CN108490626A - A kind of polarization beam splitting element and device - Google Patents

Abstract

The invention relates to the field of electromagnetic wave polarization beam splitting, and discloses a polarization beam splitting element and a device. The polarization beam splitting element includes a dielectric film and a metal film stacked alternately periodically, wherein one layer of the dielectric film and one layer of the metal film form a periodic repeating unit, and the average dielectric constant of the periodic repeating unit is to zero. When the light is incident parallel to the interface of the polarization beam splitting element, the transverse electric wave and the transverse magnetic wave in the light can be separated due to the difference in the dispersion relationship in the polarization beam splitting element. The thickness ratio in the unit is different, and the polarization beam splitting element has two working modes, which can be flexibly selected under different requirements. The polarizing beam splitting element of the structure is simple in structure, easy to prepare, wide in material selection, and far smaller in size than existing polarizing beam splitters, easy to integrate, and has broad application prospects in optical communication and integrated optical circuits.

Description

A polarizing beam splitting element and device

technical field

The invention belongs to the field of electromagnetic wave polarization beam splitting, and in particular relates to a polarization beam splitting element and device.

Background technique

Polarized light is used extensively in modern optical testing and optical applications. When an ordinary light wave propagates in an ordinary medium, it is not in a complete polarization state, but a composite state including two mutually orthogonal polarization states of a transverse electric wave (Transverse electric, TE) and a transverse magnetic wave (Transverse magnetic, TM). Polarizing beam splitters can separate these two orthogonal polarization states in space, and have extensive and important applications in traditional optical optical circuits, optical communications, and integrated optical circuits.

There are three types of traditional polarizing beam splitters, mainly including:

Polarizing beam splitter with dielectric multilayer film structure. The polarization beam splitter mainly relies on the Brewster angle to achieve polarization beam splitting. At the Brewster angle, the light waves of the TM mode can completely transmit through the dielectric multilayer film structure, while the light waves of the TE mode have a large Most of them are reflected back, so the reflected light only contains TE mode light waves, thus realizing the separation of TE mode and TM mode light waves. However, the polarizing beam splitter with a dielectric multilayer film structure also has some defects, such as being very sensitive to the dependence of the incident angle and having a large size, which makes it difficult to fully meet the needs of modern integrated optical systems.

Polarizing beam splitters based on anisotropic media. The polarization beam splitter is mainly based on the birefringence phenomenon of light waves in anisotropic media, that is, the refraction properties of light waves in TE mode and TM mode are different in anisotropic media. This type of polarization beam splitter is simple in design and mature in technology, but its size is often on the order of millimeters or even larger, making it difficult to miniaturize and cannot be applied to integrated optical circuit design.

Two-dimensional photonic crystal polarizing beam splitter. The polarization beam splitter mainly utilizes the difference in optical properties of the light waves of the TE mode and the TM mode in the two-dimensional photonic crystal to realize the separation of different polarizations. For example, at a specific frequency, the light waves of the TE mode and the TM mode are respectively in the forbidden band and the pass band of the photonic crystal, so that only the light wave with a specific polarization can be transmitted through the photonic crystal. The size of this two-dimensional photonic crystal polarization beam splitter can be on the order of microns, so it can play a certain role in photonic integration. However, its structure is relatively complex, the preparation process is relatively difficult, and the preparation cost is relatively high.

Contents of the invention

The purpose of the present invention is to overcome the above problems in the traditional polarization beam splitter, and provide a polarization beam splitting element and device with simple structure, size on the order of wavelength and two working modes.

In order to achieve the above object, the present invention provides a polarization beam splitting element and device, comprising:

A dielectric thin film and a metal thin film, wherein the dielectric thin film and the metal thin film are periodically stacked alternately to form a multilayer film structure;

Wherein, a periodic repeating unit is composed of a layer of the dielectric film and a layer of the metal film, the average dielectric constant of the periodic repeating unit is zero, satisfying,

f _d +f _m =1

f _d ε _d +f _m ε _m ＝0

Among them, f _d and f _m represent the thickness ratio of each layer of the dielectric film and each layer of the metal film in the periodic repeating unit, and ε _d and ε _m represent the dielectric film and the metal film respectively. The relative permittivity of the film;

When working, light is incident in a direction parallel to the interface of the multilayer film structure, and the light includes transverse electric waves and transverse magnetic waves, and the dispersion relations of the transverse electric waves and the transverse magnetic waves in the polarization beam splitting element are different , realizing the separation of the transverse electric wave and the transverse magnetic wave.

further,

When the transverse electric wave is incident, the dispersion relation of the transverse electric wave in the polarization beam splitting element is,

When a transverse magnetic wave is incident, the dispersion relation of the transverse magnetic wave in the polarization beam splitting element is,

Wherein, ε _⊥ = ε _d ε _m / (ε _d f _m + ε _m f _r ), a<<λ, k _x is the wave vector of the electromagnetic wave in the incident direction of the light in the polarization beam splitting element, k _y is the wave vector of the electromagnetic wave in the polarization beam splitting element perpendicular to the incident direction of the light, k ₀ is the wave vector of the light in free space, λ is the wavelength of the light, and a is the period The thickness of the repeating unit, the transverse electric wave refers to the electromagnetic wave whose electric field direction is perpendicular to the propagation direction, and the transverse magnetic wave refers to the electromagnetic wave whose magnetic field direction is perpendicular to the propagation direction.

