Field Scanning Optical Microscopy Research Articles

Micromachined Si 3N 4-tip on Cantilever for parallel SFM and NSOM applications

Micromachining of a nanoscale tip for near field scanning optical microscopy (NSOM) and scanning force microscopy(SFM) has been described. The tapered optical metal-coated fiber is generally used to provide a sub-wavelength sized aperture on tip. Several micromachining methods have been performed in order to have a metal aperture with radius less than @l/2. Apertures provided with less than @l/2 and hollow tips would provide a suitable probe for both NSOM and SFM. A tip coated with thin metal film will meet these requirements. The Si tip have been initially fabricated using reacting ion etching (RIE). The SiO2 etch masks with [email protected] and [email protected] were patterned followed by Si etching. The etched Si post was at least 3 @mm tall and the radius of the tip was found to be 30nm and ~10nm depending on the fabrication methods. A Si3N4 thin film was deposited on the fabricated Si -tip using a low pressure chemical vapor deposition technique in order to provide a capability for atomic force microscope. A Cr metal film was deposited using thermal evaporator. The thick photoresist film was coated using two-stage methods in order to cover the tall [email protected] tip. The PR film was carefully etched to have a metal aperture size with less than @l/2.

Surface modification of poly[2-methoxy,5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene] in the optical near field

We have studied the optical properties and the surface morphology of the conjugated polymer poly[2-methoxy,5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene] (MEH-PPV), under laser irradiation at 488 nm emanating from the Al coated fiber probe of a near field scanning optical microscope. Laser irradiation of the polymer in the near field induces a degradation of the material demonstrated by a dynamic change in its optical transmission. The surface of the material is modified primarily by heat conduction from the fiber probe, which under our experimental conditions is the dominant heating mechanism in the near field.

Combined electric field and near-field scanning optical microscopy: modification of silicon surfaces

A combined electric field and near-field scanning optical microscope (EF-NSOM) system has been developed for imaging and modifying surfaces. With a negative bias applied to the conducting scaling probe with respect to the substrate, the EF-NSOM system is used to modify nearly atomically flat n-type oxide-passivated Si( 111) surfaces. The electric-field-induced modification of Si surfaces, similar to that effected by a scanning tunneling microscope or an atomic force microscope, is observed by the near-field scanning optical microscopy as well as by shear-force microseopy.

Spatio-spectral mapping of multimode vertical cavity surface emitting lasers

This paper reports the spatial and spectral characteristics of multimode vertical cavity surface emitting laser (VCSEL) emission using near field scanning optical microscopy (NSOM). We have investigated 15 /spl mu/m diameter proton implanted 850 nm devices used in 2 Gb/s multimode fiber optic links. We have studied their near field spatial distribution of intensity and the lasing wavelengths of their transverse modes. False colored images were created to portray relative intensity, and spatial distribution information for each transverse mode. Correlation with shear force data allowed mapping of the optical distributions to topographical features of the device. Lasing filaments were observed at high drive currents. Spatially overlapping transverse modes were observed to compete for available gain while spatially isolated modes coexisted.

Near field scanning optical spectroscopy of InP single quantum dots

We study InP quantum dots which are prepared by strain induced self-assembly on GaAs substrates with a GaInP buffer layer using a near field scanning optical microscope operating at near liquid He bath temperatures in the collection mode. Single quantum dots are identified spatially and spectrally due to their photoluminescence spectrum. Series of luminescence lines due to single dots of different sizes are discussed in terms of dot height and width fluctuations.

A versatile stable scanning proximal probe microscope

We present a novel scanning proximal probe microscope design utilizing a piezoelectric driven coarse positioning mechanism in x, y, and z, while maintaining relatively small lateral dimensions. The instrument is suitable for insertion into a Dewar. The primary purpose of this work is to develop a stable yet versatile instrument in order to meet the signal averaging limitations imposed by low signal level measurements. We have implemented a near field scanning optical microscope with this system, whose key features include simultaneous detection of reflected and transmitted signals, unique “center of mass” tip oscillator for shear force feedback, and overall microscope stability.

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.

