A thin film electric frequency doubling device based on band edge enhancement effect

Abstract

The application relates to a thin film electric frequency doubling device based on a band edge enhancement effect. When a fundamental wave is obliquely incident, a voltage is applied to a first electrode layer and a second electrode layer. The fundamental wave passes through the transparent first electrode layer and a thin film layer, is reflected by the second electrode layer, and then passes through the thin film layer and exits from the first electrode layer. In the process, the third-order nonlinear susceptibility is enhanced by using the band edge enhancement effect of the thin film layer, so that the generation efficiency of the second harmonic wave appears a sharp peak at a certain wavelength position. The fundamental wave wavelength corresponding to the sharp peak is just covered by the energy band of the thin film layer, so that the generation efficiency of the second harmonic wave is greatly improved under the condition of applying the voltage. Further, an extremely high modulation ratio is obtained, and the modulation depth is much higher than that of a conventional electric frequency doubling device. The thin film electric frequency doubling device based on the band edge enhancement effect disclosed in the application can realize the purpose of realizing strong modulation by using electric frequency doubling in a nonlinear material.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of band-edge enhancement element, and particularly relates to a thin film electric frequency doubling device based on band-edge enhancement effect. BACKGROUND

[0002] Dynamic control of nonlinear optical signals has important application value in optical modulation and switching technology. Various switching technologies based on Kerr or free carrier nonlinearity in semiconductor materials can be developed by using all-optical control scheme. In addition, control of nonlinear optical signals can also be realized through electro-optic interaction.

[0003] In nonlinear optics, the frequency conversion process depends on the chemical composition of the material and the spatial symmetry of the optical crystal. The symmetry is particularly important for the second-order nonlinear optical process. In the electric dipole approximation, there is no second-order polarizability χ(2) in the center-symmetric material, so the second harmonic generation (SHG) has been widely studied only in natural crystals with broken center inversion symmetry. Although the inversion symmetry of traditional optical crystals can be broken by introducing stress, the modulation depth of the nonlinear optical polarizability of the hybrid system is very limited.

[0004] Electric field-induced second harmonic generation (EFISH) proposed in the early 1960s provides another way to design nonlinear optical modulators. Electric field-induced second harmonic generation is a third-order nonlinear process caused by the interaction of an external bias electric field with two incident photons. In the electric field-induced second harmonic generation process, the external static electric field can be mixed with the fundamental wave to generate the second harmonic wave in nonlinear optical materials with large third-order polarizability. Electric field-induced second harmonic generation can be used to dynamically control the nonlinear optical response of the material, so it has broad application prospects in active nonlinear devices. However, it has always been a great challenge to achieve strong modulation using electric field-induced second harmonic generation in traditional nonlinear materials. SUMMARY

[0005] In order to solve the defect that it is difficult to achieve strong modulation using electric field-induced second harmonic generation in nonlinear materials, the present application provides a thin film electric field-induced second harmonic generation device based on band-edge enhancement effect.

[0006] The technical scheme adopted by the present application is a thin film electric field-induced second harmonic generation device based on band-edge enhancement effect, which comprises a thin film layer with band-edge enhancement effect and transparency, a transparent first electrode layer and a second electrode layer capable of reflecting light are respectively arranged on both sides of the thin film layer in the thickness direction, and a voltage is applied between the first electrode layer and the second electrode layer.

[0007] Preferably, the thin film layer is made of organic semiconductor material with conjugated structure.

[0008] Preferably, the thin film layer is a PFO thin film.

[0009] Preferably, the PFO thin film has a thickness ranging from 50 nm to 500 nm.

[0010] Preferably, the first electrode layer is an indium tin oxide layer.

[0011] Preferably, the input wavelength ranges from 810 nm to 885 nm.

[0012] Preferably, the input wavelength ranges from 820 nm to 865 nm.

[0013] Preferably, the second electrode layer is an aluminum thin film.

[0014] Preferably, a direct current voltage is applied between the first electrode layer and the second electrode layer.

[0015] Preferably, the thin film electro-doubling frequency device is encapsulated in a transparent encapsulation structure.

