A metal quantum well optical switch based on quasi-continuum bound states and a preparation method thereof

Abstract

This invention discloses a metal quantum well optical switch based on quasi-continuous domain bound states, comprising a substrate and a metal quantum well metasurface array located on the substrate. The metal quantum well metasurface array is formed by periodically arranging a plurality of metal quantum well units. Each metal quantum well unit is an elliptical block structure with a deflection angle θ between its major axis and the y-direction. Every two adjacent elliptical block structures are mirror-symmetric with respect to the yz plane in the x-direction. The deflection angle θ aims to achieve structural symmetry breaking in the y-direction. When probe light polarized along the x-direction is incident normally, a quasi-continuous domain bound state is formed, and this quasi-continuous domain bound state is used to form a high-Q resonant cavity. This invention also discloses a method for fabricating the above-mentioned metal quantum well optical switch. First, a metal quantum well composed of alternating stacked metal and insulating thin films is grown on a substrate using thin-film sputtering technology. Subsequently, a metal quantum well array is obtained through photolithography or focused ion beam milling. The metal quantum well optical switch provided by this invention has high modulation speed and large modulation depth, and has good application prospects.

Description

Technical Field

[0001] This invention belongs to the field of micro-nano optoelectronics technology, specifically relating to a metal quantum well metasurface optical switch based on quasi-continuous domain bound states and its fabrication method. Background Technology

[0002] In the field of free-space optical signal processing and communication, the modulation of amplitude, phase, and polarization is mainly achieved by spatial optical modulators. Among them, all-optical modulators have a higher modulation speed compared to electro-optic modulators. An optical switch, as an optical path conversion device, is the basic unit of an all-optical modulator. Its function is to perform mutual conversion or logical operations on optical signals under the control of variables.

[0003] The modulation speed and modulation depth of optical switches play a decisive role in the performance of all-optical systems. All-optical signal processing devices are mainly driven by optical nonlinear materials, such as transparent conductive oxides with the Kerr effect or semiconductor materials with photogenerated carrier injection. However, the light-light interaction in traditional dielectrics is weak, which is not conducive to improving the efficiency and reducing the size of optical switching devices.

[0004] Patent application CN107248536A discloses an optical switch device based on a quantum well structure, including a substrate, a buffer layer grown on the substrate, and a quantum well active layer grown on the buffer layer. The quantum well active layer, from top to bottom, comprises a first barrier layer, a first isolation layer, a first channel layer, a second channel layer, a second isolation layer, and a second barrier layer. This invention's quantum well active region has two conductive channels. Therefore, the problem of low carrier mobility caused by Coulomb attraction between photogenerated electrons and holes is eliminated in the conductive channels, greatly increasing the photogenerated electron mobility in the first channel layer. Simultaneously, holes do not participate in the conduction of the first channel, thus avoiding the problem of low hole mobility and eliminating the influence of Coulomb attraction between photogenerated electrons and holes. This invention allows only high-mobility electrons to participate in the transmission of signals in the first channel, thereby improving the speed and frequency characteristics of the optical switch. This invention employs a simple multilayer semiconductor quantum well structure to design photoconductive switches, which change resistivity based on photogenerated carriers to achieve high-speed sampling of electrical signals. However, quantum wells formed from conventional materials such as semiconductors may not exhibit sufficient nonlinearity, especially at the microscale required for many chip applications.

[0005] Patent application CN114594534A discloses a metasurface optical switch based on tunable structural units. This metasurface optical switch consists of multiple tunable structural units arranged periodically. Each tunable structural unit comprises a substrate, an antenna, and a rectangular block. The substrate is located at the bottom, and the antenna and rectangular block are coplanar and situated on the substrate. The substrate is square in shape, and the antenna is an equilateral triangular ring with an opening located at the midpoint of one side of the equilateral triangular ring. The rectangular block is located at the opening, and the centers of the square and the equilateral triangular ring coincide. The modulation speed of this invention depends on changing the crystallization rate of the material, making it difficult to meet the requirements of high-speed optical signal processing.

