A plasmonic metasurface ultrafast polarization-selective optical modulator based on vanadium dioxide phase transition

Abstract

A plasmonic metasurface ultrafast polarization selective optical modulator based on vanadium dioxide phase change belongs to the field of modulator technology. It includes two layers of orthogonal gratings, the lower layer is a gold grating structure, the vanadium dioxide grating is on the gold grating, and the grating lines of the gold grating and the vanadium dioxide grating are perpendicular to each other. There are different local field enhancements for the two linearly polarized lights parallel to the two grating lines, and the generated LSPR is in different bands, which is more conducive to improving the signal-to-noise ratio. The vanadium dioxide phase change is induced by optical regulation to achieve ultrafast regulation within sub-picosecond time.

Description

Technical Field

[0001] The invention relates to a plasmon supersurface ultrafast polarization selective optical modulator based on vanadium dioxide phase change, belonging to the technical field of modulators. Background Art

[0002] Light field control usually involves the control of parameters such as intensity, phase, polarization and frequency of the light field. Polarization is an important physical quantity that describes light. It reflects the oscillation characteristics of light as a transverse wave. It is one of the inherent characteristics of electromagnetic waves and plays an important role in optical communication, computing, imaging, encryption and display. Different polarization states can load different information and realize polarization multiplexing. Traditional methods of controlling polarization states usually involve bulky optical components such as mirrors, crystals and prisms, which is not feasible for the miniaturization and integration of optical systems. Realizing large-scale integration and dynamic control of ultra-small size and low-energy photonic devices has become a research hotspot in the field of micro-nano photonics at home and abroad. Therefore, various artificial structures with different performances have been designed. In recent years, with the advancement of micro-nano processing technology and methods, the construction of micro-optical modulators with novel polarization characteristics has become a key breakthrough in solving the problem of complex on-chip polarization technology.

[0003] Recently, surface plasmons in metal structures have been used to control light fields. The localization of electromagnetic fields in subwavelength-scale nanostructures is enhanced, thereby enhancing light-matter interactions. Plasmon metasurfaces are composed of micro-nano structures distributed in a plane, and because plasmon resonance is formed between metals and dielectrics, the polarization, phase, amplitude and other characteristics of electromagnetic waves can be further controlled. Metallic nanostructures can modulate their resonant modes by adjusting the geometric parameters of the structure or the angle of the incident light, or they can make the structure anisotropic and thus realize polarization characteristics. Surface plasmon optical modulators have the advantages of large working bandwidth, fast speed and small size, and more and more research has been proposed.

[0004] Phase change material vanadium dioxide (VO 2 ) has a large contrast in optical constants when undergoing a semiconductor-to-metal phase transition. When the temperature exceeds 68°C, VO 2 The insulator-metal phase transition occurs. At low temperatures, VO 2 In the insulating phase, it has a monoclinic crystal structure (Monoclinic, M1); at high temperatures, VO 2 In the metallic phase, it has a rutile crystal structure (Rutile, R). With the change of lattice structure, VO 2 Physical properties such as conductivity, complex refractive index, dielectric constant, work function, etc. will undergo drastic changes. 2The phase transition can be controlled not only by temperature, but also by optical pumping to obtain light-speed response. Its mature growth process and stable material properties make it suitable for micro-machining. When triggered optically, this transition occurs within sub-picoseconds, which makes the device suitable for ultrafast pulsed lasers.

[0005] Optical modulators are an indispensable core device in photonics and optoelectronics, and are widely used in optical interconnection, medical treatment, biology, and environmental monitoring. Due to the increasing requirements for high-speed, compact optical modulators, the design of optical modulators is moving towards small and fast modulation. Combining the phase change material vanadium dioxide with the plasmonic metasurface to achieve ultra-small-sized ultrafast modulation of polarized light has guiding significance for the realization of low-power, integrated optical modulators, and is also beneficial to the realization of integrated photonic circuits and all-optical interconnection. Optical modulators can be divided into mechanical control, thermal control, acoustic control, electrical control, optical control, etc. according to their modulation principles. Among them, the phase change material VO is widely used in the optical field. 2 Most of the regulation is achieved by direct heating or applying current to generate resistive heat. The regulation time of this type of regulation is usually in the millisecond range, which cannot meet the requirements of fast modulation. Summary of the invention

[0006] The present invention uses femtosecond laser as the control light to irradiate the sample to generate surface plasmons to generate hot electrons injected into VO 2 , reducing VO 2 The phase change threshold is reduced, thereby reducing the requirements for light intensity. A double-layer orthogonal grating is set to produce different local field enhancements for linearly polarized light with mutually orthogonal polarization states, thereby realizing polarization-selective optical regulation of the signal light. The design of a micron-sized plasmon metasurface realizes the regulation of linearly polarized light, and the response time of the regulation is in the sub-picosecond level, achieving ultrafast modulation.

[0007] Prepare micrometer-sized plasmon metasurface light modulators to achieve ultrafast polarization-selective light regulation.