Mechanical oscillator tip-to-sample separation control for near-field optical microscopy

A nonoptical system for controlling the tip-to-sample separation in a near field scanning optical microscope is described. It consists of a tapered aluminum-coated fiber glued to a high-Q Si paddle mechanical oscillator. The paddle is driven at one of its resonances by a capacitively coupled electrode, and the amplitude of oscillation is detected through a second electrode. This resonant amplitude, which varies as the tip approaches the surface, is used as a feedback signal to control the separation between tip and sample. A separation stability better than 0.8 nm is achieved in the 0–200 nm range. We present a complete characterization of this system and demonstrate its capabilities with shear-force images of two samples.

Near field scanning optical microscopy measurements of optical intensity distributions in semiconductor channel waveguides

We present results of near field scanning optical microscopy measurements performed on single mode AlGaAs ridge channel waveguides. The optical intensity distribution just above the surface of the waveguide structure has been measured by scanning a tapered, aluminum-coated, fiber probe transverse to the waveguide propagation direction. Experimental results are compared with model calculations performed using both the effective index and beam propagation methods. An accurate description of submicron features in the intensity profile near the ridge edges, as well as the magnitude of the field outside the channel, requires the use of the beam propagation method.

A novel ultrasonic resonance sample-tip distance regulation for near field optical microscopy and shear force microscopy

We have developed a novel technique of sample-tip distance regulation by using ultrasonic resonance method to sense the shear force in the nano-Newton range. A piezoelectric quartz fork is driven by its resonance frequency without an additional dither vibrator. By combining the vibration amplitude and phase shift of the fiber tip attached to the quartz fork, the sample-tip distance can be reliably controled at 0–50 nm. The shear force images show that the lateral resolution is better than 50 nm depending of the aperture size of the tip. Being a non-optical detection mechanism, this method is very promising in the near field scanning optical microscopy, especially in the cases of reflective mode and spectroscopy, as well as being an independent shear force microscope for insulation samples.

Biological applications of near-field optical microscopy

Presents several biological applications of near field optical microscopy, in combination with force microscopy. Aperture near field scanning optical microscopy (NSOM) with fluorescence detection gives (bio)chemical specificity and orientational information, in addition to the simultaneously acquired force image. This technique has large potential for DNA sequencing, molecular organization in monolayers, and study of the role of the cytoskeleton in cellular mobility in cell growth, cell migration, formation of protrusions, etc. Fluorescence NSOM gives high resolution on flat, not too deep surfaces. Fluorescence NSOM induces virtually no bleaching, as opposed to confocal fluorescence microscopy. Bright field NSOM in transmission generally yields a complicated contrast, caused by a mixture of dielectric and topographic contributions. Shear force feedback is essential in aperture NSOM operation with fibers, and operates on soft surfaces of cells and chromosomes. Ultimately, aperture NSOM is limited by low efficiency with a source brightness of typically 100 pW to 10 nW. Thus, in spectroscopic applications (fluorescence, Raman, etc.) photon noise will be a fundamental limit in the speed of imaging. Photon tunneling in combination with force microscopy allows routine scanning with a high optical lateral resolution. However, interference effects can be dominant on surfaces which display extensive scattering. As such, the application potential of PSTM to biological surfaces is rather limited. Clearly, the virtues of optics, non-invasiveness, high spectral resolution, and high time resolution all apply to the near field optical domain with its high spatial resolution, which adds extensively to the potential of scanning probe microscopy.

Unconventional Laser Chemistry. Measurement of Chemical Reaction Processes with Localized Optical Fields.

A new microprobe of a near field scanning optical microscope has been developed for elucidating chemical reaction processes occurring in nanometer volumes. The probe is composed of a lasing microsphere, that induces the high-intensity localized optical field just outside of the sphere, and of a nanometer-sized particle fixed on the surface. Photon tunneling through the nanoparticle can be observed with high sensitivity due to the intracavity effect. Three-dimensional positioning control of the microprobe is performed by a laser manipulation technique based on radiation pressure.

Imaging of optical and topographical distributions by simultaneous near field scanning optical/atomic force microscopy with a microfabricated photocantilever

Simultaneous near field scanning optical and atomic force microscopy with a microfabricated photocantilever reveal both optical and topographical distributions. The cantilever tip changes the evanescent field into scattering light, and this scattering light is detected with a photodiode fabricated in the tip of the cantilever. The cantilever deflection signal leads to atomic force images. The resolution for imaging the evanescent field variation was 20 nm (λ/30). The near field optical and atomic force images indicate that the same point of the cantilever tip generates both optical and atomic force signals. This method is a new approach to optical and topographical microscopy with nanometer resolution.

