Photonics finds a natural solution

Rumour has it that this classified project was close to success before the Soviet regime crumbled. Their massive research effort kick-started the entire bR field.

Norbert Hampp, at the University of Marburg in Germany, has developed the first commercial bR application with biotechnology company Munich Innovation Biomaterials (MIB). Called Fringemaker, this holographic interferometry camera is used for non-destructive material testing and vibration analysis.

Fringemaker uses a bR film as an erasable holographic memory. The system is capable of 5000 lines/mm of lensless recording and German ceramics manufacturer Ceramtec has used it to test the bending characteristics of the firm's ceramic silicone nitride motor valves under a force load.

The set-up examines objects in the 2 ¥ 2 to 50 ¥ 50 cm range that are placed 20 to 80 cm in front of it. The objects under test scatter light onto the holographic bR film and the pattern that is generated can be immediately analysed and then erased by blue light from a Nd:YVO₄ laser. Double-exposure, time-averaging and real-time interferometry can all be performed.

"Where ultrasonic and X-ray analysis cannot be applied, this holographic system can be used as an alternative," said Hampp.

First commercial device

However, Fringemaker's elegance and impressive specifications have a downside - cost. The system has a resolution of 5 nm, which verges on the physical limit, but it costs more than USD 120,000 - almost USD 100,000 more than technically less impressive ultrasonic or X-ray systems.

Hampp is well aware that the system is a niche product for a niche market. He is considerably more excited about the commercial possibilities of the next product that he has in the pipeline: bR-based security inks.

He said: "It's a simple application. Bacteriorhodopsin's colour-changing ability means that we can make inks that change colour under non-ultraviolet light. If somebody tries to make a colour copy of a document containing this ink, the copy will have a different colour to the original."

Biotechnology has enabled the production of yet another security measure. Information can be easily coded in a bR ink by engineering traceable changes in the ink's amino-acid sequence. Hampp has signed contracts with a security firm that is now making test samples of security documents. The ink is expected to be commercially available by the end of this year. The headache for MIB now is to scale up its production process to manufacture kilogram quantities of bR, rather than the milligrams that it has been producing so far. The firm is diverting its emphasis from Fringemaker to inks, in view of the greater potential revenue in that area.

The emergence of biotechnology has been a huge benefit to those looking to establish commercial bR devices. Subtle changes in bR's amino acid sequences can create more stable intermediate states, tweak absorption maxima to more convenient wavelengths and make selected photochemical processes more efficient.

Marburg Castle image

Bob Birge and Jeff Stuart at Syracuse University in the US have exploited biotechnology to make bR-based data-storage devices for the US Air Force. They have developed three-dimensional volumetric memory cuvettes - which are the size of a standard absorption cell (1 ¥ 1 ¥ 3 cm) - that are based on a two-colour branched photocycle.

The capacity of this three-dimensional random-access memory is impressive: "Our sugarcube-sized memory block only costs USD 2 and can store 7 Gbyte," said Birge.

The cubes' resistance to electromagnetic effects renders them especially useful in space and military applications. The US Air Force has become Birge's first customer. It plans to use some working prototypes with global data erasure for interference-free satellite-download image storage.

Unlike conventional data-storage materials, which require a regular, repeating structure, Birge says that the key issue in producing bR memory cubes is to maintain a complete, homogeneous disorder.

"We achieved our best results in microgravity conditions on the Space Shuttle. Ironically, that led us to find a method that works well on Earth," he said. The cubes are rotated about three axes while they are being manufactured so that they have a sufficiently disordered structure for memory applications.

However, Hampp has serious doubts about the feasibility of bR-based optical data-storage devices. He told OLE:"The key issue is the reliability of these memory devices, and that requires further system integration. To achieve this, more development cycles will be necessary. I think that there will also be problems with pricing if optical memories are brought to market."

Nevertheless, Birge has set up a company called Lambdagen to manufacture the three-dimensional-cube memory and to make credit-card-sized devices that can hold 100 Mbyte. "The cards will be flexible and inexpensive to manufacture. The memory will store an image of the cardholder so that his or her identity can be easily verified," explained Birge.

