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This presentation reveals the re-writability property of azobenzene liquid crystal photoalignment. Stable in thermal fluctuations, one can change existing photoalignment by exposing it to polarized light in the visible regime. One can use this unique property by patterning photoalignment through the liquid crystal bulk in that the sample’s front and back have differing director orientations. Photoalignment using linear polarized light as well as complex polarizations will be covered. No surface alignment on sample substrates is necessary because the azobenzene is mixed with the liquid crystal in-situ in sample preparation.

It has been just over 20 years from the first disclosure of a thin and transparent film that would allow controlling propagation of light, an unpolarized and broadband light, with 100% efficiency. First it was an idea seemingly at odds with conventional optics. Then came demonstrations of the thinnest planar optics - “prisms”, lenses, beam shapers, arrays and vortices, switchable or not, - in sizes currently 8” and larger. Novel film architectures extended spectral and angular bandwidths of planar optics to unprecedented scales, changing the ways optics is made and opening avenues to long cherished dreams of adaptation, tunability, multifunctionality, and cost. History is in the making and will be presented first hand with an outline of the marvels in functionality and capabilities waiting for optics behind the horizon.

A new generation of reconfigurable optical components is conceived by bridging the photothermal properties of gold nanoparticles and the thermosensitivity of liquid crystalline materials. As such, gold nanorods (GNRs) heated using light are used to activate efficient hidden diffraction gratings realized in a blend made of a room temperature polymerizable liquid crystal (PLC) and nematic liquid crystal (NLC). Holographic liquid crystal polymer dispersed liquid crystal (HLCPDLC) gratings containing a small percentage of GNRs are fabricated with periodicity of 2.6 ?m by means of a conventional UV holographic recording setup. The HLCPDLC structures are characterized using morphological, optical, and thermooptical techniques. Because of the initial refractive index match between the polymer-rich and LC-rich regions, the "grating" is hidden and the films appear transparent and nondiffractive. Illumination of these GNR-containing structures with a suitable light source (808 nm) induces a local heating due to the plasmonic absorption of the GNRs. This heating induces a refractive index mismatch between the PLC and the NLC as the latter undergoes a phase transition from nematic to isotropic, finally resulting in a transmission diffractive structure activated over a few seconds exhibiting an efficiency of about 70%. The same HLCPDLC samples have also been tested as thermosensitive waveplates, enabling a new, fast methodology for quantifying the photoinduced plasmonic heating with a thermal sensitivity of ?0.04 °C. Moreover, thermoplasmonic driven waveplates represent a new avenue in the field of light controllable optical phase modulators.

We experimentally demonstrate that the plasmonic heat delivered by a single layer of homogeneously distributed gold nanoparticles (AuNPs), immobilized on a glass substrate, can be optically tuned by taking advantage of the properties of an organic layer based on azobenzene and nematic liquid crystal (NLC) molecules. The effect, which exploits the dependence of the NLC refractive index value on the molecular director orientation, is realized using the polarization-dependent, light-induced molecular reorientation of a thin film of photo-aligning material that the NLC is in contact with. The reversibility of the optically induced molecular director reorientation of the NLC enables an active modulation of the plasmonic photo-induced heat.

Thermo-plasmonics deals with the generation of nanoscale heating produced by light activated plasmonic nanoparticles (NPs) such as gold and silver NPs. The combination of smart-responsive materials (e.g. thermotropic liquid crystals and thermo-responsive hydrogels) and highly photo-thermal efficient plasmonic (gold) NPs has been used for the realization of light assisted, reconfigurable, thermo-plasmonic driven Bragg mirrors, diffraction gratings, waveplates and smart windows.

The contrast of astronomical coronagraphs based on vector vortex waveplates (VWs) depends critically on precision of fulfilment of half-wave retardation condition for the desired wavelength range. The structure of VWs designed for operation in broad spectral bands is typically rather complex comprising three-dimensional modulation patterns that in practice are obtained in layer-by-layer deposition of liquid crystal polymers (LCPs). Physical properties of each of those layers - thickness, retardation, and orientation modulation pattern, along with layer integrity and quality - need to be maintained with very high precision to ensure contrast requirements of coronagraphs. This inevitably reduces the yield and affects the quality of high contrast VWs. We discuss opportunities of tuning the retardation values of VWs using electric field for liquid crystal (LC) based VWs as well as temperature and optical radiation for LCPs. LC VWs allow also tuning the spectral range of operation and switching the topological charge of the effective VW system.

