Nanoscale Energy Conversion and Information Processing Devices - NanoNice - Photoacoustic response in mesoscopic systems Photonics group W. Claeys, S. Dilhair, S. Grauby, JM. Rampnoux, L. Patino Lopez, H. Michel, M. Salhi Coherent Phononics group M. Deschamps, B. Audoin, C. Rossignol, N. Chigarev, Y. Pan, M. Perton, T. Dehoux, D. Ségur Incoherent Phononics group JC. Batsale, JL. Bataglia, A. Kusiak Pont de Pierre, Bordeaux CPMOH - LMP - TREFLE Université de Bordeaux, CNRS, France
Objectives Observation of the diffraction of picosecond acoustic pulse, Generation of divergent shear waves, Measurements elastic stiffnesses. (sub-)micrometric metal films: Aluminum, gold Biological media Complex material (phononics)
Non destructive evaluation Identification (inverse problem) Material properties (stiffnesses) propagation of ultrasonic waves Signals Representation (direct problem) Direct problem is preliminary to NDE
Outline Picosecond ultrasonics Instrumentation Measurements elastic stiffnesses (sub-)micrometric metal films
Outline Picosecond ultrasonics Instrumentation Measurements elastic stiffnesses (sub-)micrometric metal films
Picosecond ultrasonics measuring the transient reflectivity changes induced in a (multi)layer structure by the propagation of an optically generated acoustic wave Pump 100 fs Probe 1986 : H.J. Maris 10! 7 # r( t) " r " 10! 3 0 Acoustic perturbation GHz THz layer µm & nm Echo ns ps substrate
Experimental set-up Laser source : 100 fs - 775 nm Moving mirrors Probe Pump Modulation Sample!r(t) r 0 Reflectometry Interferometry Detection
Tungsten (255 nm) on Silicon 1.2E-03 1.0E-03!r r 0 ( t) 1.0E-05 5.0E-06 0.0E+00 Amplitude Reflectivity 8.0E-04 6.0E-04 4.0E-04 Acoustic echoes Reflectivity -5.0E-06-1.0E-05-1.5E-05-2.0E-05 0 200 400 600 800 1000 Time delay (ps) 2.0E-04 Thermal diffusion 0.0E+00-200 0 200 400 600 800 1000 1200 Time delay (ps) Longitudinal sound velocity Acoustic reflection coefficient Ultrasonic attenuation Electron-phonon coupling Thermal conductivity Photoelastique coefficient
Optical generation 2D model Deposited energy Q( x 1,x 2,t) =!I ( 1-R) g( x 2 ) f ( t)e "!x 1 "T!C p "t # $ % Temperature rise ([ ] : $T ) = Q( x 1, x 2,t) 1 g( x 2 ) 1 f ( t) a.u. a a.u. t 0-20 -10 0 10 20 Width ( ) x 2 µm 0-500 0 500 1000 1500 Time (fs)! " 2 u " 2 t # $ C Acoustic field ([ ] : $u) = # % [ ]$T x 2 x 3 x 1
Optical detection optical relative change of reflectivity wavelength spectrum of the probe pulse wavelength selection of the detector!r( t) r 0 refl ++, 0 = P (")S " ( ) % #n #T T ( z,t) + #n & ' #$ $ ( z,t) (, ) * f ( z," )dz d" ++ 0 + Au(t,l ) photothermal coefficient temperature photoelastic coefficient acoustic strain sensitivity function
Tungsten (255 nm) on Silicon Reflectivity changes (10-5 ) 10 λ = 751 nm Selection of the Probe wavelength at 751 nm 0 100 200 300 Time delay (ps) published in JAP
Outline Picosecond ultrasonics Instrumentation BDD & PCF Measurements elastic stiffnesses (sub-)micrometric metal films
Instrumentation Measuring acoustic displacements BDD : Beam Deformation Detection Tuning optical wavelengths how? Broadband picosecond ultrasonics Time resolved spectroscopy Wavelength continuum generation Photonic crystal fiber (PCF)
BDD : Beam Distortion Detection 1. Interferometric 2. BDD 3. Reflectometric In micrometric Gold published in RSI
Photonic Crystal Fiber Nonlinear Active air-clad Airguiding hollow-core Crystal Fibre A/S w w w.blazephotonics.com
Photonic Crystal Fiber silica air holes Hexagonal arrangement Continuum generation: 777±20nm Length: 70 mm Conversion rate : 20% 1.4 µm 1.7 µm Crystal Fibre A/S
Photonic Crystal Fiber 1 (a) Intensity (a.u.) 0.1 0.01 Continuum generation Probe pulse spectrum Input: 777±20nm Output: 825 ±250nm 0.001 650 700 750 800 850 900 950 1000 Wavelength (nm)
Tungsten (255 nm) on Silicon!r( t) r 0 refl ( " #e $n &2ke 2ikv t + i'e &'v t ) * $% 4k 2 + ' 2 +, - Reflectivity changes (10-5 ) 5 λ = 922 nm λ = 905 nm λ = 816 nm λ = 751 nm 0 50 100 150 200 250 Times delay (ps) Acoustic echoes for selected probe wavelengths published in RSI
GaAs sample Reflectivity changes (10-4 ) λ = 719 nm 10 0 100 200 300 Probe Optical Interferences Acoustic perturbation Time delay (ps) f B = 2v n (!)! n(λ) : optical index v : sound velocity
