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

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

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

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

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

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

Room Temperature Operation of Microlasers Ridge length ~150 µm Devices based on Bound-to-continuum active region design - 85 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

Wavelength Tuning with Cavity Length Results based on bound-to-continuum design: 50 µm device changes in cavity length: 0.2 µm tuning over 38 cm-1 (420 nm) centered around 955 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

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

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 / Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 10

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

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

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

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

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

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

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 Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 17

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

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

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

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

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

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 Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 23

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) Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 24

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) m =82 m =83 m =83 ng = 3.797 Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 25