CN115016149A - Plasmon ultrafast polarization selective optical modulator based on vanadium dioxide phase change - Google Patents

Abstract

A plasmon ultrafast polarization selective optical modulator based on vanadium dioxide phase change belongs to the technical field of modulators. The grating structure comprises an upper layer of orthogonal grating and a lower layer of orthogonal grating, wherein the lower layer of the orthogonal grating is of a gold grating structure, a vanadium dioxide grating is arranged on the gold grating, and grid lines of the gold grating and the vanadium dioxide grating are mutually vertical. Two kinds of linearly polarized light which are respectively parallel to the two grid lines have different local field enhancement, and the generated LSPR is in different wave bands, thereby being more beneficial to improving the signal-to-noise ratio. The phase change of the vanadium dioxide is caused by adopting a light regulation and control mode, and the ultrafast regulation and control within the subpicosecond time is realized.

Description

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

technical field

The invention relates to a plasmon metasurface ultrafast polarization selective light modulator based on vanadium dioxide phase transition, and belongs to the technical field of modulators.

Background technique

Optical field regulation is usually the regulation of parameters such as the intensity, phase, polarization and frequency of the optical field. Polarization is an important physical quantity to describe light, which 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 be loaded with different information, and polarization multiplexing can be realized. Conventional methods for controlling polarization states usually involve bulky optical components such as mirrors, crystals, and prisms, which are not feasible for miniaturization and integration of optical systems. The realization of large-scale integration and dynamic regulation of ultra-small-scale, 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 properties 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 complex on-chip polarization technology problems.

Recently, surface plasmons in metallic structures have been used for optical field modulation, and the localization of electromagnetic fields in subwavelength-scale nanostructures is enhanced, thereby enhancing light-matter interactions. Plasmonic metasurfaces are composed of micro-nano structures distributed in the plane, and due to the formation of plasmonic resonance between metals and dielectrics, the polarization, phase, amplitude and other characteristics of electromagnetic waves can be further regulated. Metal nanostructures can modulate their resonance modes by adjusting the geometric parameters of the structure or the angle of incident light, and can also make the structure exhibit anisotropy to achieve polarization characteristics. Surface plasmon light modulators have the advantages of large operating bandwidth, fast speed and small size, and more and more studies have been proposed.

The phase-change material, vanadium dioxide (VO ₂ ), undergoes a semiconductor-to-metal phase transition with a large contrast in optical constants. When the temperature exceeds 68 °C, the first-order phase transition of insulator _- metal occurs in VO2. At low temperature, VO ₂ is in an insulating phase and has a monoclinic crystal structure (Monoclinic, M1); at high temperature, VO ₂ is in a metallic phase and has a rutile crystal structure (Rutile, R). With the change of the lattice structure, the physical properties of VO ₂ , such as electrical conductivity, complex refractive index, permittivity, and work function, will undergo dramatic changes. _The VO 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 microfabrication. When optically triggered, this transition occurs in sub-picoseconds, making the device suitable for ultrafast pulsed lasers.

Optical modulators are an indispensable core device in photonics and optoelectronics, and are widely used in optical interconnection, medical, biological and environmental monitoring and other fields. Due to the increasing requirements for high-speed and compact optical modulators, Therefore, the design of light modulators is researched in the direction of small and fast modulation. Combining phase-change material vanadium dioxide with plasmonic metasurfaces to achieve ultra-small-scale ultra-fast modulation of polarized light has guiding significance for the realization of low-power, integrated light modulators, and is beneficial to integrated photonic circuits , the realization of all-optical interconnection. Light modulators can be divided into mechanical regulation, thermal regulation, acoustic regulation, electrical regulation, and light regulation according to their modulation principles. Among them, the regulation of phase change material VO ₂ is mostly controlled by direct heating or by applying current to generate resistance heat. To achieve thermal regulation, the regulation time of such regulation methods is usually in the order of milliseconds, which cannot meet the requirements of fast modulation.

SUMMARY OF THE INVENTION

The invention is designed to use the femtosecond laser as the control light to irradiate the surface plasmon on the sample to generate hot electrons and inject them into VO ₂ , thereby reducing the phase transition threshold of VO ₂ and reducing the requirement for light intensity. The double-layer orthogonal grating is arranged to generate different local field enhancements for linearly polarized light whose polarization states are orthogonal to each other, thereby realizing polarization-selective optical regulation of the signal light. The design of micron-sized plasmonic metasurfaces realizes the regulation of linearly polarized light, and the response time of the regulation is in the sub-picosecond order, realizing ultrafast modulation.

