Near-field Optical Techniques Research Articles

Spatially Resolved Spectroscopy of Single and Coupled Quantum Dots

Optical properties of single and coupled quantum dots are studied with high spatial resolution. Several techniques are used to resolve spectra of individual dots. Spatially isolated dots produced by double cleaved edge overgrowth are investigated by diffraction limited spectroscopy using a special microscope setup. Self-assembled InGaAs dots have been investigated by near-field optical techniques with metallic masks and by scanning tunneling microscopy cathodoluminescence. Photoluminescence and photoluminescence excitation spectra of selected quantum dots are discussed which reveal detailed information on ground and excited states as well as their coupling with neighboring dots.

A theoretical study of near-field detection and excitation of surface plasmons

Recently, optical near-field techniques have been applied to detect or to excite surface plasmons at vacuum–metal interface. In this paper we propose a theoretical study of those experiments. We firstly describe the formalism, the sample being a multilayered structure with flat interfaces. The incident field and the field in each layer are expressed as 3D plane-wave expansions. The spatial Fourier amplitudes in each layer are linearly connected to the incident ones by transmission matrices. We then apply this formalism to describe the excitation of a surface plasmon in a Kretschmann geometry and its detection by a near-field probe. In the second experiment, the tip was used in the emission mode to excite the surface plasmons, which were thus far-field detected. We describe the experiment using the Bethe–Bouwkamp model for tip emission.

Room-Temperature Near-Field Reflection Spectroscopy of Single Quantum Wells

We report on a novel optical near-field technique to measure the local polarizability of a topographically flat sample with a spatial resolution better than 100 nm. Using an uncoated fiber probe, we implement a cross-polarization detection of the optical signal at the fiber dither frequency. This technique suppresses efficiently the otherwise dominating far-field background and reduces topographic artifacts. We demonstrate its performance on a thin, strained near-surface CdS/ZnS single quantum well at room temperature. The optical structure of these topographically flat samples is due to CdS thickness fluctuations, and is observed to be uncorrelated within the spatial resolution of the instrument.

Field propagator of a dressed junction: Fluorescence lifetime calculations in a confined geometry

The study of the fluorescence phenomenon by near-field optical techniques requires one to describe precisely the spontaneous emission change occurring when the fluorescing particle is placed in a complex optical environment. For this purpose, the field susceptibility (also called the field propagator) of a planar junction formed by a cavity bounded by two semi-infinite bodies with arbitrary optical constant is derived within the framework of linear-response theory. The field propagator associated with the junction is then modified in a self-consistent manner to account for the presence of any arbitrary object inside the junction. As a first illustration the alteration of the fluorescence lifetime of a molecule by two subwavelength-sized dielectric spheres, placed inside the junction, is presented.

A Reflection Near-Field Scanning Optical Microscope Technique for Subwavelength Resolution Imaging of Thin Organic Films

A dual mode near-field scanning optical microscopy (NSOM) technique, in which light is both emitted from and collected by a subwavelength aperture, is described and demonstrated. This dual mode configuration is capable of high contrast (via the refractive index) as well as high-resolution optical microscopy in reflection. The contrast in these images is shown to depend strongly on the interference of the light incident to and reflected from the sample surface. We show that with the dual mode configuration the topography induced contrast can be quantitatively separated from the true optical contrast resulting in a mapping of the refractive index of the sample. This separation has proven difficult, if not impossible, for other reflection NSOM geometries. To demonstrate the utility of this technique for thin organic films we present dual mode images of a nine-layer organic/inorganic self-assembled multilayer film on a silicon substrate.

Characterization of nanoparticles by microbeam analysis and microscopy

The analysis of selected, individual nanoscale particles represents the limit of performance for the most advanced microanalytical techniques. Ion beam methods are inadequate for objects with dimensions below 100 nm because of limitations on the number of atoms that can be ionized and collected. By using near-field optical techniques, photon beam methods are just beginning to gain the spatial resolution necessary to penetrate the 100 nm barrier. Electron beam techniques such as low voltage (< 5 kV) scanning electron microscopy and high voltage (≥100 kV) analytical electron microscopy are highly effective in the sub-100 nm size regime. Electron-excited X-ray spectrometry can provide accurate elemental analysis in both energy regimes. At high energy, electron energy loss spectrometry (EELS) can also be utilized, augmenting elemental analysis with molecular (compound) information. Analytical masses as small as 1 zg can be characterized with EELS.

