CREOL The College of Optics & Photonics Single-mode lasing in PT-symmetric microring resonators Matthias Heinrich 1, Hossein Hodaei 2, Mohammad-Ali Miri 2, Demetrios N. Christodoulides 2 & Mercedeh Khajavikhan 2 1 Institute of Applied Physics, Friedrich-Schiller-Universität Jena, Abbe Center of Photonics, Max-Wien-Platz 1, 07743 Jena, Germany 2 CREOL The College of Optics and Photonics, University of Central Florida, 4304 Scorpius St, Orlando FL-32816, USA Dr. Matthias Heinrich matthias.heinrich@uni-jena.de +49 (0) 3641 947990 www.iap.uni-jena.de
Motivation: Lasing in multi-moded resonators Mode competition leads to Decrease in spectral purity Lowered beam quality Intensity fluctuations Beam instabilities Thomas et al., Laser & Photon. Rev. 6, 709 (2012)
Motivation: Lasing in multi-moded resonators Underlying problem: Any mode can lase if its amplification exceeds the cumulative roundtrip losses Lasers 101: The winner takes it all Equilibrium conditions: Operation at threshold inversion Active resonators automatically evolve towards single-mode emission? Unfortunately, this rarely happens Inhomogeneous broadening, Spectral hole burning, Thermal effects Particularly in high-power applications or systems with large gain bandwidths
Motivation: Conventional approaches to mode control Introduction of additional losses Artificial narrowing of the gain bandwidth Dispersive components (Bragg gratings etc.) Filtering of undesirable components External feedback for stabilization Main drawbacks: Typically requires intricate assemblies and highly precise cavity design Additional losses decrease the system efficiency Impressive feats of engineering, but (from a fundamental physics point of view) not particularly elegant
Microring resonators High quality factors Excellent modal confinement Compact integration and small footprint Simple and scalable fabrication Variety of potential applications H. L. R. Lira, C. B. Poitras, M. Lipson, Opt. Express 19, 20115 (2011) O. Scheler, et. al., Biosensors & Bioelectronics, 36, 56 (2012)
Microring lasers Active medium spans entire resonator Large number of round trips à excellent utilization of available gain Nevertheless: Compactness = large gain required Typically: Large inhomogeneous broadening Intense mode competition despite decent spectral spacing I. Stamataki et al,. IEEE JQE 42, 1268 (2006) à Ideal showcase for the capabilities of our concept
Outline 1. PT-symmetric mode management 2. Experimental results 3. Transverse mode management 4. Efficiency & Tunability 5. Conclusion & Outlook
Microring lasers Gain envelope Maximum achievable gain contrast Radially single-moded Isolated microring cavity Net gain Threshold GAIN Free spectral range lasing passive Mul$mode lasing
Double-ring resonator Coupled symmetric microring cavities Net gain GAIN GAIN Frequency splitting Mul$mode lasing
Conventional mode control: Vernier effect Chose slightly different resonator lengths Coupled asymmetric microring cavities Modes coincide at single frequency within amplification bandwidth Requires exquisite control over both resonator dimensions Fleming et al., IEEE J. Quantum Electron. 17, 44 (1981)
Conventional mode control: Other methods Intra-cavity dispersive elements Bragg structures Distributed feedback / reflection A. Arbabi et al., Opt. Express 23, 5335 (2015) Spatial modulation of pump In particular for microdisk systems Optimization of pump profile to favor one specific mode S. F. Liew et al., Appl. Phys. Lett. 104, 231108 (2014)
PT-symmetric waveguide pair Odd mode Increased gain contrast Amplified mode LOSS GAIN LOSS GAIN Even mode Unbroken PT-symmetry Zero net gain for all modes Broken PT-symmetry Lossy mode One of the modes experiences gain El-Ganainy et al., Opt. Lett. 32, 2632 (2007)
PT-symmetric pair of ring resonators LOSS GAIN Time-periodic evolution of modal amplitudes: Eigenfrequency Coupling Gain Effective amplification coefficient: Complex eigenfrequencies of coupled system: in case of PT symmetry
PT-symmetric double-ring resonator PT-Symmetric microring cavities Net gain LOSS GAIN PT Symmetry enforces entirely real eigenvalue spectrum No lasing at all H. Hodaei at al., Science 346, 6212 (2014)
PT-symmetric double-ring resonator PT-Symmetric microring cavities Coupling before Net gain Enhanced gain contrast LOSS GAIN Spontaneous breaking of PT symmetry for the mode pair with highest gain All other mode pairs maintain unbroken PT symmetry! Single- mode lasing! H. Hodaei at al., Science 346, 6212 (2014)
PT-symmetric double-ring resonator Coupling Net gain LOSS GAIN Spontaneous breaking of PT symmetry introduces artificial lasing threshold Freely tunable via gain-coupling-balance: H. Hodaei at al., Science 346, 6212 (2014) gain-/ loss contrast between rings
Enhancement of single-mode gain Conventional single-mode amplification is limited by contrast between competing adjacent modes Eigenfrequencies of the coupled system: Symmetry breaking threshold condition: PT-enhanced single-mode gain: H. Hodaei at al., Science 346, 6212 (2014)
Enhancement of single-mode gain Conventional single-mode amplification is limited by contrast between competing adjacent modes PT-enhanced single mode gain: H. Hodaei at al., Science 346, 6212 (2014)
Coupling between microring resonators In case of negligible mode deformation, this can be derived from coupling between straight waveguides: Coupling coefficient in directional coupler Effective interaction length between rings effective mode index Coupling is freely tunable via the separation of the rings