Fabrication of micron-sized plasmonic metasurface light modulators for ultrafast polarization-selective light modulation.

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

Further, the structure of the light modulator is from bottom to top: SiO ₂ substrate, gold grating layer, vanadium dioxide grating layer, as shown in Figure 2, on the SiO ₂ substrate is a layer of gold grating layer, on the gold grating layer It is a vanadium dioxide grating layer, and the gold grating in the gold grating layer and the grid lines of 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, the period is P ₁ =400nm, the width of the grid line is a ₁ =120nm; the thickness of the vanadium dioxide grating is t ₂ =100nm, the period is P ₂ =600nm, The gate line width is a ₂ =200 nm.

Taking linearly polarized light as the incident light, when the polarization direction of the light is parallel to the grid line (y direction) direction of the gold grating, it is defined as Ey, and when the polarization direction of the light is parallel to the grid line (x direction) direction of the vanadium dioxide grating, then Defined as Ex; the incident light is normally incident on the metasurface region at 0°, the incident light passes through the vanadium dioxide grating and the gold grating in sequence, and finally exits from the _SiO2 substrate.

5. Advantages and positive effects

1) The designed light modulator utilizes the hot electrons generated by LSPR, and the injection of hot electrons reduces the phase transition threshold of vanadium dioxide and reduces the light intensity requirement for control light.

2) The light modulator is composed of two layers of orthogonal gratings, which have different local field enhancements for the two linearly polarized lights parallel to the two grating lines, and the generated LSPRs are in different wavelength bands, which is more conducive to improving the signal-to-noise ratio.

3) The light modulator induces the phase transition of vanadium dioxide by means of light regulation, and realizes ultrafast regulation in sub-picosecond time.

4) The designed light modulator size is in micrometer size.

The positive effect of the invention lies in the design of mutually orthogonal double-layer gratings, different local field enhancements are generated for the two linearly polarized lights with mutually orthogonal polarization directions, the polarization selectivity is realized, and the polarization states of the light and the signal light are controlled. They are perpendicular to each other and have different light wavelengths, which is more conducive to screening the signal light and improving the signal-to-noise ratio. The injection of hot electrons generated by LSPR into VO ₂ reduces the phase transition threshold of VO ₂ , and the phase transition of VO ₂ is induced by light regulation, which not only reduces the requirement for light intensity, but also achieves sub-picosecond level in time. The ultrafast regulation of the optical modulator is more conducive to the fast response of the light modulator and the miniaturization of the device.

Description of drawings

Figure 1. Top view of light modulator cell structure

Figure 2 Schematic diagram of the structure of the optical modulator

Figure 3-1 Absorptivity when Ey light is incident on the light modulator

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

Figure 4-1 Absorptivity when Ex light is incident on the light modulator

Figure 4-2 Transmittance and transmittance difference when Ex light is incident on the light modulator

Detailed ways

The present invention is further described below in conjunction with the examples, but the present invention is not limited to the following examples.

Example 1

The structure is shown in Figure 1-Figure 2.

1) A gold grating mask is processed by an electron beam exposure process. First, spin-coat a layer of photoresist on the quartz substrate, then use electron beam exposure to etch an electron beam photoresist grating with a period of 400 nm and a grid line width of 280 nm, and then develop and fix to obtain a structural pattern.

防止膜厚不均匀，保证Au膜的硬度。将样品放进除胶剂中剥离光刻胶，得到周期400nm、栅线宽120nm的Au光栅。2) Au with a thickness of 180-200 nm is evaporated on the surface of the photoresist grating structure by vacuum thermal evaporation process, and the current is adjusted during evaporation to keep the evaporation rate as

Prevent uneven film thickness and ensure the hardness of the Au film. Put the sample into the adhesive remover and peel off the photoresist to obtain an Au grating with a period of 400 nm and a gate line width of 120 nm.

3) Using magnetron sputtering coating technology to deposit 90-100nm thick VO ₂ film on the quartz wafer, then annealing, and finally testing to ensure that VO ₂ can phase change.

4 ₎ Soak the VO2 _- deposited quartz wafer in BOE etching solution, the VO2 film will be detached from the _SiO2 substrate, and then the detached _VO2 film will be transferred to the prepared Au grating.

5) The VO ₂ film was etched using the focused ion beam etching technology to obtain a period of 600

VO _{2 grating with a gate line width of 200 nm, the gate line of the etched VO 2 grating} is perpendicular to the gate line of the lower Au grating.