[0006] Patent application CN114577753A discloses a high-Q refractive index sensor based on quasi-continuous domain bound states and its fabrication method. The sensor includes a substrate and an array of all-dielectric metasurface structures on the substrate. The array is formed by periodically arranging several all-dielectric metasurface units, each a square block structure. Each square block structure has a notch on one side along the x-direction. The notch on each square block structure has the same position, orientation, shape, and size. The notched all-dielectric metasurface unit is used to excite a magnetic dipole moment along the z-direction when a plane wave polarized along the x-direction is incident normally, forming a bound state in the quasi-continuous domain. This bound state is then used to form a high-Q resonant cavity. This invention utilizes the high Q value of quasi-continuous domain bound states to design a high-performance sensor, but it does not explore its application in optical modulators. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a metal quantum well optical switch based on quasi-continuous bound state, which combines the high Q value of quasi-continuous bound state in the continuous (QBIC) with the high nonlinearity of metal quantum well (MQWs), and has high modulation speed and large modulation depth.

[0008] A metal quantum well optical switch based on quasi-continuous bound states includes a substrate and a metal quantum well metasurface array located on the substrate. The metal quantum well metasurface array is formed by periodically arranging a plurality of metal quantum well units. Each metal quantum well unit is an elliptical block structure, and the major axis of the ellipse of the ellipse block structure has a deflection angle θ with respect to the y-direction. In the x-direction, every two adjacent elliptical block structures are mirror-symmetric with respect to the yz plane. The deflection angle θ is intended to obtain structural symmetry breaking in the y-direction. When a probe light (plane wave) polarized along the x-direction is incident normally, a quasi-continuous bound state is formed, and a high-Q resonant cavity is formed using the quasi-continuous bound state. The x-direction is the substrate length direction, the y-direction is the substrate width direction, and the z-direction is the substrate height direction.

[0009] When the elliptical block structure of the metal quantum well metasurface array is incident with probe light (plane wave) polarized along the x-direction, a QBIC is formed, and the QBIC is used to form a resonant cavity with a high Q value.

[0010] The dielectric constant of the metal quantum well unit can be altered by pump light polarized along the y-direction, thereby changing the resonant point of the QBIC and achieving a switching effect. The probe light and pump light with different polarizations can be distinguished by a polarization beam splitter.

[0011] The substrate uses conventional substrate materials, preferably, it can be composed of dielectric materials such as silicon dioxide (SiO2).

[0012] Preferably, the deflection angle θ is >0°-30°; more preferably, when the deflection angle θ is 15°, the metal quantum well optical switch based on the quasi-continuous domain bound state has a high extinction ratio.

[0013] The metal quantum well unit is constructed by alternating stacks of metal layers and insulating layers with nanometer-thickness; the refractive index of the metal quantum well unit changes under pump light irradiation, and the greater the intensity, the greater the change.

[0014] In theory, conventional metal / insulator nanomaterials compatible with micro / nano fabrication processes, where the metal material thickness is ≤5nm, can be used to construct metal quantum well units.

[0015] Since the first energy level of the quantum well involved in the working principle is within 1 electron volt, any insulating material with a band gap of more than 1 electron volt is sufficient as an insulating layer material to construct a metal quantum well unit, preferably with a band gap of 1-10 electron volts.

[0016] Preferably, the metal layer is made of titanium nitride (TiN) film, gold (Au), silver (Ag), aluminum (Al), and indium tin oxide (ITO) film, etc., and the insulating layer is made of aluminum oxide (Al2O3) film (band gap of about 8 electron volts), silicon dioxide (SiO2) film (band gap of about 9 electron volts), etc.

[0017] Preferably, the metal quantum well unit is formed by alternating stacking of several nanoscale titanium nitride (TiN) films with a thickness of 2.0-5.0 nm and aluminum oxide films with a thickness of ≥ single atomic layer - 50 nm.

