Multiplication and division of the orbital angular momentum of light with diffractive transformation optics

We present a method to efficiently multiply or divide the orbital angular momentum (OAM) of light beams using a sequence of two optical elements. The key-element is represented by an optical transformation mapping the azimuthal phase gradient of the input OAM beam onto a circular sector. By combining multiple circular-sector transformations into a single optical element, it is possible to perform the multiplication of the value of the input OAM state by splitting and mapping the phase onto complementary circular sectors. Conversely, by combining multiple inverse transformations, the division of the initial OAM value is achievable, by mapping distinct complementary circular sectors of the input beam into an equal number of circular phase gradients. The optical elements have been fabricated in the form of phase-only diffractive optics with high-resolution electron-beam lithography. Optical tests confirm the capability of the multiplier optics to perform integer multiplication of the input OAM, while the designed dividers are demonstrated to correctly split up the input beam into a complementary set of OAM beams. These elements can find applications for the multiplicative generation of higher-order OAM modes, optical information processing based on OAM-beams transmission, and optical routing/switching in telecom.Comment: 28 pages, 10 figure

Total angular momentum sorting in the telecom infrared with silicon Pancharatnam-Berry transformation optics

Parallel sorting of orbital angular momentum (OAM) and polarization has recently acquired paramount importance and interest in a wide range of fields ranging from telecommunications to high-dimensional quantum cryptography. Due to their inherently polarization-sensitive optical response, optical elements acting on the geometric phase prove to be useful for processing structured light beams with orthogonal polarization states by means of a single optical platform. In this work, we present the design, fabrication and test of a Pancharatnam-Berry optical element in silicon implementing a log-pol optical transformation at 1310 nm for the realization of an OAM sorter based on the conformal mapping between angular and linear momentum states. The metasurface is realized in the form of continuously-variant subwavelength gratings, providing high-resolution in the definition of the phase pattern. A hybrid device is fabricated assembling the metasurface for the geometric phase control with multi-level diffractive optics for the polarization-independent manipulation of the dynamic phase. The optical characterization confirms the capability to sort orbital angular momentum and circular polarization at the same time.Comment: 15 pages, 10 figure

Non-paraxial design and fabrication of a compact OAM sorter in the telecom infrared

A novel optical device is designed and fabricated in order to overcome the limits of the traditional sorter based on log-pol optical transformation for the demultiplexing of optical beams carrying orbital angular momentum (OAM). The proposed configuration simplifies the alignment procedure and significantly improves the compactness and miniaturization level of the optical architecture. Since the device requires to operate beyond the paraxial approximation, a rigorous formulation of transformation optics in the non-paraxial regime has been developed and applied. The sample has been fabricated as 256-level phase-only diffractive optics with high-resolution electron-beam lithography, and tested for the demultiplexing of OAM beams at the telecom wavelength of 1310 nm. The designed sorter can find promising applications in next-generation optical platforms for mode-division multiplexing based on OAM modes both for free-space and multi-mode fiber transmission.Comment: 12 pages, 8 figure

Test of mode-division multiplexing and demultiplexing in free-space with diffractive transformation optics

In recent years, mode-division multiplexing (MDM) has been proposed as a promising solution in order to increase the information capacity of optical networks both in free-space and in optical fiber transmission. Here we present the design, fabrication and test of diffractive optical elements for mode-division multiplexing based on optical transformations in the visible range. Diffractive optics have been fabricated by means of 3D high-resolution electron beam lithography on polymethylmethacrylate resist layer spun over a glass substrate. The same optical sequence was exploited both for input-mode multiplexing and for mode sorting after free-space propagation. Their high miniaturization level and efficiency make these optical devices ideal for integration into next-generation platforms for mode-division (de)multiplexing in telecom applications.Comment: 4 pages, 1 extended references page, 6 figures. arXiv admin note: substantial text overlap with arXiv:1610.0744

Design, fabrication, and test of bi-functional metalenses for the spin-dependent OAM shift of optical vortices

The ability to encode different operations into a single miniaturized optical device is required to reduce the complexity and size of optical paths for light manipulation, which usually employs dynamic optical components, interferometric setups, and/or multiple bulky elements in cascade. A very efficient solution is provided by metalenses, which are flat optical elements able to generate and manipulate structured light beams in a compact and efficient way, offering a powerful and attractive tool in many fields, such as life science and telecommunications. In this work, we present the design and test of transmission dielectric bi-functional metalenses that exploit both the dynamic and the geometric phases, to enable the spin-controlled manipulation of different focused orbital angular momentum (OAM) beams, depending on the circularly polarized state in input. In detail, we provide numerical algorithms for the design and simulation of the meta-optics in the telecom infrared, the fabrication processes, and the optical characterization under different impinging polarized optical vortices. This solution provides new integrated flat optics for applications in imaging, optical tweezing and trapping, optical computation, and high-capacity telecommunication and encryption

