Abstract: An apparatus and technique are invented that enhance the sensitivity of a spectrometer for trace gas detection by employing wavelength modulation spectroscopy (WMS) and integrating the absolute value of the recorded spectra across multiple absorption lines (features) of the species. The sensitivity is further enhanced by conducting WMS with large modulation depths. This technique is implemented using a continuously tunable external cavity CW quantum cascade laser to record the second harmonic wavelength modulated spectra of NO2 across the peak of the R-branch from 1629.5 cm?1 to 1633.9 cm?1. By integrating the absolute value of the resulting spectra, the detection sensitivity of NO2 is improved by a factor of 40 compared to the sensitivity achieved using single line WMS with the same apparatus. A sensitivity of approximately 6 ppb can be obtained using a short-path cell (a 1 m absorption cell with 2 passes) which can be significantly improved using multipass cells and cavity enhanced techniques.

Abstract: An apparatus and method are used to enhance the sensitivity of a spectrometer (sensor) for trace gas species detection by employing an external cavity continuously tunable CW quantum cascade laser and integrating the absorption spectra across multiple lines of the species. With this method the absorption spectra of NO2 is continuously recorded across the R-branch from 1628.8 cm?1 to 1634.5 cm?1. By integrating the resulting spectra, the detection sensitivity for NO2 is improved by a factor of 15 compared to the sensitivity achieved using single line laser absorption spectroscopy with the same apparatus. This procedure offers much shorter data acquisition times for the real-time monitoring of trace gas species compared with adding repeated scans of the spectra to improve the signal-to-noise ratio.

Abstract: A sensor with high sensitivity and selectivity for the detection of NO2 uses a simple diode laser operating nominally at 405 nm in the visible region, a high finesse optical cavity and a low noise photon detector. The sensor employs the multimode broad output of the diode laser with the high finesse optical cavity in an essentially off-axis arrangement that can provide large path lengths of the order of a km in a small volume cell. The detected absorption signal corresponds to multiple line integrated absorption spectroscopy (MLIS). Because the sensor uses visible radiation it can employ optics in the visible region that are normally less expensive. Also, the sensor is free from interference from atmospheric water vapor which is often a severe problem for sensors based on mid-infrared quantum cascade lasers operating in the mid-infrared (for example 1650 cm?1) region.

Abstract: An apparatus and technique are invented that enhance the sensitivity of a spectrometer for trace gas detection by employing wavelength modulation spectroscopy (WMS) and integrating the absolute value of the recorded spectra across multiple absorption lines (features) of the species. The sensitivity is further enhanced by conducting WMS with large modulation depths. This technique is implemented using a continuously tunable external cavity CW quantum cascade laser to record the second harmonic wavelength modulated spectra of NO2 across the peak of the R-branch from 1629.5 cm?1 to 1633.9 cm?1. By integrating the absolute value of the resulting spectra, the detection sensitivity of NO2 is improved by a factor of 40 compared to the sensitivity achieved using single line WMS with the same apparatus. A sensitivity of approximately 6 ppb can be obtained using a short-path cell (a 1 m absorption cell with 2 passes) which can be significantly improved using multipass cells and cavity enhanced techniques.

Abstract: A sensor for NO2 with ultrahigh sensitivity of detection is created by combining off-axis integrated cavity output spectroscopy (OA-ICOS) (which can provide large path lengths of the order of several km in a small volume cell) with multiple line integrated absorption spectroscopy (MLIAS) (where the absorption spectra is integrated over a large number of rotational-vibrational transitions of the molecular species to further improve the sensitivity). Employing an external cavity tunable quantum cascade laser operating in the 1601-1670 cm?1 range and a high finesse optical cavity, the absorption spectra of NO2 over 100 transitions in the R-band have been recorded. Based on the observed linear relationship between the integrated absorption vs. concentration of NO2, the sensor has an effective sensitivity of detection of 28 ppt for NO2, which is among the most sensitive levels of detection of NO2 to date.

Abstract: An apparatus and method are used to enhance the sensitivity of a spectrometer (sensor) for trace gas species detection by employing an external cavity continuously tunable CW quantum cascade laser and integrating the absorption spectra across multiple lines of the species. With this method the absorption spectra of NO2 is continuously recorded across the R-branch from 1628.8 cm?1 to 1634.5 cm?1. By integrating the resulting spectra, the detection sensitivity for NO2 is improved by a factor of 15 compared to the sensitivity achieved using single line laser absorption spectroscopy with the same apparatus. This procedure offers much shorter data acquisition times for the real-time monitoring of trace gas species compared with adding repeated scans of the spectra to improve the signal-to-noise ratio.

Abstract: According to one exemplary embodiment of the present invention, a method for writing an arbitrary, two-dimensional pattern using interferometric lithography and classical techniques includes the steps of: (1) creating a pixel array defined by a number of pixels having specific coordinates; (2) mapping pixel information based on the desired pattern, the pixel information including a list of which pixels are activated to define the desired two-dimensional pattern; (3) controlling a relative strength of each pixel for indicating a feature height of a portion of the desired two-dimensional pattern; and (4) controlling a degree that one pixel is shifted in an x-direction and a y-direction relative to original coordinates of the pixel in order to define the desired two-dimensional pattern pixel by pixel.