[0016] Compared with the prior art, the present application has the following beneficial effects:

[0017] The present application discloses a thin film electro-doubling frequency device based on a band edge enhancement effect. When a fundamental wave is obliquely incident, a voltage is applied to a first electrode layer and a second electrode layer. The fundamental wave passes through the transparent first electrode layer and a thin film layer, is reflected by the second electrode layer, and then passes through the thin film layer and exits from the first electrode layer. In this process, the band edge enhancement effect of the thin film layer is used to enhance the third-order nonlinear susceptibility, so that the generation efficiency of the second harmonic wave appears a sharp peak at a certain wavelength position. The fundamental wave wavelength corresponding to the sharp peak is exactly covered by the energy band of the thin film layer, so that the generation efficiency of the second harmonic wave is greatly improved under the applied voltage. Further, an extremely high modulation ratio is obtained, and the modulation depth is much higher than that of a conventional electro-doubling frequency device.

[0018] Compared with the prior art, the thin film electro-doubling frequency device based on the band edge enhancement effect disclosed in the present application can achieve the purpose of realizing strong modulation by using electro-doubling frequency in a nonlinear material. BRIEF DESCRIPTION OF DRAWINGS

[0019] The present application will be described in detail below with reference to the embodiments and the accompanying drawings, in which:

[0020] Figure 1 A principle diagram of a thin film electro-doubling frequency device based on a band edge enhancement effect according to an embodiment of the present application is shown;

[0021] Figure 2 A second harmonic wave spectrum of a thin film electro-doubling frequency device based on a band edge enhancement effect according to an embodiment of the present application under two different applied voltages is shown;

[0022] Figure 3The figure shows a second harmonic intensity pattern under different applied voltages when the second harmonic wavelength is 420nm in a thin film electric frequency doubling device based on the band edge enhancement effect according to an embodiment of the present application.

[0023] Figure 4 The figure shows a processing preparation flow chart of a preparation method of a thin film electric frequency doubling device based on the band edge enhancement effect according to an embodiment of the present application.

[0024] Label explanation:

[0025] 1, substrate; 2, indium tin oxide; 3, PFO; 4, aluminum film; 5, transparent packaging structure. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical scheme and advantages of the present application clearer, the following will further describe the embodiments of the present application in combination with the drawings. The examples of the embodiments are shown in the drawings, wherein the same or similar reference signs represent the same or similar parts or parts having the same or similar functions throughout. The embodiments described below by referring to the drawings are exemplary and are only used to explain the present application, and cannot be explained as a limitation to the present application.

[0027] The present application discloses a thin film electric frequency doubling device based on the band edge enhancement effect, please refer to Figure 1 , comprising a thin film layer with band edge enhancement effect and transparency, a transparent first electrode layer and a second electrode layer capable of reflecting light are respectively arranged on both sides of the thin film layer in the thickness direction, and a voltage is applied between the first electrode layer and the second electrode layer.

[0028] When the fundamental wave is obliquely incident, a voltage is applied to the first electrode layer and the second electrode layer, the fundamental wave passes through the transparent first electrode layer and the thin film layer, is reflected by the second electrode layer, and then passes through the thin film layer and exits from the first electrode layer. In this process, the band edge enhancement effect of the thin film layer is used to enhance the third-order nonlinear susceptibility, so that the generation efficiency of the second harmonic wave appears a sharp peak at a certain wavelength position, and the frequency doubled light corresponding to the fundamental wave wavelength at the sharp peak covers the energy band of the thin film layer, thereby greatly improving the generation efficiency of the second harmonic wave under the applied voltage. Further, an extremely high modulation ratio is obtained, and the modulation depth is much higher than that of the conventional electric frequency doubling device. Compared with the prior art, the thin film electric frequency doubling device based on the band edge enhancement effect disclosed in the present application can realize the purpose of using electric frequency doubling in nonlinear materials to achieve strong modulation.

[0029] Figure 1Disclosed is a schematic diagram of a thin film electric frequency doubling device based on band edge enhancement effect for realizing electric frequency doubling. The micro-nano photoelectric device of sandwich structure is composed of a metal reflection layer of aluminum, an organic semiconductor PFO and a transparent electrode ITO. Under the action of incident transverse magnetic light wave and applied direct current electric field, a great electric frequency doubling effect is generated.

