NATO SCP programme
Opto-electronic devices
based on bR
Objectives
Planned Applications and Devices
Summary
The cooperation partners
Bacteriorhodopsin is a protein-pigment complex from the cell membrane of Halobacterium salinarium. It is a biological light energy converter: upon absorption of a photon it pumps a proton across the cell membrane, i.e. it converts the energy of light into the electrochemical energy of the created transmembrane proton concentration difference. This is its biological function, which is, however, actually irrelevant from the viewpoint of bio-electronics applications.
Bacteriorhodopsin is very easy and cheap to produce in practically unlimited quantities. The bacteria are easy to grow and the pigment is easy to separate. The isolated bacteriorhodopsin (unlike most biological samples) is extremely stable: solutions, or dried films with unlimited activity (in time) can be easily produced.
Genetic engineering techniques to produce modified proteins are well established. Now, species with advantageously modified kinetic parameters can be prepared.
The function of bacteriorhodopsin is based upon a sequence of photochemical reactions, the photocycle. Following light excitation the bacteriorhodopsin molecule changes its absorption, refractive index and charge distribution; these properties can be used separately or simultaneously in opto-electronic devices: highly specific and sensitive sensors, optically controlled optical switches, logical optical devices.
There have been numerous attempts to design devices to utilise these properties. However, to our knowledge up till now there has been no other group that attempted the combination of bacteriorhodopsin and integrated optics. We consider this to be the most promising route.
The objectives of the proposal are:
- The exploration of the nonlinear optical (NLO) properties of the chromoprotein bacteriorhodopsin with emphasis on sensor and opto-electronics applications.
- The design of integrated optical devices based on optical waveguides combined with the chromoprotein bacteriorhodopsin. The operation of both the sensor and opto-electronic devices is based on modulation of the propagation of light within optical waveguides by change of the optical properties - absorption and index of refraction - of the chromoprotein bacteriorhodopsin induced by an external perturbation (e.g. a light beam).
- Design and development of a sensor for singlet Oxygen (1O2), of a most important radical in medicine. Operation is
based on the interaction of the retinal chromophore of bacteriorhodopsin with 1O2. The radical is observed by optical
methods: the interaction initiates the isomerisation reaction in retinal and consecutive optical changes are
monitored by the optoelectronic device. On this work will be based the:
- Commercial production of a highly sensitive sensor for singlet Oxygen (1O2).
- Design and development of optically controlled optical switches based on the chromoprotein bacteriorhodopsin.
Light is coupled in and out of the optical waveguide by changing optical properties of bacteriorhodopsin.
The numerous principal possibilities will be evaluated and the best solution will be selected for industrial development.
- Production of fully operational proof-of-concept devices realising optically controlled optical switches.
- Development and production of composite otpoelectronic devices (built from the above described switch elements)
representing logical circuits operating optically. With these devices all elements of purely optical
computing can be realised. Commercial production of fully operational proof-of-concept basic logical devices will be
started with the aim of generating broad interest for large scale development built on these elements.
Planned Applications and Devices
The technology to be developed is based upon the modulation of light conductivity in optical waveguides by the nonlinear optical (NLO) properties of bacteriorhodopsin. Since optical transitions of bacteriorhodopsin can be initiated in a number of ways, light modulation in the waveguides can be achieved by different external factors, consequently, optical switching devices with significantly different properties can be constructed. To make the technology successful, a number of problems have to be solved: adapting waveguide technology to the bio-materials, optimising the complex waveguide-biomaterial system to perform the desired tasks efficiently, and building stable devices with efficient and reliable function. The planned steps of research and development are detailed below.
" 8.4.1. Production of laboratory samples of the Input/Output devices.
8.4.1.1. Waveguide preparation The waveguide consists of a planar glass support plate and a layer of material of high index of refraction, typically a SiO2 - TiO2 solid solution. The thickness of the layer is small (10 nm), much smaller than the wavelength of the guided light. The efficiency of the waveguide is determined by the layer thickness (by influencing the evanescent character of the light), the consistency of the material of the layer (depending on the method of producing the layer - evaporation, SOL-GEL technique). The waveguide parameters optimal for stable light guide properties and effective modulation by Bacteriorhodopsin have to be determined. A systematic study will be performed to establish the best qualities. The primary production and optimisation of the optical elements will be done in KFKI, subsequent production for later studies and device development will be carried out at Optilab.
8.4.1.2. Coupling of waveguides with fibre optics The active devices are connected to each other and to peripherals by fiber optics. The coupling of light between the planar waveguide and optical fibres is a major problem. There are several basic possibilities, primarily grating coupling and prism coupling. An efficient and reliable connection has to be achieved for the devices to be of practical value. These basic possibilities will all be evaluated, and the optimal waveguide geometry and the coupling unit (prism, grating or direct (by index matching, with proper index gradient lenses between the sheet and the fibre). The combination giving optimal efficiency will be established, then will be used throughout the later applications. These tasks will be primarily done in the IMFA in close contact with Optilab.
