No abstract

No abstract

No abstract

No abstract

No abstract

No abstract

We report a direct determination of the specular scattering probability of acoustic phonons at a crystal boundary by observing the escape of incident coherent phonons from the coherent state during reflection. In the sub-THz frequency range where the phonon wavelength is much longer than the lattice constant, the acoustic phonon-interface interaction is found to agree well with the macroscopic theory on wave scattering from rough surfaces. This examination thus quantitatively verifies the dominant role of atomic-scale corrugations in the Kapitza anomaly observed at 1-10 K and further opens a new path to nondestructively estimate subnanoscale roughness of buried interfaces.

Third-harmonic-generation (THG) has been emerged as an important noninvasive intravital imaging modality of in vivo biological research [1][2][3][4] in recent years with the advantages including intrinsic optical sectioning capability due to the highorder nonlinearity nature and no energy release due to the virtual-state-transition characteristic, [5][6][7][8][9] thus allowing much improved cell viability [3,4] in contrast to current absorptionbased fluorescence technologies. Although THG nonlinearity exists in all bio-materials, the Gouy phase shift effect substantially limits THG to be observed in the vicinity of interfaces where the first order or third order susceptibility discontinues.[10] Therefore, THG is generally regarded as a morphological imaging tool due to its interface-sensitive nature, with limited capability for molecular imaging. It is thus highly desirable to develop exogenous THG contrast agents to trace the functions of a specific molecule, taking advantage of the noninvasive nature of the THG process. Recently, noble metal nanoparticles have been proved to be able to enhance various nonlinear optical signals through surface plasmon resonance. [11][12][13][14][15][16] It should be ideal to adopt nanoparticles as molecular contrast agents of THG microscopy. However, these previous experiments proposed to enhance nonlinear emissions by matching excitation energy with the plasmon resonance energy of metal nanoparticles, which could induce strong laser absorption in nanoparticles while the induced temperature increase might alter the behaviors of the targeted bio-molecules or even induce thermal damages in the studied biological specimens.In this letter, we demonstrate molecular THG microscopy by using silver nanoparticles as exogenous THG contrast agents. This demonstration was performed in cultured mouse bladder carcinoma cells (MBT2) and the matched cell line with knocked-down Her2/neu expression by RNAi. Through matching surface plasmon wavelength to THG wavelength, strong contrast can be provided by the silver nanoparticles under a THG microscope, while the laser wavelength is located in the biological penetration window and laser absorption in nanoparticles is also strongly reduced due to the huge spectral difference between the laser excitation wavelength and the plasmon resonance wavelength. By successfully conjugating anti-her2 antibodies with the citrated silver nanoparticles, Her2/neu in the cancerous cell membranes is successfully imaged with THG microscopy.With the help of surface plasmon-resonance, nanometersized noble metals can serve as a nanoscopic optical resonant cavity. Metal nanoparticles with the plasmon resonance at the third harmonic of optical excitation, in the macroscopic point of view, is analogous to an optical third-harmonic oscillator. [17,18] We chose silver nanoparticles for its blue-violet plasmon resonance wavelengths when soaked in water. For nonlinear biological in vivo imaging, near-infrared (NIR) femtosecond lasers are preferred as the THG excitation sources ...