quantum cascade laser based trace gas...

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QUANTUM CASCADE LASER BASED TRACE GAS SENSORS

Christina YoungResearch Advisor: Prof. Dr. Boris Mizaikoff

Georgia Institute of TechnologySchool of Chemistry and Biochemistry

Atlanta, GA, USA

Boris [email protected] http://asl.chemistry.gatech.edu/

Christina Young [email protected]

2Why Mid-IR Technology for Trace Gas Sensing?

Analytes absorb with unique molecular signatures

Selectivity inherently based on wavelength

Single-mode, tunable QCLs promise sensitivity and miniaturization

Can be used for simultaneous quantitative monitoring of different analytes in air for environmental detection, process monitoring, biomarkers in breath, etc.

FIR

20 µm

500 cm-1

2.5 µm

4000 cm-1

800 - 400 nm

Maximum of black bodyemission at 300 KFINGERPRINT

(500 - 1300 cm )-1

NIR VISto UV

MID-INFRARED BAND



3Outline

Quantum Cascade Laser Based Trace Gas Sensors

EC-QCL HWG Multianalyte Detection

Wavelength Selection by Cavity Length Variation



EC = external cavity, HWG = hollow waveguide, QCL = quantum cascade laser

4Quantum Cascade Lasers (QCL) for Trace Gas SensingFundamentals

one period

active region

injector

minigap

probability density

miniband

Intersubband transitions between quantized conduction band states

Band Energy Diagram• Active region: Design of energylevels 3 and 2 to achieve:

• light amplification• desired laser frequency ν=(E3-E2)/h

• Injector: Supplies electrons

Young, C. et. al., Sensors and Actuators B: Chemical, 140(1), pp. 24-28.Boris [email protected] http://asl.chemistry.gatech.edu/


5Widely Tunable EC-QCLs for Spectroscopic ApplicationsFundamental Principles

EL Characteristics

Cavity Response

Output Characteristics

EC-QCL

QCL100% HRcoating

95% HRcoating

λ 1λ 2λ 3

Location of the resonant peak shifts according to the angle of the grating.

Output Lens

QCL Gain Medium

Cavity Lens Cavity Filter

Scheme Courtesy of Daylight Solutions, Inc.• QCL tuning options: current or

temperature (few cm-1), external cavity

• EC provides ~ +/- 5% cm-1 tuning range of the central emission frequency



6EC-QCL Emission CharacterizationCentral emission frequency of 1258 cm-1

Operating Conditions: Temp: 0 C, Pulse Width: 0.50 μsec, Frequency: 100.0 kHz, Duty Cycle: 5%, I: 1500 mA

EC-QCLMCT detector

BrukerIFS 66 FT-IR



7EC-QCL Trace Gas Sensor Experimental SetupIndividual analytes measured

EC-QCL

Reference

Sample

Glass Stir Bar

Exponential Dilution Flask

Analyte InjectionNitrogen Carrier Gas

Gas Flow to SensorC = C0e - αt

WhereC = final concentrationC0 = initial concentrationα = flowrate/vol of EDFt = time elapsed between initial absorption and

signal regeneration

• Ethyl chloride at 1287.25 cm-1

• Dichloromethane at 1262 cm-1

• Trichloromethane at 1220 cm-1

Exponential dilution performed for three analytes to derive limit of detection (LOD):

Charlton, C., et. al., Appl. Phys. Lett. 86, 194102 (2005);Lovelock, J., Anal. Chem. 33, 163 (1961); Young, C.., et. al., Sens. Act. B. (2009)



8Univariate ResultsEC-QCL precisely tuned to the Q-branch of CH2 wag vibrational mode for each analyte

50 60 70 80 901.80

1.95

2.10

2.25

2.40

2.55

Nor

mal

ized

Sig

nal (

V)

Time (min)30 35 40 45 50 55 60

0.14

0.16

0.18

0.20

0.22

Nor

mal

ized

Sig

nal (

V)

Time (min)40 50 60 70 80

2.3

2.4

2.5

2.6

2.7

2.8

2.9

3.0

Nor

mal

ized

Sig

nal (

V)

Time (min)

LOD

ClCl

= noise level

Ethyl ChlorideCl

LOD

13 ppb

Trichloromethane

LOD

Cl

ClCl

Variance

LODAssignment

Chemical Structure

Analyte

5 ppb

υ 7

7 ppm

5 ppm

11 ppb

15 ppb

Dichloromethane

υ 9 υ 4



9Quantitative Measurement of An Analyte in MixturePartial Least Squares (PLS) model based on 11 training set mixtures

Cross Validation, 7 Latent Variables

35 40 45 50 55 6030

35

40

45

50

55

60

65

Concentration Measured

Con

cent

ratio

n P

redi

cted

Concentration Measured (ppm)

Con

cent

ratio

n Pr

edic

ted

(ppm

)

Quasi-unknownTraining Set Standards

ClCl

R2 = 0.977

1215 1230 1245 1260 1275 1290

1.0

1.2

1.4

1.6

1.8

2.0

2.2

Abs

orba

nce

(a.u

.)

Wavenumbers (cm-1)

Dichloromethane

Trichloromethane

Ethyl Chloride

EC-QCL HWG gas spectrum, 1 cm-1 resolution PLS Model Validation

(MATLAB (Mathworks ©)



10Outline

Quantum Cascade Laser Based Trace Gas Sensors

EC-QCL HWG Multianalyte Detection

Wavelength Selection by Cavity Length Variation



EC = external cavity, HWG = hollow waveguide, QCL = quantum cascade laser

11Mounting and Measuring the QCL

FP-QCL chip

Cleave in the plane of the crystal lattice with diamond knife and microscope

Cleaved single facet

Prepare sub-mount by gluing a insulated gold pad onto a copper block

Bond gold wires from gold pad to QCL chip to transport current

Apply In0.97Ag0.3 to Cu block and align laser on top with micro-tweezers. Keep aligning as In0.97Ag0.3 is heated from 130 C to 180 C and back to 130 C, chemically bonding chip to block

Cu block with Au pad

Laser sub-mount before bonding

Laser sub-mount after bonding

CryostatFT-IR

XYZ positioner

Laser Sub-Mount

LN2

Collaboration with Mid-Infrared Photonics Group, MIRTHE, Princeton University

Photo courtesy of C. Gmachl

Young, C. et. al., Applied Physics Letters, 94(9), pp. 091109.Boris [email protected] http://asl.chemistry.gatech.edu/


12QCL Emission Tuning by Δ Cavity LengthTheoretical

QCL Band Energy Diagram

Theoretical Calculations

L

Young, C. et. al. Applied Physics Letters, 94, 091109, (2009).Boris [email protected] http://asl.chemistry.gatech.edu/


13Towards Precise Overlap of QCL and Analyte Absorption

Frequency emission shifts with respect to cavity length due to changes in n and E



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Experimental Results Confirm Theoretical Calculations Cavity Length Dependence: Δ Efield Causes Δ λem

Shorter cavity length:- Increase in Efield (Vth)- Increase in λem

Emission Frequency Vs. Vth

Vth Vs. Cavity Length



15Acknowledgments

• Applied Sensors Laboratory, Georgia Tech, Atlanta, GA

• Daylight Solutions, Inc.

• Mid-IR Photonics Group at Princeton University

• Mark Disko, Andy Riley, John Szobota, John Martin and Neil Brons atExxonMobil Research and Engineering Company, Annandale, NJ

• Adtech Optics, Inc. for providing a507BH QCL

Many thanks to colleagues and co-workers at…



16Questions?



quantum cascade laser based trace gas...

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