Angular dependence of the transverse Raman scattering in KDP and DKDP in geometries suitable for beam polarization control

The angular dependance of the transverse Raman scattering in potassium dihydrogen phosphate (KDP) and its deuterated analogue (DKDP) for the entire range of crystal configurations suitable for laser beam polarization control has been investigated via experimental and modeling tools. This work was made possible by simultaneously rotating a spherical sample and the pump polarization to effectively measure the angular dependance of the transverse Raman signal in 360°. This novel method, which is applicable for the investigation of the Raman scattering in optically anisotropic materials, demonstrates that the spontaneous Raman scattering signal exhibits strong angular dependence that is modulated by depolarization and polarization rotation effects generated as the Raman signal traverses the material due to its birefringence. The results show that the total signal generated by the pump beam is the sum of the signals generated by the two components that have polarization parallel and orthogonal to the optic axis. The peak signal intensity, which is of importance for high-power laser applications, depends on the orientation of the optic axis and can vary by a factor of about 2. The excellent agreement between experimental data and modeling results validates the associated models and enables one to consider optimal crystal cut designs for specific applications.

Electric-field enhancement caused by subwavelength-sized particles located on the surface of multilayer dielectric mirrors

A laser pulse impinging on the surface of an optical component can interact with particles, such as contamination debris, to produce a scattered electric field, which, either by itself or combined with the incident laser field, coherently can significantly increase the local field intensity. This effect can be of critical importance as it can reduce the laser-induced-damage threshold of the affected component. In this work, we use a field-propagation code to improve understanding regarding the factors that determine the magnitude and location of the electric-field enhancement for the case of subwavelength-sized particles located on the surface of multilayer dielectric mirrors.

Dynamics of electronic excitations involved in laser-induced damage in HfO2 and SiO2 films

The dynamics of electron excitations associated with the initiation of laser-induced damage in hafnia and silica monolayer films are investigated using time-resolved damage testing involving a pair of 0.7 ps pulses with adjustable delay and laser pulse fluences. Results in hafnia indicate that the relaxation profile depends on the pump-pulse fluence (initial excitation), and as a result, it exhibits an effective lifetime that is variable. Analogous experiments in silica form two different types of damage morphologies that are observed on different ranges of delay times.

Determination of the Raman polarizability tensor in the optically anisotropic crystal potassium dihydrogen phosphate and its deuterated analog

The Raman tensor of the dominant A₁ modes of the nonlinear optical crystalline material potassium dihydrogen phosphate and its 70% deuterated analog have been ascertained. Challenges in determining the A₁ mode tensor element values based on previous reports have been resolved using a specially designed experimental setup that makes use of spherical crystal samples. This novel experimental design enabled the determination of measurement artifacts, including polarization rotation of the pump and/or scattered light propagating through the sample and the contribution of additional overlapping phonon modes, which have hindered previous efforts. Results confirmed that the polarization tensor is diagonal, and matrix elements were determined with high accuracy.

Mechanisms of picosecond laser-induced damage in common multilayer dielectric gratings

The modifications of multilayer dielectric (MLD) gratings arising from laser-induced damage using 0.6-ps and 10-ps laser pulses at 1053 nm are investigated to better understand the damage-initiation mechanisms. Upon damage initiation, sections of the affected grating pillars are removed, thereby erasing the signature of the underlying mechanisms of laser damage. To address this issue, we performed paired studies using macroscopic grating-like features that are 5 mm in width to reveal the laser-damage morphology of the different grating sections: pillar side wall, sole, and pillar top. The results suggest that, similarly to MLD coatings, there are two damage-initiation mechanisms corresponding to the different pulse durations.

