TECHNOLOGY AREA(S): Electronics
OBJECTIVE: The objective of this task is to investigate the novel interaction of electromagnetic radiation with materials that have the dielectric constant close to zero, with the specific aim to understand and thereby enhance the nonlinear interactions for applications in optical limiting for sensor protection.
DESCRIPTION: A long standing application of nonlinear optics is that of optical limiting for sensor protection. Progress in this area has been hampered by the ultrafast materials available which until recently had very small nonlinear coefficient. Slow nonlinear devices can have large nonlinearities but these are not usefully because several optical pulses are needed before the limiting process engages. Recent research on nonlinear Epsilon Near Zero, ENZ, materials have demonstrated that these materials have the potential to overcome these problems to finally bring efficient optical limiters to fruition.
A recent experimental result on the nonlinear optical response of an ENZ material showed not just a large nonlinearity, but an unprecedented nonlinear response which could have implications for many nonlinear devices [1]. In particular, it was noted that for a given change in the permittivity, the resulting change in the refractive index for a lossless material is related to the inverse square root of the dielectric constant.
Therefore, the change in refractive index becomes large as the permittivity becomes small, suggesting that the ENZ frequencies of a material system should give rise to strong nonlinear optical properties. The experimentally observed change in the real part of index of refraction near the ENZ frequency of Indium Tin Oxide (ITO) was measured to be 0.7 or 170% of the linear index and had a response time on the order of a few hundred femtoseconds. Until now, typical changes in the index of refraction for ultrafast nonlinear materials were <1%. The work on ITO was quickly followed by another measurement on a transparent semiconductor similar to ITO, Al doped ZnO [2]. Like the ITO, the ZnO had an ultrafast nonlinear index change close to unity.
In practical terms, the nonlinear optical properties of an ENZ material can in principle become very large and dominate linear optical properties even at what may be considered moderate intensities, or a few megawatts per centimeter squared (MW/cm^2). In general the onset of the nonlinearity will depend on how close to zero the linear dielectric constant can be designed, via doping, material processing, or a combination of both. For instance, the ENZ crossing point of ITO changes depends on annealing temperatures. Then, if for example, the minimum magnitude of the linear dielectric constant were of order 0.01, then one might expect a boost of the local field intensity inside the ENZ material by a factor of 100, which could reduce needed threshold intensities by similar factors. Therefore, while typically tens of gigawatts per centimeter squared (GW/cm^2) may be needed to trigger the nonlinearity in ordinary materials, only tens of MW/cm^2 or less may suffice to activate the nonlinearity of an ENZ material, opening the door to lower thresholds and practical devices.
In addition to the ENZ conducting oxides, it is possible to fabricate metamaterial ENZ’s [3-4]. While little work has been done on the nonlinear properties of metamaterial ENZ’s it is likely that these will possess large and ultrafast nonlinear optical responses. The potential application of ENZ metamaterials will allow the frequency of the ENZ point to be designed and not limited by material properties of the constituents. It may also possible to use either a metamaterial or a coupled, multi-resonance approach to flatten the dispersion curve, thereby having a broadband nonlinear optical response.
PHASE I: Demonstrate a feasible design of an optical limiter based on the nonlinear properties of a conducting oxide ENZ material or an ENZ metamaterial. The device should be designed with optimal linear transmittance and an order of magnitude modulation for optical limiting.
PHASE II: Demonstrate a working ENZ optical limiter for operation in the near-infrared or mid-infrared. In addition to optimizing the dynamic range and throughput as in Phase I, the second phase shall focus on minimizing device volume and including parameters related to environmental temperature changes and impact resistance.
PHASE III: Dual Use Applications: Materials with large and fast optical nonlinearities have numerous applications including white light generation, optical frequency combs, harmonic generation, sensor protection and many others. All these applications are strongly material dependent and would benefit from materials with larger nonlinearities than those presently available.
REFERENCES: 1: M.Z. Alam, I. De Leon, R.W. Boyd, "Large optical nonlinearity of ITO in its ENZ region," Science 352, 798 (2016).2: L. Caspani, et al., "Enhanced nonlinear refractive index in ENZ materials," Phys. Rev. Lett. 116, 233901 (2016).3: P. Moitra, et al., "Realization of an all dielectric zero index optical metamaterial," Nature Photonics 7, 791 (2013).4: J. Gao, et al., "Experimental realization of ENZ metamaterial slabs with metal- dielectric multilayers," Appl. Phys. Lett. 103, (2013).KEYWORDS: Nonlinear Optics, Epsilon Near Zero, Metamaterial, Optical Limiting, Sensor And Eye Protection
CONTACT(S):
Michael Scalora
(256) 842-2140
michael.scalora.civ@mail.mil
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