(Fifth Lecture) Techno Forum on Micro-optics and Nano-optics Technologies Efficient light emission from LEDs, OLEDs, and nanolasers via surface-plasmon resonance 송석호, 한양대학교물리학과, http://optics.anyang.ac.kr/~shsong silver grating Key notes 1. How does the surface plamon resonance enhance the internal quantum efficiency of light source? 2. Understand the Fermi-Golden rule and Purcell enhancement factor in spontaneous emission 3. What are the practical difficulties in realizing SP-enhanced LEDs? 4. Summary of the five lectures
Remind! The next chip-scale technology λ limit e limit Three light-design regimes WAVE DESIGN ( d ~ λ ) Light extraction LED RAY DESIGN ( d > λ ) Internal QE PHOTON DESIGN ( d < λ )
Power conversion efficiency of III-Nitride LEDs Example: λ=530nm, I=350mA PCE ~ 12%
External efficiency of LEDs η η external extraction R nr R = ηextrac tion R+ Rnr :extraction efficiency :nonradiative-recombination rate R:spontaneous-emission rate η extraction 1 2 1 4( n f / n g ) = = 4% θ s, p 0 2 c [ 1 R( θ )] θ sin dθ 2 for GaN(2.5) - air(1.0) i(10)
Wave Design for efficient extraction of the guided light -. Geometric optics η external R = ηextraction R + Rnr -. Random scattering in surface textured structure APL 63, 2174 (1993)
Photon Design for increasing the emission rate η external R = ηextraction R + Rnr What determines spontaneous emission rate of radiating source? E i E f Energy of EM field ω ( n + 1/2) Number of photon Vacuum fluctuation ti (Stimulated emission) (Spontaneous emission) electron SE Rate : Fermi s Golden Rule R = 1 1 2 ( ) τω ( ) = 2 f p ε E i ρ ω 0 Photon DOS (density of states) Dipole moment Electric field strength of radiation source of half photon (vacuum fluctuation) Microoptics Lab Hanyang University emd Lab. 6
Photon Design for increasing the emission rate 1 1 R = = f p E i τω ( ) 2ε 0 2 ρ( ω) E, ρ increase η external R = ηextraction R + Rnr Ag p-gan Quantum Well n-gan Atoms in microcavity High Q Narrow Δν F p ~ 1 5 Low volume filling factor Photonic crystal cavity Moderate Q Wider Δν F p (Quantum wells) ~ 3 F p (Quantum dots) ~ 5 100 Off-resonant and complicated fabrication Surface plasmon coupling Low Q Narrow Δν F p ~ 5 100 lossy and off-resonant www.phys.unt.edu/research/ photonic/website/surf-plasmon-ohps-f.ppt Department of Physics, University of North Texas, Denton, Texas 76203
Photonic-crystal approach 1 1 R = = f p E i τ ( ω ) 2ε 0 2 ρ( ω) E, ρ increase η external η R = extraction R + R nr Baba LumiLed Limited by surface recombination Good scheme!!! 100 um device size achievable. Several layer of PC for extraction. Good internal quantum efficiency i Needed (>90%). Multiple pass limits device size (~10um). Small volume needed. Not so good for lighting. Surface recombination limited Surface recombination limited. Noda
Photonic-crystal assisted LEDs 1 1 R = = f p E i τω ( ) 2 ε 0 2 ρ( ω) Very small increase in E, ρ! Look like a result of wave design rather than photon design!
Surface-plasmon approach η = int R p R p + R nr η ' int = R p R p + R sp + R + R sp nr Surface Plasmons
The SP approach was started for organic LEDs ITO glass (anode) Organic molecules Cathode & Mirror SPP quenching (~40%) Conventional Structures: Strongly coupled to SPPs Main issue: SPP Radiation coupling Metallic mirror Metallic thin film SPP2 SPP1 SPP band gap ( Λ ~ π / ) k SPP Direct coupling ( Λ > π / k SPP ) SPP cross-coupling ( Λ = π /[ k k ]) SPP1 SPP2
Effect of SPP band gap on PL 11411 Angle resolved PL of dye molecule (DCM) 1 st and 2 nd order diffraction of SPPs Tracing 1 st order peaks shows SPP band gap.