8.4.1.3. Preparation of the biomaterial coating on the waveguide In all subsequent applications the key to effective function is a film of good optical quality. Hence the preparation of these films has to be optimised. There are a number of possibilities for film production: 1. Simple drying of a suspension of bacteriorhodopsin on the surface. 2. Admixture of an additive (e.g. polyvinyl alcohol (PVA), or gelatin) to improve the optical quality. 3. Using the Langmuir-Blodgett technique to produce films with very well defined properties. All the above methods are available, and have been used for basic science studies. We will try all methods and select the best for later routine use in the specific tasks. Development of the film producing technique will be done in Szeged.
8.4.2. Active optical device development As written earlier in section 8.4., the planned devices are all based on modulating light propagation in the waveguide by optical transitions in bacteriorhodopsin. There are a number of possibilities to achieve this goal, below we list and explain the basic classes.
8.4.2.1. Grating coupling modulated by the refractive index of bacteriorhodopsin In this scheme light is coupled into the waveguide by a diffraction grating formed in the waveguide. The ad-layer (layer of Bacteriorhodopsin) is deposited directly above the grating. Efficient coupling is a very sensitive function of the coupling angle. The angle at which efficient coupling takes place largely depends on the refractive index of the material around the grating. Consequently, the change of the index of refraction of the ad-layer can be very sensitively followed by measuring the coupling angle. Or equivalently, at a given coupling angle the intensity of the coupled light is modulated by the reactions.
8.4.2.2. Coupling light into and out of the waveguide by a transient grating formed in the adlayer by holographic excitation Here the grating for coupling is formed within the ad-layer by appropriate light excitation: by interference of two identical laser beams light excitation produces a holographic grating within the ad-layer. This transient grating can act as a coupling grating, achieving a grating for the duration of the photoreactions in bacteriorhodopsin. The transient grating can be used both as a phase-grating (by coupling light where there is no absorption change during the photoreaction) or as an absorption grating (where absorption at the wavelength of the coupled light changes during the photoreaction). This arrangement is clearly useful for coupling light both in and out. In the second case light coupled in by a prism can be removed from the waveguide controlled by the transient grating. Note that in this case high light intensities can be handled - when the grating is used to couple the light in, only a small portion of the light is transferred into the waveguide. This can be an important point when selecting layouts for switching applications.
8.4.2.3. Coupling light into and out of the waveguide by a grating formed in the adlayer by holographic bleaching In the presence of certain chemicals (e.g. hydroxylamine) Bacteriorhodopsin is bleached by light. This phenomenon can be used to burn permanent gratings into the bacteriorhodopsin layer by applying holographic excitation as described in the previous paragraph. When the photocycle is initiated in bacteriorhodopsin, this grating changes its efficiency at different wavelengths, according to the changes in the absorption spectrum.
8.5. Optically controlled optical switch All the ways to modulate light transfer in the waveguide by bacteriorhodopsin outlined in 8.4.2. are in principle feasible for the application in building an optically controlled light switch. Consequently, we will evaluate all methods to find out the most appropriate solution in terms of efficiency, stability, ease of operation and production. At present we believe that the most appropriate solution will be those in 8,4,2,2, and 8.4.2.3., where light is coupled into the waveguide by a prism and outcoupling is achieved by a grating generated in the ad-layer. The advantages of this layout for switching applications are the high intensity of light handled: clearly, a really useful switch has to operate with output light intensities sufficient for operating subsequent switches, too. This is the prerequisite for the construction of complex devices.
8.5.1. Controlling of optical switches Crucial for the application of optical switches is the timing control of the transients. Systems with different time characteristics will be built using different bacteriorhodopsin mutants - with characteristic reactions following different kinetics, chromoproteins with dfferent colours, etc. In all the following examples, the timing of the changes can be varied arbitrarily: it can be dynamic with characteristic times from picoseconds to infinity, static, also bistable (switching between two stable states by illumination with lights of different colours). Operation in all modes will be verified in detailed kinetic experiments with high time resolution. For all these crucial experiments (short excitation pulses with different wavelengths are needed) the tuneable pulsed laser (flashlamp pumped Nd:YAG laser with Optical Parametric Oscillator) is fundamental.
8.6. Complex logical optoelectronic devices Once efficient optical switches are developed, complex devices using them as building blocks will be constructed. It may be recalled that the preceding parts form the elements of optical computing. Here the logical circuits necessary for realising all functions of a computer will be created. The final product is a proof-of-concept device: a model computer that operates fully by light.
8.7. Sensor for singlet oxygen Retinal, the chromophore of bacteriorhodopsin belongs to the family of carotenoids, the most efficient traps for the free radical 1O2. Their action is based on a special reaction: when interacting with singlet oxygen: instead of oxidation, they undergo a cis-trans isomerisation. The accompanying optical changes of bacteriorhodopsin- and retinal-containing thin films can be utilized to establish a novel singlet oxygen sensor. In the case of a sensory sensitivity, not the optical efficiency is crucial. Consequently, the optical arrangements described in points 8.4.2.1 and 8.4.2.3. come into play - they will both be evaluated. In the sensor application the size of the detector has to be small and possibly mobile: this puts limitations on the geometry of the waveguide system. The design will rely heavily on the experience with the IO devices concerning the coupling parameters. The sensor prototypes will be tested by exposing the device to 1O2 of different known concentrations.
