RU2555503C1 - Optical passive limiter of passing radiation - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: this limiter comprises translucent insulating substrate with semiconductor film on its surface to increase electric conductivity at heating by passing radiation and to be placed in the lens focal area. Said film is structured as a grid or mosaic of spots with mean periodicity of the structure smaller than radiation wavelength.

EFFECT: higher performance, lower threshold of operation.

4 cl, 3 dwg

Description

The invention relates to optical and optoelectronic technology, to devices for protecting the photosensitive elements of optical and optoelectronic systems from the damaging effects of powerful radiation.

To protect sensitive receivers of radiation from damage by powerful radiation, optical passive limiters of transmitted radiation are studied, which are analogues of the invention, based on the use of liquid sols or solid sols in the form of thin films containing fullerenes, carbon nanoparticles or metals [Kamanina N.V. Photophysics of fullerene-containing media: laser radiation limiters, diffraction elements, dispersed liquid crystal light modulators // Nanotechnology. No. 1, 2006]. The radiation passes through a sol layer (in the form of a film), which has a transparency of 50-70%, to the protected receiver; with increasing radiation intensity, absorption in the medium nonlinearly increases to a certain opacity of the medium and the receiver is protected from damage by powerful radiation.

The disadvantage of this protection is the impossibility of limiting the radiation of the middle and far infrared ranges of the spectrum, since the limiting effect is based on the photoinduced increase in the absorption cross section of nanoparticles, which is manifested only in the visible and near infrared ranges.

An analogue of the present invention is also a mesh filter of long-wave infrared radiation [Long-wave infrared spectroscopy. Digest of articles. Per. from English edited by V.N. Murzina. - M .: World. 1966. - 320 p., Pages 292-315]. A wire mesh with a period much shorter than the radiation wavelength, and a “reverse mesh” obtained on a substrate by sputtering metal through a mesh like a mask are mirrors with high reflectivity in the long-wave infrared range. The condition for the possibility of using a mesh filter as a mirror is the requirement that the thickness of the skin layer in the metal of the grids at a given wavelength be less than the thickness of the jumpers of the grid.

The use of such a filter as a limiter of powerful radiation with transmittance depending on the radiation intensity is not known.

The prototype of the claimed invention is an optical limiter operating at a wavelength of 10 μm, using a first-order dielectric-metal phase transition [Mikheeva OP, Sidorov A.I. Features of the limitation of laser radiation by vanadium dioxide mirrors // Optical Journal. Volume 68, No. 4, 2001]. The paper presents the results of an experimental study of the limitation of laser radiation of the mid-IR range by vanadium dioxide mirrors. Vanadium dioxide is a semiconductor capable of undergoing a dielectric-metal phase transition at heating temperatures below 100 ° C with an increase in electrical conductivity and an increase in reflectance in the infrared range.

The disadvantage of this limiter is the lack of speed - the delay time of the beginning of the limitation of the order of 100 ns.

The problem solved by the present invention is the creation of an optical limiter of transmitted radiation in the infrared range of the spectrum with increased speed and a reduced threshold for limiting radiation.

The problem is solved in that in an optical passive limiter of transmitted radiation, containing a transparent insulating substrate with a semiconductor film on the surface that increases the electrical conductivity when heated by transmitted radiation and placed in the focal plane of the lens, in accordance with the invention, the film is structured in the form of a grid or mosaic of spots with an average periodicity of the structure, shorter wavelength of radiation.

It is also proposed that said structured film is thermally insulated from the substrate by an intermediate transparent dielectric layer. It is also proposed that the specified structured film is placed on both surfaces of the substrate, and the thickness of the substrate does not exceed the depth of field of the lens.

It is also proposed that the structured grid is made of elementary semiconductors or semiconductor metal oxides with a negative temperature coefficient of resistance, including vanadium, titanium, nickel, cobalt, iron, hafnium, aluminum, or oxide mixtures.