We report on new modification of visible sensitive SPAAD series photoalignment materials (BEAM Co) and a method of alignment/treatment to create a photoalignment layer that can be reversible or fixed in a polymerizable matrix. The irreversible state is stable to exposures with to visible as well as ultraviolet radiation.

A thermoresponsive large-area plasmonic architecture, made from randomly distributed gold nanoparticles (GNPs) located at the substrate interface of a cholesteric liquid crystal (CLC) cell, is fabricated and thoroughly characterized. A photo-thermal heating effect due to the localized plasmonic resonance (LPR) mechanism is generated by pumping the GNP array with a resonant light beam. The photo-induced heat, propagating through the CLC layer, induces a gradual phase transition from the cholesteric to isotropic phase. Both the plasmonic and photonic properties of the system as both the selective reflection properties and frequency of the LPR are modulated.

Thermoplasmonics deals with the generation and manipulation of nanoscale heating associated with noble metallic nanoparticles. To this end, gold nanoparticles (AuNPs) are unique nanomaterials with the intrinsic capability to generate a nanoscale confined light‐triggered thermal effect. This phenomenon is produced under the excitation of a suitable light of a wavelength that matches the localized surface plasmonic resonance frequency of AuNPs. Liquid crystals (LCs) and hydrogels are temperature‐sensitive materials that can detect the host AuNPs and their photo‐induced temperature variations. In this perspective, new insight into thermoplasmonics, by describing a series of methodologies for monitoring, detecting, and exploiting the photothermal properties of AuNPs, is offered. From conventional infrared thermography to highly sophisticated temperature‐sensitive materials such as LCs and hydrogels, a new scenario in thermoplasmonic‐based, next generation, photonic components is presented and discussed. Moreover, a new road in thermoplasmonic‐driven biomedical applications, by describing compelling and innovative health technologies such as on‐demand drug‐release and smart face masks with smart nano‐assisted destruction of pathogens, is proposed. The latter represents a new weapon in the fight against COVID‐19.

Circular dichroism (CD) is an important parameter, characterizing optically active medium. The devices for CD measurements presented on today market measures CD in wide spectral range. The high-precision standard optics, electronics and optomechanics are used in these devices, and data processing requires the application of a complex mathematical apparatus. These devices are quite expensive, have large dimensions, require specially trained staff, and measurement duration is tens of minutes. In this work we present a device for CD measurement on only one wavelength, the key element of which is polarization diffractive waveplate, the optical element of new generation. Device has a number of advantages: easy to use, compact, working in real time, and low cost. These advantages make it ideal for educational programs.

Vector vortex waveplates offer distinct advantages compared to conventional light blocking components for coronagraphs due to their continuous structure, thinness, and transparency. They allow high contrast over a broad band in any spectral region for which the available anisotropic optical materials are transparent. We present opportunities of reducing in-band leakage of vector vortex waveplates to

Diffractive waveplates (DWs) are highly efficient optical components realized by means of a polarization holography setup that makes use of UV/blue laser sources. It is more convenient to perform the holographic recording process with green lasers (e.g., continuous wave operating at 532 nm) because they offer compactness, efficiency, and high power. Unfortunately, the photo-alignment materials used for DW fabrication exhibit limited sensitivity at 532 nm. Herein, we report the realization of cycloidal diffractive waveplates (CDWs) by employing a polarization holographic setup using a high-power, diode-pumped solid-state laser operating at 532 nm. The enabling factor is the use of a photo-alignment material with a broad absorption band extending into the visible part of the spectrum. Samples are characterized in terms of morphological and optical properties. They exhibit a well-defined morphology along with very high diffraction efficiency (≈97%). The possibility to realize CDWs with larger apertures is underway, enabling the realization of a new generation of optical components for integration in modern applications.

Improvements are possible in the tradeoff between bandwidth and diffraction efficiency as well as manufacturability in diffractive waveplate optics. We report recent progress in development and fabrication of diffractive waveplate optics using multi-layer designs, with weighting of the optimization corresponding to the spectral response of the human eye.