Measurement of the angular dependence of the spontaneous Raman scattering in anisotropic crystalline materials using spherical samples: Potassium dihydrogen phosphate as a case example

A specialized experimental configuration was developed to allow for more-accurate characterization of the spontaneous Raman scattering properties in anisotropic materials. This need stems from the challenges, arising from the complexity of light propagation, in obtaining accurate measurements of the angular dependence of the Raman scattering cross section in birefringent materials. The nonlinear optical material KH₂PO₄ (KDP) is used as a model medium. This study is motivated by the need to improve our understanding and management of transverse stimulated Raman scattering in KDP crystals and its deuterated analog, DKDP, typically used for frequency conversion and polarization control in large-aperture laser systems. Key to this experimental platform is the use of high-quality spherical samples that enable one to measure the Raman scattering cross section in a wide range of geometries using only a single sample. The effect of polarization rotation of both the pump light and the collected Raman signal must be carefully considered in data analysis and can give rise to artifacts, which can, in part, be mitigated by reducing the input and collection cone angles.

Dynamics of secondary contamination from the interaction of high-power laser pulses with metal particles attached on the input surface of optical components

We investigate the interaction of 355-nm and 1064-nm nanosecond laser pulses with nominally spherical metallic particles dispersed on the input surface of transparent substrates or high-reflectivity (HR) multilayer dielectric coatings, respectively. The objective is to elucidate the interaction mechanisms associated with contaminant-induced degradation and damage of transparent and reflective optical elements for high-power laser systems. The experiments involve time-resolved imaging capturing the dynamics of the interaction pathway, which includes plasma formation, particle ejection, and secondary contamination by droplets originating from the liquefied layer of the particle. The results suggest that HR coatings are more susceptible to secondary contamination by liquid droplets produced by the particles because of the different geometry of excitation and the location of plasma initiation. Modeling results focus on better understanding the melting of the particle surface, leading to ejections of liquid droplets and the pressure applied to the substrate, leading to mechanical damage.

Influence of absorption-edge properties on subpicosecond intrinsic laser-damage threshold at 1053 nm in hafnia and silica monolayers

Owing to their relatively high resistance to laser-induced damage, hafnia and silica are commonly used in multilayered optical coatings in high-power laser facilities as high- and low-refractive-index materials, respectively. Here, we quantify the laser-induced-damage threshold (LIDT) at 1053 nm in the short-pulse regime of hafnia and silica monolayers deposited by different fabrication methods, including electron-beam evaporation, plasma ion-assisted deposition and ion-assisted deposition. The results demonstrate that nominally identical coatings fabricated by different deposition techniques and/or vendors can exhibit significantly different damage thresholds. A correlation of the LIDT performance of each material with its corresponding absorption edge is investigated. Our analysis indicates a weak correlation between intrinsic LIDT and the optical gap of each material (Tauc gap) but a much better correlation when considering the spectral characteristics in the Urbach tail spectral range. Spectrophotometry and photothermal absorption were used to provide evidence of the correlation between the strength of the red-shifted absorption tail and reduced LIDT at 1053 nm.

Interaction of short laser pulses with model contamination microparticles on a high reflector

The response of model contamination particles located on the surface of a multilayer dielectric mirror when exposed to 1053 nm laser pulses of 10 ps or 0.6 ps duration is investigated. Four different particle types were studied: stainless steel, borosilicate glass, polyethylene, and polytetrafluoroethylene, all having an average diameter of about 40 μm. Irradiation with one laser pulse caused particles to eject from the surface with an onset fluence in the range 5× to 100×, depending on the particle type, below the particle-free, laser-induced damage threshold of the mirror. Morphological analysis showed, however, that the ejection process always generated ablation craters and/or secondary contamination, both of which can degrade the performance of the optic during subsequent pulses. Ejection and damage mechanisms are discussed for each particle type.

