CN108803192B

CN108803192B - Sub-wavelength coherent signal compensation device

Info

Publication number: CN108803192B
Application number: CN201710317294.4A
Authority: CN
Inventors: 赵永生; 林贤清
Original assignee: Institute of Chemistry CAS
Current assignee: Institute of Chemistry CAS
Priority date: 2017-05-04
Filing date: 2017-05-04
Publication date: 2021-01-01
Anticipated expiration: 2037-05-04
Also published as: CN108803192A

Abstract

The invention provides a sub-wavelength coherent signal compensation device, which comprises a nonlinear material nanowire and a metal substrate; a nano-sized gap exists between the nonlinear material nanowire and the metal substrate. The strong plasma confinement effect of the invention tightly confines the optical field distribution in the device between the nonlinear medium and the gap, thereby realizing the enhancement and long-distance transmission of the second harmonic signal on the scale lower than the diffraction limit.

Description

Sub-wavelength coherent signal compensation device

Technical Field

The invention relates to an optical device, in particular to a sub-wavelength related signal compensation device.

Background

Miniaturization of widely wavelength-tunable coherent light sources has gained wide attention due to potential application potential in areas such as multi-color detection and broadband chip optical integration. The second harmonic is a frequency doubling signal for generating an incident laser signal, and has unique advantages in realizing a wide-spectrum light source, but it is still a challenge to realize long-distance transmission of the second harmonic signal on the basis of the device size breaking through the optical diffraction limit.

Disclosure of Invention

In view of the above problems, the present invention provides a sub-wavelength coherent signal compensation device, which is used for realizing efficient long-distance transmission of a second harmonic signal in a sub-wavelength scale range.

The invention is realized by the following technical scheme:

an apparatus for sub-wavelength coherent signal compensation, the apparatus comprising:

a nonlinear material nanowire and a metal substrate; a nano-sized gap exists between the nonlinear material nanowire and the metal substrate.

The above apparatus wherein the thickness of the gap is related to the material used in the gap for second harmonic enhancement.

The device as described above, wherein the width of the gap is 5-30 nm.

In the invention, the width of the gap can influence the coupling of the waveguide mode and the surface plasma mode, namely the enhancement effect on the second harmonic signal in the mixed plasma waveguide mode.

In the invention, the gap width is set to separate an optical field from a dissipation layer on the metal surface, and the domain is limited in the nanowire and the gap layer, so that the loss is reduced.

According to a preferred embodiment, the width of the gap is preferably between 5 and 25nm, more preferably between 5 and 15 nm.

The above device wherein the gap uses a low refractive index material.

In the invention, the purpose of selecting the low-refractive-index material is the same as the gap width, and the purpose is to separate an optical field from a dissipation layer on the metal surface, limit the area in the nanowire and the gap layer and reduce the loss.

If the refractive index of the material is too high, for example, above that of the nanowire material, the optical field is completely confined in the spacer layer, and the interaction with the nanowire is weak, and there is no way to complete the transformation.

The invention makes the above effect better through the selection of the gap width and the low refractive index material. The device as described above, wherein the low refractive index material comprises silicon dioxide, magnesium fluoride, aluminum oxide, or an organic polymer.

According to the invention, the organic polymer is selected from polystyrene, polymethyl methacrylate, and the like.

The apparatus as described above, wherein the nonlinear material comprises zinc selenide, potassium niobate, zinc sulfide, gallium arsenide, or sodium gallium phosphide.

The above apparatus wherein the metal substrate comprises gold, silver, aluminum or copper substrate.

Compared with the prior art, the device disclosed by the invention realizes the enhancement and long-distance transmission of second harmonic signals on the scale lower than the diffraction limit.

Drawings

FIG. 1 is a schematic diagram of a compensation apparatus according to an embodiment of the present invention;

in the figure, 1 is a zinc selenide nanowire, 2 is a magnesium fluoride gap, and 3 is a silver material substrate;

FIG. 2 is a spectrum collected at an excitation point in the embodiment of the present invention, where a very strong frequency doubling signal peak is generated at 425nm, and the half-peak width is 4 nm;

FIG. 3 is a diagram illustrating second harmonic signal intensities generated by different laser signal intensities according to an embodiment of the present invention;

FIG. 4 is a schematic diagram of the second harmonic signal enhancement effect of adjusting the gap thickness according to an embodiment of the present invention.

