Time-Domain Separation of Optical Properties From Structural Transitions in Resonantly Bonded Materials

The extreme electro-optical contrast between crystalline and amorphous states in phase change materials is routinely exploited in optical data storage and future applications include universal memories, flexible displays, reconfigurable optical circuits, and logic devices. Optical contrast is believed to arise due to a change in crystallinity. Here we show that the connection between optical properties and structure can be broken. Using a unique combination of single-shot femtosecond electron diffraction and optical spectroscopy, we simultaneously follow the lattice dynamics and dielectric function in the phase change material Ge2Sb2Te5 during an irreversible state transformation. The dielectric function changes by 30% within 100 femtoseconds due to a rapid depletion of electrons from resonantly-bonded states. This occurs without perturbing the crystallinity of the lattice, which heats with a 2 ps time constant. The optical changes are an order-of-magnitude larger than those achievable with silicon and present new routes to manipulate light on an ultrafast timescale without structural changes

Formamidinium Incorporation into Compact Lead Iodide for Low Band Gap Perovskite Solar Cells with Open-Circuit Voltage Approaching the Radiative Limit

To bring hybrid lead halide perovskite solar cells toward the Shockley-Queisser limit requires lowering the band gap while simultaneously increasing the open-circuit voltage. This, to some extent divergent objective, may demand the use of largecations to obtain a perovskite with larger lattice parameter together with a large crystalsize to minimize interface nonradiative recombination. When applying the two-stepmethod for a better crystal control, it is rather challenging to fabricate perovskites withFA+cations, given the small penetration depth of such large ions into a compact PbI2film. In here, to successfully incorporate such large cations, we used a high-concentration solution of the organic precursor containing small Cl-anions achieving,via a solvent annealing-controlled dissolution-recrystallization, larger than 1µmperovskite crystals in a solar cell. This solar cell, with a largely increasedfluorescencequantum yield, exhibited an open-circuit voltage equivalent to 93% of thecorresponding radiative limit one. This, together with the low band gap achieved(1.53 eV), makes the fabricated perovskite cell one of the closest to the Shockley-Queisser optimum.Peer ReviewedPostprint (author's final draft

Electrically Driven Varifocal Silicon Metalens

Optical metasurfaces have shown to be a powerful approach to planar optical elements, enabling an unprecedented control over light phase and amplitude. At that stage, where a wide variety of static functionalities have been accomplished, most efforts are being directed toward achieving reconfigurable optical elements. Here, we present our approach to an electrically controlled varifocal metalens operating in the visible frequency range. It relies on dynamically controlling the refractive index environment of a silicon metalens by means of an electric resistor embedded into a thermo-optical polymer. We demonstrate precise and continuous tuneability of the focal length and achieve focal length variation larger than the Rayleigh length for voltage as small as 12 V. The system time-response is of the order of 100 ms, with the potential to be reduced with further integration. Finally, the imaging capability of our varifocal metalens is successfully validated in an optical microscopy setting. Compared to conventional bulky reconfigurable lenses, the presented technology is a lightweight and compact solution, offering new opportunities for miniaturized smart imaging devices.Peer ReviewedPostprint (author's final draft

Optical switching at 1.55 µm in silicon racetrack resonators using phase change materials

An optical switch operating at a wavelength of 1.55¿µm and showing a 12 dB modulation depth is introduced. The device is implemented in a silicon racetrack resonator using an overcladding layer of the phase change data storage material Ge2Sb2Te5, which exhibits high contrast in its optical properties upon transitions between its crystalline and amorphous structural phases. These transitions are triggered using a pulsed laser diode at ¿¿=¿975¿nm and used to tune the resonant frequency of the resonator and the resultant modulation depth of the 1.55¿µm transmitted light

Grating couplers integrated on Mach-Zehnder interferometric biosensors operating in the visible range

We present the design, fabrication, and characterization of submicronic grating couplers integrated on SiN rib waveguide Mach-Zehnder interferometers (MZIs) for biosensing applications working in the visible spectral range for both TE and TM polarizations. Depending on the waveguide structure, a maximum of 11.5% of coupling efficiency has been experimentally obtained at 658 nm, while a limit of detection of 1.6 × 10 in refractive index unit is achieved for the biosensor. These results represent an important milestone toward the achievement of a truly portable and multiplexed point-of-care platform using the integrated MZI sensor

Time-domain separation of optical properties from structural transitions in resonantly bonded materials

The extreme electro-optical contrast between crystalline and amorphous states in phase-change materials is routinely exploited in optical data storage1 and future applications include universal memories2, flexible displays3, reconfigurable optical circuits4, 5, and logic devices6. Optical contrast is believed to arise owing to a change in crystallinity. Here we show that the connection between optical properties and structure can be broken. Using a combination of single-shot femtosecond electron diffraction and optical spectroscopy, we simultaneously follow the lattice dynamics and dielectric function in the phase-change material Ge2Sb2Te5 during an irreversible state transformation. The dielectric function changes by 30% within 100 fs owing to a rapid depletion of electrons from resonantly bonded states. This occurs without perturbing the crystallinity of the lattice, which heats with a 2-ps time constant. The optical changes are an order of magnitude larger than those achievable with silicon and present new routes to manipulate light on an ultrafast timescale without structural changes.Peer ReviewedPostprint (author's final draft

Nanophotonic lab-on-a-chip platforms including novel bimodal interferometers, microfluidics and grating couplers

One of the main limitations for achieving truly lab-on-a-chip (LOC) devices for point-of-care diagnosis is the incorporation of the "on-chip" detection. Indeed, most of the state-of-the-art LOC devices usually require complex read-out instrumentation, losing the main advantages of portability and simplicity. In this context, we present our last advances towards the achievement of a portable and label-free LOC platform with highly sensitive "on-chip" detection by using nanophotonic biosensors. Bimodal waveguide interferometers fabricated by standard silicon processes have been integrated with sub-micronic grating couplers for efficient light in-coupling, showing a phase resolution of 6.6 × 10 × 2π rad and a limit of detection of 3.3 × 10 refractive index unit (RIU) in bulk. A 3D network of SU-8 polymer microfluidics monolithically assembled at the wafer-level was included, ensuring perfect sealing and compact packaging. To overcome some of the drawbacks inherent to interferometric read-outs, a novel all-optical wavelength modulation system has been implemented, providing a linear response and a direct read-out of the phase variation. Sensitivity, specificity and reproducibility of the wavelength modulated BiMW sensor has been demonstrated through the label-free immunodetection of the human hormone hTSH at picomolar level using a reliable biofunctionalization process