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 Motivation A bit of background IR-QCL experiment on creatine and algae Summary Future directions

Motivation 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 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. 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 x1 X100-1000 X100,000 Brightness

IR Spectra of a Single Red Blood Cell with Synchrotron vs 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 Febry-Perot (FP) lasers Simple, high power, multi-mode at higher operating current, wavelength tunable by changing the temperature of the QC device. Distributed feedback (DFB) lasers Single mode operation, wavelength tunable by changing the temperature External cavity lasers wavelength selectable by using frequency-selective element such as gratings.

Spectrum of the Multi-mode QCL Laser Resolution: 0.1cm-1 80K, 450mA, cw, integrated power measured at the sample compartment ~50mW

Experimental Setup FT-IR Spectrometer 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 IR Single Channel Spectra through 125m Water Cell with QCL vs. with Globar Resolution: 4cm-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 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 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. Classification: Unicellular forms 5 micrometer to mm (e.g. diatoms can reach up to 2 mm). Multicellular forms Macroalgae (e.g. seaweed) longer than 50M Diatoms Seaweed

Algae Fuel Extract the biomass Continuous flow centrifuge and other approaches Grow the Algae with sunshine, water, CO2 and nutrition. Mechanical Methods or/and Chemical Methods Extract the lipids Transesterification Refine into bio-diesel and other products ”Bio-crude” oil

IR Spectra of Green Algae through 125m Aqueous Solution X1000 QCL signal through 125 m Algae solution 125m, QCL 15m, Globar

Summary Multi-mode QCL lasers can be used as a broadband MIR light source. 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. It is critical that 4cm-1 resolution is sufficient for most of the applications so that the spacing between two Fabry-Perot modes of the QCL lasers (<1cm-1) wouldn’t affect much.

Future Directions Higher brightness 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 Truly continuous to achieve high resolution spectrum Temperature tuning Better stability