Wireless Temperature Sensor Based on a Nematic Liquid Crystal Cell as Variable Capacitance.

Wireless communication is growing quickly and now allows technologies like the Internet of Things (IoT). It is included in many smart sensors helping to reduce the installation and system costs. These sensors increase flexibility, simplify deployment and address a new set of applications that was previously impossible with a wired approach. In this work, a wireless temperature sensor based on a nematic liquid crystal as variable capacitance is proposed as a proof of concept for potential wearable applications. Performance analysis of the wireless temperature sensor has been carried out and a simple equivalent circuit has been proposed. Sensor prototype has been successfully fabricated and demonstrated as the beginning of new biomedical sensors.

Tunable liquid crystal multifocal microlens array.

A novel liquid crystal microlens array with tunable multifocal capability, high optical power and fill-factor is proposed and experimentally demonstrated. A specific hole pattern design produces a multifocal array with only one voltage control. Three operations modes are possible, "Off", "Tunable Multifocal" and "Unifocal". The design is patterned in both substrates. Then, the substrates are arranged in symmetrical configuration. The result is a high optical power in comparison with typical hole patterned structures. Besides, it is proposed a hexagonal pattern that produces a high fill factor, specially indicated for some applications as Integral Imaging. The array has several useful characteristics for this type of application: tunability for the loss of resolution; multifocal for extended DOF; high fill factor for increase the number of views; and low power consumption for integration in portable devices. Moreover, the optical characteristics of the proposed device could bring new applications in other fields.

Low aberration and fast switching microlenses based on a novel liquid crystal mixture.

In this work, we present a novel kind of LC mixture (5005) for photonic applications, with emphasis on a LC microlens array. This mixture is a nematic composition of three different families of rod like liquid crystals. The key is that frequency dependence of parallel component of electric permittivity is different for each component, resulting in a strongly dependent on frequency dielectric anisotropy. The unique properties of this LC mixture are demonstrated to work in a frequency modulated LC microlens array. A hole patterned structure is used. Thanks to the special characteristics of this mixture, the microlenses are reconfigurable by low voltage signals with variable frequency. This is a first demonstration of a LC lens with tunable focal length by frequency in an analog way. The result of this type of control are microlenses with low aberrations and fast switching (the frequency switching is around 10 times faster than amplitude modulation). The tunability with frequency and the fast switching, makes this liquid crystal of special interest not only for microlenses but for all kind of optical phase modulators.

Liquid crystal spherical microlens array with high fill factor and optical power.

A novel liquid crystal spherical microlens array with high optical power and almost 100% of fill-factor is proposed and experimentally demonstrated. The combination of a specific structure and electrical waveforms applied to the electrodes generates an array of spherical microlenses with square aperture. The manufacturing process is simple (patterned electrodes) and the microlenses are reconfigurable by low voltage signals (the electrodes are in contact with the LC layer). This device could be a key for the next generation of autostereoscopic devices based on Integral Imaging technique.

Liquid Crystal Microlenses for Autostereoscopic Displays.

Three-dimensional vision has acquired great importance in the audiovisual industry in the past ten years. Despite this, the first generation of autostereoscopic displays failed to generate enough consumer excitement. Some reasons are little 3D content and performance issues. For this reason, an exponential increase in three-dimensional vision research has occurred in the last few years. In this review, a study of the historical impact of the most important technologies has been performed. This study is carried out in terms of research manuscripts per year. The results reveal that research on spatial multiplexing technique is increasing considerably and today is the most studied. For this reason, the state of the art of this technique is presented. The use of microlenses seems to be the most successful method to obtain autostereoscopic vision. When they are fabricated with liquid crystal materials, extended capabilities are produced. Among the numerous techniques for manufacturing liquid crystal microlenses, this review covers the most viable designs for its use in autostereoscopic displays. For this reason, some of the most important topologies and their relation with autostereoscopic displays are presented. Finally, the challenges in some recent applications, such as portable devices, and the future of three-dimensional displays based on liquid crystal microlenses are outlined.

Temperature-phase converter based on a LC cell as a variable capacitance.

The main characteristic of liquid crystals is that their properties, both electrical and optical, can be modified through a convenient applied signal, for instance a certain voltage. This tunable behavior of liquid crystals is directly related to the orientation of their nanometric components with respect to a director direction. However, the initial alignment is a fabrication-dependent parameter and may be either planar or homeotropic. In addition, the strong dependence of the properties of liquid crystals with the temperature is well known and widely used for several temperature sensors. This dependence is produced by the influence of the temperature on the ordering of the molecules. In this work, we have studied the temperature dependence of the electric properties of a liquid crystal cell, in particular the dielectric permittivity, with the temperature as a function of the initial alignment set during the fabrication process. Starting from experimental measurements, an equivalent circuit model including the temperature dependence has been proposed. We have observed that a good linearity in a wide temperature range is provided at a suitable exciting frequency. Finally, a proper conditioner circuit is proposed as a powerful tool for linear and high sensibility temperature measurement.

Modal liquid crystal microaxicon array.

