CN110780466A - A device for generating a surface evanescent wave field with an electrically adjustable intensity - Google Patents

Abstract

The invention belongs to the technical field of guided wave optics, in particular to a surface evanescent wave field device whose intensity can be adjusted electrically. The device of the invention comprises: a coupling prism, an electronically controlled birefringent liquid crystal cell, an AC signal driver, and a collimated laser light source; the electronically controlled birefringent liquid crystal cell is composed of two upper and lower substrates that are assembled and packaged; the surfaces of the upper and lower substrates Coated with functional films: transparent ITO conductive film layer, liquid crystal alignment layer, liquid crystal is poured between the two liquid crystal alignment layers; the coupling prism is attached to the lower substrate of the liquid crystal cell; the AC signal driver is transparent to the upper and lower sides of the liquid crystal cell The ITO conductive film layers are connected; the collimated laser beam emitted by the collimated laser light source is incident from a hypotenuse of the coupling prism, and an electrically adjustable surface evanescent wave field is generated on the outer layer of the upper substrate of the liquid crystal cell. The intensity of the evanescent field can be up to several orders of magnitude enhanced. The device has important application prospects in the fields of surface microscopic measurement, biology, surface chemistry, and microscopic spectroscopy.

Description

A device for generating a surface evanescent wave field with an electrically adjustable intensity

technical field

The invention belongs to the technical field of guided wave optics, and in particular relates to a device for generating a surface evanescent wave field.

Background technique

Research on materials, chemistry, and life sciences has been an important driving force for the development of various disciplines in recent years. New materials determine the application and development of technologies. Research on surface/interface chemical reactions and life sciences is closely related to people's quality of life. Early diagnosis of various diseases, clarification and screening of the mechanism of action of various drugs, etc. Among them, single-molecule science or nanoparticle research has become a research hotspot because it can elucidate the surface/interface structure and morphology of materials at the microscopic level, the process of chemical reactions and the changes of biological tissues. Among them, fluorescence spectroscopy and Raman spectroscopy have become the main means of research. Molecular fluorescence can study the electronic structure inside the molecule, while Raman spectroscopy can distinguish the bonding structure of the molecule.

However, surface-enhanced Raman scattering (SERS) generally cannot be obtained on flat adsorption surfaces, and samples must be deposited on rough metal surfaces or metal nanoparticle surfaces, making SERS largely dependent on sample preparation and substrate surface roughness Spend. The uncertainty of this field enhancement is the biggest weakness of SERS. In addition, the spatial resolution of SERS measurements is limited by the diffraction limit of the excitation light and the measurement system. Tip enhanced Raman Scattering (TERS) can overcome this shortcoming of spatial resolution of SERS, and can reach the nanoscale, and its signal enhancement factor can reach 10 ⁵ -10 ⁶ times. However, the tip of TERS is easily damaged, oxidized, and contaminated during the experiment, which affects the stability of signal measurement.

In recent years, field enhancement based on Bloch Surface Wave (BSW) has attracted much attention. BSW is an electromagnetic wave existing in a periodic multilayer dielectric film stack. Based on the optical waveguide effect, the optical field is confined in the periodic film layer to generate localized enhancement, and the evanescent wave field on its surface is also enhanced accordingly. effect. At present, whether it is SPR or BSW, the excitation of field enhancement is mostly carried out by Kretschmann's prism coupling method. For such a structure, its field enhancement performance is a passive enhancement mechanism, that is, once the system structure (1DPC or metal film layer) is prepared, the theoretical field enhancement factor (EF) is determined for a certain wavelength and the sample to be tested. The amount of enhancement is greatly affected by factors such as the fabrication error of the device structure, the accuracy of the measuring instrument, and the operator's ability.

Therefore, researching an in-situ tunable field enhancement mechanism to actively control the field enhancement factor, such as continuous adjustment or setting, or magnification compensation for wavelength changes, has become a problem of important scientific and practical significance. , which can reduce the uncertainty of experiments and results, facilitate the quantitative comparison of different measurements, and also facilitate the measurement of the excitation intensity dependence of fluorescence and Raman signals and the excitation spectrum of characteristic fluorescence signals. More material structure information.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a surface evanescent wave field generating device with excellent performance and convenient use with an electrically adjustable intensity.

