CN207601358U - A kind of broadband unidirectional transmission structures of photonic crystal - Google Patents

Abstract

The utility model discloses a broadband unidirectional transmission structure of a photonic crystal, which belongs to the technical field of optoelectronics. The purpose is to provide a broadband unidirectional transmission structure of a photonic crystal, a special structure. Changing the structure parameters can make the reverse transmission efficiency zero, and adjusting the structure size can get the UDT effect at any frequency. Realize the broadband unidirectional transmission structure of photonic crystal. It consists of a two-dimensional photonic crystal and a dielectric grating. Photonic crystals are designed to have a Dirac cone at point Γ, which allows transmission of normally incident light but not obliquely incident light within a certain frequency range. The goal of a dielectric grating is to diffract normally incident light into other directions. Thus, when incident light passes through the structure from one direction, it is completely blocked, and when incident light enters from the other direction, it is transmitted.

Description

A broadband unidirectional transmission structure of photonic crystal

technical field

The utility model relates to a broadband unidirectional transmission structure of a photonic crystal, which belongs to the technical field of optoelectronics. Specifically, an excellent one-way transmission device is provided.

Background technique

Unidirectional transmission (UDT) devices are fundamental to optical computing and information processing. In general, there are two basic approaches to obtain UDT effects: interactive systems and non-interactive systems. It is relatively easy to implement non-interactive systems by using Faraday effect and nonlinear effect to break the time-reversal property, but the large volume and related high-intensity requirements make the UDT effect of non-interactive systems unsuitable for modern integrated optics. In recent years, the UDT effect has attracted considerable attention in interacting linear systems with micro- and nanoscale structural dimensions. As of now, various interacting UDT structures have appeared in the literature, including asymmetric metal films, two-layer metal gratings, single dielectric gratings, metal gratings, and various optical materials. However, two practical problems still arise when applying interactive UDTs:

1. Because in a linear system, there is no energy threshold for optical transmission, so the reverse transmission cannot be completely cut off, even if it is a single operating frequency, the following conditions cannot be satisfied at the same time:

T ₁ /T ₂ ＞1000 10log(1/T ₂ )＞20

Among them, T ₁ represents the forward transmission efficiency, and T ₂ represents the reverse transmission efficiency.

2. Most of the UDT effects obtained in the receiving system are based on various resonance effects. They have no bandwidth and are only effective at a single or discrete resonance frequency. Even at THz, GHz, and even at individual visible frequencies, UDT has been able to However, there is still no general method for implementing UDT at any frequency, especially wideband UDT.

Utility model content

The purpose of the utility model is to provide a broadband unidirectional transmission structure of a photonic crystal, a special structure. Changing the structure parameters can make the reverse transmission efficiency zero, and adjusting the structure size can get the UDT effect at any frequency.

Specifically, a broadband unidirectional transmission structure for realizing photonic crystals. It consists of a two-dimensional photonic crystal and a dielectric grating. Photonic crystals are designed to have a Dirac cone at point Γ, which allows transmission of normally incident light but not obliquely incident light within a certain frequency range. The goal of a dielectric grating is to diffract normally incident light into other directions. Thus, when incident light passes through the structure from one direction, it is completely blocked, and when incident light enters from the other direction, it is transmitted.

The broadband unidirectional transmission structure also includes a dielectric grating arranged opposite to the photonic crystal. The photonic crystal is a two-dimensional photonic crystal. The photonic crystal has a Dirac cone at one point, and the Dirac cone allows vertically incident light transmission within a certain frequency range. Rather than allowing obliquely incident light to transmit, dielectric gratings are used to diffract normally incident light into other directions.

The refractive index of the dielectric grating is n=2.

The thickness of the dielectric grating is H=0.317 μm.

