Plasmonic photodetectors leverage the unique optical properties of metallic nanostructures to enhance light-matter interactions, enabling high-sensitivity detection beyond the diffraction limit. Central to their operation are localized surface plasmon resonances (LSPR), collective oscillations of conduction electrons in metals such as gold (Au) and silver (Ag), which generate intense near-field enhancements and hot electrons. These mechanisms have unlocked advancements in subwavelength imaging and ultrasensitive biosensing, driven by precise nanostructure engineering and optimized hot-electron harvesting.

Localized surface plasmon resonance occurs when incident light couples with the natural frequency of free electrons in metallic nanoparticles, producing resonant absorption and scattering. The resonance wavelength depends on the material, size, shape, and dielectric environment of the nanostructure. For instance, spherical Au nanoparticles exhibit LSPR peaks around 520–550 nm, while Ag nanoparticles resonate at 400–450 nm. Anisotropic structures like nanorods or nanostars allow tunability across visible to near-infrared spectra. A 50 nm Au nanorod can shift its resonance from 520 nm to over 800 nm by increasing its aspect ratio. This spectral flexibility is critical for applications requiring wavelength-specific enhancement.

Hot-electron generation arises from the non-radiative decay of LSPR, where plasmon energy transfers to high-energy charge carriers. These electrons, excited above the Fermi level, can be injected into adjacent semiconductor materials (e.g., TiO2, Si) or drive redox reactions in molecular systems. The internal quantum efficiency of hot-electron devices typically ranges from 1% to 10%, influenced by factors like interfacial energy barriers and electron scattering losses. Optimizing the metal-semiconductor Schottky barrier is essential; for Au-TiO2 systems, a barrier height of 1.0–1.2 eV balances efficient hot-electron transfer with minimal backscattering.

Nanostructure design plays a pivotal role in maximizing LSPR and hot-electron yield. Au and Ag remain the preferred materials due to their low optical losses and chemical stability. Common geometries include nanodisks, nanopyramids, and nanohole arrays, each offering distinct field localization profiles. For example, Ag nanodisks with 100 nm diameter and 30 nm height exhibit electric field enhancements exceeding 100-fold at their edges, ideal for surface-enhanced spectroscopy. Coupled nanostructures, such as dimer gaps or bowtie antennas, further amplify field confinement. A 5 nm gap between two Au nanoparticles can achieve field intensities 1000 times greater than the incident light, enabling single-molecule detection.

In subwavelength imaging, plasmonic photodetectors overcome the diffraction limit by converting evanescent near-fields into propagating signals. Scanning near-field optical microscopy (SNOM) integrates sharp Au or Ag tips to resolve features below 50 nm. Alternatively, plasmonic superlenses based on Ag thin films have demonstrated imaging at λ/7 resolution. These systems rely on the rapid decay of plasmonic fields, which confine light to nanoscale volumes. A 30 nm Ag layer can enhance optical transmission by 400% at 365 nm wavelength, facilitating high-resolution photolithography and nanoscale optical mapping.

Biosensing applications exploit the extreme sensitivity of LSPR to refractive index changes. When target molecules bind to functionalized Au or Ag surfaces, the resonance wavelength shifts proportionally to analyte concentration. A well-designed LSPR biosensor can detect prostate-specific antigen (PSA) at 0.1 pg/mL, surpassing conventional ELISA techniques. Nanostar-shaped Au particles, with their branched tips, provide multiple hotspots for molecular binding, yielding a 5 nm shift per nanometer of adsorbed protein layer. Real-time monitoring of DNA hybridization or antibody-antigen interactions is possible with millisecond response times, making plasmonic detectors invaluable for point-of-care diagnostics.

Challenges persist in improving hot-electron extraction and reducing thermalization losses. Recent strategies include embedding Au nanoparticles in graphene to exploit its high carrier mobility, achieving photoresponsivities of 10 mA/W at 600 nm. Alternatively, plasmonic-electronic hybrids combine Ag nanowires with silicon p-n junctions, attaining 90% absorption in ultrathin active layers. For biosensing, minimizing nonspecific adsorption through polyethylene glycol (PEG) coatings enhances selectivity, while microfluidic integration enables high-throughput screening.

Future directions focus on scalable fabrication and multifunctional designs. Roll-to-roll nanoimprinting can produce Ag nanogratings over large areas for flexible photodetectors, maintaining sub-50 nm feature uniformity. Core-shell nanoparticles, such as Au@SiO2, offer tunable plasmon resonances with reduced quenching for biomedical imaging. Advances in cryogenic plasmonics may further reduce electron-phonon scattering, boosting hot-electron efficiencies toward 20%.

Plasmonic photodetectors represent a convergence of nanophotonics and solid-state physics, where tailored nanostructures bridge the gap between light and charge. By harnessing LSPR and hot electrons, these devices push the boundaries of sensitivity and resolution, finding utility in fields ranging from early disease diagnosis to nanoscale optical engineering. Continued innovation in material synthesis and device architecture will further solidify their role in next-generation optoelectronic systems.