Figure 1 shows schematically the device and the optical application circuit of the limiter, figure 2 is a grid structure of an absorbing film of a limiter according to the invention, Figure 3 is a device according to the invention with grid structures located on two sides of the substrates.

Figure 1: 1 - transparent substrate, 2 - structure elements of the absorbing grid layer of the limiter in the form of square thin-film islands separated by gaps from changing the electrical conductivity during laser exposure to materials, 3 - radiation entering the limiter, 4 - radiation coming out of the limiter in the absence of the restriction effect , 5 - radiation reflected from the grid layer upon the occurrence of the restriction effect, L1 and L2 - confocal lenses.

Figure 2: 2 - the structure elements of the grid layer of the limiter located on the surface of the transparent substrate and having, for example, the shape of square islands separated by gaps, 6 - the structure in the form of a grid or islands of the mesh layer of the limiter located on the surface of the transparent substrate, the mesh cells can have the shape of a square, as shown in the figure as an example, or may have a different shape, 7 - zone of irradiation of the grid structure by laser focused radiation.

Figure 3: 8 - structure in the form of a grid or islands of a grid layer on the side of the substrate 1 with a thickness h opposite to the irradiated; 9 - a light beam incident on a substrate, creating a focal spot on the substrate and having a constriction with a diameter D _f ; 10 - transmitted radiation beam.

As shown in FIG. 1, the limited radiation 3 is focused by the lens L1 on the surface of the substrate 1, on which is located a thin structured film of a material capable of changing (for example, increasing) its electrical conductivity when heated. The focal spot on the surface must have dimensions at which several (on the order of several tens or more) structural elements of the grid fall on the spot area. The grid period should be a small fraction of the radiation wavelength. At a low radiation intensity, the heating of the mesh layer with a laser beam is negligible, its electrical conductivity is small, the reflection from the mesh structure is small, and the radiation passes through the structure of the limiter, practically without changing its intensity, and is collected in a parallel beam by the second lens. The optical scheme may be different, it is only necessary to ensure the concentration of incident radiation in the focal spot of small sizes on a substrate with a mesh layer. With an increase in the radiation intensity, the temperature of the structural elements of the grid that fall within the focal spot increases and their conductivity increases, which leads to an increase in the reflection coefficient of the grid in the region of the focal spot and prevents an increase in the intensity of the radiation passing through the structure and limits the radiation intensity. The right figure of Figure 1 shows the path of the rays of the reflected beam 5, corresponding to the case of reflection of radiation from the network structure. The reflection of radiation is close to specular reflection, the role of scattering is negligible.

Let us quantify the effect of radiation restriction by grids using the formula for the reflection coefficient of a wire grid with a normal incidence of radiation on the grid [Long-wave infrared spectroscopy. Digest of articles. Per. from English edited by V.N. Murzina. - M .: World. 1966 .-- 320 s. Pages 292-315]:

where the quantity G determines the dependence of reflection on the electrical conductivity σ of the wire material, and tgΦ depends on the geometry of the lattice:

where Φ is the phase of the reflected wave, a ′ is the distance between the centers of adjacent wires, d is the diameter of the wires, m is an integer, the serial number of a member of the series.

и $$G<<\frac{2a\text{'}}{\lambda }\ln \frac{a\text{'}}{\pi d}$$

, значение $$H\left(\frac{\lambda }{a\text{'}}\right)=0$$

.Provided $$\frac{a\text{'}\text{'}}{\lambda }<<\mathrm{one}$$

and $$G<<\frac{2a\text{'}\text{'}}{\lambda }\ln \frac{a\text{'}\text{'}}{\pi d}$$

, value $$H\left(\frac{\lambda }{a\text{'}\text{'}}\right)=0$$

.

Estimates show that in the case of parameter values corresponding to our problem, tgΦ = 0 and the formula for the reflection coefficient can be represented as:

At a low radiation intensity, the electrical conductivity of the grid is zero, G ² → ∞, R = 0, the radiation passes through the grid unhindered.

