RU2650352C1 - The device for the second harmonic generation of the optical radiation - Google Patents

Abstract

FIELD: electronics.

SUBSTANCE: invention relates to quantum electronics, namely to devices for doubling the frequency of optical radiation. Device for the second harmonic generation of the optical radiation comprises an active element based on aluminum nitride, and two plates of solid solution Al_xGa_1-xN. Active element is made of at least one pair of alternating layers: layer (1) consists of intrinsic conductive aluminum nitride with a thickness of 100-1400nm and a layer (2) consists of material with metallic conductivity with a thickness of 1-5nm.

EFFECT: invention provides improved efficiency of the second harmonic generation of the optical radiation.

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Description

The invention relates to quantum electronics, and in particular to devices for doubling the frequency of optical radiation.

A device is known for generating the second harmonic of optical radiation (see application WO 9950709, IPC H01S - 003/16, publ. 07.10.1999) containing a birefringent nonlinear crystal Gd _x Y _1-x Ca ₄ O (BO ₃ ) ₃ , where 0 , 01 <= x <= 0.35.

A disadvantage of the known device is the small nonlinear susceptibility of the crystal used in the device, which reduces the efficiency of second harmonic generation.

A device is known for generating the second harmonic of optical radiation (see ed. St. RU 784550, IPC G02F 1/37, publ. 05.20.2000), consisting of a waveguide layer deposited on a substrate and a comb electrode deposited on a waveguide layer. The substrate is made of semiconductor material, and the period of the structure of the comb electrode is chosen equal to

where λ is the wavelength of the pump radiation, nm;

и

- эффективные показатели преломления волноводных мод основой частоты и второй гармоники соответственно;

and

- effective refractive indices of the waveguide modes based on the frequency and second harmonic, respectively;

k ₁ and k ₂ are the moduli of wave vectors at the fundamental frequency and second harmonic.

The known device ensures the achievement of the so-called quasisynchronism, achieved by rotating the polarization vector in the opposite direction upon reaching the length of synchronism. This makes it possible to use uniaxial nonlinear crystals with a high quadratic nonlinear susceptibility for nonlinear conversion. A disadvantage of the known device is the reduction in conversion efficiency compared to true synchronism.

A device is known for generating the second harmonic of optical radiation (see application JPH 06347848, IPC G02F - 001/37, H01S - 005/00, publ. 12/22/1994), containing a nonlinear crystal with induced periodic polarization. In the known device, the domain structure of the polarization direction is already fixed in the manufacture of the crystal, which greatly simplifies the device in use.

A disadvantage of the known device is the use of quasi-synchronism in the device, which reduces the efficiency of nonlinear conversion.

A device is known for generating the second harmonic of optical radiation (see application WO 2009075363, IPC G02F - 001/35, G02F - 001/35, G02F - 001/37, G02F - 001/37, publ. 06/18/2009), which coincides with this the decision on the largest number of essential features and taken as a prototype. The prototype device contains an active element of aluminum nitride AlN having a structure with periodically changing to the opposite direction of polarization of the crystal.

The device can double or triple the frequency of optical radiation due to quasi-phase matching. A disadvantage of the known device is the use of quasi-phase matching in it for phase matching, which reduces the efficiency of second harmonic generation.

The objective of the invention is to develop such a device for generating a second harmonic of optical radiation, which would increase the efficiency of generation of the second harmonic of optical radiation, having the properties of a waveguide structure and providing true phase matching for phase matching.

The problem is solved in that the device for generating a second harmonic of optical radiation contains an active element based on aluminum nitride, the active element is located between the plates of the solid solution Al _x Ga _1-x N, where 0.15≤x≤1, and is made of one or several pairs of alternating layers of aluminum nitride intrinsic conductivity with a thickness of 100-1400 nm and a material with metal conductivity with a thickness of 1-5 nm.

New in the device is the location of the active element between the plates of the solid solution Al _x Ga _1-x N, where 0.15≤x≤1, and the execution of the active element in the form of at least one pair of alternating layers of aluminum nitride intrinsic conductivity with a thickness of 100-1400 nm and a material with metallic conductivity 1-5 nm thick.

This device is a planar waveguide with solid-state plates and metamaterial as a waveguide layer and an active element for the generation of the second harmonic. The effective refractive index of the waveguide layer turns out to depend on the direction of radiation propagation through the structure and varying from 1.8 to 2.90. The layers of the Al _x Ga _1-x N solid solution are used as waveguide plates for the active region, where 0.15≤x≤1. The range of molar fractions of the composition of the waveguide plates is determined by the fact that the refractive index of the plates must be lower than 2.1 in order to achieve synchronism in the nonlinear conversion of light in a given waveguide element. This condition is satisfied when the molar fraction of aluminum is not lower than 0.15. With an increase in the molar fraction of aluminum up to 1, a gradual decrease in the refractive index of the plates occurs up to values of 1.7-1.8. New in the device is to take into account the waveguide dispersion and the dispersion of the refractive index of the cladding material in order to ensure true phase matching for phase matching during propagation of pump radiation and second harmonic along the waveguide. Moreover, the coherence length can reach several centimeters.

