Dynamic optical gratings in two layer system: ferroelectric and nanostructured metal film

Abstract

ABSTRACT Dynamic reflection gratings, recorded by the single laser beam in ferroelectric crystals covered by nano-structured golden film generate optical and electrical pulsations. Freque ncy and amplitude of pulsations depend on laser intensity, the ambient pressure and temperature. Model explaining pulsations are developed based on the photogalvanic and pyroelectric mechanisms of holographic grating recording. Possible applications for the remote pressure and temperature sensing are discussed. Golden nanostructured film increase significantly sensitivity of the crystal pulsations to laser intensity, temperature and pressure. For a focused HeNe laser beam (intensity about 160mW/cm 2 ) an interesting phenomenon (deserving special considerations) of self-phase conjugation was observed. 1. INTRODUCTION Recording of the reflection-type hologram, using single be am and placing holographic medium between laser and object, is known as Denisyuk-Lippman holograms [1-3]. In this paper we describe recording of the dynamic reflection or “mirror” hologram (holographic grating) in ferroelectric crystal iron doped lithium niobate (Fe:LN). In our case single laser beam illuminate z-cut LN crystal sample (1x1x0.1cm ) with rear side coated by gold film. Incident and mirror-reflected from the rear surface beams form interference pattern, that is recorded in the crystal as a dynamic transient holographic grating. Self diffraction of the recording beam on this reflection-type grating change intensity of the beam reflected from the rear crystal surface. In the same time laser illumination generate photogalvanic current [4, 5], that charge crystal to high voltage, that may produce ionization of the near-surface air and electrical discharges. Electrical discharges are picked-up by the capacitance-type electrodes and are detected by oscilloscope. These electrical discharges are also influence self-diffraction, leading to quasi-periodic modulation (pulsations) in the optical reflected signal. It is important to note, that both electrical and optical pulsations are produced by the CW laser with constant intensity. Both pulsations are due to the photo induced charging processes in the volume due to the photogalvanic and pyroelectric effects and to the discharging near the crystal surface. In our experiments we observed dependence of the frequency of pulsations on the laser intensity, ambient pressure and temperature. To explain these experiments, we suggest physical model that describe both electrical and optical pulsations. In our theoretical model we w ill use two systems of equation: material photorefractive equations that include pyroelectric effect and Maxwell wave equations.