RU2371259C1 - Method of generating acoustic waves - Google Patents

Abstract

FIELD: physics; acoustics.

SUBSTANCE: invention relates to acoustoelectronics and can be used in generators, filters, delay lines and other devices which use surface acoustic waves. Oscillations are excited by effect of laser radiation on an opal matrix at wavelength in the region of its photonic band gap. The opal matrix is cooled to temperature below 200 K. Inter-spherical spaces of the opal matrix can be filled with ethanol.

EFFECT: wider range of generation of three-dimensional surface acoustic waves to at least 22,5 GHz with considerable increase in energy conversion efficiency.

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Description

The invention relates to the field of electronics, namely acoustoelectronics, and can be used in the design and manufacture of various devices that use bulk and surface acoustic waves, such as generators, filters, delay lines, radio signal processing systems and others.

There are various methods of generating in solid-state systems of bulk and surface acoustic waves [1-3]. In the case of surface acoustic waves (SAWs) propagating in a thin layer of a substrate, the generation methods differ in the mechanisms of perturbation of the surface (surface layer) of such a system. Basically, the methods used are based on the use of piezoelectric materials and interdigital transducers, consisting of metal electrodes "inserted" into each other (and placed on the surface), so that each is connected to a common contact bus to which electrical energy (voltage ) As a result, the distribution of the electric field established in the system has a periodicity determined by the spatial period of the electrodes. There are a lot of different types of interdigital converters, but the devices in which they are used mainly come down to the following: filters, delay lines, generators, resonators.

Depending on the wavelength range used, different types of lithography are used to create interdigital transducers, however, for a number of reasons, the main of which are the difficulties of obtaining short periods and large absorption coefficients, the generation and use of acoustic waves, in particular surfactants, with frequencies above 4-5 GHz is not appropriate and difficult using the indicated classical schemes [1].

Along with surface acoustic waves, bulk acoustic waves are used, for example, in devices such as resonators manufactured using piezoelectric crystals, in particular quartz. Since the action of such resonators is based on the multiple reflection of body waves from opposite surfaces, obtaining high Q factors is limited by volume losses, and the frequency range (usually not exceeding 300 MHz) is limited by the thickness of a piezoelectric resonator plate, which is comparable to the wavelength. Therefore, to expand the frequency range, resonators and SAW filters are used using film structures of various materials deposited on substrates. However, even when using diamond-like films and modern lithographs, the achieved frequencies do not exceed 5 GHz.

It is also known that the interaction of pulses of high-power laser radiation with a dielectric medium can lead (under certain conditions) to the generation of intense hypersound and cause an energy exchange between the phonon and photon modes. For example, there are known methods [1-4] for generating hypersound in isotropic and anisotropic media using photoelastic and thermoelastic effects in which the frequency range does not exceed 1 GHz, in any case, with coefficients for converting external energy into acoustic waves that are sufficiently effective for practical use.

The closest analogue (prototype) is the method described in [2], which uses an anisotropic medium in the form of a crystal or polycrystal of zinc deposited on it with layers of zinc oxide. We used a pulsed (100 femtoseconds) Ti-sapphire laser at a wavelength of 800 nm with a pump energy of 2 nJ in each pulse and a beam diameter of 40 microns. The excited hypersonic frequency corresponded to the excitation frequency (76 MHz).

The technical result of the proposed method is to significantly expand the generation range of volumetric and surface acoustic waves to at least 22.5 GHz obtained experimentally, with a significant increase in the coefficients of conversion of external energy into acoustic waves (up to 44%) (table).

The proposed method is based on the creation of acoustic vibrations corresponding to the natural frequencies of SiO ₂ nanospheres (of which the regular packing of the opal matrix consists) due to the interaction of electromagnetic waves of the optical range (photons) with acoustic modes (phonons) of this material. In this case, the used range of electromagnetic radiation is selected from the region of the forbidden photon zone, since at the edge of the zone the density of photon states increases significantly.

Since the properties of a hypersonic wave are determined by the parameters of the medium used, it is proposed to use opal matrices to generate sound waves, which are close-packed (mainly according to the cubic law) very close in diameter SiO ₂ nanospheres, the sizes of which in different samples can vary from 200 to 400 nm . Figure 1 shows the structure of the opal matrix and the sublattice of voids in the opal matrix [4]. The regular packing of SiO ₂ nanospheres forms a three-dimensional lattice with the indicated periodicity and can be described as a three-dimensional nanogrid of voids playing the role of microresonators, and the whole system as a whole is a photonic crystal or a material with a photonic band gap. In such structures, there are two types of interspherical voids forming tetrahedral and octahedral nanocavities with sizes from 50 to 150 nm for the indicated sizes of nanospheres. Such voids occupy about 24% of the total volume and can be filled with various substances.

In the experiments performed for samples with regular packing of SiO ₂ nanospheres, in which nanowells (inter-spherical space) form a system of microresonators, to observe acoustic vibrations, we used an experimental scheme based on measuring the frequency of radiation scattered by hypersound in the forced mode. Excitation was carried out by focused radiation of a ruby laser. As samples, opal matrices with a nanosphere diameter of 200-300 nm were used. The samples used were 3 × 5 × 5 mm in size and were cut parallel to the (111) plane. We studied both opal matrices consisting of close-packed nanospheres of amorphous silica with diameters in the range of 200-300 nm and monodispersity of at least 5%, and nanocomposites (opal matrices with voids filled with ethanol) in the experimental setup shown in Fig. 2, in which presents (in the form of a diagram) a method for implementing the present invention with the following notation: 1 - ruby laser, 2, 5, 11 - glass plates, 3 - system for monitoring laser parameters, 4, 12 - direct energy systems for measuring scattered light reverse, 6, 13 - Fabry-Perot interferometers, 7, 14 - registration system spectra 8 - mirror 9 - lens, 10 - opal matrix. In all experiments, a ruby laser was used, whose frequency range (694.3 nm) of the radiation is in the photonic band gap of the opal matrix (650-750 nm for the matrices used).