[0030] Specifically, the band edge enhancement effect refers to that when the incident photon energy is close to the band gap, the absorption and radiation of the semiconductor to the light of this wavelength are both enhanced, which indicates the direct transition of carriers from the conduction band to the valence band in the semiconductor. Therefore, in the nonlinear optical process, the band edge enhancement effect enhances the intensity or response of the electromagnetic field at the edge of the band structure, and enhances the radiation of the nonlinear harmonic. At this time, the frequency doubled light corresponding to the fundamental wave wavelength at the sharp peak can cover the energy band of the thin film layer, that is, the energy range of the frequency doubled light can cover or cross the energy band, so as to excite the transition of electrons in the energy band, thereby changing the electronic structure and properties of the thin film layer.

[0031] Among them, the band edge enhancement effect obtained according to the properties of the thin film layer made of different materials is not the same, and the size of the fundamental wave wavelength at the position of the sharp peak value corresponding to the sharp peak value is also not the same. The fundamental wave wavelength at the sharp peak value is mainly related to the material properties of the thin film layer, and the thickness of the thin film layer has little effect on the position of the sharp peak wavelength. In the consideration of the present project, since no complex resonance effect is involved, the peak value of the fundamental wave wavelength mainly depends on the material properties of the thin film layer.

[0032] It needs to be explained that the fundamental wave wavelength at the sharp peak value has the maximum second harmonic generation gain, and the fundamental wave wavelength on both sides of the sharp peak value also has a certain gain effect, and the maximum second harmonic generation gain obtained at this time does not change with the change of the applied voltage.

[0033] Regarding how to ensure that the frequency doubled light exactly covers the energy band of the thin film layer, first, the thin film layer material is selected: in the design of the actual device, the material is selected (such as PFO), and the band gap energy is also fixed. Second, determine the wavelength according to the band gap energy: select a suitable fundamental frequency wavelength according to the band gap energy of the material to ensure that the energy of the frequency doubled light is closest to the band gap energy of the material, thereby realizing the best frequency doubling generation. Finally, when the energy of the frequency doubled light is closest to the band gap energy, the second harmonic intensity reaches the peak value: this is the specific embodiment of the band edge enhancement effect, and the resonance enhancement of the frequency doubled light can maximize the intensity of the second harmonic signal.

[0034] In the present application, the second harmonic can be generated in the organic polymer thin film layer when the fundamental wave is obliquely incident. The reflected second harmonic intensity of the device can be described as: where Eω is the electric field of the fundamental wave; EDC For the external electric field applied to the organic polymer thin film layer through the transparent electrode (first electrode layer) and the metal electrode (second electrode layer); χ(2) and χ(3) are the effective second-order and third-order polarizabilities of the organic polymer thin film layer, respectively. χ(2) is generated by the second harmonic on the metal-organic polymer or ITO-organic polymer thin film layer interface. The first term and the third term describe the electric frequency doubling process and the conventional second harmonic process, respectively, and the second term represents the interference between the two, which depends on the χ(2) and χ(3) coefficients of the system. This is contrary to the results of most active substances, in which χ(3) plays a dominant role. Due to the coupling of the fundamental wave field and the direct current field in the second term of the equation, the strength of the electric frequency doubling can also be adjusted by the sign of the direct current field applied by the switch.

[0035] When the fundamental wave of the electric frequency doubling thin film electric frequency doubling device of the "sandwich structure" is incident, the generation of the second harmonic in the organic polymer thin film layer is prohibited, because the second-order polarizability χ(2) in the uniform thin film can be ignored.

[0036] Connecting the first electrode layer of the thin film electric frequency doubling device with the edge enhancement effect to the anode / cathode of the direct current power supply can apply positive / negative voltage. The transparent first electrode layer can act as an anode (U>0) or a cathode (U<0). As shown in Figure 2 By applying a voltage to the first electrode layer and the second electrode layer, the external static electric field can be mixed with the fundamental wave (FW) to generate a second harmonic in an organic polymer material with a large third-order polarizability by using oblique incidence of the fundamental wave, and the device can strongly enhance the generation efficiency of the second harmonic.

[0037] In other application scenarios, electric frequency doubling is a special case of third-order nonlinear optics, and third-order nonlinear effects can also be generated when an alternating current voltage is applied, which does not affect the normal operation of the device for common alternating current frequencies. The first electrode layer and the second electrode layer can also be connected to an alternating current power supply and applied to application scenarios that require rapid response or multiple switch control.

