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We explore a versatile technique for inverse designing 2D photonic crystal metasurfaces. These surfaces, known for their ability to manipulate light-matter interactions, can be precisely controlled to achieve specific functionalities. The key lies in efficiently optimizing the geometric patterns and dimensions of the metasurface. Through a composite method which exploits two well-established paradigms - Covariance Matrix Adaptation optimization and Rigorous Coupled Wave Analysis (RCWA), we demonstrate our ability to design and optimize resonances in metaelements to achieve desired optical performance such as near-perfect absorption at chosen wavelengths/optical modes, which otherwise proves to be challenging or even impossible with conventional inverse design implementations. We apply our method to design three-layered structures involving a monolayer absorber, transparent metasubstrate, and a back mirror to get near 100% absorption at one or two chosen wavelengths. For illustration, we choose black phosphorus and silicon metasurface to predict ~100% absorption in a monolayer at 1550 nm. The versatile technique can be applied to tailor reflectance and transmittance for any optical mode and wavelength. This computationally efficient design method paves the way for creating high-performance 2D metasurface-based devices with a variety of applications, including quantum technology components such as single photon sensors and biphoton sources, communication systems, and non-linear light conversion.

We discuss the behaviour of cooperative effects in the emitted light from a system of two nitrogen-vacancy (NV) centers confined in a nanopillar having dimensions close to the wavelength of the excitation pump. We experimentally observe a $g^{(2)}(0) > 0.5 \to 1$ for the coupled emitter system and a drastic decrease in the singlet and triplet lifetimes by a factor of $\approx 6$, indicating an interaction between the two emitters, which indicates superradiant behaviour. We theoretically study the dissipative dynamics of the interaction of the emitter system with the excitation light at a finite temperature for three cases (single emitter, two emitters with and without dipole-dipole coupling) by solving the Lindblad master equation and providing an analytical expression for the second-order correlation function. We observe that from the master equation, the populations and coherences mix for the two emitters' cases. Through this, we support our experimental results and discuss superradiance from our system. Finally, we discover a reliable quantum random number generation rate of $\sim 200$ kHz from the coupled emitter system, at low pump powers.

Achieving high quantum efficiency (QE) with low dark count is essential for highly sensitive photodetectors (PDs), including single photon avalanche detectors (SPADs). However, high QE requires a thicker absorber region, which leads to high dark current and noise, which in turn affects the detectivity of PDs and the photodetection efficiency and dark count of SPADs.The holy grail of photodetector and avalanche photodiode designs is to achieve highest QE with thinnest absorber and still enable large avalanche to gain as needed. We have developed a new design paradigm which exploits the coupling between dielectric Mie resonance and transverse propagating waves in thin layers. The Mie resonance launches the incident light at an angle in an ultrathin absorber, and when coupled to transverse waves, the light propagates laterally and is fully absorbed owing to the longer optical path. Consequently, with appropriate choice of materials for a chosen wavelength, a high absorption(~90%) within typically <100 nm absorber thickness is possible. For illustration, we apply our approach to design Si-based detector operating at 810 nm and InGaAs-based detector operating at 1550 nm and predict that the dark current at room temperature is reduced at least by two orders of magnitude. In addition, the lateral distances are often in a few microns and hence these designs can potentially enable avalanching for a large optical gain., Comment: 6 figures

Fluorescent atomic defects, especially in dielectric materials, such as diamond are quite promising for several emerging quantum applications. However, efficient light extraction, directional emission, and narrow spectral emission are key challenges. We have designed dielectric metasurface exploiting Mie-resonance and the Kerker condition to address these issues. Our designed diamond metasurface, tailored for nitrogen-vacancy (NV) defect centers in diamond, predicts up to 500x improvement in the collection of 637 nm (zero phonon line) photons over that from the bare diamond. Our design achieves highly directional emission, predominantly emitting in a 20 degree lobe in the forward direction. This makes light collection more efficient, including for fiber-based collection. The predicted results are stable against the position of the emitter placed in the metaelement, thus alleviating the challenging fabrication requirement of precise positioning of the defect center. Equally importantly, our design approach can be applied to enhance single photon emission also from other defects such as SiV, other materials such as hBN, and other sources such as quantum dots., Comment: 10 Pages, 5 figures, 1 table

Quantum metrology aims at achieving enhanced performance in measuring unknown parameters by utilizing quantum resources. Thus, quantum metrology is an important application of quantum technologies. Photonic systems can implement these metrological tasks with simpler experimental techniques. We present a scheme for improved parameter estimation by introducing entangled generalized coherent states (EGCS) for photonic quantum metrology. These states show enhanced sensitivity beyond the classical and Heisenberg limits and prove to be advantageous as compared to the entangled coherent and NOON states. Further, we also propose a scheme for experimentally generating certain entangled generalized coherent states with current technology.

In the following paper, we discuss a possible detection of non-locality in two-mode light states in the Bell protocol, where the local observables are constructed using displacement operators, implemented by Mach-Zender Interferometers fed by strong coherent states. We report numerical results showing that maximizing the Braunstein-Caves Chained Bell (BCCB) inequalities requires equal phases of displacements. On the other hand, we prove that non-locality cannot be detected if the phases of displacements are unknown. Hence, the Bell experiment has to be equipped with a synchronization mechanism. We discuss such a mechanism and its consequences.

We demonstrate optical detection of a broad spectrum of ferromagnetic excitations using nitrogen-vacancy (NV) centers in an ensemble of nanodiamonds. Our recently developed approach exploits a straightforward CW detection scheme using readily available diamond detectors, making it easily implementable. The NV center is a local detector, giving the technique spatial resolution, which here is defined by our laser spot, but in principle can be extended far into the nanoscale. Among the excitations we observe are propagating dipolar and dipolar-exchange spinwaves, as well as dynamics associated with the multi-domain state of the ferromagnet at low fields. These results offer an approach, distinct from commonly used ODMR techniques, for spatially resolved spectroscopic study of magnetization dynamics at the nanoscale., Comment: 6 pages and 1 page supplmentary information