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

Recent developments in solid-state image sensors and digital computers have made it possible to directly record holograms by Charge Coupled Device (CCD) camera and numerical reconstruction of the object wave front by computer. Digital holograms recorded with a CCD array are numerically reconstructed in amplitude and phase through calculation of the Fresnel-Kirchhoff integral. Two methods are usually adopted to reconstruct digital holograms called Fresnel Transformation Method (FTM) and the Convolution Method (CM). In FTM, the reconstruction pixel increases with the reconstruction distance so that the size of image, in terms of number of pixels, is reduced for longer distances, limiting the resolution of amplitude and phase reconstruction. In CM, by contrast, the reconstruction pixel does not change, but remains equal to the pixel size of recording array. The CM is more appropriate for reconstruction at small distances whereas the FTM is useful for longer distances according to the paraxial approximation necessary to apply it. The flexibility offered by the reconstruction process in Digital Holography allows exploitation of new possibilities of application in different fields. Through the reconstruction process we will show that it is possible to control image parameters as focus distance, image size and image resolution. Those newly explored potentialities open further novel prospective of application of Digital Holography in single and multi-wavelengths operation either for display and metrological applications. We demonstrate the concept of controlling parameters in image reconstruction of digital holograms in some real situations for inspecting silicon MEMS structures.

Controlling several image parameters in the digital holographic reconstruction process

A direct laser writing process has been exploited to fabricate a high order Bragg grating on the surface of a porous
silicon slab waveguide. The transmission spectrum of the structure, characterized by a pitch of 10 µm, has been
investigated by end-fire coupling on exposure to vapor substances of environmental interest. The analyte molecules
substitute the air into the silicon pores, due to the capillary condensation phenomenon, and the transmitted spectrum of
the grating shifts towards higher wavelengths. The experimental results have been compared with the theoretical
calculations obtained by using the transfer matrix method together with the slab waveguide modal calculation.

Optical sensing of chemicals by a porous silicon Bragg grating waveguide

This study was undertaken to provide cytogenetic information about onset and sequence of RBA-band replication on the inactive X-chromosomes of cattle, river buffalo and goat. Blood cultures were synchronized overnight with thymidine after 48 hours of growth. The cell block was released with fresh medium and the cells allowed to grow in the presence of BrdU and H33258 for 1, 2, 4, 6, 8, 10, 12 and 14 hours, including 20 minutes colcemide. Results show that: (a) the onset of RBA-banding replication was 12 hours before mitosis in cattle and river buffalo, 14 hours in the goat; (b) the replication process was still ‘on’ in cattle and river buffalo one hour before mitosis, whereas it was ‘off’ in the goat; consequently the length of the G2 phase was less than one hour in cattle and river buffalo and one hour or slightly longer in the goat; (c) the first band undergoing replication was identified as band Xq31 in cattle, homologous to band Xq34-36 in river buffalo and Xq24 in the goat; (d) the second replicating band was the Xp22 in cattle, homologous to band Xq21 in river buffalo and Xq34 in the goat, respectively; (e) the sequence of RBA-band replication was quite similar between cattle and river buffalo, but reversed in the goat, due to the wide chromosomal rearrangements which differentiated the X-chromosome of Caprinae from that of Bovinae.

Onset and sequence of RBA-band replication on the inactive X-chromosomes of cattle (Bos taurus L.), river buffalo (Bubalus bubalis L.) and goat (Capra hircus L.).

In this work we have investigated resonant cavity enhanced (RCE) photodetectors (PDs), exploiting the Internal Photoemission Effect (IPE) through a Single Layer Graphene (SLG) replacing metals in the Silicon (Si) Schottky junctions, operating at 1550 nm. The SLG/Si Schottky junction is incorporated into a Fabry-Perot (F-P) optical microcavity in order to enhance both the graphene absorption and the responsivity. These devices are provided of high spectral selectivity at the resonance wavelength which can be suitably tuned by changing the length of the cavity. We get a wavelength-dependent photoresponse with external responsivity ∼20 mA/W in a planar F-P microcavity with finesse of 5.4. In addition, in order to increase the finesse of the cavity, and consequently its responsivity, a new device where the SLG has placed in the middle of a Si-based F-P microcavity has been proposed and theoretically investigated. We have demonstrated that, in a properly designed device, a SLG optical absorption, responsivity and finesse of 100%, 0.43 A/W and 172 can be obtained, respectively. Unfortunately, the estimated bandwidth is low due to the planarity of the structure where both Ohmic (AI) and Schottky (SLG) contacts are placed in the same plane. In order to improve the PD bandwidth, we have fabricated and characterized a prototype of a vertical RCE SLG/Si Schottky PD where two contacts are both placed at the edges of a high-finesse 200nm-thick Si-based microcavity. Thanks to this innovative structure an increase of the responsivity-bandwidth product is expected. The insights included in this work can open the path for developing of a new family of high-performance photodetectors that can find application in silicon photonics.

Giuseppe Coppola

Papers

Controlling several image parameters in the digital holographic reconstruction process

Optical sensing of chemicals by a porous silicon Bragg grating waveguide

Onset and sequence of RBA-band replication on the inactive X-chromosomes of cattle (Bos taurus L.), river buffalo (Bubalus bubalis L.) and goat (Capra hircus L.).

Silicon Meet Graphene for a New Family of Near-Infrared Resonant Cavity Enhanced Photodetectors

Identification of chromosome abnormalities in boars with reduced litter size