Unified modeling of photothermal and photochemical damage.

Correlating damage outcomes to a retinal laser exposure is critical for diagnosis and choosing appropriate treatment modalities. Therefore, it is important to understand the causal relationships between laser parameters, such as wavelength, power density, and length of exposure, and any resulting injury. Differentiating photothermal from photochemical processes in an retinal model using cultured retinal pigment epithelial cells would be a first step in achieving this goal.

Damage integral and other predictive formulas for nonisothermal heating during laser exposure.

Significance: Physics-based models supply simulated temperature rises to photothermal damage rate models and provide comprehensive risk assessments for laser-induced damage. As the physics-based models continue to be refined, the damage rate models have not advanced. This peculiar lack of improvement is counterintuitive considering the damage integral (Ω), originally derived for isothermal heating events, and fails to accurately represent the nonisothermal heating from short laser exposures.

Mammalian complex III heme dynamics studied with pump-probe spectroscopy and red light illuminations.

The electronic or molecular mechanisms that initiate photobiomodulation (PBM) in cells are not yet fully understood. The porcine complex III (C-III) of the electron transport chain was characterized with transient absorption spectroscopy (TAS). We then applied our recently developed continuous wave laser coupled TAS procedure (CW-TAS) to investigate the effect of red light irradiances on the heme dynamics of C-III in its c reduced state.

Visualizing bleb mass dynamics in single cells using quantitative phase microscopy.

Understanding biological responses to directed energy (DE) is critical to ensure the safety of personnel within the Department of Defense. At the Air Force Research Laboratory, we have developed or adapted advanced optical imaging systems that quantify biophysical responses to DE. One notable cellular response to DE exposure is the formation of blebs, or semi-spherical protrusions of the plasma membrane in living cells.

Transient absorption spectroscopy to explore cellular pathways to photobiomodulation.

Photobiomodulation (PBM) describes the use of low irradiance light in the red to near-infrared wavelength range to stimulate biological effects in tissue, and many biological and spectroscopic techniques are used to study PBM. However, these techniques focus on the products or downstream effects rather than the electronic transitions that initiate the PBM processes. This study presents a novel approach to studying low irradiance light exposures on individual proteins and/or protein complexes by combining a continuous wave (CW) laser diode with femtosecond transient absorption spectroscopy (TAS), coined here as CW-TAS, and tests the system on reduced cytochrome c (Cyt c) for proof of principle.

Quantitative phase microscopy monitors subcellular dynamics in single cells exposed to nanosecond pulsed electric fields.

A substantial body of literature exists to study the dynamics of single cells exposed to short duration (<1 μs), high peak power (~1 MV/m) transient electric fields. Much of this research is limited to traditional fluorescence-based microscopy techniques, which introduce exogenous agents to the culture and are only sensitive to a single molecular target. Quantitative phase imaging (QPI) is a coherent imaging modality which uses optical path length as a label-free contrast mechanism, and has proven highly effective for the study of single-cell dynamics.

Evaluation of the potential eye hazard at visible wavelengths of the supercontinuum generated by an ultrafast NIR laser in water.

Lasers with ultrashort pulse durations have become ubiquitous in various applications, including ocular surgery. Therefore, we need to consider the role of nonlinear optical effects, such as supercontinuum generation during propagation within the ocular media, when evaluating their potential hazard. We used a NIR femtosecond laser to generate a supercontinuum within an artificial eye.

Continuous assessment of metabolic activity of mitochondria using resonance Raman microspectroscopy.

Dysfunctional mitochondrial activity can lead to a variety of different diseases. As such, there exists a need to quantify changes in mitochondria function as it relates to these specific diseased states. Here, we present the use of resonance Raman (RR) spectroscopy as a tool to determine changes in isolated mitochondrial activity.

Thermal damage thresholds for multiple-pulse porcine skin laser exposures at 1070 nm.

As solid-state laser technology continues to mature, high-energy lasers operating in the near-infrared (NIR) band have seen increased utilization in manufacturing, medical, and military applications. Formulations of maximum permissible exposure limits establish guidelines for the safe use of these systems for a given set of laser parameters, based on past experimental and analytical studies of exposure thresholds causing injury to the skin and eyes. The purpose of our study is to characterize the skin response to multiple-pulsed laser exposures at the NIR wavelength of 1070 nm, at a constant beam diameter of 1 cm, using anesthetized Yucatan mini-pig subjects.

Filamentation in Atmospheric Air with Tunable 1100-2400 nm Near-Infrared Femtosecond Laser Source.

Intense femtosecond pulse filamentation in open-air has been utilized for long distance optical communication and remote sensing, but it results in nonlinear-effect driven eye hazards which are not addressed by current eye safety standards. A systematic study of filamentation in atmospheric air was performed using a tunable 100 fs near-infrared laser (1100 nm-2400 nm). While undergoing filamentation, each source wavelength was spectrally broadened resulting in supercontinuum and third harmonic generation in the visible and near-IR spectrum.

Effect of ambient temperature and intracellular pigmentation on photothermal damage rate kinetics.

Computational models predicting cell damage responses to transient temperature rises generated by exposure to lasers have implemented the damage integral (Ω), which time integrates the chemical reaction rate constant described by Arrhenius. However, few published reports of empirical temperature histories (thermal profiles) correlated with damage outcomes at the cellular level are available to validate the breadth of applicability of the damage integral. In our study, an analysis of photothermal damage rate processes in cultured retinal pigment epithelium cells indicated good agreement between temperature rise, exposure duration (τ), and threshold cellular damage.