The beneficial effect of the utility model is that: the photonic crystal/grating structure of the utility model realizes the broadband unidirectional transmission of the photonic crystal. The device is not only suitable for broadband frequency, more importantly, the working frequency can be adjusted arbitrarily by adjusting the structure size. In addition, both photonic crystals and gratings are symmetrical structures, and the entire structure is dielectric, so fabrication and integration are very convenient. The present invention opens up a way to achieve high-efficiency broadband UDT effects using the most conventional optical components and materials.

Description of drawings

Figure 1a is a photonic crystal structure;

Figure 1b is the three-dimensional dispersion surface of the photonic crystal;

Figure 1c is a diagram of the dispersion relationship when light is vertically incident;

Figure 1d is a diagram of the dispersion relationship when light is obliquely incident;

Fig. 2 is the corresponding transmission spectrum of the photonic crystal layer during normal incidence and oblique incidence (K _x =0.1(2π/a)), the solid line represents the vertical incidence, and the dotted line represents the oblique incidence;

Fig. 3 a is the unidirectional (UDT) structure schematic diagram of this utility model broadband forward incidence;

Fig. 3 b is a schematic diagram of the structure of the utility model broadband reverse incident unidirectional (UDT);

Figure 4a is the transmission spectrum when the thickness of any single grating is H=0.317 μm;

Figure 4b shows the relationship between transmittance contrast and isolation and incident wavelength when H=0.317μm;

Figure 4c is the corresponding electric field distribution for forward and reverse incidence when λ=0.65 μm.

Reference signs are as follows: 1. Two-dimensional photonic crystal, 2. Photoelectron grating, 3. First band gap, 4. Second band gap, 5. Dirac point frequency.

Detailed ways

The specific embodiment of the utility model is described below in conjunction with accompanying drawing:

Firstly, the transmission characteristics near the Dirac point in the photonic crystal based on the dispersion relation are analyzed, and then a UDT structure is designed. The principle is described from the perspective of geometric optics, and the numerical simulation of the one-way transmission result and its properties is carried out by using the finite difference time domain (FDTD) technique. Figure 1a is a simple two-dimensional photonic crystal, which consists of some dielectric rods arranged in a square shape. The Cartesian coordinates are also given in the figure. The dielectric rods are arranged periodically along the X and Y directions, and their axes are along the Z direction. arrangement, f and R represent the lattice constant and the radius of the rod, respectively, ε is the relative permittivity of the dielectric rod, ε = 12.5, first, along the Z direction, we first calculate the transverse magnetic (TM) polarized light with electric The three-dimensional dispersion surface of the photonic crystal, in the calculation, R=0.2a, then, λ is the incident wavelength. The results are shown in Fig. 1b, which agree well with the dispersion relation of traditional square lattice photonic crystals. It can be seen that the Dirac cone is located at the Dirac point at K _x =0, _Ky =0, f=0.541a/λ, corresponding to the Γ point of the first Brillouin zone of the band structure of the photonic crystal. Near the Dirac point frequency, there is no band gap in the entire calculated frequency span. However, this does not mean that there is no band gap in any direction of propagation. When light propagates along the y direction, when _Kx = 0 (normal incidence), from Fig. 1b to Fig. 1c, it can be seen that there is no band gap, which means that all calculated frequencies can propagate in the photonic crystal. However, if K _x ≠ 0 (oblique incidence), assuming K _x = 0.1(2π/a), the dispersion curve is shown in Fig. 1d. It can be seen that, unlike the normal incidence case, two band gaps appear at the Dirac point frequencies above and below, leaving a small transmission band in between. For frequencies within the band, no transmission is possible in this photonic crystal.

