Third Harmonic Generation Signal Research Articles

Nonlinear optical study of the Fermi-surface oscillation in gold induced by acoustic-phonon excitation

Using a two-color ultrafast optical pump-probe technique with an amplified Ti:sapphire femtosecond laser system, we study the heat-induced structural dynamics in gold by monitoring both the fundamental and third harmonic generation (THG) signals of the probe beam. An amplitude oscillation, with a period of tens of picoseconds, is clearly observed in the THG signal, but absent in fundamental. In contrast to previous understandings from a purely mechanical deformation point of view, we demonstrate that this oscillation in THG directly results from a periodic modulation of the Fermi level in gold driven by acoustic-phonon excitation following ultrafast laser heating.

Third-harmonic generation microscopy in highly scattering media

The performance of third-harmonic generation (THG) microscopy in highly scattering media is analyzed with the Monte Carlo technique. The three-dimensional point-spread function (PSF) of the laser-scanning THG microscope with a pulsed excitation light source is derived for both isotropic and anisotropic scattering media and at different h/d(s) values, where h is the scattering depth as measured from the geometric focus of the objective lens and d(s) is the mean free path of the scattering medium. The generated THG signal is detected by a large-area photodetector. The PSF of the THG microscope is given by the third power of the normalized distribution of the excitation beam near the beam focus. The behavior of the temporal broadening of the excitation pulse on the generated THG signal is also analyzed as a function of h/d(s). The relative advantages and disadvantages of the THG microscope relative to the two-photon fluorescence microscope are discussed thoroughly.

Preparation of Thin Film of Layer Structured Bismuth Iodide with a Long Chain Alkylammonium and its Nonlinear Optical Property

We have examined to prepare a hybrid thin film consisting of bismuth triiodide and alkylammonium iodide by the conventional spin-coating technique. Their X-ray diffraction profiles and UV-Vis-NIR absorption spectra showed that the two-dimensional (Bi2I9 3−)x layer was sandwiched between alkylammonium layers by the molecular fastener effect. The third order nonlinear property of the thin film was estimated by the third harmonic generation (THG) measurements. Strong THG signals were observed from the hybrid thin film with stearylammonium iodide, which gave 10−11 esu order of χ(3) values.

Second and third optical harmonic generation in type I collagen, by nanosecond laser irradiation, over a broad spectral region

We report optical up-conversion of pulsed (ns) laser radiation in type I pure collagen, tuneable over a broad excitation spectrum covering the 760–1070 nm wavelength range. We investigated second harmonic generation (SHG) in collagen using λ p=1064, 901, 892, 828, 785 and 766 nm nanosecond pulsed laser excitation and recorded monochromatic signals at λ p/2, i.e. 532, 451, 446, 414, 393 and 383 nm, respectively, corresponding to the SHG optical process. The SHG signal intensity exhibited a quadratic dependence on the excitation radiation (log[ I 532]=1.92*log[ I 1064]). Furthermore, a weaker third harmonic generation (THG) signal from collagen was also observed at λ p/3 (355 nm) using 1064 nm nanosecond pulsed laser excitation. The THG signal was found to have near-cubic dependence upon the irradiation laser intensity (log[ I 356]=2.53*log[ I 1064]). The significance of collagen ability to exhibit broadly tuneable second harmonic generation is discussed.

Third order nonlinear optical properties of gold iodide with alongalkyl chain

Gold iodide with a long alkyl chain was fabricated into a thin film by the spin coating technique. Halogen doping was performed on the prepared film sample with a purpose of forming a halogen bridged structure of the gold complex. Their third-order nonlinear optical properties have been estimated by the thirdharmonic generation (THG). Iodine dopedgold iodideshowed large nonlinear optical property although the original undoped complex gave considerably weak THG signal.

Surface-plasmon-enhanced third-harmonic generation in thin silver films

Optical third-harmonic generation (THG) is dipole allowed. I report a 10(3) signal enhancement of THG that is due to the excitation of a surface plasmon in thin silver films by an attenuated-total-internal-ref lection geometry. Because the THG signal depends on the cube of the incident intensity and the second-harmonic generation depends on the square of the intensity, the THG overtakes the second-harmonic generation at laser intensities beyond 6 x 10(11) W/cm(2).

Optical third harmonic generation study of the hydration of DNA films

Optical third harmonic generation (THG) has been observed for the first time from DNA films. The THG signal is observed from NaDNA films exposed to relative humidities (RHs) between 0% and 98%. A strong enhancement (approximately 5x) of the THG signal from NaDNA is observed at 84% RH; no enhancement is observed for RbDNA. The most likely mechanism for such an enhancement is an increased coherence length. A model calculation using estimates of the refractive indices at both the fundamental and third harmonic frequencies supports this interpretation. The observed THG signal has the same polarization as the incident (fundamental) light. For the A conformation, the THG signal polarized perpendicular to the helical axis is approximately twice as strong as the signal polarized parallel to the helical axis. No such anisotropy is observed for either the disordered conformation (below about 50% RH) or the B conformation (above 92% RH).

Study of diacetylene monomer and polymer monolayers using second- and third-harmonic generation

Monolayers of several diacetylene monomers and polymers spread at the air–water interface have been studied using optical second-harmonic generation (SHG) and third-harmonic generation (THG). Owing to the centrosymmetry of the diacetylene core, SHG from these molecules arises mainly from their side groups. THG from a single monolayer of polydiacetylene is reported, to our knowledge, for the first time—the THG signal (in reflection) arising from the polydiacetylene monolayer is several times larger than that from the water subphase. THG was observed both when a polymer monolayer was spread directly and when a monomer monolayer was UV polymerized on the water surface. Values of the third-order nonlinearity of polydiacetylenes derived from these measurements are in agreement with earlier studies.