From microphotonics to nanophononics October 16th-28th Cargèse, France Elastic, thermodynamic and magnetic properties of nano-structured arrays impulsively excited by femtosecond laser pulses Università Cattolica del Sacro Cuore Dipartimento di Matematica e Fisica, Via Musei 41, Brescia, Italy. Claudio Giannetti

From microphotonics to nanophononics October 16th-28th Cargèse, France ARRAYS OF MAGNETIC DISKS Introduction Fundamental physics → Vortex configuration T. Shinjo et al., Science 289, 930 (2000). Magnetic eigenmodes on permalloy squares and disks K. Perzlmaier et al., Phys. Rev. Lett. 94, (2005). Technological interest →Candidates to MRAM R. Cowburn, J. Phys. D: Appl. Phys. 33, R1 (2000). 1m1m Fe 20 Ni 80

From microphotonics to nanophononics October 16th-28th Cargèse, France DIFFRACTION FROM ARRAYS OF 3D CONFINED METALLIC NANO-PARTICLES This technique strongly increases the sensitivity to the periodicity of the system, allowing to follow the mechanical and thermodynamic relaxation dynamics of the system with high accuracy. TIME-RESOLVED MEASUREMENTS OF THE DIFFRACTED PATTERN h = 800 nm  =120 fs 80 MHz Ti:Sapphire oscillator LIGHT SOURCE E PUMP ≈10 nJ/pulsefwhm≈60 µm E PROBE <1 nJ/pulsefwhm≈40 µm Reflected intensity variation Diffracted intensity variation G=2  /D

From microphotonics to nanophononics October 16th-28th Cargèse, France D=2018±30 nm 2 a =990 ±10 nm h=31±1 nm D=1020±50 nm 2 a =470 ±10 nm h=21±2 nm D=810±10 nm 2 a =380 ±20 nm h=33±5 nm D=610±3 nm 2 a =320 ±10 nm h=60±20 nm TIME-RESOLVED DIFFRACTION AS A FUNCTION OF THE ARRAY PERIODICITY 2x delay line piezomotors QPDs Feedback system for pump-probe alignment control during the long-range experiment (delay >1 m) Oscillations in the diffracted signal triggered by the impulsive heating of the metallic nanoparticles.  2D SAWs or  single modes of the dots

From microphotonics to nanophononics October 16th-28th Cargèse, France Dispersion relation of the 2D SAW excited at the center of the Brillouin zone. SURFACE WAVE VELOCITIES V SAW =4900 Si(100) [5] V SAW =5100 Si(110) [5] The damping , due to energy radiation of SAWs to bulk modes, is proportional to G 4. SAW damping SAW dispersion Initial transverse displacement u z0  h -1 2D Surface Acoustic Waves q v SAW -  /D  /D

From microphotonics to nanophononics October 16th-28th Cargèse, France CHANGING THE DISK RADIUS frequency shift 2 a =320 ±10 nm T=207.6±0.1 ps D=1000 nm; h=50 nm Constant periodicities and thicknesses 1st order perturbation theory predicts a frequency-shift, due to the mechanical loading, linear with the filling factor: r S : reflection coeff. Failure of the 1st order perturbative approach at large filling factors  =  a 2 /D 2 filling factor 2 a =395 ±7 nm T=212.4±0.1 ps 2 a =785 ±7 nm T=218.9±0.1 ps

From microphotonics to nanophononics October 16th-28th Cargèse, France Harmonic oscillator model, where the radial displacement u r (t) depends on the temperature of the disk. The solution, similarly to DECP, is given by: where  2 =   2 and  =1/  -  WAVELET ANALYSIS OF THE DIFFRACTED SIGNAL Convolution with the wavelet C-Morlet wavelet main period ≈ 220 ps ← impulsive excitation

From microphotonics to nanophononics October 16th-28th Cargèse, France time-domain dynamics FREQUENCY ANALYSIS OF THE DIFFRACTED SIGNAL G1G1 G2G2  SAW 22 D=1005±6 nm 2 a =785±7 nm h=51±2 nm Si(110) Si(100)  X M (533) (531)(311) X-ray diffraction Detection of the diagonal collective mode:  2 /  SAW =1.386±0.004 influence of the substrate anisotropy (θ=35°) 30°

From microphotonics to nanophononics October 16th-28th Cargèse, France WAVELET ANALYSIS OF THE DIFFRACTED SIGNAL DATA FIT with  SAW =4.57 GHz and  2 =6.33 GHz To reproduce the data we need to add a third highly damped frequency  3 ≈8.5 GHz (1-cos  t)-like excitation sin  t-like excitation

From microphotonics to nanophononics October 16th-28th Cargèse, France Periodic conditions on displacement, strain and stress Mode 1 Mode 3 Mode 2 Mode 4 1 µm 4.19 GHz3.78 GHz 4.52 GHz 5.80 GHz Symmetric mode  Form-factor modulation at  Asymmetric mode  Form-factor modulation at 2  Asymmetric mode  Form-factor modulation at 2  Asymmetric mode  Form-factor modulation at 2  NUMERICAL CALCULATION OF EIGENMODES Transverse mode Longitudinal mode

From microphotonics to nanophononics October 16th-28th Cargèse, France EIGENMODES DEPENDENCE ON THE DISK RADIUS Single disk modes Possible opening of a gap  TWO-DIMENSIONAL SURFACE PHONONIC CRYSTAL in the GHz The highly damped  3 frequency is close to the double of the asymmetric mode 2 frequency at the bottom of the band-gap q -  /D  /D ELASTIC-mismatch INTERACTION: opening of a gap at zone center

From microphotonics to nanophononics October 16th-28th Cargèse, France TIME-RESOLVED MAGNETO-OPTICAL KERR EFFECT M   Polarization rotation induced by the interaction with M  is the rotation  is the ellipticity → ,   M MAGNETIZATION RECOVERY DYNAMICS Static hysteresis cycle in press on Phys. Rev. Lett.

From microphotonics to nanophononics October 16th-28th Cargèse, France FUTURE Brillouin scattering measurements to evidence the opening of the gap in the 2D surface phononic crystal Decoupling the thermodynamic and mechanical contributions (double pump experiment)  CALORIMETRY of NANOPARTICLES Resonant excitation of magnetic eigenmodes of the system Applications to sub-wavelength optics

From microphotonics to nanophononics October 16th-28th Cargèse, France Acknowledgements Group leader Fulvio Parmigiani Thermodynamics F. Banfi and B. Revaz (University of Genève) Samples P. Vavassori (Università di Ferrara) V. Metlushko (University of Illinois) Ultrafast optics group (Università Cattolica, campus di Brescia) Gabriele Ferrini, Matteo Montagnese, Federico Cilento TR-MOKE Alberto Comin (LBL)