This work explores reactive ion etching parameters in order to identify and optimize key characteristics in gratings that govern their overall performance, including minimization of sidewall and trench structural defects and modification of fused silica via intrinsic molecular-level defects. This study is performed using grating-like samples with 5-µm-wide lines and trenches generated in fused silica by the photolithographic process and inductively coupled plasma-reactive ion etching. The analysis compiles metrology, simulation, and damage-testing results to obtain a better understanding of how to modify the fabrication process of gratings toward achieving better laser-induced–damage performance.
The long-term performance of high-power laser systems is adversely affected by particle contaminants that are introduced into the system during the manufacturing of optical components and the handling during installation and operation of the laser system. Such particles can absorb or focus laser energy, reducing the laser-induced–damage threshold (LIDT) values. We developed ultrathin coatings that can decrease the overall load of contamination and aid with the removal of the already-accumulated particles using simple gas-flow cleaning. These coatings do not alter the intrinsic LIDT values, and they remain stable over time and during the system operation.
KEYWORDS: Contamination, Reactive ion etching, Etching, Laser damage threshold, Chemical analysis, Systems modeling, Silica, Scanning electron microscopy, Polymers, Optical fabrication
We investigate contamination induced in grating-like structures during the etching process as a possible precursor to laser-induced damage. Our experimental model utilizes 5-mm line structures fabricated in E-beam–deposited coatings of silica using reactive ion etching (RIE) and reactive ion beam etching (RIBE). This makes it possible to compare the behavior in the pillars and trench regions. The results suggest that surface contaminants are primarily fluorinated polymers, while embedded contaminants consist primarily of carbon with very low detection of fluorine. Samples fabricated by the RIBE method exhibit significantly reduced roughness in the trenches, yet still present similar embedded contamination.
Silica substrates coated with organic thin films were exposed to stainless steel and silica micro-particles to determine the effectiveness in preventing particle contamination and cleaning efficiency by air flows. Three specially designed monolayers coatings were developed and tested. Laser induced damage tests were conducted to confirm that the coatings do not affect the LIDT values. The results suggest that although the accumulation of particles is not significantly affected, the coated substrates exhibit significantly improved cleaning efficiency with air flow. A size distribution analysis was conducted to study the adsorption and cleaning efficiency of particles of different sizes.
Understanding of the laser-induced damage threshold and impact of air–vacuum cycling of the optical components in short-pulse laser systems is of fundamental importance. We report the results of a damage-testing campaign that monitored representative pulse compression grating samples that were positioned inside the OMEGA EP grating compressor vacuum chamber during normal operation and routinely damage tested on every quarterly vent for a period of about 10 years. The evolution of their damage resistance under 10-ps and 100-ps pulse lengths are associated with a significant degradation of the laser-induced damage thresholds, which is comparatively larger at 100 ps, is described.
The standardization and comparison of laser-damage protocols and results are essential prerequisites for development and quality control of large optical components used in high-power laser facilities. To this end, the laser-induced–damage thresholds of two different coatings were measured in a round-robin experiment involving five well-equipped damage testing facilities. Investigations were conducted at the wavelength of 1 μm in the sub picosecond pulse duration range with different configurations in terms of polarization, angle of incidence, and environment (air versus vacuum). In this temporal regime, the damage threshold is known to be deterministic, i.e., the continuous probability distribution transitions from 0 to 1 over a very narrow fluence range. This in turn implies that the damage threshold can be measured very precisely. These characteristics enable direct comparison of damage-threshold measurements between different facilities, with the difference in the measured values indicating systematic errors or other parameters that were not previously appreciated. The results of this work illustrate the challenges associated with accurately determining the damage threshold in the short-pulse regime. Specifically, the results of this round-robin damage-testing effort exhibited significant differences between facilities. The factors to be taken into account when comparing the results obtained with different test facilities are discussed: temporal and spatial profiles, environment, damage detection, sample homogeneity, and nonlinear beam propagation.
The standardization and the comparison of laser-damage testing are essential prerequisites for development and quality control of large optical components used in high-power laser facilities. To this end, the laser-induced–damage thresholds of two different coatings were measured at four laboratories involved in a round-robin experiment. Tests were conducted at 1 m in the subpicosecond range with different configurations in terms of polarization, angle of incidence, and environment (air versus vacuum). In this temporal regime, the damage threshold is known to be deterministic, i.e., the continuous probability distribution transitions from 0 to 1 over a very narrow fluence range. This in turn implies that the damage threshold can be measured very precisely. These traits enable direct comparison of damage-threshold measurements between different facilities, while the difference in the measured values are not accompanied by large statistical uncertainties.
