Novel Nano and Micro scale Lasers

Transcription

1 Novel Nano and Micro scale Lasers Mercedeh Khajavikhan CREOL, The College of Optics and Photonics, University of Central Florida

2 L A S E R small size (smaller than 1 μm 3 ) 1 W. Hayenga et al, metallic coaxial lasers, Advances in Physics X, to be appeared 2 W. Hayenga et al, Measurement of the second order coherence function for metallic nanolasers, submitted 3 M. Khajavikhan, et. al., "Thresholdless Nanoscale Coaxial Lasers", Nature, 482, (2012) 4 Jin H. Lee, et al "Electrically pumped sub wavelength metallo dielectric pedestal pillar lasers", Opt. Express, 19, (2011) mid size, mid power 1 H. Hodaei et al Design considerations for single mode microring lasers using parity timesymmetry, IEEE JSTQE, to appear (2016) 2 A. U. Hassan et al, Nonlinear reversal of PT symmetric phase transition in a system of coupled semiconductor micro ring resonators, Physical Review A., 92, (2015) 3 H. Hodaei et al, Parity time symmetric coupled microring lasers operating around an exceptional point, Opt. Lett., 40, (2015). R. El Ganainy et al, Supersymetric laser arrays, Physical Review A 92, (2015) H. Hodaei et al, Parity time symmetric microring lasers, Science 346, 975 (2014) R. El Ganainy et al, Exceptional points and lasing self termination in photonic molecules, Physical Review A 90, (2014) high power 1 Khajavikhan, M.; John, K.; Leger, J.R.; JQE IEEE, 46, , Khajavikhan, M.; Leger, J.R.; " IEEE JSTQE, 15, , M. Khajavikhan, A. Hoyer Leitzel, and J. R. Leger, Opt. Lett. 33, (2008)

3 L A S E R small size (smaller than 1 μm 3 ) 1 W. Hayenga et al, metallic coaxial lasers, Advances in Physics X, to be appeared 2 W. Hayenga et al, Measurement of the second order coherence function for metallic nanolasers, submitted 3 M. Khajavikhan, et. al., "Thresholdless Nanoscale Coaxial Lasers", Nature, 482, (2012) 4 Jin H. Lee, et al "Electrically pumped sub wavelength metallo dielectric pedestal pillar lasers", Opt. Express, 19, (2011)

4 History of Nanolasers Non metallic cavities Metallic cavities Photonic mode Plasmonic mode Microdisk (1992) Photonic Crystal (1999) V=0.46λ 3 V=0.46λ 3 smallest attainable mode volume: V=2(λ/2n) 3 nano disk (2007) V=0.46λ 3 metallo dielectric (2010) nanopatch (2010) spaser (2009) nano pan (2010) Low loss Narrow linewidth Slightly sub wavelength lasers Low loss Deeply subwavelength lasers High loss 1 McCall, S. L., Appl. Phys. Lett. 60, (1992) 2 Painter, O. et al., Science, 284 (5421), (1999) 1 M.T. Hill et al, Nature photonics, 1, , Mizrahi, A. et al., Opt. Lett. vol. 33, no. 11, pp , (2008), 3- Nezhad, M. P. et al., Nat. Photon. 4, , (2010) 4 K. Yu, A. Lakhani, M. C. Wu, Opt. Express 18, (2010) 1 Noginov, M. A. et al., Nature 460, (2009) 2 Kwon S. H. et al., Nano Lett. 10 (9), (2010)

5 Nanoscale Coaxial Cavity Microwave Coaxial Waveguides Plugs generate <100% reflection

6 Fabrication Steps Results Before metal deposition Before metal deposition Cleaned Wafer Spin HSQ resist Back-side < 50 nm Patterning HSQ with e-beam Before metal deposition Dry Etching Ag deposition Chemistry: H 2 :CH 4 :Ar Silver e-beam evaporation InP Wet Etch Mounting on glass InP InGaAsP QWs HSQ Silver Microscope slide Air

7 Coaxial laser at room-temperature (R core = 175 nm, Δ= 75 nm) top view R core = 175 nm h 1 = 10 nm Degenerate Δ= 75 nm TEM like h 2 = Mode HE 210 nm 11 Modes h 3 = 30 nm side view wavelength Q=36 Γ=67.8% V mode =0.06(λ/n) 3 Q=53 Γ=68% V mode =0.076(λ/n) 3 Spectral Window of Gain 1241nm 1401 nm 1522 nm 1665nm Three modes coincide with the spectral window of the gain

