RU2267145C2 - Non-linear optical environment - Google Patents

Abstract

FIELD: the invention refers to optics.

SUBSTANCE: it may be used for protection of photo receiving devices from laser blinding by radiation of excessive intensity and at creation of non-linear optical limiters of radiation designed for protection of eyesight apparatus from damaging by laser radiation, for making low-threshold of optical switches. The purpose of the invention is in decreasing of energy threshold of the beginnings of non-linear -optical response of the environment. The non-linear -optical environment contains inorganic crystalline nanoparticles placed in transparent environment with linear optical characteristics. The nanoparticles consist of a nucleus not holding a shell having absorbing centers for example admixtures or defects. The shell and the nanoparticles are made of the same material. At impacting of laser radiation on non-linear-optical environment, photo generation of non-equilibrium electrons begins in the shell of the nanoparticle having defects out of an impurity zone into a conducting band. The changing of concentration of carriers lead to appearance of non-linear addition to the refraction index and absorbing of the shell. At photo generation of carriers a gradient of their concentration originates between the shell and the nucleus and as a result their diffusion begins deep into the nucleus. At that the thickness of the layer containing non-linear addition to the refraction index and absorption grows. As a result dielectric permittivity of a nanoparticle changes and that leads to changing of the section of its absorption and dispersion.

EFFECT: non-linear optical effect becomes apparent in limiting radiation.

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Description

The invention relates to optics and can be used in laser technology.

Known non-linear optical material selected as an analogue. Nonlinear optical material includes nanoparticles having a core of silver halides (AgCl and AgBr) with a shell of an island silver film [1]. Optical nonlinearity manifests itself in the form of radiation limitation. The mechanism of nonlinearity is the electrostriction effect enhanced by plasma resonance. The laser radiation restriction threshold is 15 μJ / cm ² , at λ = 10.6 μm, with a laser pulse duration of τ = 1 μs. The disadvantage of this material is the high energy threshold for the manifestation of a nonlinear optical response.

Also known non-linear optical material selected as an analogue. A nonlinear optical material consists of nanoparticles having an Au ₂ S core and an Au shell in a transparent polymer matrix [2, 3]. Nonlinear optical response is manifested in the form of enlightenment of the medium. The nonlinearity mechanism consists in a shift of the absorption band associated with plasma resonance with a change in the dielectric constant of the shell. The effect occurs when the radiation energy density is more than 100 μJ / cm ² , at λ = 865 nm, at τ = 50 fs. The disadvantage of this material is the high energy threshold for the manifestation of a nonlinear optical response.

Known nonlinear optical material [4], selected as a prototype, which is a suspension of semiconductor nanoparticles in the form of a core of Ag ₂ S with a shell of CdS. The threshold for the manifestation of a nonlinear optical response in such a medium is 0.5-0.7 J / cm ² , at λ = 0.532 nm, with a laser pulse duration of λ = 7 ns. The nonlinearity mechanism is absorption on free charge carriers. Nonlinearity is expressed in the limitation of laser radiation at an intensity exceeding the threshold. The disadvantage of this material is the high energy threshold for the manifestation of a nonlinear optical response.

The aim of this invention is to reduce the energy threshold for the occurrence of a nonlinear optical response of the medium.

The goal is achieved in that the core and shell of the nanoparticles consist of the same material with a band gap exceeding the photon energy by more than 1.3 times, and the core of the nanoparticle does not contain absorbing centers, and the shell contains absorbing centers that create deep impurity levels or impurity zone in the forbidden zone.

When laser radiation acts on a nonlinear optical medium containing inorganic crystalline nanoparticles, the core of which does not contain, and the shell contains impurity centers, the photogeneration of nonequilibrium electrons from the impurity band in the band gap to the conduction band begins in the shell containing defects. A change in the concentration of nonequilibrium carriers leads to the appearance of a nonlinear additive to the refractive index and absorption of the nanoparticle shell. When carriers are photogenerated in the shell, a concentration gradient arises between the shell and the nucleus, as a result of which carrier diffusion deep into the nucleus begins, and, consequently, the thickness of the layer containing a nonlinear additive to the refractive index and absorption increases. The dielectric constant of the nanoparticle changes, which leads to a change in its absorption and scattering cross sections. Since this process is single-photon, the nonlinear optical response of the medium arises at low laser radiation intensities.

This technical solution is new, and the combination of distinctive features does not follow from the known technical solutions. The essentiality of the distinguishing features is that the nonlinear optical material includes inorganic crystalline nanoparticles having a core and a shell of the same material, the core of the nanoparticle does not contain absorbing centers, and the shell contains absorbing centers that create deep levels or an impurity band in the band gap .

Specific embodiments of the invention.

