCEI increases the trap density and leads to higher absorption. The laser-induced damage threshold and nano-indenter test results indicate that the MCEI multilayer coating exhibits better laser resistance and mechanical property, despite the larger absorption. The experimental results suggest that adhesive force between layers plays a more important role in nanosecond laser damage resistance than interface absorption.

High photon conversion efficiency continuous wave lasing in an optically pumped I2 hollow fiber gas laser in the visible region.

Continuous wave lasing in the visible spectral region from a molecular iodine-filled hollow core photonic crystal fiber is demonstrated. More than an order of magnitude improvement in photon conversion efficiency has been achieved compared to previous nonfiber-based geometries in this spectral region. The laser shows strong coupling of pump and laser polarization.

Modeling third harmonic generation from layered materials using nonlinear optical matrices: erratum.

We present corrected versions of Equations (11), (13), and (14) where typos were made.

Spatio-TEmporally REsolved Optical Laser Induced Damage (STEREO LID) technique for material characterization.

A technique for measuring the ablation and laser-induced damage threshold (LIDT) by identifying the temporal onset of damage and location of initiation within the beam profile is demonstrated. This new method, dubbed Spatio-TEmporally REsolved Optical Laser Induced Damage (STEREO LID), is compared to traditional damage tests and its advantages are exemplified.

Frequency tripling mirror.

A frequency tripling mirror (FTM) is designed, fabricated and demonstrated. The mirror consists of an aperiodic sequence of metal oxide layers on a fused silica substrate tailored to produce the third harmonic in reflection. An optimized 25-layer structure is predicted to increase the reflected TH by more than five orders of magnitude compared to a single hafnia layer, which is a result of global compensation of the phase mismatch of TH and fundamental, field enhancement and design favoring reflection. Single pulse conversion efficiencies approaching one percent have been observed with the 25-layer stack for fundamental wavelengths in the near infrared and 55 fs pulse duration. The FTM is scalable to higher conversion, larger bandwidths and other wavelength regions making it an attractive novel nonlinear optical component based on optical interference coatings.

Determination of defect densities from spatiotemporally resolved optical-laser induced damage measurements.

A procedure is developed to retrieve defect densities of optical coatings and surfaces from spatiotemporally resolved optical-laser induced damage (STEREO-LID) measurements. In STEREO-LID, the temporal onset and location of nanosecond laser damage initiation is measured for each excitation event. The power of STEREO-LID relative to traditional damage tests resulting in damage probabilities is characterized with LID data from Monte Carlo simulations.

Modeling third-harmonic generation from layered materials using nonlinear optical matrices.

A matrix approach is formulated to describe third-harmonic (TH) generation in stacked materials in the small signal limit, in both transmission and reflection geometries. The model takes into account the contribution from the substrate to the total generated TH, interference of fundamental and nonlinear fields inside the stack, the nonlinear signal generation in forward and backward direction, the beam profile of the focused incident beam in the substrate, and the finite spectrum associated with short laser pulses. The model is applied to design stacks of thin films for efficient TH generation.

Characterization and χ(3) measurements of thin films by third-harmonic microscopy.

Third-harmonic (TH) generation from a thin layer on a substrate is analyzed in reflection and transmission geometry taking into account interference effects of fundamental and TH waves in the film, the backward-generated TH, and the pump-beam profile. Conditions are derived for both geometries where the signal from the film dominates, which is important for TH microscopy. The analysis results are applied to retrieve from experiment nonlinear susceptibilities χ(3) of hafnia/silica mixture (Hf(x)Si(1-x)O2), alumina (Al2O3), and scandia (Sc2O3) thin films.

Time-dependent absorption of TiO2 optical thin films under pulsed and continuous wave 790 nm laser irradiation.

The time-dependent absorption at 790 nm of TiO2 films prepared by ion-beam sputtering (IBS) and electron-beam evaporation (EBE) was measured. The pump source was a Ti:sapphire oscillator that was operated in CW and pulsed (50 fs) modes. The absorption coefficient of the IBS film under CW illumination was 8 cm-1, independent of time and power. Under pulsed illumination, there was evidence of three-photon absorption, and the total absorption increased 10-fold over time at the highest measured irradiance. The absorption of the EBE film had higher initial absorption (∼24 cm-1) and increased under both CW and pulsed illumination with time. An electron state model based on band-to-band excitation and electron trapping is presented that explains the observed results. The implications for laser-induced damage of oxide coatings are discussed.

Characterization of laser-induced air plasmas by third harmonic generation.

Third harmonic generation by a weak femtosecond probe pulse intersecting a pump laser-induced plasma in air is investigated and a general model is developed to describe such signal, applicable to a wide range of focusing and plasma conditions. The effect of the surrounding air on the generated signal is discussed. The third-order nonlinear susceptibility of an air plasma with electron density N(e) is determined to be χ(p)((3)) = χ(a)((3)) + γ(p)N(e) with γ(p) = 2 ± 1 × 10(-49) m(5) V(-2) and χ(a)((3)) being the third-order susceptibility in air. Lateral scans of the probe through the plasma are used to determine electron density profiles and the effect of focusing and phase mismatch is discussed.

