A ferroelectric spiral liquid crystal material and a method for achieving second harmonic circular dichroism therein.

Abstract

This invention discloses a ferroelectric spiral liquid crystal material and a method for achieving second-harmonic circular dichroism (BHD). This invention obtains a ferroelectric spiral liquid crystal with an ultra-high polarity spiral structure by doping ferroelectric nematic liquid crystals with chiral molecules of different helical orientations. Such liquid crystal materials possess strong nonlinear optical effects and can excite high-intensity second harmonics (e.g., with a nonlinear coefficient comparable to that of LiNbO3 nonlinear crystalline materials). Based on the helical polarization structure of the ferroelectric spiral liquid crystal, this invention achieves BHD in highly fluid liquid materials by introducing different helicalities (i.e., left-handed and right-handedness), with a BHD anisotropy factor as high as 2.0, showing broad application prospects in chiral research and spatial polarization resolution.

Description

Technical Field

[0001] This invention belongs to the field of nonlinear optical material preparation and application. It discloses a novel method for fabricating a novel ultra-high polarity fluid material with a periodic helical structure to achieve second harmonic circular dichroism, selectively controlling the intensity of the second harmonic, and its application in chirality research and spatial polarization exploration. Background Technology

[0002] Nonlinear optics, established after the advent of the laser, is a science that studies the nonlinear interactions between light and matter. Nonlinear optical processes are generally described using the polarization induced by light in a medium.

[0003]

[0004] In the formula, P and E are the polarization intensity and electric field vector, respectively, χ is the polarizability, ε0 ​​is the vacuum permittivity, and P (1) =ε0χ (1) E represents linear polarization, describing various linear optical phenomena. The remaining terms are nonlinear terms of various orders, used to describe the nonlinear interactions between light and matter, involving second-, third-, and higher-order nonlinear optical effects, corresponding to frequency doubling, difference frequency, sum frequency, optical parametric oscillations, four-wave mixing, stimulated scattering, multiphoton processes, etc. Among many nonlinear optical phenomena, second-order nonlinear optical effects are the most widely studied, especially three-wave mixing, which provides the most fundamental principle for modern laser frequency conversion technology and many extended studies. In the three-wave mixing process, when the frequencies of the two incident beams are equal, i.e., ω1 = ω2 = ω, the emitted frequency-doubled light ω3 = ω... 1+ ω2=2ω, which is the optical frequency doubling effect, also known as second harmonic generation (SHG). The incident light is usually called the fundamental frequency light, and the outgoing light is called the frequency doubling light.

[0005] The intensity of the second harmonic can be simplified to the following relationship if the walk-off effect is neglected:

[0006]

[0007] In the formula I 2ω and I ω Let d represent the intensity of the frequency-doubled light and the fundamental light, respectively, where L is the thickness of the sample through which the beam passes, and d is the intensity of the frequency-doubled light. eff For effective second-order nonlinear coefficients, Let be the phase mismatch factor, where λ is the wavelength of the incident light. Due to the dispersion effect of the medium, generally n 2ω ≠n ω That is, Δk≠0, is called the phase mismatch condition. The intensity of the frequency-doubled light changes periodically with the increase of the crystal length L, which is called L. c=π / Δk is the coherence length. Under phase mismatch conditions, the nonlinear conversion efficiency is extremely low. Only when the phase matching condition Δk = 0 is met, that is, the fundamental frequency light and the frequency-doubled light have the same propagation speed in the medium, or the same refractive index, can the intensity of the frequency-doubled light continuously increase.

[0008] As is well known, chiral materials exhibit different linear absorptivity (different refractive indices) for left-handed and right-handed circularly polarized light; this optical rotation effect is called circular dichroism (CD). Interestingly, optical rotation effects have also been observed in nonlinear optics. When incident light passes through a chiral material with different circular polarization states, the SHG signal efficiencies of left-handed and right-handed circularly polarized light differ, a phenomenon known as second-harmonic generation circular dichroism (SHG-CD). This SHG-CD technique combines nonlinear optical processes and circular dichroism, making it applicable to chiral studies. Similar to the situation in linear optics, a dimensionless anisotropy factor g can be defined. NL The difference in SH signals generated by circularly polarized (left-handed or right-handed) incident light is determined by a weighted average of the two:

[0009]

[0010] In the formula, and These are the SH signals generated by left-handed and right-handed circularly polarized incident light, respectively.