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

The finite element analysis software (COMSOL) is used to simulate the structure, and the two-layer gratings orthogonal to each other have different local field enhancements for Ex light and Ey light, which provides feasibility for realizing polarization selectivity. The absorptivity of Ey light is shown in Fig. 3-1 when the light is normally incident on the structure. The localized plasmon (LSPR) is generated near the wavelength of 1190 nm (the position of the dotted line in Fig. 3-1). The LSPR is localized in the gold grating and the vanadium dioxide grating. At the location of the contact, LSPR can provide hot electron injection into the vanadium dioxide, lowering the vanadium dioxide phase transition threshold, so that lower intensity light can be used to excite the vanadium dioxide phase transition. The transmittance of Ey light before and after the vanadium dioxide phase transition and the difference in transmittance change are shown in Figure 3-2. Most of the Ey light is absorbed to form LSPR, and the transmittance is low in the near-infrared band. The change in transmittance before and after is small. The formula for calculating the transmittance difference is as follows:

ΔT=T _{(after phase transition)} -T _{(before phase transition)}

where ΔT represents the transmittance difference, T _{(before phase change)} represents the transmittance of the light modulator to Ex light or Ey light before the vanadium dioxide phase change, and T _{(after the phase change)} represents the light modulator after the vanadium dioxide phase change Transmittance to Ex light or Ey light.

When Ex light is incident on the light modulator, the absorptivity of vanadium dioxide before and after the phase transition is shown in Figure 4-1. When no Ey light is incident, the vanadium dioxide in the light modulator does not undergo phase transition, and the LSPR when Ex light is incident appears around 900 nm. When Ey light is incident, the phase transition of vanadium dioxide occurs, and the LSPR of Ex light incident appears around 735 nm. The phase transition of vanadium dioxide causes the blue shift of the LSPR generated when the Ex light is incident, and the transmittance of the light modulator to the Ex light changes. -2 shown. The transmittance of Ex increased after the vanadium dioxide phase transition, and the transmittance at the wavelength of 945 nm changed significantly before and after the vanadium dioxide phase transition, and the modulation depth could reach 39.3%.

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

Claims (6)

1. a kind of plasmon metasurface ultrafast polarization selective light modulator based on vanadium dioxide phase transition, it is characterized in that, light modulator is made up of periodic arrangement of unit structure, and unit structure comprises up and down two orthogonal gratings , the lower layer is a gold grating structure, the gold grating is a vanadium dioxide grating, and the grid lines of the gold grating and the vanadium dioxide grating are perpendicular to each other. 2. according to a kind of plasmon metasurface ultrafast polarization selective light modulator based on vanadium dioxide phase transition according to claim 1, it is characterized in that, the structure of light modulator is sequentially from bottom to top: SiO ₂ substrate, gold grating layer, vanadium dioxide grating layer, a layer of gold grating layer on the SiO ₂ substrate, vanadium dioxide grating layer on the gold grating layer, gold grating and vanadium dioxide grating layer in the gold grating layer The grid lines of the vanadium dioxide grating are perpendicular to each other. 3. a kind of plasmon metasurface ultrafast polarization selective light modulator based on vanadium dioxide phase transition according to claim 1, is characterized in that, the specific parameter is: the thickness of gold grating is t ₁ =200nm , the period is P ₁ =400nm, the gate line width is a ₁ =120nm; the thickness of the vanadium dioxide grating is t ₂ =100nm, the period is P ₂ =600nm, and the gate line width is a ₂ =200nm. 4. a kind of plasmon metasurface ultrafast polarization selective light modulator based on vanadium dioxide phase transition according to claim 1, is characterized in that, taking linearly polarized light as incident light, when the polarization direction of light It is defined as Ey when it is parallel to the grid line (y direction) direction of the gold grating, and is defined as Ex when the polarization direction of the light is parallel to the grid line (x direction) direction of the vanadium dioxide grating; In the surface area, the incident light passes through the vanadium dioxide grating and the gold grating in sequence, and finally exits from the _SiO2 substrate. 5. according to a kind of plasmon metasurface ultrafast polarization selective light modulator based on vanadium dioxide phase transition according to claim 1, it is characterized in that, select the Ey light near wavelength 1190nm as regulating vanadium dioxide phase With variable control light, Ex near the wavelength of 945nm is regulated as a signal light, realizing the function of selective regulation of ultrafast polarization state. 6 . The plasmon metasurface ultrafast polarization-selective light modulator based on vanadium dioxide phase transition according to claim 1 , wherein the size of the modulator is in a micron size. 7 .

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