Spectral measurements were carried out using a Fabry-Perot interferometer with different bases, which made it possible to change the dispersion region from

0.42 cm ^-1 to 1.67 cm ^-1 . The exciting radiation was focused into the substance by lenses with different focal lengths (50, 90, and 150 mm). The distance of the opal matrix from the focusing system and the energy of the exciting radiation varied, which made it possible to carry out measurements for different pump power densities in the opal matrix and for different field distributions inside the sample. The studies were carried out under various energy and geometric conditions. For an unfilled opal matrix with a backward scattering geometry with a pump power density greater than 0.12 GW / cm ² , one component was found in the spectrum, shifted relative to the component corresponding to the exciting radiation, by 0.44 cm ^-1 - Fig. 3 shows the scattering spectrum in opal the matrix in the direction of the geometry "back" with the dispersion region of the Fabry-Perot interferometer 0.833 cm ^-1 .

It should be noted that the opal matrix (≈24% of the volume of which is accounted for by inter-spherical voids) filled with ethanol became almost transparent, since the refractive indices of the components of such a nanocomposite (opal matrix and liquid) are close. The latter made it possible to observe the scattering of light in the forward direction. In the case of experiments in the "back" direction at a pump power density of more than 0.12 GW / cm ² for opal matrices filled with ethanol, one component was observed with a frequency shift of 0.4 cm ^-1 ; an increase in the density of the pump power to 0.21 GW / cm ² led to the appearance of the second component with a frequency shift of 0.65 cm ^-1 .

The table shows examples of the proposed method for generating acoustic waves (for the samples used and various experimental geometries) in the form of the following data: conversion coefficients of laser generation and frequency of generation of sound vibrations, as well as frequency shifts of scattered radiation components at room temperature and upon cooling with liquid nitrogen .

Table of Examples 1-8 Example No. / conversion coefficient of laser generation,% Anisotropic medium Dispersion geometry Properties of the opal matrix The magnitude of the shift, ν, cm ^-1 Hypersonic frequency, GHz Room temperature (300 K) 1/4 Opal matrix back Diameter of nanospheres 250 nm, monodispersity - 4.5% 0.44 8.8 2/8 Opal matrix back Nanosphere diameter 250 nm, monodispersity - 8.0% 0.39 7.8 3/12 Ethanol filled opal matrix back Diameter of nanospheres 220 nm, monodispersity - 4.0% 0.63 12.6 4/10 Ethanol filled opal matrix "forward" Diameter of nanospheres 290 nm, monodispersity - 4.5% 0.37 8.8 The temperature of the opal matrix upon cooling with liquid nitrogen (120 K) 5/4 Opal matrix back Diameter of nanospheres 350 nm, monodispersity - 5.0% 0.38 7.3 6/6 Ethanol filled opal matrix "forward" Nanosphere diameter 250 nm, monodispersity - 7.5% 0.40 8.0 7/24 Ethanol filled opal matrix "forward" Diameter of nanospheres 250 nm, monodispersity - 3.0% 0.77 15.7 8/44 Ethanol filled opal matrix "forward" Diameter of nanospheres 200 nm, monodispersity - 4.5% 1.13 22.6

Under the same conditions and laser radiation powers when exposed to an anisotropic medium in the form of a ZnO sample (layers) / Zn (according to the prototype), hypersound was obtained, presumably in the 70 MHz region, with a laser generation conversion coefficient of less than 1%.

The high conversion efficiency (over 40%) of laser generation into radiation at a displaced frequency by the proposed method indicates high intensities of hypersound excited in the samples under study. The efficiency of stimulated emission in the blue-green region is ensured by anomalously high densities of photon states near the edges of the photon band.

Information sources

1. Surface acoustic waves. Edited by A. Oliner. M .: Mir, 1981, 390 p.

2. T. Pezeri, P. Ruello, S. Gougon. Generation and detection of plane coherent shear picosecond acoustic pulse by lasers: experiment and theory. Physical Rev. B 75.174307 - 1-9. (2007).

3. Gulyaev Yu.V. Acoustoelectronics. UFNu, vol. 175, No. 8, pp. 887-893 (2005).

4. Nanomaterials: Photonic crystals and nanocomposites based on opal matrices. Ed. M.I. Samoilovich. M.: Tekhnomash, 2007, 303 p.

Claims (3)

1. A method of generating acoustic waves using the action of laser radiation on an anisotropic medium, characterized in that the oscillations are excited by the action of laser radiation on the opal matrix at a wavelength in the region of its photonic band gap. 2. The method according to claim 1, characterized in that the opal matrix is cooled to a temperature of less than 200 K. 3. The method according to claim 1 or 2, characterized in that the inter-spherical voids of the opal matrix are filled with ethanol, while deviations from the average (monodispersity) for the diameters of SiO ₂ nanospheres from the range of 200-300 nm do not exceed 5%.

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