Optical networks are becoming a reality as the physical layer of high-performance telecommunication networks. The deployment of wavelength-division multiplexing (WDM) technology allows the extended exploitation of installed fibers now facing an increasing traffic capacity demand. Performances of such systems can be degraded by wide variations of the optical channel power following propagation in the network. Therefore a tilt control of optical amplifiers in WDM networks and dynamic channel power regulation and equalisation in cross-connected nodes is necessary. An important tool for the system designer is the variable optical attenuator (VOA). We present the design and the realization of newly developed VOAs using the ASOC technology. This technology refers to the fabrication of integrated optics components in silicon-on-insulator (SOI) material. The device is based on the light absorption by the free-carriers that are injected in the core of a rib waveguide from a p-i-n diode. The devices incorporate horizontally and vertically tapered waveguides for minimum fiber coupling loss. The p-i-n diode for carrier injection into the active region of the rib waveguide was optimised in order to enhance the attenuation. One major advantage of the ASOC technology is the possibility of monolithic integration of many integrated optics devices on one chip. In the light of this the paper illustrates the result of characterisation of multichannel VOAs.

In this article, we present a variety of optical CDMA photonic chips, some with the capability of random programming the CDMA codes in the 1.55um C-band. The devices are composed of two arrayed waveguide gratings with an array of thermo-optical switches in the center, which encode and decode the optical signal for optical CDMA operation. Detailed performance of the device will be discussed.

As the AWG size is reduced our experimental and theoretical work demonstrates that it becomes increasingly difficult to suppress higher order modes and birefringence using ridge dimension alone. In part, it simply becomes difficult to meet the required fabrication tolerances when the ridge dimension approaches the order of a micron. We show that a novel polarization compensator scheme similar to that previously reported for a grating based demultiplexer in InP and consisting of simple shallow etched regions in the combiner sections of an SOI AWG, can eliminate the polarization sensitivity of the device by reducing the initial polarization dispersion of 2.22 nm to 0.04 nm. By combining the polarization compensator with mode filtering using appropriate array waveguide curvature, the shape of the array waveguides is no longer constrained. This allows the size of an AWG device to be scaled down to very small dimensions (e.g. less than a millimeter) and also permits the use of simple fabrication techniques such as wet etching. Our results were obtained on AWG devices based on 1.5 micrometers thick Si-on-insulator waveguides with a typical waveguide array area of a few square millimeters.

A novel light modulator device constructed by integrating nanoscale silicon mirrors with nematic liquid crystals is reported. The mirrors are composed of porous silicon Bragg reflectors, prepared via electrochemical etching of crystalline silicon. By controlling the etching parameters, the average pore size can be easily adjusted to accommodate the liquid crystal molecules. In our device, the pores are perpendicular to the multilayer structure and the pore size is 20 - 50 nm. It is shown that when voltage is applied to this composite device, the liquid crystal molecules re-orient such that their long axis aligns along the electric field. The molecular reorientation changes the refractive index of each layer, and accordingly the reflection of the mirrors is electrically modulated. The fabrication, optical characterization, molecular alignment, and operation of this novel device are discussed.

A multidisciplinary team of end users and suppliers has collaborated to develop a novel yet broadly enabling process for the design, fabrication and assembly of Micro-Opto- Electro-Mechanical Systems (MOEMS). A key goal is to overcome the shortcomings of the polysilicon layer used for fabricating optical components in a conventional surface micromachining process. These shortcomings include the controllability and uniformity of material stress that is a major cause of curvature and deformation in released microstructures. The approach taken by the consortium to overcome this issue is to use the single-crystal-silicon (SCS) device layer of a silicon-on-insulator (SOI) wafer for the primary structural layer. Since optical flatness and mechanical reliability are of utmost importance in the realization of such devices, the use of the silicon device layer is seen as an excellent choice for devices which rely on the optical integrity of the materials used in their construction. A three-layer polysilicon process consisting of two structural layers is integrated on top of the silicon device layer. This add-on process allows for the formation of sliders, hinges, torsional springs, comb drives and other actuating mechanisms for positioning and movement of the optical components. Flip-chip bonding techniques are also being developed for the hybrid integration of edge and surface emitting lasers on the front and back surfaces of the silicon wafer, adding to the functionality and broadly enabling nature of this process. In addition to process development, the MOEMS manufacturing Consortium is extending Micro-Electro-Mechanical Systems (MEMS) modeling and simulation design tools into the optical domain, and using the newly developed infrastructure for fabrication of prototype micro-optical systems in the areas of industrial automation, optical switching for telecommunications and laser printing.

