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Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 1 Nanostructured Quantum Cascade Lasers for Longitudinal Single Mode.

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Presentation on theme: "Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 1 Nanostructured Quantum Cascade Lasers for Longitudinal Single Mode."— Presentation transcript:

1 Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 1 Nanostructured Quantum Cascade Lasers for Longitudinal Single Mode Control Motivation and structure of quantum cascade (QC) lasers Ultra-Short QC Microlaser Two segment distributed feedback (DFB) lasers J.P. Reithmaier 1,3, S. Höfling 1, J. Seufert 2, M. Fischer 2, J. Koeth 2, A. Forchel 1 1 Technische Physik, Universität Würzburg, Germany 2 nanoplus, Nanosystems and Technology GmbH, Germany 3 present address: Technische Physik, Universität Kassel, Germany

2 Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 2 Motivation Many important gases have their fundamental absorption in the mid-infrared spectral region (e.g NH 3, O 3, CO 2 ) 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 NH 3 demonstrated with single mode distributed feedback lasers in cooperation with:

3 Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 3 Active Region Designs Page et al., Appl. Phys. Lett. 78(22) (2001) Three quantum well design Resonant tunneling between lowest injector state and upper laser level 3 Fast depopulation of lower laser level 2 by interminiband scattering processes Pflügl et al., Appl. Phys. Lett. 83(23) (2003) bound-to-continuum design 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

4 Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 4 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 : Why Micro-Lasers

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

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

7 Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 7 Room Temperature Operation of Microlasers Ridge length ~150 µm Devices based on Bound-to-continuum active region design - 85 mW, 80 K mW, 293 K (20 °C) > 10 dB

8 Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 8 Wavelength Tuning with Cavity Length m=33 m=34 n g = 3.41 changes in cavity length: 0.2 µm tuning over 38 cm -1 (420 nm) centered around 955 cm -1 (10.5 µm) Results based on bound-to- continuum design: 50 µm device

9 Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 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 I th mode jump due to blue shift by increased voltage mode spacing about 30 cm -1 (340 nm)

10 Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 10 Wavelength tuning observed with: Heat sink temperature cm -1 /K Drive current -1.0 cm -1 /A Tuning with Temperature/Current Results based on three quantum well design Ridge length ~100 µm /

11 Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 11 Mode Switching with Temperature Discontinuous tuning by temperature and according drive current variation Spacing between modes 16 cm -1

12 Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 12 Two Segment Distributed Feedback Lasers Two segment distributed feedbacklLaser with different grating periods 11 front segment rear segment 22

13 Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 13 Reversible Mode Switching I f = current injected in front segment I r = current injected in rear segment IfIf

14 Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 14 Evolution of DFB Modes with Temperature

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

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

17 Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 17 Modification of Facet Reflectivities Why we use semiconductor-air Bragg mirrors to control the reflectivities at facets? Adv antages: 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

18 Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 18 Single Mode Spectra

19 Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 19 Mode Tuning Continuous tuning range: 21 nm + 50 nm = 71 nm spectral range

20 Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 20 Three Quantum Well Design three quantum well GaAs/Al 0.45 Ga 0.55 As 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 = 9.0 µm L p = 45 nm, |z 32 | = 1.7 nm,   32 = 2.1 ps,  3 = 1.4 ps,  21 = 0.3 ps I II III

21 Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 21 Bound-To-Continuum Design Bound-to-continuum GaAs/Al 0.45 Ga 0.55 As 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

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

23 Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 23 Single Mode Emission 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 Results based on three quantum well design

24 Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 24 1D Photonic Crystal Bragg Mirrors Ridge (active region) DBR-mirrors d sem d air 10µm Bragg-condition: n sem d sem +d air = k /2 k: order, n i : refractive index air thickness (nm) Semicond. thickness (nm) reflectivity 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

25 Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 25 n g = m =82 m =83 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 21 cm -1 (180 nm) centered around 1089 cm -1 (9.18 µm)


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