Modification of Spontaneous Emission Rate of Eu 3+ Main emission of Eu 3+ (614nm) SPP quenching τ ( spacer thickness ) TRPL at 614nm
Self-driven dipole (CPS) modeling d p Metal interface 2 2 d d 2 e p + b0 p + ω0 p = E 2 dt dt m ω p = p e, E = E e i( ib/ 2) t i( ω ib/ 2) t 0 r 0 2 unknowns and 2 equations r 2 e b / b0 = 1 + Im{ E0 } mω p b 0 0 2 2 b bb0 e Δ ω 8ω 4ω 2mω p 0 0 0 Re{ E0 } 14
Dipole Decay Calculation Test : Metal Mirror Cavity 10 2 10-4 10 2 pated pow wer dissi 10 1 10 0 10-1 10-2 10-3 perpendicular dipole parallel dipole 10-4 0.0 0.5 1.0 1.5 2.0 kx / k1 J. A. E. Wasey and W. L. Barnes, J. Mod. Opt. 47, 725-741, 2000 15
CPS Model Calculation for Spontaneous Emission Rates of an OLED Emission Spectrum No guided mode TM 0 TM 0 +TE 0 TM 0 +TE 0 +TM 1 70nm 100nm 200nm 390nm 30 3.0 ra adiation rat te (R 0 ) 16 2.5 2.0 1.5 1.0 0.5 0.0 total emission rate air emission emission to substrate guided modes emission to active layer guided modes 0 50 100 150 200 250 300 350 400 active layer thickness (nm) dipole ( h = h + h ) cover (medium c) h c active material h (medium a) s a s c substrate (medium s)
Comparison with an experiment 100 100 iency (% %) 90 80 ratio (%) 90 80 70 60 50 PL Effic 70 60 100 200 300 400 500 Film Thickness (nm) power 40 30 20 10 P air +P sub +1.0P guided P air +P sub +0.4P guided P air +P sub +0.8P guided P air +P sub +0.2P guided P air +P sub +0.6P guided P air +P sub +0.0P guided 0 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 active layer thickness (μm) (measured) (calculated) 17
SPP Enhanced Spontaneous Emission of Eu 3+ Ion SE rate 90% SPP coupling 25 times SE rate Dipole-SPP coupling fraction Maximum internal efficiency
Role of Preferred Orientation of the Dipole Source Adv. Mater. 14 19 1393 Angle integrated EL
Enhanced PL by Coupled SPP
Cross-Coupled vs Coupled SPP (1) (2) (3) (4)
SPP Enhanced PL of InGaAs QW Most cited paper Un-processed (a) Half-processed (b) Fully-processed (c) 480nm period (2 nd order coupling) (d) 250nm period (1 st order coupling) (160nm gap)
1 st Result of SPP enhanced PL from InGaN QW Nature Materials, VOL 3, p.601-605, 605 2004 1 1 2 R = = f p E i ρ ( ω ) τ ( ω ) 2ε 0 E, ρ increase η external R = η extraction R + Rnr Nature Materials, VOL 3, p.601-605, 2004
1 st Result of SPP enhanced PL from InGaN QW Nature Materials, VOL 3, p.601-605, 2004 40x100nm 2 133nm wide, 400nm period grating (no enhancement for 200nm wide, 600nm period grating) 0.42 0.18 x2 x28 x14 006 0.06 Average internal quantum efficiency e cy estimation
TRPL of SPP enhanced InGaN QW emission
How does the surface-plasmon resonance contribute to emission rate? R = 1 1 2 ( ) τω ( ) = 2 f p i ρ ω ε E 0 Field enhancement near the source layer High DOS due to decrease in group velocity emd Lab. Microoptics Lab Hanyang University 26
R = 1 1 2 ( ) = f p i ρ ω τω ( ) 2 ε E 0 Field enhancement near the source layer High DOS due to decrease in group velocity Requirements for enhancing SE rate B -. slow group velocity -. tight confinement of mode -. low ohmic loss A -. large field enhancement fast group velocity, low loss slow group velocity, high loss A B Q.W. Q.W.
Purcell factor defining enhancement of the spontaneous emission F p R + R R = 1+ R R original additional additional original original For a cavity mode: F p Rcav 3 Q( λc / n) = = 2 R 4π V free 3 mode_ volume For a SP mode : F p R SP 1 λ k SP / k = 1+ = 1+ R 2 π L υ / c 0 SP 0 υ SP ( ( ωε ) dz ( z) dω E SP =, L = ω 2 dk E at dipole 2 We need a slow and confined mode!
Factors influencing Purcell Enhancement F p (ω) GaN ~ ζ Ag ~ z Single Quantum Well GaN Variation with Ag thickness Variation with GaN thickness
Purcell enhancement factor: A numerical factor (F-1) estimation cover Cover = 1.0 Cover = 1.5 Cover = 2.0 Need a very thin p-gan layer!!
Improvement I-L curve F p 2.68at10 K = 1.75at 300 K No improvement I-V curve the enhanced F p can be attributed to an increase in the spontaneous emission rate due to SP-QW coupling.
Why SP-LED hasn t been successful yet? Practical Barriers (especially for InGaN/GaN devices) Thin p-gan leads to abrupt occurrence of leakage current under a certain thickness SP propagation length in blue wavelength along the Ag/GaN interface is extremely short Nanopatterning becomes a huge burden at short wavelength Damageless p GaN patterning has been impossible Damageless p-gan patterning has been impossible SQW devices are prone to leakage current due to carrier overflow Silver is a nasty material with poor adhesion to GaN and tends to agglomerate at an elevated temperature
SP propagation length Nanopatterning of SPs [nm m] Propagat tion Length 1 PL SPs = 2 k 4000 3500 3000 2500 2000 1500 1000 500 k 3 2 ω ε mε d ε m = c ε m ε + d 2( ε m 2 ) Surface Plasmon on the Ag/GaN Interface 0 450 500 550 600 650 700 750 800 Wavelength of Photon [nm] Green LEDs might be possible. Freq quency (2π πc/μm) 25 2.5 2.0 1.5 1.0 0.5 Λ = λsp, 2λsp, 3λsp, 530nm λsp~140 nm 460nm λsp~70 nm SP-dispersion dspeso on Ag/GaN 0.0 0 2 4 6 8 10 12 14 In-plane Wavevector (2π /μm) 2 nd order gratings (Λ~280nm) might be readily fabricated by Holo litho at Green.
