Phase Change Materials for Photonics

Background: Phase change materials (PCMs) can be switched rapidly and repeatedly between two states, typically a high-resistance amorphous state and a low-resistance crystalline state. Electronics applications could take advantage of several orders-of-magnitude change in resistivity. Concurrent with the changes in resistivity are changes in refractive index, with applications to optical memory. The reliability of one PCM, germanium antimony tellurium (GST), has been clearly demonstrated, with commercialization of rewritable optical disks with an endurance of more than 1013 read-write cycles. The simultaneous changes in resistivity and refractive index could also allow mixed-mode operation in which electrical excitation is provided and sensing is carried out optically or vice-versa. Photonic devices could include tunable optical modulators, filters, switches, couplers, routers, broadband absorbers, memory, and waveguides. For example, electrical actuation could allow optical modulators with extremely small footprints. Implementation on a silicon platform could also be considered, allowing the integration of memory, logic, and photonics. Progress in Si (or Group IV) photonics has been hampered by the relatively small electro-optic effects in Si. A hybrid approach including PCMs with large electro-optic coefficients could be effective. Recent reports also illustrate the potential for PCMs in tunable optical metamaterials, with the demonstration of a metasurface with near-perfect IR absorption using VO2, and reversible optical switching of IR antenna resonances using femtosecond laser pulses. 

Clearly, a number of photonic devices could benefit from PCMs, but better scientific understanding is needed to enable technological advances. Open questions include the dynamics of phase change under light or light coupled with electric fields or temperature changes, whether pure electrical activation of PCMs is possible for high-speed photonics, the reversibility of the phase change under different stimuli, and the role of defects which can significantly influence these functional properties. For example, instabilities induced by defects may affect device performance or, in some cases, create additional tuning possibilities. To date, a majority of research has focused on GST and VO2. Other materials, including several chalcogenides, oxides, and nickelates, exhibit phase-change properties and may be worthy of investigation. Important parameters to be investigated in these materials will include the speed of the phase change and the energy per unit volume required for a phase change. A coordinated interdisciplinary program is needed, with expertise in materials growth and characterization, physics of phase-change mechanisms, and optical engineering to model and test photonic devices. 

Objective: The objectives of this MURI are to advance the understanding of the electro-optic properties of phase change materials and the underlying physical mechanisms responsible for the phase change, and to explore their uses for next-generation photonics. 

Research Concentration Areas: Areas of interest include but are not limited to: (1) Discovery and ab initio-based computational modelling of “new” materials that have suitable phase-change properties for photonic applications; (2) Development/refinement of appropriate growth techniques for new and existing materials such as sputtering, pulsed laser deposition, molecular beam epitaxy, and/or atomic layer deposition; (3) Investigation of the fundamental mechanisms of the phase change process induced by optical, electrical or thermal stimuli; (4) Stability as PCMs change their strain state; (5) Novel means of reconfigurability; and (6) Modeling and testing of (nano)photonic structures that incorporate PCMs, possibly on Si platforms. 

Anticipated Resources: It is anticipated that awards under this topic will be no more than an average of $1.5M per year for 5 years, supporting no more than six funded faculty researchers. 

Research Topic Chiefs:

Dr. Brian R. Bennett, ONR 312, 703-696-4220,

Dr. Antti J. Makinen, ONR 332, 703-696-0283,