Mechanisms of laser-induced damage in absorbing glasses with nanosecond pulses

The propagation of 355-nm, nanosecond pulses in absorbing glasses is investigated for the specific case examples of the broadband absorbing glass SuperGrey and the Ce³⁺-doped silica glass. The study involves different laser irradiation conditions and material characterization methods to capture the transient material behaviors leading to laser-induced damage. Two damage-initiation mechanisms were identified: (1) melting of the surface as a result of increased temperature; and (2) self-focusing caused by a transient change in the index of refraction. Population of excited states greatly affects both mechanisms by increasing the transient absorption cross section via excited-state absorption and introducing a change of the refractive index to support the formation of graded-index lensing and self-focusing of the beam inside the material. The governing damage-initiation mechanism depends on the thermodynamic properties of the host glass, the electronic structure characteristics of the doped ion, and the laser-spot size.

Simulation of internal stress waves generated by laser-induced damage in multilayer dielectric gratings

Multilayer dielectric (MLD) gratings used in ultrahigh-intensity laser systems often exhibit a laser-induced damage performance below that of their constituent materials. Reduced performance may arise from fabrication- and/or design-related issues. Finite element models were developed to simulate stress waves in MLD grating structures generated by laser-induced damage events. These models specifically investigate the influence of geometric and material parameters on how stress waves can lead to degradation of material structural integrity that can have adverse effects on its optical performance under subsequent laser irradiation: closer impedance matching of the layer materials reduces maximum interface stresses by ~20% to 30%; increasing sole thickness from 50 nm to 500 nm reduces maximum interface stresses by ~50%.

Optical properties of oxygen vacancies in HfO2 thin films studied by absorption and luminescence spectroscopy

Hafnium oxide thin films with varying oxygen content were investigated with the goal of finding the optical signature of oxygen vacancies in the film structure. It was found that a reduction of oxygen content in the film leads to changes in both, structural and optical characteristics. Optical absorption spectroscopy, using nanoKelvin calorimetry, revealed an enhanced absorption in the near-ultraviolet (near-UV) and visible wavelength ranges for films with reduced oxygen content, which was attributed to mid-gap electronic states of oxygen vacancies. Absorption in the near-infrared was found to originate from structural defects other than oxygen vacancy. Luminescence generated by continuous-wave 355-nm laser excitation in e-beam films showed significant changes in the spectral profile with oxygen reduction and new band formation linked to oxygen vacancies. The luminescence from oxygen-vacancy states was found to have microsecond-scale lifetimes when compared with nanosecond-scale lifetimes of luminescence attributed to other structural film defects. Laser-damage testing using ultraviolet nanosecond and infrared femtosecond pulses showed a reduction of the damage threshold with increasing number of oxygen vacancies in hafnium oxide films.

Mechanisms of surface contamination in fused silica by means of laser-induced electrostatic effects

We demonstrate that fused-silica samples exposed to nanosecond laser pulses at 355 nm and 1064 nm develop long-lived electrostatic charges on their surfaces. These charges extend well beyond the area exposed to the laser beam. The results suggest this effect is dependent on laser fluence and wavelength. In addition, ejected particles generated during laser-induced breakdown are electrostatically charged. Experiments indicate that such electrostatic charges can produce forces that can support the transport of dielectric and metallic microspheres between surfaces. This in turn can promote increased contamination of optical components during operation at relevant excitation conditions.

Dynamics of transient absorption in bulk DKDP crystals following laser energy deposition

The transient changes in the optical properties of bulk DKDP material arising from its exposure to high temperatures and pressures associated with localized laser energy deposition are investigated. Two methods for initiation of laser-induced breakdown are used, intrinsic, involving relatively large energy deposition brought about by focusing of the laser beam to high intensities, and extrinsic, arising from more localized deposition due to the presence of pre-existing absorbing damage initiating defects. Each method leads to a very different volume of material being affected, which provides for different material thermal relaxation times to help better understand the processes involved.

