Presbyopia is an age-related loss of accommodation of the human eye that manifests itself as inability to shift focus from distant to near objects. Assuming no refractive error, presbyopes have clear vision of distant objects; they require reading glasses for viewing near objects. Area-divided bifocal lenses are one example of a treatment for this problem. However, the field of view is limited in such eyeglasses, requiring the user to gaze down to accomplish near-vision tasks and in some cases causing dizziness and discomfort. Here, we report on previously undescribed switchable, flat, liquid-crystal diffractive lenses that can adaptively change their focusing power. The operation of these spectacle lenses is based on electrical control of the refractive index of a 5-m-thick layer of nematic liquid crystal using a circular array of photolithographically defined transparent electrodes. It operates with high transmission, low voltage (<2 V rms), fast response (<1 sec), diffraction efficiency > 90%, small aberrations, and a power-failure-safe configuration. These results represent significant advance in stateof-the-art liquid-crystal diffractive lenses for vision care and other applications. They have the potential of revolutionizing the field of presbyopia correction when combined with automatic adjustable focusing power.ophthalmic lens ͉ switchable lens ͉ vision correction T he use of nematic liquid crystals to implement switchable lenses has been proposed previously but had limited success for ophthalmic applications (1). Hybrid liquid-crystal refractive lenses incorporating convex and concave substrates have been demonstrated (2, 3). However, the large thickness of the liquidcrystal layers (Ͼ400 m) make their response and recovery times long and their transmission low because of optical scattering. To reduce the thickness of the active layer, surface relief Fresnel lens substrates have been proposed (4). However, in this geometry the lens is optically active in the electrically off-state, which is not desirable for ophthalmic applications where a loss of electrical power could suddenly result in near-vision correction during a critical distance vision task such as driving. In other approaches, thin uniform layers of liquid crystal were used, and refractive lenses were produced by the use of discrete electrodes (5), continuous highly resistive electrodes (6), or spatially distributed electric fields (microlenses) (7). However, in these lenses either the range of focal length or their small diameter made them unsuitable for ophthalmic applications. We employ a photolithographically patterned thin diffractive lens with large aperture, fast response time, and a power-failure-safe configuration to overcome these limitations. Although high-efficiency liquid-crystal-based diffractive devices have been demonstrated for beam-steering (8, 9), less effort was given to the development of switchable diffractive lenses. The diffraction efficiencies of the lenses achieved for imaging (10-15) and other applications (16, 17) we...

Gallium selenide (GaSe) is a layered semiconductor and a well-known nonlinear optical crystal. The discovery of graphene has created a new vast research field focusing on two-dimensional materials. We report on the nonlinear optical properties of few-layer GaSe using multiphoton microscopy. Both second- and third-harmonic generation from few-layer GaSe flakes were observed. Unexpectedly, even the peak at the wavelength of 390 nm, corresponding to the fourth-harmonic generation or the sum frequency generation from third-harmonic generation and pump light, was detected during the spectral measurements in thin GaSe flakes.

When silicon strip and slot waveguides are coated with a 50 nm amorphous titanium dioxide (TiO2) film, measured losses at a wavelength of 1.55 μm can be as low as (2 ± 1)dB/cm and (7 ± 2)dB/cm, respectively. We use atomic layer deposition (ALD), estimate the effect of ALD growth on the surface roughness, and discuss the effect on the scattering losses. Because the gap between the rails of a slot waveguide narrows by the TiO2 deposition, the effective slot width can be back-end controlled. This is useful for precise adjustment if the slot is to be filled with, e. g., a nonlinear organic material or with a sensitizer for sensors applications.

Background: The periodic dielectric multilayers sustaining Bloch surface waves have been proposed as a platform for the sensing applications and the two dimensional integrated optics. In this paper, we present the experimental and theoretical investigation of propagation properties of Bloch surface waves, for example propagation length and refractive index of the surface mode, at the interface of a dielectric multilayer platform. We use thin layers (~λ/25) of titanium dioxide as an additional layer of high index material. Methods: We exploit multi-heterodyne scanning near-field optical microscopy and total internal reflection configuration as a near-field and far-field characterization tools. Results: The longest propagation length is achieved when the multilayer is designed to have the dispersion curve positioned close to the middle of the photonics band gap. We measure a Bloch surface wave mode of propagation length 3.24 mm and of an effective refractive index contrast 0.15. Conclusions: The experimental results are in conformity with theoretical results. This study paves a way to realize efficient and compact two dimensional components and systems.