Design, fabrication and characterization of Computer Generated Holograms for anti-counterfeiting applications using OAM beams as light decoders

In this paper, we present the design, fabrication and optical characterization of computer-generated holograms (CGH) encoding information for light beams carrying orbital angular momentum (OAM). Through the use of a numerical code, based on an iterative Fourier transform algorithm, a phase-only diffractive optical element (PH-DOE) specifically designed for OAM illumination has been computed, fabricated and tested. In order to shape the incident beam into a helicoidal phase profile and generate light carrying phase singularities, a method based on transmission through high-order spiral phase plates (SPPs) has been used. The phase pattern of the designed holographic DOEs has been fabricated using high-resolution Electron-Beam Lithography (EBL) over glass substrates coated with a positive photoresist layer (polymethylmethacrylate). To the best of our knowledge, the present study is the first attempt, in a comprehensive work, to design, fabricate and characterize computer-generated holograms encoding information for structured light carrying OAM and phase singularities. These optical devices appear promising as high-security optical elements for anti-counterfeiting applications.Comment: 24 pages, 9 figures. Supplementary info: 8 pages, 3 figure

Plasmonic Gratings for Sensing Devices

In last decades surface plasmon resonance has known an increasing interest in the realization of miniaturized devices for label-free sensing applications. The research in the direction of such plasmonic sensors with innovative performance in sensitivity and resolution opened to a wide range of unexpected physical phenomena. This work is aimed at understanding and modeling the physical principles of plasmonic platforms which support the exploitation of propagating plasmon modes for sensing purposes. Surface plasmon polaritons excitation and propagation on metallic gratings have been deeply studied and fully analyzed with theoretical models, numerical simulations and optical characterizations of fabricated samples. In particular the physics underlying azimuthal rotation of these nanostructures and the polarization role in this configuration have been theoretically and experimentally examined. The rotated configurations revealed considerable benefits in sensitivity and this improvement has been demonstrated by analyzing the optical response to surface functionalization and liquid solutions flowing through an embodied microfluidic cell. The exploitation of this plasmonic phenomenon in the conical mounting led to the design and realization of a promising setup for a new class of compact and innovative grating-based sensors. The different approaches, modeling – numerical – experimental, through which the problem has been examined, provided an exhaustive investigation into the physics of grating-coupled surface plasmon resonance and its innovative and original applications for advanced sensing devices.Negli ultimi decenni la risonanza plasmonica di superficie ha conosciuto un crescente interesse nella realizzazione di dispositivi minaturizzati per applicazioni sensoristiche label-free. La ricerca nella direzione di sensori plasmonici con prestazioni innovative in sensibilita’ e risoluzione ha aperto ad un vasto panorama di inattesi fenomeni fisici. Questo lavoro di tesi ha l’obbiettivo di capire e analizzare i principi fisici su cui si basano i supporti plasmonici che sfruttano l’eccitazione di onde di superficie per fini sensoristici. L’eccitazione e la propagazione di plasmoni polaritoni di superficie su reticoli metallici sono state studiate e analizzate a fondo con modelli teorici, simulazioni numeriche e caratterizzazioni ottiche di campioni nanofabbricati. Nello specifico la fisica della rotazione azimutale di queste nanostrutture e il ruolo della polarizzazione in questa configurazione sono state esaminate con strumenti sia teorici che sperimentali. La rotazione del reticolo plasmonico ha rivelato considerevoli benefici in sensibilita’ e questo effetto e’ stato testato e dimostrato analizzando la risposta ottica a funzionalizzazioni di superficie e tramite l’analisi di soluzioni liquide flussate attraverso una cella microfluidica integrata. L’applicazione di questo fenomeno plasmonico ha portato all’individuazione di una configurazione promettente per una nuova classe di sensori a base plasmonica compatti e innovativi. I differenti approcci, modellistico –numerico – sperimentale, con cui il problema e’ stato affrontato, hanno fornito un’analisi completa della fisica della risonanza plasmonica di superficie con reticoli metallici e delle sue innovative applicazioni per dispositivi sensoristici avanzati

Grating-coupled surface plasmon resonance in conical mounting with polarization modulation

A grating-coupled surface plasmon resonance (GCSPR) technique based on polarization modulation in conical mounting is presented. A metallic grating is azimuthally rotated to support double-surface plasmon polariton excitation and exploit the consequent sensitivity enhancement. Corresponding to the resonance polar angle, a polarization scan of incident light is performed, and reflectivity data are collected before and after functionalization with a dodecanethiol self-assembled monolayer. The output signal exhibits a harmonic dependence on polarization, and the phase term is used as a parameter for sensing. This technique offers the possibility of designing extremely compact, fast, and cheap high-resolution plasmonic sensors based on GCSPR. (C) 2012 Optical Society of Americ