Near-field microscopy with a single-photon point-like emitter: Resolution versus the aperture tip?

We discuss theoretically the concept of spatial resolution in near-field scanning optical microscopy (NSOM) in light of a recent work [Opt. Express 17 (2009) 19969] which reported on the achievement of active tips made of a single ultrasmall fluorescent nanodiamond grafted onto the apex of a substrate tip and on their validation in NSOM imaging. Since fluorescent nanodiamonds tend to decrease steadily in size, we assimilate a nanodiamond-based tip to a point-like single photon source and compare its ultimate resolution with that offered by standard metal-coated aperture NSOM tips. We demonstrate both classically and quantum mechanically that NSOM based on a point-like tip has a resolving power that is only limited by the scan height over the imaged system whereas the aperture-tip resolution depends critically on both the scan height and aperture diameter. This is a consequence of the complex distribution of the electromagnetic field around the aperture that tends to artificially duplicate the imaged objects. We show that the point-like tip does not suffer from this “squint” and that it rapidly approaches its ultimate resolution in the near-field as soon as its scan height falls below the distance between the two nano-objects to be resolved.     

Numerical analysis of the spectral response of an NSOM measurement

Near-field Scanning Optical Microscopy (NSOM) is a powerful tool for investigating optical field with resolution greater than the diffraction limit. In this work, we study the spectral response that would be obtained from an aperture NSOM system using numerical calculations. The sample used in this study is a bowtie nanoaperture that has been shown to produce concentrated and enhanced field. The near- and far-field distributions from a bowtie aperture are also calculated and compared with what would be obtainable from a NSOM system. The results demonstrate that it will be very difficult to resolve the true spectral content of the near-field using aperture NSOM. On the other hand, the far-field response may be used as a guide to the near-field spectrum.     

Near-field microscopy with a scanning nitrogen-vacancy color center in a diamond nanocrystal: A brief review

We review our recent developments of near-field scanning optical microscopy (NSOM) that uses an active tip made of a single fluorescent nanodiamond (ND) grafted onto the apex of a substrate fiber tip. The ND hosting a limited number of nitrogen-vacancy (NV) color centers, such a tip is a scanning quantum source of light. The method for preparing the ND-based tips and their basic properties are summarized. Then we discuss theoretically the concept of spatial resolution that is achievable in this special NSOM configuration and find it to be only limited by the scan height over the imaged system, in contrast with the standard aperture-tip NSOM whose resolution depends critically on both the scan height and aperture diameter. Finally, we describe a scheme we have introduced recently for high-resolution imaging of nanoplasmonic structures with ND-based tips that is capable of approaching the ultimate resolution anticipated by theory.     

Near-field optical characterization of GaN and InxGa1−xN/GaN heterostructures grown on freestanding GaN substrates

Using near-field scanning optical microscopy (NSOM), we report the spatial distribution of photoluminescence (PL) intensity in III-nitride-based semiconductor layers grown on GaN substrates. Undoped GaN, In_0.11Ga_0.89N, and In_0.13Ga_0.87N/GaN multi-quantum wells (MQWs) were grown by metal organic chemical vapor deposition (MOCVD) on freestanding GaN substrates. Micro-Raman spectroscopy has been used to evaluate the crystalline properties of the GaN homoepitaxial layers. The variation of the PL intensity from the NSOM imaging indicates that the external PL efficiency fluctuates from 20% to 40% in the 200 nm InGaN single layer on freestanding GaN, whereas it fluctuates from 20% to 60% in InGaN/GaN MQWs. In the NSOM-PL images, bright island-like features are observed. After deconvolution with the spatial resolution of the NSOM, the size of these features is estimated to be in the range of 150–250 nm.     