In this presentation, the results of this comparative experiment are compiled, illustrating the challenges associated with accurately determining the damage threshold in the short-pulse regime. Specifically, the results of this this round-robin damage-testing effort exhibited significant differences between facilities. The factors to be taken into account when comparing the results obtained with different test facilities are discussed: temporal and spatial profiles, environment, damage detection, samples homogeneity and nonlinear beam propagation.
Damage-test data are scarce for liquid crystalline (LC) materials at 1-ns pulse lengths and nonexistent at shorter pulselengths.
Here we describe the methodology to develop a comprehensive database of damage performance for typical
nematic LC’s for a wide range of pulse lengths at 1053 nm. This series of nematic LC materials investigates the effect of
a varying degree of π-electron delocalization. Obtaining damage-threshold measurements is of fundamental interest for
the consideration of LC materials for applications in short-pulse laser systems.
The laser damage thresholds of various HfO2/SiO2-based thin film coatings, including multilayer dielectric (MLD) gratings and high reflectors of different designs, prepared by E-beam and Plasma Ion Assisted Deposition (PIAD) methods, were investigated in vacuum, dry nitrogen, and after air-vacuum cycling. Single and multiple-pulse damage thresholds and their pulse-length scaling in the range of 0.6 to 100 ps were measured using a vacuum damage test station operated at 1053nm. The E-beam deposited high reflectors showed higher damage thresholds with square-root pulse-length scaling, as compared to PIAD coatings, which typically show slower power scaling. The former coatings appeared to be not affected by air/vacuum cycling, contrary to PIAD mirrors and MLD gratings. The relation between 1-on-1 and N-on-1 damage thresholds was found dependent on coating design and deposition methods.
The role of thin-film interfaces in the near-ultraviolet (near-UV) absorption and pulsed laser-induced damage was studied for ion-beam-sputtered and electron-beam-evaporated coatings comprised from HfO2 and SiO2 thin-film pairs. To separate contributions from the bulk of the film and from interfacial areas, absorption and damage threshold measurements were performed for a one-wave (355-nm wavelength) thick, HfO2 single-layer film and for a film containing seven narrow HfO2 layers separated by SiO2 layers. The seven-layer film was designed to have a total optical thickness of HfO2 layers, equal to one wave at 355 nm and an E-field peak and average intensity similar to a single-layer HfO2 film. Absorption in both types of films was measured using laser calorimetry and photothermal heterodyne imaging. The results showed a small contribution to total absorption from thin-film interfaces as compared to HfO2 film material. The relevance of obtained absorption data to coating near-UV, nanosecond-pulse laser damage was verified by measuring the damage threshold and characterizing damage morphology. The results of this study revealed a higher damage resistance in the seven-layer coating as compared to the single-layer HfO2 film in both sputtered and evaporated coatings. The results are explained through the similarity of interfacial film structure with structure formed during the codeposition of HfO2 and SiO2 materials.
The role of thin-film interfaces in the near-ultraviolet absorption and pulsed-laser–induced damage was studied for ion-beam–sputtered coatings comprised of HfO2 and SiO2 thin-film pairs. To separate contributions from the bulk of the film and from interfacial areas, absorption and damage threshold were measured for a one-wave (355-nm)–thick HfO2 single-layer film and for a film containing seven narrow HfO2 layers separated by SiO2 layers. The seven-layer film was designed to have a total optical thickness of HfO2 layers equal to one wave at 355 nm and an E-field peak and average intensity similar to a single-layer HfO2 film. Absorption in both types of films was measured using laser calorimetry and photothermal heterodyne imaging. The results showed a small contribution to total absorption from thinfilm interfaces, as compared to HfO2 film material. The relevance of obtained absorption data to coating near-ultraviolet, nanosecond-pulse laser damage was verified by measuring the damage threshold and characterizing damage morphology. The results of this study revealed a higher damage resistance in the seven-layer coating as compared to the single-layer HfO2 film, in agreement with data recently reported for similarly designed electron-beam–deposited coatings. The results are explained through the similarity of interfacial film structure and structure formed during the co-deposition of HfO2 and SiO2 materials.