Further, the light is visible light.

Further, the thickness of the polarization beam splitting element is of the same order as the wavelength of the light.

Further, the thickness is between 100nm and 1000nm.

Further, both the dielectric thin film and the metal thin film are made of low-loss materials.

Further, the dielectric thin film is air or silicon dioxide, and the metal thin film is gold or silver.

The present invention also provides a polarization beam splitting device, including an integrated substrate, and the polarization beam splitting element arranged on the integrated substrate.

In the present invention, a multi-layer film structure is formed by periodic and alternate stacking of dielectric thin films and metal thin films, so that the average dielectric constant is zero, and then according to the difference in the dispersion relationship between the transverse electric wave and the transverse magnetic wave in the polarization beam splitting element in the light, so as to realize the The polarization beam splitting effect of light.

Description of drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the accompanying drawings that need to be used in the technical description of the embodiments. Obviously, the accompanying drawings in the following description are only some implementations of the present invention For example, those of ordinary skill in the art can also obtain other drawings based on these drawings on the premise of not paying creative efforts.

Fig. 1 is a schematic structural view of a polarization beam splitting element of the present invention;

Fig. 2 is a schematic diagram of the working mode of Embodiment 1 of the polarization beam splitting element of the present invention;

Fig. 3 is the equal frequency graph when embodiment 1 works;

Fig. 4 is the transmission simulation figure of transverse electric wave and transverse magnetic wave when embodiment 1 works;

Fig. 5 is the schematic diagram of the field energy distribution of transverse electric wave and transverse magnetic wave at the dotted line in Fig. 4 when embodiment 1 works;

6 is a schematic diagram of the working mode of Embodiment 2 of the polarization beam splitting element of the present invention;

Fig. 7 is the equal frequency graph when embodiment 2 works;

Fig. 8 is the transmission simulation figure of transverse electric wave and transverse magnetic wave when embodiment 2 works;

Fig. 9 is a schematic diagram of the field energy distribution of the transverse electric wave and transverse magnetic wave at the dotted line in Fig. 8 when the embodiment 2 works.

The symbols in the figure are as follows:

1. Dielectric film, 2. Metal film, 3. Periodic repeat unit,

4. Polarization beam splitting element

Detailed ways

The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some, not all, embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without creative efforts fall within the protection scope of the present invention.

Referring to accompanying drawing 1, it is the schematic structural diagram of the polarization beam splitting element of the present invention. The polarization beam splitting element includes a dielectric thin film 1 and a metal thin film 2, and the dielectric thin film 1 and the metal thin film 2 are periodically and alternately stacked along the y-axis direction to form a multilayer film structure, and the multilayer film is the polarization beam splitting element 4; wherein, a One layer of dielectric film and one layer of metal film form a periodic repeating unit 3, and the average dielectric constant of periodic repeating unit 3 is zero, satisfying,

f _d +f _m =1

f _d ε _d +f _m ε _m ＝0

Among them, f _d and f _m represent the thickness proportion of each layer of dielectric film 1 and each layer of metal film 2 in the periodic repeating unit 3, ε _d and ε _m represent the relative permittivity of the dielectric film and metal film, respectively .

When the polarization beam splitting element 4 works, the light is incident parallel to the interface direction of the multilayer film structure (x-axis direction), but the light is not in a completely polarized state, but contains transverse electric waves (the electric field is along the z-axis direction, Transverseelectric, TE) and transverse magnetic wave (the magnetic field is along the z-axis direction, Transverse magnetic, TM) is a composite state of two mutually orthogonal polarization states. Therefore, for the electromagnetic wave of TE mode, its dispersion relationship (ie equal frequency curve) in the polarization beam splitting element 4 is,

For the electromagnetic wave of TM mode, its dispersion relation in the polarization beam splitting element 4 is then,

Wherein, ε _⊥ = ε _d ε _m /(ε _d f _m + ε _m f _r ), a<<λ, k _x is the wave vector of the electromagnetic wave in the direction of incident light in the polarization beam splitting element 4, and _ky is the electromagnetic wave The wave vector perpendicular to the light incident direction in the polarization beam splitting element 4, k ₀ is the wave vector of the light in free space, λ is the wavelength of the light, a is the thickness of the periodic repeating unit 3, and the transverse electric wave It refers to the electromagnetic wave whose electric field direction is perpendicular to the direction of propagation and transverse magnetic wave refers to the electromagnetic wave whose magnetic field direction is perpendicular to the direction of propagation.