Far field characterization of diffracting circular apertures

The far field angular intensity distribution I(θ) of the λ=633 nm radiation transmitted through diffracting circular apertures is measured for diameters ranging between 60 and 500 nm. The circular apertures are located at the apex of aluminum coated tapered optical fiber tips. I(θ) depends sensitively on the aperture diameters down to λ/6. This property is used to determine the optical aperture size of metal coated tapered optical fiber tips used for near field scanning optical microscopy.

Near field measurements of optical channel waveguide structures

Near field scanning optical microscopy (NSOM) has been used to investigate the guided mode intensity distribution in optical channel waveguides, phase-matched directional couplers, and symmetric Y-junctions. A near field measurement of the guided mode intensity profile across the optical channel waveguide has been performed and compared with model calculations. The near field guided mode intensity profiles above the waveguides were measured as a function of distance along the propagation direction of both a directional coupler and a Y-junction, providing a near field view of the spatial evolution of optical power in these structures.

Near field probe microscopy of porous silicon: Observation of spectral shifts in photoluminescence of small particles

Imaging of topography and locally induced photoluminescence of anodically etched porous silicon is performed using a force regulated near field scanning optical microscope. The photoluminescence signal on as-prepared wafers shows strong evidence for inherent, rather than purely geometrically induced, variations. Images of small particles display distributions of regions of relatively strong luminescence on the 100 nm lateral scale. Highly localized photoluminescence spectra obtained on these particles show spectral shifts as high as 50 nm compared with those obtained on as-prepared porous silicon wafer.

Analysis of near field tip patterns including object interaction using finite-difference time-domain calculations

Numerical simulation of the field patterns in the vicinity of a typical probe tip of a near field scanning optical microscope is performed using the finite-difference time-domain technique. Maps of electric field vectors and Poynting vectors are given for a two-dimensional 120 × 160 cell model of the tip for p-polarized incident illumination with initially no object nearby. A metal stripe object is then placed near the tip and perturbations of the field patterns are displayed, in particular those caused by the induction of surface charge density in the metal object. The metal stripe is scanned across the tip plane at three separation distances and the radiated power, determined by a sum of Poynting vectors across a rear plane, is plotted as a function of lateral position of the object. An interesting edge enhancement effect is seen for the close metal object, but not for a dielectric object.

Low temperature near field spectroscopy and microscopy

A low temperature near field scanning optical microscope has been constructed. Images of InGaAs quantum wires have been obtained at 4.2 K with a resolution < 100 nm (λ/8). Low temperature spectroscopy and images of 270 nm GaAs quantum dots show vertical dependence in both intensity and spectral shape in collection mode operation. The microscope is designed for use in high magnetic fields and top loads into a 1.2 inch cryostat. A pulled optical fiber [1] tip is mounted in a simple and novel lens collection scheme for reflection mode NSOM. Additional side mounted fibers with focusing GRIN lenses are used for detecting the fiber tip resonance, providing independent shear force microscopy.

Near Field Scanning Optical Microscopy and Spectroscopy of Electronic Materials and Structures

AbstractA Near Field Scanning Optical Microscope (NSOM) with spectroscopic capability is applied to imaging semiconductor and microelectronic structures. NSOM combined with spectroscopic analysis provides physical and chemical information of thin films and defects with ultra high spatial resolution. We have studied epitaxial and bulk samples and partially fabricated SiO2/Si CMOS structures to investigate the spatial resolution and imaging modes of NSOM. Reflected intensity contrast in NSOM yields images of defect networks in InGaAs/InAlAs/GaAs epitaxial layers and shows thickness variations in SiO2 films on Si. Surface topological changes observed in NSOM demonstrate a spatial resolution of significantly better than 0.25 μm. Fluorescence imaging is examined for chemically identifying materials and defects.

Spectroscopy of individual quantum structures with low-temperature near field optical microscopy

A low temperature Near Field Scanning Optical Microscope (NSOM) has been built and operated to study the optical properties of individual quantum structures. Near field spectroscopy of modulation doped quantum dots is performed and spectra of a single dot is obtained. Spectral maps of 100nm quantum wires resolve a well defined region where emission from the n=2 quantum confined state is enhanced. This effect is attributed to local strain in the sample.