Dynamics of material modifications following laser-breakdown in bulk fused silica

We report on the material response during the cooling phase in bulk fused silica following localized energy deposition via laser-induced breakdown.We use a time-resolved microscope system to acquire images of the region of energy deposition at delay times covering the entire timeline of events. In addition, this system is configured to perform pump-and-probe damage testing measurements to investigate the evolution of the transient absorption of the modified material. The main features of a damage site are established at approximately 30 ns after the pump pulse, i.e. cracks reach their final size within this time frame. The results reveal that the cracks and melted core exhibit a transient absorption up until about 300 ns and 200 micros delay times, respectively, and suggest that the melted region returns to solid phase at approximately 70 ms delay.

Laser damage performance of KD2-chiHchiPO4 crystals following X-ray irradiation

We investigate the laser-induced damage performance of KD(2-chi)H(chi)PO(4) crystals following exposure to X-ray irradiation. Two important issues addressed by our study are i) the performance of the material when operational conditions lead to its exposure to ionizing irradiation and ii) the way the radiation-induced transient defects interact with the pre-existing precursor defects responsible for laser-induced damage. Our results indicate that the damage performance of the material is affected by exposure to X-rays. This behavior is attributed to a change in the physical properties of the precursors which, in turn, affect their ability to initiate damage following interaction with X-ray generated defects.

Expedited laser damage profiling of KDxH(2-x)PO4 with respect to crystal growth parameters

We investigate the laser-induced damage resistance at 355 nm in deuterated potassium dihydrogen phosphate (DKDP) crystals grown with varying growth parameters, including speed of growth and temperature. The aim is to explore a new expedited method to study the growth parameters affecting the laser-induced damage resistance in DKDP material to obtain crystals with enhanced performance.

Investigation of laser annealing parameters for optimal laser-damage performance in deuterated potassium dihydrogen phosphate

Laser annealing via preexposure to laser pulses at sub-damage-threshold fluences is known to improve the resistance of KDP crystals to laser-induced damage. Using a specific damage-testing method, we investigate the laser annealing process as a function of fluence and number of preexposure pulses (at 355 nm, 2.5 ns). Our aim is to reveal the key laser parameters in order to devise a practical and efficient protocol for optimizing performance of the material for operation in laser systems in the near UV. Results suggest that a near twofold improvement to the laser-damage performance can be achieved with a limited number of preexposure pulses.

Radiation produced by femtosecond laser-plasma interaction during dielectric breakdown

Optical breakdown by femtosecond and nanosecond laser pulses in transparent dielectrics produces an ionized region of dense plasma confined within the bulk of the material. This ionized region is responsible for broadband radiation that accompanies the breakdown process. Spectroscopic measurements of the accompanying light have been used to show that, depending on the laser parameters, the spectra may originate from plasma-induced second-harmonic generation, supercontinuum generation, or thermal emission by the plasma. By monitoring the emission from the ionized region, one can ascertain the predominant breakdown mechanism and the morphology of the damage region.

Multiwavelength investigation of laser-damage performance in potassium dihydrogen phosphate after laser annealing

The laser-induced damage performance of potassium dihydrogen phosphate and deuterated potassium dihydrogen phosphate nonlinear optical crystals after pre-exposure to lower-energy laser pulses (laser annealing, also known as laser conditioning) is investigated as a function of wavelength for both the damaging and conditioning pulses. We obtain a quantitative evaluation of the bulk damage performance of these materials by measuring the density of damage events as a function of laser parameters. This new method allows for a detailed assessment of the improvement of material performance from laser conditioning and reveals the key parameters for optimizing performance depending on the operational wavelength.

Localized dynamics during laser-induced damage in optical materials

Laser-induced damage in wide band-gap optical materials is the result of material modifications arising from extreme conditions occurring during this process. The material absorbs energy from the laser pulse and produces an ionized region that gives rise to broadband emission. By performing a time-resolved investigation of this emission, we demonstrate both that it is blackbody in nature and that it provides the first direct measurement of the localized temperature of the material during and following laser damage initiation for various optical materials. For excitation using nanosecond laser pulses, the plasma, when confined in the bulk, is in thermal equilibrium with the lattice. These results allow for a detailed characterization of temperature, pressure, and electron densities occurring during laser-induced damage.