TiO ₂ nanoparticles with a size of 100 nm, with a shell of the same material containing defects placed in a matrix - VM-4 vacuum oil. Defects in the crystal structure create an impurity band in the band gap. On the transmission spectrum of the nanoparticles there is a wide absorption band associated with transitions of electrons from the impurity band to the conduction band (Figure 1). The thickness of the cell containing TiO ₃ nanoparticles in VM-4 vacuum oil was 0.6 cm. The nonlinear response of the medium was measured using a YAG: Nd laser, for two radiation wavelengths of 0.53 μm and 1.06 μm, the laser pulse duration was 10 ns. Figure 2. The dependence of the energy density of radiation transmitted through the medium on the energy density of incident radiation is shown. From Figure 2. (curve 1) it can be seen that at a wavelength of λ = 0.53 μm with Q _in <0.15 nJ / cm ^{2 the} transmission is linear. When 0.15 nJ / cm ² <Q _Rin <7.5 nJ / cm ² arises optical nonlinearity, which manifests itself in the emission constraint. When Q _I > 7.5 nJ / cm ^{2 the} process of limitation stops. The dynamic range of the limitation is 50. A similar effect is observed at a wavelength of 1.06 μm (curve 2). Here, the restriction occurs when Q _I > 0.1 nJ / cm ² and stops when Q _I > 10 nJ / cm ² (Figure 2). The dynamic range of the limitation is 100. Thus, in a medium with TiO ₂ nanoparticles having a shell containing deep impurity levels for nanosecond laser pulses of the visible and near IR ranges, optical nonlinearity arises with an energy threshold of 0.1-0.15 nJ / cm ² .

CaF ₂ nanoparticles with a size of 100 nm, with a shell of the same material containing defects placed in a matrix - VM-4 vacuum oil. Defects in the crystal structure in the shell create an impurity band in the band gap. On the transmission spectrum of the nanoparticles there is a wide absorption band associated with transitions of electrons from the impurity band to the conduction band (Figure 3). The thickness of the cell containing CaF ₂ nanoparticles in the VM-4 vacuum oil is 1 cm. The nonlinear response of the medium was measured using a YAG: Nd laser for two radiation wavelengths of 0.53 μm and 1.06 μm, and the laser pulse duration was 10 ns. Figure 4. The dependence of the energy density of radiation transmitted through the medium on the energy density of incident radiation is shown. From Figure 4. (curve 1) it can be seen that at a wavelength of λ = 0.53 μm with Q _in <0.1 nJ / cm ^{2 the} transmission is linear. When 0.1 nJ / cm ² <Q _Rin <10 nJ / cm ² arises optical nonlinearity, which manifests itself in the emission constraint. When Q _I > 10 nJ / cm ^{2 the} process of limitation stops. The dynamic range of the limitation is 100. A similar effect is observed at a wavelength of 1.06 μm (curve 2). Here, the limitation occurs when Q _I > 0.5 nJ / cm ² (Figure 4). The dynamic range of the limitation is 30. Thus, in a medium with CaF ₂ nanoparticles having a shell containing deep impurity levels for nanosecond laser pulses of the visible and near infrared, an optical nonlinearity arises with an energy threshold of 0.1-0.5 nJ / cm ² .

It follows from the examples given that the use of a nonlinear optical medium containing inorganic crystalline nanoparticles with a defective shell to limit radiation reduces the radiation restriction threshold by 10 ⁸ -10 ⁹ times compared to the limitation threshold due to absorption on free charge carriers proposed in prototype.

The invention can be used to protect photodetector devices from being blinded by high-intensity laser radiation and due to the low threshold of limitation when creating nonlinear optical radiation limiters designed to protect organs of vision from damage by laser radiation, as well as to create low threshold optical switches.

LITERATURE

1. Sidorov A.I. Optical properties of a composite with silver nanoparticles in the range of 8-12 microns. Optical Journal, vol. 70, No. 2, pp. 9-14, 2003

2. Averitt R. D., Westcott S. L. et al. Ultrafast optical properties of gold nanoshells, J. Opt.Soc.Am. B, vol. 16, No. 10, p. 1814-1822, 1999.

3. Oldenburg SJ. Averitt RD, Halas NJ Patent 6344272 USA, MKI ⁷ V 32 V 15/02, priority from 11.03.1998.

4. Han MY, Huang C. H. et al. Large nonlinear absorption in coated Ag ₂ S / CdS nanoparticles by inverse microemulsion, J. Phys. Chem. B, v. 102, p. 1884-1886,1998.

Claims (1)

A nonlinear optical medium consisting of a transparent matrix and inorganic crystalline nanoparticles having a core and a shell, characterized in that the core and shell of the nanoparticles consist of the same material with a band gap exceeding the photon energy by more than 1.3 times, moreover, the core of the nanoparticle does not contain absorbing centers, and the shell contains absorbing centers that create deep impurity levels or an impurity band in the band gap.

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