Optical Information Technology

We use the pump and probe beam and the luminescence spectroscopy to study the nonlinear response of GaAs/(AlGa)As heterostructures to quasistationary excitation conditions. The carrier induced energetic shift of the 1hh-exciton as a function of the quantum well width shows a dimensional dependence of the carrier screening properties. This shift gives a rather good criterion to decide if a system behaves more 2D or 3D like. The high excitation regime is dominated by electron-hole plasma features. Many particle effects lead to a renormalization of the fundamental bandgap. This effect is essential for understanding the physics of III–V semiconductor lasers. The carrier density and the reduced bandgap are determined via systematic evaluation of both gain and luminescence spectra. The observed behaviour can be described by a strict 2D theory using effective exciton parameters in order to account for the finite well widths of the structures. The study of the higher sub-bands reveals that both, exciton bleaching and sub-band renormalization are mainly due to direct occupation of the specific sub-band while intersub-band effects are considerably smaller. By coating the two sides of a 50×l00Å multiple quantum well with semitransparent Cr-Au electrodes we are able to control the energetic position of the 1hh-exciton as a function of the applied electric field and of the incoming light power. Several structures to optimize this effect in order to build an electrooptical switch or modulator are discussed.

The optical properties of semiconductors, for example the absorption coefficient and the refractive index, can be controlled by applying external electric fields and/or currents to the structure. This is technologically important for devices, such as electrically controlled intensity and phase modulators, directional couplers and devices for wavelength multiplexed systems.

We demonstrate a novel, efficient and fast vertical nin AlGaAs/GaAs multiple quantum well (MQW) microresonator grown by molecular beam epitaxy (MBE). The electrical current of this hybrid device is kept low by using undoped AlGaAs barrier layers cladding the active MQW material. For high-speed operation, we propose a novel traveling-wave modulator with a special coplanar metallic waveguide structure. Electrooptical modulation is studied using a Ti:Sapphire laser pumped by an argon ion laser.

In optical computing it is often difficult to place a device concept in a system context. The hardware of digital computers is based on logic and memory devices. It is within this context that work on GaAs etalons has been historically pursued [1] and the demonstration of transistor action and bistability [2] have been important events. The systems aspect of optical computing has meantime broken new ground and we need figures of merit for such devices that reveal new avenues for exploiting them. The three figures of merit which we discuss are the gain-bandwidth product (GBWP), the time-bandwidth product (TB), and the space-bandwidth product (SBWP). A section is devoted to each.

Synthetic diffractive optics has proved to be an efficient method of realizing the optical interconnections needed in modern digital optical computers [1]. The design and operation of periodic holographic diffractive elements has been widely studied on the basis of scalar (Fraunhofer and Fresnel) diffraction theory [2]. These theories are applicable if the smallest transverse features of the grating structure at least an order of magnitude larger than the wavelength of light. However, the trend towards miniaturization and integration of optical circuits calls for more compact interconnects, which no longer obey the approximate scalar theories. In particular, Fourier optics has been shown to fail when the characteristic feature sizes are comparable to the wavelength of light [3,4]. The analysis and design of gratings operating in this region, which we call the resonance domain, must be based on the electromagnetic vector diffraction theory. Several numerical methods have been proposed and applied to solve the rigorous diffraction problem [5,6,7].

A technique for the reduction of the peak intensity of noise which arises from Fourier plane array generators is introduced. This is achieved by designing the array generator as a quasi-periodic computer generated hologram. In this way an even distribution of noise is obtained while the required diffraction orders are left as tightly sampled spots. Using the method of generalised error diffusion several different quasi-periodic 4 × 4 array generators are designed. A 14.5 dB reduction in peak noise is obtained for a hologram with 8 × 8 quasi-periods. Diffraction efficiency and array uniformity are not adversely affected by this technique.

A miniaturization of refractive optical elements is interesting for various applications including 3D-integrated optical systems. In these concepts the typical functions of microlenses like image formation and collimation of light are required. In most fabrication techniques for microlenses there is a trade off between the lens size and the achievable range of focal length. A new fabrication method for microlenses with high relative apertures over a wide range of diameter sizes in polymethyl methacrylate (PMMA) by irradiation with a high energy proton beam and diffusion of monomer vapor in the irradiated domains is reported in [1].

