CN108803195A - A kind of electricity regulation and control method of graphene nonlinear optical effect - Google Patents

Abstract

The invention belongs to the technical field of nonlinear optical photoelectric modulation, and specifically relates to an electrical control method for nonlinear optical effects of graphene. The present invention uses the field effect to electrically dope graphene, that is, uses the field effect of the gate electrode to inject carriers into the single-layer graphene to adjust the chemical potential of the graphene, and is used to turn on or off the non-conductor in the graphene. The resonant transition channel of the linear optical process affects the intensity of the nonlinear optical response, thereby effectively exciting and regulating the nonlinear optical effect of graphene. The method of the present invention can greatly enhance and effectively regulate the second-order, third-order and even higher-order nonlinear optical effects of graphene, and provides a convenient, reliable and effective electrical regulation means for the application of graphene in the field of nonlinear optics .

Description

An electrical control method for the nonlinear optical effect of graphene

technical field

The invention belongs to the technical field of nonlinear optical photoelectric modulation, and in particular relates to an electrical control method for graphene nonlinear optical effects.

Background technique

As the first two-dimensional material discovered by humans that can exist stably in nature, graphene has been discovered by British scientists K.S. Novoselov and A.K. Geim in 2004. High mobility, fixed light transmittance (undoped graphene), linear energy band structure, and zero energy gap have attracted extensive attention from researchers and business circles. At present, the properties of graphene in the field of linear optics are well known and have been used in practical engineering applications, such as infrared detectors, touch screens, electronic paper, etc. In contrast, the potential of graphene in the field of nonlinear optics is still understudied and underexploited.

Due to the unique linear energy band structure of graphene, when the carriers in graphene are driven by the alternating electric field in the excitation light, the corresponding photocurrent is not a sine or cosine signal of the same frequency as in traditional semiconductors, but a square Wave signal, so it is naturally accompanied by a strong nonlinear optical effect; at the same time, due to the zero energy gap characteristics of graphene, the nonlinear optical effect in graphene can achieve resonance in a very wide band, and there are many A variety of resonant transition channels. Based on the above two considerations, since 2007, a large number of theoretical physicists have predicted that graphene has a strong third-order nonlinear optical effect. Later, experimental physicists observed the third-order nonlinear optical effect from single-layer graphene. For example, in 2010, E. Hendry et al. observed four-wave mixing signals in single-layer graphene. In 2013, N. Kumar et al. and S. Y. Hong et al. observed the third harmonic signal, and all the experimental data show that single-layer graphene has a very strong third-order nonlinear optical effect.

However, due to the central inversion symmetry of single-layer graphene, even-order nonlinear optical effects, such as second harmonics, optical parametric conversion, etc., do not exist when only the electric dipole approximation is considered. However, recent theories point out that when the contribution of electric quadrupole moment and magnetic dipole moment is further considered, there should be second-order nonlinear optical effects in graphene; not only that, in the case of oblique incident excitation, along the wave vector Looking at the direction, graphene can exhibit a strong second-order nonlinear optical effect. Furthermore, based on the special properties of graphene, it is also expected to observe higher-order nonlinear optical effects in single-layer graphene.

Based on the above research results, graphene has great application prospects in nonlinear optical devices and devices. At present, a large number of applications based on the nonlinear optical effect of graphene have been proposed and realized, such as using graphene in mode-locked lasers, or as a saturable absorption medium, and such as coating single-layer graphene on the outside of optical fibers, or placing On the photonic crystal structure, the third-order nonlinear effect of single-layer graphene is used to realize the frequency conversion of excitation light, etc. However, although a large number of devices related to graphene nonlinear optical effects have been proposed or realized, there is still a lack of an effective method to control the intensity of graphene nonlinear optical effects; in addition, single-layer graphite has not been realized before. Reports on the excitation of even-order nonlinear optical effects and their corresponding regulation greatly limit the application prospects of graphene nonlinear optical effects in aspects related to nonlinear optical modulation and switching.

