Metamaterials are artificially fabricated materials that allow for the control of light and acoustic waves in a manner that is not possible in nature. This Review covers the recent developments in the study of so-called metasurfaces, which offer the possibility of controlling light with ultrathin, planar optical components. Conventional optical components such as lenses, waveplates and holograms rely on light propagation over distances much larger than the wavelength to shape wavefronts. In this way substantial changes of the amplitude, phase or polarization of light waves are gradually accumulated along the optical path. This Review focuses on recent developments on flat, ultrathin optical components dubbed 'metasurfaces' that produce abrupt changes over the scale of the free-space wavelength in the phase, amplitude and/or polarization of a light beam. Metasurfaces are generally created by assembling arrays of miniature, anisotropic light scatterers (that is, resonators such as optical antennas). The spacing between antennas and their dimensions are much smaller than the wavelength. As a result the metasurfaces, on account of Huygens principle, are able to mould optical wavefronts into arbitrary shapes with subwavelength resolution by introducing spatial variations in the optical response of the light scatterers. Such gradient metasurfaces go beyond the well-established technology of frequency selective surfaces made of periodic structures and are extending to new spectral regions the functionalities of conventional microwave and millimetre-wave transmit-arrays and reflect-arrays. Metasurfaces can also be created by using ultrathin films of materials with large optical losses. By using the controllable abrupt phase shifts associated with reflection or transmission of light waves at the interface between lossy materials, such metasurfaces operate like optically thin cavities that strongly modify the light spectrum. Technology opportunities in various spectral regions and their potential advantages in replacing existing optical components are discussed.

Flat Optics With Designer Metasurfaces

The phase-shifting mask consists of a normal transmission mask that has been coated with a transparent layer patterned to ensure that the optical phases of nearest apertures are opposite. Destructive interference between waves from adjacent apertures cancels some diffraction effects and increases the spatial resolution with which such patterns can be projected. A simple theory predicts a near doubling of resolution for illumination with partial incoherence σ < 0.3, and substantial improvements in resolution for σ < 0.7. Initial results obtained with a phase-shifting mask patterned with typical device structures by electron-beam lithography and exposed using a Mann 4800 10× tool reveals a 40-percent increase in usuable resolution with some structures printed at a resolution of 1000 lines/mm. Phase-shifting mask structures can be used to facilitate proximity printing with larger gaps between mask and wafer. Theory indicates that the increase in resolution is accompanied by a minimal decrease in depth of focus. Thus the phase-shifting mask may be the most desirable device for enhancing optical lithography resolution in the VLSI/VHSIC era.

Improving resolution in photolithography with a phase-shifting mask

Over the years extensive studies have been carried out to apply coherent optics methods in real-time communications and image transmission. This is especially true when a large amount of information needs to be processed, e.g., in high-resolution imaging. The recent progress in data-processing networks and communication systems has considerably increased the capacity of information exchange. However, the transmitted data can be intercepted by nonauthorized people. This explains why considerable effort is being devoted at the current time to data encryption and secure transmission. In addition, only a small part of the overall information is really useful for many applications. Consequently, applications can tolerate information compression that requires important processing when the transmission bit rate is taken into account. To enable efficient and secure information exchange, it is often necessary to reduce the amount of transmitted information. In this context, much work has been undertaken using the principle of coherent optics filtering for selecting relevant information and encrypting it. Compression and encryption operations are often carried out separately, although they are strongly related and can influence each other. Optical processing methodologies, based on filtering, are described that are applicable to transmission and/or data storage. Finally, the advantages and limitations of a set of optical compression and encryption methods are discussed.

https://hal.archives-ouvertes.fr/docs/00/51/69/80/PDF/20.pdf

Optical image compression and encryption methods

We report a new, to our knowledge, technique for encoding amplitude information onto a phase-only filter with a single liquid-crystal spatial light modulator. In our approach we spatially modulate the phase that is encoded onto the filter and, consequently, spatially modify the diffraction efficiency of the filter. Light that is not diffracted into the first order is sent into the zero order, effectively allowing for amplitude modulation of either the first-order or the zero-order diffracted light. This technique has several applications in both optical pattern recognition and image processing, including amplitude modulation and inverse filters. Experimental results are included for the new technique.

Encoding amplitude information onto phase-only filters

Several attacks are proposed against the double random phase encryption scheme. These attacks are demonstrated on computer-generated ciphered images. The scheme is shown to be resistant against brute force attacks but susceptible to chosen and known plaintext attacks. In particular, we describe a technique to recover the exact keys with only two known plain images. We compare this technique to other attacks proposed in the literature.