[0011] In 1993, Hicks et al. first studied and verified the SHG-CD phenomenon in chiral 2,2′-dihydroxy-1,1′-binaphthyl molecule. Early studies of second harmonics in biological tissues (collagen), chiral thin films, etc., proposed possible mechanisms for this nonlinear circular dichroism, including interactions between electric and magnetic dipoles, and large phase shifts between nonlinear tensor elements with multiple excited states. In recent years, studies of nanoparticles, plasmas, metamaterials, and metasurfaces have revealed differences in the generated second harmonic circular dichroism signals. These nanostructures or metasurfaces are typically in subwavelength spatial dimensions, unconstrained by phase matching; their nonlinear responses are mainly caused by resonant interactions or mode overlap between light and the nanostructures or nonlinear metasurfaces. For macroscopic nonlinear optical materials, high nonlinear conversion efficiency is usually achieved through phase matching. Under conventional conditions, there is no chiral effect.

[0012] In 2017, Professor Kikuchi Hirotsugu and colleagues at Kyushu University in Japan discovered a strongly polar nematic phase state in the liquid crystal molecule DIO, which possesses an extremely high dielectric constant (~10). 4The Δε is exceptionally large (@1kHz), hundreds of times larger than that of traditional liquid crystals, which overturned the basic understanding of liquid crystals at the time. The texture of DIO molecules under a polarizing microscope is also very interesting; at high temperatures, it exhibits the traditional Schellieren texture of nematic phases, while at low temperatures it displays a novel sandy texture. Simultaneously, second-harmonic interferometry measurements confirmed that DIO possesses a ferroelectric-like polarization orientation sequence periodic structure in its polar phase at lower temperatures. Around the same time, Mandle et al. from the University of York in the UK independently reported a wedge-shaped molecule, RM734, with a large dipole moment. Similar to DIO, RM734 exhibits two distinct nematic phase states around 130℃. These novel nematic liquid crystals gradually attracted the research interest of scholars worldwide. In 2020, Clark's team at the University of Colorado in the US, through electro-optic experiments, proved for the first time that the low-temperature nematic phase formed by RM734 and others is indeed the ferroelectric nematic (N...) predicted by Born. F )liquid crystal. N F The phase is a three-dimensional uniaxial column phase with spontaneous, reorientable local polarization characteristics, and the polarization direction is parallel to the direction vector. Its polarization density can reach 6 μC / cm 2 This is the highest value ever measured in a fluid or glass material. Currently, fundamental research on this novel nematic phase is still in its early stages, but its extremely strong dielectric and nonlinear optical characteristics make it highly valuable for applications.

[0013] By adding left-handed or right-handed molecules to ferroelectric nematic liquid crystals, ferroelectric spiral liquid crystals with left-handed or right-handed periodic helical structures can be obtained. Compared with traditional nonlinear optical crystals, ferroelectric spiral liquid crystals exhibit a periodic domain structure similar to that of nonlinear optical crystals within a complete cycle (i.e., within one pitch). Furthermore, the spontaneous polarization of the ferroelectric nematic liquid crystal is aligned along the long axis of the molecules, and the polarization characteristics are preserved when the liquid crystal molecules are helically aligned along the helical axis. Quasi-phase matching techniques similar to those of traditional nonlinear optical crystals can be achieved by tuning the periodic helical structure of the ferroelectric spiral liquid crystal, i.e., pitch = Λ = 2 mL. c =mλ / 2(n 2ω -n ω (where m is an odd number), exhibiting an extremely strong second harmonic response. Considering quasi-phase matching and chiral effects, we present numerical simulations of the second harmonic circular dichroism under different incident polarization conditions. In considering the SHG, arbitrary polarization states are present. The fundamental light field propagates along the x-axis and interacts with the birefringent nonlinear optical medium (see appendix). Figure 1 The polarization state is specified by the phase difference (Δ) and angular rotation angle (Ψ) between the electric components decomposed along the y-axis and z-axis, therefore and Entering a region with positive optical uniaxiality (Δn=n) e -n o In a medium with a polarization greater than 0, each component of the fundamental light experiences a different phase velocity depending on the relative angle between the principal polarization axis of the light and the local slow axis of the medium. This leads to changes in the polarization state of the interacting wave. and The cumulative phase difference (Δ) between total We use a 4×4 matrix to calculate the Jones matrix M corresponding to each optical layer from x = X to x = X + Δx. x And obtain the nonlinear polarization caused by the oscillating fundamental light at the entrance of each layer of the propagating light field:

