Progress of Scanning Near-Field Optical Microscopy(Invited)
Significance The continuous progress in modern materials science,information technology,and biotechnology has greatly boosted societal advancement.As human understanding of the microcosm deepens,the capability to better describe and characterize the interactions between atomic-scale light and matter,and to achieve the simultaneous participation,coordinated regulation,and multi-modal coupling of multiple physical fields can significantly provide diverse methods and approaches for artificially controlling these matters.Scanning near-field optical microscopy(SNOM)with 40 years'history can serve as a promising tool for these fundamental purposes by hoping to carry out measurement and characterization of light fields and light-matter interactions at deep-subwavelength and even nanometer scales.The essential elements for in-depth exploration of multi-physical field interaction systems in experiments are listed.They include the measurement,characterization,and analysis methods for light-matter interactions at the micro and nanoscales,interactions between photons and various quasi-particles,coupling between quasi-particles,and coupling and regulation between multiple physical fields.Meanwhile,a systematic and precise experimental study on the spatio-temporal details of the interaction among light and molecules,two-dimensional materials,quantum dots,metal nanoparticles,and nonlinear structures at the micro and nanoscales can reveal deep physics and a series of new phenomena and laws.Additionally,it is imperative to develop new methods,technologies,tools,and instrumentation of SNOM for microscopic imaging and spectral analysis with higher spatial resolution(approximately 10 nm),higher temporal resolution(about 50-100 fs),and higher brightness(transmission efficiency of 1％-10％).Thus,it is vital to deeply,completely,and compactly introduce and describe the history of SNOM and its applications,with the route towards building such a fundamental high-performance SNOM machine presented.Progress Due to the limitations of traditional optical imaging methods,researchers have turned their attention to near-field imaging.Owing to its exceptional optical resolution down to 10 nm and spectral analysis capabilities,SNOM provides a powerful near-field optical characterization tool with high spatio-temporal resolution for studying many crucial frontier scientific problems in physics,chemistry,materials science,and life sciences.In 1928,Synge first proposed the concept of near-field microscopy(Fig.2)to improve the resolution of traditional microscopes.Until the 1980s,with the successful invention of lasers and scanning tunneling microscopes,Pohl,Lewis,and Betzig respectively made outstanding contributions to SNOM systems and nanoscanning tips,realized Synge's vision,and increased the optical diffraction limit by one to two orders of magnitude,thus catching great attention from both academia and the industry.However,due to the fundamental limitations that resolution and transmittance cannot be achieved simultaneously,the typical a-SNOM is difficult to apply to biology and medicine that require both advantages.Therefore,as a pioneer of a-SNOM technology,Betzig abandoned this technological route after 1993 and sought alternative methods.Then,he quickly yielded success in fluorescence microscopy imaging technology and was awarded the Nobel Prize in Chemistry in 2014.To solve this problem,many researchers have subsequently made many improvements on the s-SNOM with better performance(Fig.3).Despite decent progress in certain aspects,there is still significant room for improvement in the overall performance optimization,including resolution,throughput,and signal-to-noise ratio.Meanwhile,many studies adopt s-SNOM imaging technology to study the spatio-temporal details of light-matter interactions at the micro and nanoscales,including atoms,molecules,two-dimensional materials,quantum dots,biomolecules,and nonlinear structures.Finally,a series of new phenomena and laws in deep physics,chemistry,and biology are revealed.In recent years,we have carried out a project to build a novel SNOM tip based on the innovative concept and mechanism of SPP energy transfer(Fig.5).This tip features a metal spiral cone-shaped structure,constant high resolution(10 nm),high transmission efficiency(10％),and high signal-to-noise ratio(20 dB).Employing the nano spot of the SNOM probe as an illumination light source is expected to measure and analyze the physical,chemical,and biological properties of micro and nano substances such as single molecules.Conclusions and Prospects Generally,SNOM technology has become an important tool for studying near-field optics.Further improvement in the spatio-temporal resolution of SNOM technology will promote fundamental and applied research on the light-matter interactions at the nanoscale and even single-molecule scale.
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