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Silicon photonics has been studied in various areas by providing small-footprint, high-performance, and mass-producible optoelectronic components integrated on a chip. For the photonic packaging, ultraviolet (UV) curing techniques have been widely utilized. Meanwhile, silica cladding absorption to UV light could alter its refractive index (RI), affecting the optical testing of silicon devices during or after the device packaging. Therefore, it is significant to characterize the dynamics of RI changes induced by UV-light irradiation. However, the in-situ characterization of such devices is seldom explored. Here, we studied the influence of UV-light irradiation on silicon photonic devices by probing resonant wavelength shifts of a racetrack microring resonator. Specifically, we studied the temporary variation of the resonant wavelength and its recovery time under different UV-light exposure durations. Experimental results show that with a UV energy of 21 J/cm2, the resonator had a maximum resonant wavelength shift of 0.31 nm, corresponding to an effective RI change of 0.0009 and recovering time of 95 minutes. Based on the experimental results, we compressively analyzed and compared RI changes induced by plasmon dispersion effect, thermal optical effect and photon-induced silica densification effect. Our study is expected to provide useful guidelines for in-situ silicon photonic testing and packaging. [ABSTRACT FROM AUTHOR]

Transmission matrix can be used as an elegant tool to represent the light transmission characteristics of a scattering medium. In this paper, we propose a novel common path interferometer technique to measure the transmission matrix utilizing the polarization rotation of incident beam. Specifically, the horizontally polarized and vertically polarized components of the incident beam are used as signal light and reference light, respectively. By rotating the polarization of the incident beam, the ratio between the powers of signal and reference light can be freely adjusted and balanced. The phase shift method is employed to retrieve the transmission matrix of scattering medium. Once the transmission matrix is known, the polarization of the incident light beam can be rotated to switch off the reference and only the signal will remain. Focusing and manipulation of light through a ground glass has been experimentally demonstrated with the measured transmission matrix. An optical focus enhancement reaches 83% of the theoretical maximal value is obtained, which is about 30% improvement compared to the previous co-propagation method. The transmission matrix characterization method has the potential to become a general tool to harness the multiple scattering of light through complex medium. [ABSTRACT FROM AUTHOR]

We report an elliptical beam design for aberration mitigation in high-resolution optical coherence tomography (OCT). We polished a large angle on the fiber terminal facet in the sample arm to make a non-rotational symmetric beam with different numerical apertures (NA) for the two axes vertical to the optical axis. By sacrificing the resolution in the out-of-plane transverse direction, the elliptical beam mitigated the aberration introduced by the focusing optics in the OCT system. The elliptical beam with a doubled NA in the in-plane transverse direction promoted the axial field-of-view (FOV) by about 50% and increased the signal back-coupling efficiency by about 25%. We verified the feasibility of the design by imaging the USAF 1951 resolution chart, swine cornea ex vivo, and human skin in vivo. Results show that the proposed method relieves aberration-related problems in high-resolution OCT. [ABSTRACT FROM AUTHOR]

Focusing Fresnel lenses are used in many fields of applied optics. These devices are used in optical sensor systems for imaging and optoelectronic integration. The traditional Fresnel lens concentrates the light intensity on the center of the formed image. We present a microprismatic Fresnel lens that transforms a circular incident parallel light beam into a homogeneous light circle with the necessary diameter at a certain distance from the lens. These transforming Fresnel concentrators can be successfully used, for example, in monitoring devices to automatically adjust the output signal from four-quadrant photodetectors. Traditional focusing Fresnel structures are manufactured by photolithographic methods or adjustable direct laser recording with photoresists. These methods enable the formation of stepped optical structures, which have inherent surface defects, resulting in the formation of images that are not high in quality. The proposed specialized Fresnel concentrators can be easily fabricated via the diamond cutting method, which enables the manufacturing of flat working surfaces with exceedingly high optical quality. We also develop a method for simulating the Fresnel transforming lenses with flat conical working facets and calculate the geometric parameters of the circular concentrators. We then apply the simulation results to the diamond cutting method and fabricate the microprismatic light transforming lens samples. These samples are then investigated experimentally with a collimated laser beam. The obtained data agree with the theoretical predictions. [ABSTRACT FROM AUTHOR]

We propose a graphene-based metasurface to investigate the function of switchable polarization selective wave manipulation in THz regime by using the finite-difference time-domain method. The proposed metasurface consists of two layers of graphene arrays, polymer dielectric spacer and a gold mirror film, which can be switched between two states by simply biasing the two graphene layers on and off with the designed voltage and zero voltage without reconfiguring the Fermi energy distribution. One state is for x polarized wave manipulation and the other is for y polarization incidence. Then, the functions of switchable polarization selective anomalous reflection and focusing effect for 6 THz wave are realized by configuring the structure with specific values of Fermi energy. Moreover, we also designed the structure to focus at other spatial positions and another working frequency of 5.5 THz by reconfiguring the Fermi energy distributions.