Mesoscale structures in luminescent conjugated polymer thin films studied by near-field scanning optical microscopy

We report the studies of various conjugated polymer thin films with near-field scanning optical microscopy, NSOM. Firstly, it is shown that MEH-PPV thin film undergoes significant changes in film morphology upon thermal annealing. The once homogeneous morphology becomes inhomogeneous after annealing. Secondly, polarization near-field measurements reveal mesoscale polymer ordering in PPV thin film. The average domain size and the coefficient for linear dichroism were studied as a function of film thickness. Finally, phase separation in polymer blend film was directly observed by transmission NSOM. Time-resolved fluorescence spectra indicate that the phase domains are decomposed of different fractions of the two constituent polymers. The near-field optical microscopy was also used to write lithographic patterns with a resolution of 100 nm, exceeding the diffraction limit.     

High-Contrast Imaging of Nano-Channels Using Reflection Near-Field Scanning Optical Microscope Enhanced by Optical Interference

We demonstrated a contrast enhancement in a near-field scanning optical microscope (NSOM) by optical interference with an aperture probe in reflection (illumination-collection) mode operation. We observed a NiO film deposited on a sapphire substrate and clearly visualized 2-nm-deep nano-channel structures on the surface of the film. The reflection NSOM enhanced by optical interference is quite a promising instrument for high-resolution optical detection and estimation of low-contrast nanostructures.     

Measuring local optical properties: near-field polarimetry of photonic block copolymer morphology

Ultrahigh molecular weight polystyrene-b-polyisoprene block copolymers (BCs), noted for their photonic behavior, were imaged using transmission near-field scanning optical microscopy (NSOM) and NSOM polarimetry. Our improved scheme for polarization modulation (PM) polarimetry, which accounts for optical anisotropies of the NSOM aperture probe, enables mapping of the local diattenuation and birefringence (with separately aligned diattenuating and fast axes) in these specimens with subdiffraction limited resolution. PM-NSOM micrographs illuminate the mesoscopic optical nature of these BC specimens by resolving individual microphase domains and defect structures.     

Recent progress of nano-technology with NSOM

Recent progress of nano-technology with near-field scanning optical microscope (NSOM) is surveyed in this article. We focus mainly on NSOM, nano-scale spectroscopy with NSOM, probe technology of NSOM, and study of nano-structured metallic surface with NSOM. First, we follow developments of aperture NSOM and apertureless NSOM, and then address progress of NSOM-combined spectroscopy which is so sufficiently advanced with apertureless NSOM technology to provide chemical information on length scales of a few nanometers. Recent achievement of nano-scale Raman and IR spectroscopy will be introduced. Finally, research on nano-optic elements using surface plasmon polariton with NSOM is introduced as an example of NSOM applications to nano-structured metallic surfaces.     

Nano-manipulation performance with enhanced evanescent field close to near-field optical probes

Recently, interest in nano-manipulation using the evanescent wave generated by nano-objects has been growing, but the analyses of manipulation flexibility and performance haven't been solved. In this paper the near-field optical trap utilizing a tapered metalized probe used in NSOM is described in detail. By employing a generalization of the conservation law for momentum using three-dimensional FDTD method, rigorous calculations of field distributions and trapping forces in near-field region are conducted. Calculations show that the particle with radius larger than the aperture is pushed away from the metal-coated fiber probe, while it tends to be trapped in larger effective region as its radius becoming smaller. The particle that is placed very near the aperture and around two field peaks intends to be dragged to the aperture edge, while the particle placed at other position tends to be attracted to the center surface of the probe tip. Furthermore, a preferable method using the combination of the near-field optical fiber probe and the AFM metallic probe is proposed, for more efficient non-contact manipulation and better observation of one single nano-particle. The analyses of trapping potential along the probe axis and the near-field distribution show the possibility of particle trapping.     

A functional probe with bowtie aperture and bull's eye structure for nanolithograph

The bowtie aperture surrounded by concentric gratings(the bull’s eye structure) integrated on the near-field scanning optical microscopy(NSOM) probe(aluminum coated fiber tip) for nanolithography has been investigated using the finite-difference time domain(FDTD) method.By modifying the parameters of the bowtie aperture and the concentric gratings,a maximal field enhancement factor of 391.69 has been achieved,which is 18 times larger than that obtained from the single bowtie aperture.Additionally,the light spot depends on the gap size of the bowtie aperture and can be confined to sub-wavelength.The superiority of the combination of the bowtie aperture and the bull’s eye structure is confirmed,and the mechanism for the electric field enhancement in this derived structure is analyzed.     

