Formation Of Plasmonic Nanohotspots At The Tip Of Metal Probes And Its Interaction With Matter

Surface plasmon polaritons(SPPs)are highly localized electromagnetic field excitation modes that exist at the interface between metals and dielectric materials.In recent years,significant progress has been made in the field of SPP research.In the very small nano photonics devices,the use of nano SPP to modulate optical signals shows many advantages,and SPP has opened up a promising path in the fields of environment,energy,biology and medicine.This article provides an overview of the current research progress of SPP,including basic physics and applications.We first discuss the excitation mode and condition of SPP based on SPP dispersion relation,and study the coupling characteristics and propagation behavior of SPP through the momentum matching between photons and SPP.Based on the physical mechanism and unique performance of SPP,we have listed its main applications,such as waveguides,light sources,near-field optics,surface enhanced Raman spectroscopy,and sensors.This paper combines surface plasmons and 3D nanofabrication technology to prepare a three-dimensional(3D)spiral cone structure on the fiber end face,which is applied to fluorescence enhancement,near-field imaging,and optical sensing applications.And theoretically studied the force analysis of single molecules in the gap hot spots formed by smooth metal needle tips and metal substrates.Provide a new and powerful experimental tool and technical solution for further providing time and space resolved physical chemistry of light matter interactions at the single molecule level.The specific research content includes the following four points:1.A design and preparation scheme for optical fiber integrated gold spiral cone probes was proposed,and applied to fluorescence enhancement.The surface plasmon resonance mode enhances the electromagnetic field on the outer surface of the metal,thereby enhancing the emission of fluorescent molecules located at nano distances.I have demonstrated that the enhancement factor of the metal surface plasmon enhanced fluorescence signal in the fiber end coupled spiral grating structure can reach 38 times.It is worth mentioning that this fiber optic integrated gold spiral conical probe uses a relatively simple manufacturing technology.It is expected to find potential applications in biotechnology,clinical analysis and analytical chemistry.2.The geometric parameters and preparation scheme of the gold spiral cone probe were optimized,and combined with near-field scanning optical microscopy(SNOM),a high-performance SNOM probe with high resolution,high transmittance,and high signalto-noise ratio was successfully developed.I designed the structure of the SNOM probe by introducing a gold spiral grating on the surface of the conical tip,which helps to provide flexible momentum matching conditions,so that the internal optical signal from the fiber can excite the SPP and efficiently couple to the external metal surface of the gold spiral cone probe,independent of the polarization of the incident light.The numerical simulation and optical measurement results of this carefully designed and manufactured SNOM probe indicate that it has a near-field imaging spatial resolution of about ～5nm,a light transmittance of ～10%,a signal-to-noise ratio(SNR)of ～20d B,a fast scanning speed of 7500 pixels per second,and a transmission bandwidth of 50 nm.My self-designed and self-made gold spiral grating conical probe can greatly improve the performance of standard SNOM instruments to unprecedented levels in key performance indicators such as near-field imaging resolution,luminous flux,and signal-to-noise ratio.3.The 3D gold spiral cone probe is applied to fiber optic plasmon resonance(SPR)sensing,which has the advantages of small contact area and high sensing sensitivity,and can detect changes in micro region solution concentration.I measured the reflection spectra of fiber optic probes under white light irradiation in glycerol solutions with different refractive indices.The reflection spectrum indicates that the fiber probe responds to changes in the refractive index(RI)of the liquid in the spectral range of 1.333 to 1.398,with an average sensitivity of 333nm/RIU,which is consistent with the theoretical calculation results.In addition,the fiber optic probe RI sensor has 10 μ High spatial resolution of m and fast response speed of 20 ms.This SPR enhanced gold tip fiber optic RI sensor has a small active area,simple and reliable working principle,simple and robust operating performance,high RI sensitivity,high SNR value,and good mechanical reliability.The convective process of water ethanol solution was successfully monitored using this high temporal spatial resolution optical sensing fiber probe,and the refractive index distribution of thin interface layers in a layered liquid system was measured.This indicates that this 3D gold grating fiber SPR sensor has the characteristics of real-time in situ chemical and biological sensing,fast response speed,and high spatial resolution.This fiber optic microsensor has broad application prospects in biological and chemical microarea detection.4.Theoretical research was conducted on the optical force effect on molecules in the hot spot region of plasmons such as gold probe gold substrate nano gaps in needle tip enhanced Raman scattering(TERS)systems.The TERS structure forms a hot spot due to the needle tip effect,and the stiffness of the hot spot optical potential well can reach about 2pN/[(W /cm2 × m)](horizontal plane)and 40 pN/[(W/cm2)× m] The high value(in the vertical axis direction)is much greater than the stiffness of traditional single beam optical tweezers～0.4pN/[(W /cm2)× m].This stiffness will enable molecules to stably capture in plasmon hotspots.This result indicates that TERS has the potential to become a promising optical tweezer tool for capturing microscopic objects such as molecules,while also conducting Raman spectroscopy imaging and analysis.