8.8. Electrical devices, fast photodetectors We have discussed it in point III, the function of Bacteriorhodopsin is also connected to charge displacements. Consequently, a layer in which the molecules are oriented also acts as a photoelectric device (in practice, a dry layer of 1 mm thickness produces transient voltages of the order of 10 mV upon illumination). Orientation of the Bacteriorhodopsin layers is achieved by applying a DC electric field (in the order of 10 V - a surprisingly low field is sufficient since the molecules are contained in relatively large membrane fragments) during layer formation. On this basis photodetectors of down to picosecond time resolution can be produced. Since this electric changes are superimposed on the optical activity, a property with possible applications as even fast cameras. A major advantage of cameras with bacteriorhodopsin as the active element is that by utilising the photoreactions primary signal processing can also be made. E.g. enhanced contrast sensitivity, or motion perception can be achieved.
8.9. Production of biomaterials The experiments and devices require the production of site directed mutant versions of Bacteriorhodopsin with appropriately modified parameters concerning colour and photocycle kinetics. Highly reliable production of large amounts of material is required - this will be performed by the Witten group.
8.10. Product design Following the establishment of the best basic arrangement (in each application) by the experiments on model laboratory systems, a product design will be initiated in close collaboration by the industrial partner. The goal is to produce compact, stable devices carrying all functional parameters of the laboratory models efficiently, in large quantities. The product development process will result in intermediate prototype devices. These prototypes will be continuously tested for the required qualities: a steady interaction of the groups will result in subsequent redesigns, and by the iterative development process an optimal end-product ready for commercialisation will result. "
In conclusion we can state that optically coupled optical switches (very important in modern telecommunications) could be constructed in all of the above ways. We have tried all of them successfully on a basic science level. Also, optical logical devices (the basis of optical computers) can be designed and built. Thus all elements of optical computing can be realised.
The idea of applying of bR and carotenoids in biosensors based on integrated optics is also a novelty. At present, besides EPR spectroscopy - a complicated, bulky, cumbersome technique -, there is no alternative method available for the fast detection of singlet oxygen, a free radical playing an important role in many physiological (and pathological) processes.
Investigation is needed to decide which of the above opportunities are most promising in a practical sense and what are the physical and optical parameters to make optimal use of the promising physical properties. This is basically what the concerted action of the research groups can offer to establish the production of practical devices by the industrial partner.
Institute of Biophysics, Biological Research Centre, Szeged.
The group has intensively studied the chromoprotein
bacteriorhodopsin, the key material for the project. The experience involves spectroscopy (in all spectral regions of
light: infrared, visible, ultraviolet) with very high (ns) time resolution - with ready access to lasers permitting
subpicosecond time resolution technology. The group has renowned expertise in the production of stable oriented samples
of bacteriorhodopsin with excellent optical quality, crucial for photoelectric studies and (polarised) holography.
The accumulated experience involves detailed understanding of the reactions of the photocycle, including the molecular
motions and charge redistributions during the reactions. Recently, to utilise and extend our previous activities,
a laboratory has been set up to study the NLO properties of bacteriorhodopsin during the photoreaction. These studies were
initiated in collaboration with the group from the Institute for Materials Sciences, (KFKI), a participant of the
present proposal. In addition, a new microspectroscopy laboratory, where manipulation of microscopic optical objects
is possible, has also been built up..
Research Institute for Technical Physics and Materials Science Budapest.
The know-how of this group needed for the
projects concerns optical waveguide technology. The group has vast experience with optical waveguides with grating
coupling. This technology is extremely efficient for the study of changes in index of refraction. The technology is
crucial for the studies of the optical properties of bacteriorhodopsin which are to be utilised in the sensor
and switching applications. At the same time, this is also the technology fundamental for integrated optics upon
which all planned applications are based.
Laboratory for Applied Biotechnology, University of Witten-Herdecke, Witten, Germany.
The group of Professor E. Wolff is specialised in the growing of microorganisms under highly specific environments.
The parameters of the growth medium are computer controlled in an intelligent way to ensure well-determined qualities;
even the steering of mutations (controlled evolution) is possible. The fully computerized bioreactor of the group
offers a unique facility to produce biological material in large amounts and in a precisely controlled manner.
Optilab Ltd, Budapest The industrial partner is a typical high-tech SME operating with a staff of 8 people. The activity covers custom design and production of optical elements and systems including glass and other optical materials processing, production of optical layers for dichroic mirrors, filters, optical waveguides, etc. The role of the partner in the project will be twofold. Firstly, they will produce the waveguides necessary for the study of the NLO properties of bR. Secondly: the integrated optical sensor and optical switch end products, jointly designed by all participants, will be produced by Optilab.