Nano-antenna array for high efficiency sunlight harvesting.

Solar rectennas are promising devices for energy harvesting. Capability of rectennas to convert incident light into useful energy depends on the antenna efficiency, that is the ratio between the power transferred to the load vs the incoming power. In this work, we first emphasize that for the efficiency to be calculated accurately, antennas need to be treated as receiving devices, not as transmitting ones. Then, we propose an arrangement of antennas that differs from those published so far in three respects: (1) the proposed arrangement is formed by an array of nano-antennas with sub-wavelength inter-element spacing, (2) it comprises a reflecting mirror, and (3) it allows for dual polarization operation. Through numerical simulations, we show that the small lattice pitch we use is responsible for frequency flattening of the lattice impedance over the whole solar spectrum, eventually allowing for excellent matching with the antennas' loads. Also, the small pitch allows for a smooth dependence of the receiving efficiency on the angle of incidence of sunlight. Finally, we show numerically that the reflecting mirror also allows for an almost complete cancellation of light scattered by the receiving antennas. The final result is a polarization insensitive receiving theoretical efficiency larger than 70% over the whole 300-3000 nm spectral range, with a less than 10% energy wasting due to back-scattering of sunlight.

Method for characterization of Si waveguide propagation loss.

A new method for measuring waveguide propagation loss in silicon nanowires is presented. This method, based on the interplay between traveling ring modes and standing wave modes due to back-scattering from edge roughness, is accurate and can be used for on wafer measurement of test structures. Examples of loss measurements and fitting are reported.

Graphene sustained nonlinear modes in dielectric waveguides.

We discuss the existence of nonlinear modes sustained by graphene layers in dielectric waveguides. Taking advantage of the almost two dimensional nature of graphene, we introduce the nonlinear effect as a parameter in the continuity equations. We then apply our modeling to a simple slab waveguide to enlighten how graphene can be used to induce huge nonlinear phase shifts at easily accessible power levels.

Graphene-assisted control of coupling between optical waveguides.

The unique properties of optical waveguides electrically controlled by means of graphene layers are investigated. We demonstrate that, thanks to tunable losses induced by graphene layers, a careful design of silicon on silica ridge waveguides can be used to explore passive PT-symmetry breaking in directional couplers. We prove that the exceptional point of the system can be probed by varying the applied voltage and we thus propose very compact photonic structures which can be exploited to control coupling between waveguides and to tailor discrete diffraction in arrays.

Graphene-assisted critically-coupled optical ring modulator.

Graphene's conductivity at optical frequencies can be varied upon injection of carriers. In the present paper, this effect is used to modulate losses of an optical wave traveling inside a ring cavity. This way an optical modulator based on the critical-coupling concept first introduced by Yariv can be realized. Through numerical simulations, we show that a modulator featuring a bandwidth as large as 100 GHz can be designed with switching energy in the order of few fJ per bit. Also, we show that operations with driving voltages below 1.2 volt could be obtained, thus making the proposed modulator compatible with requirements of low-voltage CMOS technology.

Modeling of enhanced field confinement and scattering by optical wire antennas.

We describe the application of full-wave and semi-analytical numerical tools for the modeling of optical wire antennas, with the aim of providing novel guidelines for analysis and design. The concept of antenna impedance at optical frequencies is reviewed by means of finite-element simulations, whereas a surface-impedance integral equation is derived in order to perform an accurate and efficient calculation of the current distribution, and thereby to determine the equivalent-circuit parameters. These are introduced into simple circuits models, directly borrowed from radio frequency, which are applied in order to model the phenomena of enhanced field confinement at the feed gap and light scattering by optical antennas illuminated by plane waves.

Performance Appraisal and Innovative Behavior in the Digital Era.

In digital competitive environments, organizations' ability to innovate is more than ever the key to competitive advantage. One way to cope with this increased pressure for innovation is to capitalize on employees' ability to generate new ideas and use these as building blocks for new and better products, services, and work processes. Individual innovation thus emerges as a key competence required from workers, in turn crucially affecting the way managers make employees contribute to organizational goals and assess their performance. This study draws on the process-based approach to HRM (Bowen and Ostroff, 2004) suggesting that HRM practices may have a signaling effect, to address the following research question: which specific characteristics of performance appraisal are more likely to be perceived as promoting individual innovation at work? To address this issue, we carried out a survey on 865 employees working in large, multinational firms operating in digitalized sectors or industries with the potential to become digitalized. We collected data on the main characteristics of the performance appraisal systems adopted by the firm where respondents work, as perceived by employees themselves. We gathered also data on the respondents' overall perception that performance appraisal boosts innovative work behavior (IWB). Then, we employed logit analysis to test the relationship between data on performance appraisal systems and data on the effectiveness of performance appraisal as a booster of IWB. Our results reveal that, as compared to informal feedback, formal performance appraisal is more likely to reduce the perception that performance appraisal promotes individual innovation and creativity at work. In addition, we found that in the employees' perception performance appraisal focused on the achievement of pre-set, quantitative outcomes is more likely to affect positively IWB than appraisal focused on pre-defined skills that employees exhibited performing their work. However, performance assessment focused on the new competences developed by the employees has a perceived positive impact even stronger than result-oriented appraisal. Taken together, these results contribute to advance our understanding of how organizations should evaluate employees in the digitalization era.