8 Coaxial Laser at Room-temperature (R core = 175 nm, Δ= 75 nm) increasing pump power PL ASE Lasing

9 Coaxial Laser (R core = 175 nm, Δ= 75 nm) Beam Profile non-lasing Examining Polarization Lasing Far field radiation pattern of the TEM mode beam cross section

10 nano pan laser How did we get here? nano coax laser Volume: μm 3 Cavity Q factor: 4000 Operating temperature: 8K Operating wavelength: 1402 nm Threshold pump power: 942 μw Volume: 0.05 μm 3 Cavity Q factor: 53 Operating temperature: 300K Operating wavelength: 1492 nm Threshold pump power: 10 μw Nanoscale coaxial lasers with smaller volume, higher loss, and at higher temperature have lower threshold

11 Definition of Laser Threshold stimulated lowest excitation level at which spontaneous emission dominates laser s output Pump populate the excited states n ex =αpδt Spontaneous emission due to limited upper level lifetime (τ sp ) n sp n ex exp( Δt/τ sp ) Photon lifetime (τ ph =Q/ω ) n ph n sp exp( Δt/τ ph ) Increasing pump generates heat n ph αpδte Δt/τ sp e Δt/τ ph Loss in metallic plasmonic lasers is high

12 Modification of Spontaneous Emission Rate Purcell Factor To increase or suppress spontaneous emission, the strength of the zero field at the emitter must be modified. Purcell factor gives the ratio of spontaneous emission rate in the cavity to the spontaneous emission rate of bulk F P spbulk spcavity 3 ( 4 n 3 ) 2 Q V mode a a annihilation creation. E n E n E 5 2 v 2 E 3 1 v 2 1 E 0 v ( n ) v 1 2 ( n ) v 2 1 ( n ) v 2 E n 1

13 Spontaneous Emission Coupling Factor to the Lasing Mode (β) continuous spectrum of radiation modes other cavity modes laser mode sp into laser mode β= sp in other cavity modes+ sp in radiation modes+ sp in laser mode

14 Spontaneous Emission Coupling Factor (β) laser mode sp into laser mode β= sp in other cavity modes+ sp in radiation modes+ sp in laser mode 1 The laser with β=1 is called thresholdless laser to increase β: sparse set of modes > cavity design suppress coupling to continuum of radiation modes > cavity design

15 Thresholdless Thresholdless Coaxial Coaxial Laser Laser at T = 4.5 K (R core = 100 nm, Δ= 100 nm) top view side view wavelength R core = 100 nm TEM like Modeh 1 = 10 nm Δ= 100 nm h 2 = 210 nm top view h 3 = 30 nm side view Q=264 Γ=69.7% V mode =0.038 (λ/n) 3 Spectral Window of Gain 1230 nm 1427nm 1610nm only one mode coincide with the spectral window of the gain β β=1 1

16 pump power Lasing Coaxial Laser at T= 4.5 K (R core = 100 nm, Δ= 100 nm) Pump Power: 0.5 nw-100 nw

17 Coaxial Laser at T= 4.5 K (R core = 100 nm, Δ= 100 nm) Lasing pump power Pump Power: 100 nw-2 μw

18 Coaxial Laser at T= 4.5 K (R core = 100 nm, Δ= 100 nm) Lasing pump power Pump Power: 2μW-200 μw

19 Coaxial Laser at T= 4.5 K (R core = 100 nm, Δ= 100 nm) ds dt i ( g L d ) S e L d i i ( n, ) ( i) i i ( n, ) ( i) Qi dn N N P g L d S e L d dt p( n, ) ( (, ) ( ) ) p i i n i i i i ( n, ) ( i) r nr i e L d 0 ( n, ) ( 0 ) e L d 1 i ( n, ) ( i ) r S i : Number of Photons in mode i Γ i : Confinement factor of mode i g (n,ω) : material gain L (ω- ωi) : Lorentzian distribution of the mode i Q i : Quality factor of mode i ω i : Center frequency of mode i e (n,ω) : Modified spontaneous emission function N : Number of carriers α p(n,ωp) : Pump factor β= R1 core = n 100 eˆ nm, Δ= 100 nm 2 2 ( n, ) d F c Single mode cavity 1 r As V Cn 1 2 nr g g β=0.2 R core = 175 nm, Δ= 75 nm Multimode cavity

