infrared spectroscopy using quantum cascade lasers peng wang and tom tague bruker optics, billerica,...
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Infrared Spectroscopy using Quantum Cascade Lasers
Peng Wang and Tom TagueBruker Optics, Billerica, MA
Laurent Diehl, Christian Pflügl and Federico CapassoSchool 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
Brightness
x1 X100-1000 X100,000
IR Spectra of a Single Red Blood Cell with Synchrotron vs. with Globar Source
Biochimica et Biophysica Acta 1758 (2006) 846–857
S/N greatly enhanced!
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
80K, 450mA, cw, integrated power measured at the sample compartment ~50mW
Resolution: 0.1cm-1
Experimental Setup
QCL
Interferometer
Liquid cell
detector
FT-IR Spectrometer
Creatine
IR Single Channel Spectra through Water with Globar
10001500200025003000
Wavenumber cm-1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Sin
gle
chan
nel
15m liquid cell
125m liquid cell
IR Absorption Spectra of Creatine through Aqueous Solution with Globar
1320 1340 1360 1380 1400 1420 1440 1460 1480
Wavenumber cm-1
0.15
0.20
0.25
0.30
0.35
Abs
orba
nce
Uni
ts
15m liquid cell
125m liquid cell
IR Single Channel Spectra through 125m Water Cell with QCL
vs. with Globar
1320 1340 1360 1380 1400 1420 1440 1460 1480
Wavenumber cm-1
0.00
0.02
0.04
0.06
0.08
0.10
Sin
gle
chan
nel
125m liquid cell with QCL
125m liquid cell with Globar
Resolution: 4cm-1
IR Absorption Spectra of Creatine through 125m Water Cell with QCL vs. with Globar
1380 1400 1420 1440 1460 1480
Wavenumber cm-1
0.0
0.1
0.2
0.3
0.4
0.5
Abs
orba
nce
Uni
ts
125m liquid cell with QCL
125m liquid cell with Globar
15m 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
Grow the Algae
with sunshine, water, CO2
and nutrition.
Extract the biomass
”Bio-crude” oilRefine into bio-diesel
and other products
Continuous flow
centrifuge and other
approaches
Mechanical Methods or/and
Chemical Methods
Transesterificatio
n
Extract the lipids
IR Spectra of Green Algae through 125m Aqueous Solution
125m, QCL
15m, Globar
QCL signal through 125 m
Algae solution
1320 1340 1360 1380 1400 1420 1440 1460 1480
0.0
0.1
0.2
Ab
so
rba
nce
Un
it
Wavenumber (cm-1)
X1000
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 125m.
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