Near-field Scanning Optical Microscopy Studies Research Articles

Near-field scanning optical microscopy studies of thin film surfaces and interfaces

In this article, the results of the modeling of topography related artifacts appearing in near-field scanning optical microscopy measurements are presented. The results obtained for near-field scanning optical microscope operation in reflection mode with off-axis far field detector position are compared with experimental results. It is shown that the chosen numerical method – Finite Difference in Time Domain method (FDTD) – can be used for efficient modeling of main topography related artifact. It is also seen that the far field detector position can have large influence on the resulting reflection mode optical images.

Near-Field Emission Properties of a Self-Formed InAs Quantum Dot Laser Diode by Using Near-Field Scanning Optical Microscopy

Near-field scanning optical microscopy studies of a self-formed InAs quantum dot laser diode (QDLD) at room-temperature were performed. We measured the near-field electroluminescence spectra and the spectrally resolved near-field scanning images emitted from a QDLD with a spatial resolution of ∼100 nm. We show that these data have direct implications for device performance associated with the injection current.

Near-field scanning optical microscopy and time-resolved optical characterization of epitaxial lateral overgrown c-plane and a-plane GaN

Epitaxial lateral overgrowth (ELO) was employed for both c-plane and a-plane GaN layers on sapphire, and a more pronounced optical improvement was observed for the a-plane GaN as evidenced by the significantly increased band edge photoluminescence (PL). Room temperature near-field scanning optical microscopy studies explicitly showed enhanced optical quality in the wing regions of the overgrown GaN due to reduced density of dislocations, and for the a-plane ELO GaN sample the wings and the windows were clearly discernible from PL mapping. Time-resolved PL measurements revealed biexponential decays with time constants that were significantly enhanced for the a-plane ELO GaN (τ1=0.08ns, τ2=0.25ns) when compared to the non-ELO control sample but were still much shorter than those for the c-plane ELO GaN (τ1=0.26ns, τ2=0.90ns).

Single dot near‐field spectroscopy for photonic crystal microcavities

In this paper we demonstrate how near-field scanning optical microscopy (NSOM) can be utilized for tuning the emission energy of a single quantum dot (QD) using nanoindentation with the tapered optical fiber probe. Thus the use of NSOM in the nanoindentation regime can provide spectral matching of QD excitons and localised photons in photonic crystal cavities. We present results of NSOM study of confined photon modes in triangular lattice photonic crystal microcavities containing InAs quantum dots with emission wavelength λ ∼ 1.2 µm. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical and structural characterization of GaN/AlN quantum dots grown onSi(111)

GaN/AlN-based heterostructures made from stacked GaN quantum dots (QDs)have been studied by means of the cathodoluminescence (CL), photoluminescence(PL), near-field scanning optical microscopy (NSOM) and micro-Ramantechniques. The influence of the number of stacked layers (2–85) and of thedifferent electron beam injection conditions on the main optical emissionswas studied by means of CL, revealing transitions from 2.5 and 4.4 eV.Power-dependent cross-sectional CL studies revealed a large (87–180 meV)blue-shift only for the optical bands located in the 2.5 and 3.1 eV spectral range.This observation enabled us to assign a zero-dimensional character to those bands.The results were confirmed by PL and NSOM studies. Different values of theblue-shift were found for specimens with different numbers of stacked layers. Thissuggested the presence of different residual strains inside the structures, asconfirmed by micro-Raman studies. An inhomogeneous distribution of the QDemissions was also observed both in the plane and along the growth direction.

Simultaneous Near-Field Optical Birefringence and Fluorescence Contrast Applied to the Study of Dye-Doped Polymer-Dispersed Liquid Crystals

The local optical and electrooptical properties of dye-doped polymer-dispersed liquid crystal (PDLC) thin films are explored by near-field scanning optical microscopy (NSOM). Detailed information on the mechanisms for dye alignment and electric-field-induced dye reorientation in localized sample regions is sought. Dye reorientation is predicted to occur by two distinct processes: (i) via its interactions with the liquid crystal host and (ii) via the direct interactions of its permanent and induced dipole moments with the applied field. The liquid crystal is doped with a nearly isotropically aligned BODIPY dye so that contributions of both reorientation mechanisms can be explored. An order parameter of 0.10 ± 0.02 is measured for BODIPY in bulk aligned samples, suggesting that host−guest interactions are very weak. Simultaneously recorded birefringence (transmission) and fluorescence NSOM images confirm that the dye is apparently not well aligned, even on submicrometer length scales. Surprisingly, NSOM studies of dye reorientation prove that host−guest interactions still play a substantial role in controlling the dye reorientation process. The field dependence of dye reorientation is found to be correlated with liquid crystal reorientation, varying spatially across individual droplets. Analysis of the simultaneously recorded birefringence and fluorescence data shows that the dye is strongly reoriented via its interactions with the reorienting liquid-crystal host and also via its independent interactions with the applied field. The former mechanism is manifested as a rotation of the dye alignment axis while the latter leads to a field-dependent increase in the local dye order parameter. “Bistable” alignment of the dye in the nematic host is proposed as a possible explanation for the apparent coupling of the two reorientation mechanisms.