3D-microoptic systems can be realized by a thermal imprinting processes in Poly-MethylMethAcrylate (PMMA). Hereby each microoptic component is realized by thermally imprinting a metal master into the substrate. This process, though being simple, provides high quality components and allows a mass production. Experimental results are demonstrated for an interconnection scheme.

From the many studies on invariant pattern recognition, the coauthors have selected single harmonic filters [1,2]. A real-time optical implementation of such filters was demonstrated on photographic input images [3], but its 10-F length yielded a poor signal-to-noise ratio, making it unusable with the limited dynamic range and flatness of a light valve for real-time video input. For real applications, the other difficulty is the usual requirement of all invariances simultaneously: scale and projection can be processed in a single filter [1], but a pre-processing is necessary for the rotation before any object recognition, since the projection and scale invariant filter do not work on rotated objects.

Both computer-generated and optically-recorded holograms play important roles in the current generation of optical computing demonstrator systems. This paper reviews the various types of holographic interconnect components that have been used in a series of optical parallel digital processing experiments at Heriot-Watt University. Both high-level and low-level space-invariant fan-out elements are covered, along with components for space-variant interconnect systems.

Since the recognition of the importance of optical array illuminators [1] in the realization of parallel digital optical processors, a wide variety of optical components have been designed and demonstrated that convert a single laser beam into a regularly-spaced array of M × N equal-intensity light spots. Space-variant array illuminators, which form the spot array in a Fresnel plane or an image plane of the aperture are in general easy to design and fabricate, but they require uniform plane-wave illumination that is difficult to provide. Space-invariant array illuminators generate the spot array in the Fourier plane and are therefore rather immune to the exact shape of the incident beam, but they are more difficult to design and require tight fabrication tolerances.

The prospect of wide bandgap light emitting diodes (LEDs) and lasers has been transformed by recent advances in p-doping in materials such as ZnSe grown by molecular beam epitaxy. We report the growth of p-n ZnSe junctions on GaAs substrates using iodine as the n-type dopant and nitrogen from a plasma discharge source as the p-type dopant. LEDs have been fabricated using a gold contact to the p-type layer and blue CW emission has been observed under forward bias. Stripe geometry laser structures have been fabricated and blue stimulated emission has been observed for the first time at low temperatures.

Electronic dispersive optical nonlinearities of CdS platelets are investigated by wave mixing and pump-probe experiments. These nonlinearities enabled the realization of a picosecond single-wavelength Fabry-Pérot type logical gate that exploits polarisation dichroism.

Coherent nonlinear resonances due to extended and localized excitons in CdSe and CdSe _x S_1-x have been investigated by picosecond time resolved degenerate four-wave mixing and differential transmission experiments. Large nonlinear coefficients (χ(3) ≥ 10 cm2/V2) are found, with coherence times in the picosecond range. Results on picosecond optical switching in CdSe are presented and discussed.

The nonlinear spectra of absorption near the band edge are calculated for quantum well wires with up to three subbands. The calculations take into account phase space filling, plasma screening and band gap renormalization due to an optically excited electron-hole plasma. Large optical nonlinearities are obtained around the exciton ground state mainly due to state-filling by the optically excited thermal electron-hole plasma, while the plasma screening effects are found to have relatively little influence. For all plasma densities n (including n = 0), the free-carrier transition spectra differ strongly from those calculated with Coulomb interaction.

We present results concerning optical properties of II–VI semiconductor quantum dots. In pump and probe beam experiments at room and helium temperature we found no holeburning but a strong bleaching of both maxima in the absorption spectrum up to 75% of the optical density under a pump intensity up to 60 MW/cm2. This bleaching is nearly independent of the pump energy and we connect the resulting small energetic shift with the inhomogeneous broadening due to the size distribution of the crystallite. For both temperatures the halfwidth of the bleaching peak is comparable, which suggests a strong coupling of the excited states to the lattice. Corresponding results were obtained also from linear luminescence measurements. From both we calculated a Huang-Rhys factor from 1 to 2 for the system. Additionally we investigated the samples in a self diffraction experiment at room temperature. The efficiency spectrum is a very broad band corresponding to the broad structures in the absorption spectra of the sample with a halfwidth of about 20 nm. From these results we calculated the effective χ_eff(3) about 1 × 10-9 esu for the samples.

