Transcript ESLW 2004 - University of Glasgow

Nanostructured Quantum Cascade Lasers for
Longitudinal Single Mode Control
J.P. Reithmaier1,3, S. Höfling1, J. Seufert2, M. Fischer2, J. Koeth2, A. Forchel1
1 Technische
2 nanoplus,
3
Physik, Universität Würzburg, Germany
Nanosystems and Technology GmbH, Germany
present address: Technische Physik, Universität Kassel, Germany
•
Motivation and structure of quantum cascade (QC) lasers
•
Ultra-Short QC Microlaser
•
Two segment distributed feedback (DFB) lasers
Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 1
Motivation
•
Many important gases have
their fundamental absorption
in the mid-infrared spectral
region (e.g NH3, O3, CO2)
Quantum cascade lasers (QCLs)
are reliable mid-infrared lasers
capable of room temperature operation
3.2
2.8
Absorbtion (a.u.)
•
2.0
NH3
1.6
1.2
0.8
C02
0.0
900
950
1000
1050
1100
-1
Wavenumber (cm )
0.35
0.30
0.25
Signal (a.u.)
Detection of NH3 demonstrated with
single mode distributed feedback
lasers in cooperation with:
2.4
0.4
 Single mode emission is requested
for gas sensing applications
•
O3
0.20
pulse
with NH3 (250 ppm)
0.15
0.10
0.05
0.00
0
20
40
60
Time (ns)
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80
100
Active Region Designs
0.8
1.0
0.6
Energy (eV)
0.5
3
2
1
0.3
0.2
0.0
0.8
0.6
0.4
0.1
bound-to-continuum
design
Energy (eV)
Three quantum
well design
0.7
0.2
Page et al., Appl.
Phys. Lett. 78(22) (2001)
0
10
20
30
40
50
3
2
0.4
0.0
60
70
80
0
Resonant tunneling between lowest
injector state and
upper laser level 3
•
Fast depopulation of lower laser
level 2 by LO-phonon resonance
with ground state 1
20
40
60
80
100
120
140
Distance (nm)
Distance (nm)
•
Pflügl et al., Appl. Phys.
Lett. 83(23) (2003)
•
Resonant tunneling between lowest
injector state and
upper laser level 3
•
Fast depopulation of lower laser
level 2 by interminiband
scattering processes
Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 3
Why Micro-Lasers
Advantages of micro-lasers:
• Increased device density compared to conventional ridge waveguide lasers
by approximately a factor 10 is possible
• Low threshold currents
• Short cavity devices can exhibit single mode emission due to limited gain bandwidth
and large mode spacing
~FP 
m
2n g L
[Höfling et al, Electr. Lett. 40, 120 (2004)]
 Wavelength tuning should be possible
by controling the cavity length
1 /   ~FP 
1
2ng L
Use of highly reflective deeply etched semiconductor-air Bragg mirrors
allows the fabrication of ultra-short ridge waveguide micro-lasers:
Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 4
Fabrication Process
Monolithically integrated: ridge waveguide and Bragg-mirror fabrication
(1) RWG definition
(optical lithography + lift-off)
(2) Bragg mirror definition
(e-beam lithography + lift-off)
(3) Pattern transfer
(dry etching by ECR-RIE)
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Ultra-Short Microlasers
Microlasers with ridge lengths down to 30 µm (< 10 x wavelength) realized
Power (a.u.)
8
6
15 µm
4
2
0
0.0
0.5
1.0
1.5
2.0
2.5
Current (A)
•
High-quality Bragg mirrors
•
Optically smooth surfaces
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Room Temperature Operation of Microlasers
Ridge length ~150 µm
Devices based on
Bound-to-continuum
active region design
90
80
70
- 3.4 mW , 293 K (20 °C)
Intensity (a.u.)
0.2
180 K
2A
9 mW
> 10 dB
Power (mW)
- 85 mW , 80 K
0.1
0.08
0.06
0.04
60
50
40
30
20
10
0
-0.5
0.02
900
80 K
160 K
200 K
240 K
280 K
293 K
910
920
-1
Wavenumber (cm )
0.0
0.5
1.0
1.5
2.0
2.5
Current (A)
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3.0
3.5
4.0
Wavelength Tuning with Cavity Length
Results based on bound-tocontinuum design: 50 µm device
• tuning over
38 cm-1 (420 nm)
centered around
955 cm-1 (10.5 µm)
Intensity (a.u.)