The device for generating a surface evanescent wave field with an electrically adjustable intensity provided by the present invention comprises: a coupling prism, an electronically controlled birefringence (ECB) liquid crystal cell, an AC signal driver, and a collimated laser light source; The electronically controlled birefringent liquid crystal cell is composed of an upper and a lower substrate butt-packed; wherein, the upper (or lower) surface of the lower substrate is coated with a functional film: a low-refractive-index coupling layer, a transition layer; The surface is also plated (coated) with a transparent ITO conductive film layer and a liquid crystal alignment layer in turn; the lower surface of the upper substrate is sequentially plated (coated) with a transparent ITO conductive film layer and a liquid crystal alignment layer; between the coupling prism and the lower substrate of the liquid crystal cell; the AC signal driver is connected with two transparent ITO conductive film layers in the liquid crystal cell; the collimated laser beam emitted by the collimated laser light source is coupled from the One oblique side of the prism is incident, and the incident angle is greater than the critical angle of total reflection of the prism to the coupling layer of the liquid crystal cell, and an electrically adjustable surface evanescent wave field is generated on the outer layer of the upper substrate of the liquid crystal cell. See Figure 1 for details.

In the present invention, the liquid crystals in the liquid crystal cell are arranged in parallel along the plane to form a planar waveguide structure, and work in an electronically controlled birefringence (ECB) mode. Depending on the alignment layer, the liquid crystal can also work in other modes, such as TN, IPS, VA, etc., and use polarizers to control the polarization direction of light waves.

In the present invention, the material of the bottom substrate (lower substrate) of the liquid crystal cell and the material of the coupling prism both have a higher refractive index, which is higher than the refractive index of the low-refractive index coupling layer. The lower substrate can be selected from glass or other transparent materials.

In the present invention, the low refractive index coupling layer, the so-called "low refractive index" is relative to the higher refractive index of the coupling prism, the liquid crystal cell substrate and the liquid crystal, and its material can be selected from a low refractive index coating such as magnesium fluoride. Material.

In the present invention, the material of the transition layer can be selected from quartz, fused silica or magnesium fluoride.

In the present invention, the incident angle of the collimated light beam to the surface normal of the bottom substrate of the liquid crystal cell is greater than the total reflection critical angle of the prism to the coupling layer.

In the present invention, the collimated beam emitted by the collimated laser light source is guided into the device by the coupling prism in the polarization direction of the transverse magnetic wave (TM). In other working modes, the polarization direction of the collimated beam can also be the polarization direction of the transverse electric wave (TE).

In the present invention, the transition layer can be made of a transparent material such as quartz, and its refractive index is between that of the low-refractive index coupling layer and the liquid crystal cell substrate (eg glass).

In the present invention, the liquid crystal in the liquid crystal cell can be either a positive liquid crystal material or a negative liquid crystal material.

In the present invention, the material of the upper substrate can be a transparent material having a higher refractive index than the low-refractive index coupling layer, such as glass.

The working principle of the device of the present invention: the collimated beam emitted by the collimated laser light source is introduced into the device by the coupling prism in the polarization direction of the transverse magnetic wave (TM) to excite the surface evanescent wave field supported by the planar waveguide structure composed of the liquid crystal cell; The AC signal driver applies a driving voltage along the normal direction of the liquid crystal layer. When the driving voltage exceeds the orientation threshold of the liquid crystal layer, the refractive index of the extraordinary light of the liquid crystal layer changes, which changes the refractive index distribution of the planar structure of the liquid crystal cell. Changes the intensity of the surface evanescent waves. Continuously adjusting the driving voltage of the liquid crystal layer within a certain range can realize the continuous adjustment of the surface evanescent wave field intensity.