[0018] More preferably, the metal quantum well unit is composed of several layers of titanium nitride (TiN) films with a thickness of 2.4 nm and several layers of aluminum oxide (Al2O3) films with a thickness of 0.5 nm stacked alternately. In this case, the metal quantum well optical switch based on quasi-continuous domain bound states has a strong resonant cavity in the off state and the resulting high extinction ratio.

[0019] By changing the structure of the metal quantum well metasurface array, that is, by changing the period and deflection angle θ of the metal quantum well unit, the operating wavelength of the metal quantum well optical switch based on the quasi-continuous domain bound state can be adjusted and selected.

[0020] Preferably, the period Px of the metal quantum well unit in the x-direction is 500-1000 nm and the period Py in the y-direction is 750-1500 nm; the thickness of the metal quantum well unit is h = 50-200 nm, the major axis radius La of the metal quantum well unit is 180-750 nm, and the minor axis radius Lb is 40-200 nm.

[0021] More preferably, the period of the metal quantum well unit in the x-direction is Px = 610 nm and the period in the y-direction is Py = 915 nm; the height of the metal quantum well unit is h = 200 nm, the minor axis radius Lb is 96 nm, and the major axis radius La is 430 nm. At this point, the metal quantum well optical switch based on quasi-continuous domain bound states achieves a speed of 150 GW / cm². 2 Under intense pump light irradiation, the transmittance is 0.1 at a working wavelength of 1997 nm; the transmittance of the metal quantum well optical switch based on quasi-continuous domain bound states is 0.838 at a working wavelength of 1997 nm without pump light irradiation.

[0022] This invention also provides a method for fabricating a metal quantum well optical switch based on a quasi-continuous domain bound state. First, a metal quantum well composed of alternating stacked metal thin films and insulating thin films is grown on a substrate using thin film growth technology. Then, a metal quantum well array is obtained by photolithography, electron beam etching, or focused ion beam milling.

[0023] The metal quantum well unit is composed of multiple pairs of metal layers and insulating layers stacked alternately. The metal layers and insulating layers can be grown by thin film growth techniques, including magnetron sputtering, laser-assisted sputtering, electron beam evaporation, and molecular beam epitaxy. These techniques are all publicly available in the field.

[0024] Preferably, the metal quantum well is generated by high-vacuum magnetron sputtering technology, and the high-vacuum magnetron sputtering conditions are: greater than 1*10 ^-7 The reaction was carried out under Torr vacuum at a temperature of 300℃ to 600℃.

[0025] Preferably, when the metal film is a TiN film and the insulating film is an Al2O3 film, the TiN film growth power is less than 200W and it is grown under a nitrogen / argon mixed gas condition; the Al2O3 film growth power is less than 300W and it is grown under a pure argon environment.

[0026] The thickness h of the metal quantum well unit can be reduced by sacrificing the extinction ratio, thereby reducing the difficulty of actual manufacturing.

[0027] More preferably, the TiN layer of the metal quantum well can be grown at a reaction growth temperature of 350°C and a nitrogen / argon volume ratio of 7:3; the Al2O3 layer of the metal quantum well is deposited by sputtering in the same chamber as the TiN growth chamber, and the growth temperature of the Al2O3 layer is set to 350°C; under a chamber pressure of 5-mTorr and argon gas of 5 sccm, the growth power is 150W and the growth rate is about 0.4 nm / min.

[0028] Compared to existing technologies, the advantages of this invention lie in its use of a metallic quantum well to realize a tunable metasurface based on plasmonic quantum well-controlled interband transitions. Due to the large dipole moment of interband transitions provided by the quantum size effect, the nonlinear coefficient of the metallic quantum well is enhanced by several orders of magnitude compared to traditional nonlinear materials. Combining the high Q-value of the quantum well with the large nonlinear coefficient of the metallic quantum well, the modulation speed and modulation depth are effectively improved, surpassing previous modulation platforms based on continuous-domain bound states.

