KR20160094828A

KR20160094828A - Surface plasmon pulse group velocity converter

Info

Publication number: KR20160094828A
Application number: KR1020150041664A
Authority: KR
Inventors: 이병호; 김준수; 이승열; 박현수; 김휘
Original assignee: 서울대학교산학협력단
Priority date: 2015-02-02
Filing date: 2015-03-25
Publication date: 2016-08-10
Also published as: KR101677208B1

Abstract

Disclosed is a surface plasmon pulse group velocity converter which effectively converts two plasmon modes in an MIM waveguide having a symmetric SPP mode with high velocity and a semi-symmetric SPP mode with low velocity at the same time. The surface plasmon pulse group velocity converter comprises two metal layers, and a dielectric layer positioned therebetween. The dielectric layer includes a dielectric formed in a chirped asymmetric lattice structure.

Description

[0001] Surface plasmon pulse group velocity converter [0002]

The present invention relates to a surface plasmon pulse group velocity transducer capable of changing the velocity of a pulsed wave pulse.

Surface Plasmon Polariton (SPP) is a coupled mode generated by free electron plasma oscillation of a metal surface and the interaction of photons, so that the electromagnetic field is strongly focused on the surface, Limitations, and further, the diffraction limit which limits the miniaturization of optical devices.

In the case of a metal-insulator-metal (MIM) waveguide, the symmetric SPP mode is a mode in which there is no cut-off due to dielectric thickness reduction, and the opposite SPP mode is cut off. It has been reported that when the permittivity ratio of dielectric and metal falls within a certain range, there is a frequency band in which the quasi-SPP mode almost stops. Since the symmetric SPP mode is relatively fast and the energy loss is small in the corresponding frequency band, it is possible to perform a function similar to the optical buffer if the symmetric SPP mode is switched from the desired position to the opposite SPP mode or vice versa.

Patent Publication No. 2014-0147383

It is an object of the present invention to provide a surface plasmon pulse group velocity transducer for effectively causing two Plasmon mode conversions to occur in an MIM waveguide having both a symmetric SPP mode having a high speed and an opposing SPP mode having a low speed .

According to an aspect of the present invention, there is provided a surface plasmon pulse group velocity transducer comprising: a first metal layer; A second metal layer; And a dielectric layer disposed between the first metal layer and the second metal layer, wherein the dielectric layer includes a dielectric formed in a chirped asymmetric lattice structure.

According to the present invention, it is possible to efficiently convert the symmetric SPP mode and the opposite SPP mode in a wide wavelength band by converting the speed of the optical pulse to a wide range. It can also be used to implement optical memories or optical buffers that play an important role in optical computing.

1 is a diagram showing a configuration of an example of a surface plasmon pulse group velocity transducer according to the present invention,
FIG. 2 illustrates dependence of the surface plasmon pulse group velocity transducer according to the present invention on the dielectric layer thickness; FIG.
3 is a diagram showing a dispersion relationship when the pulse speed in the opposite-pitch SPP mode is about 25% of the pulse speed in the symmetric SPP mode;
FIG. 4 is a diagram showing an example of a computational simulation result for the surface plasmon pulse group velocity converter of FIG. 1, and FIG.
5 is a graph showing an example of transmittance of a surface plasmon pulse group velocity transducer according to the present invention.

Hereinafter, a surface plasmon pulse group velocity converter according to the present invention will be described in detail with reference to the accompanying drawings.

1 is a diagram showing a configuration of an example of a surface plasmon pulse group velocity converter according to the present invention.

Referring to FIG. 1, a surface plasmon pulse group velocity transducer is formed of a MIM waveguide having a three-layer structure including a first metal layer 100, a dielectric layer 110, and a second metal layer 120.

The dielectric layer 110 includes a dielectric constant distribution 130 of a chirped asymmetric lattice structure. Specifically, the dielectric layer 110 includes a dielectric rod 130 having a lattice structure longer than a few wavelengths in the y direction. The dielectric layer 110 includes grating periods required by the phase matching condition of each wavelength within the wavelength band of interest. The grating period (for example, Λ ₁ , Λ _n (≠ Λ ₁ ), etc.) required by the phase matching condition of each wavelength is expressed by the following equation (4).