Parallel architectures for cellular automata are adapted to optical computing. Electronic implementations have the interconnection as the limiting factor. Our purpose is to couple the performance of electronics (which offers nonlinear components with large-scale integration and high speed) with the performance of optics (which allows the implementation of dense and rapid interconnections, in particular for the input and output), in a compact optoelectronic cellular automaton setup. The specific case that we selected for the purpose of illustration is the “Lattice-Gas” [1] automaton.

We have demonstrated a fully functional optical adder based on systolic arrays and symbolic substitution (SSL). This application shows the feasibility of our optical design concepts based on incoherent data processing systems with optoelectronic threshold amplifier arrays. The optical setup applies four SSL rules in parallel to a 162 pixel data plane. Low hardware effort with integrable modules has been achieved.

The aim of the present work is to give an overview of existing algorithms for parallel addition and to estimate the performance of optical implementations based on self-electro-optic effect devices (SEEDs). The main conclusion is that more processing capabilities in the active elements (“smart pixels”) or new design principles are required in order to reduce the complexity of digital optical arithmetic units.

Optical interconnects are already well established in the field of long distance communication. In this paper we present a board-to-board interconnect system. The interconnect distance is in the range of some centimeters to some meters. We present an optoelectronic interconnect comprising a 2-D emitter array, a fibre bundle and a monolithic custom-designed receiver array (opto-ASIC) for application as a link between CPUs and shared memories within a multiprocessor system. Each channel has a data rate of 10 MBit/s (similar to the system clock rate), which yields with, 128 parallel channels, an overall data throughput of more than 1 GBit/s.

This paper reviews briefly the Heriot-Watt activity over the past decade on the design and construction of all-optical computational circuits, leading to the present demonstrator Optical Cellular Logic Image Processors (O-CLIP).

A cellular logic image processor was designed, constructed and successfully operated by interconnecting two symmetric self electro-optic effect device (S-SEED) arrays. This paper outlines some of the design issues associated with the implementation of a free-space digital optical system. It is argued that pixellated devices are more desirable than other devices and that if differential data representation is used, high contrast is not required.

Silicon on sapphire (SOS) has been studied at IEF for many years as a nonlinear waveguide at λ = 1.06/μm. Resonant excitation of guided waves in submicron undoped SOS films can be obtained with a Nd:YAG laser beam for precise values of the incidence angle by means of a submicron period grating coupler etched in silicon. Sharp and contrasted angular resonances can be observed near the fundamental TE_o mode excitation on either the reflected or transmitted beam. The wavelength being slightly smaller than the silicon absorption edge, fast optical and electrical switchings have been observed and studied at IEF in the nonlinear pulsed regime (20 ns and 200 ps pulse durations). These switchings can be useful for either optical or electrical pulse shaping [1]; they result, at first from electronic nonlinearities induced by a carrier density in excess of the order of 1018 cm-3 due to high optical excitation and later (with 20 ns pulses) from the competition between these effects and thermal nonlinearities of opposite sign due to the fast relaxation (lifetime < 1 ns) of the electron/hole pairs in excess.

In this contribution we will focus on thermally induced dispersive and absorptive optical nonlinearities in II–VI semiconductors. We demonstrate an optical oscillator by introducing the sample into a hybrid ring resonator with long round trip time. In the case of a bistable input-output characteristic (IOC) of the nonlinear element (we use an interference filter in reflection consisting of a glass matrix doped with CdSSe) we get strong mode locking to the resonator round trip time with a complex mode structure including Farey tree [1,2] transitions and mode coexistence. For a barely bistable IOC (realized with a CdS single crystal) we find a completely different kind of dynamics. Relaxation oscillations become important and stabilize the system to prevent a transition to chaos. Further investigations are concerned with the electrooptic bistability in ZnSe single crystals. By applying an electric field perpendicular to the light beam we observe changes of the optical properties that are due to the temperature change and to the Franz-Keldysh effect.

