Forum for Electromagnetic Research Methods and Application Technologies (FERMAT) Metamaterials, metasurfaces and plasmonic devices to efficiently control the electromagnetic waves Christos Argyropoulos Metamaterials and Integrated Nanophotonics Lab, Department of Electrical Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska, 68588-0511, USA Abstract :- Metamaterials, metasurfaces and plasmonic devices can be used to manipulate, control and tailor the electromagnetic radiation in unprecedented ways. They are artificially constructed structures able to exhibit novel functionalities not available in materials provided by nature. Their properties can lead to the design of exciting structures, such as invisible cloaks and ultrathin energy concentrators. However, these novel devices often suffer from intrinsic physical limitations, such as extremely narrowband operation, high losses, weak optical nonlinear response, low tunability and poor reconfigurable operation. In my talk, I will propose several ways to overcome these inherent limitations, based on the introduction of tunable, active and nonlinear media. The large field enhancement in the vicinity of individual and collections of plasmonic nanoparticles and inside metamaterial gratings ensures a significant boosting of nonlinear optical effects, which can lead to ultrathin optical frequency mixers, tunable optical sensors and filters, all-optical switches and nano-memories. I will also present a new ultrathin nonlinear metasurface reflector, or meta-mirror, with giant nonlinear response, a million times stronger compared to traditional nonlinear materials. Finally, I will discuss recent theoretical and experimental advances towards demonstrating large enhancement of spontaneous emission rates of molecules and quantum dots embedded in plasmonic patch nanoantennas. The experimental demonstration of Purcell factors ~1,000 will be reported, while maintaining high quantum efficiency and directional emission. Keywords: Metamaterials, metasurfaces, plasmonics, nanoantennas, non-linear optics, second harmonic generation, optical bistability, Purcell enhancement
References [1] C. Argyropoulos, P.-Y. Chen, G. D Aguanno, N. Engheta, and A. Alù, Boosting Optical Nonlinearities in ε-near-zero Plasmonic Channels, Phys. Rev. B 85, 045129, 2012. [2] C. Argyropoulos, P.-Y. Chen, F. Monticone, G. D Aguanno, and A. Alù, Nonlinear plasmonic cloaks to realize giant all-optical scattering switching, Phys. Rev. Lett. 108, 263905, 2012. [3] C. Argyropoulos, C. Ciracì, and D. R. Smith, Enhanced optical bistability with film-coupled plasmonic nanocubes, Appl. Phys. Lett. 104, 063108, 2014. [4] C. Argyropoulos, G. D Aguanno, and A. Alù, Giant second harmonic generation efficiency and ideal phase matching with a double ε-near-zero cross-slit metamaterial, Phys. Rev. B 89, 235401, 2014. [5] C. Argyropoulos, P.-Y. Chen, G. D Aguanno, and A. Alù, Temporal soliton excitation in an ε-nearzero plasmonic metamaterial, Opt. Lett. 39, 5566-5569, 2014. [6] J. Lee, M. Tymchenko, C. Argyropoulos, P.-Y. Chen, F. Lu, F. Demmerle, G. Boehm, M.-C. Amann, A. Alù, and M. A. Belkin, Giant nonlinear response from plasmonic metasurfaces coupled to intersubband polaritons, Nature 511, 65 69, 2014. [7] G. M. Akselrod, C. Argyropoulos, T. B. Hoang, C. Ciracì, C. Fang, J. Huang, D. R. Smith, M. H. Mikkelsen, Probing the mechanisms of large Purcell enhancement in plasmonic nanoantennas, Nat. Photonics 8, 835 840, 2014.
Weak linear absorption Non-directional spontaneous emission Slow and small emission Weak nonlinear interactions
P. A. Franken, et. al., Phys. Rev. Lett. 7, 118 (1961) Nonlinear optical effects: Weak mechanisms. Poor field enhancement inside elongated and bulky Fabry-Perot resonators.
