An adaptive-focus lens is a device that is capable of tuning its focal length by means of an external stimulus. Numerous techniques for the demonstration of such devices have been reported thus far. Moving beyond traditional solutions, several new approaches have been proposed in recent years based on the use of liquid crystals, which can have a great impact in emerging applications. This work focuses on the recent advances in liquid crystal lenses with diameters larger than 1 mm. Recent demonstrations and their performance characteristics are reviewed, discussing the advantages and disadvantages of the reported technologies and identifying the challenges and future prospects in the active research field of adaptive-focus liquid crystal (LC) lenses.

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. Recently, liquid crystal (LC) technology has begun to be used in nonoptical applications due to its promising features in further electronic applications ranging from kilohertz to megahertz frequencies. Intrinsic anisotropy of some LC properties, which implies different properties depending of the direction in which they are measured, allows new advanced devices to be designed with tunable features, by using these materials.The use of LC to design tunable devices at microwave frequency bands is not a new conception; nevertheless there has been an increasing interest in improving their performance, particularly in the last decade. Liquid crystals were recognized as candidates for microwave dielectric substrates in the early 1990s. 1 Although first approaches to LC devices in waveguides led to bulky and large consumption designs (due to strong magnetic fields for switching LC molecules), most of the recent prototypes, such as tunable phase shifters, 2, 3 capacitors, 4 filters, 5,6 or antennas, 7 have reported practical functions involving electric fields for orienting LC at those frequencies.Filters are very valuable devices because they represent a powerful tool for frequency response processing. They are designed in order to select or to remove bands of frequency. A band-rejection filter or band-stop filter is a filter that attenuates a frequency band, while the other frequencies remain unchanged. A notch filter is a band-rejection filter with a narrow stopband, with a high quality factor, that is mainly used to remove spurious frequencies and to filter noise signals.In this work, the design of a notch filter for about 5 GHz rejection frequency (f 0 ), based on the electric field switching of a LC, has been proposed and its feasibility has been demonstrated. Filters working at this frequency, allocated in the C-band, are intended to be used in satellite telecommunication systems or, for example, to avoid the interference a) Author to whom correspondence should be addressed. Electronic mail:vurruchi@ing.uc3m.es.between the UWB (Ultra Wide-Band) and WLAN (Wireless Local Area Networks) systems. 8 UWB systems are particularly promising for short-range high-throughput wireless communications. Multiple solutions have been proposed for providing tunability in microwave devices, such as varactors based on sem...

In this work, a novel technique to create positive-negative tunable liquid crystal lenses is proposed and experimentally demonstrated. This structure is based on two main elements, a transmission line acting as a voltage divider and concentric electrodes that distribute the voltage homogeneously across the active area. This proposal avoids all disadvantages of previous techniques, involving much simpler fabrication process (a single lithographic step) and voltage control (one or two sources). In addition, low voltage signals are required. Lenses with switchable positive and negative focal lengths and a simple, low voltage control are demonstrated. Moreover, by using this technique other optical devices could be engineered, e.g. axicons, Powell lenses, cylindrical lenses, Fresnel lenses, beam steerers, optical vortex generators, etc. For this reason, the proposed technique could open new venues of research in optical phase modulation based on liquid crystal materials.

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.

A structure based on a dimer of silicon nanoparticles, presenting directional scattering in the visible range, was studied as a new design of an all-optical switch. The combination of spherical nanoparticles satisfying, at the same incident wavelength, the zero-backward and the minimumforward scattering conditions can produce either a maximum or a minimum of the scattered field in the area between the nanoparticles. The modulation of the incident wavelength can be used as a switching parameter due to the sensitivity of these conditions to it. An optimization of the dimer setup, both in the distance between the nanoparticles and the incident wavelength, was numerically performed to obtain a maximum contrast. Also, near-field and far-field distributions of the electric field have been considered. ■ INTRODUCTIONSince the famous lecture of Richard P. Feynman, 1 so far, the understanding and control of the physics in the nanoscale have not stopped. In fact, this work is focused on one of these recent discoverings, in particular, the directionality of light scattering of magnetodielectric nanoparticles, like silicon nanoparticles, and its use in new photonic devices. Until now, in photonics, the control and manipulation of light at subwavelength dimensions has been the main challenge. Fortunately, nowadays, this is possible thanks to the interaction between light and nanoscale structures. Consequently, photonics is now present in a large amount of applications in the nanometric range, from the design of novel and high sensitive nanobiosensors 2 to photonic on-chip devices. 3 In this latter case, photonic devices are being proposed as efficient optical counterparts of current electronic devices. 4,5 Several functionalities, such as information storage, 6 switching, 7 communications, 8,9 or even the manipulation of information, 10,11 have been explored using photonic nanotools. However, the weak interaction between light and nanometric devices requires the presence of resonant systems to obtain measurable signals. Light resonances in metallic nanostructures, known as plasmon resonances, are one of the main physical phenomena used for the control and manipulation of light at the nanoscale. The collective oscillation of the free electrons on the surface of the metal produces a strong enhancement of both the scattering and absorption of light. 12 These effects are now present in a wide range of applications and devices in such diverse fields, from the visible to the terahertz range. 13 Different sensors, 14−16 subdiffraction limit imaging, 17 or nanoantennas 18,19 are some examples of a myriad of them. The use of plasmonic materials has an important disadvantage for several applications: the ohmic losses. For this reason, the interest on resonant dielectric nanostructures arose in the last years because of the interesting properties of dielectric nanoparticles with high refractive index (e.g., silicon, Germanium, etc.). They are low-losses, present light resonances, and they are CMOS compatible. In addition, these nanoparti...

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.

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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 eff ciency of ∼100%. In addition, this device is more effi ient and simpler than previously reported optical vortex generators. Moreover, the device is completely reconf gurable, 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 effi iency, simplicity, and possibility of reconfi uration.