1. 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

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

3. 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.

4. Different Types of Broadband IR Light Source
Globar
Synchrotron
QCL x1 X
X100,000
Brightness

5. 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

6. Quantum Cascade Lasers

7. 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.

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

9. Experimental Setup FT-IR Spectrometer QCL Interferometer Liquid cell
detector
FT-IR Spectrometer

10. Creatine

11. IR Single Channel Spectra through Water with Globar
15m liquid cell
125m liquid cell

12. IR Absorption Spectra of Creatine through Aqueous Solution with Globar
15m liquid cell
125m liquid cell

13. 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

14. 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

15. 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

16. 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

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

18. 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.

19. 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