CN103762220A - High-linearity degree-of-polarization quantum-well infrared detector with plasmon micro-cavity coupled structure - Google Patents

Abstract

The invention discloses a highly linearly polarized quantum well infrared detector with a plasmon microcavity coupling structure. The detector consists of a metal grating layer formed by upper metal lines, a quantum well infrared photoelectric conversion activation layer and a lower metal reflection layer. composition. The advantages of the present invention are: 1. Utilize the mode selection effect of the electromagnetic wave near-field coupling microcavity formed between the upper metal grating and the lower metal reflective layer, so that the photons that can enter the microcavity can be combined with those The detection wavelength is dominated by photons that form resonances in polarization modes. 2. The electric vector direction of the photons entering the microcavity is changed from the x direction to the z direction under the modulation of the microcavity mode, which can be absorbed by the quantum well subband transition to form a photoelectric conversion process. Due to the above characteristics, the polarization coupling structure of the present invention can greatly improve the extinction ratio of the polarization response, so that the detector has extremely high polarization resolution capability.

Description

Highly Linearly Polarized Quantum Well Infrared Detector with Plasmonic Microcavity Coupling Structure

technical field

The invention relates to a photodetector with polarization detection capability, in particular to a linear polarization detection quantum well infrared photodetector adopting a plasmon microcavity coupling structure integrated on a picture element.

Background technique

The polarization state of light can be divided into circular polarization state and linear polarization state (referred to as linear polarization state). Generally, for the measurement of the linear polarization characteristics of light, a linear polarizer is placed in front of the detector, and the relationship between the intensity of the photodetector response and the angle of the polarizer is obtained by rotating the angle of the polarizer, so as to obtain the intensity of the incident light. linear polarization properties. However, this method requires mechanical rotation and cannot obtain the polarization characteristics of the incident light in real time. Another method is to divide the incident light into 3 to 4 paths through light splitting, place a polarizer with an angle deviation increasing by 45 degrees on each branch, and then use 3 to 4 detectors to measure at the same time, by calculating the Stokes component of light And the polarization state and its change of the real-time incident light are obtained through mathematical calculation synthesis. However, the device structure of this method is complex, especially for polarization imaging detection, at least three focal plane detectors with the same performance are required to work on the confocal plane at the same time, and image stitching and correction must be completed to realize polarization detection imaging. The deviation introduced in the assembly of optical components or the inconsistency of thermal expansion and contraction of each branch when the temperature changes will affect the quality of imaging. In order to solve this problem, people integrate the polarizer on the surface of the detector pixel, so that the pixel of a single photodetector only has the largest response to one polarization direction, and the polarization response to the vertical direction is the smallest, and then through Set polarizers in 4 directions on the 4 detector pixels, usually horizontal, vertical, 45 degrees to the left and 45 degrees to the right. When working, read the photoresponse signals on 4 pixels at the same time, obtain the Stokes polarization components in all directions, and then obtain the polarization state of the incident light through mathematical calculation. This kind of integrated polarizer is usually realized by a one-dimensional transmission grating formed by fabricating periodic line grids (referred to as wire grids) on the surface of photodetector pixels. For photodetectors such as quantum well infrared detectors that cannot directly absorb normal incident light, a one-dimensional reflective grating is fabricated on the back of the photodetector. On the one hand, the propagation direction of the incident light is changed, and on the other hand, the polarization state of the reflected light is selected. resulting in polarization selectivity. Since the integrated polarizer is directly made on the pixel, it is easy to make a focal plane detector. Among them, 4 pixels are taken to detect different polarization directions respectively, and then synthesized into a signal of a single pixel, and then directly output imaging through the two-dimensional array detectors which are periodically and repeatedly arranged in groups of such 4 pixels. Although this will reduce the spatial resolution of the focal plane detector, for example, the original 640×512 element detector, the actual number of equivalent pixels after the polarizer is reduced to 160×128 elements, but due to the increase in the detection of the polarization state , which can reflect the polarization information that is not available in the original intensity image, which is equivalent to adding a dimension of the detection signal, while still maintaining the real-time, fast and direct imaging capability of the original focal plane detector. Therefore, in medicine, tumor detection, Geological detection in remote sensing and target detection in military reconnaissance have broad application prospects.