Detailed Description

The apparatus of the present invention (which is the same as the disclosed concept in the present invention) is used to achieve efficient long-distance transmission of second harmonic signals in the sub-wavelength scale range. The device of the present invention will be explained below with reference to the accompanying drawings.

In one embodiment, a sub-wavelength coherent signal compensation device is provided, the device comprises three parts as shown in fig. 1, namely a zinc selenide nanowire (1), a magnesium fluoride gap (2) and a silver material substrate (3) from top to bottom. The device constitutes a hybrid plasma waveguide structure. The silver substrate has smaller plasma vibration damping in visible and near infrared wave bands, and is beneficial to low-loss long-distance transmission of SPPs (surface plasmons). The single crystal zinc selenide is of a cubic sphalerite structure, grows into a nanowire along the (111) crystal face, has a large second-order nonlinear coefficient, and is used as a nonlinear material in the embodiment to generate a second harmonic signal.

In this example, a glass substrate was used at the same time for comparison. A titanium gem laser with the wavelength of 850nm (the pulse width is 150 femtoseconds, and the frequency is 80 megahertz) is adopted as a pumping light source to respectively excite zinc selenide nanowires on a silver substrate and a glass substrate, and the polarization of the electric field component of the excitation light is vertical to the long axis direction of the nanowires. The intensity of frequency doubling light emitted by the zinc selenide nanowires on the glass substrate after being excited is far weaker than that of a sample on the silver substrate, so that the zinc selenide nanowires in the hybrid plasma waveguide composite structure generate second harmonic frequency doubling signals which are far stronger than those in the glass substrate composite waveguide. Clear interference fringes can be seen from the nano-wires in the mixed plasma wave composite waveguide structure, and the second harmonic signals are proved to have good coherence. If distribution simulation diagrams of electric field components of two types of substrate material 850nm wavelength light are obtained, the local type of optical field distribution of the composite structure of the glass substrate can be seen to be poor, and compared with the distribution simulation diagrams, a hybrid plasma mode in the hybrid plasma waveguide is well limited in a gap between a metal substrate and a nanowire, so that the generation of second harmonic signals in a sub-wavelength scale range is facilitated, and the transmission of the second harmonic signals is facilitated.

FIG. 2 is a spectrum collected at an excitation point, and a very strong frequency doubling signal peak is generated at 425nm, and the half-peak width is 4 nm.

Further, in this embodiment, the polarization direction of the excitation light is changed, and the second harmonic frequency doubling signal is collected, and under the precondition that the excitation power is fixed and not changed, the half-wave plate is used to change the polarization of the incident light, i.e., the fundamental laser, so that the polarization is respectively parallel to the long axis direction of the zinc selenide nanowire. The frequency doubling signals collected by the two composite waveguides adopting the silver substrate and the glass substrate show polarization dependence. When the polarization of the incident fundamental laser is changed from being vertical to the main axis of the nanowire to being parallel, the second harmonic frequency doubling signal is greatly enhanced. The signal is enhanced by 8 times in the glass substrate composite waveguide. The signal enhancement ratio in the silver substrate composite waveguide is as high as 46, which is close to 6 times of that on a glass substrate under the same condition.

For the glass substrate composite waveguide, due to the difference between the dielectric constant of the nanowire and the dielectric constant of the surrounding environment, the coupling efficiency of the fundamental frequency light with different polarizations is different, thereby affecting SHG (c: (g) ()Second Harmonic Generation) The signal strength. For the hybrid type plasma waveguide, in addition to the difference of the coupling efficiency of the fundamental frequency light with different polarization caused by the difference of the dielectric constant of the nanowire and the environment, the hybrid type plasma mode in the hybrid type waveguide structure has high polarization characteristic, so that the polarization of the incident fundamental frequency light signal is changed.

Further, the method can be used for preparing a novel materialAs can be seen from fig. 3, the signal strength of the second harmonic increases with the increase of the pump energy, so the emergent signal is a second-order nonlinear phenomenon. In fig. 3, the first composite mode is a silver-based composite waveguide, and the second composite mode is a glass-based composite waveguide. Rough calculations indicate that the nonlinear conversion efficiency is about 5 × 10 when the pump energy is 5Mw in the hybrid plasmon waveguide^-6%, the efficiency is improved by 120 times compared with the glass substrate mixed waveguide composite structure. In addition, the substrate without the zinc selenide nanowire is pumped by 850nm laser signals, and 425nm frequency doubling signals are not collected, which indicates that the second harmonic frequency doubling signals collected in the embodiment completely originate from pumping excitation of the zinc selenide nanowire.