We have developed a novel wafer-scale single-crystalline silicon micromirror bonding process to fabricate optically flat micromirrors on polysilicon surface-micromachined 2D scanners. The electrostatically actuated 2D scanner has a mirror area of 450 micrometers x 450 micrometers and an optical scan angle of +/- ±7.5 degree(s). Compared to micromirrors made with a standard polysilicon surface-micromachining process, the radius of curvature of the micromirror has been improved by 1 50 times from 1.8 cm to 265 cm, with surface roughness < 10 nm. Besides, single-crystalline honeycomb micromirrors derived from silicon on insulator (SOI) have been developed to reduce the mass of the bonded mirror.

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.

We have designed and fabricated waveguide optical modulators using cubic silicon carbide-(3C-SiC)-on-insulator rib waveguides. A refractive index change is induced in the rib via the plasma dispersion effect. These types of devices have potential for relatively high-speed silicon-based photonics compatible with silicon processing technology, as compared to pure silicon. Furthermore, the wide bandgap (2.2 eV) of 3C-SiC makes the devices suitable for use over the visible and near infrared spectrum range as well as the longer communication wavelengths. We have demonstrated waveguiding in 3C-SiC, fabricating the waveguides by ion implantation of oxygen into a silicon carbide layer. We have also established a processing recipe for the SiC wafers which enables fabrication of 3- dimensional devices. The work reported here describes the fabrication of the devices and presents preliminary experimental results for the waveguide losses and the modulation of the refractive index as a function of applied current. An efficient waveguide modulator for a single polarization is reported.

Ultrathin silicon/germanium (SiGe) quantum well (QW) and short-period Si_mGe_n superlattice structures have been grown by molecular beam epitaxy (MBE) on <100$GTR Si substrates. Si/SiGe detectors in the near infrared (IR; 1.3(mu) ) for optical communication and mid-infrared (3(mu) -5(mu) ; 8(mu) -12(mu) ) regime for heat sensing applications have been fabricated and characterized. The SiGe detectors for the mid IR are based on hetero-internal photoemission (HIP) from a highly p-doped SiGe quantum well into an undoped Si layer. These SiGe HIP-heterostructures allow the possibility to tailor the photoresponse and cut-off wavelength for IR-detectors by changing the Ge-content and QW width of the active layers. External quantum efficiencies up to 0.6% at 77K have been achieved from HIP-detectors and detectivities in excess of 8x10¹¹cmHz^0.5/W at 77 Kelvin have been obtained for Si/SiGe multiple quantum well (MQW) detectors. We have also studied nano-scaled, three dimensional Ge islands grown by self-organized Stranski-Krastanov growth. The Ge-islands are deposited in the base of a Si solar cell to increase the quantum efficiency and are investigated by atomic force microscopy (AFM), photoluminescence and photocurrent measurements. They have been grown with varying conditions and exhibit three dimensional growth in a small temperature regime between 500 degree(s)C and 700 degree(s)C for Ge-thicknesses above 4ML.

We report the observation of electroluminescence from intersubband transitions in the valence band of Si/SiGe quantum cascade structures. The samples were grown by molecular beam epitaxy at 350 degree(s)C and reveal good crystal quality as determined by transmission electron microscopy and high resolution x-ray diffraction. The 4-fold quantum cascade structure is repeated 3 times interspersed by two Si spacer layers to reduce the high strain. Electrical contact is provided by the doped back and top contact layers. The electroluminescence of three samples is investigated. The peak energy of 130 meV to 150 meV is found to be close to the calculated value of the intended heavy hole (HH) 2 to HH1 transition of the respective sample. The luminescence signal is TM polarized as expected for intersubband transitions between HH levels. By comparison with a III-V quantum cascade structure the lifetime of the upper state could be determined; it was found that it depends strongly on the design, but it can reach values comparable to III-V quantum cascade structures.

We analyse theoretically the feasibility of a vertical (SiGe/Si)n/Si quantum well structures for enhanced spontaneous emission light emitting diodes. The structure can be grown by selective epitaxy on silicon-on-insulator substrate. A design of a LED emitting at 1.3 micrometers wavelength is carried out.

Photonic crystal planar circuits designed and fabricated in silicon on silicon dioxide are demonstrated. Our structures are based on two-dimensional confinement by photonic crystals in the plane of propagation, and total internal reflection to achieve confinement in the third dimension. These circuits are shown to guide light at 1550 nm around sharp corners where the radius of curvature is similar to the wavelength of light.