Enhanced light confinement in a near-field optical probe with a triangular aperture

We present a probe concept for scanning near-field optical microscopy combining the excellent background suppression of aperture probes with the superior light confinement of apertureless probes. A triangular aperture at the tip of a tetrahedral waveguide (full taper angle approximately 90 degrees ) shows a strong field enhancement at only one rim when illuminated with light of suitable polarization. Compared to a circular aperture of equivalent size, the resolution capability is doubled without loss of brightness. For a approximately 60 nm sized triangular aperture, we measured an optical resolution <40 nm and a transmission of approximately 10(-4).     

Hydrophobic optical elements for near-field optical analysis (NOA) in liquid environment--a preliminary study.

Near-field Scanning Optical Microscopy (NSOM) in liquid environment is expected to allow time resolved morphological mappings on cellular surfaces on the nanoscale level. Near-field Optical Analysis (NOA) via NSOM exploits the energy transfer from the tip of an optical element (tip diameter > or = 20nm), oscillating within the range of the characteristic length of the energy transfer ( approximately 10nm) in the near-field of the surface to be analysed. In NOA, a molecular assembly is monitored by visible light with a resolution far below the wavelength of visible light. Actually, NOA is successfully applied in mapping local optical contrasts, for instance in photonic crystals with dielectric periodicities on the nanoscale. NSOM could in principle be performed in two different modes: tapping mode, with tip-oscillations perpendicular, or shear force mode, with tip-oscillations parallel to the substrate. Both basic modes have specific advantages and disadvantages. In biological systems (e.g. in cell cultures), where scanning in liquids is prevalent, elongated optical elements non-invasively operated in the shear force modus could have some specific advantages when compared to contact modus systems. While tapping mode NSOM provides satisfactory nanoscale images even on solid surfaces covered with millimetres of liquids, the performance of shear force mode NSOM is presently largely confined to operations on dry samples. This is due to the inability of conventional shear force mode NSOM systems to provide sharp topographic images of sample surfaces substantially covered with liquids. By equipping a conventional NSOM system with hydrophobic optical elements, shear force mode based topographic images could be obtained on biological samples in dry as well as in aqueous environment, and with resolutions on the nanoscale level.     

Fluorescence near-field microscopy of DNA at sub-10 nm resolution

We demonstrate apertureless near-field microscopy of single molecules at sub-10 nm resolution. With a novel phase filter, near-field images of single organic fluorophores were obtained with approximately sixfold improvement in the signal-to-noise ratio. The improvement allowed pairs of molecules separated by approximately 15 nm to be reliably and repeatedly resolved, thus demonstrating the first true Rayleigh resolution test for near-field images of single molecules. The potential of this technique for biological applications was demonstrated with an experiment that measured the helical rise of A-form DNA.     

Near-Field Scanning Optical Microscopy Combined with a Tapping Mode Atomic Force Microscope

Tapping mode atomic force microscopy is used to control the tip-sample distance in near field scanning optical microscopy (NSOM), which gives both topographic and near-field images simultaneously. The evanescent waves are scattered by a vibrating silicon-nitride tip in the proximity of sample surfaces and are detected through a microscope objective. This NSOM allows the observation of opaque samples with reflection illumination. A glass grating of 1-μm pitch and an InP grating of 0.5-μm pitch are observed with a lateral resolution of 100 nm.Presented at 1996 International Workshop on Interferometry (IWI ‘96), August 27-29, Saitama, Japan     

Nanoscopic study of second-harmonic generation in organic crystals with collection-mode near-field scanning optical microscopy

Collection-mode near-field scanning optical microscopy (NSOM) is used to map nanoscopic second-harmonic generation (SHG) in N -(4-nitrophenyl)- (L) -prolinol crystals. A spatial resolution of 98 nm is achieved. Near-field polarization-dependent SHG measurement is performed, and a local effective SHG susceptibility of 224+/-18 pm/V is obtained.     