[0014]

[0015] ε₀ is the dielectric constant of vacuum, and d is the second-order nonlinear optical tensor. The symmetry for helical ferroelectric nematic liquid crystals belongs to C. ∞ ,

[0016]

[0017] When the optical (polarization) axis is oriented along the z-direction, rotation of the polarization axis will cause... The relative phase shift, when the polarization rotates θ away from the z-axis, is achieved by... The expression is modified to depend on the nonlinear polarization of θ.

[0018]

[0019] Where R(θ) is the rotation matrix with a rotation angle of θ. Then... The component is projected onto the polarization plane, i.e. Generates incremental second harmonics. The second harmonic wave with phase shift is generated from x = X to the exit surface x = L of the medium, and is represented as follows: therefore, The effective nonlinear polarization corresponds to the position x = X. The effective second-order nonlinear coefficient d eff It can be done We obtain. We write the equation with complex amplitude. and time-independent phase The fundamental frequency and the second harmonic are respectively and but Substituting these waves into Maxwell's equations, which have second-order nonlinear terms, under a slowly varying amplitude approximation, yields the equation describing the positional variation of the complex amplitude of the second harmonic:

[0020]

[0021] Where λ 2ω It is the wavelength of the second harmonic. This is the phase difference between the fundamental and second harmonic waves, which accounts for all optical effects such as different optical lengths, birefringence, and optical rotation. Under the small-signal approximation, the conversion from the fundamental to the second harmonic light is very low, and the amplitude of the pump fundamental can be considered as... The constant is given. As a result, we obtained the overall second harmonic optical output at the medium outlet: For any given structure, the spatial variation of the second harmonic output at any location can be determined by using a method that reflects the polarization direction. This can be calculated. From this relationship, it is clear that in order to obtain a high conversion efficiency SHG, all projected electric fields along x, y, and z, or at least one of them, should be non-zero and increase with thickness.

[0022] Compared to traditional nonlinear optical crystals, liquid crystals exhibit excellent fluidity, are easy to fabricate into devices, and their chiral and periodic polarization structures can be easily tuned by changing the rotation and concentration of chiral molecules. The discovery of ferroelectric nematic liquid crystals makes them a potential nonlinear medium with chiral periodic polarization structures, which is beneficial for realizing second-harmonic circular dichroism and has broad application prospects in chirality research and the exploration of spatial polarity distribution. Summary of the Invention

[0023] Currently, achieving second-harmonic circular dichroism is mainly focused on biological tissues (collagen), nanoparticle dimers, plasmonic metasurfaces, and metamaterials. The inherent properties of these materials limit their application in flexible devices. This invention utilizes the coupling of ferroelectric nematic liquid crystals and chiral molecules to prepare a ferroelectric helical liquid crystal material with a chiral helical structure. The ferroelectric helical liquid crystal exhibits a periodic domain structure similar to that of a nonlinear optical crystal within a complete cycle (i.e., one pitch), and the spontaneous polarization of the ferroelectric nematic liquid crystal is aligned along the long axis of the molecules. When the liquid crystal molecules are helically aligned along the helical axis, the polarization characteristics are preserved. In the ferroelectric helical liquid crystal system, extremely strong second-harmonic response is achieved by tuning the periodic helical structure of the ferroelectric helical liquid crystal, similar to the quasi-phase-matching technique of traditional nonlinear optical crystals, i.e., pitch = A = 2mL. c =mλ / 2(n 2ω -n ω(where m is an odd number), and by combining the different responses of chiral molecules to different circularly polarized light, second-harmonic circular dichroism is achieved, with an anisotropy factor as high as 2.0. This ferroelectric spiral liquid crystal also exhibits fluidity, making it easy to fabricate devices. This ferroelectric spiral liquid crystal material requires no complex equipment, its chiral and periodic structures are easily adjustable, and its fabrication is simple. This is the first application of second-harmonic circular dichroism in this field.