Femtosecond pulse damage thresholds of dielectric coatings in vacuum.

The dielectric breakdown behavior of dielectric coatings in studied for different ambient gas pressures with femtosecond laser pulses. At 10(-7) Torr, the multiple femtosecond pulse damage threshold, Fm, is about 10% of the single pulse damage fluence F(1) for hafnia and silica films compared to about 65% and 50%, respectively, at 630 Torr. In contrast, the single-pulse damage threshold is pressure independent. The decrease of Fm with decreasing air pressure correlates with the water vapor and oxygen content of the ambient gas with the former having the greater effect. The decrease in Fm is likely associated with an accumulation of defects derived from oxygen deficiency, for example vacancies. From atmospheric air pressure to pressures of ~3x10(-6) Torr, the damage "crater" starts deterministically at the center of the beam and grows in diameter as the fluence increases. At pressure below 3x10(-6) Torr, damage is initiated at random "sites" within the exposed area in hafnia films, while the damage morphology remains deterministic in silica films. A possible explanation is that absorbing centers are created at predisposed sample sites in hafnia, for example at boundaries between crystallites, or crystalline and amorphous phases.

Mid-infrared gas filled photonic crystal fiber laser based on population inversion.

We demonstrate for the first time an optically pumped gas laser based on population inversion using a hollow core photonic crystal fiber (HC-PCF). The HC-PCF filled with 12C2H2 gas is pumped with ~5 ns pulses at 1.52 µm and lases at 3.12 µm and 3.16 µm in the mid-infrared spectral region. The maximum measured laser pulse energy of ~6 nJ was obtained at a gas pressure of 7 torr with a fiber with 20 dB/m loss near the lasing wavelengths. While the measured slope efficiencies of this prototype did not exceed a few percent due mainly to linear losses of the fiber at the laser wavelengths, 25% slope efficiency and pulse energies of a few mJ are the predicted limits of this laser. Simulations of the laser's behavior agree qualitatively with experimental observations.

Comparative study of femtosecond and nanosecond laser-induced breakdown spectroscopy of depleted uranium.

We present spectra of depleted uranium metal from laser plasmas generated by nanosecond Nd:YAG (1064 nm) and femtosecond Ti:sapphire (800 nm) laser pulses. The latter pulses produce short-lived and relatively cool plasmas in comparison to the longer pulses, and the spectra of neutral uranium atoms appear immediately after excitation. Evidence for nonequilibrium excitation with femtosecond pulses is found in the dependence of spectral line intensities on the pulse chirp.

Nanolaminate-based design for UV laser mirror coatings.

With ever-increasing laser power, the requirements for ultraviolet (UV) coatings increase continuously. The fundamental challenge for UV laser-resistant mirror coatings is to simultaneously exhibit a high reflectivity with a large bandwidth and high laser resistance. These characteristics are traditionally achieved by the deposition of laser-resistant layers on highly reflective layers. We propose a "reflectivity and laser resistance in one" design by using tunable nanolaminate layers that serve as an effective layer with a high refractive index and a large optical bandgap. An Al2O3-HfO2 nanolaminate-based mirror coating for UV laser applications is experimentally demonstrated using e-beam deposition. The bandwidth, over which the reflectance is >99.5%, is more than twice that of a traditional mirror with a comparable overall thickness. The laser-induced damage threshold is increased by a factor of ~1.3 for 7.6 ns pulses at a wavelength of 355 nm. This tunable, nanolaminate-based new design strategy paves the way toward a new generation of UV coatings for high-power laser applications.

Determination of the transient electron temperature in a femtosecond-laser-induced air plasma filament.

The transient electron temperature in a weakly ionized femtosecond-laser-produced air plasma filament was determined from optical absorption and diffraction experiments. The electron temperature and plasma density decay on similar time scales of a few hundred picoseconds. Comparison with plasma theory reveals the importance of inelastic collisions that lead to energy transfer to vibrational degrees of freedom of air molecules during the plasma cooling.

Photobleaching resistance of stimulated parametric emission in microscopy.

Photobleaching of organic fluorescent labels is a ubiquitous problem in fluorescence microscopy, limiting imaging capabilities and presenting hurdles to quantitative biophysical measurements. We report here that a nonlinear optical signal from some organic fluorophores persists in the presence of photobleaching. Specifically, a four-wave mixing process that is enhanced by a two-photon absorption resonance in the target fluorophore, termed stimulated parametric emission (SPE), is essentially unaffected by the photobleaching of the fluorophore, for rhodamine 6G and other commercial green and red fluorophores. The stability of the SPE signal, and the ability to image weakly or nonfluorescent chromophores, should make this nonlinear microscopy useful for quantitative biophysical measurements.

Dispersion-compensating beam shaper for femtosecond optical vortex beams.

A beam shaper suitable for sub-20 fs pulses based on a prism pair and a computer-generated hologram was developed to produce vortex beams. Comparatively high throughput and the ability to tune the group-velocity dispersion make this shaper suitable for pulses from femtosecond oscillators and amplifiers and where there is a need for additional postchirp or prechirp compensation.