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

We describe a new configuration for an optoelectronic router integrated in a silicon-on-silicon waveguide structure. The device is based on the mode-mixing principle together with the injection-induced phase shift. The structure is composed by a single mode input optical channel waveguide, a two-mode active region and an output Y branch to separate the two output channels. The active region is designed to allow a (pi) shift between the two modes that it supports, by means of a Bipolar Mode Field Effect Transistor, which injects and controls the free carrier plasma inside the active region. By doing so it is possible to steer light from one output channel to the other, when switching from the OFF state to the ON state. The numerical device simulator MEDICI has been used for evaluation of the electrical performances of the device, while different numerical analyses have been exploited to understand its optical behavior. These simulations show optical losses of about 3dB/cm, an overall crosstalk of -10.5 dB and a maximum switching frequency of about 200 MHz.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Electrically controlled optoelectronic Y-switch based on a BMFET device

A novel type of high-sensitivity Surface Plasmon Resonance (SPR)-based refractive index sensor is demonstrated, with a Kretschmann prism-coupling setup enclosed in an optical cavity and interrogated by a telecom wavelength laser source. Two thin layers of Au and SiO2 are deposited on the prism (SPR chip). Analogously to the sensors commonly referred as “Intensity modulated SPR sensors”, in the presented setup refractive index variations of the sample induce a shift of the SPR central wavelength, which in turn leads to a variation of the SPR chip reflectivity. Since the SPR chip is one of the cavity mirrors, the corresponding loss can be sensitively determined by measuring the internal photon lifetime by a cavity ring-down technique [1]. The interrogating laser is periodically shut down and the subsequent exponential decay of the on-resonance transmitted intensity is recorded to retrieve the ring-down time. In order to improve the repetability of the measurement and allow for long-term averaging, the laser is frequency locked to a cavity resonance by means of the Pound-Drever-Hall scheme[2]. The setup is sketched in fig.1.

Optical cavity-enhanced surface plasmon resonance refractive index sensing

Observing at a microscope object having a three-dimensional (3D) complex shape only a portion of it appears in focus since essentially a single image plane is imaged. Up to now only two approaches have been developed to obtain an extended focused image (EFI). An EFI shows all details of the object in focus. Both methods having severe limitations since one requires mechanical scanning while the other needs specially designed optics. We demonstrate that an EFI image of an object can be obtained by means of digital holography (DH) in a microscope configuration overcoming the mentioned limitations. The novelty of the proposed approach lies in the fact that it is possible to build an EFI by exploiting the unique feature of DH in extracting all the information content stored in hologram (phase and amplitude).

Coherent imaging by digital holographic microscopy: focusing capabilities and depth of focus in the reconstructed images

Flow focusing is a fundamental prior step in order to sort, analyze and detect particles or cells. In traditional flow-cytometry flow focusing is achieved by hydrodynamic focusing. An additional sheath fluid is used in order to confine the sample one. The main drawback of the method is the need of at least two inlets, one for the sheath and one for the sample fluid. This adds a big limitation: it is difficult to realize a device with multiple parallel micro-channels, and therefore exploit one of the main advantages introduced by microfluidic technology. As a matter of fact, the possibility of having more channels working simultaneously helps with the clogging problems, and at the same time improves the throughput. In this work we propose a parallelized microfluidic device that allows getting hydrodynamic focusing by needing of only one inlet. This is achieved by introducing a cross-filter region at each micro-channel.

Self-hydrodynamic focusing in a parallel microfluidic device

The realization of a novel microfluidic device for inducing cell rotation is reported. It exploits the hydrodynamic forces acting on cells moving in a flow characterized by a parabolic velocity profile. The device operation was tested on yeast cells, since they have an asymmetrical shape due to a bud formation during the duplication phase. Image processing analysis of the experimental data allowed to precisely evaluate the cell angular orientation and rotation rate.

Giuseppe Coppola

Papers

Electrically controlled optoelectronic Y-switch based on a BMFET device

Optical cavity-enhanced surface plasmon resonance refractive index sensing

Coherent imaging by digital holographic microscopy: focusing capabilities and depth of focus in the reconstructed images

Self-hydrodynamic focusing in a parallel microfluidic device

Inducing cell rotation in a microfluidic device by hydrodynamic forces