Paper like cholesteric interferential mirror

Multiple Anti-Counterfeiting Composite Film Based on Cholesteric Liquid Crystal and QD Materials

A composite film with multiple anti-counterfeiting features was demonstrated by superposing quantum dots (QDs) polymer matrix (film A) and cholesteric liquid crystal film (film B) together. The first-line and second-line anti-counterfeiting characteristics were successfully implemented by employing thermochromic, angular photochromic, and circularly polarized discoloration of film B, respectively. By initiatively utilizing the different relative positions between the fluorescence emission peak (λ_em) of film A and the central selective reflection wavelength (λ_m) of film B at different temperatures, which resulted in changes in the fluorescence spectra or the different presence of latent patterns, the most important third-line anti-counterfeiting feature was successfully achieved.

Characterization of Second-Order Reflection Bands from a Cholesteric Liquid Crystal Cell Based on a Wavelength-Swept Laser

We report the results of an experimental study of the characterization of second-order reflection bands from a cholesteric liquid crystal (CLC) cell that depends on the applied electric field, using a wide bandwidth wavelength-swept laser. The second-order reflection bands around 1300 nm and 1500 nm were observed using an optical spectrum analyzer when an electric field was applied to a horizontally oriented electrode cell with a pitch of 1.77 μm. A second-order reflection spectrum began to appear when the intensity of the electric field was 1.03 V_rms/μm with the angle of incidence to the CLC cell fixed at 36°. The reflectance increased as the intensity of the electric field increased at an angle of incidence of 20°, whereas at an incident angle of 36°, when an electric field of a predetermined value or more was applied to the CLC cell, it was confirmed that deformation was completely formed in the liquid crystal and the reflectance was saturated to a constant level. As the intensity of the electric field increased further, the reflection band shifted to a longer wavelength and discontinuous wavelength shift due to the pitch jump was observed rather than a continuous wavelength increase. In addition, the reflection band changed when the angle of incidence on the CLC cell was changed. As the angle of incidence gradually increased, the center wavelength of the reflection band moved towards shorter wavelengths. In the future, we intend to develop a device for optical wavelength filters based on side-polished optical fibers. This is expected to have a potential application as a wavelength notch filter or a bandpass filter.

Simple model for estimating band edge wavelengths of selective reflection from cholesteric liquid crystals for oblique incidence

A simple model for estimating band edge wavelengths of selective reflection from cholesteric liquid crystals (CLCs) for oblique incidence is proposed. The proposed model and calculation method are based on a geometrical optics model and Bragg's law. Average refractive indices and Bragg angles in a CLC for the short- and long-wavelength edges of a reflection band are calculated using the model. The band edge wavelengths are determined by substituting the average refractive indices and Bragg angles into Bragg's law. The angular dependences of the band edge wavelengths show good agreement with those calculated by Berreman and Scheffer's [Phys. Rev. Lett. 25, 577 (1970)10.1103/PhysRevLett.25.577] 4×4 matrix method. Although the derived equations are not exact solutions of Maxwell's equations, the proposed method can approximately predict the angular dependences of the center wavelength, bandwidth, and band edge wavelengths of the selective reflection in CLCs.

Liquid crystal blue phases interconversions based real-time thermal imaging device

The real-time thermal imaging systems, which allow the rapid conversion, acquisition, and manipulation of obtained optical information, are the emerging technologies that offer a variety of imaging applications. Here, we present a novel type of thermal imaging device, based on the thermo-optical properties of liquid crystal blue phases. Herewith, the novelty lies in the use of a weak first-order phase transition between the blue phases controlled by external thermal fields. In turn, the stimulated interconversions of the selective reflections between the blue phases enable the visualization of the two-dimensional spatial distribution of the thermal fields. The real-time room temperature operation capabilities of the proposed thermal imaging device may enable applications in areas such as medicine, astronomy, security, surveillance, people tracking, aerospace monitoring, and artworks inspection.

High-performance dichroic dye-doped flexible cholesteric polymer film optical filter for laser protection application

High-performance optical filters are quite useful in laser protection application that is critical in military and civilian fields for protection of human eyes and photonic devices. However, most existing technologies are often angle-dependent and complicated, which hinder the useful range of real-time application. Here, we report the development of a dichroic dye-doped flexible cholesteric polymer film optical filter with double-layer structure consisted of two films with opposite handedness for laser protection application. The fabricated films exhibit high and stable optical density (>4.6) in large angle range from 0 to 70°, indicating a good angle-independence. The proposed films show excellent stability in a broad temperature range from -100 °C to 100 °C, and high mechanical stress up to 9.8 × 105 Pa. As a laser protection, the optical filter can resist pulse pump energy up to 110 mJ/pulse, showing a high damage threshold. The flexible and free-standing film optical filters as laser protection device are integrated by simple fabrication, excellent stability to temperature and mechanical stress, angle-independence, high optical density, and good visibility simultaneously, showing dramatically application potential in laser protection and other flexible photonic devices.

Optical thermal sensor based on cholesteric film refilled with mixture of toluene and ethanol

We demonstrate an optical thermal sensor based on cholesteric film refilled with mixture of toluene and ethanol. The thermal response mechanism is mainly based on the thermal expansion effect induce by toluene, where the ethanol is used for refractive index adjustment to determine the initial refection band position of cholesteric film. The ethanol-toluene mixture was used to adjust the color tunability with the temperature in relation with the habits of people (blue as cold, green as safe and red as hot). A broad temperature range of 86 °C and highly sensitivity of 1.79 nm/ °C are achieved in proposed thermal sensor, where the reflective color red-shifts from blue to red when environmental temperature increases from -6 °C to 80 °C. This battery-free thermal sensor possesses features including simple fabrication, low-cost, and broad temperature sensing range, showing potential application in scientific research and industry.

Visual micro-thermometers for nanoparticles photo-thermal conversion

We present a method to calibrate the light to heat conversion in an aqueous fluid containing nanoparticles. Accurate control of light and heat is of dramatic importance in many fields of science and metal nanoparticles have acquired an increased importance as means to address heat in very small areas when irradiated with an intense light. The proposed method enables to measure the temperature in the environment surrounding nanoparticles, as a function of the exposure time to laser radiation, exploiting the properties of thermochromic cholesteric liquid crystals. This method overcomes the problems of miscibility of nanoparticles in liquid crystals, provides temperature reading at the microscale, since the cholesteric liquid crystal is confined in microdroplets, and it is sensitive to a temperature variation, 28°C-49°C, suitable for biological applications.