Diffraction of circularly polarized light from near-field optical probes

Diffracted fields from 100-nm aperture near-field scanning optical microscopy (NSOM) probes and uncoated tapered fibres are measured and analysed. Using a solid angle scanner, the two-dimensional intensity distribution and polarization state of the diffracted light are resolved experimentally. Polarization analyses show that circularly polarized input light does not maintain its polarization state for all diffraction angles, and is completely filtered into linearly polarized light at large polar diffraction angles. This drastic decomposition originates from the vector nature of light diffracted by the sub-wavelength aperture. There is a fundamental difficulty in generating circularly polarized light near the aperture of NSOM probes owing to polarization-dependent diffraction in the near-field regime. This is illustrated by the Bethe-Bouwkamp model using circularly polarized input light.     

Fundamental differences between micro- and nano-Raman spectroscopy

Electric field polarization orientations and gradients close to near-field scanning optical microscope (NSOM) probes render nano-Raman fundamentally different from micro-Raman spectroscopy. With x -polarized light incident through an NSOM aperture, transmitted light has x, y and z components allowing nano-Raman investigators to probe a variety of polarization configurations. In addition, the strong field gradients in the near-field of a NSOM probe lead to a breakdown of the assumption of micro-Raman spectroscopy that the field is constant over molecular dimensions. Thus, for nano-Raman spectroscopy with an NSOM, selection rules allow for the detection of active modes with intensity dependent on the field gradient. These modes can have similar activity as infra-red absorption modes. The mechanism can also explain the origin and intensity of some Raman modes observed in surface enhanced Raman spectroscopy.     

Single molecule mapping of the optical field distribution of probes for near-field microscopy

The most difficult task in near-field scanning optical microscopy (NSOM) is to make a high quality subwavelength aperture probe. Recently, we have developed high definition NSOM probes by focused ion beam (FIB) milling. These probes have a higher brightness, better polarization characteristics, better aperture definition and a flatter end face than conventional NSOM probes. We have determined the quality of these probes in four independent ways: by FIB imaging and by shear-force microscopy (both providing geometrical information), by far-field optical measurements (yielding throughput and polarization characteristics), and ultimately by single molecule imaging in the near-field. In this paper, we report on a new method using shear-force microscopy to study the size of the aperture and the end face of the probe (with a roughness smaller than 1.5 nm). More importantly, we demonstrate the use of single molecules to measure the full three-dimensional optical near-field distribution of the probe with molecular spatial resolution. The single molecule images exhibit various intensity patterns, varying from circular and elliptical to double arc and ring structures, which depend on the orientation of the molecules with respect to the probe. The optical resolution in the measurements is not determined by the size of the aperture, but by the high optical field gradients at the rims of the aperture. With a 70 nm aperture probe, we obtain fluorescence field patterns with 45 nm FWHM. Clearly, this unprecedented near-field optical resolution constitutes an order of magnitude improvement over far-field methods like confocal microscopy.     

Single molecule mapping of the optical field distribution of probes for near-field microscopy

The most difficult task in near-field scanning optical microscopy (NSOM) is to make a high quality subwavelength aperture probe. Recently, we have developed high definition NSOM probes by focused ion beam (FIB) milling. These probes have a higher brightness, better polarization characteristics, better aperture definition and a flatter end face than conventional NSOM probes. We have determined the quality of these probes in four independent ways: by FIB imaging and by shear-force microscopy (both providing geometrical information), by far-field optical measurements (yielding throughput and polarization characteristics), and ultimately by single molecule imaging in the near-field. In this paper, we report on a new method using shear-force microscopy to study the size of the aperture and the end face of the probe (with a roughness smaller than 1.5 nm). More importantly, we demonstrate the use of single molecules to measure the full three-dimensional optical near-field distribution of the probe with molecular spatial resolution. The single molecule images exhibit various intensity patterns, varying from circular and elliptical to double arc and ring structures, which depend on the orientation of the molecules with respect to the probe. The optical resolution in the measurements is not determined by the size of the aperture, but by the high optical field gradients at the rims of the aperture. With a 70 nm aperture probe, we obtain fluorescence field patterns with 45 nm FWHM. Clearly, this unprecedented near-field optical resolution constitutes an order of magnitude improvement over far-field methods like confocal microscopy.     