The realisation of two-dimensional Si/Si_1-xGe_x/Si strained layer low-loss waveguides (1.7 dB/cm at 1.3micrometers ) is reported. The waveguide structure is grown using selective epitaxy. This fabrication method insures loosened cut-off and critical thickness conditions as demonstrated previously by the room-tem-perature operation of vertical emitting SiGe/Si LED. The main difference from other fabrication methods is the local deposition of the SiGe in a finite stripe region while in the conventional fabrication of rib waveguides the SiGe layer is deposited on an entire wafer and then patterned by reactive ion etching. The relative high amount of Ge (19%) incorpo-rated in selectively grown waveguides, and reduced thickness (0.6micrometers ) of Si cap layer are improvements from the previous reported SiGe/Si waveguides where thick Si cap layers (few microns) and reduced Ge concentrations (<10%) are necessary in order to obtain waveguiding.

Novel Si waveguide MSM photodetector suitable for high speed/high quantum efficiency applications is proposed and demonstrated. Silicides are formed on a silicon-on-insulator (SOI) substrate through metal/Si reaction under heat treatment, in two areas separated by a narrow gap. The silicide sidewalls on the two sides of the narrow gap provide lateral waveguide confinement, and also serve as electrodes. The silicide/Si interface forms a Schottky junction, making the structure a MSM diode. The waveguide structure provides a long optical path length to increase the quantum efficiency at near infrared wavelengths. The distance between electrodes can be changed easily through photolithography, and can be made very small to reduce the transit time between electrodes for high-speed operation. Since the devices are made on SOI substrates, the drift component of the photocurrent can be eliminated, further facilitating high-speed operation. First set of photodetectors was made using PtSi on commercially available SOI substrates with 0.34micrometers Si layer. Initial experiments have demonstrated a responsivity of near 200mA/W at (lambda) equals980 nm for a detector with 486micrometers long electrodes and 2 micrometers gap size. The dark current was on the order of 0.1 nA/micrometers ² at 5V bias.

This paper reports the growth, fabrication and characterization of integrated Ge detectors with rib waveguides based on SOI technology. The MBE Ge diode structures were first grown on different graded buffers on SOI wafers. These structures were then fabricated into individual and integrated diodes with various kinds of rib waveguides. Analysis of the performance of the integrated detectors indicates that Ge detectors with quantum efficiency over 70% can be achieved at 1.55um. Major obstacle for practical applications of these Ge detectors will be discussed.

We present a technology for the integration of high performance near-infrared Ge P-I-N photodetectors on Si for Si microphotonics. High quality Ge epilayers were grown on Si by a two-step ultrahigh-vacuum / chemical-vapor-deposition (UHV/CVD) process. Two-step UHV/CVD allows the epitaxial growth of Ge on Si without islanding. Threading-dislocations in Ge epilayers were reduced by cyclic thermal annealing. The reduction of threading-dislocations can be understood in terms of thermal stress induced dislocation glide and reactions. We found that sessile threading-dislocations are not permanent and can be removed.

We report on the integration of Ge p-i-n heterojunction photodiodes on Si substrates. The crucial role of interface defects at the Ge/Si interface on the performance of photodetectors is analyzed and taken into account in the design of the devices. We have designed and fabricated high performance p-i-n Ge photodiodes for the near infrared. Pure Ge is grown by ultra-high-vacuum CVD followed by a cyclic thermal annealing and ion implantation. Devices are fabricated using standard photolithography. The photodiodes exhibit maximum responsivity of 0.8 A/W at 1.3 micrometers and 0.7 A/W at 1.55 micrometers , reverse dark currents in the 20 mA/cm² range at 1V and response times as short as 520 ps. Our devices are the first p-i-n Ge on Si photodetectors fabricated by CVD and exhibit high performances for a wide range of applications.

Integration of near infrared (NIR) photodetectors on a silicon substrate is a key step for the fabrication of an all silicon based NIR transceiver. To this extent, polycrystalline germanium (poly-Ge) technology is attractive due to the low deposition temperature and cost. Poly-Ge detectors demonstrated broad response, covering the whole NIR spectrum to 1.55 micron, fast, subnanosecond, speed and excellent versatility. In this work we present our recent results on the integration of a poly-Ge photodetector on a SOI silicon waveguide. The use of a waveguide for light in-coupling is appealing for telecom applications where signal is transported on an optical fibre, but, at the same time, it allows to increase detector responsivity. In fact, in this device the light is absorbed into the thin sensitive layer of the poly-Ge/Si heterojunction in a distributed way, during propagation. This releases the strong constraint of the absorption length being smaller than photocarrier collection length typical of normal incidence photodetectors. In the paper, both design issues and experimental results are reported.

