Picosecond-Pulse Damage Studies of Diffraction Gratings

Metasurfaces, introduced as a compact 2D alternative of metamaterials, have developed into a vast field in recent times for light manipulation at an ultra-compact scale. Metasurface applications have found a place in the literature for compact alternatives to lens, holograms, polarizers, color filters. Plasmonic metasurfaces consisting of noble metals such as gold and silver provide light confinement on an unprecedented scale. Gold and silver grown conventionally on transparent substrates are polycrystalline, and exhibit losses and limit performance of the device. Moreover, these materials have a lower damage threshold and melting point. To circumvent the lower melting point and damage thresholds, new materials, and material growing techniques need to be researched. <br>In the first part of this work, a metasurface for color holography with an epitaxially grown silver thin film on a transparent substrate is shown. The demonstrated metasurface has been the first ever epitaxial silver metasurface that operated in the transmission mode. This plasmonic hologram has also been the thinnest metasurface hologram operating in transmission mode at the time of its reporting. The holographic image of all three basic color components of red, green, and blue has been demonstrated in the transmission mode. The control of color has been achieved by resonant sub-wavelength slits and the phase can be manipulated through altering slit orientation. This amplitude and phase control pave the way to applications of ultra-compact polychromatic plasmonic metasurfaces for advanced light manipulation. In the second part, we explore temperature rise due to the optical absorption in plasmonic structures. Titanium Nitride based metasurfaces structures are fabricated, that work in harsh environmental conditions and high temperature. A time domain thermo reflectance technique for rapid measurement of temperature is explored. Finally, a practical design prototype for thermo-photovoltaic (TPV) emitters using plasmonic metasurfaces is fabricated and characterized.<br><br>

The energy densities which can be obtained in a laser resonator before optical damage occurs are usually much lower than the optical damage thresholds measured in extracavity laser experiments. We have measured intracavity optical damage thresholds in an alexandrite laser operating in both single- and multitransverse mode and found the multitransverse mode laser has a much lower damage threshold. We studied the temporal and spatial variations of a multitransverse mode laser and found intensity hot spots which may be an important factor in the difference in the intracavity damage thresholds.

The energy of the acoustic pulse generated by laser-surface interactions and measured by probe beam deflection was used to investigate laser surface damage thresholds of fluoride crystals with optical quality. It was found that damage thresholds decrease with increasing density of surface states. The defect density also controls the energy absorption mechanism: for surfaces with few defects, like polished MgF2 and CaF2, avalanche breakdown occurs at above 1 GW/cm2, whereas for materials with lower damage thresholds, such as LiF, BaF2, and roughened or incubated surfaces of CaF2, multiphoton absorption across the band gap is observed.

ABSTRACTBoron nitride thin films on sapphire substrates were investigated for their tribological and optoelectronic applications. A gridless end Hall gun source and an electron cyclotron resonance (ECR) source were used for nitrogen species delivery while pure boron was evaporated at a rate of 0.2 Å/s. The surface stability of these thin films was investigated by high temperature annealing. Atomic force microscopy (AFM), friction force microscopy (FFM), and Knoop microhardness measurements were performed on the materials in order to assess their merits as tribological coatings. Finally, BN thin films were subjected to laser transient photoconductivity (TPC) experiments to determine both their optical laser damage threshold as well as their photoconductivity characteristics. For both single-pulse shot and multiple-pulse irradiation regimes, preliminary tests showed the higher the ion beam current used during growth (70–150 mA), the higher the optical damage threshold. The lower damage threshold was typical of BN films grown using an ECR plasma source and was measured to be in the range of ∼50 MW/cm2. Optical damage of films grown at ion beam currents above 100 mA was not observed at laser intensities up to few hundreds MW/cm2. A multiphoton excitation technique was utilized to obtain PC signals from this wide band gap material and preliminary results show that unusual PC voltage amplitudes as high as 0.5 V were observed.

Examination of the effects of cochlear implantation in the animal model has made it increasingly clear that biological changes may be induced by the device. In a preliminary study we established a damage threshold at 400 microA RMA (70 microCoul/cm2/phi) after a single three-hour exposure to continuous 1 kHz sinusoidal stimulation. By systematic modification of stimulation parameters in the present study we have sought to establish damage thresholds and patterns of histological change in the chronically stimulated animal and further elucidate the mechanisms underlying the histopathological change in a second acute animal preparation. In part one, chronic stimulation at 1 000 Hz for a total of 12 hours distributed over a four-week period resulted in a lower damage threshold at 15-20 microCoul/cm2/phi, the lowest value tested. In addition to sensory and supporting cell degeneration osteoneogenesis and spiral ganglion cell degeneration were also observed. These findings are of some relevance under conditions of prolonged implantation and stimulation in the human. In part two of the study, acute stimulation for a three-hour period at 100 Hz reduced the damage threshold from 400 microA to 200 microA RMS. This inverse relationship between threshold and frequency suggests that electrophoretic effects are, at least in part, responsible for the histological changes observed. The threshold shift was less than predicted, however, making it less likely that a single factor is responsible for the stimulation-induced damage.

The diffraction gratings are widely used in Space-flight satellites for spectrograph instruments or in ground-based telescopes in astronomy. The diffraction gratings are one of the key optical components of such systems and have to exhibit very high optical performances. HORIBA Jobin Yvon S.A.S. (part of HORIBA Group) is in the forefront of such gratings development for more than 40 years. During the past decades, HORIBA Jobin Yvon (HJY) has developed a unique expertise in diffraction grating design and manufacturing processes for holographic, ruled or etched gratings. We will present in this paper an overview of diffraction grating technologies especially designed for space and astronomy applications. We will firstly review the heritage of the company in this field with the space qualification of different grating types. Then, we will describe several key grating technologies developed for specific space or astronomy projects: ruled blazed low groove density plane reflection grating, high-groove density holographic toroidal and spherical grating, and finally transmission Fused Silica Etched (FSE) grism-assembled grating. We will not present the Volume Phase Holographic (VPHG) grating type which is used in Astronomy.