[0024] The objective of this invention is achieved through the following measures:

[0025] A ferroelectric spiral liquid crystal material is obtained by uniformly mixing chiral molecules and ferroelectric nematic liquid crystal in a certain ratio (mass / mass). Based on 100 parts by mass, the chiral molecule accounts for 1.1 or 0.61 parts, and the ferroelectric nematic liquid crystal accounts for 98.9 or 99.39 parts. This uniformly mixed material satisfies the second harmonic circular dichroism condition, meaning that the second harmonic intensity of left-handed or right-handed circularly polarized light is different.

[0026] Furthermore, the main body of the ferroelectric spiral liquid crystal material is a ferroelectric nematic liquid crystal, which has ferroelectric properties and thus a very strong second harmonic response.

[0027] Furthermore, the ferroelectric catenary liquid crystal RM734 or DIO provides ferroelectric properties.

[0028] Furthermore, the ferroelectric spiral liquid crystal material has different chiral polarization structures, namely a right-handed polarization structure and a left-handed polarization structure.

[0029] Furthermore, the ferroelectric spiral liquid crystal material has a right-handed polarization structure provided by right-handed molecules and a left-handed polarization structure provided by left-handed molecules.

[0030] Furthermore, the ferroelectric spiral liquid crystal material can periodically and uniformly distribute a certain number of polarized spiral structures, and the number of such polarized spiral structures increases with the increase of thickness, as detailed in the appendix. Figure 1 .

[0031] Furthermore, the ferroelectric spiral liquid crystal material exhibits circular dichroism, and its second harmonic intensity is selective depending on the rotation direction of the circularly polarized incident light.

[0032] Furthermore, we can prepare ferroelectric spiral liquid crystal materials with chiral structures simply by mixing chiral molecules with ferroelectric nematic liquid crystals.

[0033] Furthermore, the spiral orientation of the aforementioned ferroelectric spiral liquid crystal material can be achieved by mixing chiral molecules with different spiral orientations and ferroelectric nematic liquid crystals.

[0034] Furthermore, the aforementioned ferroelectric spiral liquid crystal material exhibits circular dichroism for lasers with different incident polarization states, thereby enabling second-harmonic circular dichroism.

[0035] A method for fabricating second-harmonic circular dichroism using ferroelectric spiral liquid crystal materials:

[0036] Two glass substrates, each made of polyimide and rubbed together in parallel, are used to create liquid crystal cells of different thicknesses. Ferroelectric spiral liquid crystal is then injected into the cells using capillary action. The liquid crystal is then slowly cooled using a temperature controller to achieve a uniform planar alignment texture. The slow cooling rate is 0.1–2 °C / min.

[0037] Furthermore, the chiral direction of the ferroelectric spiral liquid crystal can be adjusted by chiral molecules. When left-handed or right-handed molecules are mixed with ferroelectric nematic liquid crystal, the chiral direction of the ferroelectric spiral liquid crystal will also become left-handed or right-handed accordingly, thus exhibiting second harmonic circular dichroism.