In order to verify the above properties, Fig. 2 shows the corresponding transmission spectra of the photonic crystal layer at normal incidence and oblique incidence (K _x = 0.1(2π/a)), including the first band gap 3 and the second band gap in Fig. 2 4. The lattice constant a=0.36 μm, and the thickness of the photonic crystal layer is selected as 10 layers of dielectric rods. Solid and dashed lines indicate normal incidence and oblique incidence, respectively. Clearly, for normal incidence, there are no transmission gaps across the frequency band. While for oblique incidence _Kx = 0.1(2π/a), there are two distinct transmission gaps next to the Dirac point frequency. For frequencies within these gaps, photonic crystals behave as zero-index materials (except for Dirac point frequencies, there is no true zero-index). When light is incident at normal incidence, it is almost completely transmitted. When the light is angled, it is fully reflected.

From the above analysis, it can be seen that for the frequency in the band gap, the direction of the incident light determines whether it can be transmitted. There are various methods to control the propagation direction of incident light, and grating diffraction is one of the most convenient methods. Here we choose a dielectric diffraction grating to realize this function. Figure 3a and Figure 3b show the combined structure of photonic crystal and grating. The rectangles represent photonic crystals 1 , the arrays represent independent dielectric gratings 2 (actually the UDT effect is not affected if there is a substrate with gratings of the same material), and the Dirac point frequency 7 is included in the figure. It can be seen that for normal incidence, the light normally reaches the photonic crystal 1, it passes through the photonic crystal 1 layer and reaches the dielectric grating 2, and then diffracts to the right with a different diffraction order. The overall transmission is the sum of all diffraction orders. When light comes in from behind, it is first diffracted into many directions. When these diffracted lights are incident on the photonic crystal, except for the zero sequence, the other diffracted lights are oblique. This oblique incident light is reflected. Therefore, if the period of the grating is larger than the incident wavelength, the asymmetric transmission (AT) effect is easily obtained. The more diffraction orders, the better the effect. In addition, the intensity distribution between diffraction orders is another factor affecting the AT effect. The lower the diffraction efficiency of the zero order, the better the AT effect. According to our previous research, the diffraction efficiency of the zeroth order can be adjusted to 0 by choosing an appropriate grating thickness, so that a perfect UDT effect can be obtained.

According to the above principles, the UDT structure is now designed and its parameters are optimized. In the simulation, only one cycle surrounded by the dashed line is counted. The lattice constant and radius were kept at a = 0.36 μm, R = 0.2a = 0.072 μm, and 10 layers of rods were still used in the incident direction. For the sake of simplicity, the fill factor of the grating is selected as L/P=0.5, and the refractive index of the grating material is n=2. First, in order to obtain a better AT effect, the grating period should be greater than the incident wavelength, which is set as P=6a=2.16μm, which is about three times the incident wavelength. Figure 4a shows the transmission spectrum for any single grating with a thickness of H = 0.317 μm. It can be clearly seen that the transmissions of the two opposite directions are very different in a certain frequency range. The forward transmittance fluctuates at higher numerical levels, reaching almost unity at some frequencies. While for the reverse direction from 0.62 μm to 0.66 μm, the transmittances are all close to 0. This is because the grating almost diffracts the incident energy into the higher-order bandgap of the photonic crystal. Thus, unidirectional transmission within a certain frequency range is obtained.

As an excellent optical one-way device, the transmission difference ΔT=T ₁ -T ₂ , and the reverse transmittance T ₂ should be large enough. Transmittance contrast (T ₁ /T ₂ ) and isolation (D=10log(1/T ₂ )) are two parameters for judging the performance of UDT devices. Figure 4b shows the relationship between these two parameters and the incident wavelength when H=0.317μm. It can be seen that the peaks of the two curves occur at the same position. The maximum contrast and isolation can reach 8000 and 40 respectively. This is because at this wavelength value, the condition H=λ/2 is satisfied, and the efficiency of the zero order is almost 0, so the reverse transmittance is very small, resulting in the maximum contrast and isolation. This means that for a single frequency point, a perfect UDT effect can be achieved by optimizing the thickness of the grating. In addition, it can be seen from the figure that the isolation is almost higher than 20 and the contrast is almost higher than 1000 in the entire calculation range. Even for a single wavelength in other UDT structures, it is difficult to satisfy these two conditions at the same time in a certain frequency range. It certainly verifies that this UDT structure works for a wide range of bands.