This paper proposes an innovative autofocus method to ensure the image of an integrated circuit is correctly in focus under an infrared microscope. It discusses the difficulties inherent to the optical system and explores several inefficient methods used for natural scenes. It will also present a Focus Metric based on POlynomial Decomposition (FMPOD) adapted to our context. This approach relies on analyzing the projection of images on an orthonormal polynomial basis. Its robustness is tested using different magnifying lenses in addition to multiple distortions. In conclusion, we will demonstrate how this novel approach outperforms existing methods related to our work environment.

In this manuscript, a method of generating an ultra-long optical needle (depth-to-width ratio 37.5:1) is proposed and demonstrated by focusing an azimuthally polarized beam. In theory, the action mechanism between the incident beam and the amplitude modulation element, the spiral phase modulation element, the focusing lens were studied based on the Richards and Wolf's theory. The relationship between the intensity distribution of the optical needle and the structure parameter of the element were obtained, thus leading to the complete design model and design standard. In experiment, the annular amplitude modulation element and spiral phase modulation element were fabricated by lithography. The optical needle was obtained based on a custom-designed optical setup in our laboratory. The optical system consists of an annular aperture (3.9-mm inner diameter, 80-µm annular width), a spiral phase plate (topological charge of 1), and an objective lens with numerical aperture of 0.95. Finally, an optical needle with a subwavelength size (0.416λ) and an ultra-long depth of focus (15.6λ) was obtained, showing an excellent agreement with our theoretical model. [ABSTRACT FROM AUTHOR]

Integrating multiple functionalities into one single device is of great importance for ever-growing demand of photonic device. Optical vortex beam contains orbital angular momentum (OAM) and spin angular momentum (SAM) associated with phase and polarization singularities respectively, which has contributed to various application including optical manipulation, optical trapping and high-speed optical communication. Traditional methods to detect its phase and polarization mostly focus on the functionality of single dimensional optical field. However, quite different from the previous reports, we here propose a novel approach to measure the phase and polarization state based on the full use of near and far-field light functionalities in one single metadevice, which supports spin-controlled surface plasmon polaritons (SPPs) and topological charge-dependent focus spot position. On the one hand, this design makes the most of multidimensional field optical scattering characteristics to rich the functions of metadevice. On the other hand, it provides a novel concept to recognize various angular momentum information of vortex beam. We believe that such design will have great potential for the integration of different sub-wavelength optical components. [ABSTRACT FROM AUTHOR]

Design and analysis of metasurface-based devices mainly rely on the ideal ray theory and full-wave simulation technique. However, for near-field and low-frequency applications, these idealized design and time-consuming full-wave simulation methods often suffer from inaccuracy and inefficiency, which hinders the rapid prototyping and further evolution of metasurface-based devices. Here, dipole-based wavelet superposition (DWS) is proposed and formulated to accurately and efficiently design the metasurface and analyze its diffraction field, by treating unit cell on the metasurface and every wavefront representing the target field as Hertzian dipole. We further define quantitatively an effective zone, within which the proposed DWS offers sufficiently accurate results comparable to the full-wave simulation. A series of numerical experiments on design and analysis of metasurface are then carried out to validate our DWS method. Excellent and more superior consistence with the full-wave analysis and theoretical results confirms that the DWS method provides an advanced closed-loop design and analysis tool with significantly improved accuracy and efficiency for metasurface-based devices. [ABSTRACT FROM AUTHOR]

Metalenses based on dielectric nanostructures operating in the transmission mode are of great practical significances. As large-area, multifunctional metalenses are increasingly demanded at higher volumes nowadays, the quality and efficiency of the nanofabrication processes for these lenses are made even more important. Yet, no fabrication is without defects or errors. By using finite-difference time-domain (FDTD) calculation method, we study, in simulation, a set of common fabrication-induced errors such as inclined sidewall, critical dimension (CD) bias and process defects, and analyze their influences on the optical performances of two types of metalenses – one made of amorphous silicon and the other of silicon nitride. The study concludes that the lens behavior generally relies on the resonant nature of the nanostructure array. While most types of defects discussed in this work linearly decrease the focusing efficiency, a few would also influence the quality of the focal spot. Potential solutions to mitigate the effect of the process-induced defects are also investigated. The study provides the particular knowledge to understand the actual metalens performance when fabricated. [ABSTRACT FROM AUTHOR]

