Transcript QCL laser.ppt

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.
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
QCL
Interferometer
Liquid cell
detector
FT-IR Spectrometer
Creatine
Single channel
0.2 0.3 0.4 0.5
0.6
IR Single Channel Spectra through Water with Globar
0.1
15m liquid cell
0.0
125m liquid cell
3000
2500
2000
W avenumber cm-1
1500
1000
15m liquid cell
125m liquid cell
0.15
Absorbance Units
0.20
0.25
0.30
0.35
IR Absorption Spectra of Creatine through Aqueous Solution with Globar
1320
1340
1360
1380
1400
1420
Wavenumber cm-1
1440
1460
1480
IR Single Channel Spectra through 125m Water Cell with QCL
Resolution: 4cm-1
125m liquid cell with QCL
125m liquid cell with Globar
0.00
Single channel
0.02 0.04 0.06 0.08
0.10
vs. with Globar
1320
1340
1360
1380
1400
1420
Wavenumber cm-1
1440
1460
1480
0.5
IR Absorption Spectra of Creatine through 125m Water Cell
with QCL vs. with Globar
0.0
Absorbance Units
0.1
0.2
0.3
0.4
15m liquid cell with Globar
125m liquid cell with QCL
125m liquid cell with Globar
1380
1400
1420
1440
Wavenumber cm-1
1460
1480
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.
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 50M
Seaweed
Algae Fuel
Extract the biomass
Continuous flow
centrifuge and other
approaches
Grow the Algae
with sunshine, water, CO2
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
QCL signal through 125 m
X1000
Algae solution
Absorbance Unit
0.2
125m, QCL
0.1
15m, Globar
0.0
1320 1340 1360 1380 1400 1420 1440 1460 1480
-1
Wavenumber (cm )
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