In this integrated micro-polarizer structure, due to the limitation of the pixel size of the detector, the size of the polarizer cannot exceed the pixel size and thus is also limited, resulting in the working conditions of the one-dimensional wire grid polarizer far away from that of the ideal polarizer. The working state, that is, the ideal working state of the polarizer requires that the size of the wire grid in the length direction is much larger than the period of the wire grid and larger than the diameter of the incident light spot. Therefore, when the integrated micro-polarizer forms the best transmission condition for a certain polarization state, the polarized light in the vertical direction can also pass through the polarizer through pixel edge coupling, etc., causing a photoelectric response in the detector pixel . This reduces the polarization selectivity of the integrated polarizer and correspondingly reduces the extinction ratio of the polarization detector (defined as the ratio between the transverse magnetic TM polarized light response and the transverse electric TE polarized light response, expressed as the extinction ratio ρ=R _TM /R _TE , R _TM , R _TE are the detector's response to TM polarized light and TE polarized light respectively).

In addition, polarizers in the visible light band can use linear arrangements of organic macromolecules to make one-dimensional transmission gratings. This linear arrangement of macromolecules has the characteristics of simple fabrication and low cost. However, in the infrared band, especially in the mid-infrared band (400-4000cm ^-1 , 2.5-25μm), these macromolecules will strongly absorb the incident light, so they cannot be used to prepare infrared polarizers. In the infrared band, only metal or non-absorbing dielectric materials can be used to make one-dimensional wire grids to form infrared polarizers.

Among the currently reported polarization detectors integrated with conventional all-solid-state one-dimensional wire grid micropolarizers, silicon-based CCD detectors can achieve an extinction ratio of 60 in the visible band [for details, see the literature Viktor Gruev, RobPerkins, and Timothy York, CCD polarization imaging sensor with aluminum nanowire optical filters, OPTICS EXPRESS Vol.18, P.19087], and the maximum extinction ratio in the infrared band is 10 [specifically see the literature Thomas Antoni, Alexandru Nedelcu, Xavier Marcadet, Hugues Facoetti, and Vincent Berger, High contrast polarization sensitive quantum well infrared photodetectors, APPLIED PHYSICS LETTERS, Vol.90, P.201107]. In order to detect the polarization state of light more sensitively, people are looking forward to the emergence of detectors with higher polarization extinction ratios. The present invention proposes a highly linearly polarized quantum well infrared photodetector using a plasmonic microcavity coupling structure, utilizing the mode selection function of the plasmonic microcavity structure and the quantum selection rule for intersubband transitions in quantum well detectors The determined intrinsic characteristic of selective transition absorption, the superposition of the two characteristics, realizes the maximization of the TM polarization coupling of the incident light in the direction perpendicular to the wire grid, and successfully suppresses the incident intensity of the TE polarized light in the vertical direction , so a high polarization extinction ratio in the infrared band can be obtained, thereby realizing the photodetection of infrared light with a high degree of linear polarization. The polarization extinction ratio obtained in the embodiment of the present invention near the wavelength of 14.2-14.9 microns in the long-wave infrared band reaches 65, which is comparable to the best level in the visible band.

Contents of the invention

The object of the present invention is to propose a quantum well infrared photodetector using a plasmonic microcavity coupling structure integrated on a picture element to realize high linear polarization photodetection.

The present invention adopts a quantum well infrared photodetector with a plasmon microcavity coupling structure, and its structure is in the order of incident light passing through: a metal grating layer 1 formed by upper metal lines, and a quantum well infrared photoelectric conversion active layer 2 , the lower metal reflective layer 3 .