High-intensity bowtie-shaped nano-aperture vertical-cavity surface-emitting laser for near-field optics

We report a high-intensity nano-aperture vertical-cavity surface-emitting laser (VCSEL) utilizing a bowtie-shaped aperture. A maximum power of 188 microW is achieved from a 180 nm bowtie aperture at a wavelength of 970 nm. The near-field full width at half-maximum intensity spot size 20 nm away from the bowtie aperture is 64 nm x 66 nm from simulation, and the peak near-field intensity is estimated to be as high as 47 mW/microm(2). This intensity is high enough to realize near-field optical recording, and the small spot size corresponds to storage densities up to 150 Gbits/in(2). The bowtie-aperture VCSEL also enables other applications, such as compact high-intensity probes for ultrahigh-resolution near-field imaging and single molecule fluorescence and spectroscopy.     

Near-field time-resolved fluorescence studies of poly(phenylmethyl silane) with subwavelength resolution

We present an example of the first time-correlated single-photon counting (TCSPC) near-field optical measurement. The aperture size of our prepared aluminun-coated fiber-optic probe was approximately 50 nm, which represents a spatial resolution of _ex/7 for our UV measurements. Near-field fluorescence decays of poly(phenylmethyl silane) in solid thin films excited in the range 325–360 nm were obtained and the steady-state excitation spectra compared with the excitation spectral information obtained in the far field. Fluorescence decays showed single exponential lifetimes ranging from 45 to 277 ps, which was dependent on the excitation wavelength and the selected near-field tip. The proximity of the metal-coated tip to the sample may be the reason for the modulation in fluorescence lifetime.     

Cloaked near-field scanning optical microscope tip for noninvasive near-field imaging

Near-field imaging is a well-established technique in biomedical measurements, since closer to the detail of interest it is possible to resolve subwavelength details otherwise unresolved by regular lenses. A near-field scanning optical microscope (NSOM) tip may indeed overcome the resolution limits of far-field optics, but its proximity inherently perturbs the measurement. Here, we apply the recent concept of a "cloaked sensor" to an NSOM device in collection mode, showing theoretically how a proper plasmonic cover applied to an NSOM tip may drastically improve its overall measurement capabilities.     

Fabrication of a Near-Field Optical Fiber Probe with a Nanometric Metallized Protrusion

We have developed a novel probe with a nanometric metallized protrusion extending through a subwavelength aperture to increase optical near-field excitation and collection efficiencies. The apex diameter of the fabricated metallized protrusion was 35 nm. The Intensity distribution of the optical near-field at the apex of the probe was measured by scanning another probe across the apex, and it was observed that strong optical near-field was generated at the apex of the metallized protrusion. The width of the intensity distribution was 150 nm including instrumental resolution. Probes with spherical and ellipsoidal metallized protrusion were also fabricated, by which enhancement of the optical near-field is expected due to localized plasmon excitation.     

阶梯型纳米孔径的近场光学局域增强特性研究

提出一种高分辨力与高通光效率兼备的阶梯型纳米孔径设计方法 ,孔径的尺寸从膜层的入射表面向出射表面呈阶梯型逐渐减小 ,直到在膜层的出射表面形成一个亚波长的小孔。采用三维时域有限差分 (FDTD)方法对方形阶梯型纳米孔径及三角形阶梯型纳米孔径进行了数值模拟计算。结果表明 ,由于近场光学很强的局域场增强效应 ,其通光效率与输出光强极大值在具有相同近场光斑尺寸情况下 ,较普通的非阶梯型纳米孔径提高了两个数量级 ,甚至更高 ,有效地提高了输出光功率。采用四台阶三角形阶梯型纳米孔径 ,当光斑半峰全宽为 97nm× 74nm时 ,出射光强极大值达到 10 4 9.76 ,较入射光增强了 10 0 0倍 ,而通光效率大于 1,达到 1.6 7。这种阶梯型纳米孔径可以直接作为纳米孔径激光器的出射孔径提高其输出光功率 ,也可以作为独立的光学屏对入射光进行整形得到具有高输出功率的亚波长尺度光源 ,在纳米尺度光学成像、光谱探测、数据存储、光刻、光学操作等近场光学应用领域具有潜在的应用前景。