Near-field scanning optical microscopy studies of electronic and photonic materials and devices

Probing optical properties of materials and optical characterization of crystallographic defects at the nanometer scale have been inaccessible until recently due to the diffraction limit of light. With the invention of near-field scanning optical microscopy (NSOM), resolution at the 50–100 nm level using visible or near infrared light is now practical. In addition to describing the NSOM technique, this review focuses on the application of NSOM to the characterization of electronic and photonic materials and devices, with particular emphasis on defects. The unique capability of NSOM to simultaneously measure surface topography and local optoelectronic properties, thereby eliminating the need to perform cross correlation analysis on results obtained using different techniques, is particularly useful in this area. Several examples are discussed. By performing near-field photocurrent (NPC) measurements, NSOM is used to probe electrical activities associated with individual threading dislocations and dislocation networks in strain relaxed, compositionally graded GeSi films. The non-destructive nature of NSOM helps elucidate how microstructural defects in the SrTiO3 bicrystal substrates affect YBa2Cu3O7 film growth and GBJJ performance. Characterization of III–V and II–VI semiconductors, guantum dots grown by strain epitaxy, laser diodes, waveguides, and photonic crystals is also included. The advantages and disadvantages of NSOM in each application will be outlined. Throughout the review, emphasis is placed on how NSOM complements existing materials characterization techniques, as well as how quantitative results can be obtained from NSOM measurements.

Atomic Force Microscopy and Near-Field Scanning Optical Microscopy Study of Quantum-Dot Assemblies and Fractal Films

Quantum dots and nanocrystalline films are interesting materials due to their novel properties, not achievable from the bulk materials [1]. New materials fabricated by assembling of quantum dots and nanostructured materials exhibit, for example, high optical nonlinearities. Fractal structures [2] can be formed from quantum-dot aggregates or films. Scanning probe microscopy is the essential technique to characterize these nanometer-scaled materials?Gold nanocluster colloids are synthesized in the interior of surfactant aggregates known as inverse micelles, without the use of water to solubilize the metal salt [3]. The size of the Au quantum dots is well controlled in the colloid synthesis and selected using chromatography. The Au clusters are then sprayed onto a glass slides to form thin films made of Au quantum dots. We have also used the laser ablation technique [4] to grow Ag nanoparticles, fractal aggregates and thin films. These samples have been studied using atomic force microscopy (AFM) and nearfield scanning optical microscopy.

Time-Resolved and Near-Field Scanning Optical Microscopy Study on Porphyrin J-Aggregate

Time-Resolved and Near-Field Scanning Optical Microscopy Study on Porphyrin J-Aggregate

Hexakis Porphyrinato Benzenes. A New Class of Porphyrin Arrays

A new type of porphyrin array has been synthesized by the coupling of six porphyrin moieties to a central benzene core via an ether linkage. The resulting porphryin supermolecule has a diameter up to 80 Å and a mass of 8500 daltons. In solution, the six porphyrins around the central benzene ring arrange themselves into three sets of offset overlapping dimers, which are rapidly interconverting at room temperature. Solution UV−vis and fluorescence studies, however, indicate that there are no electronic interactions between the individual porphyrin molecules. Upon spreading a chloroform solution of these porphyrin molecules on a surface, they self-assemble to form ring-shaped architectures on a micrometer scale. Near-field scanning optical microscopy studies reveal that the porphyrin moieties within the rings have an ordered arrangement with respect to their position in the ring after the sample has been annealed at 80 °C for 2 days.

Near-field scanning optical microscopy studies of V-grooved quantum wire lasers

Detailed near-field scanning optical microscopy studies of GaAs/AlGaAs V-grooved quantum wire lasers at room temperature were performed. We measured the spectrally resolved near-field intensity distributions, emitted from the complex epitaxially grown optical waveguide in the V groove with a spatial resolution of ∼0.1 μm. Distinct regions emitting at different wavelengths were identified, and a heart shaped modal distribution of less than 0.5 μm in diameter was measured and well matched the calculated light distribution of this structure.