The metal grating layer 1 is a one-dimensional metal line grating arranged periodically with period p, line width s, and thickness h1, and its material includes but not limited to highly conductive gold or silver. In order to improve its adhesion, a layer of adhesive metal with a thickness of 10-30 nanometers can be added between it and the quantum well infrared photoelectric conversion active layer 2, and its material includes but not limited to titanium. Its period p, line width s, and thickness h1 are determined by the optimization results obtained from theoretical calculations. The goal of optimization calculations is to enable the incident light wave to collectively oscillate with the electrons in the metal to form a localized surface mode of the plasmon (Localized Surface Plasmon, LSP) and the surface plasmon polariton mode (Surface Plasmon Polariton, SPP) undergo resonant coupling, and enter the coupled microcavity under the induction of the two modes, forming a transverse standing wave cavity mode. For the mid-to-far infrared band (50-4000cm ^-1 , 2.5-200μm), the theoretical calculation gives the following size parameter design range of the metal grating: ①The value of the stripe width s is between one-tenth and ten-tenths of the detection wavelength . For the resonant coupling mode, the relationship between the metal stripe width s and the detection wavelength λ satisfies s=kλ/2n, where k is the resonance order, and n is the refractive index of the active material in the quantum well infrared photoelectric conversion active layer 2, and the general value is 3-5, depending on layer 2 thickness. For k=1, s is at least one tenth of the detection wavelength. When the resonance order k>2n, s can reach a detection wavelength at most. But when k>6, the resonant coupling effect will gradually weaken, resulting in a weakened polarization effect. Therefore the maximum value of s does not exceed ten tenths of the detection wavelength. ② The value of the period p is one-tenth to thirty-tenths of the detection wavelength. This is because the period p must be greater than the fringe width s, and therefore must be greater than one-tenth of the detection wavelength. However, when the stripe width s remains unchanged, increasing the period means increasing the slit width. When the slit width is larger than twice the stripe width, theoretical calculations show that the TE polarization will increase to be comparable to the resonant wavelength position of the TM polarization, resulting in a decrease in the extinction ratio. The value of the period p therefore does not exceed thirty-tenths of the detection wavelength at most. ③ The value of the thickness h1 of the metal grating layer 1 is not less than 0.0096 times the square root of the detection wavelength in microns. This is because the resonant coupling condition requires that there is no interaction between the electromagnetic fields on the upper and lower surfaces of the metal grating layer 1, that is, the value of the thickness h1 is required to be not less than twice the skin depth. In the mid-to-far infrared band, the theoretically given skin depth of electromagnetic waves in metals is d ~ 0.0048·λ ^1/2 .

The quantum well infrared photoelectric conversion activation layer 2 refers to a semiconductor quantum well thin film material capable of absorbing incident light and generating photogenerated carriers, which is formed by sandwiching a single or multiple quantum wells in a barrier layer. Under the light, the electrons in the ground state sub-level (or sub-band) of the conduction band absorb photons and transition to the excited state, forming photo-generated carriers, and transporting the photo-generated carriers through the electric field between the upper and lower metal electrodes In the external circuit, the signal of photoelectric conversion is formed. Its thickness h2 is determined by the optimization result obtained from theoretical calculation, and the goal of optimization calculation is to make the transverse standing wave mode formed by the electromagnetic wave coupled into the coupled microcavity to be the strongest. According to the near-field coupling requirements of the plasmonic microcavity, h2 must be smaller than the equivalent light wavelength of the detected incident light, that is, the light wavelength in vacuum divided by the refractive index of the layer material. When the minimum value of the refractive index is 3, h2 should not be greater than one-third of the detection wavelength.

The lower metal reflective layer 3 refers to a complete metal layer with a thickness of h3. The value of h3 must be greater than the skin depth of the probe light in the metal, that is, 0.0048 times the square root of the probe wavelength in microns. The metal layer at this time can serve as a reflective layer for electromagnetic waves. Its material includes but not limited to highly conductive gold or silver. In order to improve its adhesion, a layer of adhesive metal with a thickness of 10-30 nanometers can be added between it and the quantum well infrared photoelectric conversion active layer 2, and its material includes but not limited to titanium.

The working principle of the present invention is: the metal wire grid designed for a specific infrared detection wavelength is arranged periodically, and its period is smaller than the wavelength of the detected light, so that the plasmons formed by the collective oscillation of electrons in the grating metal can It resonantly couples with the transverse magnetic (TM) polarized light whose polarization direction is perpendicular to the direction of the grating lines in the incident infrared light, and cooperates with the lower metal reflective layer to form a new modulation for the distribution of the light field, so that the incident light can be coupled It propagates into the microcavity and forms a cavity mode in the form of a standing wave. Its propagation direction changes from the z direction perpendicular to the detector plane in free space to the x direction along the detector plane, and is absorbed by the quantum well infrared photoelectric conversion active layer 2 and converted into an electrical signal. However, the transverse electric (TE) polarized light whose polarization direction is parallel to the y direction of the grating line in the detected infrared light cannot be resonantly coupled, so it cannot enter the coupling microcavity formed by the upper and lower metal layers, so it will not be detected by the photoelectric active layer. Absorption, it can not generate electrical signals. Thus, the structure forms a photoelectric response signal that is highly dependent on the polarization state of incident infrared light, constituting a photodetector with a high degree of polarization.