Contents of the invention

In order to overcome various deficiencies in existing graphene nonlinear optical devices, the purpose of the present invention is to provide a practical method for stimulating and regulating graphene nonlinear optical effects.

The method for stimulating and regulating the nonlinear optical effect of graphene proposed by the present invention is to use the field effect to electrically dope graphene, that is, to use the field effect of the grid electrode to inject carriers into the single-layer graphene to regulate the graphene. The chemical potential of graphene can open or close the resonance transition channel of nonlinear optical process in graphene and affect the intensity of nonlinear optical response, so as to effectively stimulate and regulate the nonlinear optical effect of graphene.

The physical principles of the inventive method are as follows: the linear energy band and zero energy gap characteristics of graphene cause a variety of resonant transition channels when nonlinear optical processes occur in graphene (for example, when using an angular frequency of and The excitation angular frequency of When the additive four-wave mixing signal of , there is a corresponding energy of and There are five resonant transition channels in total), and the nonlinear optical effect in graphene is dominated by these resonant transition channels; and when the chemical potential of graphene is adjusted by the method of electrical doping, if a part of the resonant transition channels are controlled by the chemical potential When turned off, the intensity of the nonlinear optical effect changes significantly.

In the present invention, by adjusting the chemical potential of graphene to the energy corresponding to the transition channel close to the resonance, the shut-off effect and the resonance effect appear; wherein the shut-off effect refers to the intensity of the nonlinear effect of graphene when the chemical potential of graphene is close to the resonance transition There will be a step-like increase or decrease before and after the energy corresponding to the channel, and the resonance effect means that the strength of the nonlinear effect of graphene will be enhanced when the chemical potential of graphene is near the energy corresponding to the resonance transition channel.

In the present invention, the third-order nonlinear optical effect and the second-order nonlinear optical effect in graphene can be adjusted by adjusting the chemical potential of graphene according to physical principles and experimental results. The third-order nonlinear optical effects include the third harmonic process, Kerr effect and four-wave mixing process, etc., while the second-order nonlinear optical response includes the second harmonic process, and the sum frequency and difference frequency process, etc.; and the modulation The effect is not limited to third-order and second-order nonlinear optical effects, but can be generalized to more and broader nonlinear optical effects.

In the present invention, since the resonant transition channels have different intensities and phases, and the chemical potential energies corresponding to the resonant transition channels of different nonlinear optical effects are different, when different nonlinear optical processes adjust the chemical potential of graphene, the change of its intensity Trends are also different. Taking the third-order nonlinear optical effect as an example, for the third harmonic signal, when the chemical potential of graphene is adjusted away from the electrical neutral point of graphene, the signal intensity is continuously enhanced within a certain range; for the subtractive four-wave mixing signal, when When the chemical potential of graphene is adjusted away from the electrical neutral point of graphene, the signal intensity decreases continuously.

The realization of the method of the present invention requires the use of graphene devices, electrical control equipment, and excitation light paths. Among them, the graphene device includes graphene, substrate material, dielectric material and gate electrode. When in use, by applying a voltage to the gate electrode, the chemical potential of graphene is adjusted by the field effect of the dielectric material; wherein, for different applications , different kinds of dielectric materials can be used. The electrical regulation device is used to apply a gate voltage to the graphene device. The excitation light path can use normal incidence or oblique incidence excitation light, and the excitation light of normal incidence or oblique incidence can be used to excite the third-order nonlinear optical effect of graphene; while the oblique incidence excitation light should be used for the second-order nonlinear optical effect of graphene.

The present invention has the following beneficial effects:

1. The electrical control means for the nonlinear optical effect of graphene provided by the present invention realizes the effective control of the nonlinear optical response of graphene. The strength of nonlinear optical effects from graphene can be significantly changed when the chemical potential of graphene is changed by electrical doping;

2. The electrical control means provided by the present invention have different adjustment effects on different nonlinear optical effects, so this property can be used to control the relative intensity between different effects, or to selectively excite or suppress some nonlinear optical processes;

3. The oblique incident excitation optical path provided by the present invention can effectively excite the second-order nonlinear optical response from single-layer graphene. On this basis, electrical control means can effectively control the intensity of the second-order nonlinear optical effect from single-layer graphene;

4. The electrical control method provided by the present invention can be applied to the generation of optical parameter conversion, terahertz, infrared light, and nonlinear optics such as optical switches and optical information storage in the field of optical communication, so as to enhance or control the nonlinear optical effects.