[0038] In some embodiments, the thin film layer is made of conjugated structure organic semiconductor material.

[0039] On the one hand, the conjugated structure material usually has a delocalized π electron system, and these π electrons can move freely throughout the molecular skeleton. The delocalization of these electrons significantly enhances the third-order nonlinear polarizability χ(3) of the material, resulting in higher nonlinear optical response; on the other hand, the conjugated structure material usually has good solution processability, which is very advantageous in preparing optical thin films.

[0040] The conjugated organic semiconductor has the following advantages: 1. Compared with ordinary organic matter, the conjugated structure has a larger nonlinear polarization rate; 2. Compared with inorganic semiconductors, the organic semiconductor can be processed and realized by simple spin coating; 3. The conjugated organic semiconductor can adjust the wavelength of the band edge effect by changing the chemical synthesis method.

[0041] In some embodiments, the thin film layer is a PFO thin film.

[0042] The fundamental wavelength at the sharp peak is related to the characteristics of PFO itself. When the fundamental wavelength is 840 nm, the wavelength of the second harmonic (frequency doubling) light is 420 nm, which is closest to the band gap of PFO, which enhances the nonlinear polarization response of the material, resulting in a peak in the generation efficiency of the second harmonic at this wavelength.

[0043] Specifically, PFO, i.e. Poly(9,9-di-n-octyl-2,7-fluorene), PFO, as a conjugated organic semiconductor material, has good optical absorption and emission performance in the visible light range due to the π electron conjugated system in its molecular structure, and the molecular chain of PFO can exhibit various chain conformations with different torsion angles. The stability of these chain conformations has an important influence on the improvement of the band edge enhancement effect of PFO, so the thin film layer is selected as a PFO thin film.

[0044] In other embodiments, other organic conjugated polymer materials with band edge enhancement effect and transparency can also be selected. In addition to PFO, other conjugated organic semiconductor thin films can also be selected as nonlinear materials, such as BEHP-PPV, MEH-PPV, etc. These organic semiconductors have a larger nonlinear polarization rate, and the wavelength of the band edge effect can be adjusted by adjusting the mixing ratio of these organic semiconductors, covering almost the entire visible light range. In addition to the following listed organic conjugated polymers such as MEH-PPV and BEHP-PPV, some perovskite materials and two-dimensional materials such as ZnO and transition metal dichalcogenides (WS2, MoS2, etc.) can also be selected as the thin film layer.

[0045] In some specific embodiments, the thickness of the PFO thin film ranges from 50 nanometers to 500 nanometers.

[0046] It should be noted that the thickness of the PFO thin film is limited to a range of 50 nanometers to 500 nanometers. On the one hand, this thickness range can meet the basic band edge enhancement effect strength of the PFO thin film. On the other hand, the PFO thin film with this thickness range can exhibit a more transparent material to meet better optical properties during operation. On the other hand, the thin film electro-optical frequency doubling device has a more lightweight texture, which is suitable for use in various scenarios.

[0047] Preferably, the PFO film has a thickness of 100 nm, and the optical properties and the band edge enhancement effect of the PFO film are in the best match, so that the thin film electro-optical frequency doubling device has stronger performance.

[0048] In some embodiments, the first electrode layer is an optical crystal having spatial symmetry.

[0049] In other embodiments, the first electrode layer has a first part having non-spatial symmetry and a second part having spatial symmetry, so that the second harmonic values generated when the fundamental wave is incident and reflected in the first part or the second part are different, providing more choices for the input and output modes of the thin film electro-optical frequency doubling device. It should be noted that some materials can form crystals having spatial symmetry and optical crystals having non-spatial symmetry according to different preparation processes, so the first part and the second part can be made of the same material or different materials.

[0050] In some more specific embodiments, the first electrode layer is an indium tin oxide layer.