The advantages of the present invention are:

1 A brand-new design is proposed in the photoelectric coupling structure of the infrared integrated polarized photodetector, which can replace the traditional wire grid coupling as the main way, and utilize the plasmon resonance formed between the upper metal grating and the lower metal reflective layer The mode selection effect of the electromagnetic wave near-field coupling microcavity makes the photons that can enter the microcavity mainly those photons that can form resonance with the detection wavelength polarization mode.

2 The electric vector direction of the photons entering the microcavity is changed from the x direction to the z direction under the modulation of the microcavity mode, which can be absorbed by the quantum well subband transition to form a photoelectric conversion process. Those photons that cannot resonate with this polarization mode cannot enter the microcavity, and therefore cannot form a photoelectric conversion. Adopting the polarization coupling method of the present invention can greatly improve the extinction ratio of the polarization response. In the embodiment of the present invention, the polarization extinction ratio of the detector can be realized to be 65, which has extremely high polarization resolution capability.

Description of drawings

FIG. 1 is a schematic diagram of a cross-sectional structure of a quantum well infrared polarization detector of the present invention. In the figure 1: the metal grating layer formed by the upper metal lines, 2: the quantum well infrared photoelectric conversion active layer, 3: the lower metal reflective layer.

Fig. 2 is a schematic diagram of the three-dimensional structure of the pixel of the quantum well infrared polarization detector of the present invention. In the figure 1: the metal grating layer formed by the upper metal lines, 2: the quantum well infrared photoelectric conversion active layer, 3: the lower metal reflective layer.

Fig. 3 is a curve of the current response spectrum of the quantum well infrared polarization detector to incident polarized light as a function of the angle of the polarizer in the incident light path, which is actually measured in the second embodiment of the present invention.

Fig. 4 is the relationship between the average value of the photocurrent with the wavelength between 14.2-14.9 μm and the angle of the polarizer in the incident light path in the second embodiment of the present invention. The solid circle points are the experimental points, and the data are taken from the experimental spectra in Fig. 4. The solid line is the normalized calculation result of the function Sin ² θ, where θ is the angle of the polarizer. This curve represents the transmittance of an ideal polarizer as a function of angle.

Detailed ways

Taking the high degree of linear polarization GaAs/ _AlxGalxAs quantum well infrared photodetector with a peak detection wavelength of 13.6 μm and _a plasmonic microcavity coupling structure as an example, the specific implementation of the present invention will be further described in conjunction with the accompanying drawings Detailed description.

See Fig. 1 and Fig. 2, the highly linearly polarized quantum well infrared photodetector of the plasmonic microcavity coupling structure involved in the present invention includes: a metal grating layer 1, which has a period of p, a linewidth of s, and a thickness of is a one-dimensional periodic metal line grating of h1, and the metal used in this embodiment is gold. In order to improve its adhesion, a layer of metal titanium is added between it and the quantum well infrared photoelectric conversion active layer 2 . The period p, line width s and thickness h1 of the metal grating layer 1 are calculated and determined by the finite difference time domain (FDTD) method. The value is between one-tenth and ten-tenths of the detection wavelength in microns, and the value of the thickness h1 is not less than 0.0096 times the square root of the detection wavelength in microns. The metal grating layer 1 is prepared by thin film deposition, and a grating pattern is formed by photolithography and etching.

The quantum well infrared photoelectric conversion active layer 2 refers to a semiconductor quantum well film material capable of absorbing incident light and generating photogenerated carriers, and is formed by sandwiching a single or multiple quantum wells in a barrier layer. Under the light, the electrons in the ground state sub-level (or sub-band) of the conduction band absorb photons and transition to the excited state, forming photo-generated carriers, and transporting the photo-generated carriers through the electric field between the upper and lower metal electrodes In the external circuit, the signal of photoelectric conversion is formed. Its thickness h2 is determined by the optimization result obtained from theoretical calculation, and the goal of optimization calculation is to make the transverse standing wave mode formed by the electromagnetic wave coupled into the coupled microcavity to be the strongest. According to the near-field coupling requirements of the plasmonic microcavity, h2 must be smaller than the equivalent light wavelength of the detected incident light, that is, the light wavelength in vacuum divided by the refractive index of the layer material. The quantum well film is prepared on the GaAs substrate by molecular beam epitaxy (MBE) or metal organic chemical vapor phase epitaxy (MOCVD), and then a separate epitaxial film is formed by substrate lift-off.

