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.     

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.     

High-resolution constant-height imaging with apertured silicon cantilever probes

We present high-resolution aperture probes based on non-contact silicon atomic force microscopy (AFM) cantilevers for simultaneous AFM and near-infrared scanning near-field optical microscopy (SNOM). For use in near-field optical microscopy, conventional AFM cantilevers are modified by covering their tip side with an opaque aluminium layer. To fabricate an aperture, this metal layer is opened at the end of the polyhedral probe using focused ion beams (FIB). Here we show that apertures of less than 50 nm can be obtained using this technique, which actually yield a resolution of about 50 nm, corresponding to λ/20 at the wavelength used. To exclude artefacts induced by distance control, we work in constant-height mode. Our attention is particularly focused on the distance dependence of resolution and to the influence of slight cantilever bending on the optical images when scanning at such low scan heights, where first small attractive forces exerted on the cantilever become detectable.     

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.     

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.     

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.     

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.     

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).     

Femtosecond near-field scanning optical microscopy

We have developed an instrument for optically measuring carrier dynamics in thin-film materials with ≈150 nm lateral resolution, ≈250 fs temporal resolution and high sensitivity. This is accomplished by combining an ultrafast pump–probe laser spectroscopic technique with a near-field scanning optical microscope. A diffraction-limited pump and near-field probe configuration is used, with a novel detection system that allows for either two-colour or degenerate pump and probe photon energies, permitting greater measurement flexibility than that reported in earlier published work. The capabilities of this instrument are proven through near-field degenerate pump–probe studies of carrier dynamics in GaAs/AlGaAs single quantum well samples locally patterned by focused ion beam (FIB) implantation. We find that lateral carrier diffusion across the nanometre-scale FIB pattern plays a significant role in the decay of the excited carriers within ≈1 μm of the implanted stripes, an effect which could not have been resolved with a far-field system.     

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.     

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.     

Image contrast of dielectric specimens in transmission mode near-field scanning optical microscopy: imaging properties and tip artefacts

Near-field scanning optical microscopy (NSOM) is a scanned probe technique utilizing a subwavelength-sized light source for high-resolution imaging of surfaces. Although NSOM has the potential to exploit and extend the experimental utility of the modern light microscope, the interpretation of image contrast is not straightforward. In near-field microscopy the illumination intensity of the source (probe) is not a constant value, rather it is a function of the probe–sample electronic environment. A number of dielectric specimens have been studied by NSOM to elucidate the contrast role of specimen type, topography and crystallinity; a summary of metallic specimen observations is presented for comparative purposes. Near-field image contrast is found to be a result of lateral changes in optical density and edge scattering for specimens with little sample topography. For surfaces with considerable topography the contributions of topographic (Z) axis contrast to lateral (X,Y) changes in optical density have been characterized. Selected near-field probes have also been shown to exhibit a variety of unusual contrast artefacts. Thorough study of polarization contrast, optical edge (scattering) contrast, as well as molecular orientation in crystalline specimens, can be used to distinguish lateral contrast from topographic components. In a few cases Fourier filtering can be successfully applied to separate the topographic and lateral contrast components.     

Optical microcantilever consisting of channel waveguide for scanning near-field optical microscopy controlled by atomic force

We develop a novel optical microcantilever for scanning near-field optical microscopy controlled by atomic force mode (SNOM/AFM). The optical microcantilever has the bent channel waveguide, the corner of which acts as aperture with a large tip angle. The resonance frequency of the optical microcantilever is 9 kHz, and the spring constant is estimated to be 0.59 N/m. The optical microcantilever can be operated in contact mode of SNOM/AFM and we obtain the optical resolution of about 200 nm, which is as same size as the diameter of aperture. We confirm that the throughput of optical microcantilever with an aperture of 170 nm diameter would be improved to be more than 10⁻⁵.     

Imaging 'intact' myofibrils with a near-field scanning optical microscope

Fluorescently labelled myofibrils were imaged in physiological salt solution by near-field scanning optical microscopy and shear-force microscopy. These myofibrils were imaged in vitro , naturally adhering to glass while retaining their ability to contract. The Z-line protein structure of the myofibrils was antibody labelled and easily identified in the near-field fluorescence images. The distinctive protein banding structure of the myofibril was also seen clearly in the shear-force images without any labelling requirement. With the microscope in the transmission mode, resolution of the fluorescence images was degraded significantly by excessive specimen thickness (>1 μm), whereas the shear-force images were less affected by specimen thickness and more affected by poor adherence to the substrate. Although the exact mechanism generating contrast in the shear-force images is still unknown, shear-force imaging appears to be a promising new imaging modality.     