High-Q microcavities Field enhancement due to confined mode, resonant with absorber/emitter Quantum dot Hennessy et al., Nature 445, 896 (2007) Englund et al., PRL 95, 013904 (2005) Some limitations: Chang et al., PRL 96, 117401 (2006). Modest radiative rate enhancements (~10) Requires precise fab to match emitter resonance Incompatible with broadband, room temp emitters
Propagating surface plasmons Coherent electron oscillations at metaldielectric interface Plasmon wavelength is small l p l 0 Localized surface plasmons Large field enhancement E / E 0 Lu et al., Annu. Rev. Phys. Chem. 2009, 60, 167 192. Enhanced optical effects: Absorption Spontaneous emission Nonlinear generation Raman scattering
Receiver Transmitter Schuck et al., Phys. Rev. Lett. 94, 017402 (2005) Alù, Engheta, Phys. Rev. Lett. 104, 213902 (2010) Jamshidi, Nature Photonics 2, 85 (2008) N Yu et al, Appl. Phys. Lett., 91; 173113 (2007).
Alù, Engheta, Nature Photonics 2, 307 (2008) N. Engheta, A. Salandrino, A. Alù, Phys. Rev. Lett., 95, 095504 (2005) N. Engheta, Science, 1133268, 1698 (2007) The recent concept of metactronics extends the well-established microwave engineering ideas to optical frequencies. Metal and dielectric particles at optical frequencies act, respectively, as lumped nanoinductors and nanocapacitors. RF RF Antenna Optical Nanoantenna C Optics L Matching, loading, radiation and frequency tuning by nanocircuit lumped elements.
Robust, efficient and flexible tunability and tailoring of the scattering response. Relative enhanced nonlinear operation. Moderate optical bistability and switching effects. Novel all-optical nanodevices (memory, all-optical switching, optical transphasor, logical cell). Relative high input intensities required I in = 112 MW/cm 2 I in = 470 MW/cm 2 P.-Y. Chen, C. Argyropoulos, and A. Alù, Enhanced Nonlinearities Using Plasmonic Nanoantennas, Nanophotonics, vol. 1, no. 3-4, pp. 221-233, 2012
S / S S / S x x y y E or P.-Y. Chen, A. Alù, Phys. Rev. B, 82, 235405 (2010)
e The fascinating properties of ε-near-zero materials Source Tailoring Plasmonic Cloaking Directivity Enhancement A. Alù, et al., Phys. Rev. B 75, 155410 (2007) A. Alù, N. Engheta, Phys. Rev. E 72, 016623 (2005) A. Alù, et al., IEEE TAP 54, 1632 (2006)
Ag Glass e Array of Plasmonic Channels e eff 2 2 pp ech 2 k0 w 2 e / Re e pp ch Ag 2 E H l 500 nm, a b 400 nm, w 200 nm, t 40 nm, e ch 2.2 C. Argyropoulos et al, Phys. Rev. B, 85, 045129, 2012
E ch /E 0 e ENZ FP z x 0 E 1 z x E ch ba tw E in z-axis (nm) at ENZ from power conservation 0 E 1 C. Argyropoulos et al, Phys. Rev. B, 85, 045129, 2012
FP E ENZ E l 500 nm, a b 400 nm, w 200 nm, t 40 nm, e 2.2 ch All-optical memory I in = 2400 MW/cm 2 All-optical switch Still high input intensities required C. Argyropoulos et al, Phys. Rev. B, 85, 045129, 2012 C. Argyropoulos et al, Phys. Rev. Lett., 108, 263905, 2012
c (2) -NOM I 258.3 W / cm in 2 c (2) l 20 pm / V Perfectly phase matched dispersion n2 n 1.5 c a 440 m t 44 m l 380m b 440 m w 334m (2) d 53m h165m Orthogonal Polarizations both ENZ SH 20 pm / V t PEC 2ω ω c (2) -NOM a Infinite Coherence Length n2 n 0 I 258.3 W / cm in 2 FF C. Argyropoulos et al, Phys. Rev. B 89, 235401, 2014 C. Argyropoulos et al, Opt. Lett. 39, 5566, 2014.
