RU2809776C1 - Optically transparent device for ir signal modulation - Google Patents

Abstract

FIELD: optical devices.

SUBSTANCE: invention is intended for use as part of glazing elements and means of visual display of information to provide parametric noise of IR radiation. An optically transparent device is a multilayer coating that includes layers of a transparent piezoelectric, transparent conducting layers or layers of a wide-gap semiconductor, matching layers. Operation of the device consists in applying voltage to the layers of metal or wide-gap semiconductor resulting in a change in the geometric and, as a consequence, optical thickness of the piezoactive layers, which leads to modulation of the phase of the transmitted IR radiation. To reduce acoustic noise, control voltage can be applied to the layers in such a way that changing the geometry of the piezoactive layers does not lead to a significant change in the total thickness of the coating and displacement of its centre of mass.

EFFECT: generation of parametric noise in order to prevent remote reading of information.

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Description

Field of technology

The invention relates to infrared technology and can be used in electrically controlled devices for modulating passing electromagnetic radiation, including parametric noise in order to prevent remote reading of information.

State of the art

An electro-optical modulator based on a twisted liquid crystal is known, described in US patent US6094246A, G02F 1/1335; S09K 19/0, date 07/25/2000. The modulator acts as a switchable half-wave plate that can selectively (in response to the application or non-application of an electric field) rotate the input linear polarization to 0° or 90°. In the absence of an electric field, the modulator rotates the polarization of the incoming light by 90°. When an electric field is applied, the polarization rotates by a smaller amount. This modulator provides relatively fast switching times (about 50 ms), and relatively large attenuation coefficients (up to minus 25 dB). The disadvantage of this structure is its opacity in the visible wavelength range. Also, the patent does not consider the possibility of using this structure for modulating signals in the IR wavelength range.

A waveguide structure with phase modulation is known, described in US patent US8014636B2, G02F 1/035; H01L 29/06; Н04 В 10/04, date 09/06/2011, which includes either layers of p- and n-doped silicon, between which an oxide layer is located, or p- and n-doped SiGe layers, epitaxially grown on a Si substrate, between which there is also a layer of oxide. In the first case, when a voltage is applied between the layers of p- and n-doped silicon, charge carriers accumulate on both sides of the thin oxide layer acting as a gate, which leads to a change in the refractive indices of these regions and, accordingly, to modulation of the energy passing through the waveguide signal. In the second case, a voltage is applied to the doped SiGe layers, as a result of which the energy of bound states inside a thin oxide layer (quantum well) is reduced due to the Coulomb interaction of electron-hole pairs. The effective band gap is modulated and a red shift is observed in the interband absorption, resulting in a decrease in the peak absorption value. As a result, the intensity of the laser light passing through the quantum well is modulated. In this case, light can be directed through the oxide/SiGe and SiGe/Si interfaces. The disadvantages of the proposed structures include the fact that the materials used are opaque in the visible wavelength range. Also, the patent does not consider the possibility of using these structures to modulate a signal in the IR wavelength range.

The closest analogues are the compact electro-optical modulator described in the US patent US 10732441 B2, G02F 1/025, date 04/08/2020 and the photonic integrated circuit described in the US patent US9535308B2, G02F 1/035; G02B 6/122; G02F 1/225, date 01/03/2017.

The compact electro-optical modulator is based on a visible-wavelength metal-oxide-semiconductor (MOS) integrated capacitor with a one-dimensional silicon photonic nanocavity. A MOS capacitor is a sequence of layers of ITO/SiO ₂ /Si (hereinafter ITO: indium tin oxide). Electro-optical modulation is achieved by switching the voltage across the plates of a given capacitor, resulting in a change in the refractive index of the semiconductor layers.

The photonic integrated circuit includes a diode based on a semiconductor-dielectric-semiconductor structure (SDI-structure) as part of an optical waveguide. This optical waveguide is part of an optical modulator, for example, a Mach-Zehnder interferometer. The dielectric barrier layer is an oxide or material with a high absorption rate. The semiconductor layers in a DMA diode may include geometric elements (such as a periodic pattern of holes or grooves) that form an optical waveguide on a lattice-shifted photonic crystal having a group speed of light lower than that of another semiconductor layer without geometric features. The application of an electrical voltage to the semiconductor layers leads to a change in the refractive index of part of the material in the optical waveguide, which in turn causes modulation of the optical signal passing through the waveguide.

The disadvantages of these technical devices (compact electro-optical modulator and photonic integrated circuit) include the fact that the device designs are designed to modulate signals in the visible and/or near-IR wavelength ranges passing through the waveguide path. This leads to a small size of the area in which the signal is modulated (about 1 μm), which does not allow these solutions to be used, for example, for parametric noise of large-area glazing objects in order to prevent remote reading of information.

Thus, no technical solutions have been discovered that represent a film structure that provides modulation of IR signals, while the film structure is transparent in the visible wavelength range.

The technical result of the operation of the proposed device is the modulation of the phase of IR radiation passing through a transparent multilayer film structure, when a voltage - a potential difference - is applied to the conductive layers, between which a transparent piezoelectric layer of this structure is located. As a result of using the proposed technical solution, a transparent multilayer film structure allows for parametric noise of IR signals in order to prevent remote reading of information due to phase changes.

