Raman spectroscopy is a powerful analytical technique used for molecular detection, identification, and characterization, but its broader utility has been limited by the intrinsically weak spontaneous Raman scattering intensity. In this work, we demonstrate significant Raman signal enhancement using a novel high-performance integrating cavity constructed with newly developed Lambertian materials exhibiting exceptionally high reflectivity. Cavity ringdown measurements yield a peak average reflectivity of 99.943 ± 0.0004% at 610 nm. Raman measurements of bulk methanol, magnesium sulfate, and glycine demonstrate μmol sensitivity using a compact, fiber-coupled 405 nm diode laser delivering 17 mW of optical power. Additionally, limit-of-detection studies performed using a 532 nm diode-pumped solid-state laser at 150 mW demonstrate nanomole-level sensitivity for two common polycyclic aromatic hydrocarbons─benzo[a]pyrene and pyrene. These findings establish integrating cavity-enhanced Raman spectroscopy as a promising approach for compact, high-sensitivity systems in medical, environmental, industrial, and space-based applications.

OF ICY WORLDS (ERSO) CONCEPT. C. M. Phillips-Lander1, T. Z. Moore1, U. Raut1, P. M. Molyneux1, M. A. Miller1, K. Nowicki3, R. C. Blase1, M. W. Davis1, T. J. Veach1, G. J. Dirks1, K. B. Persson1, Y. D. Tyler1, R. A. Klar1, P. L. Karnes1, M. A. Freeman1, C. J. A. Howett3, A. Soto3, K. Mandt4, L. Roth5, B. Schmidt6, E. Spiers, A. Templeton7, J. D. Mason8, E. S. Fry8, K. Retherford1, 1Southwest Research Institute, San Antonio, TX (clander@swri.edu), 2University of Texas at San Antonio, San Antonio, TX, 3Southwest Research Institute, Boulder, CO, 4Johns Hopkins University, Applied Physics Laboratory, Laurel, MD, 5Department of Space and Plasma Physics, KTH, Stockholm, Sweden,6Earth and Atmospheric Sciences, Georgia Tech, Atlanta, GA, 7Geological Sciences, University of Colorado, Boulder, CO, 8Texas A&M University, College Station, TX.

The Raman spectra of trace gases and bulk compounds are measured using an integrating cavity enhanced Raman spectroscopy technique. The technique utilizes an integrating cavity formed from fumed silica providing significant Raman signal enhancement. © 2019 The Author(s)

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.

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.

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.

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.

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.

Introduction: The desire to understand planetary atmospheres, terrestrial chemistry, or search for potential biological markers on Ocean Worlds often involves some form of optical spectroscopy. Common spectroscopic methods include absorption, fluorescence, or Raman based measurements. We present a new approach to planetary spectrographic instruments based on a novel high reflectivity integrating cavity. The exploration of planetary surfaces using Raman based spectroscopy techniques is relatively new, and promises to play an important role in NASA’s future exploration of the Solar System. Similar to the spectral fingerprints available through infrared imaging spectroscopy the Raman excited emissions provide distinct signatures, including those of more complex molecules typically found in trace abundances. In many ways the initial survey of the Solar System with Raman spectrographs is just beginning at Mars. Future missions including UV fluorescence measurements will have increasingly focused objectives related to biologically interesting species and related design requirements for highly sensitive (low abundance level) measurements in increasingly extreme environments. The need to better adapt UV Raman and fluorescence measurement techniques combined with IR Raman for the high sensitivity needed for life detections motivates us to develop a new instrument optimized for high sensitivity and with multiple excitation source wavelengths. While many optical spectroscopy techniques have the capability to provide very sensitive measurements, many suffer from significant practical challenges such as scattering within the sample region, small sample volume interaction, weak excitation sources, or weak optical signals. For example, most optical absorption instruments do not measure absorption directly but measure attenuation where the presence of scattering will significantly degrade the measurement. Weak optical signals from Raman or fluorescence based instruments in a laboratory setting require sensitive low noise detectors and often long integration times. Several techniques have been developed to address many of the issues commonly encountered with optical spectroscopy instruments. However, these techniques often come with an additional cost such as increased complexity. Integrating Cavity Enhanced Spectroscopy (iCES): Integrating cavities are utilized extensively in optics from irradiance measurements, to detection systems, to the generation of uniform light sources.[1] However, until recently, commonly available integrating cavity materials generally did not have the optical characteristics to be used effectively for sensitive spectroscopy measurements and/or for space flight instrumentation. Recently, our colleagues at Texas A&M University have developed a new process for fabricating a highly reflective Lambertian reflector from commercially available fumed silica powders. This disruptive advancement in the state-of-the-art integrating cavity material has resulted in significantly higher reflectance with very low absorption. In addition, the cavity has been shown to have excellent performance in the deep UV region of the spectrum.[2] After fumed silica powder is pressed and sintered, it can be formed into any shape as shown in Figure 1. Cavity ring-down measurements have demonstrated reflectivity of 99.98% throughout the visible spectrum and into the near infrared, from 400 nm to 1.2 m, and 99.96% into the deep UV, the highest reflectivity of any known Lambertian reflector.[3]

We report the development of a diffuse reflecting material with measured reflectivity values as high as 0.99919 at 532 nm and 0.99686 at 266 nm. This material is a high-purity fumed silica, or quartz powder, with particle sizes on the order of 40 nm. We demonstrate that this material can be used to produce surfaces with nearly Lambertian behavior, which in turn can be used to form the inner walls of high-reflectivity integrating cavities. Light reflecting off such a surface penetrates into the material. This means there will be an effective "wall time" for each reflection off the walls in an integrating cavity. We measure this wall time and show that it can be on the order of several picoseconds. Finally, we introduce a technique for absorption spectroscopy in an integrating cavity based on cavity ring-down spectroscopy. We call this technique integrating cavity ring-down spectroscopy.

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.

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.

A number of imaging modalities rely on the exact knowledge of both the absorption and scattering properties of cells and organelles. We report a simple method for accurate and precise measurement of the optical absorption coefficient of biological samples, even in the presence of strong scattering. The technique is based on cavity ring-down spectroscopy, but the traditional mirrored cavity is replaced with a high-reflectivity integrating cavity. The Lambertian behavior of the cavity walls creates an isotropic field inside the cavity, thereby eliminating the effects of scattering in the sample. Thus, integrating cavity ring-down spectroscopy (ICRDS) provides a true, direct measurement of the absorption coefficient, as opposed to the net attenuation. We demonstrate the effectiveness of this technique by measuring the absorption coefficient of retinal pigmented epithelium cells. Furthermore, we demonstrate that ICRDS is insensitive to scattering effects using suspensions of copolymer microspheres and an absorbing dye solution. These results are compared with measurements made using a more traditional transmission-style setup. This technique will have an impact on the field of nanoscience, where optical characterization of nanoparticles is still done using a conventional spectrometer that is only capable of providing measurements of the extinction coefficient.

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.

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.