Raman spectral imaging is increasingly becoming the tool of choice for field-based applications such as threat, narcotics and hazmat detection; air, soil and water quality monitoring; and material ID. Conventional fiber-coupled point source Raman spectrometers effectively interrogate a small sample area and identify bulk samples via spectral library matching. However, these devices are very slow at mapping over macroscopic areas. In addition, the spatial averaging performed by instruments that collect binned spectra, particularly when used in combination with orbital raster scanning, tends to dilute the spectra of trace particles in a mixture. Our design, employing free space line illumination combined with area imaging, reveals both the spectral and spatial content of heterogeneous mixtures. This approach is well suited to applications such as detecting explosives and narcotics trace particle detection in fingerprints. The patented High Throughput Virtual Slit1 is an innovative optical design that enables compact, inexpensive handheld Raman spectral imagers. HTVS-based instruments achieve significantly higher spectral resolution than can be obtained with conventional designs of the same size. Alternatively, they can be used to build instruments with comparable resolution to large spectrometers, but substantially smaller size, weight and unit cost, all while maintaining high sensitivity. When used in combination with laser line imaging, this design eliminates sample photobleaching and unwanted photochemistry while greatly enhancing mapping speed, all with high selectivity and sensitivity. We will present spectral image data and discuss applications that are made possible by low cost HTVS-enabled instruments.
Remote sensing has moved out of the laboratory and into the real world. Instruments using reflection or Raman imaging modalities become faster, cheaper and more powerful annually. Enabling technologies include virtual slit spectrometer design, high power multimode diode lasers, fast open-loop scanning systems, low-noise IR-sensitive array detectors and low-cost computers with touchscreen interfaces. High-volume manufacturing assembles these components into inexpensive portable or handheld devices that make possible sophisticated decision-making based on robust data analytics. Examples include threat, hazmat and narcotics detection; remote gas sensing; biophotonic screening; environmental remediation and a host of other applications.
Hyperspectral imaging (HSI) device users often require both high spectral resolution, on the order of 1 nm, and high light-gathering power. A wide entrance slit assures reasonable étendue but degrades spectral resolution. Spectrometers built using High Throughput Virtual Slit™ (HTVS) technology optimize both parameters simultaneously. Two remote sensing use cases that require high spectral resolution are discussed. First, detection of atmospheric gases with intrinsically narrow absorption lines, such as hydrocarbon vapors or combustion exhaust gases such as NOx and CO2. Detecting exhaust gas species with high precision has become increasingly important in the light of recent events in the automobile industry. Second, distinguishing reflected daylight from emission spectra in the visible and NIR (VNIR) regions is most easily accomplished using the Fraunhofer absorption lines in solar spectra. While ground reflectance spectral features in the VNIR are generally quite broad, the Fraunhofer lines are narrow and provide a signature of intrinsic vs. extrinsic illumination.
The High Throughput Virtual Slit enables higher spectral resolution than is achievable with conventional spectrometers by manipulating the beam profile in pupil space. By reshaping the instrument pupil with reflective optics, HTVS-equipped instruments create a tall, narrow image profile at the exit focal plane, typically delivering 5X or better the spectral resolution achievable with a conventional design.
Many DMD-based programmable light sources consist of a white light source and a pair of spectrometers operating in subtractive mode. A DMD between the two spectrometers shapes the delivered spectrum. Since both spectrometers must (1) fit within a small volume, and (2) provide significant spectral resolution, a narrow intermediary slit is required. Another approach is to use a spectrometer designed around a High Throughput Virtual Slit, which enables higher spectral resolution than is achievable with conventional spectroscopy by manipulating the beam profile in pupil space. Conventional imaging spectrograph designs image the entrance slit onto the exit focal plane after dispersing the spectrum. Most often, near 1:1 imaging optics are used in order to optimize both entrance aperture and spectral resolution. This approach limits the spectral resolution to the product of the dispersion and the slit width. Achieving high spectral resolution in a compact instrument necessarily requires a narrow entrance slit, which limits instrumental throughput (étendue). By reshaping the pupil with reflective optics, HTVS-equipped instruments create a tall, narrow image profile at the exit focal plane without altering the NA, typically delivering 5X or better spectral resolution than is achievable with a conventional design. This approach works equally well in DMD-based programmable light sources as in single stage spectrometers. Assuming a 5X improvement in étendue, a 500 W source can be replaced by a 100 W equivalent, creating a cooler, more efficient tunable light source with equal power density over the desired bandwidth without compromising output power.
Transient infrared methods are used to observe directly fast reactions and vibrational relaxation processes in solution with high spectral resolution. Bimolecular reactions of the CN radical are observed following photolysis of ICN in chloroform. The CN radicals react with solvent molecules to form HCN. In the deuterated solvent, some of the nascent DCN molecules are formed in a vibrationally excited state, which constitutes the first-observation of vibrationally excited products in a condensed phase bimolecular reaction. In a second experiment, photolysis of s-tetrazine in solution creates vibrationally hot HCN in a unimolecular reaction. Deuterated s-tetrazine creates an initially inverted vibrational distribution of DCN products. Both experiments indicate that the dynamics of reactive barrier crossings and vibrational energy redistribution are significantly altered upon solvation.