The lower metal reflective layer 3 is a complete metal layer with a thickness h3 not less than the skin depth of the detection light wave in the metal, that is, the thickness h3 is not less than 0.0048 times the square root of the detection wavelength in microns. The metal reflective layer is prepared by thin film deposition method.

In the above size range, strict FDTD theoretical calculations show that due to the common coupling effect of the metal periodic wire grid and the metal reflective layer, the incident TM light is coupled and resonantly modulated, so that the incident light with a wavelength greater than 10 microns can enter the thickness In a coupled microcavity of less than 1 micron, the propagation direction changes from the z direction perpendicular to the detector surface to the x direction parallel to the detector surface, and a transversely oscillating column wave is formed. The electric vector of the electromagnetic wave in the microcavity is along the z direction, parallel to the growth direction of the quantum well, and can be absorbed by the subband transition of the quantum well.

The coupling structure used in this embodiment has an area of 230×200 μm ² , which can be changed according to actual needs. In this embodiment, the manufacturing process of the highly linearly polarized quantum well infrared photodetector with the plasmonic microcavity coupling structure can be made according to the invention patent (patent number: 201110082811.7, patent name: for photoelectric function) obtained by the inventor. It can be realized by the step method in the technological process of metal waveguide microcavity optical coupling structure of the device), and can also be realized by other micromachining technological processes.

Aiming at the peak detection wavelength of 13.6 μm, the present invention uses different coupling structure size parameters in three embodiments to prove the feasibility and effectiveness of the present invention. The thickness dimension parameters h1, h2 and h3 are fixed, and the period p, line width s and thickness of the viscous metal grating are changed.

The thickness dimension parameter h1 is taken as 0.1 micron in this embodiment, which satisfies the condition that it is not less than 0.0096 times the square root of the detection wavelength in micron.

For the thickness dimension parameter h2, in the embodiment of the present invention, the direction of the incident light is taken as the upper direction, then the layer consists of 5 sub-layers from bottom to top, which are respectively: the n-type doped GaAs lower electrode with a sub-layer thickness of 490 nanometers layer with a doping concentration of 5.0×10 ¹⁷ cm ^-3 ; an Al _x Ga _lx As lower barrier layer with a sublayer thickness of 100 nanometers, where x=0.15; an n-type doped GaAs potential well with a sublayer thickness of 7 nanometers layer with a doping concentration of 2.0×10 ¹⁷ cm ^-3 ; an Al _x Ga _lx As upper barrier layer with a sublayer thickness of 100 nm, where x=0.15; an n-type doped GaAs upper electrode with a sublayer thickness of 190 nm layer with a doping concentration of 5.0×10 ¹⁷ cm ^-3 . The total thickness of the 5 sublayers constitutes h2, which has a value of 887 nanometers, or 0.887 micrometers. This value satisfies the condition that the thickness is smaller than the equivalent wavelength (for the detection wavelength range of 10-16 μm in this embodiment, if the refractive index of the quantum well is 3.3, the equivalent wavelength is 3-4.5 μm).

The thickness dimension parameter h3 is taken as 0.1 micron in this embodiment, which satisfies the condition that it is not less than 0.0048 times the square root of the detection wavelength in micron.

Embodiment 1: The line width s of the metal grating on the upper layer is set to 1.36 microns, which is one-tenth of the detection wavelength. The period p is taken as 2.6 microns, which satisfies the condition of being greater than the line width s. The sticky metal titanium is 10 nanometers thick.

Embodiment 2: The line width s of the upper metal grating is 5.5 microns, the period p is 10.6 microns, and the thickness of the viscous metal titanium is 20 nanometers.

Embodiment 3: The line width s of the metal grating on the upper layer is set to 13.6 microns, which is ten tenths of the detection wavelength. The period p is taken as 40.8 microns, which is thirty-tenths of the detection wavelength. The viscous titanium metal is 30 nanometers thick.

The results obtained in the above three embodiments are similar, and the test results of the second embodiment are shown in the accompanying drawings.