[0008] A plasmonic metasurface ultrafast polarization selective optical modulator based on vanadium dioxide phase transition, the optical modulator is composed of a periodic arrangement of unit structures, and the top view of the unit structure is shown in the figure Figure 1 As shown, the unit structure includes two layers of orthogonal gratings, the lower layer is a gold grating structure, and the vanadium dioxide grating is on the gold grating. The grating lines of the gold grating and the vanadium dioxide grating are perpendicular to each other.

[0009] The structure of the optical modulator is as follows from bottom to top: SiO 2 Substrate, gold grating layer, vanadium dioxide grating layer, such as Figure 2 As shown, in SiO 2The substrate is a gold grating layer, and the gold grating layer is a vanadium dioxide grating layer. The grating lines of the gold grating in the gold grating layer and the vanadium dioxide grating in the vanadium dioxide grating layer are perpendicular to each other. Further specific parameters are: the thickness of the gold grating is t 1 =200nm, period is P 1 =400nm, the gate line width is a 1 =120nm; the thickness of the vanadium dioxide grating is t 2 =100nm, period is P 2 =600nm, the gate line width is a 2 =200nm.

[0010] The linearly polarized light is taken as the incident light. When the polarization direction of the light is parallel to the grating lines of the gold grating (y direction), it is defined as Ey. When the polarization direction of the light is parallel to the grating lines of the vanadium dioxide grating (x direction), it is defined as Ex. The incident light is incident on the metasurface area at 0°. The incident light passes through the vanadium dioxide grating and the gold grating in turn, and finally exits from the SiO 2 Base ejection.

[0011] 5. Advantages and positive effects

[0012] 1) The designed optical modulator utilizes hot electrons generated by LSPR. The injection of hot electrons reduces the phase change threshold of vanadium dioxide and reduces the intensity requirement for controlling light.

[0013] 2) The optical modulator is composed of two layers of orthogonal gratings, which have different local field enhancements for the two linear polarized lights parallel to the two grating lines. The generated LSPR is in different bands, which is more conducive to improving the signal-to-noise ratio.

[0014] 3) The optical modulator uses light control to induce phase change of vanadium dioxide, achieving ultrafast control within sub-picosecond time.

[0015] 4) The designed light modulator size is in micrometer size.

[0016] The positive effect of the present invention is that the mutually orthogonal double-layer grating is designed, which produces different local field enhancements for linear polarized light with two polarization directions perpendicular to each other, realizes polarization selectivity, and the polarization states of the control light and the signal light are perpendicular to each other and have different wavelengths, which is more conducive to screening the signal light and improving the signal-to-noise ratio. 2 Reduced VO 2 The phase change threshold of VO is induced by light regulation. 2 Phase change not only reduces the requirements for light intensity but also realizes ultrafast regulation in sub-picosecond time, which is more conducive to the rapid response of optical modulators and the miniaturization of devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Top view of the optical modulator unit structure

[0018] Figure 2 Schematic diagram of the structure of the optical modulator

[0019] Figure 3-1 Absorption rate when Ey light is incident on the light modulator

[0020] Figure 3-2 Transmittance and transmittance difference when Ey light is incident on the light modulator

[0021] Figure 4-1 Absorption rate when Ex light is incident on the light modulator

[0022] Figure 4-2 Transmittance and transmittance difference when Ex light is incident on the light modulator DETAILED DESCRIPTION

[0023] The present invention is further described below in conjunction with embodiments, but the present invention is not limited to the following embodiments.

[0024] Example 1

[0025] Structure Figure 1-Figure 2 .

[0026] 1) Processing the gold grating mask using electron beam exposure technology: First, a layer of photoresist is spin-coated on a quartz substrate, and then an electron beam photoresist grating with a period of 400nm and a grid line width of 280nm is etched using electron beam exposure, and a structural pattern is obtained after development and fixing.

[0027] 2) Using vacuum thermal evaporation process to evaporate 180-200nm thick Au on the surface of the photoresist grating structure, adjust the current during evaporation to keep the evaporation speed at Prevent uneven film thickness and ensure the hardness of the Au film. Put the sample into the adhesive remover to strip the photoresist and obtain an Au grating with a period of 400nm and a line width of 120nm.

[0028] 3) Using magnetron sputtering technology to deposit 90-100nm thick VO on the quartz wafer 2 The film is then annealed and tested to ensure that VO 2 Capable of phase change.

[0029] 4) Deposit VO 2 The quartz plate is immersed in BOE etching solution, VO 2 The film will be made from SiO 2 The detached VO 2 The film was transferred onto the prepared Au grating.

[0030] 5) Using focused ion beam etching technology to VO 2 The film is etched to obtain a cycle of 600

[0031] nm, VO with a gate width of 200 nm 2 Grating, etched VO 2 The lines of the grating are perpendicular to the lines of the underlying Au grating.

[0032] In the Au plating step, in addition to vacuum thermal evaporation coating technology, magnetron sputtering coating technology can also be used; to prepare the Au grating structure, gold can be plated on the quartz wafer first, and then the gold film on the quartz wafer can be etched in a small area using focused ion beam etching technology.

