Optics (physics.optics)

Abstract: The terahertz (THz) frequency range is key to study collective excitations in many crystals and organic molecules. However, due to the large wavelength of THz radiation, the local probing of these excitations in smaller crystalline structures or few-molecular arrangements, requires sophisticated methods to confine THz light down to the nanometer length scale, as well as to manipulate such a confined radiation. For this purpose, in recent years, taking advantage of hyperbolic phonon polaritons (HPhP) in highly anisotropic van der Waals (vdW) materials has emerged as a promising approach, offering a multitude of manipulation options such as control over the wavefront shape and propagation direction. Here, we demonstrate the first THz application of twist-angle-induced HPhP manipulation, designing the propagation of confined THz radiation between 8.39 and 8.98 THz in the vdW material alpha-molybdenum trioxide ($\alpha-MoO_{3}$), hence extending twistoptics to this intriguing frequency range. Our images, recorded by near-field optical microscopy, show the frequency- and twist-angle-dependent change between hyperbolic and elliptic polariton propagation, revealing a polaritonic transition at THz frequencies. As a result, we are able to allocate canalization (highly collimated propagation) of confined THz radiation by carefully adjusting these two parameters, i.e. frequency and twist angle. Specifically, we report polariton canalization in $\alpha-MoO_{3}$ at 8.67 THz for a twist angle of 50{\deg}. Our results demonstrate an unprecedented control and the manipulation of highly-confined collective excitations at THz frequencies, offering novel possibilities for nanophotonic applications.

Abstract: Perovskite solar cells (PSCs) are the most promising technology for advancing current photovoltaic performance. However, the main challenge for their practical deployment and commercialization is their operational stability, affected by solar illumination and heating, as well as the electric field that is generated in the PV device by light exposure. Here, we propose a transparent multispectral photonic electrode placed on top of the glass substrate of solar cells, which simultaneously reduces the device solar heating and enhances its efficiency. Specifically, the proposed photonic electrode, composed of a low-resistivity metal and a conductive layer, simultaneously serves as a highly-efficient infrared filter and an ultra-thin transparent front contact, decreasing devices' solar heating and operating temperature. At the same time, it simultaneously serves as an anti-reflection coating, enhancing the efficiency. We additionally enhance the device cooling by coating the front glass substrate side with a visibly transparent film (PDMS), which maximizes substrate's thermal radiation. To determine the potential of our photonic approach and fully explore the cooling potential of PSCs, we first provide experimental characterizations of the absorption properties (in both visible and infrared wavelengths) of state-of-the-art PSCs among the most promising ones regarding the efficiency, stability, and cost. We then numerically show that applying our approach to promising PSCs can result in lower operating temperatures by over 9.0 oC and an absolute efficiency increase higher than 1.3%. These results are insensitive to varying environmental conditions. Our approach is simple and only requires modification of the substrate; it therefore points to a feasible photonic approach for advancing current photovoltaic performance with next-generation solar cell materials.

Abstract: The frozen mode regime is a unique slow-light scenario in periodic structures, where the flat-bands (zero group velocity) are associated with the formation of high-order stationary points (aka exceptional points). The formation of exceptional points is accompanied by enhancement of various optical properties such as gain, Q-factor and absorption, which are key properties for the realization of wide variety of devices such as switches, modulators and lasers. Here we present and study a new integrated optical periodic structure consisting of three waveguides coupled via micro-cavities and directional coupler. We study this design theoretically, demonstrating that a proper choice of parameters yields a third order stationary inflection point (SIP). We also show that the structure can be designed to exhibit two almost-overlapping SIPs at the center of the Brillouin Zone. We study the transmission and reflection of light propagating through realistic devices comprising a finite number of unit-cells and investigate their spectral properties in the vicinity of the stationary points. Finally, we analyze the lasing frequencies and threshold level of finite structures (as a function of the number of unit-cells) and show that it outperforms conventional lasers utilizing regular band edge lasing (such as DFB lasers).

Abstract: Near-field optics can overcome the diffraction limit by creating strong optical gradients to enable the trapping of nanoparticles. However, it remains challenging to achieve efficient stable trapping without heating and thermal effects. Dielectric structures have been used to address this issue, but they usually offer weak trap stiffness. In this work, we exploit the Fano resonance effect in an all-dielectric quadrupole nanostructure to realize a twenty-fold enhancement of trap stiffness, compared to the off-resonance case. This enables a high effective trap stiffness of $1.19$ fN/nm for 100 nm diameter polystyrene nanoparticles with 3.5 mW/$\mu$m$^{2}$ illumination. Furthermore, we demonstrate the capability of the structure to simultaneously trap two particles at distinct locations within the nanostructure array.