The advantages of the device of the present invention:

1. It can realize the electric control continuous adjustment of evanescent wave field strength. There is no need for any movement adjustment of the mechanism during the adjustment process, so it has good adjustment stability;

2. The adjustment range of field enhancement is large. Under the condition of high angle adjustment accuracy, the adjustment range of field enhancement can reach several orders of magnitude;

3. The device of the present invention is convenient to use and adjust.

The device of the present invention proposes an adjustable mechanism of field enhancement, so that some uncertainties of field enhancement can be effectively suppressed, and at the same time, the electric adjustment mechanism can eliminate the need for additional mechanism movement to realize field enhancement after the excitation conditions are determined. adjustment, so it has good stability. In addition, depending on the coupling at different angles, the field enhancement magnification can reach several orders of magnitude. Therefore, the present invention can have important application prospects in the fields of surface microscopic measurement, biology, surface chemistry, microscopic spectroscopy and the like.

Description of drawings

FIG. 1 is a schematic diagram of a surface evanescent wave field generating device with an electrically adjustable intensity according to the present invention.

FIG. 2 shows the spatial distribution of the refractive index along the normal (Z) direction of the liquid crystal cell on both sides of the liquid crystal cell for the designed structure without applying a driving voltage. At this time, the incident angle of the collimated light is 51.817°. The critical angle of total reflection of the incident prism to the coupling layer is 46.984°.

FIG. 3 is a partial enlargement of the thickness direction of the liquid crystal cell of FIG. 2 to show the spatial division of the refractive index inside the liquid crystal cell along the normal direction.

Fig. 4 shows the spatial distribution of the refractive index along the normal (Z) direction of the liquid crystal cell inside and outside the liquid crystal cell in Fig. 3 when a driving voltage of 3.6-4.7 volts is applied to the liquid crystal cell. At this time, the incident angle of the collimated light is 51.817°. The critical angle of total reflection of the incident prism to the coupling layer is 46.984°.

Figure 5 shows the intensity of the surface evanescent field generated at the outer surface of the upper substrate of the liquid crystal cell when a driving voltage of 4.7 volts is applied to the liquid crystal cell varies along the outer normal (Z) direction of the liquid crystal cell. At this time, the incident angle of the collimated light is 51.817°. The critical angle of total reflection of the incident prism to the coupling layer is 46.984°.

FIG. 6 shows the intensity of the surface evanescent field generated at the outer surface of the upper substrate of the liquid crystal cell as a function of the driving voltage when a driving voltage of 3.6-4.7 volts is applied to the liquid crystal cell. At this time, the incident angle of the collimated light is 51.817°. The critical angle of total reflection of the incident prism to the coupling layer is 46.984°.

Labels in the figure: 1 is the collimated incident beam, 2 is the coupling prism, 3 is the lower substrate of the liquid crystal cell, 4 is the low-refractive index coupling layer, 5 is the transition layer (can be multi-layered), and 6 is the upper and lower substrates of the liquid crystal cell The transparent conductive film (ITO) layer on the top, 7 is the liquid crystal alignment layer (PI) on the upper and lower substrates of the liquid crystal cell, 8 is the upper substrate of the liquid crystal cell, 9 is the AC signal driver, 10 is the liquid crystal layer, and 11 is the liquid crystal cell. The evanescent wave region of the upper substrate surface.

Detailed ways

The invention includes: a coupling prism, an electronically controlled birefringence (ECB) liquid crystal cell, an alternating current signal driver, and a collimated laser light source. The structure is shown in Figure 1.