[0033] The structure was simulated using finite element analysis software (COMSOL). The orthogonal double-layer gratings have different local field enhancements for Ex and Ey light, which provides feasibility for achieving polarization selectivity. The absorption rate of Ey light when it is incident on the structure is as follows: Figure 3-1 , at 1190nm wavelength ( Figure 3-1 The localized plasmon (LSPR) is generated near the dotted line position in the figure. The LSPR is localized at the contact position between the gold grating and the vanadium dioxide grating. The LSPR can provide hot electron injection into the vanadium dioxide, reducing the phase transition threshold of the vanadium dioxide. Therefore, a lower intensity of light can be used to excite the phase transition of the vanadium dioxide. The transmittance of the Ey light before and after the phase transition of the vanadium dioxide and the difference in transmittance change are shown in Figure 2. Figure 3-2 , most of the Ey light is absorbed to form LSPR, the transmittance is low in the near-infrared band, and the transmittance of the light modulator changes little before and after the vanadium dioxide phase transition. The formula for calculating the transmittance difference is as follows:

[0034] ΔT=T (相变后) -T (相变前)

[0035] Where ΔT represents the transmittance difference, T (相变前) represents the transmittance of the light modulator to Ex light or Ey light before the vanadium dioxide phase change, T (相变后) It represents the transmittance of the optical modulator to Ex light or Ey light after the vanadium dioxide phase change.

[0036] When Ex light is incident on the light modulator, the absorption rate of vanadium dioxide before and after the phase change is as follows: Figure 4-1 As shown. When there is no Ey light incident, the vanadium dioxide in the optical modulator does not undergo a phase change, and the LSPR when Ex light is incident appears near 900nm. When Ey light is incident, the vanadium dioxide undergoes a phase change, and the LSPR when Ex light is incident appears near 735nm. The phase change of vanadium dioxide causes the LSPR generated when Ex light is incident to blue shift, and the transmittance of the optical modulator to Ex light changes. The transmittance of vanadium dioxide before and after the phase change when Ex light is incident and the transmittance difference are shown as follows Figure 4-2 As shown in the figure, the transmittance of Ex increases after the vanadium dioxide phase transition, and the transmittance at a wavelength of 945nm changes significantly before and after the vanadium dioxide phase transition, with a modulation depth of up to 39.3%.

[0037] The LSPR generated by the normally incident 1190nm wavelength Ey light on the optical modulator provides hot electron injection to reduce the phase change threshold of vanadium dioxide and cause the phase change of vanadium dioxide. Whether the phase change of vanadium dioxide has little effect on the transmittance of Ey light, so Ey light is more suitable as control light. When Ex light is normally incident, the phase change of vanadium dioxide will change the position of LSPR on the spectrum. What is important is that the transmittance of the optical modulator changes greatly before and after the phase change, so it is more suitable as signal light. Therefore, Ey light with a wavelength near 1190nm is selected as the control light for regulating the phase change of vanadium dioxide, and Ex light with a wavelength near 945nm is regulated as the signal light. The designed optical modulator can realize the function of ultrafast selective regulation of polarization state.

Claims

1. A plasmonic metasurface ultrafast polarization selective optical modulator based on vanadium dioxide phase transition, characterized in that: The optical modulator is composed of a periodic arrangement of unit structures, wherein the unit structure includes two layers of orthogonal gratings, the lower layer is a gold grating structure, and the vanadium dioxide grating is placed on the gold grating, and the grating lines of the gold grating and the vanadium dioxide grating are perpendicular to each other; The specific parameters are: the thickness of the gold grating is t1 = 200 nm, the period is P1 = 400 nm, and the grid line width is a1 = 120 nm; the thickness of the vanadium dioxide grating is t2 = 100 nm, the period is P2 = 600 nm, and the grid line width is a2 = 200 nm; Take linear polarized light as incident light. When the polarization direction of light is parallel to the grating direction of the gold grating, it is defined as Ey. When the polarization direction of light is parallel to the grating direction of the vanadium dioxide grating, it is defined as Ex. The incident light is incident on the metasurface area at 0°. The incident light passes through the vanadium dioxide grating and the gold grating in turn, and finally exits from the SiO2 substrate. Ey light with a wavelength near 1190 nm is selected as the control light for regulating the phase change of vanadium dioxide, and Ex light with a wavelength near 945 nm is regulated as the signal light to achieve the function of ultrafast selective regulation of polarization states.

2. A vanadium dioxide phase-change based plasmon metasurface ultrafast polarization selective optical modulator according to claim 1, characterized in that: The structure of the optical modulator is from bottom to top: SiO2 substrate, gold grating layer, vanadium dioxide grating layer. There is a gold grating layer on the SiO2 substrate, and there is a vanadium dioxide grating layer on the gold grating layer. The grating lines of the gold grating in the gold grating layer and the vanadium dioxide grating in the vanadium dioxide grating layer are perpendicular to each other.

3. A vanadium dioxide phase-change based plasmon metasurface ultrafast polarization selective optical modulator according to claim 1, characterized in that: The modulator size is in the micrometer dimension.

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