Brett H. Hokr

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. Furthermore, Monte Carlo simulations are used to study the complex spatial and temporal dynamics of nonlinear processes in turbid media. In addition to providing a large signal, characteristic of the Raman medium, the random Raman laser offers us an entirely new tool for studying the dynamics of gain in a turbid medium.

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

Significance The long-range stand-off chemical identification of materials has been a high profile goal of science in recent years. In this article, we demonstrate the stand-off identification of chemical compounds from kilometer-scale distances in a single laser pulse by detecting the emission from random Raman lasing processes in the target. This technique opens up the door to rapid identification of potentially hazardous chemicals from a safe distance. 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. The bright nature of random Raman lasing renders directionality unnecessary, allowing for the detection and identification of chemicals from large distances in real time. In this article, the single-shot remote identification of chemicals at kilometer-scale distances is experimentally demonstrated using random Raman lasing.

Assessment of tissue heating under tunable near-infrared radiation

Abstract. 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.

Raman signal enhancement via elastic light scattering

The enhanced generation of a spontaneous Raman signal by way of elastic scattering is demonstrated. Using Monte Carlo simulations, we show that elastic scattering, by increasing the path length of light through the medium, enhances the generation of a Raman signal. This is investigated over a large parameter space, demonstrating that this effect is robust, and providing additional physical insight into the dynamics of light propagation in a turbid medium. Both the temporal and spatial profiles of the Raman signal are shown to depend heavily on the amount of scattering present.

Ultrasensitive detection of waste products in water using fluorescence emission cavity-enhanced spectroscopy

Significance Clean water is paramount to human health. Contaminants, such as human waste products in drinking water, can result in significant health issues. In this article, we present a technique for detection of trace amounts of human or animal waste products in water. This technique could allow for real-time assessment of water quality without the need for expensive laboratory equipment. Clean water is paramount to human health. In this article, we present a technique for detection of trace amounts of human or animal waste products in water using fluorescence emission cavity-enhanced spectroscopy. The detection of femtomolar concentrations of urobilin, a metabolic byproduct of heme metabolism that is excreted in both human and animal waste in water, was achieved through the use of an integrating cavity. This technique could allow for real-time assessment of water quality without the need for expensive laboratory equipment.

Optimization of focusing through scattering media using the continuous sequential algorithm

The ability to control the propagation of light through scattering media is essential for atmospheric optics, astronomy, biomedical imaging, and remote sensing. The optimization of focusing light through a scattering medium is of particular interest for the case of highly scattering materials. Optical wavefront beam-shaping plays a critical role in optimizing such a propagation; however, an enormous field of adjustable parameters makes the overall task complicated. Here, we propose and experimentally evaluate several variations on the standard continuous sequential algorithm (CSA) that hold a promise of revealing new, faster, and more efficient optimization algorithms for selecting an optical wavefront to focus light through a scattering medium. We demonstrate that the order in which pixels are chosen in the CSA can lead to a two-fold decrease in the number of iterations required to reach a given enhancement.

Wavefront shaping enhanced Raman scattering in a turbid medium

Spontaneous Raman scattering is a powerful tool for chemical sensing and imaging but suffers from a weak signal. In this Letter, we present an application of adaptive optics to enhance the Raman scattering signal detected through a turbid, optically thick material. This technique utilizes recent advances in wavefront shaping techniques for focusing light through a turbid media and applies them to chemical detection to achieve a signal enhancement with little sacrifice to the overall simplicity of the experimental setup. With this technique, we demonstrate an enhancement in the Raman signal from titanium dioxide particles through a highly scattering material. This technique may pave the way to label-free tracking using the optical memory effect.

A narrow-band speckle-free light source via random Raman lasing

Currently, no light source exists which is both narrowband and speckle free with sufficient brightness for full-field imaging applications. Light-emitting diodes are excellent spatially incoherent sources, but are tens of nanometers broad. Lasers, on the other hand, can produce very narrow-band light, but suffer from high spatial coherence which leads to speckle patterns, which distort the image. Here, we propose the use of random Raman laser emission as a new kind of light source capable of providing short-pulsed narrow-band speckle-free illumination for imaging applications.

Modeling focusing Gaussian beams in a turbid medium with Monte Carlo simulations.

Monte Carlo techniques are the gold standard for studying light propagation in turbid media. Traditional Monte Carlo techniques are unable to include wave effects, such as diffraction; thus, these methods are unsuitable for exploring focusing geometries where a significant ballistic component remains at the focal plane. Here, a method is presented for accurately simulating photon propagation at the focal plane, in the context of a traditional Monte Carlo simulation. This is accomplished by propagating ballistic photons along trajectories predicted by Gaussian optics until they undergo an initial scattering event, after which, they are propagated through the medium by a traditional Monte Carlo technique. Solving a known problem by building upon an existing Monte Carlo implementation allows this method to be easily implemented in a wide variety of existing Monte Carlo simulations, greatly improving the accuracy of those models for studying dynamics in a focusing geometry.

A proposal for a random Raman laser

The concept of a random Raman laser is investigated using a Monte Carlo simulation. Stimulated Raman scattering is taken into account by introducing an interaction between Raman and pump photons. Due to a lack of experimental data on the subject, validation of this model is accomplished via comparison to analytical solutions in the no scattering, no absorption limit. Random Raman lasing is discussed, and several distinctive properties are predicted.