Select a laser working wavelength for the collimated laser light source, such as: 632.8nm, select the coupling prism material such as flint glass, the refractive index is 1.778, and the lower substrate of the liquid crystal cell is also selected to match the coupling prism. The upper surface of the lower substrate of the liquid crystal cell is coated with a low refractive index coupling layer, the material is fused silica, the refractive index is 1.457, and the thickness is 500 nm. Then, a transition layer is plated on it, which are quartz, fused silica, magnesium fluoride, fused silica, quartz, the refractive index is 1.52, 1.457, 1.30, 1.45, 1.52, and the thickness is 500, 500, 100, 200 , 100nm. A layer of transparent conductive film (ITO) was deposited again with a refractive index of 1.9 and a thickness of 100 nm, and then a liquid crystal alignment layer (PI) was spin-coated with a refractive index of 1.54 and a thickness of 100 nm. The loss value of both is 10 ^-5 . The upper substrate of the liquid crystal cell is made of glass with a refractive index of 1.52 and a thickness of 1 mm. The ITO layer and the liquid crystal alignment layer PI are successively plated on its surface, and the parameters are the same as those of the lower substrate. Take the anti-parallel direction and rub the liquid crystal alignment layer. After the above process is completed, the liquid crystal cell is formed into a cell, and the liquid crystal is poured. The liquid crystal is made of 5CB, the refractive indices of ordinary light and extraordinary light are 1.532 and 1.692, respectively, and the thickness is 5 μm. The evanescent field area on the upper surface of the liquid crystal cell is the sample area to be tested, set as an aqueous solution environment, and the refractive index is 1.33.

According to the above structural parameters, the critical angle of total reflection of the coupling prism to the low-refractive index coupling layer is 46.984°. Take the incident angle of the collimated light as 51.817°. The TM incident light will be coupled into the liquid crystal structure in the form of an evanescent wave, and conducted in the liquid crystal layer to its upper surface, and form a surface evanescent wave field outside the upper surface.

A normal driving voltage is applied to the parallel-aligned liquid crystal layers. After the effective value of the voltage is greater than its orientation threshold, the positive liquid crystal molecules will tilt toward the direction of the electric field. The magnitude of the tilt angle depends on the effective value of the driving voltage. According to the elastic continuum theory of liquid crystal, (see D.K.Yang and S.T.Wu, Fundamentals of Liquid Crystal Devices Wiley 2006), and the elastic parameters of liquid crystal materials, the tilt angle distribution of liquid crystal molecules under different driving voltages can be obtained, which can be obtained The spatial distribution of the refractive index of the liquid crystal layer in the thickness direction of the liquid crystal cell. On this basis, using the calculation method of the optical transmission matrix, the spatial distribution of the wave field of the TM light wave in the above structure can be calculated. For specific methods, please refer to our published paper: Opt. Express 25(11), 12121-12130 (2017)). Here, we focus on the intensity of the light field in the evanescent wave field closest to the surface of the glass plate, which is defined as the product of the square of the light field amplitude and the refractive index. (See M. Born, and E. Wolf, Principles of Optics, 7th ed. (Cambridge Univ. Press, 2007)).

Under different driving voltages, the different orientations of the liquid crystal molecules cause changes in the refractive index of the liquid crystal layer, thereby changing the light-guiding properties of the liquid crystal layer, and ultimately affecting the field strength of the surface evanescent waves. According to the above theory, we can calculate the corresponding relationship between the driving voltage and the surface evanescent wave field strength. In the case of the incident angle of collimated light being 51.817°, Fig. 2 shows the spatial distribution of the refractive index in the thickness direction of the liquid crystal cell when the driving voltage is zero. FIG. 3 is a partial enlargement of the thickness direction of the liquid crystal cell of FIG. 2 to show the spatial division of the refractive index inside the liquid crystal cell along the normal direction. Under the same incident angle, Fig. 4 shows the spatial distribution of the refractive index along the normal (Z) direction of the liquid crystal cell inside and outside the liquid crystal cell in Fig. 3 when a driving voltage of 3.6-4.7 volts is applied to the liquid crystal cell.

Keeping the incident angle of the collimated light, Fig. 5 shows the intensity of the surface evanescent field generated at the outer surface of the upper substrate of the liquid crystal cell under the condition of applying a driving voltage of 4.7 volts to the liquid crystal cell along the direction of the outer normal (Z) of the liquid crystal cell. Case. FIG. 6 shows the intensity of the surface evanescent field generated at the outer surface of the upper substrate of the liquid crystal cell as a function of the driving voltage when a driving voltage of 3.6-4.7 volts is applied to the liquid crystal cell. This intensity is a ratio relative to the intensity of the optical field within the coupling layer. Therefore, it is also called the enhancement factor of the light field intensity. It varies continuously and monotonically with the driving voltage. That is, a surface evanescent wave field with an electrically tunable intensity is realized.