FIG. 3 shows the variation curve of the current response spectrum of the quantum well infrared polarization detector to incident polarized light as a function of the angle of the polarizer in the incident light path, which is actually measured in Embodiment 2 of the present invention. It can be seen that in the range of 10-16 microns, the intensity of the photocurrent spectrum of the detector changes gradually with the angle of the polarizer on the incident light path, showing clear polarization selectivity.

Fig. 4 is the relationship between the average value of the photocurrent with the wavelength between 14.2-14.9 μm and the angle of the polarizer in the incident light path in the second embodiment of the present invention. The solid circle points are the experimental points, and the data are taken from the experimental spectra in Fig. 3. The solid line is the normalized calculation result of the function Sin ² θ, where θ is the angle of the polarizer. This curve represents the transmittance of an ideal polarizer as a function of angle. Since almost no photons can be coupled into the detector when the angle of the polarizer is zero, the signal detected by the detector is extremely weak, and R _TE is close to zero, resulting in large fluctuations in the value of the extinction ratio (extinction ratio ρ=R _TM / R _TE , R _TM , R _TE are the detector's response to TM polarized light and TE polarized light respectively). In order to obtain a definite value of the polarization extinction ratio, the average value of the photocurrent in the wavelength range of 14.2-14.9 μm is obtained, and the variation relationship of the average value with the angle of the incident light path polarizer is given. The maximum extinction ratio is obtained by dividing the value of the polarizer 90 by the value of the polarizer 0 degree. The wavelength range of the maximum polarization extinction ratio is between 14.2-14.9 μm and does not appear at the peak detection wavelength of 13.6 μm. This is because the peak detection wavelength is not only modulated by the electromagnetic wave mode in the plasmon-coupled microcavity, but also modulated by the absorption peak shape of the quantum well itself. The maximum polarization extinction ratio obtained in the embodiment of the present invention near the wavelength of 14.2-14.9 μm in the long-wave infrared band reaches 65, which is comparable to the best level in the visible band.

Claims (4)

1. the high Linear polarization quantum trap infrared detector of a phasmon microcavity coupled structure, its structure is successively as order is the metal grating layer (1) that upper strata metal wire forms successively take incident light process, quantum well infrared electro conversion active coating (2) and lower metal reflector (3), is characterized in that:

The metal grating layer (1) that described upper strata metal wire forms for the cycle be that p, live width are the metal wire grating that One Dimension Periodic that s, thickness are h1 is arranged, its material includes but not limited to the golden or silver-colored of high conductivity;

Described quantum well infrared electro conversion active coating (2) is the single or multiple quantum well layers of N-shaped semiconductor based on conduction band intersubband transitions, composition material includes but not limited to GaAs/AlGaAs, InGaAs/InAlAs/InP or InGaAs/GaAs, and the numerical value of its thickness h 2 should be not more than surveys 1/3rd of wavelength;

Described lower metal reflector (3) refers to that thickness is the metal level of the intact of h3, and the value of h3 is not less than subduplicate 0.0048 times of detection wavelength take micron as unit, and its material includes but not limited to the gold of high conductivity or silver-colored.

2. the high Linear polarization quantum trap infrared detector of a kind of phasmon microcavity coupled structure according to claim 1, it is characterized in that: the stickiness metal level that is attached with the adhesion of the metal grating layer 1 that improves upper strata metal wire formation between the metal grating layer (1) forming at described upper strata metal wire and quantum well infrared electro conversion active coating (2), its thickness is 10～30 nanometers, and material includes but not limited to titanium.

3. the high Linear polarization quantum trap infrared detector of a kind of phasmon microcavity coupled structure according to claim 1, it is characterized in that: the numerical value of the period p of the metal grating layer (1) that described upper strata metal wire forms is to survey 30/1/10ths to ten of wavelength, the numerical value of live width s be survey wavelength 1/10th to 10/10ths between, the value of thickness h 1 is not less than subduplicate 0.0096 times of detection wavelength take micron as unit.

4. the high Linear polarization quantum trap infrared detector of a kind of phasmon microcavity coupled structure according to claim 1, it is characterized in that: between described quantum well infrared electro conversion active coating (2) and lower metal reflector (3), be attached with the stickiness metal level that improves lower metal reflector (3) adhesion, its thickness is the stickiness metal of 10～30 nanometers, and its material includes but not limited to titanium.

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