Laser scanning gradient index optics microscope: submicrometre scale spectroscopy and imaging at cryogenic temperatures

A laser scanning far-field optical microscope for low-temperature imaging and spectroscopy based on gradient index optics is presented. A rod-shaped gradient index microlens is used as a zero-working-distance solid immersion objective lens. The obtained lateral resolution is 310 nm of the FWHM at a wavelength of 545 nm. A laser scanning mechanism located outside an optical cryostat enables one to achieve large scanning ranges independent of temperature. The use of the microscope for submicrometre-scale spectroscopy and low-temperature photochemistry performed on molecular J aggregates in thin polymer films is presented.     

Shear force imaging of DNA in a near-field scanning optical microscope (NSOM)

A near-field scanning optical module has been constructed as an accessory for a Nanoscope IIIa commercial scanning probe microscope. Distance feedback and topographic registration are accomplished with an uncoated optical fibre scanning tip by implementation of the shear force technique. The tip is driven by a piezoelectric actuator at a resonance frequency of 8–80 kHz. A laser diode beam is scattered by the tip and detected by a split photodiode, with lock-in detection of the difference signal. The amplitude ( r ) and phase (τ) responses were characterized as a function of the calibrated tip–sample separation. Using an r cos τ feedback signal, imaging of pUC18 relaxed circular plasmid DNA spread on mica precoated with cetylpyridinium chloride was achieved. The apparent width (28 ± 5 nm) was approximately four times that achieved by scanning force measurements with the same instrument; the apparent height of the DNA (0.6 ± 0.3 nm) was similar with the two techniques. These results demonstrate the applicability of the shear force signal for imaging biological macromolecules according to topography and in conjunction with the optical signals of a near-field scanning optical microscope (NSOM).     

Fluorescence imaging with a laser trapping scanning near-field optical microscope

We investigated fluorescence imaging using a near-field scanning optical microscope which uses a laser-stabilized gold nanoparticle as a near-field probe. This microscope is suitable for observations of biological specimens in aqueous solutions because the probe particle is held by a noncontact force exerted by a laser beam. Theoretical calculations based on Mie scattering theory are presented to evaluate the near-field enhancement by a gold particle of 40 nm diameter. We also present fluorescence images of a single fluorescent bead and discuss the near-field contribution to the fluorescence image in this type of microscope.     

A novel light source for SICM–SNOM of living cells

We have developed a novel light source for use in a scanning near‐field optical microscope (SNOM or NSOM) based on a nanopipette whose distance from the sample surface is controlled using scanning ion conductance microscopy. The light source is based on the general principle of the chemical reaction between a fluorophore in the pipette and ligand in the bath, to produce a highly fluorescent complex that is continually renewed at the pipette tip. In these experiments we used fluo‐3 and calcium, respectively. This complex is then excited with an Ar⁺ laser, focused on the pipette tip, to produce the light source. This method overcomes the transmission problem of more traditional SNOM probes and has been used to acquire simultaneous high‐resolution topographic and optical images of biological samples in physiological buffer. A resolution of ～220 nm topographic and ～190 nm optical was determined through imaging fixed sea‐urchin sperm flagella. Live A6 cells were also imaged, demonstrating the potential of this system for SNOM imaging of living cells.     

A versatile multipurpose scanning probe microscope

A combined scanning probe microscope has been developed that allows simultaneous operation as a non‐contact/tapping mode atomic force microscope, a scattering near‐field optical microscope, and a scanning tunnelling microscope on conductive samples. The instrument is based on a commercial optical microscope. It operates with etched tungsten tips and exploits a tuning fork detection system for tip/sample distance control. The system has been tested on a p‐doped silicon substrate with aluminium depositions, being able to discriminate the two materials by the electrical and optical images with a lateral resolution of 130 nm.     

Fibre-optic two-photon scanning fluorescence microscopy

Two geometries of a novel two‐photon fluorescence microscope incorporating single‐mode fibre optics for the delivery of ultrashort‐pulsed illumination to a remote sample are characterized. First, a 785 nm single‐mode optical fibre is implemented in a scanning microscope, which demonstrates that an improvement in axial resolution is achieved due to the non‐linear response of the fibre to intense ultrashort‐pulsed light. Second, a 785 nm single‐mode optical fibre coupler is adapted, in which case spectral broadening and blue shifting of the ultrashort‐pulsed laser beam caused by the non‐linear response of the fibre to ultrashort‐pulsed illumination are experimentally characterized. An investigation into the impact of temporal broadening of the ultrashort‐pulsed beam on the systems is also considered. The coupling efficiency of both geometries for various illumination wavelengths is also presented. The introduction of the fibre coupler to the system has significant advantages, including an improved optical sectioning effect and a reduction in the number of bulk optical components resulting in a low‐cost, compact instrument. Sets of three‐dimensional images of fluorescent polymer microspheres and biological material confirm these features.     

