Virginia Urruchi

Note: Tunable notch filter based on liquid crystal technology for microwave applications

In this work, a compact design of an electrically tunable notch filter, based on liquid crystal (LC) technology, has been designed, manufactured, and characterized. The proposal has been achieved through particular configuration schemes with low cost inverted-microstrip structures and conventional spurlines structures due to its ease of integration. Central frequency tunability has been induced by applying low ac voltages, thus involving low power consumption. For these devices, filter responses have been approached specifically at microwave C-band frequency allocated for many satellite communications applications. Also, it has taken advantage of new highly anisotropic nematic LC mixtures at those frequency ranges.

Modal liquid crystal array of optical elements

In this study, a novel liquid crystal array based on modal control principle is proposed and demonstrated. The advanced device comprises a six striped electrode structure that forms a configurable 2D matrix of optical elements. A simulation program based on the Frank-Oseen equations and modal control theory has been developed to predict the device electrooptic response, that is, voltage distribution, interference pattern and unwrapped phase. A low-power electronics circuit, that generates complex waveforms, has been built for driving the device. A combined variation of the waveform amplitude and phase has provided a high tuning versatility to the device. Thus, the simulations have demonstrated the generation of a liquid crystal prism array with tunable slope. The proposed device has also been configured as an axicon array. Test measurements have allowed us to demonstrate that electrooptic responses, simulated and empirical, are fairly in agreement.

Note: Electrical modeling and characterization of voltage gradient in liquid crystal microlenses

In this work, a novel equivalent electric circuit for modeling liquid crystal microlenses is proposed. This model is focused on explaining a lens behavior at the micrometric scale, using its manufacturing parameters. It suggests an approach to predict the solution of the voltage gradient distribution across a microlens. An interesting feature of the model is that it provides an analytical solution for microlenses with modal and hole-patterned electrode schemes, by a simple software tool. The model flexibility allows lens designers to apply complex waveform signals with different harmonics. The voltage distribution has been tested. The simulated and measured voltage profiles are fairly in agreement.

Electrooptic Characterization of Tunable Cylindrical Liquid Crystal Lenses

In this work, one-dimensional arrays of cylindrical adaptive liquid crystal lenses were manufactured and characterized; and test devices were filled with nematic liquid crystal. Comb interdigitated electrodes were designed as a mask pattern for the control electrode on the top glass substrates. A radial graded refractive index along each microsized lens was achieved by fabricating a layer of high resistance sheet deposited as a control electrode. These tunable lenses were switched by applying amplitude and frequency optimized waveforms on the control electrode. Phase profiles generated by the radial electric field distribution on each lens were measured by a convectional interferometric technique.

Reflective sidewall electrodes for low voltage and high transmittance blue-phase liquid crystal displays

The typical sidewalls produced in the fabrication of protrusion electrodes are proposed to create a low voltage (4.5 Vrms) and high transmittance (93%) blue-phase liquid crystal display (BP-LCD). The tilted electrodes produce a strong horizontal electrical field that reduces the operating voltage considerably. The common problem of the ‘dead zones’ is solved by reflecting the light onto the electrodes. In order to estimate the phase retardation of the reflected light, a ray tracing simulation program for anisotropic mediums has been developed. The proposed device is more competitive than vertical field switching based BP-LCD and also, has the advantages of protruded in-plane-switching structures. These facts make this technology a potential candidate for the next generation of BP-LCDs.

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 voltages 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.

Generation of Optical Vortices by an Ideal Liquid Crystal Spiral Phase Plate

An ideal spiral phase plate based on liquid crystals and two high resistivity layers is proposed and theoretically analyzed. The proposed structure generates a spiral-like voltage with simple voltage control. The liquid crystal layer produces an optical phase shift that depends on the voltage distribution. These two effects cause light passing through the device to be twisted like a corkscrew around its travel axis. Because of the continuous phase shift, the proposed device is expected to exhibit a conversion efficiency of ~100%. In addition, this device is more efficient and simpler than previously reported optical vortex generators. Moreover, the device is completely reconfigurable, i.e., the operating wavelengths and topological charges are tunable. The device can be used to reduce the fabrication costs of current devices and generate different orbital angular momentum modes with improved light efficiency, simplicity, and possibility of reconfiguration.

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.

Fiber Optic Temperature Sensor Based on Amplitude Modulation of Metallic and Semiconductor Nanoparticles in a Liquid Crystal Mixture

The response of an amplitude modulation temperature sensor based on a liquid crystal doped with either metallic or semiconductor nanoparticles has been theoretically analyzed. The effects of the concentration, the type of nanoparticle material, and liquid crystal compound have been studied in detail. The high sensitivity of light resonances to refraction index changes, in collaboration with the high thermooptic coefficients of liquid crystal materials, has resulted in the design of an optical fiber sensor with high temperature sensitivity. This sensitivity has been demonstrated to be dependent on nanoparticle concentration. A maximum theoretical sensitivity of 64 × 10-2 dB/°C has been observed. Moreover, the sensitivity is highly linear with a regression coefficient of 99.99%.

Cylindrical Liquid Crystal Microlens Array With Rotary Optical Power and Tunable Focal Length

A novel cylindrical liquid crystal microlens array with rotary power and tunable capability is proposed and experimentally demonstrated. This structure is capable of generating tunable cylindrical microlenses in two different axis by low voltage control. For this, complex electrical signals are required (electrical phase shift of 180°). A control over the maximum phase shift from 4π to 34π radians has been demonstrated. This effect modifies the effective focal length (from 0.2 to 1.2 mm). Moreover, an OFF-state is also possible (behaving like a transparent medium). The structure is simple and does not require a complex fabrication process. The proposed device has several advantages over the existing microlens arrays and is simple and low cost. The device could contribute to developing new applications and to reducing the fabrication costs of current devices. For example, this device can be used in the next generation of mobile displays with autostereoscopic capability.