Optical techniques for information security have drawn a lot of attention in recent years. The development of cryptanalysis strategies will also have a positive promotion for the cryptosystems themselves. However, most of the present publications on cryptanalysis of optical cryptosystems are focusing only on theoretical analysis but not on experimental verification. In this paper, a practical optical cryptanalysis method using wavefront shaping is proposed. Considering that the confusion and diffusion of information in the optical cryptosystem are actually caused by scattering effect of random phase mask(s), the proposed method employs a point source function and the feedback-based optimization algorithm, which can obtain an equivalent key of the optical cryptosystem. In such a way, the confusion and diffusion of the random phase mask(s) in the cryptosystem could be removed successfully. To the best of our knowledge, this is the first time the attack strategy from the perspective of wavefront shaping can be seen, and one of the biggest advantages is that it could get an equivalent key of the cryptosystem without deducing its original keys.

This paper proposes an innovative autofocus method to ensure the image of an integrated circuit is correctly in focus under an infrared microscope. It discusses the difficulties inherent to the optical system and explores several inefficient methods used for natural scenes. It will also present a Focus Metric based on POlynomial Decomposition (FMPOD) adapted to our context. This approach relies on analyzing the projection of images on an orthonormal polynomial basis. Its robustness is tested using different magnifying lenses in addition to multiple distortions. In conclusion, we will demonstrate how this novel approach outperforms existing methods related to our work environment. [ABSTRACT FROM AUTHOR]

In this paper, the tight focusing of high-order cylindrical vector beams (beams with polarization singularity) was investigated. Using the Richards–Wolf formalism, there were obtained expressions for all projections of the electric and magnetic light fields with m-order polarization singularity in the focus of the aplanatic system. Also expressions for the longitudinal component of the Poynting vector were obtained. It was shown that these beams produce in the focal areas with the direction of the Poynting vector opposite to the direction of propagation of the beam. Moreover, the negative values could be comparable in absolute value with positive values; however, this strong inverse energy flow is obtained only while laser light is focused by a lens with high numerical aperture. Of particular interest is the case when the beam order is two (m = 2). In this case, the region where the Poynting vector longitudinal projection is negative is located on the optical axis. If the order of the beam is more than two (m > 2), than the reverse flow occurs near the optical axis and has a shape of a “tube.” Moreover, the width of the negative values region (the diameter of the “tube”) increases with increasing order of the beam, however, the absolute value of energy backflow decreases. Earlier, we reported on the observation of a spiral negative energy flow from the center of the focal plane of a focused vortex beam. In this paper, the negative propagation of a laminar near-axis energy flow is reported. [ABSTRACT FROM AUTHOR]

Focusing technique is the key factor in improving the resolution of projection lithography. In order to achieve high accuracy of focusing on the nanometer level, we present a focusing method based on dual-channel light intensity modulation. Two superposed grating fringes are formed with a phase difference of π/2. The shift of the wafer can be resolved by the phase variation of corresponding fringes. The conventional focusing method with light intensity modulation is influenced by disturbing effects such as fluctuations of the light intensity and stray light. These adverse factors are overcome by the signal ratio of two channels with the same optical components. The focusing accuracy in our experiments is ±8 nm.

We report an improved scheme for the measurement of the transmission matrix of multimode waveguides in which the reference field copropagates with the signal wave. The performance of the technique is demonstrated with the measurement of a 1.6-m long multimode optical fiber guiding 104 LP modes at 1064 nm. The transmission matrix permitted efficient focusing of the light delivered at the fiber output as well as shaping in the fiber's transmission channels. [ABSTRACT FROM AUTHOR]