Resistance of the double random phase encryption against various attacks

Switchable, continuous, complex-amplitude modulation is demonstrated with two cascaded, twisted nematic liquid-crystal televisions (LCTV's), both operating in phase- and amplitude-coupled modulation modes The condition for full-range complex modulation is that one of the LCTV's must provide a 2π-range phase modulation A look-up table encoding method is proposed that permits the compensation of phase-amplitude coupling and nonlinearity in the two individual LCTV modulations Experimental techniques for determining the LCTV-device parameters, for maximizing the phase-mostly modulation range and the amplitude-mostly modulation contrast, and for testing the complex-amplitude modulation are developed Optical complex-amplitude Fresnel holograms are shown

Full-range, continuous, complex modulation by the use of two coupled-mode liquid-crystal televisions

Cellular and porous polymers with internal bipolar space charge can exhibit large piezoelectric thickness coefficients and have therefore led to significant advances in the understanding and the application of piezoelectricity in polymer electrets. As possible alternatives to these cellular ferroelectrets, other potentially useful configurations of space-charge electrets, such as bi-layer or multi-layer stacks of at least one "soft" porous and one "hard" non-porous electret film have been suggested and investigated. Extending the concept of cellular or porous polymer electrets with microscopic voids, we propose and describe novel piezoelectret structures with regular arrays of millimeter-sized bubbles that are formed between fluoro-ethylene-propylene (Teflonreg-FEP) films via a vacuum-assisted thermal process. After internal charging by means of high impulse voltages, the bubble structures exhibit rather large piezoelectric activities in their thickness direction, with phenomenological quasi-static d33 coefficients of up to 500 pC/N (or pm/V) which quite strongly decrease with the amplitude of the applied force. The electromechanical behavior of the new piezoelectrets has been modeled in analogy to the operation of electret microphones

https://www.researchgate.net/profile/Luiz_Neto7/publication/3340734_Piezoelectrets_from_thermo-formed_bubble_structures_of_fluoropolymer-electret_films/links/54dbbf280cf28d3de65c78ed.pdf

Piezoelectrets from thermo-formed bubble structures of fluoropolymer-electret films

In the optical implementation of image encryption using ran- dom phase encoding proposed by Refregier and Javidi the random phase mask is not band limited. Speckle noise can thus destroy optical reconstruction of the encrypted images. We demonstrate encryption of speckle-free kinoforms by smoothed random phase masks. Optical veri- fication of the encrypted images is shown. © 1996 Society of Photo-Optical

Optical implementation of image encryption using random phase encoding

We design and fabricate efficient, narrow-band, transmission color filters whose operating principle resides in a narrow-band guided-mode resonance associated with a surface-plasmon resonance. The fundamental device consists of an aluminum grating over a 200-nm-thick aluminum oxide film on a glass substrate. Numerical simulations show a sharp resonance-derived spectral profile that is additionally shaped by a neighboring Rayleigh anomaly. Besides the Rayleigh effect, we show numerically that the narrow bandwidth is predominantly due to the low refractive-index contrast between the waveguide film and the substrate. Red, green, and blue filters are fabricated using ultraviolet holographic lithography followed by a lift-off process. The experimental spectral efficiency in transmission exceeds 80% with full-width-at-half-maximum linewidths near 20 nm. We provide color images of the zero-order transmitted spectra, and illustrate the pure colors associated with the modal resonance extracted as side-coupled output light.

Efficient band-pass color filters enabled by resonant modes and plasmons near the Rayleigh anomaly

Commercial twisted nematic liquid-crystal television provides coupled phase and amplitude modulation. We propose a simple wedged shear plate interferometer for in situ measurement of its phase modulation and operating curve. For a given operating curve, the coupled-mode modulation holograms are designed with an iterative method. We adjust the operating curve by rotating the polarizer and analyzer to obtain the optimal operating curve. The phase-mostly holograms yield good-quality reconstructed images with the zero-order spot reduced to a minimum. Experimental results are shown.

Luiz Gonçalves Neto

Papers

Full-range, continuous, complex modulation by the use of two coupled-mode liquid-crystal televisions

Piezoelectrets from thermo-formed bubble structures of fluoropolymer-electret films

Optical implementation of image encryption using random phase encoding

Efficient band-pass color filters enabled by resonant modes and plasmons near the Rayleigh anomaly

Programmable optical phase-mostly holograms with coupled-mode modulation liquid-crystal television