Enhanced second-order nonlinearities Enhanced third-order nonlinearities ω 2ω Absorption enhancement of molecules Emission rate enhancement (Purcell effect) of molecules
ω 2ω Plasmonic metasurfaces + Nonlinear multi-quantum well (Metamaterial reflector)+(quantum-engineered nonlinear medium) Efficient second harmonic generation at IR frequencies from ultra-thin metasurface J. Lee, M. Tymchenko, C. Argyropoulos et al., Nature 511, 65 69, 2014
ω Quantum-engineering electronic intersubband transitions Al 0.48 In 0.52 As(barrier)/In 0.53 Ga 0.47 As(well) Total 400nm thick, 19 repetitions 2ω Nonlinear susceptibility 3 orders of magnitude larger than natural optical materials ω e z z z 3 (2) 12 23 31 zzz ( 2 ) Ne 2 e0 31 31 21 21 c 2 i i Strongly couples only to the normal electric field component to the semiconductor layers J. Lee, M. Tymchenko, C. Argyropoulos et al., Nature 511, 65 69, 2014
400nm Units [nm] Top structure design Resonant at ω Resonant at 2ω Good mode overlap at ω and 2ω Lack inversion symmetry
Normalized E z field component induced in the MQW q = 0 o q = 90 o q = 0 o 3 2 2 a b c d 1.5 1 1 q = 90 o 2 1 Eq = 90 o ) y x Eq = 0 o ) 0 0 0 0-1.5-1 -1-1 e FF FF q = 0 o FF -3-2 -2-2 10 q = 90 o 7 q = 0 o q = 90 3 o 3 f g h 5 3.5 1.5 2 Au Au Au Au 0 0 0 0 MQW MQW MQW MQW -5 FF -3.5 SH -1.5 SH Pt Pt Pt Pt -2 Au Au Au Au InP substrate InP substrate InP substrate InP substrate -10-7 -3-3 SH SH ω 2ω Optimized design for strong E z overlap at FF and SH J. Lee, M. Tymchenko, C. Argyropoulos et al., Nature 511, 65 69, 2014
Highly Nonlinear Metasurface ω 2ω Effective second-order nonlinear surface susceptibility Calculated based on the Lorentz reciprocity theorem c c (2) eff (2) ijk zzz V,,,, 2,, E x y z E x y z E x y z dv z(k) z(j) z( i) E E E V 2 k ( inc) j( inc) i( inc), i, j,k x or y Absorption spectrum 2 Input polarizations: x-polarization y-polarization cross-pol yxx >x 2>y I 2 2 (2) eff ˆ ˆ ˆ 2 e 3 2c ee 8e 0c 2 2 2 I L
SH power [W] FF intensity squared [kw 2 /cm 4 ] 0 50 100 150 200 250 0.16 yyy xxx 0.06 yxx xyy 0.12 23 W/W 2 0.08 0.04 57 W/W 2 11 W/W 2 24 W/W 2 5 W/W 2 2 W/W 2 0.04 0.02 0.00 0.00 0.0 2.0x10-3 4.0x10-3 6.0x10-3 FF power squared [W 2 ] SH inensity [W/cm 2 ] SH power [nw] 120 100 80 60 40 20 yyy 0 0.00 1160 1200 1240 1280 1320 FF wavenumber [cm -1 ] SHG power for four polarization combinations was measured Highest SHG efficiency was achieved for yyy polarization Maximum SHG efficiency at input pump wavenumber 1240cm -1 0.05 0.04 0.03 0.02 0.01 SH intensity [W/cm 2 ]
Plasmonic metasurfaces + Nonlinear multi-quantum well (Metamaterial absorber) (Quantum-engineered nonlinear medium) The highest nonlinear response from ultrathin nonlinear metasurface up to date Efficient second harmonic generation 400nm from ultra-thin metasurfaces Conversion efficiency [%] FF intensity [kw/cm 2 ] 0 3 6 9 12 15 4x10-4 3x10-4 2x10-4 1x10-4 yyy 0 0 10 20 30 40 50 60 70 FF power [mw] J. Lee, M. Tymchenko, C. Argyropoulos et al., Nature 511, 65 69, 2014
