Infrared Spectroscopy using Quantum Cascade Lasers Peng Wang
- Slides: 19
Infrared Spectroscopy using Quantum Cascade Lasers Peng Wang and Tom Tague Bruker Optics, Billerica, MA Laurent Diehl, Christian Pflügl and Federico Capasso School of Engineering and Applied Sciences, Harvard University, Cambridge, MA
Overview Ø Motivation Ø A bit of background Ø IR-QCL experiment on creatine and algae Ø Summary Ø Future directions
Motivation Ø Current mid-infrared spectroscopy methods: • Large spectral range yet broadband light source with low brightness • Laser source with high optical power but narrow spectral range Ø A need exists for a broadband light source with high brightness • Measure through optically dense media, such as aqueous solution • Transmission through or reflection from strongly absorbing and poorly reflecting samples, such as tablets, polymers, films, cells, etc. • Stand-off analysis of surface adsorbents, chemical agents or pollutions through the atmosphere. Ø Resolution • Combine a spectrally broad and bright light source with a wavelength dispersive element like FT-IR spectrometer.
Different Types of Broadband IR Light Source Globar Synchrotron QCL x 1 X 100 -1000 X 100, 000 Brightness
IR Spectra of a Single Red Blood Cell with Synchrotron vs. with Globar Source S/N greatly enhanced! Biochimica et Biophysica Acta 1758 (2006) 846– 857
Quantum Cascade Lasers
Laser Types Ø Febry-Perot (FP) lasers Simple, high power, multi-mode at higher operating current, wavelength tunable by changing the temperature of the QC device. Ø Distributed feedback (DFB) lasers Single mode operation, wavelength tunable by changing the temperature Ø External cavity lasers wavelength selectable by using frequency-selective element such as gratings.
Spectrum of the Multi-mode QCL Laser Resolution: 0. 1 cm-1 80 K, 450 m. A, cw, integrated power measured at the sample compartment ~50 m. W
Experimental Setup QCL Interferometer Liquid cell detector FT-IR Spectrometer
Creatine
IR Single Channel Spectra through Water with Globar 15 m liquid cell 125 m liquid cell
IR Absorption Spectra of Creatine through Aqueous Solution with Globar 15 m liquid cell 125 m liquid cell
IR Single Channel Spectra through 125 m Water Cell with QCL vs. with Globar Resolution: 4 cm-1 125 m liquid cell with QCL 125 m liquid cell with Globar
IR Absorption Spectra of Creatine through 125 m Water Cell with QCL vs. with Globar 15 m liquid cell with Globar 125 m liquid cell with QCL 125 m liquid cell with Globar
Algae: • • Autotrophic organisms, photosynthetic, like plants. Because of lack of many distinct organs found in land plants, they are currently excluded from being considered plants. Diatoms Classification: • Unicellular forms • 5 micrometer to mm (e. g. diatoms can reach up to 2 mm). • Multicellular forms • Macroalgae (e. g. seaweed) longer than 50 M Seaweed
Algae Fuel Extract the biomass Continuous flow centrifuge and other approaches Grow the Algae with sunshine, water, CO 2 and nutrition. Extract the lipids Mechanical Methods or/and Chemical Methods Transesterification Refine into bio-diesel and other products ”Bio-crude” oil
IR Spectra of Green Algae through 125 m Aqueous Solution X 1000 QCL signal through 125 m Algae solution 125 m, QCL 15 m, Globar
Summary Ø Multi-mode QCL lasers can be used as a broadband MIR light source. Ø The feasibility of using multi-mode QCL laser and FT-IR spectrometer to measure the absorption of creatine and algae through aqueous solutions are demonstrated. The measured thickness is up to 125 m. Ø It is critical that 4 cm-1 resolution is sufficient for most of the applications so that the spacing between two Fabry-Perot modes of the QCL lasers (<1 cm-1) wouldn’t affect much.
Future Directions Ø Higher brightness Ø Broader band coverage • FP laser Operated in the regime of Risken-Nummedal-Graham-Haken (RNGH) instabilities • An array of FP lasers operated at different wavelength range Ø Truly continuous to achieve high resolution spectrum • Temperature tuning Ø Better stability
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