Lambdagen supplies another US start-up - Virginia-based Starzent - with the raw protein to produce high-density holographic memories. Starzent, whose principal scientist is Nikolai Vsevolodov, a "Project Rhodopsin" veteran, manufactures memory devices based on bR for data storage and optical processing under a DARPA-funded military project. Birge also has plans for a second company to commercialize the development of an artificial retina. Bacteriorhodopsin-based devices have a great advantage because they would come supplied with their own integral power source. This arises from the massive 250 mV that each bR molecule generates when it changes state. Current attempts to build artificial retinas are based on semiconductors that need to be powered by an external battery.

Hampp believes that such photoelectric applications - which could also include solar energy cells - have greater commercial potential than optical memories.

bR photocycle

Optical processing presents yet another set of applications for bR. Hampp cites pattern-recognition applications, such as genetic library scanning, as a promising area. In principle the material's stable photocycle is perfect for such applications, but the old problem of converting optical information to and from electronics is the stumbling block. The speed of the processor needs to be matched by the databus. Hampp says that for optical processors to become technically viable an increase in the speed of spatial light modulators is necessary.

Despite his scepticism over the commercial potential of many bR applications, Hampp is generally optimistic about the future. "Certainly the number of patents based on bR that are applied for and granted every year is increasing, and that is a clear indication of bR's future commercial success. Filing patents is expensive and many people obviously think that they will make a return on their money." If bR devices do turn out to be a success, then Europe may come to rue its relative sluggishness to adopt the technology. Of the 100 or so bR patents only 13 have been filed by Europeans. Hampp believes this is a cause for concern: "The numbers mark the weak strategic position of Europe in this field and confirm the European attitude of waiting until the commercial aspects of new technologies are fully recognized."

The computer industry will be reluctant to change its methods and a biological random-access memory in everybody's laptop computer remains a distant possibility, so a younger technology might offer the best platform to exploit bR's optical trickery.

One possibility is in photonic bandgap crystals. The natural structure of bR is similar to that of photonic bandgap materials and the precise spacing of microscale structures can be altered through genetic engineering - which is much simpler than painstaking crystal-growth techniques.

André Persoons and Koen Clays of Leuven University reported this potential application of bR at last year's CLEO Europe conference. They found that the photonic-crystal coherence length is limited only by the size of the bR crystal or purple membrane patch. These can be fine-tuned genetically for a specific application.

Bacteriorhodopsin has already been around for an estimated 3.5 billion years. It seems inevitable that at least part of its future will lie at the heart of a new generation of optical systems.

Bob Birge's memory cell

Light from the green source hits the cuvette via a spatial light modulator, converting selected areas, or "pages", of the protein into the O state, which is exceptionally stable. This stability can be enhanced by genetically engineering the protein.

However, the O state absorbs in the red, and two milliseconds after the green pulse a red laser - which also passes through a spatial light modulator - irradiates the protein to form the P state, which subsequently decays to the Q state.

This two-stage illumination process produces a protein volume where the "1" bits are represented by proteins in the P and Q states and "0" bits correspond to the O and bR states.

Reading the data back is performed in exactly the same manner, except that the red source has a reduced intensity. The green paging light is only absorbed by proteins in the O state and the low-intensity red light images the P and Q states onto a charged-coupled-device chip rather than convert any O proteins.

Blue light erases the encoded data. The P and Q states absorb the blue light to return to their original bR state. Individual data can be erased using a blue laser or a global wipe can be performed with an incoherent blue source. Erasure is a highly efficient process and the memory cuvette must be shielded for protection.

The read step has proved to be the most problematic. Birge said: "The Q state's red absorption is inefficient in the natural form of bR, so a high light intensity is required to carry out the read cycle." Adversely, higher intensities make the storage of data unreliable. By using genetic modifications Birge has improved the efficiency of the Q absorption by 200-fold and says that this is "within striking distance" of achieving the required increase for reliable storage.