Nanoscale Energy Conversion and Information Processing Devices - NanoNice - Photoacoustic response in mesoscopic systems

Transcription

1 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

2 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)

3 Non destructive evaluation Identification (inverse problem) Material properties (stiffnesses) propagation of ultrasonic waves Signals Representation (direct problem) Direct problem is preliminary to NDE

4 Outline Picosecond ultrasonics Instrumentation Measurements elastic stiffnesses (sub-)micrometric metal films

5 Outline Picosecond ultrasonics Instrumentation Measurements elastic stiffnesses (sub-)micrometric metal films

6 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

7 Experimental set-up Laser source : 100 fs nm Moving mirrors Probe Pump Modulation Sample!r(t) r 0 Reflectometry Interferometry Detection

8 Tungsten (255 nm) on Silicon 1.2E E-03!r r 0 ( t) 1.0E E E+00 Amplitude Reflectivity 8.0E E E-04 Acoustic echoes Reflectivity -5.0E E E E Time delay (ps) 2.0E-04 Thermal diffusion 0.0E Time delay (ps) Longitudinal sound velocity Acoustic reflection coefficient Ultrasonic attenuation Electron-phonon coupling Thermal conductivity Photoelastique coefficient

9 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 Width ( ) x 2 µm Time (fs)! " 2 u " 2 t # $ C Acoustic field ([ ] : $u) = # % [ ]$T x 2 x 3 x 1

10 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" Au(t,l ) photothermal coefficient temperature photoelastic coefficient acoustic strain sensitivity function

11 Tungsten (255 nm) on Silicon Reflectivity changes (10-5 ) 10 λ = 751 nm Selection of the Probe wavelength at 751 nm Time delay (ps) published in JAP

12 Outline Picosecond ultrasonics Instrumentation BDD & PCF Measurements elastic stiffnesses (sub-)micrometric metal films

13 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)

14 BDD : Beam Distortion Detection 1. Interferometric 2. BDD 3. Reflectometric In micrometric Gold published in RSI

15 Photonic Crystal Fiber Nonlinear Active air-clad Airguiding hollow-core Crystal Fibre A/S w w w.blazephotonics.com

16 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

17 Photonic Crystal Fiber 1 (a) Intensity (a.u.) Continuum generation Probe pulse spectrum Input: 777±20nm Output: 825 ±250nm Wavelength (nm)

18 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 Times delay (ps) Acoustic echoes for selected probe wavelengths published in RSI

19 GaAs sample Reflectivity changes (10-4 ) λ = 719 nm Probe Optical Interferences Acoustic perturbation Time delay (ps) f B = 2v n (!)! n(λ) : optical index v : sound velocity

20 GaAs sample Reflectivity changes (10-4 ) 5 (a)! = 858 nm! = 819 nm! = 755 nm! = 734 nm! = 719 nm Frequency (GHz) Times delay (ps) Wavelenght (nm) Brillouin oscillations for selected probe wavelengths

21 Outline Picosecond ultrasonics Instrumentation Measurements elastic stiffnesses (sub-)micrometric metal films Aluminum & Gold films Imagining Biological media

22 Picosecond ultrasonics : 1D 1D problem? Pump substrate Surface heated a=50 µm layer µm & nm

23 Picosecond ultrasonics : 1D 1D problem! Longitudinal waves only Pump substrate Surface heated a=50 µm layer µm & nm

24 Picosecond ultrasonics -> 3D 3D: acoustic diffraction Longitudinal & shear waves Pump Blue light substrate Surface heated a=0.5 µm layer µm only

25 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)

26 Directivity function Sound generation Surface Optical penetration L S S L Reflection R SL R LS

27 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) 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 Time delay (ps)

28 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 Time delay (ps) Time delay (ps) Published in PRL

29 Aluminum plate 0.9 µm Waves calculation inside the plate 0.9 µm Free Al standing film Source: a=δ µm Snapshot 250ps

30 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

31 Group velocities Probe tan! = x Nh x θ X 2 T L! = 40 Pump V = 2 Nh T cos! 4 2 h A X 1

32 Diffraction in gold thin films Vs. thickness Vs. Source width µm µm x 50x µm x t, ps t, ps Diffraction effects 0.5µm Acoustic diffraction in thin gold films

33 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

34 Group velocities in gold 2µm2 1st and 2nd longitudinal echoes 5µm 2µm displacement, a.u. Group velocity 3400m/s

35 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

36 Ray theory 0,4 0,3 0,2 X 2 T Waves propagate at group velocities X 2 T L! = 40 0,1 L 2 X ,1 0,2 0,3 0, 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

37 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

38 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, ,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

39 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)

40 Outline Picosecond ultrasonics Instrumentation Measurements elastic stiffnesses (sub-)micrometric metal films Imagining Biological media

41 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)