[0051] The indium tin oxide (ITO) is a substitutional solid solution and also a metal oxide material with excellent electrical properties. In a thin film state, the indium tin oxide has excellent transparency. Using the indium tin oxide layer as the transparent first electrode layer can minimize the influence of the first electrode layer on the PFO film layer. Preferably, the thickness of the indium tin oxide layer is in the range of 50 nm to 500 nm. It should be noted that the thickness of the indium tin oxide layer needs to be selected according to the size of the applied voltage, because high concentration of charge carriers can increase the conductivity of the material, but also reduce its transparency. Controlling the thickness of the indium tin oxide layer in the range of 50 nm to 500 nm can help to control the size of the applied voltage in a smaller range, so that the indium tin oxide can maintain its excellent transparent properties.

[0052] As shown in FIG. 1, the thin film electro-optical frequency doubling device 100 includes a first electrode layer 110, a PFO film layer 120, and a second electrode layer 130. Figure 3 Figure 3 ​The image shows the second harmonic intensity spectrum under different applied voltages when the second harmonic wavelength is 420 nm. The experimental second harmonic intensity conforms to the nonlinear polarization theory as the applied DC voltage changes. At the resonant wavelength of 840 nm, the second harmonic consistently exhibits the highest efficiency for this electro-optical device, regardless of the applied DC electric field. When the applied voltage is changed, the second harmonic intensity decreases to a minimum at U = 1.5 V, then rapidly increases, reaching its maximum at U = 6 V, indicating an extremely high modulation ratio. Applying a reverse voltage to the device reveals highly unusual electro-optical frequency doubling behavior. As U sweeps from 0 to -6 V, the second harmonic intensity continues to increase, exhibiting a very high SHG negative modulation depth.

[0053] In some more specific embodiments, the input wavelength ranges from 810 nanometers to 885 nanometers.

[0054] Specifically, please refer to Figure 2 The spectra of the second harmonics under two different applied voltages are shown. Without an applied DC electric field, the sandwich-structured micro / nano device exhibits extremely weak second harmonic efficiency (triangular broken line in the figure). When the applied DC voltage is 6V (dotted broken line in the figure), the second harmonic generation efficiency is significantly improved, reaching its maximum at a pump wavelength of 840nm. The second harmonic generated at this wavelength (wavelength 420nm) corresponds to the bandgap of the organic semiconductor PFO, indicating band-edge enhanced electro-frequency doubling. When the input wavelength range is 810nm to 885nm, its SHG gain is above 0.3, thus achieving a better gain effect.

[0055] In some particularly specific embodiments, the input wavelength ranges from 820 nanometers to 865 nanometers.

[0056] Among them, by Figure 2 It can be seen that when the input wavelength range is 820 nm to 865 nm, the SHG gain is above 0.6, thus achieving a better gain effect. For optimal gain, the input wavelength can be set to 840 nm, at which point the efficiency of second harmonic generation is at its maximum.

[0057] In some embodiments, the second electrode layer is an aluminum thin film.

[0058] Specifically, the second electrode layer can be made of metal, preferably an aluminum thin film, which has excellent electrical properties and can reflect the fundamental wave. Preferably, the aluminum thin film has a thickness of 100 nanometers to maintain the nanometer-level thickness of the thin-film electro-frequency doubling device.

[0059] In some embodiments, a DC voltage is applied between the first electrode layer and the second electrode layer.

[0060] Wherein, the direct current voltage is applied to the first electrode layer and the second electrode layer, so that the frequency-doubling device can obtain stable second harmonic, and the robust performance is higher. At the resonant wavelength of 840 nm, no matter how the direct current field is applied, the second harmonic always has the highest efficiency on the optoelectronic device of the frequency-doubling. As shown in Figure 3 , when the applied voltage is changed, the intensity of the second harmonic decreases to a minimum at U = 1.5 V, and then increases rapidly, and reaches the highest value at U = 6 V. The thin film frequency-doubling device has a very high modulation ratio.

[0061] In some embodiments, the thin film frequency-doubling device is packaged in a transparent packaging structure.

[0062] Wherein, the thin film frequency-doubling device is placed in the cavity surrounded by the substrate and the transparent packaging structure, so as to well protect the thin film frequency-doubling device from damage and affect the normal implementation of its function.

[0063] The application also discloses a preparation method of the thin film frequency-doubling device based on the band edge enhancement effect, comprising the following steps:

[0064] Preparation of an indium tin oxide film on the substrate as a first electrode layer;

[0065] Coating a uniform PFO film layer on the first electrode layer;

[0066] Plating an aluminum film on the PFO film layer as a second electrode layer.