Near-ultraviolet absorption in hafnium oxide and silica oxide thin-film pairs in a configuration strongly departing from the regular quarter-wave–thickness approach has been studied with the goal of separating film and interfacial contributions to absorption and pulsed laser damage. For this purpose, we manufactured a model HfO2 SiO2 thin-film coating containing seven HfO2 layers separated by narrow SiO2 layers and a single-layer HfO2 film in one coating run. The two coatings were characterized by a one-wave total optical thickness for the HfO2 material and similar E-field peak intensity inside the film. Absorption in the electron-beam–deposited films was measured using photothermal heterodyne imaging. By comparing absorption for the seven-layer and single-layer films, one can estimate the partial HfO2 SiO2 interface contribution. Relevance of obtained data to the thin-film pulsed-laser damage was verified by conducting 351-nm, nanosecond-laser–damage measurements and damage-morphology characterization using atomic force microscopy.
Hafnium oxide (HfO2) is the most frequently used high-index material in multilayer thin-film coatings for high-power laser applications ranging from near-infrared to near-ultraviolet (UV). Absorption in this high-index material is also known to be responsible for nanosecond-pulse laser-damage initiation in multilayers. In this work, modification of the near-UV absorption of HfO2 monolayer films subjected to irradiation by continuous-wave (cw), 355-nm or 351-nm laser light focused to produce power densities of the order of ∼100 kW/cm2 is studied. Up to a 70% reduction in absorption is found in the areas subjected to irradiation. Temporal behavior of absorption is characterized by a rapid initial drop on the few-tens-of-seconds time scale, followed by a longer-term decline to a steady-state level. Absorption maps generated by photothermal heterodyne imaging confirm the permanent character of the observed effect. Nanosecond-pulse, 351-nm and 600-fs, 1053-nm laser-damage tests performed on these cw laser–irradiated areas confirm a reduction of absorption by measuring up to 25% higher damage thresholds. We discuss possible mechanisms responsible for near-UV absorption annealing and damage-threshold improvement resulting from irradiation by near-UV cw laser light.
Hafnium oxide is the most frequently used high-index material in multilayer thin-film coatings for high-power laser
applications ranging from near-infrared to near-ultraviolet. Absorption in this high-index material is also known to be
responsible for nanosecond-pulse laser-damage initiation in multilayers. In this work, modification of the near-ultraviolet
absorption of HfO2 monolayer films subjected to irradiation by continuous-wave (cw) 355-nm or 351-nm laser light
focused to produce power densities of the order of ~100 kW/cm2 is studied. Up to a 70% reduction in absorption is
found in the areas subjected to irradiation. Temporal behavior of absorption is characterized by a rapid initial drop on the
few-tens-of-seconds time scale, followed by a longer-term decline to a steady-state level. Absorption maps generated by
photothermal heterodyne imaging confirm the permanent character of the observed effect. Nanosecond-pulse, 351-nm
and 600-fs, 1053-nm laser-damage tests performed on these cw laser-irradiated areas confirm reduction of absorption by
measuring up to 25% higher damage thresholds. We discuss possible mechanisms responsible for near-ultraviolet
absorption annealing and damage-threshold improvement resulting from irradiation by near-ultraviolet cw laser light.
Multilayer-dielectric (MLD) diffraction gratings are used in high-power laser systems to compress laser-energy pulses.
The peak power deliverable on target for these short-pulse petawatt class systems is limited by the laser-damage
resistance of the optical components in the system, especially the MLD gratings. Recent experiments in our laboratory
have shown that vapor treatment of MLD gratings at room temperature with organosilanes such as hexamethyldisilazane
(HMDS) produces an increase in their damage threshold at 1054 nm (10-ps, 370- μm spot size) as compared to uncoated
MLD grating control samples. The 1-on-1 laser-damage threshold of an HMDS-treated grating increased by 4.5% as
compared to the uncoated control sample, while the N-on-1 damage threshold of an MLD grating treated with
tetramethyldisilazane increased by 16.5%. For an MLD grating treated with bis-(trifluoropropyl)tetramethyldisilazane,
the N-on-1 and 1-on-1 damage thresholds increased by 4.8% and 5.3%, respectively. Such increases in laser-damage
threshold are unprecedented and counterintuitive because it is widely believed that the presence of organic materials or
coatings on the surfaces of optical substrates will inevitably lead to reduced laser-damage resistance.
Multilayer dielectric (MLD) diffraction gratings are an essential component for the OMEGA EP short-pulse, highenergy
laser system. The MLD gratings must have both high-optical-diffraction efficiency and high laser-damage
threshold to be suitable for use within the OMEGA EP Laser System. Considerable effort has been directed toward
optimizing the process parameters required to fabricate gratings that can withstand the 2.6-kJ output energy delivered by
each beam.