; это означает, что проходящее излучение ограничивается.Under the influence of high-power radiation due to the phase transition of the insulator-metal substance of the network, the conductivity of the network can increase to a value of one hundredth of the conductivity of copper (σ (Cu) = 5.14 · 10 ¹⁷ s ^-1 (in the Gaussian system)) and more. At a wavelength of λ = 10 μm and with mesh parameters a ′ = 2 μm, d = 0.5 μm, we obtain a decrease in G ² to 0.01, an increase in reflection to $$R=\frac{\mathrm{one}}{\mathrm{one}+0.01}\approx \mathrm{one}$$

; this means that the transmitted radiation is limited.

The grid structure of the film leads to light scattering due to diffraction effects on periodic inhomogeneities. At a low radiation intensity, film inhomogeneities affect the incident radiation as phase inhomogeneities of the surface of a transparent substrate. In our case, the inhomogeneities are in the form of disks on the surface or indentations in a substrate with a heterogeneity diameter much smaller than the radiation wavelength. In the work [Born M., Wolf E. Fundamentals of optics. M .: Science. Ch. ed. Phys.-Math. lit. - 1970. - 855 p.] Presents the results of the calculation of the cross section for scattering of light on single dielectric spheres, based on the Mie theory. With the diffraction parameter of the sphere 2πρ / λ << 1 (ρ is the radius of the sphere), the scattering cross section is much smaller than the real cross section of the sphere, which indicates the insignificance of the scattering effect. We extend this conclusion to scattering by particles in the form of small disks, as well as scattering by small depressions (the latter - in accordance with the Babinet principle).

Thus, the scattering of transmitted light through the grid structure of the limiter at a low radiation intensity is negligible.

A necessary condition for the functioning of the mesh structure on the surface as a limiter of transmitted radiation is the smallness of the periodicity parameter in comparison with the wavelength of the limited radiation:

On figa) shows the network structure of the film on the surface of the substrate, in which the occurrence of electrical conductivity is induced during laser irradiation, the network jumpers are shaded, in fig.2b) - the island structure of the same film, the islands are shaded. The structure is shown to be strictly periodic with periodicity a ′ as one of the possible options, since a random arrangement of the gaps of the grid or islands is possible, provided that the average distance between the centers of these cavities in the grid or islands remains much less than the radiation wavelength. The shape of the inhomogeneities can also be different, for example, the gaps in the grid can be round, and the islands can be disks. The focal spot of radiation is shown by shaded circles.

It is assumed that the mesh and island structures are not located directly on the surface of the substrate, but are separated from the surface by a layer of bonding or heat-insulating material.

We will compare with the prototype according to the threshold of operation of the limiter, that is, by the magnitude of the pulse energy of the radiation being limited, at which the limiter goes into non-linear mode and begins to limit the transmitted radiation.

In the prototype, it is necessary to bring the mass of material in the volume of a continuous film on the entire surface of the substrate of the limiter to heating to the temperature of the dielectric-metal phase transition and to the occurrence of specular reflection;

in the proposed solution, it is necessary to heat only the mass of jumpers of the mesh structure, the volume of which is much smaller than the volume of the continuous film, since there are gaps in the structure (or it consists of islands with gaps between the islands). Thus, the threshold of operation of the limiter according to the invention can be significantly less than that of the prototype.

Figure 3 shows a device in which mesh structures are applied on a substrate on both sides thereof. The mesh structure of each side is a radiation limiter, as a result, the total transmittance of the device is equal to the product of the transmittance of the structures on both sides. To ensure the same thermal effect of laser radiation on the mesh structures on the sides of the substrate, it is necessary that the sides are within the depth of field of the image created by the lens. Depth of field characterizes some selected acceptable level of image blur; image quality deteriorates gradually as the amount of defocus increases. The depth of field tolerance, corresponding, for example, to the Rayleigh quarter-wave criterion, is given by the equation

where U _m is the angle of inclination of the side beam in the image (focal spot), λ is the wavelength of light, n is the refractive index of the medium.