With the aim of realizing a temperature sensor based on thermally induced optical bistability that can be operated by a conventional diode laser, we investigated the absorptive behaviour of GaAs/(AlGa)As multiple quantum well structures. Even at room temperature, these structures exhibit a steep excitonic absorption edge in the near infrared spectral region. With rising temperature, this absorption edge shifts strongly enough to the red to lead to a sharp increase in the absorption of suitable photon energies below the absorption edge of the quantum wells. This opens the possibility of observing thermally induced optical bistability in the sample with moderate pumping intensities. The bistability is observed by focusing an infrared laser beam on the multiple quantum well structure. The transmitted laser intensity shows the switching of the sample between the two possible absorptive states. The incident laser intensities at which this switching occurs are extremely sensitive to the temperature of the material surrounding the sample. Using this effect, an optical temperature sensor can be realized in a very practical design using an optical fiber to guide the incident and reflected laser beam that can possibly be provided by a diode laser.

The development of optical switching elements has been directed, until now, mostly to applications in the field of signal processing with very high data capacity. But optical switching elements could also offer great advantages if used in optical or fibre optical sensor systems. The discussion of two relatively simple examples is intended to demonstrate the chances of nonlinear optical switching elements for applications as sensor elements or as sensor specific signal processing elements.

Shaping of 1.9 ns laser pulses and optical switching behaviour with about 100 ps switching on/off times are measured in a bulk n-GaAs Fabry-Perot etalon. The etalon structure is not optimised, having a finesse of about 4 at low incident power. The impurity-related fast nonlinearity causes optical switching at wavelengths slightly longer than that of the fundamental absorption edge with incident intensities of ~ 106 W/cm2, corresponding to a switch energy of ~ 1 pJ/μm2.

Epitaxial microcavity devices, such as surface emitting lasers [1,2], vertical cavity modulators [3,4], and all-optical bistable microresonators [5,6], are currently subject to a lot of interest for their potential application in photonic switching. Due to their small active volume and their optical access perpendicular to the surface, they can be integrated as two-dimensional arrays, with a device density of 106 devices/cm2 or larger. Such arrays of all-optical bistable devices could be useful as opto-optical modulators, wavelength converters, optical logic gates, or fast access digital optical memories.

In “type II” heterostructures, potential profiles of conduction and valence bands are such that electrons are separated from holes, in real space. As a consequence, illumination induces charge separation and thus internal electric fields, which, in turn, modify the properties of the structure. On the other hand, it is now well established that such type II heterostructures may be obtained from various associations of GaAs, GaAlAs, and AlAs [1,2,3]. This happens as far as X-point conduction states are concerned. As a consequence, new tools for “band gap engineering” are available in this well controlled family of material; for instance, an indirect-to-direct transition has been observed in a GaAs/AlAs superlattice under external electric field [4], with consequences on the photoluminescence intensity. As a further example, in a more sophisticated and asymmetric structure, spatial separation of charges has been demonstrated to create an internal electric field with subsequent action on the photoluminescence emission energy of the structure [5]. More recently, Zrenner and coworkers [6] have observed an intrinsic optical bistability under external electric field, thought to be due to hot carriers.

The structure and fabrication of a vertical cavity surface emitting laser diode is described. The lowest threshold currents are 16mA. The light emission is single longitudinal mode at 915 nm wavelength. Under pulsed excitation the maximum output power is 3.5 mW. A wavelength selective photodetector of related structure shows a quantum efficiency of 10% at resonance with 1.5nm spectral width at half maximum.

The Double Heterostructure Optical Thyristor is a promising device for optical information processing. It has the important advantage of combining the receiving, transmitting, and memory functions in a single device, and simultaneously achieving a considerable optical gain. We show in this paper the dynamics of the switching of a single device and of several devices in a Winner-Takes-All network. We solve the problem of the dramatic surface recombination losses, without passivation or regrowth schemes, but by low doping of the outermost layers. This improvement led to arrays of sensitive devices (5 × 5, and 16 × 16 up to 32 × 64 elements) which are used in some interesting applications, such as locating the maximum light intensity (and optical dose) of optical input patterns, and transcribing parallel optical information from one array to another.

An architecture for a massively parallel computer is presented. The basic processing element is a pnpn device. The privileged application area is two dimensional vision: each element of the processing arrays performs Boolean operations on one single pixel and its nearest neighbors. The structure is SIMD (single instruction, multiple data), which means that all pixels undergo the same Boolean operation simultaneously. The logic architecture and the controlling structure needed to perform operations on two dimensional binary images, and the extension to gray value images, will be presented.

