John D. Mason

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.

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.

Ultraviolet (250–550 nm) absorption spectrum of pure water

Data for the spectral light absorption of pure water from 250 to 550 nm have been obtained using an integrating cavity made from a newly developed diffuse reflector with a very high UV reflectivity. The data provide the first scattering-independent measurements of absorption coefficients in the spectral gap between well-established literature values for the absorption coefficients in the visible (>400  nm) and UV (<200  nm). A minimum in the absorption coefficient has been observed in the UV at 344 nm; the value is 0.000811±0.000227  m-1.

Utilizing scattering to further enhance integrating cavity-enhanced spectroscopy

Spectroscopic optical characterization and identification of molecular structures and complex systems would greatly benefit from new technologies capable of analyzing molecular species in small quantities with maximum sensitivity and specificity. Integrating cavity-enhanced spectroscopy has recently been shown as a viable tool for achieving this goal. This technique could greatly benefit from methods for further enhancing the desired spectroscopic signal, allowing for lower detection limits. Here, we present a simple method to further enhance fluorescence signal generated inside an integrating cavity by introducing additional scattering to the sample of interest.

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.

Chemical, biological, and trace gas detection and measurement with a newly developed integrating Cavity Enhanced Raman (iCERS) technique

Raman spectroscopy is routinely used in the laboratory for detection, chemical identification, and quantitative measurements of complex molecular compounds. One key advantage of the method is that a single laser wavelength can be used to identify and measure several different molecular compounds simultaneously. While Raman spectroscopy is a powerful technique, it is a very inefficient process where only one in 1011 scattered photons contain the desired vibrational information. Several techniques have been developed to enhance Raman scattering, which are typically applied to liquids and solids such as surface enhanced Raman spectroscopy and coherent anti-Stokes Raman spectroscopy. For gas phase measurements, photonic crystals, cavity enhanced Raman spectroscopy and functional waveguides have been developed to provide Raman enhancement. However, Raman spectroscopy has seen limited use in commercial and military applications due to instrument complexity, sample preparation, acquisition time, and spatially localized point measurements. A recently developed technique to enhance spontaneous Raman scattering utilizing a highly reflective integrating cavity is presented. Elastically scattered light circulates within the cavity volume continuously interacting with the sample, whether a bulk sample or gas, resulting in significant Raman enhancement. In addition, the Raman scattered light is collected from all directions before being coupled out of the cavity. Enhancements of 107 have been realized with the use of inexpensive low power diode lasers and a modest CCD based spectrometer. Application of the iCERS technique operating near 400 nm providing near real-time detection and measurement of trace gases, chemicals, and biological compounds is discussed.

High speed fluorescence imaging with compressed ultrafast photography

Fluorescent lifetime imaging is an optical technique that facilitates imaging molecular interactions and cellular functions. Because the excited lifetime of a fluorophore is sensitive to its local microenvironment,1, 2 measurement of fluorescent lifetimes can be used to accurately detect regional changes in temperature, pH, and ion concentration. However, typical state of the art fluorescent lifetime methods are severely limited when it comes to acquisition time (on the order of seconds to minutes) and video rate imaging. Here we show that compressed ultrafast photography (CUP) can be used in conjunction with fluorescent lifetime imaging to overcome these acquisition rate limitations. Frame rates up to one hundred billion frames per second have been demonstrated with compressed ultrafast photography using a streak camera.3 These rates are achieved by encoding time in the spatial direction with a pseudo-random binary pattern. The time domain information is then reconstructed using a compressed sensing algorithm, resulting in a cube of data (x,y,t) for each readout image. Thus, application of compressed ultrafast photography will allow us to acquire an entire fluorescent lifetime image with a single laser pulse. Using a streak camera with a high-speed CMOS camera, acquisition rates of 100 frames per second can be achieved, which will significantly enhance our ability to quantitatively measure complex biological events with high spatial and temporal resolution. In particular, we will demonstrate the ability of this technique to do single-shot fluorescent lifetime imaging of cells and microspheres.

Integrating cavity ring-down spectroscopy (ICRDS) and the direct measurement of absorption coefficients

Absorption of light at various wavelengths (i.e. absorption spectroscopy) is a powerful tool for identifying the presence of chemical compounds or specific substances in a sample. Cavity ring down spectroscopy (CRDS) is a well-known technique for very high sensitivity absorption spectroscopy. Another technique, integrating cavity spectroscopy has the additional unique feature of providing accurate absorption data even in the presence of severe scattering. This paper describes a combination of these two techniques that has led to an extremely powerful and useful new technology—integrating CRDS.

Low cost integrating cavity for monitoring of environmental toxins

Contamination of the water source and air pollution are two major problems that must be faced in the coming years. The increasing worldwide contamination of freshwater systems with thousands of industrial and natural chemical compounds is one of the key environmental problems facing humanity today. It is estimated that pathogens in water cause more than 2 million deaths annually. Additionally, traditional water quality assessment methods, such as liquid chromatography and mass spectroscopy, are expensive and time consuming from sample collection to analysis. Low cost tools are needed which can provide high sensitivity in sensing, while remaining portable and providing near real time analysis. Here, we present a low cost integrating cavity that can be used for highly sensitive environmental sensing.

Measuring the Absorption Coefficient of Strongly Scattering Samples Using Integrating Cavity Ring-Down Spectroscopy

We report a novel technique for the precise and accurate measurement of the optical absorption coefficient of strongly scattering samples via ring-down spectroscopy inside ultra-high reflectivity integrating cavities. Experimental results are discussed.