Description of drawings

FIG. 1 is a schematic structural view of a single-layer graphene device and its electrical regulation in Embodiment 1 and Embodiment 2.

Fig. 2 is an optical path diagram of the excitation light used for the third-order nonlinear optical effect of graphene and the collection and analysis of nonlinear optical signals in Embodiment 1 of the present invention. Wherein, the excitation light is normal incident on the graphene device.

Fig. 3 shows the variation of the intensity of the third-order nonlinear optical effects of the third harmonic and subtractive four-wave mixing in graphene with the chemical potential of graphene in Example 1 of the present invention. Among them, the third harmonic signal is derived from the excitation of the excitation light with a wavelength of 1300nm; the four-wave mixing signal is derived from the excitation of the excitation light with a wavelength of 1040nm and 1300nm. In the figure, μ represents the chemical potential of graphene with the electrical neutral point of graphene as the reference point.

Fig. 4 is an optical path diagram of the excitation light used for the second-order nonlinear optical effect of graphene and the collection and analysis of nonlinear optical signals in Embodiment 2 of the present invention. Wherein, the excitation light is obliquely incident at 45 degrees relative to the graphene device.

Fig. 5 shows the variation of the intensity of the second harmonic effect in graphene with the chemical potential of graphene in Example 2 of the present invention. Wherein, the second harmonic signal is derived from excitation with excitation light with a wavelength of 1308nm. In the figure, μ represents the chemical potential of graphene with the electrical neutral point of graphene as the reference point.

Detailed ways

In order to illustrate the present invention more clearly, the present invention will be further described below in conjunction with preferred embodiments and accompanying drawings. Similar parts in the figures are denoted by the same reference numerals. Those skilled in the art should understand that the content specifically described below is illustrative rather than restrictive, and should not limit the protection scope of the present invention.

Example 1

This embodiment includes a graphene device, an excitation optical path, and an electrical regulation and measurement device, as shown in FIG. 1 . The graphene device includes a single-layer graphene sample, a substrate material, a source electrode, a drain electrode, a top gate and a dielectric material. The excitation optical path is used to excite nonlinear optical effects in single-layer graphene. The electrical regulation and measurement device is used for applying grid voltage and simultaneously measuring graphene transport properties.

In the above-mentioned graphene device, the source, drain and top gate are evaporated onto the single-layer graphene sample and the substrate material by using electron beam evaporation combined with a mask method. Wherein, the substrate material is fused silica.

Among the graphene devices mentioned above, the graphene device uses ion gel as the dielectric material to obtain the optimal tuning ability, which is due to the chemical potential in monolayer graphene when using ion gel as the top gate dielectric material. The tuning capability is much greater than when using other dielectrics.

The preparation and testing method of the graphene device that can regulate graphene nonlinear optical effect in the present embodiment comprises the following steps:

(1) Obtain a single-layer graphene sample by chemical vapor deposition, and transfer it on a fused silica substrate by a wet transfer method;

(2) Evaporate electrodes for single-layer graphene through electron beam evaporation and match the mask plate, and use the dot line machine as the electrode lead;

(3) Prepare ion gel, which is composed of ionic liquid [EMIM][TFSI] and PS-PEO-PS. Afterwards, a small amount of ion gel was dropped on the graphene sample. The ion gel needs to cover the graphene sample and the top grid electrode area, and then put it in a drying cabinet for a period of time to wait for the ion gel to solidify and dry;

(4) According to the connection method in Figure 1, connect the graphene device with the electrical control and measurement device. The electrical control and measurement device is used to provide the top gate voltage required by the graphene device and detect the resistance value of the graphene device in real time;

(5) Use the excitation and signal collection optical path in Figure 2, and select a suitable position in the graphene device to focus the excitation light under the condition that the excitation light is incident;

(6) While adjusting the top gate voltage, use a spectrometer or an avalanche photodiode to collect corresponding nonlinear optical signals. Different excitation lights and optical filters can be used to obtain different nonlinear optical signals varying with the intensity of the top gate voltage.