In the case of this embodiment, the lattice period gradually decreases in the z-axis direction. However, the present invention is not limited thereto, and may be modified in various forms such as increasing or increasing and decreasing irregularly. Although the width (z-axis direction length) of the dielectric rods 130 in the lattice structure also gradually decreases in this embodiment, the present invention is not limited thereto, and may be increased or increased or decreased irregularly And can be implemented in various forms.

Due to the chirped grating dielectric 130, the symmetric SPP mode 140 with a pulse width of several hundreds of femtoseconds can be converted to the opposite SPP mode 150. At this time, the chirping rate and the variation range of the dielectric constant of the dielectric rod 130 can be adjusted to achieve a higher mode conversion efficiency in the wavelength band of interest. For example, the dielectric constant of the dielectric rods 130 of the grid structure of the dielectric layer 110 may gradually decrease or increase in the z-axis direction. The chirping rate may also gradually increase or decrease in the z-axis direction.

When the dielectric layer 110 has a constant thickness, it has a symmetric SPP mode 140 and an opposite SPP mode 150 in the waveguide. The thickness of the dielectric layer 110 suitable for the wavelength band of interest can be determined through the relationship between the wave numbers of the symmetric and anti-symmetric SPP modes and the angular frequencies as shown in FIG. 2 depending on the thickness of the dielectric layer. For example, the thickness of the dielectric layer 110 may be determined such that the pulse rate of the symmetric SPP mode 140 is four times faster than the pulse rate of the opposite SPP mode 150, as shown in FIG.

The metal and the dielectric constituting the metal layers 100 and 120 and the dielectric layer 110 can be selected through the following process. First, it is assumed that the dielectric rod 130 is formed to be longer than a few wavelengths in the y direction, and that the metal layers 100 and 120 are sufficiently thick that the thickness of the metal layer is infinite.

As is known in the prior art, the existence condition of the anti-symmetric SPP mode pulse having a group velocity of 0 is given by the dielectric constant ratio of the dielectric layer 110 and the metal layers 100 and 120 as shown in the following Equation 1:

Here,? _M and? _D represent the dielectric constants of the metal layers 100 and 120 and the dielectric layer 110, respectively. The metal layers 100 and 120 can be modeled as a Drude model ignoring damping as shown in Equation (2).

Here, ε _∞ and ω _p represent the permittivity and the plasma frequency of the metal layers 100 and 120 in the high frequency region, respectively. Equation (2) corresponds to Equation (1), Equation (3) is satisfied.

Accordingly, the materials of the metal layers 100 and 120 and the dielectric layer 110 that the wavelength band of interest can fall within the range of Equation (3) are selected.

FIG. 2 is a graph showing the dependency of the surface plasmon pulse group velocity transducer according to the present invention on the dielectric layer thickness. FIG. More specifically, FIG. 2 shows the relationship between the frequencies of the symmetric and anti-symmetric SPP modes and the angular frequencies according to the thickness of the dielectric layer.

2, the solid lines 210, 212, and 214 and the broken lines 200, 202, and 204 represent the wave-angular frequency relationship (hereinafter referred to as a dispersion relationship) of the symmetric and anti-SPP modes, respectively. As the thickness of the dielectric layer 110 increases, the dispersion relationships 200, 202 and 204 of the opposite SPP mode move from the uppermost dashed line 200 to the lower dashed line 204 and the dispersion relations 210, 212 and 214 of the symmetric SPP mode And moves from the lower solid line 210 to the upper solid line 214.

When the thickness of the dielectric layer 110 is too thick, the dispersion relation of the opposite SPP mode tends to become similar to that of the symmetric SPP mode in the wavelength band of interest. When the dielectric layer 110 is too thin, So that the phase velocity and energy velocity are in opposite directions.

If the thickness is somewhat appropriate, a very flat dispersion relation like the broken line 202 located at the middle in FIG. 2 can be obtained, which means that the pulse rate is very slow. However, in this case, since the pulse rate in the symmetric SPP mode is also generally slow to the pulse rate in the opposite SPP mode, the thickness should be selected so that the deceleration ratio between the two modes increases. For example, referring to FIG. 2, it is possible to select the thickness of the dielectric layer in which the pulse speed of the opposite SPP mode is about 25% of the pulse speed of the symmetric SPP mode.

Fig. 3 is a graph showing the dispersion relationship when the pulse speed in the opposite-pitch SPP mode is about 25% of the pulse speed in the symmetric SPP mode.