Enhanced second-order nonlinearities Enhanced third-order nonlinearities ω 2ω Absorption enhancement of molecules Emission rate enhancement (Purcell effect) of molecules
Largest field enhancements occur in <10 nm gaps between metals Acimovic, et al., ACS Nano 3, 1232 (2009). Kim et al., Nature 453, 757 (2008) Kinkhabwala et al., Nat. Phot. 3, 654 (2009) Challenges of plasmonic gaps: - small gaps (< 10 nm) difficult to define with lithography - difficult to make on large scale Need system with gap dimensions controlled on the nanometer scale
Vertically defined nanoscale gap Allows for (sub)nanometer control of metal gaps Challenge: Top-down fabrication will have rough features What is the fabrication strategy?
Reaction precursors Good size uniformity NaSH PVP HCL AgC 2 F 3 O 2 Nanocubes form by reduction of AgC 2 F 3 O 2 2.5 hour reaction, 150C Re-suspend in water Crystalline growth results in atomically flat facets Reproducible and robust fab process Yugang Sun, Younan Xia, Science 298, 2176 (2002) Zhang, Q. et. al. Chemistry 16, 10234 (2010) 50 nm
Vertical gap allows (sub)nanometer control using: Layer-by-layer polymer deposition Atomic layer deposition Self-assembled monolayers Moreau et al., Nature (2012) Optically active materials can be readily integrated: Nonlinear Materials Molecules Quantum dots Chalcogenides 2D materials, other
Sides of nanocube define Fabry-Pérot cavity Fundamental resonance Strong field enhancement Field enhancement over large area Tunable throughout visible and near-infrared Strong radiative coupling Lassiter et al., Nano Letters (2013)
Other resonances of the nanopatch Independent of polarization Q ~25 Independent of angle of incidence Relatively high quality factor Lassiter et al., Nano Letters (2013)
Tuning the gap thickness: Dark field scattering microscopy of single cubes on metal film Tuning the nanocube size: Moreau et al., Nature (2012) Theory Lassiter et al., Nano Letters (2013). Gap thickness Expt. System allows for nanometer precision control of resonance
Radiation pattern of nanopatch antenna Theory Experiment Radiation pattern of dipole on glass Single lobe directional emission normal to surface Radiation pattern measured using Fourier space imaging 84% collection efficiency with NA=0.9 objective Akselrod, Argyropoulos et al., Nat. Phot. 8, 835 840, 2014
l = 80nm g = 2nm h = 50nm E / E 0 0 100 200 Ag e e c L (3) E 2 Ag e L = 2.2 c (3) =0 c (3) = 4.4x10-18 m 2 /V 2 I in = 1 MW/cm 2 C. Argyropoulos et al, Applied Physics Letters, 104, 063108, 2014
I in (MW/cm 2 ) l = 80nm g = 2nm h = 50nm e e c L (3) E 2 e L = 2.2 c (3) = 4.4x10-18 m 2 /V 2 fsec detectors needed! t (fsec) Scattered Power (mw) Scattering jump C. Argyropoulos et al, Applied Physics Letters, 104, 063108, 2014
Enhanced second-order nonlinearities Enhanced third-order nonlinearities ω 2ω Absorption enhancement of molecules Emission rate enhancement (Purcell effect) of molecules