Polarization contrast in reflection near-field optical microscopy with uncoated fibre tips

Using cross-hatched, patterned semiconductor surfaces and round 20-nm-thick gold pads on semiconductor wafers, we investigate the imaging characteristics of a reflection near-field optical microscope with an uncoated fibre tip for different polarization configurations and light wavelengths. It is shown that cross-polarized detection allows one to effectively suppress far-field components in the detected signal and to realize imaging of optical contrast on the sub-wavelength scale. The sensitivity window of our microscope, i.e. the scale on which near-field optical images represent mainly optical contrast, is found to be ≈100 nm for light wavelengths in the visible region. We demonstrate imaging of near-field components of a dipole field and purely dielectric contrast (related to well-width fluctuations in a semiconductor quantum well) with a spatial resolution of ≈100 nm. The results obtained show that such a near-field technique can be used for polarization-sensitive imaging with reasonably high spatial resolution and suggest a number of applications for this technique.     

Femtosecond near-field scanning optical microscopy

We have developed an instrument for optically measuring carrier dynamics in thin-film materials with approximately 150 nm lateral resolution, approximately 250 fs temporal resolution and high sensitivity. This is accomplished by combining an ultrafast pump-probe laser spectroscopic technique with a near-field scanning optical microscope. A diffraction-limited pump and near-field probe configuration is used, with a novel detection system that allows for either two-colour or degenerate pump and probe photon energies, permitting greater measurement flexibility than that reported in earlier published work. The capabilities of this instrument are proven through near-field degenerate pump-probe studies of carrier dynamics in GaAs/AIGaAs single quantum well samples locally patterned by focused ion beam (FIB) implantation. We find that lateral carrier diffusion across the nanometre-scale FIB pattern plays a significant role in the decay of the excited carriers within approximately 1 microm of the implanted stripes, an effect which could not have been resolved with a far-field system.     

Inducing superconductivity at a nanoscale: photodoping with a near-field scanning optical microscope

The local modification of an insulating GdBa₂Cu₃O_6.5 thin film, made superconducting by illumination with a near-field scanning optical microscope (NSOM), is reported. A 100-nm aperture NSOM probe acts as a sub-wavelength light source of wavelength λ_exc = 480–650 nm, locally generating photocarriers in an otherwise insulating GdBa₂–Cu₃O_6.5 thin film. Of the photogenerated electron–hole pairs, electrons are trapped in the crystallographic lattice, defining an electrostatic confining potential to enable the holes to move. Reflectance measurements at λ = 1.55 μm at room temperature show that photocarriers can be induced and constrained to move on a ≈200 nm scale for all investigated λ_exc. Photogenerated wires present a superconducting critical temperature T _c = 12 K with a critical current density J _c = 10⁴ A cm⁻². Exploiting the flexibility provided by photodoping through a NSOM probe, a junction was written by photodoping a wire with a narrow (≈ 50 nm) under-illuminated gap. The strong magnetic field modulation of the critical current provides a clear signature of the existence of a Josephson effect in the junction.     

Near field optical microscopy in aqueous solution: implementation and characterization of a vibrating probe

Near field optical microscopy (NSOM) is one of the possible solutions to circumvent the diffraction limit, but the control of the optical probe in solution has been a technical challenge for practical applications. Most recently, it has been shown that the pipette used in the scanning ion conductance microscope can be modified to form a high resolution near field optical probe. When combined with a novel distance modulation mechanism, a robust near field microscope can be constructed for operation in aqueous solution. In this paper, we present technical details of this design and a further characterization of the NSOM system for imaging in solution. Fundamental limitations of this approach in comparison to other systems are also discussed. Based on the current technology, it is concluded that better than 50 nm resolution should be achievable with this technique for fluorescence, as well as fluorescence resonance energy transfer, imaging of biological specimens.     

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.     

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.