Unique laser-scanning optical microscope for low-temperature imaging and spectroscopy

Low-temperature optical characterization of single quantum nanostructures can reveal detailed information on structure-dependent properties of these materials. We describe the development of a unique laser-scanning optical microscope capable of low-temperature single molecule/particle imaging and spectroscopy. Making use of the magnification of a microscope objective, the laser- scanning scheme of the present microscope allows for high-repeatability imaging over large sample areas. The microscope is utilized to measure the low-temperature Raman scattering spectra of individual single-walled carbon nanotubes and single molecule fluorescence spectra of conjugated polymers. The developed low-temperature microscope can be applied to study a wide array of nanomaterials at a single particle level.     

Cathodoluminescence imaging and spectroscopy by near-field detection

Using near-field techniques, we have developed an experimental set-up for spatially resolved cathodoluminescence (CL) spectroscopy and monochromatic imaging. It combines a scanning near-field optical/force microscope with a scanning electron microscope equipped with a field emission gun. The potentialities of this scanning near-field cathodoluminescence microscope are demonstrated on two kinds of sample: an indented MgO crystal and AlGaN/GaN quantum wells grown on GaN/sapphire. Monochromatic CL imaging allows a clear distinction between the emission of quantum wells and the GaN substrate, and for the MgO crystal, the localization on the slip bands, near the indentation, of luminescent centres emitting at 450 nm.     

An evanescent-field optical microscope

The classic diffraction limit of resolution in optical microscopy (～γ/2) can be overcome by detecting the diffracted field of a submicrometre-size probe in its near field. The present stage of this so-called scanning near-field optical microscopy (SNOM) is reviewed. An evanescent-field optical microscope (EFOM) is presented in which the near-field regime is provided by the exponentially decaying evanescent field caused by total internal reflection at a refractive-index transition. A sample placed in this field causes a spatial variation of the evanescent field which is characteristic for the dielectric and topographic properties of the sample. The evanescent field is frustrated by a dielectric probe and thus converted into a radiative field. In our case the probe consists either of an etched optical fibre or of a highly sharpened diamond tip. The probe is scanned over the sample surface with nanometre precision using a piezo-electric positioner. The distance between probe and sample is controlled by a feedback on the detected optical signal. The resolution of the microscope is determined by both the gradient of the evanescent field and the sharpness of the tip. Details of the experimental set-up are discussed. The coupling of the evanescent field to the submicrometre probe as a function of probe-sample distance, angle of incidence and polarization has been characterized quantitatively. The observed coupling is generally in agreement with presented theoretical calculations. Microscopy has been performed on a regular latex sphere structure, which clearly demonstrates the capacity of the evanescent-field optical microscope for nanometre-scale optical imaging. Resolution is typically 100 nm laterally and 10 nm vertically. The technique is promising for biological applications, especially if combined with optical spectroscopy.     

Near-field and confocal surface-enhanced resonance Raman spectroscopy at cryogenic temperatures

For laser spectroscopy at variable temperatures with high spatial resolution a combined scanning near‐field optical and confocal microscope was developed. Rhodamine 6G (R6G) dye molecules dispersed on silver nano‐particles or nano‐clusters were investigated. For optical excitation of the molecules, either an aperture probe or a focused laser spot in confocal arrangement were employed. Raman spectra in the wavenumber range between 300 cm^?1 and 3000 cm^?1 at room temperatures down to 8.5 K were recorded. Many of the observed Raman lines can be associated with the structure of the adsorbed molecule. Intensity fluctuations in spectral sequences were observed down to 77 K and are indicative of single molecule sensitivity.     

Fabrication of near-field optical apertures in aluminium by a highly selective corrosion process in the evanescent field

A simple, one‐step process to fabricate high‐quality apertures for scanning near‐field optical microscope probes based on aluminium‐coated silicon nitride cantilevers is presented. A thin evanescent optical field at a glass–water interface was used to heat the aluminium at the tip apex due to light absorption. The heat induced a breakdown of the passivating oxide layer and local corrosion of the metal, which selectively exposed the front‐most part of the probe tip from the aluminium. Apertures with a protruding silicon nitride tip up to 72 nm in height were fabricated. The height of the protrusion was controlled by the extent of the evanescent field, whereas the diameter depended on the geometry of the probe substrate. The corrosion process proved to be self‐terminating, yielding highly reproducible tip heights. Near‐field optical resolution in a transmission mode of 85 nm was demonstrated.     

Fluorescence anisotropy near-field scanning optical microscopy (FANSOM): a new technique for nanoscale microviscometry

A near-field scanning optical microscope system was implemented and adapted for nanoscale steady-state fluorescence anisotropy measurement. The system as implemented can resolve 0.1 cP microviscosity variations with a resolution of 250 nm laterally in the near field, or 10 μm when employed in a vertical scanning mode. The system was initially used to investigate the extent of microviscous vicinal water over surfaces of varying hydrophilicity. Water above a cleaved mica surface was found to have a decreased microviscosity, while water above a hydrophobic surface showed no change (detection limit 0.1 cP at 30+ nm from the surface).     

