Infrared Spectroscopy using Quantum Cascade Lasers Peng Wang and Tom Tague Bruker Optics, Billerica, MA Laurent Diehl, Christian Pflügl and Federico Capasso.

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