In this paper, we discuss a number of conventional semiconductor chemical cleaning processes that have been
investigated for grating cleaning, and present evidence of their effectiveness in the critical cleaning of MLD gratings
fabricated at LLE. Diffraction efficiency and damage-threshold data were correlated with both scanning electron
microscopy (SEM) and time-of-flight secondary ion-mass spectrometry (ToF-SIMS) to determine the best combination
of cleaning process and chemistry. We found that using these cleaning processes we were able to exceed both the LLE
diffraction efficiency (specification >97%) and laser-damage specifications (specification >2.7 J/cm2).
Multilayer dielectric (MLD) diffraction gratings are a key component for the construction of high-peak-power, pulse-compressed laser systems. While a great deal of effort has been devoted to the design of optimal grating structures and the etching of these structures into the MLD coating, there has not been the same effort put into the optimization of the MLD coating itself. The primary characteristics of the multilayer that must be considered during design include minimization of the standing wave created in the photoresist because of the reflectivity of the coated optical surface, creation of a sufficiently high reflectivity at the use wavelength and incidence angle in a dry environment, proper balance of the individual layer materials to yield a coating with an overall neutral or slightly compressive stress, and a high laser-damage threshold for the wavelength and pulse duration of use. This work focuses on the modification of a standard MLD mirror, while considering these characteristics, to allow the fabrication of a diffraction grating with higher efficiency and laser-damage threshold than is typically achieved. Scanning electron microscopy (SEM) images of the grating structures demonstrate smoother shapes with lower roughness due to the holographic exposure. Damage testing performed at 1053 nm with a pulse width of 10 ps demonstrates the MLD coating has a sufficiently high laser-damage threshold to form the basis of reflection gratings that survive in high-fluence applications.
The OMEGA EP Facility includes two high-energy, short-pulse laser beams that will be focused to high intensity in the OMEGA target chamber, providing backlighting of compressed fusion targets and investigating the fast-ignition concept. To produce 2.6-kJ output energy per beam, developments in grating compressor technology are required. Gold-coated diffraction gratings limit on-target energy because of their low damage fluence. Multilayer dielectric (MLD) gratings have shown promise as high-damage-threshold, high-efficiency diffraction gratings suitable for use in high-energy chirped-pulse amplification [ B. W. Shore et al., J. Opt. Soc. Am. A14, 1124 (1997).] Binary 100-mm-diam MLD gratings have been produced at the Laboratory for Laser Energetics (LLE) using large-aperture, holographic exposure and reactive ion-beam etching systems. A diffraction efficiency of greater than 99.5% at 1053 nm has been achieved for gratings with 1740 grooves/mm, with a 1:1 damage threshold of 5.49 J/cm2 diffracted beam fluence at 10 ps. To demonstrate the ability to scale up to larger substrates, several 100-mm substrates have been distributed over an aperture of 47 × 43 cm and successfully etched, resulting in high efficiency over the full aperture. This paper details the manufacture and development of these gratings, including the specifics of the MLD coating, holographic lithography, reactive ion etching, reactive ion-beam cleaning, and wet chemical cleaning.
Thin-film polarizers are essential components of large laser systems such as OMEGA EP and the NIF because of the need to switch the beam out of the primary laser cavity (in conjunction with a plasma-electrode Pockels cell) as well as providing a well-defined linear polarization for frequency conversion and protecting the system from back-reflected light. The design and fabrication of polarizers for pulse-compressed laser systems is especially challenging because of the spectral bandwidth necessary for chirped-pulse amplification.
The design requirements for a polarizer on the OMEGA EP Laser System include a Tp greater than 98% over a spectral range of 1053±4 nm while maintaining a contrast ratio (Tp/Ts) of greater than 200:1 (500:1 goal) over the same range. An allowance must be made for the uniformity of the film deposition such that the specifications are met over the aperture of the component while allowing for some tolerance of angular misalignment. Production results for hafnia/silica designs will be shown, illustrating high transmission and contrast over an extended wavelength/angular range suitable for the 8 nm spectral bandwidth of OMEGA EP. Difficulties in production will also be illustrated, as well as the methods being implemented to overcome these challenges. A key challenge continues to be the fabrication of such a coating suitable for use on fused-silica substrates in a dry environment. Laser-damage thresholds for 1-ns and 10-ps pulse widths will be discussed.
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