Under the condition that the precision of the coupling angle reaches (1/1000)°, the enhancement of the surface evanescent wave intensity can reach 70 times. This enhancement is a ratio relative to the intensity of the light field in the coupling layer. The minimum to maximum range of this ratio is the adjustable range of the surge intensity. Selecting other coupling angles, the intensity enhancement factor of evanescent wave may have a larger enhancement rate. In addition, if the control accuracy of the coupling angle can be further improved, the maximum enhancement range of the evanescent field intensity can reach several orders of magnitude. The device has important application prospects in the fields of surface microscopic measurement, biology, surface chemistry, and microscopic spectroscopy.

Claims (6)

1. A surface evanescent wave field generating device with an adjustable intensity is characterized in that, comprising: a coupling prism, an electronically controlled birefringence liquid crystal cell, an alternating current signal driver, and a collimated laser light source; wherein, The electronically controlled birefringence liquid crystal cell is composed of two upper and lower substrates that are assembled and encapsulated; wherein, the upper surface or the lower surface of the lower substrate is coated with a functional film: a low-refractive index coupling layer, a transition layer; The surface is also plated (coated) with a transparent ITO conductive film layer and a liquid crystal alignment layer in turn; the lower surface of the upper substrate is sequentially plated (coated) with a transparent ITO conductive film layer and a liquid crystal alignment layer; between the coupling prism and the lower substrate of the liquid crystal cell; the AC signal driver is connected with two transparent ITO conductive film layers in the liquid crystal cell; the collimated laser beam emitted by the collimated laser light source is coupled from the One oblique side of the prism is incident, and the incident angle is greater than the critical angle of total reflection of the prism to the coupling layer of the liquid crystal cell, and an electrically adjustable surface evanescent wave field is generated on the outer layer of the upper substrate of the liquid crystal cell. 2 . The surface evanescent wave field generator with an electrically adjustable intensity according to claim 1 , wherein the liquid crystals in the liquid crystal cell are arranged in parallel along the plane to form a planar waveguide structure and operate in an electrically controlled birefringence mode. 3 . 3 . The surface evanescent wave field generating device with an electrically adjustable intensity according to claim 1 , wherein the bottom substrate material of the liquid crystal cell and the coupling prism material have higher refractive indices than the low refractive index coupling. 4 . the refractive index of the layer. 4. The surface evanescent wave field generating device with an electrically adjustable intensity according to claim 1, wherein the incident angle of the collimated beam to the surface normal of the bottom substrate of the liquid crystal cell is greater than the full angle of the prism to the coupling layer. critical angle of reflection. 5 . The surface evanescent wave field generating device with an electrically adjustable intensity according to claim 1 , wherein the liquid crystal in the liquid crystal cell is a positive liquid crystal material or a negative liquid crystal material. 6 . 6. The surface evanescent wave field generating device with an electrically adjustable intensity according to one of claims 1-5, is characterized in that, its work flow is: a collimated laser light source emits a collimated beam, and the polarization direction of the transverse magnetic wave The surface evanescent wave field supported by the planar waveguide structure composed of the liquid crystal cell is excited by the coupling prism introduction device; a driving voltage is applied along the normal direction of the liquid crystal layer by the AC signal driver, when the magnitude of the driving voltage exceeds the orientation threshold of the liquid crystal layer , the extraordinary light refractive index of the liquid crystal layer changes, which changes the refractive index distribution of the planar structure of the liquid crystal cell, thereby changing the intensity of the surface evanescent wave, that is, the driving voltage of the liquid crystal layer is continuously adjusted within a certain range, and the surface evanescent wave can be adjusted continuously. Continuous adjustment of the evanescent wave field strength.

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