A novel tunable liquid crystal microaxicon array is proposed and experimentally demonstrated. The proposed structure is capable of generating tunable axicons (thousands of elements) of micrometric size, with simple control (four control voltages) and low voltage, and is totally reconfigurable. Depending on the applied voltages, control over the diameter, as well as the effective wedge angle, can be achieved. Controls over the diameter ranging from 107 to 77 µm have been demonstrated. In addition, a control over the phase profile tunability, from 12π to 24π radians, has been demonstrated. This result modifies the effective cone angle. The diameter tunability, as well the effective cone angle, results in a control over the nondiffractive Bessel beam distance. The RMS wavefront deviation from the ideal axicon is only λ/3. The proposed device has several advantages over the existing microaxicon arrays, including being simple having a low cost. The device could contribute to developing new applications and to reducing the fabrication costs of current devices.

A novel high-sensitivity, low-power, liquid crystal temperature sensor.

A novel temperature sensor based on nematic liquid crystal permittivity as a sensing magnitude, is presented. This sensor consists of a specific micrometric structure that gives considerable advantages from other previous related liquid crystal (LC) sensors. The analytical study reveals that permittivity change with temperature is introduced in a hyperbolic cosine function, increasing the sensitivity term considerably. The experimental data has been obtained for ranges from -6 °C to 100 °C. Despite this, following the LC datasheet, theoretical ranges from -40 °C to 109 °C could be achieved. These results have revealed maximum sensitivities of 33 mVrms/°C for certain temperature ranges; three times more than of most silicon temperature sensors. As it was predicted by the analytical study, the micrometric size of the proposed structure produces a high output voltage. Moreover the voltage's sensitivity to temperature response can be controlled by the applied voltage. This response allows temperature measurements to be carried out without any amplification or conditioning circuitry, with very low power consumption.

Liquid Crystal Lensacons, Logarithmic and Linear Axicons.

Until now, several attempts have been made to obtain axicons by using liquid crystals. Previous results had always a considerable deviation from the linear response and the resulting aperture is square. In addition, classical fabrications methods are expensive and only produce fixed phase profiles. In this study, a novel structure to obtain tunable axicons with a perfect conical shape and a circular aperture is proposed and theoretically studied. The proposed optical device is based on nematic liquid crystal and phase shifted electrical signals. A simulation program consisted of Finite Elements Method to solve the voltage distribution combined with the Frank-Oseen equation to solve the molecular position of the nematic liquid crystal is employed. This device is totally reconfigurable by using low voltage signals. The focus depth and the position of this one can be controlled electrically.

Directional Scattering of Semiconductor Nanoparticles Embedded in a Liquid Crystal.

Light scattering by semiconductor nanoparticles has been shown to be more complex than was believed until now. Both electric and magnetic responses emerge in the visible range. In addition, directional effects on light scattering of these nanoparticles were recently obtained. In particular, zero backward and minimum-forward scattering are observed. These phenomena are very interesting for several applications such as, for instance, optical switches or modulators. The strong dependence of these phenomena on the properties of both the particle and the surrounding medium can be used to tune them. The electrical control on the optical properties of liquid crystals could be used to control the directional effects of embedded semiconductor nanoparticles. In this work, we theoretically analyze the effects on the directional distribution of light scattering by these particles when the refractive index of a surrounded liquid crystal changes from the ordinary to the extraordinary configuration. Several semiconductor materials and liquid crystals are studied in order to optimize the contrast between the two states.

Improving the Pass-Band Return Loss in Liquid Crystal Dual-Mode Bandpass Filters by Microstrip Patch Reshaping.

In this paper, the design and experimental characterization of a tunable microstrip bandpass filter based on liquid crystal technology are presented. A reshaped microstrip dual-mode filter structure has been used in order to improve the device performance. Specifically, the aim is to increase the pass-band return loss of the filter by narrowing the filter bandwidth. Simulations confirm the improvement of using this new structure, achieving a pass-band return loss increase of 1.5 dB at least. Because of the anisotropic properties of LC molecules, a filter central frequency shift from 4.688 GHz to 5.045 GHz, which means a relative tuning range of 7.3%, is measured when an external AC voltage from 0 Vrms to 15 Vrms is applied to the device.

Note: series and parallel tunable resonators based on a nematic liquid crystal cell as variable capacitance.

In this work, tunable series and parallel resonators based on a nematic liquid crystal cell as variable capacitance are proposed and characterized. Tunable resonance frequencies in the range of kHz have been obtained for the combination of the inductance and the liquid crystal cell (capacitance) used in the proposed circuits. Tuning range in frequency obtained is around an octave.

Note: Phase-locked loop with a voltage controlled oscillator based on a liquid crystal cell as variable capacitance.

A phase-locked loop is demonstrated using a twisted-nematic liquid crystal cell as a capacitance that can be varied as a function of applied voltage. The system is formed by a phase detector, a low-pass filter, as well as a voltage controlled oscillator including such variable capacitance. A theoretical study is proposed and experimentally validated. Capture and locked ranges of hundreds of kHz have been obtained for the configuration used in this circuit. An application as frequency demodulator using a practical implementation of this circuit has been demonstrated.