Disclosure of the Invention

The optically transparent device is a multilayer structure that includes layers of transparent piezoelectric material, transparent conductive layers and dielectric matching layers. The operation of the device consists of applying voltage to transparent conductive layers, between which a layer of transparent piezoelectric is located, resulting in a change in the geometric and, as a consequence, optical thickness of the transparent piezoelectric layers, which leads to modulation of the phase of IR radiation passing through this device.

As transparent piezoelectric layers, layers of a piezoactive medium transparent in the visible range can be used, for example, based on ceramics with a perovskite structure (divalent metal titanate-zirconate, for example, lead (PZT) or others) or a polymer (for example, based on polyvinylidene fluoride and its copolymers) up to several tens of micrometers thick. Transparent conductive layers can be nanometer-scale (transparent to visible wavelengths) layers of metals such as silver or copper, or layers of transparent conductive wide-gap semiconductors such as ITO, zinc-doped alumina, indium oxide, gallium oxide, zinc oxide, and indium, gallium and zinc oxide. Dielectric media transparent in the visible range with significantly different refractive indices, for example, such as SiO ₂ , TiO ₂ and ZiO ₂ , can be used as matching layers. The thickness of the layers is selected based on the requirements for the amount of signal modulation and the frequency range.

To reduce acoustic noise, control voltage is applied to the layers in such a way that changing the geometry of the piezoactive layers (transparent piezoelectric layers) does not lead to a significant change in the total thickness of the structure and a displacement of its center of mass.

A special feature of this device is that it allows signal modulation in the IR wavelength range, while being transparent in the visible wavelength range. In addition, the size of the active region of the device can be made quite large (for example, an area of several square meters). Size restrictions are determined by the technological capabilities of the installation on which the layers of this device are applied.

Let us explain the operation of the device using the example of calculating the phase modulation of the transmitted IR signal through a multilayer structure consisting of layers arranged in the following order: ITO layer, piezoelectric material layer (PZT), ITO layer, SiO2 layer, ITO layer, piezoelectric material layer (PZT ), ITO layer, in the calculations it was assumed that voltage is applied to the ITO layers.

Numerical experiments were carried out on the example of structures with the following layer thicknesses:

- 20 nm ITO, 200 nm PZT, 200 nm SiO ₂ (structure No. 1);

- 200 nm ITO, 200 nm PZT, 200 nm SiO ₂ (structure No. 2);

- 500 nm ITO, 200 nm PZT, 200 nm SiO ₂ (structure No. 3);

The resistivity of the ITO layer was taken equal to 3⋅10 ^-5 Ohm⋅m.

The piezoelectric coefficient of the PZT layers was chosen to be 289 pC/N.

To calculate the properties of multilayer “one-dimensional” systems (such as a shielding layer on a thick substrate or multilayer optical coatings), it is convenient to use transfer matrices [Landau L.D., Lifshits E.M. Electrodynamics of continuous media. M.: Nauka, 1982], [Kaliteevsky M.A., Kavokin A.V. // FTT. 1995. T. 37. Issue 10. P.2721-2728], since the transfer matrix of a system of sequentially connected subsystems can be found by matrix multiplication of the transfer matrices of subsystems in the order corresponding to the sequence of their connection.

Further calculations are based on the following Maxwell equations (written for the SI system) and boundary conditions:

where H is the magnetic field strength, E is the electric field strength, D is electrical induction, B is magnetic induction, j is the current density due to macroscopic charge transfer - vectors, in addition, ρ is the charge density, n is the normal of the interface, and the subscripts (1 and 2) denote the number of the environment, ∇ is the differential operator, and:

∇ × Н ≡ rot Н, similarly for E,

∇ ⋅ D ≡ div D, similarly for B.

Also, unless explicitly stated otherwise, only linear effects will be taken into account; magnetoelectric effects, spatial dispersion and some other phenomena that complicate the description will not be taken into account, and therefore it is further assumed:

Where And - relative dielectric and magnetic permeabilities (tensors of the second rank). For isotropic media, the scalar relative permeabilities ε and μ can be used instead of these tensors.

ε ₀ and μ ₀ are the electric and magnetic constants, respectively.

From conditions (5) and (6) in the problem of a single incident plane wave it follows that the wave phase incursion on both sides of the boundary is equal when displaced along the interface between the media, and therefore the projection of the wave vector onto the interface between the media must be the same on both sides of the boundary . At the same time, for an isotropic material, regardless of the orientation of the wave vector, the following equality holds:

where k is the wave vector, k ₀ ≡ ω/s, ω is the frequency of the electromagnetic wave, c is the speed of light.

Having chosen a coordinate system (Cartesian coordinate system 0xyz) so that the wave vector lies in the yz plane, we find:

where θ is the angle of incidence of the wave relative to the z axis in free space, that is, the angle of incidence of the wave on the interface between media from space (half-space) outside the multilayer structure.

