Silicon (Si) based microelectronics, that had revolutionized electronics, computing and communications, has reached its limits and researchers are exploring various technologies to replace silicon, that can satisfy humanity’s need for faster, smaller, greener and more powerful computers. One of class of materials, that are being explored are Phase-change materials (or PCMs) that can switch between two structural phases with different electrical states, one crystalline and conducting and the other glassy and insulating, in billionths of a second.
Chalcogenide glass is an increasingly important tool for the optical designer, providing a versatile material for many applications—from thermal imaging to hyperspectral imaging. The properties of these amorphous glasses are useful over a broad spectral range, from the near-infrared (NIR) at 700 nm well into the LWIR spectrum. Chalcogenide glasses consist of mixtures of the Group 16 elements selenium (Se), sulfur (S), and tellurium (Te), and various Group 14 and 15 elements such as arsenic (As), germanium (Ge), tin (Sn), and others.
As traditional IR materials such as Ge and zinc selenide (ZnSe) rise in cost, the use of chalcogenide glasses is becoming more widespread. Chalcogenide materials offer substantial savings today in both the raw material cost and in fabrication methods such as molding technology. They also provide numerous benefits to systems with stringent specifications.
Scientists at the University of Southampton have made a major step forward in the development of digital data storage that is capable of surviving for billions of years. Using nanostructured glass, scientists from the University’s Optoelectronics Research Centre (ORC) have developed the recording and retrieval processes of five dimensional (5D) digital data by femtosecond laser writing.
The storage allows unprecedented properties including 360 TB/disc data capacity, thermal stability up to 1,000°C and virtually unlimited lifetime at room temperature (13.8 billion years at 190°C ) opening a new era of eternal data archiving. As a very stable and safe form of portable memory, the technology could be highly useful for organisations with big archives, such as national archives, museums and libraries, to preserve their information and records
The chalcogenides materials currently, are being used solely as either electronic materials or optical materials, with different types for each application. In the future, their properties give them the potential to combine the excellent optical properties of one material with the excellent electronic properties of another and vice versa.
Thus, these materials have potential for development of hybrid ‘optoelectronic’ technology, which is a hybrid of the optical and electronic systems, but without the current limitations imposed by the two current technologies working independently.
PCMs were developed in the 1960s and found their way into optical-memory devices and more recently in electronic-memory applications. They are just now starting to replace silicon-based flash memory in some smart phones. They are already being developed and used in next-generation computer memory technology known as chalcogenide random-access memory (CRAM). As researchers continue to identify speed enhancements, non-volatile PCM could eventually supplant the more energy-intensive DRAM.
PCM based non-volatile devices have potential to replace both DRAM and logic processors in computers. This would also lead to the development of green processors as they would obviate the need of DRAM refreshing that wastes large amount of energy globally, which is costly, both financially and environmentally.
Scientists from the University of Cambridge, the Singapore A*STAR Data-Storage Institute and the Singapore, University of Technology, have created a PCM-based processor using chalcogenide glass, which can be melted and recrystallized in as little as half a nanosecond (billionth of a second) when the correct voltage is applied. The team showed that logic-processing operations can be performed in non-volatile memory cells using particular combinations of ultra-short voltage pulses, which is not possible with silicon-based technology. This works because the PCM devices put logic operations and memory in the same location
Eternal 5D data storage could record the history of humankind
A copy of the Universal Declaration of Human Rights (UDHR) encoded to 5D data storage was recently presented to UNESCO by the ORC at the International Year of Light (IYL) closing ceremony in Mexico. The documents were recorded using ultrafast laser, producing extremely short and intense pulses of light. The file is written in three layers of nanostructured dots separated by five micrometres (one millionth of a metre).
Coined as the ‘Superman memory crystal’, as the glass memory has been compared to the “memory crystals” used in the Superman films, the data is recorded via self-assembled nanostructures created in fused quartz. The information encoding is realised in five dimensions: the size and orientation in addition to the three dimensional position of these nanostructures.