• changes in cavity
length: 0.2 µm
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
51.4
50.4
51.2
80 K
m=34
m=33
m=33
940
960
ng = 3.41
980
-1
Wavenumber (cm )
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1000
Single Mode Emission Stability
Lasers with of ~50 µm ridge length
based on bound-to-continuum design
• single mode operation
0.3
3.2 A
• mode jump due to blue shift
by increased voltage
• mode spacing about
30 cm-1 (340 nm)
Intensity (a.u.)
up to 1.5 x Ith
3.2 A
0.2
2.9 A
0.1
2.6 A
x2
2.2 A
0.0
910
920
930
940
950
-1
Wavenumber (cm )
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960
970
Tuning with Temperature/Current
Results based on three
quantum well design
Ridge length ~100 µm
Wavelength tuning
observed with:
• Heat sink temperature
-0.062 cm-1/K
~/
• Drive current
-1.0 cm-1/A
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Mode Switching with Temperature
10.6
10.5
8
• Spacing between
modes 16 cm-1
200 K
7
180 K
6
Intensity (a.u.)
• Discontinuous tuning by
temperature and according
drive current variation
10.3
10.4
5
160 K
4
140 K
3
120 K
2
100 K
1
80 K
0
940
950
970
960
-1
Wavenumber (cm )
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Two Segment Distributed Feedback Lasers
1
2
~i 
front segment
rear segment
Two segment distributed feedbacklLaser with different grating periods
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1
2 n i
Reversible Mode Switching
Wavelength (µm)
10.78
10.74
If + Ir = 3.5 A
If : Ir= 0.5 : 1
8
6
4
2
0
926
10.72
0.6 : 1
0.7 : 1
200
0.8 : 1
0.9 : 1
Power (mW)
Intensity (a.u.)
10
10.76
1:1
1.1 : 1
1.3 : 1
1.5 : 1
100
120 K
50
0
0.0
1.7 : 1
2.0 : 1
927
150
If
0.5
120 K
928
929
930
931
932
933
934
-1
Wavenumber (cm )
If= current injected in front segment
Ir= current injected in rear segment
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1.0
1.5
Current (A)
2.0
Evolution of DFB Modes with Temperature
932
10.74
Mode1
-1
10.76
928
10.78
926
10.80
Mode2
10.82
924
10.84
922
10.86
920
100
120
140
160
180
200
220
240
Temperature (K)
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260
Wavelength (µm)
2.5 cm
-1
Wavenumber (cm )
930
Quasi-Continuous Tuning
• Tuning with temperature
and segment drive
current control
Wavelength (µm)
10.86
10
5
23 dB
120 K
Intensity (a.u.)
intensity, a.u.
10.80
10.78
10.76
10.74
-1
1.0
• Side mode suppresion
ratio (SMSR) up to 23 dB
1:1
10.82
9 cm
• Single mode emission
over > 9 cm-1
1
0.5
10.84
0.8
0.6
0.4
0.2
0.1
0.05
900
920
940
960
wavenumber, cm
0.0
-1
920
922
924
926
928
-1
Wavenumber (cm )
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930
932
Summary
• QC Microlasers with monolithically integrated Bragg mirrors
• Single mode emission achieved due to large mode spacing and limited gain bandwidth
• Wavelength tuning demonstrated with:
- Temperature
- Drive current
- Cavity length
• Room temperature operation achieved (>3 mW @ 20 °C, > 10 dB SMRS @ 180 K)
• Two segment QC distributed feedback lasers
• Mode switching over 1.5 and 2.5 cm-1
• Quasi-continuous tuning over 9 cm-1 (105 nm); SMRS up to 23 dB
Acknowledgement:
A. Wolf, M. Emmerling, S. Kuhn, C. König, J. Goertz, B. Rösener
Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 16