20 Where is the lasing threshold? One clear cut way to tell is through the second order coherence function. anti bunched Poissonian t bunched 0 1 for chaotic light 0 1 for coherent light 0 1 for quantum light 20

21 Cavity Design and EM Simulations: Metallic coaxial nanolaser structure: A ring shaped gain medium (InGaAsP Quantum Wells) is sandwiched between an inner and outer silver cladding. ( : 50, Δ: 200, : 20 : 210, : 30 ) SEM image of coaxial laser FEM simulation of modes in this particular structure. One of the modes at 1303 nm lases due to its larger quality and confinement factors 21

22 Laser Characterization Spectral Evolution: As predicted in the simulation, the mode at 1373 nm is first to emit due to its higher overall Q. The mode at 1303 nm nm then emerges and dominates due to its better confinement with the gain region. Light Light Curve: The logarithmic and linear L L curves for the mode at 1303 nm. From the log log graph, the threshold appears to be ~80 μw. Linewidth vs. Pump Power: The linewidth scales by until threshold is reached, where it then slightly increases with additional pumping. 22

23 Second order coherence unambiguously determines the nature of light 0 1(chaotic light), 0 1 (coherent light), and 0 1(quantum light) A micro photoluminescence with an incorporated modified Hanbury Brown Twiss setup. Lightisfilteredto~0.5 pm by means of a diffraction grating and a Fabry Perot filter. 23

24 Calibration of the setup: An ASE source with a 50 nm bandwidth was used to establish that the filtering and the Hanbury Brown Twiss setup is functioning correctly. A 1.86 was measured, confirming the setup. Agilent 81600B Tunable Laser Source ASE source (Amonics) 24

25 Second order coherence function measurement for coaxial nanolaser 215 μw Pump Power: At very high pump powers μw Pump Power: Near threshold, The second order coherence decreases gradually as the pump power increases. 66 μw Pump Power: Lowest pump power measured. Slightly below threshold, measured 25

26 Metallic nano disk Laser :20, : 210, :30 Spectrum (log scale) of nanodisk laser (r=250 nm) Above threshold second order coherence function Normalized spectral emission from nanodisk lasers, ranging in radius from 150 nm to 900 nm. 26

27 Future direction: Our group in collaboration with Professor Likamwa s group is currently working to demonstrate electrically pumped coaxial nanolasers. 27

28 L A S E R mid size, mid power 1 H. Hodaei et al Design considerations for single mode microring lasers using parity timesymmetry, IEEE JSTQE, to appear (2016) 2 A. U. Hassan et al, Nonlinear reversal of PT symmetric phase transition in a system of coupled semiconductor micro ring resonators, Physical Review A., 92, (2015) 3 H. Hodaei et al, Parity time symmetric coupled microring lasers operating around an exceptional point, Opt. Lett., 40, (2015). R. El Ganainy et al, Supersymetric laser arrays, Physical Review A 92, (2015) H. Hodaei et al, Parity time symmetric microring lasers, Science 346, 975 (2014) R. El Ganainy et al, Exceptional points and lasing self termination in photonic molecules, Physical Review A 90, (2014)

29 Single frequency semiconductor lasers DFB Laser DBR laser and VCSEL Photonic crystal lasers Multi section cavity Y branch lasers Microrings Vernier lasers and many more Frequency selectivity is achieved by using Intra Cavity Dispersive Elements

30 Micro ring lasers Stamataki, I. et al. JQE IEEE, 42 (2006) Drawback: Multi longitudinal mode operation within the broad gain bandwidth (over 300 nm for InGaAsP system)

31 Single mode PT symmetric microring laser H. Hodaei, M. A. Miri, M. Heinrich, D. N. Christodoulides, M. Khajavikhan, Parity time symmetric microring lasers, Science 346, 975 (2014)

32 PT symmetric micro ring laser Net gain (arbitrary unit) loss gain Eigenfrequencies of the supermodes for two coupled resonators Frequency ω ω 0 (arb. unit) Eigenfrequencies of the super modes in the PT symmetric case 32

33 Measurement setup diffuser CCD camera Pump laser 33

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36 (vertical axis: Log scale) Normalized Intensity Wavelength λ (nm) 36