Wavelength dependence of laser-induced damage: determining the damage initiation mechanisms

A novel experimental approach is employed to understand the mechanisms of laser induced damage. Using an OPO (optical parametric oscillator) laser, we have measured the damage thresholds of deuterated potassium dihydrogen phosphate (DKDP) from the near ultraviolet into the visible. Distinct steps, whose width is of the order of k(B)T, are observed in the damage threshold at photon energies associated with the number of photons (3-->2 or 4-->3) needed to promote a ground state electron across the energy gap. The wavelength dependence of the damage threshold suggests that a primary mechanism for damage initiation in DKDP is a multiphoton process in which the order is reduced through excited defect state absorption.

Electron- or hole-assisted reactions of H defects in hydrogen-bonded KDP

We present an ab initio study of the stability and defect reactions of neutral and charged H interstitial (H(i)) and H vacancy (H(v)) in KH2PO4 (KDP). We find that while there is no interaction between the neutral H(i) and the host, the addition of an electron leads to the ejection of a H host atom and the subsequent formation of an interstitial H2 molecule and a H(v). In sharp contrast, the addition of a hole results in the formation of a hydroxyl bond. Thus, H(i) in both charged states severs the H-bonded network. For the H(v), the addition of a hole leads to the formation of a peroxyl bridge. The neutral H(i) and the positively charged H(v) induce states in the gap. The results elucidate the underlying atomic mechanism for the defect reactions suggested by experiment.

Optical spectroscopy and imaging of the dentin-enamel junction in human third molars

A 351-nm laser excitation source was used to perform autofluorescence microscopy of dentin, enamel, and the dentin-enamel junction (DEJ) to obtain information regarding their morphology and spectral characteristics. The emission spectra of these calcified dental tissues were different from one another, and this enabled the DEJ to be imaged and dimensionalized. The DEJ displayed sharp and clearly delineated borders at both its enamel and dentin margins. The dentinal tubules and the enamel prisms appeared to terminate abruptly at the DEJ. The median DEJ width was 10 microm, ranging from 7 to 15 microm, and it did not appear to depend on intratooth position.

Imaging of laser-induced reactions of individual defect nanoclusters

The response of individual defect nanoclusters located in the bulk of a dielectric material following exposure to 355-nm, 3-ns high-power laser irradiation is investigated by use of microscopic fluorescence imaging. Experiments were carried out on KH(2)PO(4) crystals. We provide direct imaging of the reaction to an external stimulus of individual defect clusters and demonstrate a novel method of studying the dynamic behavior of bulk defects.

Local mechanical and optical properties of normal and transparent root dentin

The mechanical and optical properties of healthy and transparent root dentin are compared using atomic force microscopy (AFM), micro-Raman and emission spectroscopies and fluorescence microscopy. The elastic modulus and hardness of intertubular and peritubular transparent and healthy dentin did not differ appreciably. The tubule filling material in the transparent zone, however, exhibited values between peritubular and intertubular dentin. Raman spectroscopy revealed a shift in the 1066 cm(-1) band to 1072 cm(-1) from normal to transparent intertubular dentin. The material filling the tubule lumen in transparent dentin showed an increase in frequency of the band near 1070 cm(-1) as well. The emission spectral characteristics under 351 nm photoexcitation indicate differences between normal and transparent intertubular dentin. A transition region of about 300 microm between normal and transparent dentin was identified. In this region the intertubular emission properties were the same as for normal dentin, but tubules were filled. The filling material had emission characteristics closer to the normal intertubular than to transparent intertubular dentin.