Moulded photoplastic probes for near-field optical applications

The inexpensive fabrication of high-quality probes for near-field optical applications is still unsolved although several methods for integrated fabrication have been proposed in the past. A further drawback is the intensity loss of the transmitted light in the 'cut-off' region near the aperture in tapered optical fibres typically used as near-field probes. As a remedy for these limitations we suggest here a new wafer-scale semibatch microfabrication process for transparent photoplastic probes. The process starts with the fabrication of a pyramidal mould in silicon by using the anisotropic etchant potassium hydroxide. This results in an inverted pyramid limited by < 111 > silicon crystal planes having an angle of ∼ 54°. The surface including the mould is covered by a ∼ 1.5 nm thick organic monolayer of dodecyltrichlorosilane (DTS) and a 100-nm thick evaporated aluminium film. Two layers of photoplastic material are then spin-coated (thereby conformal filling the mould) and structured by lithography to form a cup for the optical fibre microassembly. The photoplastic probes are finally lifted off mechanically from the mould with the aluminium coating. Focused ion beam milling has been used to subsequently form apertures with diameters in the order of 80 nm. The advantage of our method is that the light to the aperture area can be directly coupled into the probe by using existing fibre-based NSOM set-ups, without the need for far-field alignment, which is typically necessary for cantilevered probes. We have evidence that the aluminium layer is considerably smoother compared to the 'grainy' layers typically evaporated on free-standing probes. The optical throughput efficiency was measured to be about 10⁻⁴. This new NSOM probe was directly bonded to a tuning fork sensor for the shear force control and the topography of a polymer sample was successfully obtained.     

突破衍射极限的成像方法综述

"衍射极限"实际上不是一个真正的障碍,除非处理远场和定位精度。这种衍射障碍并不是坚不可摧的,可以利用一些智能技术来突破光学衍射极限。讨论了四种技术,近场扫描光学显微镜(NSOM)法,受激发射损耗(STED)显微镜法,光激活定位显微镜(PALM)法或随机光学重建显微镜(STORM)法和结构照明显微镜(SIM)法,并且介绍了各自的基本原则与优劣。NSOM利用纳米级探测器检测通过光纤的极小汇聚光斑,从而获得单个像素的分辨率;PALM和STORM利用荧光探针,实现暗场和荧光的转换,从而观察到极小的荧光团;SIM则是利用栅格图案与样品叠加成像来实现。其中,STORM具有相对较高的潜力,能够更为有效地突破衍射极限。     

Near-field fluorescence imaging of single molecules with a resolution in the range of 10 nm

We demonstrate fluorescence imaging of single molecules, by near-field scanning optical microscopy (NSOM), using the illumination-collection mode of operation, with an aperture probe. Fluorescence images of single dye molecules were obtained with a spatial resolution of 15 nm, which is smaller than the diameter of the aperture (20 nm) of the probe employed. Such super-resolution may be attributable to non-radiative energy transfer from the molecules to the coated metal of the probe since the resolution obtained in the case of conventional NSOM is limited to 30–50 nm due to penetration of light into the metal.     

Study of the focused laser spots generated by various polarized laser beam conditions

In this work, three‐dimensional near‐field imaging of the focused laser spot was studied theoretically and experimentally. In the theoretical simulation, we use the electromagnetic equivalent of the vectorial Kirchhoff diffraction integral to calculate the intensity distribution of the focal region, and a high depolarization is found in high numerical aperture systems (NA = 0.85). The experimental set‐up is based on a near‐field scanning optical microscope (NSOM) system. A high‐NA objective lens is used to focus incident light of various polarizations, and a tapered near‐field optical fibre probe of the NSOM system is used to determine the intensity of the focal field. The results show an asymmetric distribution of the focused intensity with the linear polarized laser beam.     

Photoluminescence imaging of phosphor particles using near-field optical microscope with UV light excitation

We have developed fibre probes suitable for 325 nm UV light excitation and a photoluminescence near-field scanning optical microscope (NSOM) and demonstrated the photoluminescence imaging of phosphor BaMgAl₁₀O₁₇:Eu²⁺ (BAM) particles. The probe was fabricated by a two-step-etching method that we developed. The probe had a large taper angle at the top of the probe and a small taper angle at the root. The NSOM image was different from the topographical structure but roughly reflected the corresponding features of the particles. The inhomogeneity of the photoluminescence intensity between BAM particles was observed in the NSOM image. The photoluminescence intensity with various bandpass filters showed differences between the individual particles, which means that they have different spectra.     