[0029] The quasi-continuous bound state-based metal quantum well optical switch of the present invention combines the high Q value of QBIC with the extremely high Kerr nonlinear coefficient of metal quantum well, effectively improving the modulation depth and achieving an extinction ratio of 9dB. Due to the instantaneous response of Kerr nonlinearity, the modulation speed of the quasi-continuous bound state-based metal quantum well optical switch can reach the terahertz level.

[0030] This invention successfully applies quasi-continuous bound states to a plasmonic metasurface, achieving a high modulation speed and high extinction ratio optical switch. The metal quantum well optical switch of this invention, with its high nonlinear coefficient, plays an effective role in micro / nano integrated optical switches and optical modulators, and shows promising application prospects in nonlinear neuron metasurface networks, deep learning metasurfaces, and ultrafast computing in integrated photonic networks. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the structure of the metal quantum well optical switch based on continuous domain bound states of the present invention.

[0032] Figure 2 This is a schematic diagram of a metal quantum well metasurface array, in which... Figure 2 a is a schematic diagram of the parameters of a metal quantum well metasurface array; Figure 2 b is a sample cross-sectional view of the metal quantum well unit.

[0033] Figure 3 This is a graph showing the refractive index variation of a metal quantum well under different pump light intensities.

[0034] Figure 4 This describes the transmittance, absorptivity, and quality factor Q of a metallic quantum well metasurface. Figure 4 a and Figure 4 b represents the dispersion plots of the transmittance and absorptance of the metal quantum well metasurface as a function of wavelength and deflection angle θ. Figure 4 c represents the evolution of the normalized transmission spectrum as the deflection angle θ changes from 0° to 30°; Figure 4 d represents the quality factor Q of the QBIC resonance under different deflection angles θ.

[0035] Figure 5 This is a performance test diagram of the optical switch of the present invention, wherein, Figure 5 a is the transmittance spectrum of the optical switch in the on and off states; Figure 5 b is a graph showing the change in the transmittance of the metasurface with pump intensity at a wavelength of 1997 nm; Figure 5 c to Figure 5 f represents the metasurface at 0 GW / cm². 2 Up to 150GW / cm 2 The normalized electric field distribution under the pump intensity.

[0036] Figure 6 The graph shows the trend of extinction ratio variation of the light switch at different deflection angles θ. Detailed Implementation

[0037] Exemplary embodiments of the present invention will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the present invention are shown in the drawings, it should be understood that the present invention may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0038] Depend on Figure 1 As shown, the metal quantum well optical switch based on continuous domain bound states in this embodiment includes a substrate 1 and a metal quantum well metasurface array 2 located on the substrate. The metal quantum well metasurface array is formed by a number of metal quantum well (MQW) units 3 arranged periodically. The metal quantum well unit 3 has an elliptical block structure.

[0039] like Figure 2 As shown, the MQWs array is composed of pairs of elliptical MQWs units. The angle θ between the major axis of the ellipse of the elliptical block structure and the y-direction is the deflection angle θ. In the x-direction, every two adjacent MQWs nanounits are mirror-symmetrical with respect to the yz plane. The x-direction is the substrate length direction, the y-direction is the substrate width direction, and the z-direction is the substrate height direction.

[0040] For conventional metal / insulator nanomaterials compatible with micro / nano fabrication processes, MQWs can be constructed when the thickness of the metal material is 5 nm or less.

[0041] In this embodiment, the MQWs unit consists of multiple pairs of ultrathin TiN layers and Al2O3 layers. The MQWs, which are composed of alternating stacked metal and insulating films, can be grown on a substrate first using magnetron sputtering technology. The TiN layer can be grown at a reaction growth temperature of 350°C and a nitrogen / argon volume ratio of 7:3. The Al2O3 layer is deposited by sputtering in the same chamber as the TiN layer, and the growth temperature of the Al2O3 layer is set to 350°C. Under a chamber pressure of 5-mTorr and an argon atmosphere of 5 sccm, the growth power is 150W and the growth rate is approximately 0.4 nm / min.