Silicon nanocrystals, formed by ion implantation and subsequent thermal annealing, show positive optical gain under intense laser excitation. Gain has been measured by the variable strip length method where the amplified spontaneous emission intensity, which is emitted from the sample edge, is measured as a function of the excitation volume. Exponential increase, line narrowing and directionality of stimulated emission have been measured. In addition, by growing silicon nanocrystals in a quartz substrate, single pass gain in pump and probe transmission experiments has been measured. Material gain values as high as those typically found in III-V semiconductors quantum dots have been measured. We claim that population inversion is realized between the fundamental and the recently identified Si equals O interface state. This model explains the gain observations and could account for the lack of auger saturation, free carrier absorption and size dispersion. Critical issues to obtain sizable gain are (1) high oxide quality, (2) high areal density of silicon nanocrystals, and (3) nanocrystals placed in the core region of a waveguide.

We have recently observed an increase in the visible electroluminescence from a two-junction Si n⁺pn CMOS structure. The device emit visible light in the 450 - 750 nm wavelength region at intensities up to 1 nWmicrometers ^-2 and operate at 8 - 20V and 50 (mu) A - 10mA. The device utilizes the injection of electrons from a slightly forward biased and adjacently positioned pn junction into a second hot-carrier avalanching reverse-biased junction. The observed observation is explained in terms of a physical model that propose that direct interband recombination of low energy (cool) electrons recombine or interact with high energy (hot) carrier valance band holes in the silicon indirect bandgap structure. Although the emission is subsurface at this stage, the luminescence intensity appears to be about 250 times brighter than the luminescent intensity resulting from surface emitting Si pn avalanching junctions. The experimental observations and model predicts that the electrical-to-optical power conversion and quantum efficiencies as associated with present Si CMOS LED's may be increased by several orders of magnitude. The present levels of this Si LED is about three to four orders higher than the low frequency detectability of standard pn silicon detector utilizing the same area on chip.

We report room-temperature electroluminescence at Si bandgap energy from Metal-Oxide-Semiconductor (MOS) tunneling diodes. The ultrathin gate oxide with thickness 1 to approximately 3 nm was grown by rapid thermal oxidation (RTO) to allow significant current to tunnel through. The measured EL efficiency of the MOS tunneling diodes increases with the injection current and could be in the order of 10^-5, which exceeds the limitation imposed by indirect bandgap nature of Si. We also study the temperature dependence of the electroluminescence and photoluminescence. The electroluminescence is much less dependent on temperature than photoluminescence from Si. The applied external field that results in the accumulation of majority carriers at Si/SiO₂ interface in the case of electroluminescence could be the reason for such difference. The involved physics such as optical phonon, interface roughness, localized carriers, and exciton radiative recombination are used to explain the electroluminescence from silicon MOS tunneling diodes.

Room temperature (RT) electroluminescence (EL) was obtained for the first time from Mn enriched Si/SiO₂ structure. Si⁺ or Ar⁺ stimulated knock-on implantation through 20 nm Mn film with the subsequent annealing was used for EL device fabrication. Devices exhibit bright emission band at the 2.06 eV. The position does neither depend on implanted ion dose nor annealing procedure. EL is visible by naked eye even at current density as low as 1.5x10^-6 Acm^-2. Continuous wave external quantum efficiency 1.1x10^-3 and power efficiency 1.5x10^-5 have been achieved.

Photoemissive heterostructures solid films of polymer PMA, Al₂O₃, ZnS on porous silicon surfaces were investigated. Photosensitive and tenzosensitive heterostructures on the base of porous silicon were created by method of optical contact of GaSe plates and heterostructures porous silicon silicon substrate. Photoemission, photosensitive and tenzosensitive properties of heterostructures based on porous silicon were investigated.

A comparative study between theory and experiment is presented for transmission through lossy frequency selective surfaces (FSSs) on silicon in the 2 - 15 micrometer range. Important parameters controlling the resonance shape and location are identified: dipole length, spacing, impedance, and dielectric surroundings. Their separate influence is exhibited. The primary resonance mechanism of FSSs is the resonance of the individual metallic patches. There is no discernable resonance arising from a feed-coupled configuration. The real part of the element's impedance controls the minimum value of transmission, while scarcely affecting its location. Varying the imaginary part shifts the location of resonance, while only slightly changing the minimum value of transmission. With such fine-tuning, it is possible to make a good fit between theory and experiment near the dipole resonance on any sample. A fixed choice of impedance can provide a reasonable fit to all samples fabricated under the same conditions. The dielectric surroundings change the resonance wavelength of the FSS compared to its value in air. The presence of FSS on the substrate increases the absorptivity/emissivity of the surface in a resonant way. Such enhancement is shown for dipole and cross arrays at several wavelengths.