Further, changing the width of the gap can affect the coupling of the waveguide mode and the surface plasmon mode, that is, the enhancement effect on the second harmonic signal in the hybrid type plasmon waveguide mode. The second harmonic signal can also be caused to vary significantly in intensity by adjusting the coupling strength between the waveguide mode and the plasma mode. Meanwhile, the distribution of the mixed plasma mode in the waveguide is carefully adjusted to meet the phase matching condition, and the long-distance transmission of the second harmonic signal can be successfully realized. The principle is based on that in the hybrid plasma mode, the fundamental frequency light can be efficiently converted to the second harmonic signal, so that the transmission loss compensation of the second harmonic signal is realized. As can be seen in fig. 4, the optimal gap thickness is around 10 nm. While between 5nm and 30 nm, both the waveguide mode and the surface plasmon mode are well coupled. Coupling effects greater than this 30 nm are so weak that the confinement ability of light at the diffraction limit is deteriorated, resulting in insignificant enhancement effects. The optimal value and range of the gap thickness for the light to achieve the best enhancement may vary from material to material.

In this embodiment, zinc selenide can be replaced with non-linear materials such as potassium niobate, zinc sulfide, gallium arsenide, and sodium gallium phosphide. The interstitial magnesium fluoride may be replaced by a low refractive index material such as silicon dioxide, aluminum oxide, or an organic polymer. Instead of metallic silver, gold, aluminum, copper, or the like may be used.

The device with the structure constructs a mixed plasma waveguide, namely, a gap with a nanometer size is introduced on the surfaces of the semiconductor nanowire and the metal, so that a local plasma mode and a dielectric waveguide mode can be overlapped to form a mixed mode. The strong plasma confinement effect causes the optical field distribution in the hybrid plasma mode to be tightly confined between the nonlinear medium and the gap. The small mode area and enhanced light, substance interaction make it possible to achieve ultra-strong second harmonics on device dimensions that break the diffraction limit. More importantly, the electric field component in the hybrid plasma mode is confined in the gap region, so that the ohmic loss of the metal surface is sharply reduced, and the loss compensation of a second harmonic signal can be realized by utilizing an effective nonlinear gain process in the hybrid plasma waveguide.

The device can realize the generation, enhancement and long-distance transmission of second harmonic signals, but in the using process of the device, the selection of the light source can also influence the enhancement effect of the second harmonic. Due to the small size of the nanowires, different polarized light has different coupling efficiencies, and thus, SHG: (Second Harmonic Generation) The intensity of the signal may vary significantly with the polarization of the incident light.

The present invention has been described in detail, and the specific examples have been provided to illustrate the principles and embodiments of the present invention, but the above description is only provided to help understand the method and the core concept of the present invention; meanwhile, for those skilled in the art, any changes, variations and improvements in the specific embodiments and the application range according to the idea of the invention are within the protection scope of the invention, and the content of the present description should not be construed as a limitation of the present disclosure.

Claims

1. A sub-wavelength coherent signal compensation apparatus, comprising:

the device comprises a nonlinear material nanowire and a metal substrate;

a nano-sized gap exists between the nonlinear material nanowire and the metal substrate;

the width of the gap is related to the material of the gap, and when the width of the gap is changed, the coupling of the waveguide mode and the surface plasma mode is influenced; and the width of the gap is based on the following principle:

a. so as to separate the optical field from the dissipation layer of the metal surface, and the confinement is in the nanowire and the gap layer;

b. the distribution of the mixed plasma mode in the waveguide is adjusted to meet the phase matching condition;

wherein the content of the first and second substances,

the nonlinear material is zinc selenide;

the metal substrate is silver;

the material of the gap is magnesium fluoride;

the width of the gap is between 5 nanometers and 30 nanometers, and both a waveguide mode and a surface plasma mode can be well coupled;

the electric field component in the hybrid plasma mode is confined in the gap region, so that the ohmic loss of the metal surface is sharply reduced, and the loss compensation of a second harmonic signal can be realized by utilizing an effective nonlinear gain process in the hybrid plasma waveguide.

2. The apparatus of claim 1, wherein:

the width of the gap is 5-25 nanometers.

3. The apparatus of claim 1, wherein:

the width of the gap is 5-15 nanometers.