Surface modification in the optical near field

The potential use of optical near-field techniques for lithographic purposes is reviewed. After a brief introduction of the experimental schemes used in scanning near-field optical microscopy (SNOM), we discuss near-field exposure of conventional photo resists, local surface modifications in ultra-thin organic layers, and near-field magneto-optic recording. The state of the art in terms of lateral resolution and speed is discussed in view of possible implementation in technological processes. A brief sketch of possible future developments is given.

Spatially and spectrally resolved imaging of GaAs quantum-dot structures using near-field optical technique

We present experimental result on spatially and spectrally resolved imaging of GaAs quantum dot (QD) structures using a near-field optical microscope. Three photoluminescence (PL) peaks which originate from QD, quantum well (QW), and bulk regions were observed at liquid-He temperature. Carriers were observed to diffuse effectively from the AlGaAs barriers to the QD and QW regions. The region of the intension carrier capture manifests itself by emitting a sharp PL peak, and is estimated to be about 300 nm.

Near-field optical data storage

A recently developed near-field optical technique, the solid immersion lens (SIL), is utilized in a realistic demonstration of near-field optical data storage. Using 830 nm light, a 360 nm optical spot size is obtained at the exit surface of the SIL and is transferred across a small air gap to the surface of a spinning magneto-optical disk. Reading and writing of data are achieved at a density of 3.8× 108 bits/cm2 with a data rate of 3.3×106 bits/s. The subwavelength gap between the lens and the disk is maintained at a media velocity of 1.25 m/s by incorporating the lens into an air-bearing slider.

Development of a scanning near-field optical probe for localised Raman spectroscopy

The scanning near-field optical probe technique has made it possible to perform conventional optical spectroscopies with unprecedented spatial resolution. We present the development of an instrument for scanning near-field Raman spectroscopy and present near-field Raman linescans and spectra of a range of materials with subwavelength resolution.

A simple reproducible technique for producing sub-micrometre fibre-optic probes for near-field optical microscopy and chemical sensors

Near-field optical techniques enable the optical diffraction limit to be surpassed, thus allowing objects to be resolved with a spatial resolution of better than lambda /50. Many of these techniques involve the scanning of probes with sub-micrometre tips or apertures in close proximity (<20 nm) to the object surface in order to generate images of sub-wavelength resolution. We present a simple one-step method for producing optical probes with excellent reproducibility for applications in super-resolution microscopy and near-field chemical sensing. In addition, we demonstrate via experiment the light delivery and collection ability of probes manufactured using this technique.

Investigation of ultrasonic pulses with an optical probe

This paper presents the theory and experimental results for an optical ‘‘near-field’’ technique applied to pulsed ultrasound. This technique was originally developed for continuous-wave ultrasound [B. Cook, J. Acoust. Soc. Am. 60, 95–99 (1976)]. The distinguishing feature of the optical near-field technique is the placement of the photodetector in the optical near field of the optical grating induced by the sound. A photodetector, with a photodiode small compared to the ultrasonic wavelengths and a frequency bandwidth sufficient to detect the ultrasonically induced irradiance variations, combined with a low power laser comprise the optical probe. In this study, the optical probe behaved as a line receiver of ultrasonic pulses propagating in water and as a linear device for the limited range of sound frequencies and amplitudes studied. Investigation of selected portions of the acoustic field confirm previous models of ultrasonic pulses.

OPTICAL DATA STORAGE USING A SOLID IMMERSION LENS

A near-field optical technique, using a new type of Solid Immersion Lens (SIL), has been developed and applied to the writing and reading of domains in a magneto-optic material. The SIL is a truncated glass sphere which serves to increase the numerical aperture of the optical system by n2, where n is the index of refraction of the lens material. By mounting a 1 mm SIL on a slider, we have been able to fly the SIL on a magneto-optical disk. We have demonstrated writing and reading with such a lens at data rates exceeding 1 Mbit/s with a carrier-to-noise ratio of approximately 50 dB. Using a 830 nm laser and a n = 1.5 SIL, the focused spot size was reduced by close to a factor of 2 and was measured to be 600 nm.