[0042] The MQWs array can then be obtained through photolithography or focused ion beam milling; the thickness h of the MQWs unit can be reduced by sacrificing the extinction ratio to reduce the difficulty of actual manufacturing.

[0043] Figure 2 This is a schematic diagram of a metal quantum well metasurface array. Figure 2As shown in Figure a, the period of the MQWs cell in the x-direction is Px = 610 nm and the period in the y-direction is Py = 915 nm; the height of the MQWs cell is h = 200 nm, the width is 2 × Lb = 2 × 96 nm, and the length is 2 × La = 2 × 430 nm. Figure 2 As shown in the representative cross-section of sample b, the MQWs unit is composed of multiple layers of 2.4 nm thick titanium nitride (TiN) film 5 and 0.5 nm thick aluminum oxide (Al2O3) film 6 stacked alternately.

[0044] The operating wavelength of the QBIC-based MQWs optical switch can be adjusted and selected by changing the period and deflection angle θ of the MQWs unit.

[0045] When a metasurface array of MQWs is normally incident with probe light (plane wave) polarized along the x-direction, a QBIC is formed, and this QBIC is used to form a high-Q resonant cavity. The dielectric constant of the MQWs unit can be changed by pump light polarized along the y-direction, thereby changing the resonant point formed by the QBIC to achieve a switching effect. On the other hand, due to the instantaneous response of Kerr nonlinearity, the modulation speed of the MQWs optical switch based on quasi-continuous bound states can reach the terahertz level.

[0046] Optical constants of MQWs as a function of pump intensity, such as Figure 3 As shown, n / / and k / / are the in-plane components of the refractive index, while n⊥ and k⊥ are the out-of-plane components. At 150 GW / cm² 2 At this time, the loss of MQWs is minimized. Therefore, the metal quantum well optical switch in this embodiment achieves a loss of 150 GW / cm². 2 Under intense pump light illumination, a QBIC and a high-Q resonant cavity are formed relative to the normally incident probe light.

[0047] Figure 4 This shows the dispersion plots of transmittance and absorptivity of a metallic quantum well metasurface as a function of wavelength and deflection angle θ. Experimental properties, including transmittance and reflectance, can be measured using an experimental setup with a pump probe configuration.

[0048] Figure 4 The pump intensity on MQWs units a and 4b is 150 GW / cm². 2 The transmission and absorption spectra of the metasurface were calculated. In the transmission spectrum, the width of the dark lines (indicating low transmittance) disappears at the dashed circle (θ = 0°). Based on the invisibility of BIC under plane waves, this indicates a symmetry-protected BIC. As θ gradually increases from 0° into the region of in-plane symmetry destruction, the metasurface system enters the quasi-BIC region.

[0049] like Figure 4As shown in the normalized transmittance curve of c, the resonance peak of the transmittance curve gradually narrows until it disappears at 0° as the asymmetry weakens, corresponding to the transition from quasi-BIC to BIC. When θ decreases to 0.1°, the Q value reaches its maximum value (approximately 74, mainly limited by the radiation loss and inherent loss of the MQW cell).

[0050] like Figure 5 As shown, due to the Q-value enhancement effect of QBIC and the wide range of refractive index variations of MQWs under different pump intensities, the metal quantum well optical switch of the present invention exhibits strong modulation capability. The MQWs unit exhibits relatively high loss in the absence of pump light, leading to the destruction of the QBIC resonant mode and high transmittance (i.e., the on state).

[0051] like Figure 5 As shown by the solid line in a, when the pump intensity on the MQWs unit is 150 GW / cm² 2 At this time, the losses of MQWs will be minimized, resulting in QBIC mode and low passivity (i.e., off state). Figure 5 The dashed line in a shows the QBIC resonating at 1997 nm in the off state, as designed with the current structural parameters.