Near-Field Scanning Optical Microscopy Studies of Individual Dislocations in Relaxed GeSi Films

Abstract Due to the submicron size of crystallographic defects, characterization of dislocations has been done. mostly by electron microscopy techniques. Transmission electron microscopy has generated invaluable structural information at the atomic scale. However, the influence of these electrically active defects on carrier transport can only be learned from lower resolution (∼ 1 μm) techniques such as electron beam induced current (EBIC) and photocurrent measurements. Near-field scanning optical microscopy (NSOM) is a novel optical technique that circumvents the diffraction limit. In this talk, we will present the application of NSOM to perform near-field photocurrent (NPC) measurements on strain-relaxed GeSi films on Si substrates to study the electrical activity of individual threading dislocations. Photoexcited carriers are generated locally by NSOM light and are collected by the builtin p-n junctions in the sample resulting in an external photocurrent. As the tip moves across the sample, topographic and NPC images were acquired simultaneously.

Submicron Structure in l- α-Dipalmitoylphosphatidylcholine Monolayers and Bilayers Probed with Confocal, Atomic Force, and Near-Field Microscopy

Langmuir-Blodgett (LB) monolayers and bilayers of l- α-dipalmitoylphosphatidylcholine (DPPC), fluorescently doped with 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate (diIC 18), are studied by confocal microscopy, atomic force microscopy (AFM), and near-field scanning optical microscopy (NSOM). Beyond the resolution limit of confocal microscopy, both AFM and NSOM measurements of mica-supported lipid monolayers reveal small domains on the submicron scale. In the NSOM studies, simultaneous high-resolution fluorescence and topography measurements of these structures confirm that they arise from coexisting liquid condensed (LC) and liquid expanded (LE) lipid phases, and not defects in the monolayer. AFM studies of bilayers formed by a combination of LB dipping and Langmuir-Schaefer monolayer transfer exhibit complex surface topographies that reflect a convolution of the phase structure present in each of the individual monolayers. NSOM fluorescence measurements, however, are able to resolve the underlying lipid domains from each side of the bilayer and show that they are qualitatively similar to those observed in the monolayers. The observation of the small lipid domains in these bilayers is beyond the spatial resolving power of confocal microscopy and is complicated in the topography measurements taken with AFM, illustrating the utility of NSOM for these types of studies. The data suggest that the small LC and LE lipid domains are formed after lipid transfer to the substrate through a dewetting mechanism. The possible extension of these measurements to probing for lipid phase domains in natural biomembranes is discussed.

Near-Field Scanning Optical Microscopy Studies of Materials and Devices

Near-field scanning optical microscopy (NSOM) provides a means to study optical and optoelectronic properties of materials at the nanometer scale. The key to achieving resolution higher than the diffraction limit is to place a subwavelength-sized light source—e.g., an aperture—within the near-field zone of the sample. In this case, the area of the sample illuminated is determined by the aperture size and not by the wavelength (see Figure 1). An image can then be formed by moving the sample and light source with respect to each other. While the principle of near-field optics is straightforward, its realization at visible-light wavelengths was not achieved until the invention of scanning-probe techniques in the 1980s. Since Betzig et al. demonstrated in 1991 that bright subwavelength apertures can be made by tapering and metal-coating single-mode optical fibers, research activities involving NSOM have increased tremendously. The later incorporation of shear-force feedback to regulate tip-sample separation adds another strength to NSOM. Using this distance regulation, a topographic image similar to that obtained by a conventional scanning force microscope is acquired simultaneously with the optical image. This provides a way to correlate structural and physical properties at the same sample positions and greatly simplifies interpretation of the NSOM data.

Near-field scanning optical microscopy studies of Cu(In,Ga)Se2 solar cells

A near-field scanning optical microscope (NSOM) is used to study the local photoresponse of Cu(In,Ga)Se2 thin film solar cells. The grain boundaries of the small grains (<1 μm) show some reduction in photoresponse; however the photoresponse is significantly reduced near most crevices separating large grains (>10 μm). In addition, NSOM images show response variations from grain to grain and areas of reduced photoresponse which have no corresponding topography. Photovoltage imaging of the cleaved side of the solar cells reveals the depth and nonuniformities of the actual p-n junction. It is found that the response of the p-n junction varies on a 0.5 μm length scale.

A near-field scanning optical microscopy study of the photoluminescence from GaN films

We have achieved spatially resolved photoluminescence from GaN films using a near-field scanning optical microscope (NSOM). GaN films grown by hydride vapor phase epitaxy (HVPE) and metal-organic vapor phase epitaxy (MOVPE) on sapphire substrates have been studied. We have performed spatial scans of topography, band edge, and yellow luminescence signals. Atomic force microscopy measurements were also made and compared with the NSOM topography. We have found spatial variations in photoluminescence characteristics at the submicron scale for both HVPE and MOVPE GaN. The observed enhancement of yellow luminescence at multiatomic step edges on the HVPE GaN surface suggests that the yellow luminescence is associated with chemical impurities incorporated during the growth of GaN films.