Azobenzene-based complex dyes (PAAD) have found many applications, in particular in liquid crystal based waveplates and flat lenses [1-2]. We investigated PAAD layers, both as individual thin films and as an alignment layer in liquid crystal waveplates, for modulating and steering laser beams. Thin (15-35 nm), stand-alone films of PAAD, illuminated by an interference fringe pattern, show considerable photo-sensitivity, in spite of limited thickness. We exploited this effect to record polarization patterns and observed considerable diffraction efficiency, with gratings formed through the refractive index change rather than surface relief. Diffraction efficiency depended strongly on the relative polarization of the pump versus the probe beam. The ratio between diffraction efficiencies in the two polarization states reached two orders of magnitude. These measurements were used to carry out preliminary modelling to investigate grating formation and optical axis distribution in the PAAD layer, thus opening the possibility of using optical measurements to study the PAAD thin film molecular dynamics. We also developed an optically controlled, half-wave plate based on a twisted nematic liquid crystal cell with a single PAAD alignment layer. The cell was bistable, with the two states controlled by one-step illumination of with visible light. The photo-alignment properties of the layer led to reversible switching between two perpendicular alignment states at the cell surface, resulting in controllable polarization manipulation. Reproducible modulation of the transmitted polarization with an optical contrast of up to 90-100% was achieved for different wavelengths of the probe beam, from visible to near infrared.

Diffractive waveplates (DWs) generally diffract light in different directions for the two circular polarizations of light, and the diffraction efficiency of such devices generally falls when the angle through which light is diffracted becomes large. It would be desirable to be able to make fast (low f-number), high-efficiency lenses with DW type structures. We study the dependence of the efficiency of DWs and polarization volume gratings (PVGs) on the polarization and angle of incidence of light, and identify structures with polarization independent function — the diffraction efficiency and angle of diffraction. The results are obtained for DWs and PVGs periodic in one linear coordinate, however, they are applicable to other optical devices, including lenses based on these structures.

Devices based on diffractive waveplates exhibit diffraction characteristics that are dependent on the polarization and wavelength of light. In applications, switchability, polarization independence, and wavelength independence are often desirable. Here we report on recent developments of diffractive optical elements that have various combinations of switchability, polarization independence, and broad wavelength coverage.

Diffractive waveplates enable stackable ultrathin light-weight large area optics fast-switchable with low-voltage/low power fields. State-of-the-art systems and low-cost manufacturing opportunities will be presented aimed at next generation augmented, virtual reality, flexible, polarizer-free displays, LiDARs, etc.

Controlling light propagation at the nanoscale is a fascinating opportunity offered by modern photonics, more than a challenge to face off. This study is aimed at investigating a particular kind of nanocomposite and reconfigurable optical metamaterials that can be exploited for the realization of a new class of switchable photonic devices, representing a breakthrough with respect to the state of the art. Existing photonic devices exhibit, in general, a drawback in the absence of tunability; this work aims to the design and characterization of metamaterials exploiting reconfigurable media, like LCs, which enable realization of a tunable, high quality, photonic quasi-crystal based switchable mode selector. It turned out that, starting from an unpolarized white light source, through a light extraction mechanism based on the diffraction of light, the high quality structure, combined with a uniformly aligned Photo-responsive Liquid Crystal (PLC), is able to give rise to an extremely narrow (FWHM ≈5 nm) and linearly polarized single mode peak of the extracted light intensity. Moreover, we have shown that the spectral properties (switching) of the samples can be finely controlled by using both an external applied voltage and a suitable pump light source with a maximum increase of 45% of the extracted light. Finally, both Scanning Electron Microscopy (SEM) and Far Field Diffraction (FFD) analysis have shown the high quality morphology of the realized structure.

