Distinguishing between ultrafast optical harmonic generation and multi-photon-induced luminescence from ZnO thin films by frequency-resolved interferometric autocorrelation microscopy

Abstract

The nonlinear optical properties of thin ZnO film are studied using interferometric autocorrelation (IFRAC) microscopy. Ultrafast, below-bandgap excitation with 6-fs laser pulses at 800 nm focused to a spot size of 1 µm results in two emission bands in the blue and blue-green spectral region with distinctly different coherence properties. We show that an analysis of the wavelength-dependence of the interference fringes in the IFRAC signal allows for an unambiguous assignment of these bands as coherent second harmonic emission and incoherent, multiphoton-induced photoluminescence, respectively. More generally our analysis shows that IFRAC allows for a complete characterization of the coherence properties of the nonlinear optical emission from nanostructures in a single-beam experiment. Since this technique combines a very high temporal and spatial resolution we anticipate broad applications in nonlinear nano-optics.

Near-field-assisted localization: effect of size and filling factor of randomly distributed zinc oxide nanoneedles on multiple scattering and localization of light

“…Here, the phase relation between the exciting laser pulses and the emitted electric field from the sample is lost [35]. Therefore, the incoherent emission after two-pulse excitation does not show any oscillations at all when being detected at a fixed time delay.…”

“…This MPL consists of a comparably narrow emission band around 380 nm arising from direct photoemission from ZnO excitons and of a 100 nm broad emission band around 500 nm originating from point-defect-induced photoluminescence [30,41]. Since for our broadband excitation laser pulses, both coherent SH and incoherent MPL occur in the same spectral region, the reconstruction of the local fundamental electric field from the SH intensity requires a careful separation of both contributions [23,35]. For this, we excite the sample with coherent phase-locked pair of laser pulses separated by a delay τ.…”

“…The SH radiation generated in the ZnO needle array is collected in reflection geometry by the same Cassegrain objective, separated from the fundamental reflected laser light using a dichroic beam splitter and analysed in a spectrometer. By using a phase-locked pulse pair for excitation, coherent SH emission from the needles can be distinguished from incoherent multiphoton luminescence [23,35].…”

“…The ZnO sample is mounted on a 3-D piezo-stage (Physik-Instrumente GmbH, NanoCube) at an oblique angle of 30° with respect to the linearly p-polarized incident beam, maximizing the electric field component along the z-axis of the ZnO needles and enhancing the SH signal [35]. By raster-scanning the ZnO sample through the focus along the x-and y-direction in the coordinate system of the sample with a precision of 5 nm two-dimensional maps of the linear and/or nonlinear optical emission of the investigated sample are obtained (Fig.…”

Nonlinear plasmon-exciton coupling enhances sum-frequency generation from a hybrid metal/semiconductor nanostructure

“…To time resolve the surface plasmon fields of the nanosponge, we performed interferometric frequency-resolved autocorrelation (IFRAC) measurements ( Fig. 1a) [24][25][26] . The sample is excited by a pair of broadband, phase-locked 8-fs pulses, time-delayed by τ.…”