Volume 13, Issue 25 - Special Issue: Current Trends in Nanophotonics

Nucleic acids (deoxyribonucleic acid – DNA and ribonucleic acid – RNA) are essential components of all living organisms, with DNA encoding genetic information and RNA facilitating vital biological processes. The detection of nucleic acids having a specific sequence is crucial for identifying organisms and diagnosing genetic diseases. Because surface-enhanced Raman spectroscopy (SERS) is considered as one of the most promising analytical methods that offers important benefits such as short analysis time and exceptional sensitivity compared to other techniques, many groups are trying to apply SERS for nucleic acid detection. This review discusses how SERS spectroscopy can be used for DNA/RNA detection. Beginning with an overview of SERS theory, we delve into various SERS DNA/RNA sensors, including those based on a direct analysis of the SERS spectra of nucleic acids, and many types of sensors based on a selective hybridisation of probe and target nucleic acids. We describe how various types of sensors with increased sensitivity and reliability have evolved (from the first SERS DNA/RNA sensors described in the literature to recently developed ones). Challenges and future directions in SERS sensor development for nucleic acid detection and determination are also discussed. This comprehensive review aims to help researchers understand the field’s nuances, and to foster advancements in the use of SERS spectroscopy in the medical sector.

The design of optical materials in nonlinear devices represents a fundamental step for their optimization and miniaturization, that would significantly contribute to the progress of advanced nanophotonics and quantum technologies. In this work, the effect of geometry and composition of multilayer hyperbolic metamaterials on their third-order nonlinear optical properties, i.e. the optical Kerr effect, is investigated. One figure of merit is provided to be used as a predictive tool to design and best exploit the local intensity enhancement in low-loss metamaterials to be used for various applications in nonlinear nanophotonics.

We add a missing element to the set of directly computable scenarios of light-matter-interaction within classical numerical Maxwell solvers, i.e., light scattering from hybrid systems of resonators and individual Fourier-limited emitters. In particular, individual emitters are incorporated as tiny polarizable and resonant spherical scatterers. This emitter model is based on well-known extremal properties of Mie modes. The spherical emitter is made from an artificial Drude metal with ϵ ( ω ) = ϵ b − ω p 2 / ( ω 2 + i Γ ω ) ${\epsilon}(\omega )={{\epsilon}}_{b}-{\omega }_{p}^{2}/({\omega }^{2}+i{\Gamma }\omega )$ . By tuning ϵ b and ω p we adjust the resonance frequency and the Fourier-limited linewidth and by adjusting Γ we may add non-radiative damping or dephasing. This approach automatically reproduces the ideal text book coherent scattering cross-section of Fourier-limited two level quantum systems of σ 0 = 3 λ 2 /(2 πϵ out ) which is not possible with typically used Lorentz permittivities which only mimic optical resonances. Further, the emitter’s linewidth adopts to the surrounding optical local density of states (LDOS). To demonstrate this we successfully benchmark our approach with prominent examples from the literature.

In this work, we analyze the spontaneous emission dynamics of a V-type quantum emitter near a graphene nanodisk based on the combination of electromagnetic and quantum dynamical calculations. The presence of the graphene nanodisk gives strong anisotropy to the Purcell factors of the quantum emitter, leading to interference effects in spontaneous emission appearing as coupling between the emitter’s upper levels. This effect is further enhanced by the strong light–matter interaction of the quantum emitter with the modified electromagnetic mode continuum, which induces non-Markovian spontaneous emission dynamics. We have studied the population dynamics of the quantum emitter at a specific distance from the center of the graphene nanodisk for various free-space decay widths and different quantum emitter’s initial conditions and have shown weak coupling results appearing with Markovian decay dynamics, obtained for quantum emitters with small free-space decay widths, and population dynamics that exhibits distinctly non-Markovian features, such as prominent decaying Rabi oscillations in the population evolution of the quantum emitter’s excited states and energy exchange between them during the overall population decay into the photonic mode continuum for largest free-space decay widths. Also, for the largest value of the free-space decay width, we obtain significant population trapping effects in the excited states of the quantum emitter. Furthermore, we find that the population dynamics for specific light–matter interaction strength conditions between the quantum emitter and the graphene nanodisk depend distinctively on the initial state of the quantum emitter, whether it is a single state or a superposition state.

Strong coupling between light and molecular matter is currently attracting interest both in chemistry and physics, in the fast-growing field of molecular polaritonics. The large near-field enhancement of the electric field of plasmonic surfaces and their high tunability make arrays of metallic nanoparticles an interesting platform to achieve and control strong coupling. Two dimensional plasmonic arrays with several nanoparticles per unit cell and crystalline symmetries can host topological edge and corner states. Here we explore the coupling of molecular materials to these edge states using a coupled-dipole framework including long-range interactions. We study both the weak and strong coupling regimes and demonstrate that coupling to topological edge states can be employed to enhance highly-directional long-range energy transfer between molecules.

Light carries momentum and, upon interaction with material structures, can exert forces on them. Here, we show that a planar structure with asymmetric optical response is spontaneously accelerated when placed in an environment at a different temperature. This phenomenon originates from the imbalance in the exchange rates of photons between both sides of the structure and the environment. Using a simple theoretical model, we calculate the force acting on the planar structure and its terminal velocity in vacuum, and analyze their dependence on the initial temperature and the geometrical properties of the system for different realistic materials. Our results unravel an alternative approach to manipulating objects in the nano and microscale that does not require an external source of radiation.