The depth of field can also be set in units of the diameter of the focal spot as 2-3 diameters of the focal spot, which requires that the substrate has an optical thickness of no more than 2 diameters of the focal spot (diameter of the laser beam waist).

As examples of limiter designs, let us consider films based on the use of metal oxides (vanadium dioxide, titanium dioxide) and based on the use of an amorphous or polycrystalline silicon film.

In the first case, polished wafers are used as a substrate from an infrared-transparent material such as calcium fluoride, magnesium fluoride, and germanium. On one or both sides of the substrate, a layer of vanadium dioxide or titanium dioxide with a thickness of about 0.1-0.25 microns is sprayed and the required structures are created in the sprayed layers by photolithography. When creating a limiter for the wavelength range of 5-10 μm, the required parameter of the periodicity of the structure is no more than 1-2 μm (a ′ = 2 μm, d = 0.5 μm, λ = 10 μm). When using TiO ₂ [Physicochemical properties of oxides. Directory. Ed. G.V. Samsonova. M .: Metallurgy. - 1969. - 456 p.] The calculation gives the results:

573 K: σ = 10 Ohm ^-1 · m ^-1 = 9 · 10 ¹⁰ s ^-1 , G ² = 530 → R = 0;

1273 K: σ = 10 ⁵ Ohm ^-1 · m ^-1 = 9 · 10 ¹⁴ s ^-1 , G ² = 0,053 → R = 1.

It follows from the calculation that when laser heating the grid structure to 1273 K, the mirror becomes completely reflective and limits the transmitted radiation.

In the second case, the substrate is a plane polished single-crystal silicon wafer on both sides; on one or both surfaces there is a lattice or mosaic of spots made of amorphous silicon with a thickness of 0.1-0.2 microns; the size of the structural elements and the gaps between them are of the order of λ / 5. When exposed to laser radiation, islands of amorphous silicon are heated to a higher temperature than the surface of the substrate between them, due to significantly lower thermal conductivity than single-crystal silicon. This is facilitated by the embodiment of the device, when an amorphous material is deposited on top of a layer of silicon oxide on the surface of single-crystal silicon, since silicon dioxide has low thermal conductivity and low thermal diffusivity.

In a design with double-sided deposition of a mesh structure, the thickness of the substrate can be in the range h = 10-60 μm in the limiter for the mid-IR range.

In addition to silicon, other semiconductors can be used as mesh materials, including polycrystalline or amorphous ones, including germanium, double and triple semiconductor compounds, since all semiconductors have a negative temperature coefficient of resistance. Semiconductor metal oxides with a negative temperature coefficient of resistance, including oxides of vanadium, titanium, nickel, cobalt, iron, hafnium, aluminum, or mixtures of oxides, can also be used as mesh materials.

Thus, it is shown that the distinctive features of the invention allow to solve the tasks.

An optical passive limiter can be used in optoelectronics as an optical fuse that protects photodetector devices from possible radiation damage.

The technical result of the invention consists in the creation of an optical limiter of infrared radiation with increased speed, having a low threshold.

Claims (4)

1. An optical passive limiter of transmitted radiation, containing a transparent insulating substrate with a semiconductor film on the surface, increasing the electrical conductivity when heated by transmitted radiation and placed in the focal plane of the lens, characterized in that the film is structured in the form of a grid or mosaic of spots with an average periodicity of the structure, less radiation wavelengths. 2. The device according to claim 1, characterized in that said structured film is thermally insulated from the substrate by an intermediate transparent dielectric layer. 3. The device according to claim 1, characterized in that the specified structured film is placed on both surfaces of the substrate, and the thickness of the substrate does not exceed the depth of field of the lens. 4. The device according to claim 1, characterized in that the structured grid is made of elementary semiconductors or semiconductor metal oxides with a negative temperature coefficient of resistance, including oxides of vanadium, titanium, nickel, cobalt, iron, hafnium, aluminum, or mixtures of oxides.

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