37 Light Light Curves No observable power loss in the PT symmetric laser The PT symmetric laser directs almost four time power into a single mode

38 In conclusion: non Hermitian exceptional points provide a new paradigm in laser mode management. Robust, versatile, broadband and self adapting single mode operation in demonstrated in microring laser systems. 38

39 Dark State Laser Cale M. Gentry and Miloš A. Popović, "Dark state lasers," Opt. Lett. 39, (2014) H. Hodaei, W. Hayenga, M. Miri, A. Ulhassan, D. Christodoulides, and M. Khajavikhan "Dark state microring lasers: Using non Hermitian exceptional points for mode management in Postdeadline Paper CLEO (2015) 39

40 Dark State Lasers Two dissimilar rings and a central waveguide 40

41 Temporal Coupled Mode Analysis* ; Conservation of energy => Eigenvalues: Eigen modes: *One can arrive to the same results using exact continuous wave coupled mode analysis 41

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43 Standard Vernier technique: two dissimilar rings are directly coupled Dark state technique: two dissimilar rings coupled through a waveguide 43

44 Sample Preparation

45 Experimental Characterization

46 (vertical axis: Log scale) R 1 =10 μm R 2 =9 μm Lower ring Normalized Intensity upper ring Dark state Wavelength λ (nm) 46

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49 Clock Wise or Counter Clock Wise

50 Tunable Dark State Lasers More than 8 nm continuous wavelength tuning by simply changing the ambient temperature 50

51 Thank You! Our team Mercedeh Khajavikhan Demetrios Christodoulides Hossein Hodaei M. Ali Miri Absar U. Hassan Matthias Heinrich William Hayenga 51

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54 PL spectrum changes due to pump mW 0.3mW 0.5mW 0.7mW pump power is the average power. Pulsed 20 ns, Repetition Rate: 295 KHz (Peak power: &112 mwatt)

55 PL spectrum changes due to temperature 10 4 Output Power (a.u.) K 270K 250K 230K 210K 190K Wavelength (nm) 1600 at average pump power of 100 uw Pulsed 20 ns, Repetition Rate: 295 KHz (Peak power: 16 mwatt)

56 Exceptional Point example i da dz i g 2 a b 0 i db dz i g 2 b a 0 g /2 cosh 1 Z z i da dz icosh a b 0 i db dz icosh b a 0 1,2 sinh

57 Spectrum Evolution Single ring Evenly pumped double rings PT symmetric double rings

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60 Phase Locked Laser Array waveguide laser In best scenario, the highest order mode (outof phase) mode is the first to lase 60

61 Super Symmetric Waveguide Partners Elimination of fundamental mode Perfect global phase matching 61

62 Super Symmetric Laser Array gain Loss The lossy partner couples to all modes but the fundamental mode of the gain array 62

63 Conclusion Using properties of non Hermitian systems, we are solving some of the long standing problems in laser design High coherence on chip sources are demonstrated using PT symmetric structures, dark state configuration, and SUSY arrays 63

64 Thank You 64

65 Transverse modes in broad area micro ring resonators

66 PT symmetric laser structures A pair of multimode waveguides Loss Gain Propagation constant Gain waveguide Loss waveguide Coupled mode equations 66

67 i da dz i g 2 a b 0 i db dz i g 2 b a 0 Two level PT systems supermodes at phase transition 1 1 ie, 2 1 ie g /2 cosh 1 Z z a b 1 c1 ie e Z sinhθ c 2 1 ie e Z sinhθ i da dz icosh a b 0 i db dz icosh b a 0 From intial conditions c be determined 1 and c 2 can 1,2 sinh

68 PT symmetric laser structures Exact PT phase Neutral modes Broken PT symmetry Amplified/attenuated modes 68

69 0 sin 0 sin ) 2 / ( sin 1 sin 2 / a b i dz db i b a i dz da i z Z g g a b g i dz db i b a g i dz da i supermodes below phase transition

70 sinh 0 cosh 0 cosh 1 cosh 2 / ,2 1 a b i dz db i b a i dz da i z Z g a b g i dz db i b a g i dz da i be determined can and c conditions c From intial , sinh 2 sinh 1 θ Z θ Z e ie c e ie c b a ie ie Two level PT systems supermodes above phase transition

71 s R R=10 m, width=500 nm, height=200 nm d: minimum ring separation 71

72 Rings separation and coupling 72

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