Outline 1. PT-symmetric mode management 2. Experimental results 3. Transverse mode management 4. Efficiency & Tunability 5. Conclusion & Outlook
Sample fabrication Active medium: InGaAsP quantum wells Photoluminescence spectrum Substantial amplification bandwidth Inhomogeneously broadened Optical pumping (1064 nm) High optical gain Lithographic patterning H. Hodaei at al., Science 346, 6212 (2014)
Waveguide dimensions n=1.0 (air) n=1.45 (SiO 2 ) n=3.4 TM-polarizes modes reside primarily in the SiO 2 cladding Preferential overlap with active medium enforces TE-only laser operation Influence of ring curvature is negligible
Experimental setup Single ring Double ring PT configuration Double ring PT configuration
Characterization of the coupling coefficient Ring radius 10µm, waveguide width 500 nm Frequency splitting
Characterization of the coupling coefficient
Experimental results: Conventional microring laser (R=10µm) Conventional microring resonator Intensity distribution H. Hodaei at al., Science 346, 6212 (2014)
Experimental results: Double-ring resonator (R=10µm) Evenly pumped system GAIN GAIN Intensity distribution PT-symmetric configuration LOSS GAIN H. Hodaei at al., Science 346, 6212 (2014)
Experimental results: Double-ring resonator (R=5µm) Evenly pumped system GAIN GAIN Intensity distribution PT-symmetric configuration LOSS GAIN H. Hodaei at al., Science 346, 6212 (2014)
Experimental results: Double-ring resonator (R=5µm) Losses in passive ring suppress competing modes in active ring Single-mode fidelity of up to 30dB Lasing confined to the active resonator Intensity distribution PT-symmetric configuration LOSS GAIN H. Hodaei at al., Science 346, 6212 (2014)
Outline 1. PT-symmetric mode management 2. Experimental results 3. Transverse mode management 4. Efficiency & Tunability 5. Conclusion & Outlook
Transverse modes Can spontaneous PT symmetry breaking deal with transverse modes? Actually, that was the initial idea!
Multiple transverse modes n=1.0 (air) n=3.4 n=1.45 (SiO 2 ) Straight waveguide Microring resonator (R=6µm) Ring curvature acts as radial force to deform guided modes
Distinguishing between mode sets Distance-dependent coupling Microring resonator (R=6µm) Ring curvature acts as radial force to deform guided modes Separation d Transverse mode sets exhibit different coupling strengths for given separation
Distinguishing between mode sets Distance-dependent coupling Coupling dispersion Mode confinement Virtual lasing threshold
Transverse mode management TE 0 Broken PT LOSS GAIN Virtual lasing threshold Threshold of spontaneous PT symmetry breaking TE 1 LOSS GAIN Unbroken PT TE 2 LOSS GAIN
Transverse mode management TE 0 LOSS GAIN Threshold of spontaneous PT Virtual symmetry lasing threshold breaking Broken PT TE 1 LOSS GAIN TE 2 Unbroken PT LOSS GAIN à Selective suppression of entire transverse mode sets
Transverse mode management: Experimental results Isolated multimode microring PT-symmetric configuration TE 0 TE 1 TE 0 only GAIN 5.0 mw pump power LOSS GAIN 3.2 mw pump power single-mode TE 0
What I see is a multimode waveguide that is coupled to a loss mechanism [...] This approach is functionally identical to existing and known methods in semiconductor lasers and fiber lasers [...] There is no fundamental difference between the methods, only in the geometric implementation. or is there?
Outline 1. PT-symmetric mode management 2. Experimental results 3. Transverse mode management 4. Efficiency & Tunability 5. Conclusion & Outlook
Experimental results: Lasing efficiency Overall power Mode competition is avoided at virtually no cost to the overall lasing efficiency H. Hodaei at al., Science 346, 6212 (2014)
Experimental results: Lasing efficiency Power in desired mode Efficiency increase Mode competition is avoided at virtually no cost to the overall lasing efficiency Entire gain is supplied to desired mode H. Hodaei at al., Science 346, 6212 (2014)
Modal stabilization ß à Tunability Contradictory goals in conventional realizations Continuous shifting of modes versus mode hopping In resonance-based designs: Each part of the system has to be tuned in synchrony with others Temperature-dependent amplification profile of InGaAsP quantum wells
Tunability of a PT-symmetrically stabilized laser Measured intensity (arb. units) Single ring PT rings Wavelength (nm)
Outline 1. PT-symmetric mode management 2. Experimental results 3. Transverse mode management 4. Efficiency & Tunability 5. Conclusion & Outlook
Conclusion Enhanced mode selectivity in highly multimode systems Systematically increased single-mode gain Freely tunable artificial lasing threshold Lossless operation: Overall lasing efficiency is maintained Self-adapting operation & continuous hopping-less tunability Robust with respect to deviations from ideal PT symmetry H. Hodaei at al., Science 346, 6212 (2014)
Outlook: Manifold approaches to PT microrings Actively doped microtoroids B. Peng at al., Science 346, 323 (2014) Bragg gratings in the complex domain L. Feng at al., Science 346, 6212 (2014)
Outlook PT microring resonators as integrated, highly efficient single-mode laser sources Fundamental principle is independent of type of laser cavity Stable single-mode operation for any class of competing modes: longitudinal / transversal, radial / azimuthal Critical-point dynamics at PT threshold can amplify minute changes between competing modes Enhanced sensing and detection schemes
CREOL The College of Optics & Photonics Thank you for your attention! Funding: W911NF-14-1-0543 ECCS-1128520 FA9550-12-1-0148 FA9550-14-1-0037 LPDS 2012-01 LPDR 2014-03