The self-assembled nanostructures change the way light travels through glass, modifying polarisation of light that can then be read by combination of optical microscope and a polariser, similar to that found in Polaroid sunglasses.
Professor Peter Kazansky, from the ORC, says: “It is thrilling to think that we have created the technology to preserve documents and information and store it in space for future generations. This technology can secure the last evidence of our civilisation: all we’ve learnt will not be forgotten.”
Australian researchers at the University of Adelaide have developed a method for embedding light-emitting nanoparticles into glass without losing any of the nanoparticles’ unique properties, according to a university press release.
And this development could have potential applications in next-gen technology like 3-D displays or remote radiation sensors. This “smart glass” combines the properties of special light-emitting nanoparticles with well-known aspects of glass, like transparency and malleability, the release explains.
They are also widely used as optical materials in applications such as photovoltaics (solar cells), and advanced optical devices (e.g. lasers). The research by the University of Surrey, in collaboration with the University of Cambridge and the University of Southampton, has found it is possible to change the electronic properties of amorphous chalcogenides which are naturally p-type semiconductors. The researchers by using an ion – implantation process, which allows precise control of the type of impurity introduced, implanted bismuth ions in GaLaSO chalcogenide glass, to form n-type chalcogenide. They were able to create p-n junctions and demonstrate rectification and photocurrent in the device.
This development could enable the direct electronic control of nonlinear optical devices, and could lead to chalcogenide light sources and photodetectors, thus allowing a single material to produce, transmit, and detect light, leading to all-optical systems.
Optical communication and data transfer is widely known as being much quicker as information can be moved at the speed of light. However, whenever it interacts with electronics, such as when broadband optical fibre is connected to a computer the data transfer and processing must slow down to the speed of the microelectronic processors.
Manufacturing high-volume IR optics
The processes used for the precision molding of oxide glasses can be transitioned to the chalcogenide glasses. However, variations may exist in process profiles because of the difference in physical properties between the materials. The most notable of these is the lower glass transition (or transformation) temperature exhibited by the chalcogenides. As an example, the transition temperature (Tg) of the oxide glass BK7 is 557°C compared to Tg of 185°C for the chalcogenide IG6/IRG26. This significant delta can have positive effects on the following molding parameters:
Cycle time: shorter ramp times to achieve required temperatures;
Tooling choices: more potential materials for optical inserts (cheaper, easier to machine, more readily available); and
Tooling lifetimes: “softer” glass can cause less wear and tear on surface of optical inserts.
When undertaking a high-volume manufacturing project, consider whether or not the part will lend itself to a multicavity mold configuration. A multicavity molding process is possible and desirable for the small, short-focal-length landscape lenses needed for personal IR imaging devices. The upper limit on cavity number is dependent upon the lens diameter as well as the mold base diameter. Fine-tuning the molding processes to achieve acceptable parts from all cavities is more challenging for a multicavity mold than for a single-cavity mold, but the outcome effectively divides the cycle time per lens by the number of cavities in use and substantially reduces costs.
Precision-molded chalcogenide lens elements made in high volumes present distinct challenges regarding qualification of performance prior to shipment to the customer. Firstly, molded optical surfaces are typically aspheric, which may require the use of profilometry and/or aspheric interferometry, both in line to ensure surface figure and end-of-line testing as specified by the customer, from acceptable quality limit (AQL) to 100% of outgoing product. Secondly, performing any type of transmitted image quality testing requires not only a test system that is acceptable for use in the IR, but also one that replicates the in situ use of the lens in the final product. Lastly, should the customer desire 100% testing at the lens element level, the test system must be able to keep pace with lens production so as to maintain acceptable cycle times at production volumes.
There are many sources for chalcogenide glasses, including Vitron GmbH (Jena, Germany), SCHOTT North America (Duryea, PA), and IRradiance Glass (Orlando, FL), which produces a number of glass types along with custom melts.