Referring to Fig. 3 and Fig. 7, the thick solid line and the thin dotted line represent the equal frequency curves of the polarization beam splitting element 4 in TE mode and TM mode respectively. When a<<λ, the equal frequency curve of the polarization beam splitting element 4 in TE mode is a very small circle; the equal frequency curve of the polarization beam splitting element 4 in TM mode is two parabolas. Therefore, the optical properties of the TE wave and the TM wave in the polarization beam splitting element 4 are completely different, so that the beam splitting effect can be realized.

Combining the above formula, since the light is incident in the direction of the x-axis, _ky = 0, at this time the k _x of the equal-frequency curve in the TE mode always has a solution greater than zero, and there are two cases for the TM mode:

One is when f _m <0.5<f _d , because ε _⊥ >0, the two parabolas intersect, and the equal frequency curve in TM mode has two solutions with k _x greater than zero. At this time, the energy flow directions corresponding to the two solutions are different , the propagation direction of the corresponding TM wave is towards both sides of the x-axis, at this time, the TE wave has high transmission in the light incident direction and the TM wave has high transmission in both sides of the light incident direction;

Second, when f _m > 0.5 > f _d , since ε _⊥ < 0, the two parabolas do not intersect, and there is no k _x solution for the equal frequency curve in TM mode, at this time TM waves cannot propagate in the polarization beam splitting element 4 , TE waves are highly transmitted in the direction of light incidence and TM waves are totally reflected in the direction of light incidence.

Therefore, the polarizing beam splitting element 4 has two working modes according to the thickness ratio of the dielectric thin film and the metal thin film. The two working modes are described respectively below through two preferred embodiments.

Example 1

The parameters of the dielectric thin film and the metal thin film are respectively ε _d =6, ε _m =-2, f _d =0.25 and f _m =0.75, and the thickness of the periodic repeating unit is a=λ6. At this time, the equal frequency curves of the polarization beam splitting element 4 are shown in the thick solid line and the thin dashed line in Figure 3 respectively under the TE and TM modes (normalized frequency fa/c=0.167), and it is very small under the TE mode. A small circle, in TM mode, is two disjoint parabolas. In this case, when the mixed light of TE mode and TM mode is incident on the side of the multilayer film structure along the direction parallel to the interface of the multilayer film structure (i.e. the x direction), since _ky = 0, according to Fig. It can be seen from the equal frequency curve in 3 that there is a k _x solution greater than zero in the TE mode at this time, indicating that the TE wave can propagate in the polarization beam splitting element 4; and for the TM mode, there is no k _x solution at this time solution, which indicates that TM waves cannot propagate in the polarization beam-splitting element, as shown in Figure 2. Based on such characteristics, the two kinds of polarized light can be effectively separated, as shown in FIG. 4 and FIG. 5 . The black solid line and the gray solid line in Figure 5 represent the field energy distribution of the TE wave and the TM wave at the dotted line in Figure 4, respectively. It can be seen that the amplitude of the TE wave in the transmission region is much larger than that of the TM wave, indicating that the TE The TM wave has high transmission and the TM wave is mostly reflected on the incident surface, and basically no TM wave passes through the polarization beam splitting element 4 .

The thickness of the polarization beam splitting element 4 is only a quarter wavelength, which belongs to a sub-wavelength structure. When visible light is incident, its thickness is only between 100nm and 200nm, which is far smaller than the existing polarization beam splitting device.

Example 2

The parameters of the dielectric thin film and the metal thin film are respectively ε _d =1, ε _m =-4, f _d =0.8 and f _m =0.2, and the thickness of the periodic repeating unit is a=λ/8. At this time, the equal frequency curves of the polarization beam splitting element 4 are shown in the thick solid line and the thin dashed line in Fig. 7 respectively in TE and TM modes (normalized frequency fa/c=0.125), and the TE mode is very The small circle, in TM mode, is two intersecting parabolas. In this case, when the mixed light of the TE mode and the TM mode is incident on the side of the multilayer film structure along the direction parallel to the interface of the multilayer film structure (that is, the x direction), there is _ky = 0, according to Fig. It can be seen from the equal frequency curve in 7 that there is a solution of k _x greater than zero in the TE mode at this time, indicating that the TE wave can propagate in the polarization beam splitting element 4; for the TM mode, the intersection point of the two parabolas is is the solution of k _x , but the TM waves corresponding to the intersection point have different energy flow propagation directions, and can still be regarded as two k _x solutions. At this time, the propagation mode corresponding to the TM wave deviates from the x axis and transmits toward both sides of the incident direction ,As shown in Figure 6. Based on such characteristics, the two polarized lights can also be effectively separated in this mode. As shown in FIG. 8 , the TE wave transmitted from the polarization beam splitting element 4 mainly propagates along the direction of the incident wave, while the TM wave transmitted from the polarization beam splitting element is divided into upper and lower beams. The black solid line and the gray solid line in Fig. 9 respectively represent the field energy distribution of TE wave and TM wave at the dotted line in Fig. ), while the energy of the transmitted TM wave is mainly distributed on both sides of the incident direction. According to different positions of energy distribution, the polarization beam splitting element of the multilayer film structure can realize the beam splitting effect of TE mode and TM mode.