[0067] In some particularly specific embodiments, comprising:

[0068] Preparation of an indium tin oxide (ITO) film on the substrate 1 as a transparent electrode layer 2, see Figure 4 a;

[0069] Spin coating a uniform 100 nm organic polymer PFO film 3 on the transparent electrode layer 2, see Figure 4 b;

[0070] Thermally evaporating a 100 nm aluminum film on the organic polymer film layer as an electrode layer 4, see Figure 4 c;

[0071] Packaging the prepared thin film frequency-doubling device by using a transparent packaging structure 5 such as epoxy resin, and connecting an external power supply to obtain a thin film frequency-doubling device, see Figure 4 d.

[0072] In the description of the specification, if the terms "embodiment one", "this embodiment", "in one embodiment", etc. are described, it means that the specific features, structures, materials or characteristics described in conjunction with this embodiment or example are included in the invention or at least one embodiment or example of the invention. In this specification, the illustrative description of the above terms does not necessarily refer to the same embodiment or example; moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in an appropriate manner.

[0073] In the description of the specification, the terms "connection", "installation", "fixation", "setting", "have" and the like are understood in a broad sense, for example, "connection" can be fixed connection, or detachable connection, or integral connection; can be mechanical connection, or electrical connection; can be directly connected, or indirectly connected through intermediate medium, can be the communication inside two elements. For those skilled in the art, the specific meaning of the above terms in this application can be understood according to the specific circumstances.

[0074] In the description of the specification, the relationship terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between the entities or operations. Moreover, the terms "include", "contain" or any other variant thereof are intended to cover non-exclusive inclusion, so that the process, method, article or device including a series of elements not only includes those elements, but also includes other elements not explicitly listed, or further includes elements inherent to such process, method, article or device. Without more limitations, the element defined by the statement "including a" does not exclude the presence of other identical elements in the process, method, article or device including the element.

[0075] The above description of the embodiments is to facilitate the understanding and application of the present technology for those skilled in the art, and those skilled in the art can easily make various modifications to these examples and apply the general principles described herein to other embodiments without creative labor. Therefore, the present application is not limited to the above embodiments, and the following modifications should be within the scope of protection: ① new technical solutions based on the technical solutions of the present application and combined with existing common knowledge, the technical effects produced by the new technical solutions do not exceed the technical effects of the present application; ② equivalent replacement of part of the features of the technical solutions of the present application using known technology, the technical effects produced are the same as the technical effects of the present application; ③ expandable based on the technical solutions of the present application, the essential content of the expanded technical solutions does not exceed the technical solutions of the present application; ④ equivalent transformation using the contents of the present application specification and drawings, direct or indirect application in other related technical fields.

Claims

1. A thin-film electro-frequency doubling device based on band-edge enhancement effect, characterized in that, It includes a transparent thin film layer with an edge enhancement effect, and a transparent first electrode layer and a light-reflecting second electrode layer are respectively provided on both sides of the thickness direction of the thin film layer, and a voltage is applied between the first electrode layer and the second electrode layer; The thin film layer is a PFO thin film; the thickness of the PFO thin film ranges from 50 nanometers to 500 nanometers. The input wavelength range is 810 nm to 885 nm; A DC voltage is applied between the first electrode layer and the second electrode layer.

2. The thin-film electro-frequency doubling device based on the band-edge enhancement effect according to claim 1, characterized in that, The first electrode layer is an optical crystal with spatial symmetry.

3. The thin-film electro-frequency doubling device based on the band-edge enhancement effect according to claim 1, characterized in that, The first electrode layer is an indium tin oxide layer.

4. The thin-film electro-frequency doubling device based on the band-edge enhancement effect according to claim 1, characterized in that, The input wavelength ranges from 820 nanometers to 865 nanometers.

5. A thin-film electro-frequency doubling device based on band-edge enhancement effect according to claim 1, characterized in that, The second electrode layer is an aluminum thin film.

6. A thin-film electro-frequency doubling device based on the band-edge enhancement effect according to any one of claims 1 to 5, characterized in that, The thin-film electro-frequency doubling device is encapsulated within a transparent packaging structure.

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