Figure 3 shows the third harmonic signal when using a femtosecond laser with a wavelength of 1300 nm as excitation light, and the subtractive four-wave mixing signal (wavelength of 867 nm) when using a femtosecond laser with a wavelength of 1040 nm and ) as a function of the chemical potential of graphene. As shown in Figure 3, both the third harmonic signal and the four-wave mixing signal are significantly regulated by the chemical potential of graphene, and the two have different trends with the chemical potential of graphene. Figure 3 shows that the third harmonic signal is enhanced when the graphene chemical potential shuts off some resonant transition channels (shutdown effect), and further enhanced when the graphene chemical potential is further adjusted to approach other resonant transition channels (resonance effect). At the same time, the subtractive four-wave mixing signal will be greatly weakened when the graphene chemical potential turns off part of the resonant transition channel.

It should be pointed out that in this embodiment, due to the high photon energy of the excitation light used, it is necessary to adjust the chemical potential of graphene in a large range in order to turn off the corresponding resonance transition channel, so ion gel is used as the dielectric material, but it can be The selected dielectric material is not limited to ion gels. For example, when the excitation light photon energy is small, other dielectric materials can be used to control the nonlinear optical response of graphene.

Example 2

Same as Example 1, except that the 45-degree oblique incidence device shown in FIG. 4 is used to excite the second-order nonlinear optical effect of the graphene device. Fig. 5 shows the variation of the intensity of the second-order nonlinear optical effect when controlled by the chemical potential of graphene in this embodiment. Figure 5 shows that the second harmonic effect in graphene is also significantly regulated by the graphene chemical potential, and it will be enhanced when the graphene chemical potential shuts down part of the resonant transition channel.

Apparently, the above-mentioned embodiments of the present invention are only examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those of ordinary skill in the art can also make It is impossible to exhaustively list all the implementation modes here, and any obvious changes or changes derived from the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims (4)

1. the electricity of graphene nonlinear optical effect a kind of regulates and controls method, which is characterized in that using field-effect to graphene into Row electricity is adulterated, that is, utilizes the field-effect of gate electrode, carrier is injected for single-layer graphene, to adjust the chemistry of graphene Gesture, to open or turn off the resonant transition channel of nonlinear optical process in graphene and influence nonlinear optical response Intensity, effectively to excite and regulate and control the nonlinear optical effect of graphene.

2. the electricity of graphene nonlinear optical effect according to claim 1 regulates and controls method, which is characterized in that pass through tune There is shutdown effect and resonance effects to close to before and after the corresponding energy in resonant transition channel, making it in arthrolith ink alkene chemical potential；Its Middle shutdown effect refer to graphene nonlinear effect intensity in graphene chemical potential close to the corresponding energy in resonant transition channel The step-like increase or reduction of front and back appearance, resonance effects refers to the intensity of graphene nonlinear effect in graphene chemistry Enhanced when gesture is near the corresponding energy in resonant transition channel.

3. the electricity of graphene nonlinear optical effect according to claim 1 regulates and controls method, which is characterized in that pass through tune Arthrolith ink alkene chemical potential, can regulate and control the third-order nonlinear optical effect and second order nonlinear optical effect in graphene；Its In, third-order nonlinear optical effect includes triple-frequency harmonics process, Kerr effect and four-wave mixing process, second nonlinear optic Effect includes second harmonic process, and and frequency and difference frequency process.

4. the electricity of graphene nonlinear optical effect according to claim 1 regulates and controls method, which is characterized in that exciting In light path, using normal incidence or oblique incidence exciting light, graphene third-order nonlinear optical effect is excited；It is excited using oblique incidence Light excites graphene second order nonlinear optical effect.