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

A novel temperature optical sensor configuration, based on mode mixing principle, is theoretically discussed. The sensing element consists of a single-mode all-silicon waveguide, followed by a two-modes section, which acts as sensing region, and an output Y branch to separate the two output channels. The fundamental mode coming from input waveguide excites both the fundamental mode and the first higher order mode in the sensing region. The interference between the two propagating modes in the sensing region produces a periodically repeated optical intensity distribution along the propagation axis. It is possible to observe a light steering from one output channel to the other caused by the temperature change of the structure itself. This fact is related to the variation of the material refractive index with the temperature, that is the thermo-optic effect, which implies a variation of the mode effective refractive indices, and, consequently, a phase shift between the modes themselves. In this way, a continuous variation of the power distribution on the two output waveguides as a function of the temperature can be observed. A simultaneous acquisition of both output signals, followed by a simple elaboration, allows obtaining a temperature evaluation independent on light source instability. Moreover, it is possible to design the device in order to obtain the desired initial output power distribution on two channels. This permits to design sensors characterized by greater accuracy, if we use the linear part of the optical transfer function, or larger measure range, if we utilize the whole output excursion.

Temperature optical sensor based on a silicon bimodal Y branch

In the recent years microfluidic technology has affirmed itself as a powerful tool in medical and biological research. Among the different applications, cell manipulation has been widely investigated. Micro-flowcytometers, micromixers, cell sorters and analyzers are only few examples of the developed devices. Various methods for cell manipulation have been proposed, such as hydrodynamic, magnetic, optical, mechanical, and electrical, in this way categorized according to the manipulating force employed. In particular, when cells are manipulated by hydrodynamic effects, there is no needing of applying external forces. This brings to a simplification in the design and fabrication phase, and at the same time undesired effects on the biological sample are limited. In this paper, we will discuss the physics of the relevant hydrodynamic effects in microfluidics, and how they are exploited for cell manipulation.

Microfluidic technology for cell hydrodynamic manipulation

In this chapter, we investigate the possibility to realize a holographic solar concentrator by using a new photopolymeric material as recording medium. Therefore, two different configurations of holographic lenses (lenses with spherical and cylindrical symmetry) are described in terms of both recording process and optical response characterization. Finally, we propose the possibility to use this new photopolymer to realize holographic solar concentrator for space applications.

/pdf/volume-holographic-optical-elements-as-solar-concentrators-3kgc0parrm.pdf

Volume Holographic Optical Elements as Solar Concentrators

Micro-ring resonators have been widely employed, in recent years, as wavelength filters, switches and frequency converters in optical communication circuits, but can also be successfully used as transducing elements in optical sensing and biosensing. Their operation is based on the optical coupling between a ring-shaped waveguide and one or more linear waveguides patterned on a planar surface, typically an input and an output waveguide. When incoming light has a wavelength which satisfies the resonance conditions, it couples into the micro-ring and continuously re-circulates within it. A fraction of this resonant light escapes the micro-ring structure and couples into the output waveguide. The presence of a target analyte over the top surface of the micro-ring (i.e. within the evanescent field) changes the effective refractive index of the mode propagating into the structure, thus causing a shift in resonance wavelength which can be determined by monitoring the spectrum at the output port. Proper functionalization of the micro-ring surface allows to add selectivity to the sensing system and to detect specific interaction between a bioprobe and its proper target (e.g. protein-ligand, DNA-cDNA interactions). We present our preliminary results on the design of micro-ring resonators on silicon-oninsulator substrate, aimed at selective detection of several biomolecules. The design of the structure has been accomplished with the help of FDTD 2D numerical simulations of the distribution of the electromagnetic fields inside the waveguides, the micro-ring and near the micro-ring surface. Furthermore, all the functionalization reactions and the bio/non-bio interfaces have been studied and modelled by means of spectroscopic ellipsometry.

Label-free biosensing by means of optical micro-ring resonator

Early pregnancy diagnosis and monitoring play an important role following embryo transfer in sheep. The aims of the current study were to investigate (i) the pattern of serum progesterone profiles in sheep carrying somatic cell nuclear transfer (SCNT)-derived (clone) pregnancies, and (ii) the frequency of pregnancy loss during development following SCNT embryo transfer. Sheep SCNT embryos were made using standard nuclear transfer techniques. Day 7 embryos were surgically transferred to oestrus-synchronized recipients (n = 27). As a control, normal fertile ewes (n = 12) were bred by natural breeding. Serum was collected from all the ewes on the day of estrus (day 0 sample), 7 days post-estrus (day 7 sample) and 19 days post-estrus (day 19 sample) and every 10 days thereafter until lambing or pregnancy loss occurred. Serum progesterone (P4) was assessed using enzyme immunoassay. Pregnancy was confirmed by ultrasound scanning on day 35 of pregnancy followed by subsequent scanning every 10 days. In control ewes, pregnancy rate on day 35 was 83.3% (10/12), whereas in the ewes that received SCNT embryos, it was 22.2% (6/27; p < 0.05). The day 45 pregnancy rate in the control ewes was 83.3%, whereas in the SCNT embryo recipients it was 11.0% (p < 0.05). Hormone analysis revealed that SCNT embryo recipients exhibited a significantly lower P4 profiles at different time points in pregnancy compared to controls (p < 0.05). This study highlights the use of serum progesterone in combination with ultrasound for the investigation of embryo loss and crucial times during development of normal and SCNT embryos in sheep. Further, the serum P4 levels directly reflect the degree of placental development in these two groups.

Giuseppe Coppola

Papers

Temperature optical sensor based on a silicon bimodal Y branch

Microfluidic technology for cell hydrodynamic manipulation

Volume Holographic Optical Elements as Solar Concentrators

Label-free biosensing by means of optical micro-ring resonator

Early Pregnancy Diagnosis by Serum Progesterone and Ultrasound in Sheep Carrying Somatic Cell Nuclear Transfer-Derived Pregnancies