In the device, a material with metal conductivity can be made of aluminum nitride alloyed to a concentration of 1 × ^{10 12} cm ^-2 to 1 × 10 ¹⁴ cm ^-2 or metal.

The present device can be grown on a substrate, for example, from Si, or GaAs, or AlN, or InP, or AlGaN, or Al ₂ O ₃ , or β-Ga ₂ O ₃ . Then the substrate can be removed.

The present device is illustrated by drawings, where

in FIG. 1 is a schematic sectional view of the present device;

in FIG. 2 shows the dependence of the coherence length on the thickness a of the dielectric layer of the meta material;

in FIG. Figure 3 shows the dependence of the coherence length on the thickness h of the waveguide;

in FIG. Figure 4 shows the dependence of the coherence length on the wavelength λ of the fundamental harmonic.

A device for generating a second harmonic of optical radiation (Fig. 1) contains an active element based on aluminum nitride in the form of one or several pairs of alternating layers - layer 1, consisting of intrinsic aluminum nitride with a thickness of 100 to 1400 nm, and layer 2 having a metal conductivity and consisting of aluminum nitride doped to a concentration of 1⋅10 ¹² cm ^-2 to 1⋅10 ¹⁴ cm ^-2 or of metal 1-5 nm thick. The device is basically a semiconductor optical metamaterial - a structure that can be grown by epitaxial methods, which ensures the comparative simplicity of the process and low cost. The range of thicknesses of the materials of layers 1 and 2 is determined from the following conditions. Light propagating in such structures has a characteristic dispersion law, determined by the properties of the materials used and the ratio between the thicknesses of the semiconductor and metal layers. The thickness of the dielectric must be of the order of the wavelength of the light propagating in the material, so that the disturbance of the refractive index in the periodic structure affects the light. Thus, the lower boundary of the thickness of aluminum nitride intrinsic conductivity of 100 nm is determined by the absorption edge of aluminum nitride for radiation of the second harmonic. The upper limit of the thickness of the aluminum nitride layer 1400 nm is determined in order to provide a waveguide thickness of not more than 10-15 μm with the number of pairs of alternating layers of not less than 10. The choice of the maximum waveguide thickness is determined by the need to maintain a high pump power density, which ensures an increase in the second harmonic generation efficiency. The choice of the thickness of the layer with metal conductivity is determined by the fact that it must be more than an order of magnitude thinner than the dielectric layer in order to reduce absorption in it, but at the same time have a sufficient thickness satisfying the phase matching condition (1), which can be written as follows: 2к ₁ = k ₂ . The lower boundary of a layer with a metallic conductivity of 1 nm is determined by technological attainability, and the upper boundary of 5 nm is due to the requirement of low absorption in these layers. The boundaries of the concentration level are set by the fact that upon reaching the lower level of doping concentration of 1 см10 ¹² cm ^-2 aluminum nitride acquires metallic conductivity, the upper limit of 1⋅10 ¹⁴ cm ^-2 is determined by the technologically achievable level. The molar fraction of aluminum x in the Al _x Ga _1-x N solid solution is determined from the condition that the required values of the refractive index in the plates are achieved.

This device operates as follows.

The pump radiation is introduced into an active element based on aluminum nitride, which is a waveguide consisting of several pairs of layers of a semiconductor material of intrinsic conductivity and a material with metallic conductivity. The propagation of pump radiation along the waveguide ensures the preservation of a high power density and, as a result, increases the efficiency of second harmonic generation. The matching of the phases of the pump radiation and the second harmonic is ensured by the compensation of the dispersion of the refractive index of the waveguide and the materials that make up the active element by matching the effective refractive indices of the waveguide modes of the pump radiation and the second harmonic, achieved by the proper choice of layer thicknesses, their number and permittivity of intrinsic and metal conductivity.

Theoretical calculations were carried out, confirming the attainability of true phase matching, which ensures the maximum efficiency of second harmonic generation in the proposed device for second harmonic generation. Typically, the calculation of the dispersion of light in metamaterials is carried out by means of expansion in plane waves. This method is effective for structures with correlated layer thicknesses. For the structure of the proposed device for generating the second harmonic of optical radiation, the plane wave method cannot be applied, therefore, the Bloch amplitude method is used for theoretical calculations. For light propagating in the plane of the layers, such a structure is an ordinary plenary waveguide. Modification of the dispersion law occurs when light propagates through the active region of the waveguide at a certain angle ϕ, as shown in FIG. 1. The dielectric constant in a metal layer is defined as the permeability of a plasma with a characteristic plasma frequency ω _p

and L.