What happens when emitters are coupled to the nanogap? (1) Local field enhancement leads to excitation rate enhancement: g ex g ex E ex µ E ex 0 0 2 y Simulations are done in COMSOL based on finite element x y Maximum excitation enhancement of ~10,000 when on resonance with fundamental mode Rose et al., Nano Lett., 14, 4797 (2014)
(2) Spontaneous emission rate enhancement (Purcell): g sp (r, ˆn) µ ˆn Im éë G(r,r) ù û ˆn sp r nr QY r sp Enhancement of spontaneous emission rate in 8 nm gap Quantum yield determined by plasmonic properties, not internal decay rate Effectively reduced quenching from metal losses using engineered structure
Emission Ru dye l Ag Au g z glass x sp Ag Ru dye Au glass 2 2 np nˆim G( r, r) nˆ 2 e 0c Im[ Ez ] Im G( rr, ) 2 p p 1 C / m np 0 Dipole moment p 2 y x Point-dipole emitters are placed inside the nanogap and excite the nanocube. They radiate at nanoantenna resonance λ = 650 nm
quasi-two level system Straight lines: photonic transitions (Good) Wavy lines: Non-radiative relaxation (Bad) W 0 r 0 r 4 p 0 2 12e c 3 p 3 2 3 (3 e 0c ) Total power radiated by a dipole in free space Radiation rate of a dipole in free space Integration of the absorbed power over the entire domain Integration of the power flowing out of the simulation domain nr r 1 2 W 1 2 W 0 r 0 r 0 r 0 r * Re( J ) E dv * Re( ) sp r nr E H ds Quantum Yield QY(r) 1 (r) / (r) nr sp r (r) (r) Akselrod, Argyropoulos et al., Nat. Phot. 8, 835 840, 2014 sp 0 int
What is the fluorescence intensity of a molecule in the nanogap relative to glass? Collection efficiency enhancement EF = h g ex (r,q) h 0 g 0 ex (q) QE(r) QE 0 Excitation enhancement Radiative quantum yield enhancement Maximum EF of ~70,000 when on resonance with fundamental mode x y Rose et al., Nano Lett., 14, 4797 (2014)
Fabrication steps: (1) Evaporation of silver/gold film (2) Alternating layers of positive/negative polymers (PAH/PSS) - each layer ~ 1 nm thick SEM of final structure (high density) (3) Polymer film immersed in solution of anionic Cy5 fluorescent dye. (4) Adhesion of anionic nanocubes from solution 200 nm Rose et al., Nano Lett., 14, 4797 (2014)
Time-resolved fluorescence Dark field scattering image: Scattering spectra
Fluorescence enhancement per unit area relative to Cy5 on glass: 632 nm cw excitation Correlation with nanopatch resonance observed Average fluorescence enhancements > 30,000 compared to glass substrate Relative agreement with simulation <EF> measured from 48 isolated nanopatches Rose et al., Nano Lett., 14, 4797 (2014)
1 0.1 0.01 Fluorescence lifetimes measured from ensemble of nanopatches Excitation: 80 MHz Ti:Sapphire laser at 632 nm. Detection: fast-timing APD and time-correlated single-photon counting module. 74-fold reduction in lifetime vs. glass - limited by detector No cubes: fluorescence intensity reduced ~20x Cubes: fluorescence intensity enhanced ~30,000x Emission lifetime is detector limited How large is the real enhancement in spontaneous emission rate?
