Near-field imaging of the evanescent electric field on the surface of a quantum cascade laser

V. Moreau, M. Bahriz, and R. ColombelliInstitut d’Électronique Fondamentale (IEF)

Université Paris Sud – 91405 Orsay – FRANCE

P.-A. Lemoine, Y. De Wilde Laboratoire d’Optique Physique,

ESPCI, 75005 Paris - France

R. Perahia, O. PainterDepartment of Applied Physics,

California Institute of Technology, USA

L. Wilson, A. KrysaUniversity of Sheffield, Sheffield – UK

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Outline

Motivations

I. Implementation of lasers « with evanescent wave »

II. Observation by SNOM of the evanescent wave

III. Proof of principle: Application to surface detection

Conclusion

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Developing an ‘evanescent field’ waveguide with reasonable low loss

Observation of the mode inside a QC laserwaveguide with evanescent field

Developing sensor devicesproof of principle with solvents

Motivations

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InP

Active region

-4-2

02

46

Mode intensity

Dis

tanc

e (µ

m)

Active region

Evanescent Electric Field

Air

Dielectric

1% of the mode intensity into air

I. The waveguide: air guiding

D. Hofstetter et al., PTL (2000)W. Schrenk et al., APL (2000)

The surface evanescent wave reflects the presence of the standing wave inside the

laser ridge

Fabry-Perot : standing wave Ez inside the cavity

x

z

modulation along y

⎟⎠⎞

⎜⎝⎛ yneff

λπ2cos

y

ITQW 20075

TEMActive Region

Growth AxisE

nerg

y

ωLO

4

21

Gro

wth

Axi

sGrowth: MOCVD Material: InGaAs/AlInAs Active region:

double phonon resonance

ωLO

3

Emission around 7.5 µm

I. The QC material

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I. Device characterization

Jth(78K) = 1.5 kA.cm-2 and Tmax = 300 K

1280 1300 1320 1340 1360 1380 1400

Wavelength (μm)

78K

140K

200K

260K

Wavenumber (cm-1)

Out

put (

a.u.

) 300K

7.8 7.7 7.6 7.5 7.4 7.3 7.2

met

al

met

al

Sem

ico

nduc

tor

l

Fabrication

Measurements(50ns @ 84 kHz)

V. Moreau, accepted in Optics Express

Wavelength correctly redshiftswith the temperature

l = 26, 31, 36 and 41 µm

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I. Device characterization

Sem

ico

nduc

tor

l

Far field analysis

0

max

min-2 -1 0 1 2

-6-4-2

02

x (µm)y

(µm

)

Θx

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I. Device characterization

Sem

ico

nduc

tor

l

Far field analysis

0

max

min-2 -1 0 1 2

-6-4-2

02

x (µm)y

(µm

)

Θx

simulation

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I. Device characterization

Sem

ico

nduc

tor

l

Far field analysis

0

max

min-2 -1 0 1 2

-6-4-2

02

x (µm)y

(µm

)

Θx

simulation experiment

The optical mode is predominantly air guided

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II. The a-SNOM setup (Aperturless Scanning Near-field Optical Microscopy)

Oscillator

Lock-in

refΩ

refΩ

Piezo excitation – ~ 8kHzTungsten Tip

Sample (laser)

Detector HgCdTe

Feedback

Tuning fork

Piezo xy

Cassegrain

84kHz-50ns

Mirror

Lens

Measurements performed at ESPCI:P.-A. Lemoine and Y. De Wilde

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II. The a-SNOM scanning zone

AFM top view

InP

Active region

30 µm60 µm

Cut view Top view

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II. a-SNOM imaging below laser threshold

0.0 0.5 1.0 1.5 2.0 2.5

Out

put P

ower

(a.u

.)

Current (A)

Near Field

SNOM

(b) top view

AFM30 µm

60 µm

top view

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0.0 0.5 1.0 1.5 2.0 2.5

Out

put P

ower

(a.u

.)

Current (A)

Near Field

(b) (c)

(b) (c) AFM top view

(a)

II. a-SNOM imaging at laser threshold

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0.0 0.5 1.0 1.5 2.0 2.5

Out

put P

ower

(a.u

.)

Current (A)

Near Field

(b) (c) (d)

(b) (c)

(d)

AFM top view

(a)

II. a-SNOM imaging above laser threshold

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0.0 0.5 1.0 1.5 2.0 2.5

Out

put P

ower

(a.u

.)

Out

put P

ower

(a.u

.)

Current (A)

Far-field Near-field

(b) (c) (d)

(b) (c)

(d),(e)

AFM top view

(a)

II. a-SNOM imaging below and above laser threshold

V. Moreau, APL 90, 201114 (2007)

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0.0 0.5 1.0 1.5 2.0 2.5

Out

put P

ower

(a.u

.)

