Direct observation of low frequency confined acoustic phonons in silver nanoparticles: Terahertz time domain spectroscopy

Terahertz time domain spectroscopy has been used to study low frequency confined acoustic phonons of silver nanoparticles embedded in poly (vinyl alcohol) I. INTORDUCTION Confined acoustic phonons in nanoparticles are fingerprints of their shape and size. The frequencies of these phonon modes are proportional to longitudinal/transverse sound velocity in the material and inversely proportional to the size of the nanoparticles. These modes have been studied extensively by low frequency Raman spectroscopy In this paper, we report on the observation of two confined acoustic phonon modes with frequency ~ 0.60 THz and ~ 2.12 THz in silver nanoparticles of average diameter ~ 3.7 nm 3 embedded in poly(vinyl alcohol) (PVA) matrix by using THz-TDS. These observed frequencies are compared with the estimated frequencies of the spheroidal and the toroidal vibrational modes of a free elastic sphere in the Lamb's model II. EXPERIMENTAL The polymer embedded silver nanoparticle film (Ag-PVA) was cast from the mixture of aqueous solutions of PVA and high purity silver nitrate (AgNO 3 ) (both purchased from Aldrich chemicals). Solution of PVA was obtained by adding 3 gm of granular PVA in 30 ml of deionized water followed by continuous heating at 90 0 C and stirring for one hour. 5 mg of In the THz-TDS we have used photoconductive emitter and detector (EKSPLA), both driven by 100 fs laser pulses with central wavelength at 800 nm from a 76 MHz Ti:sapphire laser. The set-up generates THz radiation in the spectral bandwidth of 0.1 to 2.5 THz with good signal to noise ratio of ~ 10 3 at 0.8 THz. III. RESULTS AND DISCUSSION The time domain electric fields, E s (t) and E r (t), associated with the THz pulse, recorded with and without the film are shown in where A(ω) is the amplitude and φ(ω) is the phase of the transmission coefficient. The spectral transmission coefficient of an optically thin film can be written as Here ω k /2π is the resonance frequency, Γ k /2π is the spectral width, F k is the oscillator strength, and ∞ ε is the high frequency dielectric constant. The solid lines in Figs. 2(b) and 2(c) are the fits with fitting parameters given in [27]. The other two resonance bands centered at ~ 0.6 THz and ~ 2.12 THz are attributed to the 6 confined acoustic phonons in silver nanoparticles as discussed below. We note that the large spectral width of ~ 0.4 THz of these observed phonon modes could possibly be due to the large size distribution of the nanoparticles. A. Confined acoustic phonons in silver nanoparticles The vibrational motion of a homogeneous elastic sphere with free surface has been theoretically studied by Lamb are infrared active 7 The lowest order (m = 0) and a few higher order (m ≥ 1) Raman active, spheroidal radial (l = 0) and quadrupolar (l = 2) modes have been routinely observed either by Raman spectroscopy [40] using various models for silver nanoparticles embedded in a glass matrix also showed that the mode frequencies blue-shift up-to ~ 10% as compared to those obtained using a free sphere model. Our present experimental observation of vibrational modes of silver nanoparticles at frequencies of ~ 0.6 THz and ~ 2.12 THz have a broad spectral width of ~ 0.4 THz due to the broad particle size distribution and hence free sphere model can suffice. Theoretical investigation of the Raman and infrared activity along with the comparative study of absorption and scattering cross-section of various possible modes is needed for quantitative understanding of the observed modes. IV. CONCLUSIONS In summary, we have studied the terahertz frequency response of silver nanoparticles embedded in a PVA matrix. For the first time, we have observed the confined acoustic phonons in silver nanoparticles by measuring the real and imaginary parts of the dielectric function of the nanoparticle-doped polymer film using THz-TDS. The observed phonons with frequency 0.6 and 2.12 THz are attributed to the spheroidal and toroidal vibrational modes. The LAM associated with the crystalline lamellae in PVA has been observed at frequency 1.11 THz. Our experiments demonstrate the use of THz-TDS as complementary technique to Raman scattering to characterize the nanoparticles. We note that the phonons observed in the THz-TDS are the equilibrium phonons of the system. The next step will be to study the time evolution of these 9 modes of nanoparticles once created coherently in the ISRS by using an optical-pump. The THz absorption spectrum of the nanoparticles is measured with and without the pump as a function of time-delay between the optical-pump pulse and the THz-probe pulse in an optical-pump THzprobe experiment. It is possible that in the THz spectrum new modes can appear due to the pump excitation induced infrared activity of certain modes. ACKNOWLEDGEMENT