Schematic structure Photon Sapphire n-gan Exciton generation Radiation InGaN MQW p-gan Metal (Ag-based) e-h Surface plasmon excitation Silicon submount Λ D h
High output directionality by grating with non-even fill-factor 1 st order grating, fill factor=0.1 1 st order grating, fill factor=0.5 2 nd order grating, fill factor=0.1 2 nd order grating, fill factor=0.7
Extraction efficiency of a metal grating Data sampling at λ = 530 nm / w = 5 nm η int 1+ ηext γ sp = 1 + γ γ nr sp 1+ ext sp η γ FDTD ηint = 1+ γ sp η ext 1 1 η ext η = FDTD int (1 + γ ) 1 γ sp sp 0 180 0 10 γ nr η η γ int ext sp : nonradiative re-comb. rate : internal quantum eff. : extraction efficiency of metal grating : re-comb. rate to surface plasmon 60 100 Max ~ 80% (at 140 nm / 40 nm)
단일원기둥구조계산 Two-dimensional silver-grating (2 nd order) Λ = 250nm Grating depth = 50nm Gap to QW = 30 nm Normaliz zed LT / In nternal QE 1.2 Normalized LifeTime 2.2 1.1 Internal Quantum Efficiency 20 2.0 1.0 Upward Emitted Power 1.8 0.9 1.6 08 0.8 14 1.4 0.7 1.2 0.6 1.0 0.5 0.8 0.4 0.6 0.3 0.4 0.2 50 100 150 200 250 300 350 400 450 500 Upward em mitted pow wer (a.u.) Diameter (nm) 169 nm
Optimum gap distance between metal and QW Upward enhanc cement 2.5 2.0 1.5 1.0 0.5 λ = 530 nm d = 20 nm 00 0.0 0 5 10 15 20 25 30 Distance [nm] coupling to lossy surface wave coupling to surface plasmons 6nm is a theoretical limit given by self-driven dipole (CPS) modeling [W. L. Barens and P. T. Worthing, Optics Communications 162, 16 (1999)]
Grating on p-gan Substrate mount Aperture Little damage to p-gan Enlarged surface area for low contact resistance Rotation o stage Linear stage X Z Y θ Mirro r L-Shape mount
EL Measurement Power(a arb.) 0.0045 0.004 0.0035 0.003 0.0025 0.002 0.0015 Higher output power up to 70 % ref 250A_3 250B_2 250C_2 270A_4 270B_2 270C_3 290A_3 0.001 290B_2 0.0005 0 0 0.1 0.2 0.3 0.4 Current(A)
Sample images
An Optimistic Estimation for SP-enhanced LEDs At green (530 nm) with a 1 st order grating MQW 5nm 10 nm grating de epth FDTD calculation l 20 nm 2.3 times more Photons generated 60 nm 140 nm 100 nm grating period 180 nm Good directionality Surface plasmon ed Ph hotons escap 1.0 0.8 0.6 0.4 02 0.2 0.0 82 % 34.1% within 20 o after escape 1/(2n 2 ) = 8 % 400 500 600 700 800 Wavelength (nm) (Bare-chip LED with 8 % extraction) (82 % / 8 %) x 2.3 ~ 24 times Brighter ( Optimized LED with 50 % extraction) (82 % / 50 %) x 2.3 ~ 4 times Brighter
Nanocavity lasers
Nanocavity lasers
Key notes Final comments 1. How does the surface plamon resonance enhance the internal quantum efficiency of light source? 2. Understand the Fermi-Golden rule and Purcell enhancement factor in spontaneous emission 3. What are the practical difficulties in realizing SP-enhanced LEDs? 4. Summary of the five lectures External Efficiencies Conventional LED SP LED η ' = R p η = Ep R nr + R p ER + E R p p SP SP R + R + R nr p SP An Optimistic Estimation for SP-enhanced LEDs At green (530 nm) with a 1 st order grating MQW 5 nm 10 nm grating depth FDTD calculation 20 nm 2.3 times more Photons generation 60 nm 140 nm 100 nm grating period 180 nm
Final comments Summary of the five lectures (06/23) Introduction: Micro- and nano-optics based on diffraction effect for next generation technologies (06/30) Guided-mode resonance (GMR) effect for filtering devices in LCD display panels (07/07) Surface-plasmons: A basic (07/14) Surface-plasmon waveguides for biosensor applications (07/21) Efficient light emission from LED, OLED, and nanolasers by surface-plasmon resonance R 0 T 0 GMR grating Micros D cor e metal strip SPP mode cladding core metal slab cladding
Final comments Summary of the five lectures Now, let s get back to Macros with Nanos and Micros.