Suprathreshold laser injuries in excised porcine skin for millisecond exposures at 1070 nm.

Skin injury response to near-infrared (NIR) laser radiation between the minimum visible lesion threshold and ablation onset is not well understood. This study utilizes a 1070-nm diode-pumped Yb-fiber laser to explore the response of excised porcine skin to high-energy exposures in the suprathreshold injury region without inducing ablation. Concurrent high-speed videography is employed to determine a dichotomous response for three progressive damage categories: observable surface distortion, surface bubble formation due to contained intracutaneous water vaporization, and surface bubble rupture during exposure.

Z-scan measurements of water from 1150 to 1400 nm.

Understanding the nonlinear properties of water is essential for laser surgery applications, as well as understanding supercontinuum generation in water. Unfortunately, the nonlinear properties of water for wavelengths longer than 1064 nm are poorly understood. We extend the application of the Z-scan technique in water to determine its nonlinear refractive index (n) and nonlinear absorption (β) for wavelengths in the 1150-1400 nm range, where linear absorption is also significant.

Single-shot chemical detection and identification with compressed hyperspectral Raman imaging.

Raman imaging is a powerful method to identify and detect chemicals, but the long acquisition time required for full spectroscopic Raman images limits many practical applications. Compressive sensing and compressed ultrafast photography have recently demonstrated the acquisition of multi-dimensional data sets with single-shot detection. In this Letter, we demonstrate the utilization of compressed sensing for single-shot compressed Raman imaging.

Probe beam deflection optical imaging of thermal and mechanical phenomena resulting from nanosecond electric pulse (nsEP) exposure in-vitro.

Electric-field induced physical phenomena, such as thermal, mechanical and electrochemical dynamics, may be the driving mechanism behind bioeffects observed in mammalian cells during exposure to nanosecond-duration electric pulses (nsEP) in-vitro. Correlating a driving mechanism to a biological response requires the experimental measurement and quantification of all physical dynamics resulting from the nsEP stimulus. A passive and electromagnetic interference (EMI) immune sensor is required to resolve these dynamics in high strength electric fields.

Enabling time resolved microscopy with random Raman lasing.

Optical imaging of fast events and processes is essential for understanding dynamics of complex systems. A bright flash of illuminating light is required to acquire sufficient number of photons for superior image quality. Laser pulses can provide extreme brightness and are typically employed to achieve high temporal resolution; however, the high degree of coherence associated with the lasing process degrades the image quality with speckle formation.

Trends in nanosecond melanosome microcavitation up to 1540 nm.

Thresholds for microcavitation of bovine and porcine melanosomes were previously reported, using single nanosecond (ns) laser pulses in the visible (532 nm) and the near-infrared (NIR) from 1000 to 1319 nm. Here, we report average radiant exposure thresholds for bovine melanosome microcavitation at additional NIR wavelengths up to 1540 nm, which range from ∼0.159 J∕cm2 at 800 nm to 4.

Robust commercial diffuse reflector for UV-VIS-NIR applications.

We report the development and testing of a new commercially available diffuse reflecting material with reflectivities in the visible comparable to industry-leading products. This new diffuse reflector consists of solid quartz in which there is a dense distribution of tiny pockets of air. The multiple reflections by the quartz-air interfaces of these air pockets transforms a highly transmissive base material into a highly diffuse reflecting material.

Thermal evaluation of laser exposures in an in vitro retinal model by microthermal sensing.

A temperature detection system using a micropipette thermocouple sensor was developed for use within mammalian cells during laser exposure with an 8.6-μm beam at 532 nm. We have demonstrated the capability of measuring temperatures at a single-cell level in the microscale range by inserting micropipettebased thermal sensors of size ranging from 2 to 4 μm into the membrane of a live retinal pigment epithelium (RPE) cell subjected to a laser beam.

Single-shot stand-off chemical identification of powders using random Raman lasing.

The task of identifying explosives, hazardous chemicals, and biological materials from a safe distance is the subject we consider. Much of the prior work on stand-off spectroscopy using light has been devoted to generating a backward-propagating beam of light that can be used drive further spectroscopic processes. The discovery of random lasing and, more recently, random Raman lasing provide a mechanism for remotely generating copious amounts of chemically specific Raman scattered light.

Bright emission from a random Raman laser.

Random lasers are a developing class of light sources that utilize a highly disordered gain medium as opposed to a conventional optical cavity. Although traditional random lasers often have a relatively broad emission spectrum, a random laser that utilizes vibration transitions via Raman scattering allows for an extremely narrow bandwidth, on the order of 10 cm(-1). Here we demonstrate the first experimental evidence of lasing via a Raman interaction in a bulk three-dimensional random medium, with conversion efficiencies on the order of a few percent.

Assessment of tissue heating under tunable near-infrared radiation.

The time-temperature effects of laser radiation exposure are investigated as a function of wavelength. Here, we report the thermal response of bulk tissue as a function of wavelength from 700 to 1064 nm. Additionally, Monte Carlo simulations were used to verify the thermal response measured and predict damage thresholds based on the response.

High-resolution in vivo imaging of regimes of laser damage to the primate retina.

Purpose. To investigate fundamental mechanisms of regimes of laser induced damage to the retina and the morphological changes associated with the damage response. Methods.