[0052] according to Figure 5 The relationship between transmittance change and pump intensity in b, when the pump laser changes from 0 GW / cm² 2 (Without pump) increased to 150GW / cm 2 At that time, the transmittance decreased from 0.838 to 0.104, which is an extinction ratio of 9.06 dB.

[0053] Figure 5 The cf figure shows the normalized electric field distribution of the metasurface on the xy plane as a function of pump intensity. It can be seen that the loss of MQWs gradually decreases under higher pump intensity, and the Fano-type QBIC resonance becomes stronger.

[0054] Figure 6 This graph shows the extinction ratio variation trend of the metasurface optical switch at different deflection angles θ. The deflection angle θ has a certain influence on the modulation depth of the metasurface optical switch, and the extinction ratio of the metasurface optical switch reaches its maximum when θ = 15°. At a deflection angle θ of 15°, it approaches the "maximum absorption rate" (e.g., ...). Figure 4 The brightest region in b) is beneficial for improving modulation efficiency. Compared with deflection angles greater than 15°, the conduction state has higher transmittance at a deflection angle of 15°, resulting in lower energy loss and further improving the extinction ratio.

[0055] The above are merely embodiments of this application and are not intended to limit the scope of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of the claims of this application.

Claims

1. A metal quantum well optical switch based on quasi-continuous bound states, comprising a substrate and a metal quantum well metasurface array located on the substrate, wherein the metal quantum well metasurface array is formed by a periodic arrangement of several metal quantum well units; the metal quantum well units are elliptical block structures, wherein the major axis of the elliptical block structure has a deflection angle θ with respect to the y-direction, and every two adjacent elliptical block structures are mirror-symmetric with respect to the yz plane in the x-direction; the deflection angle θ is intended to obtain structural symmetry breaking in the y-direction, and when a probe light polarized along the x-direction is incident normally, a quasi-continuous bound state is formed, and a high-Q resonant cavity is formed using the quasi-continuous bound state.

2. The metal quantum well optical switch based on quasi-continuous domain bound states according to claim 1, characterized in that, The deflection angle θ is >0˚-30˚.

3. The metal quantum well optical switch based on quasi-continuous domain bound states according to claim 1, characterized in that, The metal quantum well unit is constructed by alternating stacks of metal layers and insulating layers with nanoscale thickness. The materials of the metal / insulator layers are metal / insulator nanomaterials compatible with micro-nano fabrication processes, wherein the thickness of the metal material is ≤5nm and the band gap of the insulating material is 1-10 electron volts.

4. The metal quantum well optical switch based on quasi-continuous domain bound states according to claim 3, characterized in that, The metal layer is a titanium nitride, gold, silver, aluminum or indium tin oxide film, and the insulating layer is an aluminum oxide film or a silicon dioxide film.

5. The metal quantum well optical switch based on quasi-continuous domain bound states according to claim 4, characterized in that, Preferably, the metal quantum well unit is formed by alternating stacking of several layers of titanium nitride thin film with a thickness of 0.5-5.0 nm and aluminum oxide thin film with a thickness of ≥ single atomic layer - 50 nm.

6. The metal quantum well optical switch based on quasi-continuous domain bound states according to claim 1, characterized in that, The metal quantum well metasurface array structure is as follows: the period Px of the metal quantum well unit in the x direction is 500-1000 nm and the period Py in the y direction is 750-1500 nm; the height of the metal quantum well unit is h=50-200 nm, the major axis radius La of the metal quantum well unit is 180-750 nm, and the minor axis radius Lb is 40-200 nm.

7. The metal quantum well optical switch based on quasi-continuous domain bound states according to claim 6, characterized in that, The period of the metal quantum well unit is Px=610 nm in the x direction and Py=915 nm in the y direction; the height of the metal quantum well unit is h=200 nm, the minor axis radius Lb is 96 nm, and the major axis radius La is 430 nm.

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