Photo-anisotropic properties of a particular command layer for Liquid Crystals (LCs), based on azo-benzene material, are exploited to control the photo-thermal response of a single layer of homogeneously and uniformly distributed Au nanoparticles, immobilised on a glass substrate. Experiments demonstrate that the intrinsic anisotropy of materials can influence the photo-thermal response of plasmonic systems. Indeed, the resonant absorption of radiation by plasmonic subunits is followed by a noticeable increase of their temperature. However, the thermal response observed in presence of a homogeneous and random array of AuNPs directly exposed to air or embedded in ice is typically isotropic; on the contrary, a homogenous, yet thin, coating made of a particular command layer for LCs, deposited on a large-area carpet of AuNPs, influences their thermal response in an anisotropic way. In particular, the temperature increase, induced by pumping with a laser source of resonant wavelength with the plasmonic AuNPs, strongly depends on the alignment direction of the command layer. This effect makes the command layer of particular interest for its capability to drive intriguing optically induced ‘thermal-reorientational' effects in a liquid crystal film.

Technique for real-time spectropolarimetry is developed. An embedded system for circular dichroism registration is designed based on National Instruments single board reconfigurable I/O platform for ultraviolet and visible spectroscopic needs. The basic functional unit of the proposed device is liquid crystal polarization diffraction grating. In order to expand the dynamic range the control of data accumulation time is provided. Two NMOS sensor linear arrays provide an excellent linearity of response.

The optical power of diffractive waveplate structures is limited not as much by fabrication technology issues as by the fundamental features of light propagation in complex anisotropic structures. The infinitely thin two-dimensional film approximation does not apply, and the efficiency of 4G lenses, prisms, etc., is reduced for geometries corresponding to sharp focusing lenses and large diffraction angles. Due to thin-film nature, these films can be combined to reduce effective focal length, increase effective diffraction angle, topological charge, etc. Along with this, we will discuss the opportunity of increasing optical power of 4G lenses, prisms, etc. without compromising efficiency.

We report about developing long-wave infrared diffractive optical components based on liquid crystals. The components show high efficiency and high transparency for the 10.6 μm wavelength of CO2 laser beam.

In this review paper,we report recent progress on Pancharatnam-Berry (PB) phase optical elements, such as lens, grating, and deflector. PB lenses exhibit a fast switching time between two or more focal lengths with large diopter change and aperture size, which is particularly attractive for addressing the accommodation mismatch in head-mounted display devices. On the other hand, PB gratings and deflectors offer a large-angle beam deflection with wide acceptance cone and high efficiency, as compared to conventional volume gratings. Such merits provide great advantages for waveguide-coupling augmented reality headsets. Moreover, the thickness of PB optical elements is only a few micrometers, thus they can be conveniently integrated into modern wearable display systems.

Near 100% broadband efficiency of polarization holograms provides opportunities for shaping light beams, imaging, and displays. Thin, flexible and switchable between different optical states and functions, novel optical components and systems are smarter than ever.

Light is a readily available and sustainable energy source. Transduction of light into mechanical work or electricity in functional materials, composites, or systems has other potential advantages derived from the ability to remotely, spatially, and temporally control triggering by light. Toward this end, this work examines photoinduced piezoelectric (photopiezoelectric) effects in laminate composites prepared from photoresponsive polymeric materials and the piezoelectric polymer polyvinylidene fluoride (PVDF). In the geometry studied here, photopiezoelectric conversion is shown to strongly depend on the photomechanical properties inherent to the azobenzene-functionalized polyimides. Based on prior examinations of photomechanical effects in azobenzene-functionalized polyimides, this investigation focuses on amorphous materials and systematically varies the concentration of azobenzene in the copolymers. The baseline photomechanical response of the set of polyimides is characterized in cantilever deflection experiments. To improve the photomechanical response of the materials and enhance the electrical conversion, the polyimides are drawn to increase the magnitude of the deflection as well as photogenerated stress. In laminate composites, the photomechanical response of the materials in sequenced light exposure is shown to transduce light energy into electrical energy. The frequency of the photopiezoelectric response of the composite can match the frequency of the sequenced light exposing the films.

This feature issue of the journal is the first one to bring together ongoing research from many groups developing diffractive waveplate technology. This technology combines the best of three worlds—the thinness of diffraction gratings, the broad bandwidth and high efficiency of conventional refractive optics, and the low-cost fast manufacturing of polymer optics even in large area. The technology is at the intersection of polarization holography and polarization gratings, Pancharatnam–Berry or geometrical phase optics, waveplates, metasurfaces and planar optics. Optics will never be the same again!