Let's consider two cases: an s-polarized (TE - from Transverse Electric) wave and a p-polarized (TM - from Transverse Magnetic) wave falling at an angle θ to the normal of the interface. In further discussions, we will denote the amplitudes and other characteristics of forward and backward propagating waves by the indices “+” and “-”, and the designations of their values valid for one of the half-spaces will be provided with an apostrophe (the designations of the corresponding values for the second half-space will be given without an apostrophe).

In the case of s-polarization in the chosen coordinate system, the vector E is everywhere directed along the x axis. Thus, in accordance with equation (2), for the H component directed along the interface, the following is true:

Taking conditions (5) and (6) into account, we find:

Thus, the transfer matrix of the electrical component of waves with s-polarization at the interface has the following form:

The transfer matrix connecting the amplitudes of waves (of any polarization) at the boundaries of a layer of thickness h on the inner side has the following form:

where i is the imaginary unit.

Thus, the transfer matrix relating the electrical amplitudes of s-polarized waves outside the boundaries of a layer of thickness h located in a vacuum can be calculated as follows:

Multiplying the matrices in equation (18) and making substitutions based on the above equalities, we find:

where I is the identity matrix.

In a similar way, transfer matrices can be obtained for the magnetic component of p-polarized waves at the interface between the media and outside the boundaries of a layer of thickness h:

In a homogeneous isotropic material, the following relationship between the electric and magnetic components of a plane electromagnetic wave is valid:

Where - wave impedance of vacuum.

Consequently, the transition from magnetic transfer matrices to electric ones can be carried out as follows:

Applying expression (23) to (20) and (21), we find:

Thus, the propagation of a plane monochromatic wave incident at an angle θ to the normal through a homogeneous isotropic layer of material with thickness h in a vacuum is described by the following equations:

Where or - components of the transfer matrix (T) for s- and p- polarization of the incident wave, respectively.

In the case of a thin electrically conductive layer, it is necessary to take into account the contribution of specific conductivity to the effective dielectric constant. At frequencies significantly lower than the plasma frequency (i.e., when the charges in the conductor can be considered to respond to the field instantly), this can be expressed as follows:

where ε ₂ is the imaginary part of the dielectric constant, σ is the specific conductivity, ε ₀ is the electrical constant.

At sufficiently low frequencies, when the contribution of specific conductivity to the effective dielectric constant is decisive, the thickness of the skin layer can be determined as follows:

where μ ₀ is the magnetic constant.

In accordance with this, equation (26) for the electrically conductive layer is rewritten as follows:

where R _S is the surface resistance of the material.

The passage of a wave through a structure can be described by the following expression:

where a is a vector describing the field at the output of the system, b is a vector describing the field at the input to the system, T is the transfer matrix for the dielectric (26) or electrically conductive (32) layer.

where b[0] is the electric field strength of the incident wave at the input, b[1] is the electric field strength of the reflected wave at the input, and [0] is the electric field strength of the transmitted wave at the output, and [1] is the electric field strength of the backpropagating wave waves at the output (if there is a vacuum behind the system under consideration, then a[1] = 0).

Since the transfer matrix is square 2x2, we can write:

Since a[1]=0, expressions (37) and (38) are rewritten as follows:

To calculate the change in phase of a wave passing through a structure, it is necessary to look at the change in the transmitted wave relative to the incident wave, which is described by equation (39). Information about the phase change will be contained in T ₁₁ .

Since the matrix T is complex, the change in phase of the transmitted wave can be calculated as follows:

The phase modulation of the transmitted signal as a function of the applied voltage can be calculated as follows:

where MF is phase modulation, V ₁ and V ₂ are voltages applied to the conductive layers (indices indicate different values of applied voltages).

Based on the model presented above, a numerical experiment was carried out to calculate the modulation of the phase of the transmitted signal of the multilayer structure described above in the spectral range of 780-2600 nm. The phase of the wave passing through the structure in the simulated spectral range ranges from -π to π. The phase modulation in a given spectral range, which is the phase difference passed through the wave structure under consideration at voltages of 0 and 50 V, lies in the range from 0 to π.

The average value of phase modulation for the spectral range under consideration was 0.1244 rad for structure No. 1, 0.1017 rad for structure No. 2 and 0.1304 rad for structure No. 3.

Thus, the technical result indicated above has been achieved.

Claims (1)

A device for modulating IR radiation, which is a multilayer structure consisting of layers arranged in the following order: ITO layer, PZT piezoelectric material layer, ITO layer, SiO ₂ layer, ITO layer, PZT piezoelectric material layer, ITO layer, and layers transparent piezoelectric PZT have a thickness of 200 nm, wide-gap semiconductor ITO layers have a thickness from 20 to 500 nm, the dielectric matching layer - a transparent medium such as SiO ₂ - has a thickness of 200 nm, and the functioning of such a multilayer structure is that when A voltage is applied to the layers of the wide-gap semiconductor ITO, between which a layer of transparent piezoelectric PZT is located, and a change occurs in the geometric and, as a consequence, optical thickness of the layers of transparent piezoelectric PZT, which leads to modulation of the phase of the transmitted IR radiation.