The challenge of visualizing nanoscale objects using conventional light microscopy stems from their weak signals and sub-diffraction-limit sizes. Existing techniques, such as ultraviolet imaging, immersion objectives, nonlinear methods, fluorescence dyes, and point-spread function engineering, each have limitations. This research presents a new approach based on electromagnetic canyons (ECs) and non-resonance amplification to directly image nanoscale objects with a widefield microscope. The ability to visualize nanoscale objects, including viruses, molecules, nanoparticles, and semiconductor defects, is crucial for advancements in integrated photonics, quantum chips, disease diagnosis, medicine, and security systems. Current methods like electron microscopy are destructive, slow, and costly; while super-resolution techniques like fluorescence microscopy require fluorescent labels and near-field scanning optical microscopy is inherently slow. Photothermal imaging, although label-free, necessitates a resonant transition. The proposed framework aims to overcome these limitations by providing a label-free, fast, non-destructive, simple, inexpensive, easily integrable, and large field-of-view visualizable detection method.

Existing optical far-field microscopy approaches often struggle due to the weak Rayleigh scattering from nanometric objects. Interferometric scattering microscopy (iSCAT) enhances detection by coherently interfering the background noise with the scattering signal. Interferometric cross-polarization microscopy (ICPM) uses interference of cross-polarized optical paths for background-free detection. Digital microarrays increase sensitivity and dynamic range using functionalized gold nanorods and interferometric scanning. These methods show potential for detecting isolated nanoscale objects but may lack the visualizability needed for simple, non-contact classification and real-time observation of dynamics.

The proposed framework uses a two-element add-on illumination apparatus and a nanowire assembly to enhance conventional low-NA microscopes. It creates an electromagnetic canyon (EC) where the background electromagnetic field is null, allowing the amplified far-field scattering of nanoscale objects to be directly imaged. Non-resonance amplification, a broadband phenomenon, is utilized, eliminating the need for precise environmental control. The EC is generated by anti-symmetric excitation, achieved using two-beam interference to create a standing wave with periodic phase jumps. Positioning nanowire pairs symmetrically with respect to an intensity null in the standing wave creates the anti-symmetric excitation. The method is based on the principle that two point sources emitting 180° out of phase are resolvable regardless of gap size. The paper provides a detailed physical model based on eigenmode expansion analysis and coupled mode perturbation theory, explaining the non-resonance amplification and the generation of ECs. Simulations are performed using near-field computation and Fourier optics to compare the proposed method with darkfield, brightfield, and iSCAT imaging. The simulations demonstrate significantly higher contrast for the proposed framework. Experiments utilize electron beam lithography (EBL) to fabricate nanowire structures with diverse objects. A two-beam far-field interference system with a 785-nm laser and a low-NA objective lens is used for imaging. Scanning electron microscopy (SEM) is employed to corroborate the findings. The system calibration involves aligning the two-beam interference apparatus and evaluating the imaging performance with NIST artifact patterns to confirm its limitations. The detailed sample fabrication process is outlined, including EBL, metal deposition, and lift-off.

Simulations show the proposed framework offers superior contrast compared to darkfield, brightfield, and iSCAT for visualizing various fixed-form objects. Experimental results validate the generation of ECs using two-beam interference, demonstrating transitions between EC and non-EC states as the sample is moved. The experiments successfully visualize nanoscale objects with features as small as 25 nm radius. Various fixed-form objects (dots, line expansions, tilted gaps) and a free-form nanoparticle are successfully imaged, with clear differentiation based on size and location. The visualization is achieved even with edge-to-edge gaps smaller than the Abbe diffraction limit (981 nm). The system uses a low-performance camera, showcasing its robustness. The results highlight the capability of the proposed framework for visualizing diverse objects, including isolated, bonded, expanded, and defective ones. The method’s sensitivity to nanoscale variations is also demonstrated. While iSCAT and ICPM may have stronger signals, the proposed approach excels in visualizing fixed-form objects and shows adequate signal strength for detection in many applications using readily available components.

The experimental results demonstrate the effectiveness of the proposed framework for visualizing nanoscale objects in a sub-diffraction-limited volume using a conventional microscope. The non-resonance amplification ensures the method works at various wavelengths and is less sensitive to environmental fluctuations. The ease of fabrication and small device footprint make it highly practical. The widefield approach allows for large fields of view, further enhancing its utility. The high contrast even with embedded nanoscale objects makes it suitable for applications like semiconductor defect inspection. The method’s limitations, such as the near-field requirement, can be addressed using arrays of nanowire pairs. The framework opens up opportunities in nanoparticle sensing, biosensing, material characterization, virus counting, and microfluidic monitoring, demonstrating superior performance compared to existing methods in certain scenarios.

The study successfully demonstrates a novel visualizable nanoscale detection method using anti-symmetric excitation and non-resonance amplification. The framework offers significant advantages in simplicity, cost-effectiveness, and applicability. Future research could focus on optimizing nanowire geometry and materials for enhanced sensitivity, exploring applications with even smaller objects such as viruses, and extending the technique to three-dimensional imaging.

The current implementation requires objects to be in the near-field of the nanowire pair. While arrays of nanowire pairs can address this, fabrication complexity increases. The sensitivity to nanoscale imperfections in nanowire fabrication is demonstrated, impacting image quality. Optimizing fabrication techniques to reduce imperfections is crucial for higher-resolution imaging. For extremely small objects, a higher dynamic range camera might be necessary to further enhance signal detection.