A narrow - linewidth, hot isostatic treated Cr:ZnSe laser was non - mechanically tuned over a total wavelength range of 195 nm by a novel organic liquid crystal etalon. The narrow - linewidth laser was continuously tuned by applying a 1 kHz square wave signal of 1 – 5 Vpp to the intracavity 9 THz nominal free spectral range liquid crystal etalon. The maximum and minimum lasing wavelengths were 2650 nm and 2455 nm, respectively, and a maximum average output power of 475 mW was recorded at 2503 nm. This work demonstrated continuous tuning, while maintaining an intrinsic narrow-linewidth ≤ 900 MHz. The results presented in this work are attributed to the low insertion loss of the liquid crystal etalon at ≤ 5%. It is believed that this is the first demonstration of purely electronic tuning of a mid-IR laser by an organic liquid crystal etalon.

A general geometric phase singularity array structure is presented and discussed. For any two-dimensional point lattice, a singularity array is defined as a summation of helical phase singularities with alternating handedness. The phase angle is the slow-axis orientation of a varying half-waveplate. Arrays are demonstrated in photoaligned polymer liquid crystal films. Simple square and biomimetic spiral lattices are characterized for diffraction behavior. Pattern selection rules based on topological charge are discovered. Numerical decomposition shows that diffracted beams are composites of a few ordered planewaves for square phase arrays and Bessel beams for spiral phase arrays. Orbital angular momentum spectra of spiral diffraction are directly measured.

We present a collinear, common-path image-inversion interferometer using the polarization channels of a single optical beam. Each of the channels is an imaging system of unit magnification, one positive and the other negative (inverted). Image formation is realized by means of a set of anisotropic lenses, each offering refractive power in one polarization and none in the other. The operation of the interferometer as a spatial-parity analyzer is demonstrated experimentally by separating even- and odd-order orbital angular momentum modes of an optical beam. The common-path configuration overcomes the stability issues present in conventional two-path interferometers.

Soft composite materials combined with a holographic photopolymerization process are used for realizing an innovative switchable periodic structure made of slices of almost pure polymer alternated with films of well aligned nematic liquid crystals named POLICRYPS. It exhibits negligible scattering losses, while the effect of the spatial modulation of the refractive index (from polymer to nematic liquid crystal) can be switched on and off by applying a low (few V/μm) electric field. The diffractive properties of the POLICRYPS structure are characterized in terms of cell thickness, impinging probe angle and wavelength revealing a strong correlation between the diffraction efficiency and all the above mentioned parameters. These results are very attractive for many applications such as switchable Bragg gratings for telecom devices, phase modulators, and displays. Other advantages of the technology include absence of an alignment layer, absence of haze, robust structure, and inexpensive manufacturing. In addition, no special alignment layers are required. This is a unique opportunity and a big advantage compared to conventional liquid crystal devices. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 158–162

Circularly polarized light is employed to write information into cholesteric liquid crystal (CLC) cells that initially exhibit homogeneous planar alignment and are fabricated with a photoaligning boundary layer. Overlap of the writing beam with light reflected from the CLC phase disrupts the reflective Grandjean texture, resulting in a localized region that strongly scatters light. These scattering areas can be geometrically patterned using circularly polarized light and a mask, erased and brought back to the original reflective condition by exposure to linearly polarized light, and then repatterned with a different mask using circularly polarized light. The light-induced scattering is localized to the side of the cell being exposed. Within the scattering region a microscopic texture is formed, which exhibits an average feature, or “domain”, size related to cell thickness and helical pitch. The behavior occurs only when the CLC and incoming writing radiation exhibit the same handedness and when the reflection bandwidth of the CLC is commensurate with the wavelength of the incoming radiation. This new phenomena provides the ability to reconfigure reflectivity from a CLC cell and is expected to be useful for systems where all optical control of dynamic information is needed.

We study the opportunity of using electrohydrodynamic instabilities in a nematic liquid crystal mixture with negative dielectric anisotropy for controlling laser beams. Switching between naturally transparent and diffuse light scattering states is achieved by application of low frequency, low amplitude voltages. The specifics of diffuse light scattering state depending on the orientation and thickness of the liquid crystal layer are revealed. The switching occurs on a milliseconds time scale. Combination of thin, flexible liquid crystal cells allows polarization independent, high contrast, fast switching in a broad band of visible wavelengths.

