Subwavelength sorting of full-color based on anti-Hermitian metasurfaces

Splitting the spectrum of incident light at nanoscale has been of great scientific and practical interest due to its potential application in various optical sensors. For many years, researchers have been striving to realize the full-color sorting of light at subwavelength scale, while keeping the loss of incident photons to a minimum. In this article, we present semiconductor-based metasurfaces that facilitate the efficient sorting of full-color by inducing anti-Hermitian coupling between multiple nanoantenna arrays. To achieve this, we first explore how the coherent interactions between maximally crafted nanoantennas in the metasurfaces can be effectively controlled by judiciously positioning them in both lateral and vertical directions, which leads to the switched coupling of light at each target position. Based on the analysis, we demonstrate a metasurface-based absorber that features efficient, spectropolarimetric detections over the entire visible spectrum, ranging from 470 to 630 nm. In addition, the metasurface detects relatively narrow spectral linewidth of 60 nm and shows the sensitivity up to 70%, which surpasses the previous works on subwavelength photon sorting or color filter-based detection system. We envision that our approach provides guidelines for realizing the metasurfaces with enhanced functionalities, that is the increase of spectral channels for detection in a given subwavelength-scaled unit cell.

Determination of the Best Empiric Method to Quantify the Amplified Spontaneous Emission Threshold in Polymeric Active Waveguides

Amplified Spontaneous Emission (ASE) threshold represents a crucial parameter often used to establish if a material is a good candidate for applications to lasers. Even if the ASE properties of conjugated polymers have been widely investigated, the specific literature is characterized by several methods to determine the ASE threshold, making comparison among the obtained values impossible. We quantitatively compare 9 different methods employed in literature to determine the ASE threshold, in order to find out the best candidate to determine the most accurate estimate of it. The experiment has been performed on thin films of an homopolymer, a copolymer and a host:guest polymer blend, namely poly(9,9-dioctylfluorene) (PFO), poly(9,9-dioctylfluorene-cobenzothiadiazole) (F8BT) and F8BT:poly(3- hexylthiophene) (F8BT:rrP3HT), applying the Variable Pump Intensity (VPI) and the Variable Stripe Length (VSL) methods. We demonstrate that, among all the spectral features affected by the presence of ASE, the most sensitive is the spectral linewidth and that the best way to estimate the ASE threshold is to determine the excitation density at the beginning of the line narrowing. We also show that the methods most frequently used in literature always overestimate the threshold up to more than one order of magnitude.