Nonlinear transmission of light through synthetic colloidal suspensions Zhigang Chen. San Francisco State Univ., California, USA & Nankai Univ.

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1 Nonlinear transmission of light through synthetic colloidal suspensions Zhigang Chen San Francisco State Univ., California, USA & Nankai Univ. China

2 What do we do with light? Spatial solitons & dynamics (PRL, OL, ) Condensed-matter photonics (Nat Mat 2014, Nat Comm 2015) Optical beam engineering (PRL 2012,2015, OL ) Trapping and manipulation (Opt Lett, OE, ) Tweezers & biophotonics (BioMed OE, 2012, 2014) Soft matter NLO (PRL 2013, Nano Lett 2014)

3 Outline Motivation & background Synthetic colloidal suspensions (optical forces, polarizibility, tunable NL) Optofluidic manipulation, transport through scattering media Metallic suspensions Tunable polarizabilities Plasmonic resonant solitons Dielectric suspensions Negative and mixed polarizibilities Self-induced transparency effects Biological suspensions Penetrating or killing? Nonlinear effects?

4 Optical nonlinearity in soft-matter systems - Motivation Colloidal Physics Soft-matter Statistical Mechanics An interdisciplinary field Fluid mechanics Nonlinear Optics Life sciences Chemistry/ electrochemistry Brownian motion Optical forces Nonlinear scattering Soliton effects

5 Historical Overview A. Ashkin, Acceleration and trapping of particles by radiation pressure, Phys. Rev. Lett. 24, (1970). Foundation for optical tweezers Nonlinear effects: Four-wave mixing in artificial Kerr Media P.W. Smith, A.Ashkin, and W.J. Tomlinson, Opt. Lett. (1981) Self focusing in artificial Kerr media A.Ashkin, J.M. Dziedzic, and P.W. Smith, Opt. Lett. (1982) Soliton-like beams in aqueous suspensions V.E. Yashin et al, Optics and spectroscopy (2005) Optical Spatial Solitons in Soft Matter C.Conti, G. Ruocco, and S. Trillo, PRL (2005) Soliton dynamics and self-induced transparency in nonlinear nanosuspensions R. El-Ganainy, D.N. Christodoulides, C. Rotschild, and M. Segev, OE (2007) Spatial solitons and light-induced instabilities in colloidal media M. Matuszewski, W. Krolikowski, and Y. S. Kivshar,OE (2008). Experimental Observation of Modulation Instability and Optical Spatial Soliton Arrays P. J. Reece, E. M. Wright, and K. Dholakia, PRL (2007)

6 In the Rayleigh regime (dipole approx.) Optical Forces ε 1 ε 2 p = α E 0 m 1 p = 3Vpε n E m b 2 0 α = 3V p m ε n m b n > n => α > 0 p b Positive polarizability (PP) n < n => α < 0 p b m = n n p b Negative polarizability (NP)

7 Optical gradient force: F = α I 4 Optical Forces 2 2 m 1 α = 3Vpε0 nb 2 m + 1 m = n n p b n p >n b Positive polarizability (Attractive!) n > n => α > 0 p b Negative polarizability (Repulsive!) n < n => α < 0 p b The origin of the nonlinearity! S. Stenholm, Rev. Mod. Phys. (1986) J.P. Gordon, Phys. Rev. A (1973) Optical Radiation Pressure: F rad σ = S c 5 2 a nb a m π 1 σ s = 3 λ m + 2 2

8 Optical Forces vs Polarizability PP α > 0 Refractive index increases at beam center in both cases (artificial self-focusing) NP 8 α < 0

9 Engineering polarizability in colloidal suspensions? Positive Polarizability (PP) Large scattering loss Super-Kerr => unstable Negative Polarizability (NP) Self-cleaned channel (Enhanced transparency) Saturable => stable! Tunable nonlinearity Mixed polarizability

10 Our experimental work: Suspension synthetic nanosuspensions Particle Size n p n b n p - n b PP Polystyrene in Glycerin Water (3:1 ratio) 200nm NP PTFE in Glycerin Water (3:1 ratio) 200nm > < ZP PTFE in Glycerin Water (1:6 ratio) = 200nm n pure Glycerin = 1.47 n water = 1.33 PTFE:Poly(tetrafluoroethylene) n ptfe = 1.35 First stable colloidial nanosuspensions with negative polarizibility!

11 Beam propagation in tunable nanosuspensions Experimental side-view n p > n b Beam Collapse PP catastrophic self-focusing collapse severe scattering losses NP n p < n b Enhanced transmission ZP n p = n b Diffraction Thermal effect? No!