Resolution enhancing using cantilevered tip-on-aperture silicon probe in scanning near-field optical microscopy

Scanning near-field optical microscopy (SNOM) achieves a resolution beyond the diffraction limit of conventional optical microscopy systems by utilizing subwavelength aperture probe scanning. A problem associated with SNOM is that the light throughput decreases markedly as the aperture diameter decreases. Apertureless scanning near-field optical microscopes obtain a much better resolution by concentrating the light field near the tip apex. However, a far-field illumination by a focused laser beam generates a large background scattering signal. Both disadvantages are overcome using the tip-on-aperture (TOA) approach, as presented in previous works. In this study, a finite difference time domain analysis of the degree of electromagnetic field enhancement is performed to verify the efficiency of TOA probes. For plasmon enhancement, silver is deposited on commercially available cantilevered SNOM tips with 20nm thicknesses. To form the aperture and TOA in the probes, electron beam-induced deposition and focused ion beam machining were applied at the end of the sharpened tip. The results show that cantilevered TOA probes were highly efficient for improvements of the resolution of optical and topological measurement of nanostructures.     

Evanescent field excitation and measurement of dye fluorescence in a metallic probe near-field scanning optical microscope

We introduce a method of dye fluorescence excitation and measurement that utilizes a near-field scanning optical microscope (NSOM). This NSOM uses an apertureless metallic probe, and an optical system that contains a high numerical aperture (NA) objective lens (NA= 1.4). When the area which satisfies NA < 1 is masked, the objective lens allows for the rejection of possible transmitted light (NA < 1) through the sample. In such conditions, the focused spot consists of only the evanescent field. We found that this NSOM system strongly reduces the background of the dye fluorescence and allows for the measurement of the fluorescence intensity below the diffraction limit of the excitation source.     

太赫兹超表面近场等离子体涡旋偏移器件设计

为了提升太赫兹（terahertz,THz）通讯容量,设计了一种基于单层超表面的激发近场等离子体涡旋偏移的太赫兹器件。基于几何相位超表面,采用FITD（时域有限积分）软件,对该器件的近场涡旋偏移进行了仿真研究。结果表明,所设计的器件在圆偏振光的入射下,能够实现空间任意位置的偏移。该类功能器件在一定程度上提升了太赫兹通讯容量,可应用于6G技术中。     

Evanescent field excitation and measurement of dye fluorescence in a metallic probe near-field scanning optical microscope

We introduce a method of dye fluorescence excitation and measurement that utilizes a near-field scanning optical microscope (NSOM). This NSOM uses an apertureless metallic probe, and an optical system that contains a high numerical aperture (NA) objective lens (NA = 1.4). When the area which satisfies NA < 1 is masked, the objective lens allows for the rejection of possible transmitted light (NA < 1) through the sample. In such conditions, the focused spot consists of only the evanescent field. We found that this NSOM system strongly reduces the background of the dye fluorescence and allows for the measurement of the fluorescence intensity below the diffraction limit of the excitation source.     

Nano-slit probes for near-field optical microscopy fabricated by focused ion beams

The near-field probes described in this paper are based on metallized non-contact atomic force microscope cantilevers made of silicon. For application in high-resolution near-field optical/infrared microscopy, we use aperture probes with the aperture being fabricated by focused ion beams. This technique allows us to create apertures of sub-wavelength dimensions with different geometries. In this paper we present the use of slit-shaped apertures which show a polarization-dependent transmission efficiency and a lateral resolution of < 100 nm at a wavelength of 1064 nm. As a test sample to characterize the near-field probes we investigated gold/palladium structures, deposited on an ultrathin chromium sublayer on a silicon wafer, in constant-height mode.     

