We present an approach for wafer-level rapid multi-modal defect non-destructive imaging of device-relevant GaN defects with high resolution and high sensitivity. The scanning GaN defects detection system is based on laser pump-and-probe photoluminescence and photothermal measurements that are compared to diode device reliability data from accelerated lifetime testing. This work hypothesis is that defects probed at optical frequencies can reliably predict reliability or performance issues of power electronic devices at near DC frequencies. Imaging, growth, and device data are correlated to validate the proposed multi-modal defect detection approach for detection of GaN defects relevant to power electronic devices.
We present a method for producing spatially patterned glass engraved meta-surfaces, which is scalable, has high mechanical stability and high laser damage durability, and thus promising for ultra-thin optics implementation for high-power lasers. The process is based on laser raster-scan of a thin metal film on a glass, followed by dry-etching and removal of the metal mask. We present fabricated structures, characterization of their optical performance, mechanical stability and laser damage performance.
We present a simple and scalable method for the production of optics with incorporated metasurfaces, resulting in durable all-dielectric based meta-optics. The scalability and robustness of this method overcome limitations imposed by current technology when fabricating metasurfaces for high power laser applications, while the simplicity of the fabrication process makes it an exciting technique for metasurface generation. This talk will describe the method, show resultant fabricated metasurfaces and the sensitivity introduced by processing parameters – i.e. control over generated surfaces, and discuss the laser damage performance of these engineered large-scale metasurfaces.
We are investigating conductive gallium nitride films grown on c-plane sapphire for use in a new area of application, high-power optoelectronics. It was found that optically-induced damage in gallium nitride-based transparent conductive thin films occurs at incident laser intensities significantly greater than in conventional metal-oxide based thin films. Furthermore, damage in gallium nitride epi-layers displays a unique morphology consisting of discrete, faceted pits which appear to initiate within fast-grown layers when exposed to high intensity near-infrared laser irradiation. We developed an integrated laser damage system with in-situ diagnostics to probe this damage mode and conducted damage tests of aluminum nitride and gallium nitride/aluminum nitride samples grown under various conditions. Through in-depth analyses using optical microscopy and results from high-throughput damage tests, this paper elucidates some of the prevailing damage processes and design considerations for gallium nitride transparent conductive films important for emerging high-power laser applications.
Transparent conducting films with superior laser damage performance have drawn intense interests toward optoelectronic applications under high energy density environment. In order to make optoelectronic applications with high laser damage performance, a fundamental understanding of damage mechanisms of conducting films is crucial. In this study, we performed laser damage experiments on tin-doped indium oxide films (ITO, Bandgap = 4.0 eV) using a nanosecond (ns) pulse laser (1064 nm) and investigated the underlying physical damage mechanisms. Single ns laser pulse irradiation on ITO films resulted in common thermal degradation features such as melting and evaporation although the laser photon energy (1.03 eV, 1064 nm) was smaller than the bandgap. Dominant laser energy absorption of the ITO film is attributed to free carriers due to degenerate doping. Upon multi-pulse irradiation on the film, damage initiation and growth were observed at lower laser influences, where no apparent damage was formed upon single pulse, suggesting a laser-induced incubation effect.
The ablation of magnetron sputtered metal films on fused silica substrates by a 1053 nm, picosecond class laser was studied as part of a demonstration of its use for in-situ characterization of the laser spot under conditions commonly used at the sample plane for laser machining and damage studies. Film thicknesses were 60 and 120 nm. Depth profiles and SEM images of the ablation sites revealed several striking and unexpected features distinct from those typically observed for ablation of bulk metals. Very sharp thresholds were observed for both partial and complete ablation of the films. Partial film ablation was largely independent of laser fluence with a surface smoothness comparable to that of the unablated surface. Clear evidence of material displacement was seen at the boundary for complete film ablation. These features were common to a number of different metal films including Inconel on commercial neutral density filters, stainless steel, and aluminum. We will present data showing the morphology of the ablation sites on these films as well as a model of the possible physical mechanisms producing the unique features observed.
Recent work on laser-induced crystallization of thin films and nanostructures is presented. Characterization of the morphology of the crystallized area reveals the optimum conditions for sequential lateral growth in a-Si thin films under
high-pulsed laser irradiation. Silicon crystal grains of several micrometers in lateral dimensions can be obtained
Laser-induced grain morphology change is observed in silicon nanopillars under a transmission electron microscopy (TEM) environment. The TEM is coupled with a near-field scanning optical microscopy (NSOM) pulsed laser processing system. This combination enables immediate scrutiny on the grain morphologies that the pulsed laser
irradiation produces. The tip of the amorphous or polycrystalline silicon pillar is transformed into a single crystalline
domain via melt-mediated crystallization. The microscopic observation provides a fundamental basis for laser-induced
conversion of amorphous nanostructures into coarse-grained crystals.
A laser beam shaping strategy is introduced to control the stochastic dewetting of ultrathin silicon film on a foreign
substrate under thermal stimulation. Upon a single pulse irradiation of the shaped laser beam, the thermodynamically
unstable ultrathin silicon film is dewetted from the glass substrate and transformed to a nanodome. The results suggest that the laser beam shaping strategy for the thermocapillary-induced de-wetting combined with the isotropic etching is a
simple alternative for scalable manufacturing of array of nanostructures.
We present a method to repair damaged optics using laser-based chemical vapor deposition (L-CVD). A CO2 laser
is used to heat damaged silica regions and polymerize a gas precursor to form SiO2. Measured deposition rates and
morphologies agree well with finite element modeling of a two-phase reaction. Along with optimizing deposition
rates and morphology, we also show that the deposited silica is structurally identical to high-grade silica substrate
and possesses high UV laser damage thresholds. Successful application of such a method could reduce processing
costs, extend optic lifetime, and lead to more damage resistant laser optics used in high power applications.
Recent research results are presented where lasers of different pulse durations and wavelengths have been coupled to
near-field-scanning optical microscopes (NSOMs) through apertured bent cantilever fiber probes and atomic force
microscope (AFM) tips in apertureless configurations. Experiments have been conducted on the surface modification of
metals and semiconductor materials. By combining nanoscale ablative material removal with subsequent chemical
etching steps, ablation nanolithography and patterning of fused silica and crystalline silicon wafers has been
demonstrated. Confinement of laser-induced crystallization to nanometric scales has also been shown. In-situ observation of the nanoscale materials modification was conducted by coupling the NSOM tips with a scanning electron
microscope (SEM). Nucleation and growth of semiconductor materials have been achieved by laser chemical vapor
deposition (LCVD) at the nanoscale level. Locally selective growth of crystalline silicon nanowires with controlled size,
heterogeneity and nanometric placement accuracy has been accomplished.