Annika Buchheit

Metasurface Reconfiguration through Lithium-Ion Intercalation in a Transition Metal Oxide

In the latest years the optical engineers toolbox has welcomed a new concept, the metasurface. In a metasurface, properly tailored material inclusions are able to reshape the electromagnetic field of an incident beam. Change of amplitude, phase and polarization can be addressed within a thickness of only a fraction of a wavelength. By means of this concept, a radical gain in compactness of optical components is foreseen, even of the most complex ones; other unique features like that of analog computing have also been identified. With this huge potential ready to be disclosed, lack of tunability is still a main barrier to be broken. Metasurfaces must now be made reconfigurable, i.e. able to modify and memorize their state, possibly with a small amount of energy. In this Communication we report low-energy, self-holding metasurface reconfiguration through lithium intercalation in a vanadium pentoxide layer integrated within the photonic device. By a proper meta-atom design, operation on amplitude and phase of linearly polarized light has been demonstrated. In addition, manipulation of circularly polarized light in the form of tunable chirality and tunable handedness-preserving reflection has been implemented. These operations are accomplished using as low as 50 pJ/{\mu}m^{2}, raising lithium intercalation in transition metal oxides as one of the most energy efficient self-holding tuning mechanisms known so far for metasurfaces, with significant perspectives in the whole field of nanophotonics.

Controlling the optical properties of sputtered-deposited LixV2O5 films

This study examines the influence of lithium intercalation on the optical properties of vanadium pentoxide films. The films with a thickness between 400 and 1000 nm were prepared by DC magnetron sputter deposition. Cyclic voltammetry and chronopotentiometry were used to set a well defined lithiation state of the LixV2O5 films between x = 0 and x = 1. The optical properties of these films were measured by optical reflectometry in the wavelength range between 500 and 1700 nm. From the reflectance data, the refractive index and the extinction coefficient of the films were finally calculated as a function of the wavelength using Cauchys dispersion model. The results confirm that the optical behavior of LixV2O5 films varies significantly upon lithium insertion. It is demonstrated that the changes produced in the optical properties are completely reversible within the limits of permanent structure changes.

A dynamically tunable chiral mirror enabled by electrochromic metasurfaces operating at telecommunication wavelengths

In this paper we introduce the concept of tunable chiral mirror and provide an experimental evidence of its implementation. A chiral mirror, i.e., a device which reflects circularly polarized light of a single handedness without inverting it, has been synthesized by numerical optimization of a metasurface composed of non-centrosymmetric meta-atoms. Operation has been targeted to the telecommunication wavelength through a judicious rescaling and reshaping of the meta-atom geometry. Embedding an electrochromic material (V2O5) in the layer stack, dynamical reconfiguration of the chiral mirror has been achieved, with a theoretical modulation depth of more than 35 dB. Reconfiguration is self-holding and reversible, showing that electrochromism in V2O5 could prove to be a powerful tool to modulate the response of complex beam shaping and polarization handling metasurface-based ultrathin photonic components.

Noninvasive monitoring and control in silicon photonics

Advanced technologies to implement on-chip monitoring and feedback control operations are required to make silicon photonics scale to large-scale-of-integration. Transparent detectors and energy saving actuators are key ingredients of this paradigm. On-chip detectors are required to be minimally invasive in order to allow their integration in key spots of the circuit, thus easing control operation through the partitioning of complex architectures in smaller cluster of devices and the realization of local feedback control loops. Non volatile integrated actuators, which are reversible switching devices that can maintain the state without the need of “always on” power dissipation, are also needed to reduce the power consumption required by tuning, reconfiguration and stabilization operations. Addressing these issues, in this contribution we report on the performance of a recently developed transparent detector, named ContacLess Integrated Photonic Probe (CLIPP), that can monitor in line the intensity of the light in silicon waveguides without introducing any photon absorption in excess to the waveguide propagation loss. A systematic characterization of the CLIPP detector is here presented, specifically addressing the dependence of the CLIPP performance on the waveguide geometry and on the polarization and wavelength of the light. Concerning the development of non-volatile integrated actuators, we demonstrate the possibility to manipulate the light transmission in silicon waveguides by electrochemical insertion of mobile ions in a mixed ionic and electronic conductor (MIEC) used as upper cladding of a silicon waveguide. A finely controllable and reversible change of the imaginary part of the refractive index of the MIEC film is exploited to trim the loss of a silicon waveguide and to modify the frequency response of a silicon microring resonator.