Non-destructive reference-free grazing incidence X-ray fluorescence (RF-GIXRF) is proposed as a highly effective analytical technique for extracting molecular arrangement density in self-assembled monolayers. The establishment of surface density standards through RF-GIXRF impacts various applications, from calibrating laboratory XRF setups to expanding its applicability in materials science, particularly in surface coating scenarios with molecular assemblies. Accurate determination of coverage density is crucial for proper functionalization and interaction, such as in assessing the surface concentration of probes on plasmonic nanostructures. However, limited synchrotron radiation access hinders widespread use, prompting the need for molecular surface density standards, especially for benchmarking substrates for surface-enhanced Raman and infrared absorption spectroscopies (SERS and SEIRA) as well as associated surface-enhanced techniques. Using reproducible densities on gold ensures a solid evaluation of the number of molecules contributing to enhanced signals, facilitating comparability across substrates. The research discusses the importance of employing molecular surface density standards for advancing the field of surface-enhanced spectroscopies, encouraging collaborative efforts in protocol development and benchmarking in surface science.

This study reports on the fabrication of quantum dot light-emitting diodes (QLEDs) with an ITO/Ni 1− x Mg x O/SAM/TFB/QDs/ZnMgO/Al structure and investigates the effects of various Mg doping concentrations in NiO on device performance. By doping Mg into the inorganic hole-injection layer NiO (Ni 1− x Mg x O), we improved the band alignment with the hole-injection layer through band tuning, which enhanced charge balance. Optimal Mg doping ratios, particularly a Ni 0.9 Mg 0.1 O composition, have demonstrated superior device functionality, underscoring the need for fine-tuned doping levels. Further enhancements were achieved through surface treatments of Ni 0.9 Mg 0.1 O with UV-Ozone (UVO) and thermal annealing (TA) of the ZnMgO electron transport layer. Consequently, by optimizing Mg-doped NiO in QLED devices, we achieved a maximum external quantum efficiency of 8.38 %, a brightness of 66,677 cd/m 2 , and a current efficiency of 35.31 cd/A, indicating improved performance. The integration of Mg-doped NiO into the QLED structure resulted in a device with superior charge balance and overall performance, which is a promising direction for future QLED display technologies.

Photonic funnels, microscale conical waveguides that have been recently realized in the mid-IR spectral range with the help of an all-semiconductor designer metal material platform, are promising devices for efficient coupling of light between the nanoscales and macroscales. Previous analyses of photonic funnels have focused on structures with highly conductive claddings. Here, we analyze the performance of funnels with and without cladding, as a function of material properties, operating wavelength, and geometry. We demonstrate that bare (cladding-free) funnels enable orders-of-magnitude higher enhancement of local intensity than their clad counterparts, with virtually no loss of confinement, and relate this phenomenon to anomalous reflection of light at the anisotropic material–air interface. Intensity enhancement of the order of 25, with confinement of light to wavelength/20 scale, is demonstrated. Efficient extraction of light from nanoscale areas is predicted.

Optical standing waves are intrinsically nanometric, spatially fixed interference-field patterns. At a commensurate scale, metallic nanotips serve as coherent, atomic-sized electron sources. Here, we explore the localized photofield emission from a tungsten nanotip with a transient standing wave. It is generated within an optical cavity with counter-propagating femtosecond pulses from a near-infrared, 100-MHz frequency comb. Shifting the phase of the standing wave at the tip reveals its nodes and anti-nodes through a strong periodic modulation of the emission current. We find the emission angles to be distinct from those of a traveling wave, and attribute this to the ensuing localization of emission from different crystallographic planes. Supported by a simulation, we find that the angle of maximum field enhancement is controlled by the phase of the standing wave. Intra-cavity nanotip interaction not only provides higher intensities than in free-space propagation, but also allows for structuring the light field even in the transverse direction by selection of high-order cavity modes.

The optical cross sections of plasmonic nanoparticles are intricately linked to their morphologies. Accurately capturing this link could allow determination of particles’ shapes from their optical cross sections alone. Electromagnetic simulations bridge morphology and optical properties, provided they are sufficiently accurate. This study examines key factors affecting simulation precision, comparing common methods and detailing the impacts of meshing accuracy, dielectric function selection, and substrate inclusion within the boundary element method. To support the method’s complex parameterization, we develop a workflow incorporating reconstruction, meshing, and mesh simplification, to enable the use of electron tomography data. We analyze how choices of reconstruction algorithm and image segmentation affect simulated optical cross sections, relating these to shape errors minimized during data processing. Optimal results are obtained using the total variation minimization (TVM) reconstruction method with Otsu thresholding and light smoothing, ensuring reliable, watertight surface meshes through the marching cubes algorithm, even for complex shapes.

The ability of surface polaritons (SPs) to enhance and manipulate light fields down to deep-subwavelength length scales enables applications in optical sensing and nonlinear optics at the nanoscale. However, the wavelength mismatch between light and SPs prevents direct optical excitation of surface-bound modes, thereby limiting the widespread development of SP-based photonics. Free electrons are a natural choice to directly excite strongly confined SPs because they can supply field components of high momentum at designated positions with subnanometer precision. Here, we theoretically explore free-electron–SP coupling mediated by small scatterers and show that low-energy electrons can efficiently excite surface modes with a maximum probability reached at an optimum surface–scatterer distance. By aligning the electron beam with a periodic array of scatterers placed near a polariton-supporting interface, in-plane Smith–Purcell emission results in the excitation of surface modes along well-defined directions. Our results support using scattering elements to excite SPs with low-energy electrons.