Referring to FIG. 3, the dispersion relationship 300, 310 is shown when the pulse rate in the symmetric SPP mode and the pulse rate in the opposite SPP mode are about four times in the wavelength band of interest. In this embodiment, the temporal pulse width of the input pulse is about 200 fs. In the computer simulation, the repetition period of the pulse laser is set to 10 ps so that the response of a single pulse can be seen.

As can be seen from the dispersion relation of Figure 3, in order to convert between symmetric SPP mode 310 and the anti-symmetric mode SPP 300 occurs is necessary frequency compensation of approximately Δq _center. Thus, the grating period L of the chirped dielectric 130 located in the dielectric layer 110 has the range defined by Equation (4).

Here, Δq (λ) represents the wave number difference between the symmetric SPP mode and the opposite SPP mode at each wavelength of the wavelength band of interest, and λ ₀ and Δλ represent the center wavelength and bandwidth of the wavelength band of interest, respectively. In order to reduce the efficiency reduction at the end of the bandwidth of the wavelength band of interest, the period of the dielectric lattice may be implemented in a slightly wider range than that of equation (4) according to the embodiment.

In the chirped grating structure 130, if the chirping rate is too fast, the efficiency is low because there is not enough distance for the mode conversion to occur. If the chirping rate is too slow, the mode is converted once, The efficiency may be reduced by the process of conversion, so a selection of an appropriate chirping rate is necessary according to the embodiment.

FIG. 4 is a diagram showing an example of a computational simulation result for the surface plasmon pulse group velocity converter of FIG. 1. FIG.

Referring to FIG. 4, since the grating period Λ of the dielectric layer 110 of the surface plasmon pulse group of FIG. 1 gradually decreases, the front portion of the grating 130 satisfies the phase matching condition at a short wavelength, The phase matching condition at the wavelength is satisfied. Therefore, as shown in FIG. 4, the point where the mode conversion occurs moves backward as the wavelength length increases.

In Figure 4, the dashed and solid lines represent the power flow of the symmetric and counter-SPP modes within the chirped grids 130, respectively. Each frequency component passes through the grating where the phase matching condition does not fit. If all of the frequency components deviate from the position where the energy flow inversion appears, the energy exchange between the symmetric SPP mode and the opposite SPP mode is significantly weakened, . As a result, a transmittance spectrum having a high efficiency over the entire wavelength band of interest can be obtained as shown in FIG.

5 is a graph showing an example of transmittance of a surface plasmon pulse group velocity transducer according to the present invention.

Referring to FIG. 5, the dotted line and the solid line indicate the energy transmittance of the symmetric and anti-SPP mode, respectively, when the symmetric SPP mode is incident. FIG. 5 shows a case where the chirping rate and dielectric constant of the grating 130 of the dielectric layer 110 are constantly changed as shown in FIG. The spectrum of FIG. 5 can be made even more uniform by adjusting the permittivity variation or the chipping speed of the chirped grating 130.

The present invention has been described with reference to the preferred embodiments. It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. Therefore, the disclosed embodiments should be considered in an illustrative rather than a restrictive sense. The scope of the present invention is defined by the appended claims rather than by the foregoing description, and all differences within the scope of equivalents thereof should be construed as being included in the present invention.

Claims

A first metal layer;
A second metal layer; And
And a dielectric layer disposed between the first metal layer and the second metal layer,
Wherein the dielectric layer comprises a dielectric formed in a chirped asymmetric lattice structure.

The method according to claim 1,
Wherein the chirped asymmetric lattice structure comprises a plurality of different lattice periods.

The method according to claim 1,
Wherein the chirped asymmetric lattice structure comprises a plurality of dielectrics having different permittivities.

The method according to claim 1,
Wherein the chirped asymmetric lattice structure comprises a plurality of dielectrics having different widths.

The method according to claim 1,
The local lattice period of the chirped asymmetric lattice structure can be expressed as:

(Where λ is the lattice period of the chirped asymmetric lattice structure, Δq (λ) represents the symmetric SPP at each wavelength and the wave number difference in the opposite SPP mode, λ ₀ and Δλ represent the center wavelength and bandwidth of the wavelength band of interest, respectively )
Wherein the surface plasmon pulse group velocity transducer satisfies the above expression.

The method according to claim 1,
The material constituting the metal layer and the dielectric layer,

(Where ε _m and ε _d are the permittivities of metals and dielectrics, respectively)
Wherein the surface plasmon pulse group velocity transducer is a metal and a dielectric material satisfying the above expression.