The examination of inherent defects in classical computing structures leads to the proposition of an intuitive computational machine based on distributed statistical processing. The implications of distributed processing and inter-model statistics are considered, and the usual fundamental requirement for computational inversion is discounted. A possible form of primary relational database is proposed, and the possibilities of differential model-fit mapping and the auto-generation of model rules are suggested. The desirable decomposition of computation into interrelational and decision-making processes presupposes an intermediate structure capable of linking the two in a bi-directional communicative manner. We propose a query-reflection architecture to achieve this and describe its required characteristics. The pseudo-implementation of such a structure demands a statistical treatment of the combination of counter-propagating data and knowledge, which suggests a new approach to the design of fast optical computers.

In order to design the very complex systems which occur in optical or optoelectronic interconnection and processing systems computer aided design tools are necessary. There are two main approaches to the design of such systems. One approach emphasizes a hybrid concept, known as smart pixels, in which communication is performed optically and processing is performed electronically. The other approach, known as symbolic substitution logic, tries to eliminate the electronics as far as possible. HADLOP (Hardware Description Logic for Optical Processing) is a software design tool for the modelling, simulation and evaluation of both approaches. In contrast to hardware description languages for pure electronic designs, with HADLOP it is possible to model the two-dimensional nature of optics. HADLOP works at the gate level because systems are described as a sequence of two-dimensional gate layers which are connected with optical connection modules. We present results for an opto-electronic broadcasting network, which has been evaluated in terms of the possible degree of parallelism, the energy requirements, and the speed of the system.

In this paper, an overview is given on the fundamental concepts of a special kind of ultrafast photonic device based upon the interaction of propagating microwave signals with optical beams. The characteristic properties are due to coplanar transmission lines leading to cut-off frequencies in the THz range. In particular, high speed travelling-wave photodetectors are first discussed, followed by a presentation of novel modulators and SEED — self electrooptic effect device — elements with interesting properties.

This has been one of the first topics considered by sub-group I “Silicon devices for optical logic” in ESPRIT WOIT. If the active region of an avalanche photodiode (APD) is a plane parallel silicon plate with good optical quality faces, a Fabry-Pérot resonator (FP) is realized by putting a reflection coating in place of the usual AR one on the input face giving the front mirror, the back mirror of the FP being the rear metallization of the silicon plate. Optical nonlinearities in silicon have two different origins — electronic and thermal — if silicon temperature is fixed: its refractive index decreases proportionally to the photo-carrier density. High optical excitation can lead to excess carrier density of the order of 1018 cm-3 and optical switching can be observed in pulsed regime where competition between electronic and thermal effects is present [1].Under cw illumination, optical nonlinearities in a silicon APD are of thermal origin and greatly enhanced by Joule effect due the large reverse applied voltage necessary to get photo-carrier multiplication in the preavalanche regime. In these conditions, an optothermal gain very much larger than in references [2,3] is expected giving much lower power thresholds for optical bistability. Such experiments have been carried out at IEF with a slightly modified RCA C30817 supplied by EG&G Canada. The experimental results have been analyzed and the potentiality of pixelated thin film APDs operating as optical bistable devices in 2D array has been evaluated.

We discuss some silicon Avalanche Photodiode (APD) structures where electronic optical nonlinearity could be exploited for optical bistability. The basic idea is to use the avalanching junction as a pilot of other devices where the carriers are effectively stored. We show that these structures can reach a switching power in the milliwatt range. However, thermal effects are always expected to overcome the electronic response if the sample temperature is not kept constant. We show the principle of operation of thermally-compensated structures with switching times in the microsecond range.

In order to optimize an optical switching device, several quantities need to be considered; these include up and down switching times (connected with the non-linearity mechanism), pixellation feasibility (which depends on device structure, heat dissipation and wafer uniformity), sensitivity and switching contrast (which is important for fan-out), technological complexity and cost.

Preferentially etched silicon structures are proposed which would enable the partial interchange of nearest neighbour optical image pixels and a degree of binary optical computation. Simple image processing is simulated, and noise reduction and image matching are described.