Based on the inverse Faraday effect, tunable light-induced magnetization chains are investigated by focusing narrow annular azimuthally polarized vortex beams with a 4 $\pi$ focusing system composed of double annular parabolic mirrors. Using Richards–Wolf vector diffraction theory, approximate analytical expressions of all parameters of the magnetization chains are theoretically calculated, realizing that all parameters are tunable. The calculated results show that the parameters are independent of the amplitude distribution function of the incident beam and the apodization factor of the focusing system. Hence, different kinds of long (about $100\lambda$) chains with sub-wavelength lateral size can be achieved by adjusting the central angular position, the angular width, and the phase difference of two counter-propagating incident beams. Compared with lenses, parabolic mirrors can generate multi-needle fields that lenses cannot. [ABSTRACT FROM AUTHOR]

Optical imaging and focusing through complex samples is a challenge with important applications in many fields. The critical problem is that turbid media such as biological tissue and multimode optical fibers randomly scatter and diffuse light, preventing the formation of diffraction-limited focus and image. We demonstrate with numerical simulations that a multiple scattering can be controlled via a serial optical transmission matrix and a parallel optical transmission matrix in order to obtain an achromatic focus and image at an arbitrary position. Simulation results of three-dimensional achromatic focusing and imaging based on this approach are agreed with the theoretical analysis. A scattering diffuser can be used as functional optical elements for focusing and imaging, moreover it can be manipulated and designed for multifunctional optical elements beyond traditional design. Our method will pave a way for using scattering media as unique optical achromatic elements for various applications, as well as for guiding the light and tailoring the light field. [ABSTRACT FROM AUTHOR]

A novel plasmonic lens consisting of metallic nanoslits with a patterned dielectric substrate is proposed. In the structure, all metallic nanoslits have the same geometrical parameters and interspacing. The phase of incident light is modulated beforehand by the substrate before they bump on the metal slits. The surface plasmon polaritons (SPPs) wave excited by the modulated light will be focused after passing through the slits. Numerical simulation demonstrates that the size of the generated focal spot is very close to half wavelength. The overriding advantage of the proposed structure is that it significantly reduces the difficulty in fabrication while the focusing properties are comparable to the ones with filling materials in the slits.

An easily multiplexed fiber-optic Fabry–Perot interferometer (FPI)-based ultrasonic wave (UW) sensor has been proposed and used to experimentally demonstrate ultrasound wave imaging of seismic-physical models. The sensor probe comprises an interferometer formed by a free-standing gold foil and a cleaved optic-fiber tip. An acoustic focusing lens enhances the sensor's ultrasound sensitivity and assists with capability to multiplex multiple probes. Theoretical modeling of the focusing effect of the acoustic lenses and the ultrasonic responses of the probes are presented and analyzed. We present an experimental demonstration of time-division multiplexing of three fiber-optic Fabry–Perot sensors operating in air to reconstruct the shapes of physical models. The compact sensor has a symmetrical structure so that it presents a good directionality of ultrasound detection, making it valuable as an effective sensor for ultrasound scanning of physical models in air. [ABSTRACT FROM PUBLISHER]

A light beam transmitted through scattering media usually turns into disordered speckles. Fortunately, there are iterative algorithms to focus light transmitted through scattering media using a spatial phase modulator (SPM). In this work, we optimized the conventional iterative algorithms to achieve greater efficiency via theory and simulations. Two independent processes, characterizing the transmission matrix (TM) of the scattering media and calculating the phase compensation of the SPM, are included in our optimized algorithm. While the TM of the scattering media is determined through the interference effect and the light reciprocity principle, the phase compensation of the SPM is calculated using the Rayleigh–Sommerfeld diffraction theory. By addressing these two processes separately, accelerating the process of characterizing the TM of the scattering media, and theoretically calculating the phase compensation of the SPM, our optimized algorithm is helpful for fast imaging through scattering media. [ABSTRACT FROM PUBLISHER]

Optical techniques for information security have drawn a lot of attention in recent years. The development of cryptanalysis strategies will also have a positive promotion for the cryptosystems themselves. However, most of the present publications on cryptanalysis of optical cryptosystems are focusing only on theoretical analysis but not on experimental verification. In this paper, a practical optical cryptanalysis method using wavefront shaping is proposed. Considering that the confusion and diffusion of information in the optical cryptosystem are actually caused by scattering effect of random phase mask(s), the proposed method employs a point source function and the feedback-based optimization algorithm, which can obtain an equivalent key of the optical cryptosystem. In such a way, the confusion and diffusion of the random phase mask(s) in the cryptosystem could be removed successfully. To the best of our knowledge, this is the first time the attack strategy from the perspective of wavefront shaping can be seen, and one of the biggest advantages is that it could get an equivalent key of the cryptosystem without deducing its original keys. [ABSTRACT FROM PUBLISHER]

