There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

http://ieeexplore.ieee.org/iel5/6354/16969/00781867.pdf

Photonic crystals

Metal interconnections are expected to become the limiting factor for the performance of electronic systems as transistors continue to shrink in size. Replacing them by optical interconnections, at different levels ranging from rack-to-rack down to chip-to-chip and intra-chip interconnections, could provide the low power dissipation, low latencies and high bandwidths that are needed. The implementation of optical interconnections relies on the development of micro-optical devices that are integrated with the microelectronics on chips. Recent demonstrations of silicon low-loss waveguides, light emitters, amplifiers and lasers approach this goal, but a small silicon electro-optic modulator with a size small enough for chip-scale integration has not yet been demonstrated. Here we experimentally demonstrate a high-speed electro-optical modulator in compact silicon structures. The modulator is based on a resonant light-confining structure that enhances the sensitivity of light to small changes in refractive index of the silicon and also enables high-speed operation. The modulator is 12 micrometres in diameter, three orders of magnitude smaller than previously demonstrated. Electro-optic modulators are one of the most critical components in optoelectronic integration, and decreasing their size may enable novel chip architectures.

Micrometre-scale silicon electro-optic modulator

Optical technology is poised to revolutionize short-reach interconnects. The leading candidate technology is silicon photonics, and the workhorse of such an interconnect is the optical modulator. Modulators have been improved dramatically in recent years, with a notable increase in bandwidth from the megahertz to the multigigahertz regime in just over half a decade. However, the demands of optical interconnects are significant, and many questions remain unanswered as to whether silicon can meet the required performance metrics. Minimizing metrics such as the device footprint and energy requirement per bit, while also maximizing bandwidth and modulation depth, is non-trivial. All of this must be achieved within an acceptable thermal tolerance and optical spectral width using CMOS-compatible fabrication processes. This Review discusses the techniques that have been (and will continue to be) used to implement silicon optical modulators, as well as providing an outlook for these devices and the candidate solutions of the future.

/pdf/silicon-optical-modulators-578o7o6kvr.pdf

Silicon optical modulators

Silicon has long been the optimal material for electronics, but it is only relatively recently that it has been considered as a material option for photonics1. One of the key limitations for using silicon as a photonic material has been the relatively low speed of silicon optical modulators compared to those fabricated from III–V semiconductor compounds2,3,4,5,6 and/or electro-optic materials such as lithium niobate7,8,9. To date, the fastest silicon-waveguide-based optical modulator that has been demonstrated experimentally has a modulation frequency of only ∼20 MHz (refs 10, 11), although it has been predicted theoretically that a ∼1-GHz modulation frequency might be achievable in some device structures12,13. Here we describe an approach based on a metal–oxide–semiconductor (MOS) capacitor structure embedded in a silicon waveguide that can produce high-speed optical phase modulation: we demonstrate an all-silicon optical modulator with a modulation bandwidth exceeding 1 GHz. As this technology is compatible with conventional complementary MOS (CMOS) processing, monolithic integration of the silicon modulator with advanced electronics on a single silicon substrate becomes possible.

A high-speed silicon optical modulator based on a metal–oxide–semiconductor capacitor

Digital Holographic Microscopy (DHM) is an optical interferometric technique for not destructive testing of micro-electro-mechanical systems (MEMS). A characterization process based on a no-contact technique allows us to analyze deformations, warping, residual stress, cracks and more other defects of MEMS, without destroy them. The flexibility of this technique allows us to improve novel numerical reconstruction algorithm for the recovery of more information. The post processing of the acquired holograms allows to reduce noise, optical aberrations, defocusing. In particular, the hologram reconstruction process has been modified to obtain Extended Focus Images (EFI). In Digital holographic microscopy, the use of microscopy objectives with high magnifications, reduces the focus depth. This means that for extended object a single reconstructed image with all the details in focus is not possible to obtain. Using a multiple reconstruction process and opportune resizing algorithms a full focused reconstructed images of extended object has been obtained without any mechanical movement. In particular, the advantages of the EFI technique are unique for dynamical characterization by DHM of extended objects, where the techniques based on multiple acquisitions fail. The EFI technique has been applied to obtain a best focused reconstructed image and profile of some micromechanical systems. It is demonstrated that this new approach allows to improve the accuracy in the EFI image when compared to the previous experimental results. Focusing of zones at different quote has been obtained evidencing, shape, crack and deformation impossible to observe otherwise at the same time. Moreover, these technique of reconstruction and analysis can be advantageous in many other fields of application.