GaAs sample Reflectivity changes (10-4 ) 5 (a)! = 858 nm! = 819 nm! = 755 nm! = 734 nm! = 719 nm Frequency (GHz) 50 48 46 44 42 0 50 100 150 200 250 300 350 Times delay (ps) 40 700 750 800 850 Wavelenght (nm) Brillouin oscillations for selected probe wavelengths
Outline Picosecond ultrasonics Instrumentation Measurements elastic stiffnesses (sub-)micrometric metal films Aluminum & Gold films Imagining Biological media
Picosecond ultrasonics : 1D 1D problem? Pump substrate Surface heated a=50 µm layer µm & nm
Picosecond ultrasonics : 1D 1D problem! Longitudinal waves only Pump substrate Surface heated a=50 µm layer µm & nm
Picosecond ultrasonics -> 3D 3D: acoustic diffraction Longitudinal & shear waves Pump Blue light substrate Surface heated a=0.5 µm layer µm only
Improvement of acoustic diffraction with a better laser focalization Lens x20 Objective x100 2µm without focalization with objective x100 laser 8µm 1µm λ=780 nm for a point source (virtual)
Directivity function Sound generation Surface Optical penetration L S S L Reflection R SL R LS
Aluminum plate 0.54 & 0.9 µm 10 shear waves detected LT and 2T 0 2L h = 0.54 µm - a = 5 µm Phase (0.1 mrad) -10-20 LT h = 0.54 µm - a = 2 µm h = 0.54 µm - a = 1 µm -30 2T & diffraction h = 0.9 µm - a = 1 µm 0 200 400 600 800 1000 Time delay (ps)
Aluminum plate 0.54 & 0.9 µm Measurements Calculations h = 0.54 µm - a = 5 µm h = 0.54 µm - a = 5 µm Phase (0.1 mrad) 5 h = 0.54 µm - a = 2 µm h = 0.54 µm - a = 1 µm Phase (a.u.) h = 0.54 µm - a = 2 µm h = 0.54 µm - a = 1 µm h = 0.9 µm - a = 1 µm h = 0.9 µm - a = 1 µm 0 200 400 600 800 1000 Time delay (ps) 0 200 400 600 800 1000 Time delay (ps) Published in PRL
Aluminum plate 0.9 µm Waves calculation inside the plate 0.9 µm Free Al standing film Source: a=δ µm Snapshot 250ps
Aluminum plate 0.9 µm Waves calculation inside the plate 0.9 µm L LL H R LT T Free Al standing film Source: a=δ µm Snapshot 250ps movies
Group velocities Probe tan! = x Nh x θ 10 8 6 X 2 T L! = 40 Pump V = 2 Nh T cos! 4 2 h 0 0 2 4 6 8 10 A X 1
Diffraction in gold thin films Vs. thickness Vs. Source width 6 3 0.45µm 5 4 2 0.8µm 3 2 1 100x 50x 1 1.25µm 0-1 -2 20x 0 500 1000 1500 2000 2500 t, ps -3 0 500 1000 1500 2000 2500 t, ps Diffraction effects 0.5µm Acoustic diffraction in thin gold films
Au polycrystalline Gold plate Orientation (1,1,1) (x-ray diffraction) size of laser beam 1 µm 100 nm AFM picture of the surface Sample considered isotropic transverse (1,1,1) X-ray diffraction figure Homogenization of the stiffness tensor coefficients (Reuss): C 11 =227, C 22 =210, C 66 =18, C 12 =142, C 23 =160
Group velocities in gold 2µm2 1st and 2nd longitudinal echoes 5µm 2µm displacement, a.u. Group velocity 3400m/s
Stiffness with group velocities Fitting experimental data with group arrival time calculated from stiffness tensor leads to a first quick evaluation of coefficients of the stiffness tensor Simulations C 11 = 237 ±1 GPa, C 22 = 170 ±30 GPa, C 66 = 21 ±1 GPa, C 12 =170 ±30 GPa Experiments
Ray theory 0,4 0,3 0,2 X 2 T Waves propagate at group velocities 10 8 6 4 X 2 T L! = 40 0,1 L 2 X 0 1 0 0 0,1 0,2 0,3 0,4 0 2 4 6 8 10 phase slownesses (ms/m) multiple arrivals of quasi-shear mode rays concentrate at the cusp edges A group velocities (km/s) 5 4 T 4 5 X 1
Synthesis of plane wavefronts source j detection 1- sampling in space domain: laser source is scanned, with a constant dx, 2- signals are shifted in time by a constant δt d N 3- s(t) =! s n (t + n!t) n= 0 such as would have been recorded if the source had moved along the interface with a constant velocity δx/δt refraction conditions
Determination of the phase slowness (a.u.) L T D 0 0,5 1 1,5 2 2,5 3 time (µs) 1 0,8 L 2LT 0,25 0,2 T a.u. 0,6 0,4 T 3L X2 0,15 0,1 L 0,2 0,05 0 1 1,5 2 2,5 3 time (µs) signal processing: modulus of the convolution with a wavelet of 5 MHz central frequency 0 0 0,05 0,1 0,15 0,2 0,25 phase slowness for various refracted directions X1!t /!x movies
Simulations (ns) Stiffness with phase velocities Experiments (ns) k s (s/km) C 11 =237 GPa, C 22 =170 GPa, C 66 =21.8 GPa, C 12 =150 GPa k s (s/km)
Outline Picosecond ultrasonics Instrumentation Measurements elastic stiffnesses (sub-)micrometric metal films Imagining Biological media
Sumary Observation of the diffraction of picosecond acoustic pulse, Generation of divergent shear waves, Measurements elastic stiffnesses. (sub-)micrometric metal films: Aluminum, gold Biological media Complex material (phononics)