To further illustrate the UDT effect, Fig. 4c shows the corresponding electric field distributions for the two incident directions when λ = 0.65 μm. from The 0.65 μm was chosen arbitrarily, and it can be seen that if the light is incident from the side of the photonic crystal, the light can propagate through the whole structure and be diffracted into many directions. Interference occurs between them, and a certain interference pattern is formed after passing through the grating. When light is incident from the side of the grating, some energy still enters the photonic crystal. But these energies will fall into the band gap of the photonic crystal and cannot pass through the photonic crystal. Therefore, the transmittance is almost zero.

For the dispersed surface of this photonic crystal, as shown in Fig. 1b, the frequency is normalized by the inverse of the lattice constant. This means that when the entire structure is scaled up or down, the red-shifted or blue-shifted dispersion relationship remains unchanged, as does the transmission spectrum shown in Figure 2. In addition, the diffraction energy distribution between diffraction orders can also be controlled by optimizing the thickness of the grating, and the zero diffraction efficiency of the zero order can be obtained at any frequency. Therefore, by adjusting the size of the structure, the UDT effect can be obtained at any frequency position. In information processing systems, 1.55μm is usually selected as the working wavelength. Here, another UDT device working at λ=1.55μm is designed to prove that the UDT effect can be obtained at any frequency. First, enlarge the structure size by 1.55/0.634=2.44 times, and the parameters become a=0.88μm, R=0.176μm, P=5.28μm, L=2.64μm, H=0.772μm. The same transmission shape was obtained. In order to be more convincing, we changed the thickness of the grating to H=0.775μm, and calculated the transmitted light. It can be seen that although the transmission spectrum changes slightly, the excellent UDT effect at a wavelength of 1.55μm is still obtained. In addition, similar UDT effects on microwave, THz, GHz, and infrared are obtained by adjusting the structure size. It is proved that the UDT effect can be obtained at any frequency.

All in all, high-efficiency UDT structures can be obtained by utilizing the unique Dirac cone dispersion surface of photonic crystals. For a single operating frequency, the forward transmittance can be as high as 95%, while the reverse transmittance is less than 10 ^-4 , so the contrast and isolation can reach 8000 and 40 respectively. Even in a relatively large frequency range, the contrast and isolation can reach 1000 and 20 at the same time. The device is not only suitable for broadband frequency, more importantly, the working frequency can be adjusted arbitrarily by adjusting the structure size. In addition, both photonic crystals and gratings are symmetrical structures, and the entire structure is dielectric, so fabrication and integration are very convenient. This paper opens the way to realize the efficient broadband UDT effect using the most conventional optical components and materials.

The foregoing is a preferred embodiment of the present utility model, and it should be pointed out that for those of ordinary skill in the art, some improvements and modifications can be made without departing from the principle of the present utility model. Retouching should also be regarded as the scope of protection of the present utility model.

Claims (3)

1. A kind of 1. broadband unidirectional transmission structures of photonic crystal, including photonic crystal, which is characterized in that the broadband one-way transmission Structure further includes the dielectric grating being oppositely arranged with photonic crystal, and the photonic crystal is 2 D photon crystal, photonic crystal There are one dirac on one point to bore, and dirac cone allows vertical incidence optical transport oblique without allowing in certain frequency range Optical transport is penetrated, it is other directions that dielectric grating, which is used for vertical incidence optical diffraction,.
2. 2. the broadband unidirectional transmission structures of photonic crystal as described in claim 1, which is characterized in that the dielectric grating Refractive index is n=2.
3. 3. the broadband unidirectional transmission structures of photonic crystal as claimed in claim 2, which is characterized in that the dielectric grating is thick H=0.317 μm of degree.