Approaches and challenges in identifying, quantifying, and manipulating dynamic mitochondrial genome variations.

Mitochondria, the cellular powerhouses, possess their own unique genetic system, including replication, transcription, and translation. Studying these processes is crucial for comprehending mitochondrial disorders, energy production, and their related diseases. Over the past decades, various approaches have been applied in detecting and quantifying mitochondrial genome variations with also the purpose of manipulation of mitochondria or mitochondrial genome for therapeutics. Understanding the scope and limitations of above strategies is not only fundamental to the understanding of basic biology but also critical for exploring disease-related novel target(s), as well to develop innovative therapies. Here, this review provides an overview of different tools and techniques for accurate mitochondrial genome variations identification, quantification, and discuss novel strategies for the manipulation of mitochondria to develop innovative therapeutic interventions, through combining the insights gained from the study of mitochondrial genetics with ongoing single cell omics combined with advanced single molecular tools.

Advanced active polymer probe for near-field optics.

We report on a novel, to the best of our knowledge, active probe for scanning near-field optical microscopy (SNOM). A fluorescent nanosphere, acting as the secondary source, is grafted in an electrostatic manner at the apex of a polymer tip integrated into the extremity of an optical fiber. Thanks to the high photostability and sensitivity of the secondary source, the near-field interaction with a gold nanocube is investigated. It is shown that the spatial resolution is well defined by the size of the fluorescent nanosphere. The polarization-dependent near-field images, which are consistent with the simulation, are ascribed to the local excitation rate enhancement. Meanwhile, measurement of the distance-dependent fluorescence lifetime of the nanosphere provides strong evidence that the local density of states is modified so that extra information on nano-emitters can be extracted during near-field scanning. This advanced active probe can thus potentially broaden the range of applications to include nanoscale thermal imaging, biochemical sensors, and the manipulation of nanoparticles.

Single Photon Source from a Nanoantenna-Trapped Single Quantum Dot.

Single photon sources with high brightness and subnanosecond lifetimes are key components for quantum technologies. Optical nanoantennas can enhance the emission properties of single quantum emitters, but this approach requires accurate nanoscale positioning of the source at the plasmonic hotspot. Here, we use plasmonic nanoantennas to simultaneously trap single colloidal quantum dots and enhance their photoluminescence. The nano-optical trapping automatically locates the quantum emitter at the nanoantenna hotspot without further processing. Our dedicated nanoantenna design achieves a high trap stiffness of 0.6 (fN/nm)/mW for quantum dot trapping, together with a relatively low trapping power of 2 mW/µm2. The emission from the nanoantenna-trapped single quantum dot shows 7× increased brightness, 50× reduced blinking, 2× shortened lifetime, and a clear antibunching below 0.5 demonstrating true single photon emission. Combining nano-optical tweezers with plasmonic enhancement is a promising route for quantum technologies and spectroscopy of single nano-objects.

Plasmonic nano-optical trap stiffness measurements and design optimization.

Plasmonic nano-optical tweezers enable the non-invasive manipulation of nano-objects under low illumination intensities, and have become a powerful tool for nanotechnology and biophysics. However, measuring the trap stiffness of nanotweezers remains a complicated task, which hinders the development of plasmonic trapping. Here, we describe an experimental method to measure the trap stiffness based on the temporal correlation of the fluorescence from the trapped object. The method is applied to characterize the trap stiffness in different double nanohole apertures and explore the influence of their design parameters in relationship with numerical simulations. Optimizing the double nanohole design achieves a trap stiffness 10× larger than the previous state-of-the-art. The experimental method and the design guidelines discussed here offer a simple and efficient way to improve the performance of nano-optical tweezers.

Quantifying the Role of the Surfactant and the Thermophoretic Force in Plasmonic Nano-optical Trapping.

Plasmonic nanotweezers use intense electric field gradients to generate optical forces able to trap nano-objects in liquids. However, part of the incident light is absorbed into the metal, and a supplementary thermophoretic force acting on the nano-object arises from the resulting temperature gradient. Plasmonic nanotweezers thus face the challenge of disentangling the intricate contributions of the optical and thermophoretic forces. Here, we show that commonly added surfactants can unexpectedly impact the trap performance by acting on the thermophilic or thermophobic response of the nano-object. Using different surfactants in double nanohole plasmonic trapping experiments, we measure and compare the contributions of the thermophoretic and the optical forces, evidencing a trap stiffness 20× higher using sodium dodecyl sulfate (SDS) as compared to Triton X-100. This work uncovers an important mechanism in plasmonic nanotweezers and provides guidelines to control and optimize the trap performance for different plasmonic designs.

Adhesion layer influence on controlling the local temperature in plasmonic gold nanoholes.

Gold films do not adhere well on glass substrates, so plasmonics experiments typically use a thin adhesion layer of titanium or chromium to ensure a proper adhesion between the gold film and the glass substrate. While the absorption of light into gold structures is largely used to generate heat and control the temperature at the nanoscale, the influence of the adhesion layer on this process is largely overlooked. Here, we quantify the role of the adhesion layer in determining the local temperature increase around a single nanohole illuminated by a focused infrared laser. Despite their nanometer thickness, adhesion layers can absorb a greater fraction of the incoming infrared light than the 100 nm thick gold layer leading to a significant increase of the local temperature. Different experimental designs are explored, offering new ways to promote or avoid the temperature increase inside nanoapertures. This knowledge further expands the plasmonic toolbox for temperature-controlled experiments including single molecule sensing, nanopore translocation, polymerization, or nano-optical trapping.

Directional light beams by design from electrically driven elliptical slit antennas.

We report on the low-energy, electrical generation of light beams in specific directions from planar elliptical microstructures. The emission direction of the beam is determined by the microstructure eccentricity. A very simple, broadband, optical antenna design is used, which consists of a single elliptical slit etched into a gold film. The light beam source is driven by an electrical nanosource of surface plasmon polaritons (SPP) that is located at one focus of the ellipse. In this study, SPPs are generated through inelastic electron tunneling between a gold surface and the tip of a scanning tunneling microscope.

Highly efficient singular surface plasmon generation by achiral apertures.

We report a highly efficient generation of singular surface plasmon (SP) fields by an achiral plasmonic structure consisting of Λ-shaped apertures. Our quantitative analysis, based on leakage radiation microscopy (LRM), demonstrates that the induced spin-orbit coupling can be tuned by adjusting the apex angle of the Λ-shaped aperture. Specifically, the array of Λ-shaped apertures with the apex angle 60° is shown to give rise to the directional coupling efficiency. The ring of Λ-shaped apertures with the apex angle 60° was found to generate the maximum extinction ratio (ER=11) for the SP singularities between two different polarization states. This result provides a more efficient way for developing an SP focusing and an SP vortex in the field of nanophotonics such as optical tweezers.

Coherence and aberration effects in surface plasmon polariton imaging.

We study theoretically and experimentally coherent imaging of surface plasmon polaritons using either leakage radiation microscopy through a thin metal film or interference microscopy through a thick metal film. Using a rigorous modal formalism based on scalar Whittaker potentials, we develop a systematic analytical and vectorial method adapted to the analysis of coherent imaging involving surface plasmon polaritons. The study includes geometrical aberrations due index mismatch which played an important role in the interpretation of recent experiments using leakage radiation microscopy. We compare our theory with experiments using classical or quantum near-field scanning optical microscopy probes and show that the approach leads to a full interpretation of the recorded optical images.