Study of femtosecond laser-induced grating in lead silicate glasses

Summary form only given. In previous studies, we have shown that the grating photo-induced by femtosecond laser pulses in a SF59 lead silica glass is not formed instantaneous and continues to evolve at the millisecond time scales after excitation. The grating reflectivity was shown to depend on the pump power density and on the number of exciting pulses [1]. The grating was directly written onto the SF glasses by imaging a master grating, which is illuminated by femtosecond laser pulses (1mJ, ~50fs, 1KHz, ~800nm) [2]. The formation of the grating was shown to result from defects photo-induced by the two photon-absorption of the femtosecond laser pulses [1,2]. Theses defects induce a change of the real (Δnr) and imaginary (Δni) part of refractive index of the glass. However our previous experiments were not able to reveal the time scale for the formation of these defects or to evidence the impact of the two photons absorption on Δnr or Δni. Indeed for a given laser power density, one expects to observe an increase of Δnr and therefore of the grating reflectivity as two-photon absorption coefficient α2 is increased. To evidence the time scale for the formation of these defects, we have measured the evolution of the reflectivity of the photo-induced grating on the picoseconds time scale. As shown in Fig.1a, we found that depending on the pump power density, a rapid growth of the reflectivity is recorded in about 1.5ps to 4ps (Fig.1a). In a good agreement with the formation of the effects in glasses, this indicates that photo-induced defects are produced on few picoseconds [3]. Now Δnr should depends on the number of defects Ndef induced by two-photon absorption: The higher the α2 of the glass, the higher Ndef and the larger (Δnr). In lead silica glasses α2 is linearly proportional to the PbO molar density [4]. This phenomenon is related to the shift of the edge absorption band of the glass toward the longer wavelength with the PbO glass content (Fig.1b). As expected, Fig.1c indicates that, at 1 kHz femtosecond pulse repetition rate and for moderate power density, Δnr increases linearly with PbO molar fraction. Moreover, as the power density is further increased, we notice that Δnr saturate. The value of Δnr at saturation depends on and increases with PbO molar density. The ensemble of measurements that contributes to the understanding of the index modification of glasses photo-induced by femtosecond pulses will be discussed in more details during our presentation

Spatiotemporal Control of THz Pulses by Shaping the Laser Beam Transverse Profile in a Non Linear Rectifying Crystal

We demonstrate that spatiotemporal properties of Terahertz (THz) pulses generated in a ZnTe crystal are controlled in the intermediate field by shaping the transverse spatial profile of the near infrared (NIR) femtosecond (fs) optical pump

Combinaison cohérente intracavité de quatre faisceaux lasers avec un élément d'optique diffractive

Les milieux amplificateurs des lasers de puissance pompés par diodes sont soumis à de forts gradients de température. Ceux-ci entrainent à la fois une dégradation de la qualité spatiale des faisceaux émis et une baisse du rendement opto-optique du laser. Pour s'affranchir des aberrations thermiques, nous proposons une méthode qui consiste à répartir la puissance de pompage sur plusieurs zones distinctes. Les performances d'une cavité “T.F.” multiaxe pompée longitudinalement par quatre diodes lasers fibrées sont présentées. Les quatre faisceaux élémentaires engendrés sont combinés intracavité de façon cohérente par un élément d'otique diffractive qui permet de focaliser 82% de la puissance laser émise en un seul faisceau de divergence minimale

Impact of dispersion, free carriers and two photon absorption on the generation of intense THz pulses in ZnTe Crystals

The evolution of THz spectrum generated in a ZnTe crystal versus the intensity of ultra-short laser pulses is reported. Free carrier absorption and phase matching shift induced by two-photon absorption account for the observed phenomena

Ultrafast, broadband and tunable terahertz reflector and neutral density filter based on high resistivity silicon

We report THz transmission and reflection properties of an ultrafast optically excited highly resistive silicon wafer. Amplified Ti:Sapphire femtosecond laser pulses at 800 nm were used to create fluence-dependent carrier density on the front surface of the wafer which modifies the dielectric properties at the THz frequencies. Time-resolved experiments in the optical pump-THz probe configuration were conducted in which THz pulses reflected off from the surface at 0°and 45°angles of incidence make it possible to measure the pump-fluence dependent ultrafast evolution of the reflection and transmission coefficients in 0.5-6 THz range. An analytical model, where both the Drude contributions from the photoexcited electrons and holes account for the change of the dielectric constant of the photoexcited silicon, has been used to evaluate the THz reflection and transmission coefficients at steady state. Thus obtained results match well with the experimental results and demonstrate an all-optical means to convert a silicon wafer into an ultrafast, tunable and broadband neutral density filter or reflector in the THz frequency range

Impact of dispersion, free carriers and two photon absorption on the generation of intense THz pulses in ZnTe Crystals

The evolution of THz spectrum generated in a ZnTe crystal versus the intensity of ultra-short laser pulses is reported. Free carrier absorption and phase matching shift induced by two-photon absorption account for the observed phenomena

High emission and detection efficiency of terahertz beam with heavy-ion-irradiated InP material excited at 0.8 mu m

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