Enhanced second-order nonlinearities Enhanced third-order nonlinearities ω 2ω Absorption enhancement of molecules Emission rate enhancement (Purcell effect) of molecules
Overcome detector resolution by using a dye with a long intrinsic lifetime Intrinsic fluorescence of Ru dye t 0 = 600 ns Ruthenium metal complex dye: Long intrinsic lifetime Large separation between absorption and emission Good photostability Akselrod, Argyropoulos et al., Nat. Phot. 8, 835 840, 2014
Electric field components at excitation wavelength ex 535 nm Always plane wave excitation in these simulations Dominant electric field component at resonance 650 nm Akselrod, Argyropoulos et al., Nat. Phot. 8, 835 840, 2014
Maximum Purcell factor of Not detector limited g max sp / g 0 sp = 860 BUT, simulations predict much larger Purcell factors! 860x enhancement What are we missing? Akselrod, Argyropoulos et al., Nat. Phot. 8, 835 840, 2014
Coupling to gap plasmon mode depends strongly on orientation of transition dipole: ( q ) ( zˆ )cos q 2 sp a sp a Emitted intensity: Emission term Excitation term Angular distribution of emitters This is what we need to solve for Barritault et al., Appl. Opt. (2002)
Simultaneous fit all 6 of the measured angular profiles: C(q a ) Most molecules are lying flat in the gap Not optimally coupled
Variation in field under single cube Dipole orientation distribution ~2 nm distribution of emitters inside gap Simulations of decay dynamics include all three effects Akselrod, Argyropoulos et al., Nat. Phot. 8, 835 840, 2014
Time-resolved fluorescence: (8 nm gap) Intrinsic decay 600 ns Distribution of emission rates: Most likely Max decay: decay: 10 ns 0.7 ns, 860x enh. ~60x enhancement Simulation 8 nm gap Experiment Simulations incorporating these effects describes observed behavior Akselrod, Argyropoulos et al., Nat. Phot. 8, 835 840, 2014
2 sp ( r, nˆ )t ˆ ex r r I( r, q, t) E (r) n ( ) e Only the field components oriented along the z-axis radiate significantly g sp (r, ˆn) = g sp (r,ẑ)cos 2 q Distribution of emitter orientations: C(q) = expé ë -(q -q 0 ) 2 / 2s 2 Averaged emitted power over all directions: 1 2 2 2 sp ( )cos t I(, t) E ( ) ex cos ( ) ( )sin 2 0 r e r r r q r q C q qdq Averaged emission of all the molecules inside the volume of the nanogap: I np (t) = N V ò V ù û I(r,t)dV Emitted power at an arbitrary position r
Gap size d g Lifetime can be tuned by nanoscale control of gap thickness Key question: Metal nanocavities are typically lossy, so Are the reduced lifetimes due to non-radiative quenching or radiative enhancement? Akselrod, Argyropoulos et al., Nat. Phot. 8, 835 840, 2014
h h 0 g ex (r,q) 0 g ex QE(r) QE 0 = EF Simulated average excitation enhancement Simulated quantum yield Simulated and measured enhancement factor g ex (r,q) / g 0 QE(r) EF QE 0 Measured <EF> agrees with simulations Radiative quantum yield is high (~0.5), as predicted Akselrod, Argyropoulos et al., Nat. Phot. 8, 835 840, 2014
Maximum radiative rate enhancement r sp QE Maximum total rate enhancement Most likely rate enhancement ~1000-fold enhancement of radiative rate due to high radiative efficiency Rate enhancements are tunable by nanoscale control of gap size Akselrod, Argyropoulos et al., Nat. Phot. 8, 835 840, 2014
Nonlinear metasurface coupled to multi-quantum well Plasmonic nanopatch antenna based on colloidal nanocubes Flat nonlinear optics paradigm Efficient frequency mixing over subwavelength films Significantly relaxed phase-matching conditions High conversion efficiency using very low pump intensity christos.argyropoulos@unl.edu High radiative quantum yield Directional emission (ultrafast LED, nanolaser) Efficient nonlinear operation with low intensities <1MW/cm 2 30,000X fluorescence intensity enhancement ~1000X Purcell enhancement
Dr. Thang Hoang Prof. Maiken Mikkelsen Dr. Gleb Akselrod Prof. David Smith Dr. Cristian Ciracì Prof. Andrea Alu Prof. Mikhail Belkin
THANK YOU QUESTIONS Several PhD vacancies exist in my group Contact me: christos.argyropoulos@unl.edu http://argyropoulos.unl.edu/