The fourth generation optics (4G optics) enables the realization of novel optical components (lenses, gratings, vector vortices, etc.) by patterning the optical axis orientation in the plane of an anisotropic film. Such components exhibit near 100% diffraction efficiency for wavelengths meeting half-wave retardation condition. In this framework, we have advanced a step-forward by realizing different diffractive waveplates (DWs) with arbitrary spatial patterns of the optical axis orientation by exploiting the capability of a Digital Spatial Light Polarization Converter (DSLPC). The DSLPC is based on a reflective, high resolution Spatial Light Modulator (SLM) combined with an “ad hoc” optical setup. The most attractive feature of the use of a DSLPC for photoalignment is that the orientation of the alignment layer, and therefore of the fabricated liquid crystal (LC) or liquid crystal polymer (LCP) DWs, can be specified on a pixel-by-pixel basis. By varying the optical magnification or de-magnification between the SLM and the alignment layer, the spatial resolution of the photoaligned layer can be adjusted to be optimal for each application. We show that with a simple “click” it is possible to record different high resolution optical components as well as arbitrary patterns ranging from lenses to invisible and even dual labels.

The thickness of functional layers in liquid crystal photonics devices is negligibly small compared to the substrates. New opportunities provided by multilayer 4G optical systems require minimizing the thickness of each layer. We report about our progress made by developing technology of thin flexible substrates, functional polymer films, solid electro-optical layers, and graphene oxide based electro-conductive coatings.

We report on lenses that operate over the visible wavelength band from 450 nm to beyond 700 nm, and other lenses that operate over a wide region in the near-infrared from 650 nm to beyond 1000 nm. Lenses were recorded in liquid crystal polymer layers only a few micrometers thick, using laser-based photoalignment and UV photopolymerization. Waveplate lenses allowed focusing and defocusing laser beams depending on the sign of the circularity of laser beam polarization. Diffraction efficiency of recorded waveplate lenses was up to 90% and contrast ratio was up to 500:1.

We report on photo-thermal effects observed in gold nanoparticles (GNPs) dispersed in Nematic Liquid Crystals (NLCs). Under a suitable optical radiation, GNPs exhibit a strong light absorption/scattering; the effect depends on the refractive index of the medium surrounding the nanoparticles, which can be electrically or optically tuned. In this way, the system represents an ideal nano-source of heat, remotely controllable by light to adjust the temperature at the nanoscale. Photo-induced temperature variations in GNPs dispersed in NLCs have been investigated by implementing a theoretical model based on the thermal heating equation applied to an anisotropic medium; theoretical predictions have been compared with results of experiments carried out in a NLC medium hosting GNPs. Both theory and experiments represent a step forward to understand the physics of heat production at the nanoscale, with applications that range from photonics to nanomedicine.

The fourth generation of optics uses modulation of optical axis orientation in anisotropic materials for achieving a wide variety of optical functions. 4G optical components blend the thinness of zero-order waveplates with the high efficiency one gets with Bragg volume holographic gratings and in a broad band of wavelengths typical of conventional glass lenses, prisms, etc. (the first generation of optics). We will discuss the technology for applications in communications and astronomy. Particularly, we will present challenges and opportunities for fabrication of vector vortex waveplates and cross-waveplates for coronagraphy, and waveplate lenses — Pancharatnam-Berry phase lenses — for optical communications and imaging.

We report on the thermo-optical properties of gold nanorods (GNRs) dispersed in a thermotropic cholesteric liquid crystal (CLC). We have characterized the CLC reflection band behavior for two different cell thicknesses under the influence of a suitable (resonant) pump beam. It turns out that for the 1.6 mu m thick cell there is a suppression of the CLC reflection band for both pure CLC and CLC/GNRs. For the 10 mu m thick cell, the presence of GNRs desensitizes the shift of the CLC reflection band to temperature. Suitable cell design enables one to 'turn off' the wavelength shift of the peak reflection, thereby turning the system into a pure amplitude measurement tool. This has implications where the probe wavelength is fixed at a common, single wavelength.