The current research by the Rice Laser Science Group is focused on the further development of advanced mid-infrared trace gas sensors for applications in four technical areas: environmental monitoring, medical diagnostics and the life sciences, industrial process analysis and control as well as nuclear security. This research will take advantage of recent, significant advances in the commercial availability of high performance mid-infrared laser diodes, quantum cascade laser and interband cascade laser devices in the 3 to 11 µm spectral range. It is planned to utilize mid-infrared semiconductor devices capable of high continuous wave output power at room temperature with wall plug efficiencies of > 10%. The work associated with the research is relevant to the training of post-doctorial fellows, graduate students and undergraduates in real world applications. The experiences not only involve laboratory experience, but also the ability to report research as posters and oral presentations at national and international conferences as well as at specialized workshops.

Detection of methane (CH4) and nitrous oxide (N2O) using a quartz enhanced photoacoustic spectroscopy (QEPAS) mid-infrared sensor system

CH4 and N2O next to CO2 are the most important greenhouse gases because of their global warming potential. Therefore, the detection of CH4 and N2O at low parts per billion (ppb) concentration levels is of great interest in environmental and agricultural monitoring. Furthermore, exposure to N2O can lead to health issues such as short-term decrease in mental performance, audiovisual ability, and manual dexterity. N2O is also a processing gas in electronics and medicine as well as in aerospace applications and therefore any leaks of N2O during these processes must be controlled and monitored. Recently the development and performance evaluation of a cost-effective, compact, autonomous QEPAS based sensor system for the detection of CH4 and N2O was completed. This sensor uses a continuous wave, thermoelectric cooled, distributed feedback quantum cascade laser mounted in a high heat load package. Second harmonic detection was implemented to perform sensitive and background free measurements. For the targeted CH4 and N2O absorption lines located at 1275.04 cm-1 and 1275.49 cm-1 detection limits (1σ) of 13 ppbv and 6 ppbv were achieved with a 1 second data acquisition time, respectively. The normalized noise equivalent absorption (NNEA) for CH4 and N2O was 2.74×10-6 cm-1 W/Hz1/2 and 2.9×10-6 cm-1 W/Hz1/2 respectively. In the near future we expect to implement novel, ultracompact control electronics. Field deployment in the Greater Houston of the QEPAS based sensor for urban and regional environmental monitoring of CH4/N2O is planned from September 1, 2013-August 30, 2014.

Detection of hydrogen peroxide (H2O2) using mid-infrared laser absorption spectroscopy

Sensitive detection of H2O2 at 7.73 μm is of interest in atmospheric chemistry applications, where it has a significant role as an atmospheric reservoir of HOx and is associated with biomass burning. Furthermore H2O2 can partition into cloud and particle water resulting in the formation of sulfate and organic arerosol. H2O2 is also useful in explosive detection and in medical diagnostics, such as early diagnosis as well as monitoring of asthma, chronic obstructive pulmonary disease (COPD), and in accute respiratory distress syndrome. A HHL packaged CW TEC DFB QCL was characterized and incorporated in a prototype sensor system based on an astigmatic multipass absorption cell with an effective optical pathlength of 36 m. By employing a calibration free 2f wavelength modulation spectroscopy (WMS) model, it is possible to accurately infer the H2O2 concentration. The model includes both the linear and nonlinear tuning characteristics of the QCL source as well as normalizes the 2f WMS signal with the 1f WMS signal. In order to measure the laser-specific tuning characteristics, a self-designed characterization system, which consists of an etalon with a free spectral range of 0.011 cm-1, two mid-infrared photo detectors, and a simultaneous data acquisition were integrated into the sensor system. To optimize the sensor performance, the influence of modulation parameters will be investigated.