[0038] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0039] The ferroelectric spiral liquid crystal material described in this invention exhibits an extremely strong second harmonic response, and its nonlinear optical properties are comparable to those of quartz crystals, which is extremely rare in fluid soft materials. By changing the helix direction of chiral molecules, the chirality of the ferroelectric spiral liquid crystal can be adjusted, thereby achieving second harmonic circular dichroism, with an anisotropy factor as high as 2.0. Compared with existing second harmonic circular dichroism materials, this technology has a simpler preparation process, easily adjustable chiral direction, lower equipment requirements, and low temperature sensitivity, allowing it to operate over a wider temperature range. Ferroelectric spiral liquid crystals allow for convenient adjustment of molecular pitch through chiral molecule doping concentration, and possess softness, ease of processing, and film-forming properties, enabling applications in many scenarios where crystals are unsuitable. It also offers a cost advantage and is well-suited for applications in chiral research and spatial polarization resolution. Attached Figure Description

[0040] Figure 1 This is a schematic diagram of the circular dichroism of the second harmonic. The intensity of the second harmonic varies depending on the direction of rotation of the incident light.

[0041] Figure 2 The change of SHG signal value of a 1.1% R811 / RM734 right-handed sample with temperature at a parallel optical path (the quartz signal is almost 0 at a parallel optical path, but the sample still has a strong signal).

[0042] Figure 3The SHG signal value of a 1.1% R811 / RM734 right-handed sample at the parallel optical path varies with temperature under different circularly polarized incident conditions (the quartz signal at the parallel optical path is almost 0, but the sample still has a very strong signal).

[0043] Figure 4 The change of SHG signal value of a 1.1% S811 / RM734 left-handed sample with temperature at a parallel optical path (the quartz signal is almost 0 at a parallel optical path, but the sample still has a very strong signal).

[0044] Figure 5 The SHG signal value of a 1.1% S811 / RM734 left-handed sample varies with temperature at a parallel optical path under different circularly polarized incident conditions (the quartz signal at the parallel optical path is almost 0, but the sample still has a very strong signal).

[0045] Figure 6 The SHG signal value of a 0.61% R811 / DIO right-handed sample at a parallel optical path varies with temperature under different circularly polarized incident conditions (the quartz signal at the parallel optical path is almost 0, but the sample still has a very strong signal).

[0046] Figure 7 The second harmonic anisotropy factor g of 1.1% R811 / RM734 and 1.1% S811 / RM734 in the ferroelectric spiral nematic phase is... NL The value of the curve changes with temperature (where 1.1% R811 / RM734 g) NL Up to 2.0 and 1.1% S811 / RM734 g NL It can reach 1.5). Detailed Implementation

[0047] The present invention will be further described in detail below with reference to embodiments, but these are not intended to limit the scope of the invention.

[0048] Example 1

[0049] The preparation method of right-handed ferroelectric spiral liquid crystal with a mixing ratio of 1.1% is as follows:

[0050] A certain mass of right-handed molecules and ferroelectric nematic liquid crystals were weighed and dissolved separately in chloroform solvent. After sonication, a homogeneous solution was obtained. Then, a mixed solution was prepared according to the mass ratio of chiral molecules to ferroelectric nematic liquid crystals of 1.1:98.9. After vacuum drying, a homogeneous mixture was obtained and labeled as 1.1%R811 / RM734.

[0051]

[0052] The ferroelectric nematic liquid crystal RM734 has R1 and R2 as -CH3.

[0053]

[0054] The chiral molecule R811, where R1 and R2 are -C6H 13

[0055] Example 2

[0056] The preparation method of left-handed ferroelectric spiral liquid crystal with a mixing ratio of 1.1% is as follows:

[0057] A certain mass of right-handed molecules and ferroelectric nematic liquid crystals were weighed and dissolved separately in chloroform solvent. After sonication, a homogeneous solution was obtained. Then, a mixed solution was prepared according to the mass ratio of chiral molecules to ferroelectric nematic liquid crystals of 1.1:98.9. After vacuum drying, a homogeneous mixture was obtained and labeled as 1.1% S811 / RM734.

[0058]

[0059] The ferroelectric nematic liquid crystal RM734 has R1 and R2 as -CH3.