Characterization of power induced heating and damage in fiber optic probes for near-field scanning optical microscopy

Tip-induced sample heating in near-field scanning optical microscopy (NSOM) is studied for fiber optic probes fabricated using the chemical etching technique. To characterize sample heating from etched NSOM probes, the spectra of a thermochromic polymer sample are measured as a function of probe output power, as was previously reported for pulled NSOM probes. The results reveal that sample heating increases rapidly to approximately 55-60 degrees C as output powers reach approximately 50 nW. At higher output powers, the sample heating remains approximately constant up to the maximum power studied of approximately 450 nW. The sample heating profiles measured for etched NSOM probes are consistent with those previously measured for NSOM probes fabricated using the pulling method. At high powers, both pulled and etched NSOM probes fail as the aluminum coating is damaged. For probes fabricated in our laboratory we find failure occurring at input powers of 3.4+/-1.7 and 20.7+/-6.9 mW for pulled and etched probes, respectively. The larger half-cone angle for etched probes ( approximately 15 degrees for etched and approximately 6 degrees for pulled probes) enables more light delivery and also apparently leads to a different failure mechanism. For pulled NSOM probes, high resolution images of NSOM probes as power is increased reveal the development of stress fractures in the coating at a taper diameter of approximately 6 microm. These stress fractures, arising from the differential heating expansion of the dielectric and the metal coating, eventually lead to coating removal and probe failure. For etched tips, the absence of clear stress fractures and the pooled morphology of the damaged aluminum coating following failure suggest that thermal damage may cause coating failure, although other mechanisms cannot be ruled out.     

Near-field effects in single molecule emission

We present the first experimental proof of the influence of a nearby nano-sized metal object on the angular photon emission by a single molecule. A novel angular sensitive detection scheme is implemented in an existing near-field scanning optical microscope (NSOM). The positioning accuracy (∼1 nm) of the NSOM allows a systematic investigation of the intensity ratio between two different half-spaces as a function of the position of the metal–glass interfaces of the probe with respect to the single emitter. The observed effects are shown to be particularly strong for molecules that are excited mainly below the rims of the aperture. An excellent agreement is found between experiments and numerical simulations for these molecules. The observed angular redistribution of the angular emission of a single molecule could explain the alteration of the emission polarization observed for certain molecules in earlier experiments (Veerman et al. (1999) J. Microsc. 194 , 477–482).     

基于几何相位的线偏振聚焦全介质超透镜的设计

超透镜凭借其独特的优势逐渐取代或补充了传统的折光和衍射透镜,从而领先小型化高性能光学设备和系统。这种小型化有望带来紧凑的纳米级光学设备,应用于相机、照明、显示器和可穿戴光学。但在传统的超透镜设计中,几何相位通常需要左右旋圆偏振光的手性限制。为了使其不再局限于左旋圆偏振光(LCP)和右旋圆偏振光(RCP)入射,设计了基于几何相位的全介质超透镜,实现了线偏振光的近场聚焦,其与超表面滤光片主要输出波长(可见光波段)一致。采用有效时域差分法研究并验证了超透镜各变量对光场聚焦特性的影响,超透镜等其他超表面光学元器件前所未有的设计自由度将极大地扩大微光学和集成光学的应用领域。     

Spatial distribution and polarization dependence of the optical near-field in a silicon microfabricated probe

This paper reports on the spatial distribution and polarization behaviour of the optical near-field at the aperture of a Si micromachined probe. A sub-100 nm aperture at the apex of a SiO₂ tip on a Si cantilever was successfully fabricated by selective etching of the SiO₂ tip in a buffered-HF solution using a thin Cr film as a mask. The aperture, 10–100 nm in size, can be reproducibly fabricated by optimizing the etching time. The optical throughput of several apertures was measured. For a 100 nm aperture, a throughput of 1% was approved. The probe shows a very high optical throughput owing to the geometrical structure of the tip. The spatial distribution of the near-field light is measured and simulated using a finite difference-time domain method. The polarization behaviour of apertures with different shapes was analysed using a photon counting camera system.     

State-selective optical near-field resonant ionization spectroscopy of atoms near a dielectric surface

Spin-sensitive optical near-field microscopy and spectroscopy are proposed based on the study on the conserved quantities in optical near-field interactions of atoms with dielectric surfaces. A two-step photoionization spectra of Cs atoms resolving hyperfine structures are demonstrated near a planar dielectric surface by using evanescent waves. These techniques of state/spin-selective excitation and highly sensitive detection, combined with the techniques of optical pumping, will open up possibilities of space- and polarization-sensitive detection of optical near‐fields using atomic probes. This novel method provides us with a useful technique for the observation of polarization nature of the optical near-field and controlling the spin states of mesoscopic electronic systems.