1. 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 Physik, Universität Würzburg, Germany
2 nanoplus, Nanosystems and Technology GmbH, Germany
3 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
Bla bla
Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 1

2. 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
 Single mode emission is requested for gas sensing applications
Detection of NH3 demonstrated with single mode distributed feedback lasers in cooperation with:
Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 2

3. Active Region Designs Three quantum well design
bound-to-continuum design 3 2 1
Page et al., Appl. Phys. Lett. 78(22) (2001)
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
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
Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 3

4. 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
[Höfling et al, Electr. Lett. 40, 120 (2004)]
 Wavelength tuning should be possible by controling the cavity length
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

5. 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)
Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 5

6. Ultra-Short Microlasers
Microlasers with ridge lengths down to 30 µm (< 10 x wavelength) realized
15 µm
High-quality Bragg mirrors
Optically smooth surfaces
Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 6

7. Room Temperature Operation of Microlasers
Ridge length ~150 µm
Devices based on Bound-to-continuum active region design mW , 80 K
- 3.4 mW , 293 K (20 °C)
> 10 dB
Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 7

8. Wavelength Tuning with Cavity Length
Results based on bound-to-continuum design: 50 µm device
changes in cavity length: 0.2 µm
tuning over cm-1 (420 nm) centered around cm-1 (10.5 µm)
m=34
m=33
ng = 3.41
m=33
Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 8

9. Single Mode Emission Stability
Lasers with of ~50 µm ridge length based on bound-to-continuum design
single mode operation
up to 1.5 x Ith
mode jump due to blue shift by increased voltage
mode spacing about 30 cm-1 (340 nm)
Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 9

10. Tuning with Temperature/Current
Results based on three quantum well design
Ridge length ~100 µm
Wavelength tuning observed with:
Heat sink temperature cm-1/K
Drive current cm-1/A /
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11. Mode Switching with Temperature
Discontinuous tuning by temperature and according drive current variation
Spacing between modes 16 cm-1
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12. Two Segment Distributed Feedback Lasers1 2
front segment
rear segment
Two segment distributed feedbacklLaser with different grating periods
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13. Reversible Mode SwitchingIf
If= current injected in front segment
Ir= current injected in rear segment
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14. Evolution of DFB Modes with Temperature
Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 14

15. Quasi-Continuous Tuning
Tuning with temperature and segment drive current control
Single mode emission over > 9 cm-1
Side mode suppresion ratio (SMSR) up to 23 dB
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16. 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 20 °C, > 10 dB 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
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17. Modification of Facet Reflectivities
Why we use semiconductor-air Bragg mirrors to control the reflectivities at facets?
Advantages:
Semiconductor-air Bragg mirrors act as 1D-photonic crystals
Fabrication of reflection- and antireflection structures (ECL) possible
Full monolithically integrated fabrication process with a single etch step
Problems by using alternative possibilities:
Conventional dielectric coatings suffer by high absorption in the 10 µm spectral region
Metallic coatings are technologically complex (insulation?). Only reflection coatings are feasible.
High facet reflectivity enables:
Reduction of mirror losses
Fabrication of ultra-short microlasers
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18. Single Mode Spectra
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19. Mode Tuning Continuous tuning range:
21 nm + 50 nm = 71 nm spectral range
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20. Three Quantum Well Design
three quantum well GaAs/Al0.45Ga0.55As active region design [Page et al. Appl. Phys. Lett. 78(22) (2001)]
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 I II
III 3 2 1
 = 9.0 µm Lp = 45 nm, |z32| = 1.7 nm, 32 = 2.1 ps, 3 = 1.4 ps, 21= 0.3 ps
Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 20

21. Bound-To-Continuum Design
Bound-to-continuum GaAs/Al0.45Ga0.55As active region design [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 21

22. Ultra-Short Microlasers
Microlasers with ridge length down to 30 µm (< 10 x wavelength) realized
High-quality Bragg mirrors
Optically smooth surfaces
15 µm
Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 22

23. Single Mode Emission Results based on three quantum well design
Ridge length ~100 µm
longitudinal single mode emission due to large mode spacing and limited gain bandwidth
up to 5.6 mW output power at 200 K
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24. 1D Photonic Crystal Bragg Mirrors
Bragg-condition:
nsemdsem+dair= k /2
k: order, ni :refractive index
Ridge (active region)
DBR-mirrors
dsem
dair
10µm
air thickness (nm)
broad stopband near emission wavelength
Here: third order Bragg mirrors
Reflectivity limited by diffraction losses to  0.80
Diffraction losses strongly depend on semiconductor-air aspect ratio
reflectivity
Semicond. thickness (nm)
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25. Tuning with Cavity Length I
Results based on three quantum well design
changes in cavity length: 1 µm (98 µm to 105 µm)
tuning over cm-1 (180 nm) centered around cm-1 (9.18 µm)
m =82
m =83
m =83
ng = 3.797
Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 25