Detection of low-concentration nitric oxide (NO) emitted by cancer cells using a QCL based TDLAS sensor

Detection of nitric oxide (NO) emission in cancer cells is of significant interest due to its integral role in novel methods of cancer therapy. Monitoring the concentration of NO is therefore crucial to understanding the effectiveness of such treatments. However, accurate measurements of NO concentration in cancer cells in real-time have not yet been achieved with existing sensing methods. A highly sensitive prototype NO sensor was developed using tunable diode laser absorption spectroscopy (TDLAS) combined with second harmonic wavelength modulation spectroscopy (2f-WMS). The system uses a 182-pass 100 m Herriott multipass cell in order to improve the detection sensitivity to the ppb NO concentration level. A continuous wave, distributed feedback (DFB) quantum cascade laser (QCL) was used to target the optimum NO absorption line at 1900 cm-1 (5.26 μm). In the future, a novel, ultra-compact multipass cell and surface mounted, digital electronics will be used to perform real-time NO emission measurements from different cancer cell samples.

Detection of ethane (C2H6) using a laser absorption spectroscopy (LAS)

C2H6 is an important molecule for atmospheric chemistry and climate modelling. Furthermore, oil and gas prospecting has been explored based on naturally occurring C2H6 seepages that accompany hydrocarbon reservoirs particularly in desert environments. Ultra-sensitive detection of ethane has also found application in medical breath analysis. Monitoring elevated levels of C2H6 in exhaled human breath can be used as a non-invasive method to identify and detect a variety of diseases, such as asthma, schizophrenia, or lung cancer. The spectral region between 3μm and 4 μm is attractive for targeting hydrocarbons since their fundamental absorption bands occur in this region. In the case of C2H6, a strong fundamental absorption band is located at ~ 3.36 μm (2977 cm-1). This spectral range can be targeted with different types of short wavelength mid-infrared, CW or pulsed room temperature laser sources such as QCLs, interband cascade lasers (ICLs) and laser diodes. For ultra-sensitive and selective C2H6 concentration measurements a CW TEC DFB GaInAsSB/AlGaInAsSb based semiconductor laser diode based spectroscopic trace-gas sensor was developed. The sensor platform employed tunable diode laser absorption spectroscopy (TDLAS) based on a 2f wavelength modulation (WM) detection technique. TDLAS was performed using an ultra-compact 57.6 m effective optical path length innovative spherical multipass cell capable of 459 passes between two mirrors separated by 13 cm and optimized for the 2.5-4 μm range TE-cooled mercury-cadmium-telluride (MCT) detector. For an interference free C2H6 absorption line located at 2976.8 cm-1 a 1σ minimum detection limit of 740 pptv with a 1 second lock-in amplifier time constant was achieved. By using a state-of-the art sensitive photovoltaic MCT detector and integrated electronics, significant noise reduction and signal quality enhancement was achieved. A further improvement of the reported C2H6 sensor will include implementing absorption line-locking functionality in the control software and further miniaturization of the sensor architecture that will result in an ultracompact, robust, and portable sub-ppb level C2H6 sensor platform.

Development of a spectroscopic QCL source for uranium hexafluoride (UF6) analytical enrichment measurements based on laser absorption spectroscopy

State-of-the-art semiconductor laser technology based on infrared absorption spectroscopy offers the opportunity to detect both in situ and remotely trace gases of specific interest to the International Atomic Energy Agency (IAEA) charged with the detection and verification of nuclear materials and activities on a global basis. A comprehensive evaluation of sensor characteristics with an appropriate simulant target analyte of UF6, such as CH4, C2H2 , Freon 134a or Freon 125 will determine the lowest detectable absorbance for both short and longtime laboratory measurements, using a 7.74 μm (1292 cm-1) DFB-QCL from AdTech Optics. Research is being conducted by our Princeton NSF-ERC-MIRTHE collaborators (C. Gmachl of room temperature continuous wave long wavelength quantum cascade lasers. Designs for a CW TEC DFB QCLs at longer wavelengths are being investigated and when available will be able to access the fundamental band at 16 μm (ν3) of UF6. The same target spectral region is useful to target benzene, toluene and xylene, which are molecules of considerable interest to the petrochemical industry.