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(Al,In)GaN laser diodesin spectral, spatial, and time domain:near-field measurements and basic simulationsUlrich SchwarzDepartment of Experimental and Applied PhysicsRegensburg UniversityOptical gain spectraAbove threshold spectraFilaments / lateral dynamics1Ulrich T. Schwarz, RegensburgUniversity
375 nm to 470 nm set of gain spectra-1Mode gain (cm )100520 5002.37 kA/cm480Wavelength (nm)4604404203.08 kA/cm224003.05 kA/cm23802.70 kA/cm2-10Hakki-Paoli method-20Nichia LDs@ 470 nm, 440 nm,405 nm, and 375 nm.-30-40-502.3DCBART2.42.52.62.7 2.8 2.9 3.0Photon energy (eV)3.13.23.33.4 Impact of band-tail states on optical gain spectra Const. carrier density vs. const. Fermi level gain model Gain insensitive to inhomogeneous broadening at 1000 cm-1 levelK. Kojima et al., Optics Express 15, 7730 (2007),B. Witzigmann et al., IEEE J. Quantum Electronics 44, 144 (2008).2Ulrich T. Schwarz, RegensburgUniversity
Substrate ModesSimulationExperimentV. Laino, et al., IEEE J. Quantum Electronics 43, 16 (2007).3Ulrich T. Schwarz, RegensburgUniversity
Microscopic analysis of gain spectraBernd Witzigmann, ETH Zürich Quantum-kinetic Maxwell-Bloch equations Many-body effects (Coulomb, exciton,bandgap renormalization), 6 band kp Self-consistent treatment of piezoelectric fields Homogeneous broadening parameter freeSubstrate modesB. Witzigmann, et al. APL 88, 021104 (2006)IEEE Phot. Tech. Lett. 18, 1600 (2006)Nitride Semiconductor Devices, edited by J. Piprek, Wiley-VCH 2007IEEE J. Quantum Electronics 44, 144 (2008)4Ulrich T. Schwarz, RegensburgUniversity
Parameter setDirectly from gain spectra:Internal lossesLinear gainAntiguiding factorMode shift with IGain shift with IGain dispersionGain fluctuationsαint 25 cm-1a 9.8 x 10-13 m3s-1α 3.53.5 x 10-3 nm/mA2.3 x 10-2 nm/mA1.25 x 1042 m4s-15 x 10-4From microscopic simulationsof gain spectra:RadiativeNonratidativeIntraband relax.Dipole momentτs 4.5 nsτnr 9 nsτin 25 fsRcv2 2.8 x 10-57 C2m2Other:Cavity lengthMirror reflectivitiesThreshold currentConfinement factorGroup ref. indexEff. ref. indexL 600 µm17% & 98 %Ith 50 mA0.0173.592.53T. Meyer, et al., Optics Express 16, 6833 (2008).D. Scholz, et al., Optics Express 16, 6846 (2008).5Ulrich T. Schwarz, RegensburgUniversity
Spectra of LD on SiC and GaN substratesSame epitaxial structure!Epitaxy on SiC substrateEpitaxy on GaN substrateSiC: single longitudinal modeGaN: 15 longitudinal modes6Ulrich T. Schwarz, RegensburgUniversity
Spectra of LD on SiC and GaN substratesEpitaxy on SiC substrateEpitaxy on GaN substrateSiC: Mode hopping between single modesGaN: broad, asymmetric spectrum7Ulrich T. Schwarz, RegensburgUniversity
Multimode rate equation modelRate equation for electronsRate equations for photonsConsideration of several longitudinal modes pGain fluctuationsLinear gainGain saturation effects8
Gain fluctuations below thresholdGaN: low fluctuationsSiC: high fluctuationsGain fluctuations: 10 x higher for SiC compared to GaN9T. Meyer et al.,Optics Express 16, 6833 (2008)
Spectra of LD on SiC and GaN substratesSiC, experimentSiC, simulationGaN, experimentGaN, simulation10Ulrich T. Schwarz, RegensburgUniversity
Relaxation OscillationsRed: experimentBlack: simulationComparison with simplerate equation model.Same set of parametersas used for simulation of spectra.λtime11Ulrich T. Schwarz, RegensburgUniversity
Filaments in a 10 µm wide, blue LDI 0.9 IthI Ith12Ulrich T. Schwarz, RegensburgUniversityI 1.04 Ith
Laser diode dynamics measurement4-dimensional data hyper-cube: x – y – t – λCCDPhotomultiplierMonochromatorScanning near-field microscope (SNOM)Or: imaging with high NA aspheric lensePMT or Monochromator & PMT or CCDor: monochromator & streak camera13Ulrich T. Schwarz, RegensburgUniversity
HP 8114A or HP8131Apulse generatorDetection withHamamatsustreak cameraFiber tip14Ulrich T. Schwarz, RegensburgUniversity
Filaments inridge waveguide LDsLateral direction1.5 µm2.5 µmtime10 µm5 µm15Ulrich T. Schwarz, RegensburgUniversity
Switching of configuration@ critical temperaturesT(t) fromlong. modesD. Scholz et al.Optics Express 16, 6846 (2008)16Ulrich T. Schwarz, RegensburgUniversity
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Temperature as driving parameter for dynamicsThermal induced ref. index profile determines filament configuration.¾ width & number of filaments¾ narrower filaments at edges¾ evolution with time & temperature18Ulrich T. Schwarz, RegensburgUniversity
Coherent / incoheren filamentsNear-field to far-fieldTransition (experiment)¾ Near-field phase reconstruction¾ Modes / Filaments¾ Phase locking of filaments19Ulrich T. Schwarz, RegensburgUniversity
Filaments in a 10 µm wide, blue LDincoherentPhase-locked, coherentSingle longitudinal mode combMultiple longitudinal mode combs20Ulrich T. Schwarz, RegensburgUniversity
Spectral separation of filaments21Ulrich T. Schwarz, RegensburgUniversity
AcknowledgementHarald Braun, Evi Sturm, Markus Pindl (PhD students)Tobias Meyer, Dominik Scholz, Christoph Lauterbach,Stephan Rogowsky, Bernd Schmidtke (Diploma students)Werner Wegscheider (Regensburg University)Uwe Strauß, Stephan Ludgen, Georg Brüderl, (Osram OS)Bernd Witzigmann (ETH Zürich)Yoichi Kawakami, Kazunobu Kojima (Kyoto University)Funding: BMBF, DFG, JSPS22Ulrich T. Schwarz, RegensburgUniversity
Summary¾ Simple models sufficient to model (w/o too much parameter bending)¾ Advanced models necessary (microscopic, multi-dimensional)¾ Experiments as input / for calibration Optical gain spectra Modal behavior above threshold Relaxation oscillations, delay, decay, mode comp. Filaments: static, dynamics, near-field and far-fieldEnjoy NUSOD 2008!23Ulrich T. Schwarz, RegensburgUniversity
Similar picture for longitudinal modes:Switching of mode combs@ critical temperaturesH. Braun, et al.,J. Appl. Phys. 103, 073102 (2008).24Ulrich T. Schwarz, RegensburgUniversity
(Al,In)GaN laser diodes in spectral, spatial, and time domain: . Nichia LDs @ 470 nm, 440 nm, 405 nm, and 375 nm. Impact of band-tail states on optical gain spectra . Laser diode