Far-field focusing with superresolution aided by a proposed metallic plate is investigated in this paper. The proposed near-field structure can be employed to distinguish point-like sources with subwavelength distance and enhance the field strength based on the spoof surface plasmon polaritons theory, and by the simulations of near- and far-field spectrums, it can be found that some near-field evanescent waves carried the superresolution information can be converted to propagation waves and propagate to the far-field. Therefore, with the help of time reversal imaging process, the far-field superresolution focusing is validated by the experiments in laboratory environment. A resolution of $\sim$ $\lambda$/13 is achieved from the last results, which is well beyond the diffraction limit. [ABSTRACT FROM PUBLISHER]

A method of focal-shift resonance with substrate based on two commonly used structures of nano-optical lens is presented with numerical simulations. This resonance exhibits periodicity with the variation of substrate refractive index. Both types of lenses are based on gradual changed nanoslits and analyzed by the extraordinary optical transmission theory. This displacement of focal point can reach the order of microns. A periodic intensity change of focal light is observed between two absorption peaks in metal nanolens. The origin of this focal shift is also discussed with the analysis of the Fresnel number in diffraction. The finite element method is utilized in this paper. [ABSTRACT FROM PUBLISHER]

Focusing technique is the key factor in improving the resolution of projection lithography. In order to achieve high accuracy of focusing on the nanometer level, we present a focusing method based on dual-channel light intensity modulation. Two superposed grating fringes are formed with a phase difference of $\pi/2$. The shift of the wafer can be resolved by the phase variation of corresponding fringes. The conventional focusing method with light intensity modulation is influenced by disturbing effects such as fluctuations of the light intensity and stray light. These adverse factors are overcome by the signal ratio of two channels with the same optical components. The focusing accuracy in our experiments is ±8 nm. [ABSTRACT FROM PUBLISHER]

We experimentally demonstrate apodized focusing subwavelength grating couplers for both the fundamental transverse electric ( TE00) mode and the fundamental transverse magnetic ( TM00 ) mode. A measured insertion loss of 3.2 dB with a 1-dB bandwidth of 36 nm has been obtained for the TE00 mode, and a measured insertion loss of 3.3 dB with a 1-dB bandwidth of 37 nm has been obtained for the TM00 mode. Back reflections of $-$24 dB and $-$21 dB have been obtained for the TE00 and TM00 modes, respectively. [ABSTRACT FROM PUBLISHER]

We propose and experimentally demonstrate a robust mode converter based on liquid crystal-on-silicon (LCOS), which is not placed at the focal plane of the collimating lens. We present theoretical investigations to evaluate the performance of proposed configuration, and our experimental results verify the favorable function of mode-selective conversion with orthogonal linear polarizations, as well as two-mode multiplexing. The robust device configuration with an insertion loss of less than 3 dB, together with cascade capability, can be used for multidimensional fiber-optical transmission system. In particular, in terms of LP 11 mode conversion, the off-focus operation allows the range of position offset for input fiber with graded-index (GRIN) lens, LCOS, and few-mode fiber to be 7.5 mm, 151 \mum , and 40.4 mm, respectively, indicating easy alignment and maintenance. [ABSTRACT FROM PUBLISHER]

Focusing and leveling are two imperative processes to adjust the wafer onto the ideal focal plane of projection lithography tools. Based on moiré fringes formed by particularly designed dual-grating marks, the four-channel focusing and leveling scheme is proposed and demonstrated. These relationships between the tilted amount of wafer, the vertical defocusing amount, and the phase distributions of moiré fringes are deduced. A single-channel experimental setup is constructed to verify the performances of proposed method. Results indicate that the tilted amount and the vertical defocusing amount can be precisely detected with accuracy at 10-4 rad and several nanometers level, respectively, and therefore meet the demand of the high-demanding focusing and leveling processes. [ABSTRACT FROM PUBLISHER]

In this paper, we present a new kind of bifocal kinoform lenses in which the phase distribution is based on the Fibonacci sequence. The focusing properties of these DOEs coined Kinoform Fibonacci lenses (KFLs) are analytically studied and compared with binary-phase Fibonacci lenses (FLs). It is shown that, under monochromatic illumination, a KFL drives most of the incoming light into two single foci, improving in this way the efficiency of the FLs. We have also implemented these lenses with a spatial light modulator. The first images obtained with this type of lenses are presented and evaluated. [ABSTRACT FROM PUBLISHER]