The Dept. of Energy has maintained a continual R&D effort since the late 1970's to develop photovoltaic (PV) solar cells of ever increasing efficiency. Initial developments in silicon PV material have ultimately given way to higher efficiency III-V multi-junction materials - raising efficiency levels from 12% to nearly 42%. Although these new materials show great promise, they are ultimately restricted by the naturally occurring materials available to convert incident sunlight into electricity. These natural materials, which provide discrete and limited bandgap options, are poor converters of broad spectrum energy. Metamaterials offer the potential to shift solar spectrums toward wavelengths that are more suitable for energy conversion or to design manmade nano-crystalline lattices that are optimized for a given spectrum. As such, the common sources of PV inefficiency - thermalization of supra-bandgap photons and transparency of sub-bandgap photons - can be minimized for a given spectrum, significantly improving the performance of a PV cell. In addition, the simultaneous thermal conductance, electrical conductance, and transparency of metamaterials can be applied in the near-term to various areas of photocell design. The Optical Metamaterials Program is currently looking at metamaterial designs aimed at:
Transparent conductors for reduced PV material obscuration
PV optical coatings for improved thermal transfer to heatsinks
Lattice compatible interstitial reflectors and filters for increased photon absorption in CPVs
Light Emitting Diode (LED) Lighting
Nearly 30% of the electricity consumed in the United States is associated with artificial lighting. Light Emitting Diodes (LEDs) have the potential to dramatically reduce this usage by utilizing diode emitters that are more energy efficient and longer lasting than traditional incandescent or fluorescent sources. However, LEDs must overcome a number of technical challenges before they can achieve their optimal efficiency. These factors include optimizing internal quantum efficiency, outcoupling of emitted light, chip shape, and LED packaging. Ultimately, metamaterials can be a powerful new tool for addressing each and every one of these challenges. However, in the near-term, the benefits of metamaterials are likely to first be seen in the area of optimized outcoupling of emitted light. Light emitted at an LED cavity must be coupled to an integrated lens so that the divergence of the LED output can be controlled and waste heat (due to internal optical absorption) can be minimized. To date, limited success has been achieved with naturally occurring materials and significant optical loss and heat gain result due to total internal reflection (TIR) of the internally generated light. The Optical Metamaterials Program is focused on using metamaterial coatings and metamaterial micro-lenses to provide a negative index of refraction optical interface to significantly reduce TIR losses. These coatings/materials can be applied directly to the face of the LED surface or at an internal interstitial layer making them applicable to a variety of LED types.
Thermal Photovoltaics (PV)
Thermal photovoltaics (TPV) convert heat into electricity using direct long-wave photon conversion or an intermediate thermal-to-near IR converter/emitter. Recent research has shown that the absorption of thermal wavelengths for re-emission at near IR wavelengths can be achieved with highest efficiency when captured in a metamaterial as opposed to a naturally occurring materials. The design of metamaterial nanoscale lattices, as opposed to the atom-scale lattices of natural materials, is better suited to the thermal wavelengths frequently encountered in waste heat recovery applications. The development of improved metamaterials for thermal PV applications is still in its infancy but has the potential to significantly improve the efficiency of thermal PV cells within the next 5-10 years.
Optical microscopy is a well-established field that began nearly 400 years ago. With naturally occurring materials, optical microscopy has been limited in resolution to the optical diffraction limits of light. However, recent research has demonstrated that metamaterial plano lenses (called SuperLenses) can focus optical light well below the diffraction limit. This exciting phenomenon, first predicted by Victor Veselago in 1967, has just recently been demonstrated for some limited metamaterial coatings. A bulk version of these coatings could one day lead to optical microscopes that can image at resolutions approaching today's most sophisticated and expensive scanning electron microscopes, atomic force microscopes, photon tunneling microscopes, etc. The Optical Metamaterials Program is working to develop metamaterial layer fabrication techniques that can be "stacked" to produce a superlens with low-efficiency but sub-diffraction performance nonetheless. Early research is focused around specialized glass drawing techniques developed for other areas of DOE research.
Sensors have numerous applications relevant to the DOE mission. Whether it's the development and integration of distributed sensors for the emerging smart grid, improved sensors for greater energy efficient industrial processes, or sensors for detecting and tracking nuclear materials, sensors are a key component of DOE's R&D activities. Metamaterials promise to open an entirely new field of sensor capabilities based on the unique index of refraction characteristics of this material. The Optical Metamaterials Program is developing planar waveguide sensors that interact the evanescent fields of weakly guided light with newly developed cladding metamaterials. These interactions results in optical sensors with high sensitivity to environmental factors (i.e. strain, pressure, temperature, etc.) beyond the limits of conventional sensor. In addition, the "optical magnetism" properties of metamaterials are being explored for nano-sized electrical power measurement devices for potential integration into the emerging SmartGrid.
Quantum computers have the potential to solve critical problems much more quickly than classical computers (i.e. Shor's algorithm). Although quantum computing is still in its infancy, experiments have been carried out in which quantum computational operations are being executed on a small number of qubits (quantum binary digits). Of particular interest to our program are optical quantum computing techniques based on entangled photons. One of the key components to realizing this approach is the development of a variable optical delay material capable of trapping photons for long periods of time (i.e. milliseconds). A metamaterial with a sufficiently high index of refraction could achieve these levels of delay and, when incorporating electro-optical materials (i.e. liquid crystals), could also be adjustable. Currently, the Optical Metamaterials Program is working to better characterize the relationship between plasmonics and metamaterials on trapping and entanglement phenomena associated with this field.