The thickness of the polarization beam splitting element 4 is 1.6λ, and its size is on the order of wavelength. In the case of incident visible light, its thickness is between 600nm and 1000nm, which is also much smaller than the existing polarization beam splitting device.

The polarizing beam splitting element and device of the present invention form a multilayer film structure through periodic and alternate stacking of dielectric thin films and metal thin films, so that the average dielectric constant is zero, and then according to the transverse electric wave and transverse magnetic wave in the light beam in the polarizing beam splitting element The dispersion relationship is different, so as to realize the polarization beam splitting effect on the light; further, according to the different thickness ratios of the dielectric film and the metal film in the periodic repeating unit, the polarization beam splitting element has two working modes, which can be used in practical applications. Flexible selection; at the same time, the polarization beam splitting element of this structure is simple in structure, easy to prepare, and has a variety of material choices. For example, low-loss materials such as air and silicon dioxide can be selected for dielectric films, and low-loss materials such as gold and silver can be selected for metal films. Noble metal; in addition, the size of the polarization beam splitting element of the present invention is on the order of wavelength (thickness is 100nm-1000nm), easy to integrate, and has extensive and important application prospects in traditional optical optical paths, optical communications, and integrated optical paths.

The above is only a preferred embodiment of the present invention, and is not intended to limit the present invention. It should be pointed out that for those of ordinary skill in the art, some improvements can be made without departing from the technical principle of the present invention. and modifications, these improvements and modifications should also be considered as the protection scope of the present invention.

Claims (8)

1. a kind of polarization beam splitting element, which is characterized in that including：

Periodically alternately superposition forms multilayer film for thin dielectric film and metallic film, the thin dielectric film and the metallic film Structure,

Wherein, a cycle repetitive unit is made of one layer of thin dielectric film and one layer of metallic film, the week The average dielectric constant of phase property repetitive unit is zero, is met,

f_d+f_m=1

f_dε_d+f_mε_m=0

Wherein, f_dAnd f_mIndicate that every layer of thin dielectric film and every layer of metallic film are repeated cyclically list described respectively Thickness accounting in member, ε_dAnd ε_mThe relative dielectric constant of the thin dielectric film and the metallic film is indicated respectively；

When work, light is incident with the direction for being parallel to the multi-layer film structure interface, and the light includes H mode and horizontal magnetic Wave, it is different with dispersion relation of the transverse magnetic wave in the polarization beam splitting element by the H mode, realize the transverse electric The separation of wave and the transverse magnetic wave.

2. polarization beam splitting element according to claim 1, it is characterised in that：

When the H mode incidence, dispersion relation of the H mode in the polarization beam splitting element be,

When the TM waves incident, dispersion relation of the transverse magnetic wave in the polarization beam splitting element be,

Wherein, ε_⊥=ε_dε_m/(ε_df_m+ε_mf_r), a ＜ ＜ λ, k_xIt is that electromagnetic wave light described in the polarization beam splitting element is incident Wave vector on direction, k_yIt is wave vector of the electromagnetic wave in the polarization beam splitting element in the light incident direction, k₀It is The wave vector of the light in free space, λ are the wavelength of the light, and a is the thickness for being repeated cyclically unit, described H mode refers to electromagnetic wave of the direction of an electric field perpendicular to the direction of propagation, and the transverse magnetic wave refers to magnetic direction perpendicular to the direction of propagation Electromagnetic wave.

3. polarization beam splitting element according to claim 1, it is characterised in that：The light is visible light.

4. polarization beam splitting element according to claim 3, it is characterised in that：The thickness of the polarization beam splitting element with it is described The same magnitude of wavelength of light.

5. polarization beam splitting element according to claim 4, it is characterised in that：The thickness is between 100nm~1000nm.

6. polarization beam splitting element according to claim 1, it is characterised in that：The thin dielectric film and the metallic film It is prepared by low-loss material.

7. polarization beam splitting element according to claim 6, it is characterised in that：The thin dielectric film is air or titanium dioxide Silicon, the metallic film are gold or silver.

8. a kind of polariz-ing beam splitting device, it is characterised in that：Including integral substrate, and on the integral substrate according to power Profit requires the polarization beam splitting element described in 1-3,6 any one.