Near-field imaging of the evanescent electric field on the surface … · 2018. 6. 12. ·...
Transcript of Near-field imaging of the evanescent electric field on the surface … · 2018. 6. 12. ·...
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
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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