Out

put P

ower

(a.u

.)

Current (A)

Far-field Near-field

(b) (c) (d)

(b) (c)

(d),(e)

AFM top view

(a)

II. a-SNOM imaging below and above laser threshold

V. Moreau, APL 90, 201114 (2007)

(e)

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II. Standing wave details and effective indexDemodulation at the

tip frequency

my μδ 25.1=yδ

3D

11.3=effn

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II. Standing wave details and effective indexDemodulation at the

tip frequency

1270 1280 1290 1300

Wavelength (μm)

Out

put P

ower

(a.u

.)

Wavenumber (cm-1)

7.85 7.8 7.75 7.7

yδ

3D

µm

ng

78.7

4.3

=

=

λ1037.0 −−=

∂∂ mn μλ

my μδ 25.1=

11.3=effn

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II. Evanescent wave decay length: ~500 nm

Simulations

500 nm

0

0.2

0.4

0.6

0.8

1

-15 150x (µm)

z (µ

m)

0

Max

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II. Evanescent wave decay length: ~500 nm

Simulations a-SNOM Measurementstopography laser

500 nm

0

0.2

0.4

0.6

0.8

1

0

0.2

0.4

0.6

0.8

1

-15 150-15 150x (µm) x (µm)

z (µ

m)

z (µ

m)

0

Max

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III. Framework: Intra-cavity absorption spectroscopy

Goal: Laser based intra-cavity sensing and absorption spectroscopy of biological and chemical molecules.

Motivation:•The mid infrared region of the spectrum is rich in vibrational and rotational resonances of chemical and biological molecules.

•Proteins, carbohydrates, and nucleic acids can be both identified and their structure probed by infrared absorption spectroscopy.

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Gain

Lase

r cav

ity

0

III. Application: surface detection

λ

Injected current

Output powerA

bsorption medium

Detector

Absorption spectroscopy

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Gain

Lase

r cav

ity

0

III. Application: surface detection

λ

Output powerA

bsorption medium

Detector

Gain

Absorbing material/liquid

Lase

r cav

ity Detector

?

Injected current

Output power

Injected currentAbsorption

spectroscopy

Intra cavity spectroscopy

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III. Set up of the surface detection

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III. Proof of principle with solvents

1250 1260 1270 1280 1290 1300

Air

Lase

r int

ensi

ty (a

.u.)

Wave number (cm-1)

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Isopropanol

0

200

400

600

800

1250 1260 1270 1280 1290 1300

Air IPA

Lase

r int

ensi

ty (a

.u.)

Wave number (cm-1)

Flui

d lo

sses

(cm

-1)

Red shift

III. Proof of principle with solvents

1250 1260 1270 1280 1290 1300

Air

Lase

r int

ensi

ty (a

.u.)

Wave number (cm-1)

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Isopropanol

0

200

400

600

800

1250 1260 1270 1280 1290 1300

Air IPA

Lase

r int

ensi

ty (a

.u.)

Wave number (cm-1)

Flui

d lo

sses

(cm

-1)

0

200

400

600

800

1250 1260 1270 1280 1290 1300

Air Ethanol

Lase

r int

ensi

ty (a

.u.)

Wave number (cm-1)

Flui

d lo

sses

(cm

-1)

Ethanol

III. Proof of principle with solvents

Red shift Blue shift

1250 1260 1270 1280 1290 1300

Air

Lase

r int

ensi

ty (a

.u.)

Wave number (cm-1)

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III. Proof of principle with solvents

Significant shift in the lasing envelope to minimize loss. Ethanol and isopropanol can be distinguished.

0

200

400

600

800

1250 1260 1270 1280 1290 1300

Air IPA Ethanol

Lase

r int

ensi

ty (a

.u.)

Wave number (cm-1)

Flui

d lo

sses

(cm

-1)

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•Model predicts the different behavior of isopropyl alcohol (IPA) vs. ethanol

III. Experiment vs model

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• The model predicts the new laser emission frequency – following fluid deposition – as a function of the “unperturbed” lasing frequency.

III. Experiment vs model

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•Model predicts lasing frequency dependence with fluid on initial lasing frequency of lasers without fluid

III. Experiment vs model

1

1

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•Model predicts lasing frequency dependence with fluid on initial lasing frequency of lasers without fluid

III. Experiment vs model

2

1

2

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•Model predicts lasing frequency dependence with fluid on initial lasing frequency of lasers without fluid

III. Experiment vs model

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Conclusions and perspectives

Observation of the evanescent wave on top of the device via a-SNOM microscopy

Proof-of-principle of surface-sensing with QC lasers

Integrate surface sensitive lasers in a microfluidic system

Possibility of studying surface plasmon by SNOM microscopy

European Young Investigator AwardEuropean Science Foundation