Evaluation of nano-optical probe from scanning near-field optical microscope images

We studied a nanometre-sized optical probe in a scanning near-field optical microscope. The probe profile is determined by using a knife-edge method and a modulated transfer function evaluation method which uses nanometre-sized line-and-space tungsten patterns (with spaces 1 μm to 50 nm apart) on SiO₂ substrates. The aluminium-covered, pipette-pulled fibre probe used here has two optical probes: one with a large diameter (350 nm) and the other with a small diameter (10 nm). The small-diameter probe has an optical intensity ≈63 times larger than that of the large-diameter probe, but the power is about 1/25 of that of the large probe.     

Near-field scanning optical microscopy and near-field induced photocurrent investigations of buried heterostructure multiquantum well lasers

Buried heterostructure multiquantum well laser devices are investigated utilizing a near-field scanning optical microscope to characterize and correlate the surface topography, optical output and electronic properties of the device. Near-field photocurrent imaging has been used to accurately measure the unbiased buried heterostructure multiquantum well device in cross-section, successfully revealing the distribution of pn-junctions and their associated fields. Moreover, this has been accurately correlated with the physical structure of the device determined by simultaneous shear-force imaging of the surface. Topographic structure is manifested as a result of strain relaxation (∼10⁻¹⁰ m) of the cleaved cross-section. These imaging modes are similarly correlated with the optical output of the operational device mapped with 50 nm lateral resolution. The collection-mode measurements detected electroluminescence external to the active region, highlighting the existence of carrier recombination away from the multiquantum well device region. The combination and correlation of different near-field scanning optical microscope imaging modes proved powerful in the analysis of the buried heterostructure multiquantum well device, and was shown to assist in the identification of current leakage pathways within the structure.     

Writing subwavelength-sized structures into aluminium films by thermo-chemical aperture-less near-field optical microscopy

An optical near-field at the tip of an atomic force microscope probe is utilised to pattern aluminium thin films on glass substrates by photo-thermally induced corrosion in water. Aluminium forms a thin passivating oxide layer when immersed into neutral water at room temperature. Owing to the high energy density of the near-field, the metal below the probe tip can be heated to 100°C due to absorption of the light, which then provokes breakdown of the passivation and metal corrosion. The localised near-field is generated by tip-induced enhancement of an evanescent field originating from a laser beam, that is totally internally reflected at the glass–aluminium–water interface. The process is governed by surface plasmons excited in the aluminium film by the evanescent waves and the field enhancement of the probe tip. Holes of 40 nm diameter and lines below 100 nm width have been written into a 20-nm-thick aluminium film. Applications of the scanning probe lithography process may include the one-step fabrication of point contacts or contact masks for near-field optical lithography and reactive ion etching.     

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.     

Fluorescence imaging with a laser trapping scanning near-field optical microscope

We investigated fluorescence imaging using a near-field scanning optical microscope which uses a laser-stabilized gold nanoparticle as a near-field probe. This microscope is suitable for observations of biological specimens in aqueous solutions because the probe particle is held by a noncontact force exerted by a laser beam. Theoretical calculations based on Mie scattering theory are presented to evaluate the near-field enhancement by a gold particle of 40 nm diameter. We also present fluorescence images of a single fluorescent bead and discuss the near-field contribution to the fluorescence image in this type of microscope.     

Apertureless near-field Raman spectroscopy

We report on the tip‐enhanced Raman spectra of C60 obtained on a custom‐built apertureless scanning near‐field optical microscope. A commercial atomic force microscope tip coated with 100 nm thickness of gold was used to enhance locally the Raman signal and permit topographic and spectral information to be acquired simultaneously. We present preliminary data which demonstrate the tip enhancement effect using C60 as a test sample.     

Analysis of fluorescence from algae fossils of the Neoproterozoic Doushantuo formation of China by confocal laser scanning microscope

Chinese algae fossils can provide unique information about the evolution of the early life. Thin sections of Neoproterozoic algae fossils, from Guizhou, China, were studied by confocal laser scanning microscopy, and algae fossils were fluorescenced at different wavelengths when excited by laser light of 488 nm, 476 nm, and 568 nm wavelength. When illuminated by 488 nm laser light, images of the algae fossils were sharper and better defined than when illuminated by 476 nm and 568 nm laser light. The algae fossils fluoresce at a wide range of emission wavelengths. The three-dimensional images of the fluorescent algae fossils were compared with the transmission images taken by light microscope. We found that the fluorescence image of the confocal laser scanning microscope in a single optical section could pass for the transmission image taken by a light microscope. We collected images at different sample depths and made a three-dimensional reconstruction of the algae fossils. And on the basis of the reconstruction of the three-dimensional fluorescent images, we conclude that the two algae fossils in our present study are red algae.