Microscopic fluorescence imaging of bulk defect clusters in KH(2)PO(4) crystals

A microscopic fluorescence imaging system is used to detect optically active centers located inside a transparent dielectric crystal. Defect centers in the bulk of KH(2)PO(4) crystals are imaged based on their near-infrared emission following photoexcitation. The spatial resolution of the system is 1mum in the image plane and 25mum in depth. The experimental results indicate the presence of a large number of optically active defect clusters in different KH(2)PO(4) crystals, whereas the concentration of these clusters depends on the crystal sector and growth method.

Time-resolved and nonlinear optical imaging for medical applications

In this article, we have presented an overview of emerging novel techniques for early-light transillumination imaging as well as nonlinear optical tomography of body organs. The use of light for probing and imaging biomedical media offers the promise for development of safe, noninvasive, and inexpensive clinical imaging modalities with diagnostic ability. The strong scattering of light by biological tissues buries the shadowgram formed by forward-propatating image-bearing photons in the background noise of multiple-scattered light. Several methods for extraction of image-bearing light that capitalize on spatial, temporal and polarization characteristics of transmitted light are reviewed. More recently emerging nonlinear-optical histopathology methods for imaging subsurface structures of tissues in terms of its local spatial symmetry and molecular content are introduced. The progress made so far indicates that some of these techniques are apt to make a transition from laboratory to useful clinical modalities.

Imaging objects hidden in scattering media with fluorescence polarization preservation of contrast agents

A method for fluorescence polarization difference imaging is demonstrated for enhancing the image quality of a luminous object embedded in a random medium. The polarization preservation of light propagating in the scattering medium leads to partially polarized light emission by a contrast-agent dye located inside the object. Subtraction of the images of the luminous object detected at two orthogonal polarization directions improves the image resolution compared with a conventional optical imaging approach.

Polarization filter for biomedical tissue optical imaging

A technique based on the degree that light is depolarized when propagating inside tissues is demonstrated for optical imaging in biomedical systems. The difference in the degree of polarization of the emerging light allows for the discrimination of different types of tissues. The technique was investigated in the transillumination and back-scattering geometry and in both cases the potential of this method to image and separate out different types of tissues is demonstrated.

Advances in optical imaging of biomedical media

In this article, we have presented an overview of fundamental issues involved in mediphotonic imaging, and reviewed some of the emerging techniques for early-light transillumination imaging of body organs. The results on human breast tissues presented here, together with the data accumulated and advances made by researchers around the globe, not only demonstrate the feasibility of optical imaging as a clinical procedure but indicate a road map to reach that goal. The milestones include evaluation of relative merits of available approaches for a particular imaging application; selection of diagnostic wavelengths, as well as sources to generate and detectors to monitor light at those wavelengths; accumulation of data on optical, spectroscopic, and transport properties of tissues and organs; in vivo testing; prototype instrumentation development; clinical trials; governmental approval; cost analysis and marketing; and finally system improvement based on feedback from end users. A new era of optical clinical imaging is at the door.

Optical polarization imaging

The temporal profiles of the parallel and perpendicular polarization components of a light pulse backscattered from a scattering medium are different. The depth of penetration into the tissue and depolarization of the backscattered light depend on the scattering and absorption characteristics of the tissue. Based on these facts, a novel technique is demonstrated for noninvasive surface and beneath-the-surface imaging of biological systems.

Temporal gating in highly scattering media by the degree of optical polarization

The temporal profiles of polarized laser pulses propagating through a scattering medium differ for scattered light parallel and perpendicular to the incident polarization. The degree of polarization is conserved over 100 ps after the arrival of the ballistic component. This observation can be used as a time gate for the early portion of the scattered light, reducing the diffusive component.

Upconverted hot-luminescence spectroscopy investigation of nonradiative relaxation in forsterite

Upper-state nonradiative relaxation in the 3.2-1.9-eV energy region of the excited-state manifold of Cr(4+)-doped forsterite is investigated. Measurements of the upconverted hot-luminescence spectra reveal two electronic bottlenecks and several phonon modes involved in the initial steps of the vibrational relaxation at different portions of the excited-state manifold.