Improvement of the reconstruction algorithm for extended focus image of MEMS by digital holography

Recent advances in nanoscience and nanotechnology have led to the development of ultrasensitive plasmonic substrates with a theoretical limit of detection of a single molecule. However, the use of such devices in clinical practice and medicine is still hampered by their low selectivity and poor specificity. Even with the support of statistical data analysis methods, identifying specific biomarkers among hundreds of antagonists using an inherently label-free technology is still challenging. Here we develop a microfluidic device to optimize functionalization of the plasmonic substrate with a cysteine-folic acid complex for selective capture of anti-folic acid in solution. Since the latter is expressed by tumor cells, the device can potentially be used to detect and isolate circulating tumor cells from liquid biopsy samples. The plasmonic substrate is a 2D array of gold nanoparticle clusters fabricated by optical lithography and electrolytic deposition techniques. The system combines the sensitivity of a plasmonic substrate with the selectivity of a ligand recognition scheme to achieve identification of specific biomarkers in complex, low-abundance mixtures. The shape of the microfluidic chamber and the flow rate to be used were designed to functionalize the plasmonic substrate and to deliver biological solutions to the active sites of the device in an optimized and uniform manner. Fluorescence and SERS measurements were conducted on the functionalized samples. The results confirmed good and uniform immobilization and colocalization of receptors on plasmonic structures, enabling effective recognition of the target molecule. 

Optimization of microfluidic functionalization of a plasmonic-based device for selective capture of anti-folic acid in solution

Digital Holographic Microscope has been employed to obtain an accurate characterization of a micro-hotplate for gas sensing applications. The fabrication of these sensors needs different materials, with different properties and different technological processes, which involve high temperature treatments. Consequently, the structure is affected by the presence of residual stresses, appearing in form of undesired bowing of the membrane. Moreover, when the temperature of the sensor increases, a further warpage of the structure is observed. DHM allows to evaluate, with high accuracy, deformations due to the residual stress and how these deformations are affected by thermal loads. In particular, profiles of the structure have been evaluated both in quasi-static condition and the profile variation due to the biasing of the heater resistor has been measured.

Digital holographic microscope for thermal characterization of silicon microhotplates for gas sensor

The realization of on-chip optical interconnects requires the integration of active micro-optical devices with
microelectronics. However, it is not clear yet how silicon photonics could be integrated within CMOS chips. In this
context the non-crystalline forms of silicon, such as laser-annealed polycrystalline and hydrogenated amorphous silicon
(a-Si:H), can deserve some advantages as they can be included almost harmlessly everywhere in a CMOS typical run-sheet,
yielding low-cost and flexible fabrication. In particular, a-Si:H can be deposited using the CMOS-compatible low
temperature plasma enhanced chemical vapour deposition (PECVD) technique, which brings clear advantages
particularly for a back-end photonic integrated circuit (PIC) integration. However, till now a-Si:H has been mainly
considered for the objective of passive optical elements within a photonic layer at λ=1.55 μm. Only a small number of
examples have been reported, in fact, on waveguide integrated active devices. In this paper we detail about an effective
refractive index variation obtained through an electrically induced carrier depletion in an as-deposited a-Si:H-based p-i-n
waveguiding device. For this device switch-on and switch-off times of ~2 ns were measured allowing a modulation rate
higher than 150 MHz.yy

CMOS-compatible electro-optical Mach-Zehnder modulator based on the amorphous silicon technology

The design of an amorphous silicon-based Mach-Zehnder electro-optic modulator including two guiding p-i-n structures integrated inside a two-dimensional (2-D) photonic crystal (PhC) working at 1.55 μm, is reported. Electrically induced free carrier dispersion effect in this photonic material with a very cost-effective technology, is investigated for modulation. Our numerical analysis, performed by a time-domain (FDTD)-based software, proves that the voltage-length product can be remarkably reduced by taking advantage of both the strong PhC confinement and the wide refractive index tunability of amorphous silicon.

Giuseppe Coppola

Papers

Improvement of the reconstruction algorithm for extended focus image of MEMS by digital holography

Optimization of microfluidic functionalization of a plasmonic-based device for selective capture of anti-folic acid in solution

Digital holographic microscope for thermal characterization of silicon microhotplates for gas sensor

CMOS-compatible electro-optical Mach-Zehnder modulator based on the amorphous silicon technology

Design of amorphous silicon photonic crystal-based M-Z modulator operating at 1.55 μm