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II11.07 - Superabsorbers with Minimized Scattering: How to Hide a Good Receiver 
April 24, 2014   4:00pm - 4:15pm

The ability of a sensor to efficiently detect or absorb an external signal is granted through strong interactions with the applied electromagnetic field. Intuitively, this implies that the presence of the sensor should also be strongly felt in its surrounding, and the more "visible" a receiver is, the better it may detect or absorb. However, this property is undesirable in many situations, such as in optical near-field imaging (e.g. using an NSOM tip), biomedical sensing and closely spaced energy harvesters. In all these examples the level of perturbation imparted on the primary field, intrinsically limits the performance of the system and directly affects the validity and efficiency of its overall response. In this presentation, we propose the concept of minimum-scattering superabsorbing nanostructures and demonstrate the possibility of realizing highly efficient sensors, which absorb as much as desired, but at the same time keeping their presence imperceptible for an external observer. We begin by discussing the principles of operation of conventional sensors and antennas and show that the limitations associated with undesired scattering from these structures is due to the fundamental trade-off between maximum achievable absorption and minimum total scattering in a receiving dipole. In this context, we briefly explain the possibility of adjusting typical receivers - e.g. through proper loading - to scatter much less than they absorb, but at the price of having very low absorption levels. Next, we propose a solution to overcome these inherent limitations, discussing the possibility of maintaining absorption at any desired high level, while arbitrarily reducing the total scattering from the sensor, by staggering few absorption channels in a single receiver. Our study reveals the "optimal" combination of these channels that guarantees the least perceivable detector design. We show that, unlike traditional single-channel receivers, all contributing harmonics must be excited at the same, largely mismatched level. Consequently, each scattering harmonic supplies only a small portion of the total absorption, proportional to its order, and the overall scattering can decrease to any arbitrarily small level by proper interference among more harmonics. Even more importantly, we show that, due to the large level of mismatch, the system shows a moderate overall Q-factor even in the presence of higher-order scattering harmonics, making the proposed optimal minimum-scattering absorbers practically realizable over reasonable bandwidths. We present several examples of realistic structures based on core-shell plasmonic nanoparticles with absorption levels comparable to current sensor designs but scattering 7 to 13 times less, each suitably designed to include different absorption channels. Our findings may have a broad range of applications in sensor design, opening up exciting possibilities in biomedical technology, energy harvesting, security and imaging.

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