. Refraction index measurements on AIN single crystals, Phys. Stat. Sol. 14, K5-K8, 1966) с помощью коэффициентов Селлмайера. Дисперсия в обкладках учитывается также с помощью коэффициентов Селлмайера (G.М. Laws, Е.С. Larkins, and I. Harrison. Improved refractive index formulas for the

and

alloys, Journ. Of Appl. Phys., 89 (2), 2001, 1108-1115). Результаты расчета показывают, что для каждой моды волновода существует свой закон дисперсии и свои области разрешенных и запрещенных значений длин волн. Задача генерации второй гармоники сводится к подбору таких значений параметров, при которых одновременно осуществляются два условия: свет на основной и удвоенной частотах распространяется под одним углом к плоскостям волновода; на основной и удвоенной частотах выполняется условие фазового синхронизма, которое в терминах длины когерентности мы формулируем следующим образом:and for a semiconductor material, the chromatic dispersion is formally taken into account (J.

and L.

. Refraction index measurements on AIN single crystals, Phys. Stat. Sol. 14, K5-K8, 1966) using the Sellmeier coefficients. The dispersion in the plates is also taken into account using the Sellmayer coefficients (G.M. Laws, E.S. Larkins, and I. Harrison. Improved refractive index formulas for the

and

alloys, Journ. Of appl. Phys., 89 (2), 2001, 1108-1115). The calculation results show that for each waveguide mode there is its own dispersion law and its own regions of allowed and forbidden wavelengths. The task of generating the second harmonic is reduced to the selection of such parameter values under which two conditions are simultaneously satisfied: light at the fundamental and doubled frequencies propagates at the same angle to the planes of the waveguide; at the fundamental and doubled frequencies, the condition of phase synchronism is fulfilled, which in terms of the coherence length we formulate as follows:

The controlled parameters of the problem are the thicknesses, the number and material of layers in the metamaterial, the lining material, and the pump radiation wavelength. As an example, the results of a theoretical calculation carried out for the fraction of aluminum x = 0.2 in the plates of the waveguide, consisting of Al _x Ga _1-x N., are shown. The results of this calculation show that with a proper choice of parameters, a coherence length of up to 25 mm can be achieved. In FIG. Figure 2 shows the dependence of the coherence length L on the thickness a of the dielectric layer of the metamaterial (calculation parameters: thickness of the layer with metal conductivity 1 nm, pump wavelength 550 nm, the active region consists of 10 pairs of alternating layers, the waveguide thickness is 12.7 μm. The calculation shows that a change in thickness due to inaccuracy in the manufacturing technology leads to a decrease in the coherence length, while the peak width at a height of L = 1 cm is about 1.2 nm, which is much more than the technological limit of accuracy of the thickness of the epitaxial layer. Figure 3 shows the dependence of the coherence length L on the change in the waveguide thickness h under the assumption that the thickness of the layer with metallic conductivity is 1 nm, the intrinsic conductivity layer is 1272 nm, the active region of the waveguide consists of 10 alternating layers, and the pump wavelength is 550 nm. the level of L = 1 cm is about 250 nm, which also fits into technological limitations, Fig. 4 shows the dependence of the coherence length L on the pump radiation wavelength λ (calculation parameters: thickness of a layer with a metal conductivity of 1 nm, intrinsic conductivity layer thickness 1272 nm, active region contains 10 layers, waveguide thickness 12.7 μm). The device demonstrates high sensitivity to the pump wavelength: a deviation of 1 nm reduces the coherence length L from 25 mm to 1.5 mm. Theoretical calculations for other fractions of aluminum content of 0.15≤x≤1 in the plates of the waveguide layer show similar results.

Claims (3)

1. A device for generating a second harmonic of optical radiation containing an active element based on aluminum nitride, characterized in that the active element is located between the plates of the solid solution Al _x Ga _1-x N, where 0.15≤x≤1, and made at least one pair of alternating layers of aluminum nitride intrinsic conductivity with a thickness of 100-1400 nm and a material with metal conductivity with a thickness of 1-5 nm. 2. The device according to p. 1, characterized in that the material with metal conductivity is made of aluminum nitride alloyed to a concentration of from 1 ^{10 12} cm ^-2 to 1 10 ¹⁴ cm ^-2 . 3. The device according to p. 1, characterized in that the material with metallic conductivity is made of metal.

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