An experimental study of a high-power forward Raman amplifier pumped by a multibeam KrF laser is presented. The active medium is either CH4 at 3-bar pressure or H2 at 1-bar pressure. As many as 56 pump beams confined in a light guide were combined into a single low-divergence Stokes beam. In CH4 a gain of up to 300 and a power-conversion efficiency of 70% was obtained. The maximum Stokes output energy was 8.4 J. The dependence of gain on pump bandwidth was investigated in H2. A gain of 500 and a conversion efficiency of 65% were obtained. The maximum output energy was 5.0 J.

High-power erbium-doped fiber laser with a carbon nanotubes-doped sol-gel glass mode-locker

Dielectric sesquioxide films (Sc2O3, Y2O3, and Lu2O3) were fabricated by pulsed-laser deposition and tested in terms of their laser damage properties for pulses of 500 fs duration, at a wavelength of 1030 nm and at a 10 Hz repetition rate. Comparable tests were performed with magnetron-sputtered thin films of established optical-coating materials (SiO2, HfO2, and Nb2O5), whose results served as a benchmark. The laser-induced damage thresholds of the sesquioxides are comparable to each other, and in the multi-pulse test regime show values close to ones of HfO2 coatings. A lower damage threshold was observed for the polycrystalline Lu2O3 film grown on sapphire compared to single-crystal Lu2O3 grown on yttrium aluminium garnet (Y3Al5O12), attributed to the highly textured morphology and potential for a greater density of defect states in these films. We conclude that pulsed-laser deposition is a potential fabrication method of sesquioxides for use in high-power resistant optical components for ultrashort-pulse lasers.

In this paper we discuss how different feedback gratings affect the tuning range and the output power of external feedback diode laser systems. A tunable high-power narrow-spectrum external-cavity diode laser system around 455 nm is investigated. The laser system is based on a high-power GaN diode laser in a Littrow external-cavity. Both a holographic diffraction grating and a ruled diffraction grating are used as feedback elements in the external cavity. The output power, spectral bandwidth, and tunable range of the external cavity diode laser system are measured and compared with the two gratings at different injected currents. When the holographic grating is used, the laser system can be tuned over a range of 1.4 nm with an output power around 530 mW. When the ruled grating is used, the laser system can be tuned over a range of 6.0 nm with an output power around 80 mW. The results can be used as a guide for selecting gratings for external-cavity diode lasers for different requirements.

Improper back-coupling of optical feedback is a known cause for accelerated gradual degradation as well as catastrophic optical mirror damage of GaAs-based high-power semiconductor lasers. For those material systems emitting below the GaAs bandgap (870 nm at room temperature), such damage is induced by feedback-light absorption in the non-transparent cap layers and substrate, which intensifies the heating of the area close to the emission facet. For InGaAs/AlGaAs-based diode lasers in the spectral range from 880 to 1060 nm, however, all passive layers as well as the substrate are, with respect to interband absorption, transparent for the lasing wavelength so that, in this case, the degrading mechanism of misaligned optical feedback could not be explained entirely. By use of an experimental multi-device study of 950-nm broad-area lasers, we demonstrate that the threshold of catastrophic optical damage is decreased the most by positioning the feedback return spot such that it covers the p-side solder and the highly p-doped semiconductor layers. By means of thermoreflectance microscopy in conjunction with numerical simulations we find that such low damage thresholds are caused by a strong localization of the absorption of feedback light close to the laser front facet.

Plasma impulse chemical vapour deposition—a novel technique for the production of high power laser mirrors

We show here that a highly effective diffraction grating can be embedded into a nanoparticulate TiO2 film via imprinting combined with TiCl4 treatment. A thin TiO2 film spin-coated on a glass substrate was patterned by imprinting and this patterned layer was TiCl4-treated with a higher concentration than for a thicker over-coated TiO2 film. Due to the refractive index difference between the two layers, the incident light could be strongly diffracted. Different types of gratings have been incorporated into the films. A line grating with a 2 μm period and 1.2 μm thickness generated diffracted beams up to 3rd-order, with total diffraction efficiencies over 90% at 633 nm. This method has potential applications in TiO2-based photovoltaic and photocatalytic devices.

Application of holographic sub-wavelength diffraction gratings for monitoring of kinetics of bioprocesses

Q‐switched and mode‐locked erbium‐doped fibre lasers (EDFLs) are demonstrated by using non‐conductive graphene oxide (GO) paper as a saturable absorber (SA). A stable and self‐starting Q‐switched operation was achieved at 1534.4 nm by using a 0.8 m long erbium‐doped fibre (EDF) as a gain medium. The pulse repetition rate changed from 14.3 to 31.5 kHz, whereas the corresponding pulse width decreased from 32.8 to 13.8 µs as the pump power increased from 22 to 50.5 mW. A narrow spacing dual‐wavelength Q‐switched EDFL could also be realised by including a photonics crystal fibre and a tunable Bragg filter in the setup. It can operate at a maximum repetition rate of 31 kHz, with a pulse duration of 7.04 µs and pulse energy of 2.8 nJ. Another GOSA was used to realise mode‐locked EDFL in a different cavity consisting of a 1.6 m long EDF in conjunction with 1480 nm pumping. The laser generated a soliton pulse train with a repetition rate of 15.62 MHz and pulse width of 870 fs. It is observed that the proposed fibre lasers have a low pulsing threshold pump power as well as a low damage threshold.