12 Nonlinearity in Colloidal Suspensions Nernst-Planck Eq: (diluted sample) (neglecting thermal effects) Drift due to gradient force Fgrad α = I 4 Diffusion due to Brownian motion At steady state condition: ρ I α = ρ I kt ( ) 0 exp 4 B Maxwell-Garnett formula: f( I) = V ρ( I) p R. El-Ganainy, D. N. Christodoulides, C. Rotschild, and M. Segev, OE 2007

13 NLS like Equation - beam propagation in tunable nanosuspensions Helmholtz eq: under SVEA: + = E kn 0 eff E 0 n = (1 f ) n + fn eff b p Exyz (,, ) = ϕ( xyz,, )exp( iknz) 0 b ϕ 1 ( ) i + ϕ + ϕ + σ ϕ = 2 i k0 np nb f f z 2kn 0 b 2Vp 0 Exponential nonlinearity Tunable NL scattering losses α I f = f 0 exp ratio 0 - pure NP; 1 pure PP 4k B T Saturable for NP Non-ideal gas model Virial coefficients B 2,3

14 α < 0 α > 0 Tunable nonlinearity by mixing PTFE and PS particles NP: Four-fold enhancement in transmission Self-induced transparency (Fair comparison: starting at same initial linear transmission) Colloidally Trapped Light Needles

15 Interaction of self-trapped beams in NP suspensions input (out-of-phase repulsion ) output y 20µm x Simulation (side-view propagation) Experiment (transverse patterns) Drive by optical forces - particle-density dependent; Not mediated by thermal effects! Such interaction not possible in PP suspensions. (in-phase attraction) S. Fardad, et al. Optics Letters. 38 (2013)

16 Deep penetration achieved by forming dense shock fronts of particle concentration in PP (polystyrene) suspensions! Opt. Express 21, (2013)

17 Plasmonic resonant solitons in metallic nanosuspensions Advantages: much larger poalrizibility, much deeper penetration (over 25 diffraction lengths ), much lower power needed (mw), much more flexible in tuning optical response (through composition, size, and shape) S. Fardad, et al. Nano Letters (2014)

18 Previous work: Metallic nanosuspensions CW laser beam Closed aperture z-scan n2 < 0 Why so? gold nanosuspension ~150mW - M.H. Majles Ara, et al, Journal of Quantitative Spectroscopy & Radiative Transfer 113,366, (2012) - Z. Mao, et al. Opt. Lett. 7, 949 (2009). - T. Jia, et al, Opt. Laser Technol. 40, 936 (2008). - R. F. Souza, et al, Proc. of SPIE Vol. 6323, 63231T, (2006) - T. Jia et al, Optics & Laser Technology 40, 936, (2008) - L. Sarkhosh et al, Phys. Status Solidi A 207, No. 10, 2303, (2010) 10mm Due to thermal effects Strong defocusing

19 Samples used 50nm 100nm 19

20 NP sample: gold nano-rod 1 E 0 Normalized field 20

21 NP samples: Gold nano-rod suspension: linear: 10mW 10 mm nonlinear: 250mW 10mW 100mW 200mW 250mW 3 mm 1 mm 0.3mm 0.1mm 21

22 NP sample: Silica core-gold shell suspension Polarizability tuning 22

23 Core-shell nanosuspension: Fivefold larger NP α r = mW 250mW 23 Nano-rod suspension (direct comparison) α r =

24 PP Samples: gold & silver spheres: φ 1 ( ) iσρ + φ + ρ φ φ + φ = i k0 np nb V k0 nt z kn 0 b 0 Polarizability Scattering and absorption Beam does not collapse as in dielectric PP suspensions! Gold sphere 24 Silver sphere

25 Guiding light by light in metallic nanosuspensions: Pump (soliton) beam: λ = 532nm, P=40mW SPR Peak: 527 nm Guided: FWHM 150 µm Probe (guided) beam: λ = 1064nm, P=50mW (green filtered, IR sideview as superposition of many frames) Probe beam itself has no (or weak) NL self-action! Controlling strong IR beams by a weak green beam! Unguided: FWHM 670 µm

26 Outlook Dielectric, metallic, and biological colloidal suspensions are fun to play with; Nonlinear optical manipulation in such highly scattering suspensions is still unexplored; New opportunities open up in developing soft-matter systems with engineered nonlinearities.

27 Thank you for your attention! Website: Co-worker: Shima Fardad and D.N. Christodoulides - CREOL Weining Man, Z. Zhang, Anna Bezryadina - SFSU