[0060]

[0061] The chiral molecule S811, where R1 and R2 are -C6H 13

[0062] Example 3

[0063] The preparation method of right-handed ferroelectric spiral liquid crystal with a mixing ratio of 0.61% is as follows:

[0064] A certain mass of right-handed molecules and ferroelectric nematic liquid crystals were weighed and dissolved separately in chloroform solvent. After sonication, a homogeneous solution was obtained. Then, a mixed solution was prepared according to the mass ratio of chiral molecules to ferroelectric nematic liquid crystals of 0.61:99.39. After vacuum drying, a homogeneous mixture was obtained and labeled as 0.61%R811 / DIO.

[0065]

[0066] For the ferroelectric nematic liquid crystal DIO, R1 is -C3H7

[0067]

[0068] The chiral molecule R811, where R1 and R2 are -C6H 13

[0069] Example 4

[0070] The realization of the second harmonic circular dichroism is as follows:

[0071] Prepare two glass substrates (1×1cm) 2 A polyimide film is spin-coated and oriented using a velvet cloth to form a parallel-oriented liquid crystal cell. The thickness of the liquid crystal cell can be tuned (0–100 μm) using spacer beads. The dried ferroelectric spiral liquid crystal is heated to the liquid phase, and the liquid crystal enters the liquid crystal cell under capillary action, as shown in Figure 1. Slow cooling using a temperature controller results in the formation of a uniform planar alignment texture in the ferroelectric spiral liquid crystal.

[0072] The laser source uses a 1064nm pulsed laser. Due to the nonlinear optical characteristics of the ferroelectric spiral liquid crystal, a second harmonic at 532nm is generated. Correspondingly, the rotation direction of the chiral molecules is changed, adjusting the chirality of the ferroelectric spiral liquid crystal, thereby achieving the effect of second harmonic circular dichroism. The emitted second harmonic is detected using a photomultiplier detector under parallel light path (see appendix). Figure 2 , 3 4, 5, 6).

[0073] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. For those skilled in the art, several simple deductions or substitutions can be made without departing from the concept of the present invention, and all such deductions or substitutions should fall within the protection scope of the present invention.

Claims

1. A ferroelectric spiral liquid crystal material, characterized in that, Based on a mass of 100 parts, the chiral molecules account for 0.61 parts, and the ferroelectric nematic liquid crystal accounts for 99.39 parts. After uniform mixing, a ferroelectric spiral liquid crystal material is obtained, which can satisfy the second harmonic circular dichroism condition, that is, the second harmonic intensity of left-handed or right-handed circularly polarized light is different. The ferroelectric spiral liquid crystal material is mainly composed of ferroelectric nematic liquid crystal, which has ferroelectric properties and thus exhibits a very strong second harmonic response. Ferroelectric nematic liquid crystals (DIOs) provide ferroelectric properties; The ferroelectric spiral liquid crystal material has different chiral polarization structures, namely a right-handed polarization structure and a left-handed polarization structure; The ferroelectric spiral liquid crystal material described above has a right-handed polarization structure provided by right-handed molecules and a left-handed polarization structure provided by left-handed molecules; The ferroelectric spiral liquid crystal material can periodically and uniformly distribute a certain number of polarized spiral structures, and the number of such polarized spiral structures increases with the increase of thickness. The ferroelectric spiral liquid crystal material exhibits circular dichroism, and its second harmonic intensity is selective depending on the direction of rotation of the circularly polarized incident light. The method for achieving second harmonic circular dichroism in the described ferroelectric spiral liquid crystal material involves creating liquid crystal cells of different thicknesses from two glass substrates with parallel rubbing alignment of polyimide. Ferroelectric spiral liquid crystal is then injected into the liquid crystal cells using capillary action. A temperature controller is used to slowly cool the liquid crystal, causing it to form a uniform planar orientation texture. The chiral direction of the ferroelectric spiral liquid crystal is adjusted by chiral molecules. When left-handed or right-handed molecules are mixed with the ferroelectric nematic liquid crystal, the chiral direction of the ferroelectric spiral liquid crystal will correspondingly change to left-handed or right-handed, thus exhibiting second harmonic circular dichroism. The slow cooling conditions are: 0.1~2℃ / min.

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