Near-Field Spectroscopy of the Quantum Constituents of a Luminescent System

White-Light Spectral Interferometry for Characterizing Inhomogeneity in Solutions and Nanocolloids

We demonstrate the use of white-light spectral interferometry as an effective technique involving only linear optical interactions and a partially coherent light source to measure the complex transmission response function of optical resonance and to determine the corresponding variation in the refractive index relative to a reference. We also discuss experimental arrangements to increase the accuracy and sensitivity of the technique. The superiority of the technique over single-beam absorption measurements is demonstrated by the accurate determination of the response function of the chlorophyll-a solution. The technique is then applied to chlorophyll-a solutions of varying concentrations and gold nanocolloids to characterize inhomogeneous broadening. Results on the inhomogeneity of gold nanocolloids are also supported by transmission electron micrographs, showing distributions of the size and shape of the constituent gold nanorods.

Phase Recovery Guarantees from Designed Coded Diffraction Patterns in Optical Imaging

Phase retrieval is an inverse problem that consists in estimating a scene from diffraction intensities. This problem appears in optical imaging, which has three main diffraction zones where the measurements can be acquired, i.e., near, middle and far. Recent works have theoretically solved this inverse problem for the far zone, creating redundancy in the measurement process by including a coded aperture, which allows to modulate the scene and acquire coded diffraction patterns (CDP). However, in the state-of-the-art, the PR problem has not been theoretically studied for CDP at the near and middle zones. Moreover, the structure of the coded aperture is selected at random, leading to suboptimal estimations. Indeed, some of the coding elements employed in the literature are unfeasible because they increase the power of the scene in the modulation process. This paper provides theoretical guarantees for the recovery of a scene from CDP acquired at the three diffraction zones using admissible modulations. Based on the theoretical results, it is established that the image reconstruction quality directly depends on the coded aperture structure; therefore, a design strategy is proposed. In fact, when the scene can be sparsely represented in some basis, its support can be better estimated for a suitable choice of the coding elements in the modulation process. Experimental results show that the scene is successfully recovered by using designed coded apertures with up to 40% less measurements compared to non-designed ensembles.

Super-Resolution Phase Retrieval from Designed Coded Diffraction Patterns

Super-resolution phase retrieval is an inverse problem that appears in diffractive optical imaging (DOI) and consists in estimating a high-resolution image from low-resolution phaseless measurements. DOI has three diffraction zones where the data can be acquired, known as near, middle, and far fields. Recent works have studied super-resolution phase retrieval under a setup that records coded diffraction patterns at the near and far fields. However, the attainable resolution of the image is mainly governed by the sensor characteristics, whose cost increases in proportion to the resolution. Also, these methodologies lack theoretical analysis. Hence, this work derives super-resolution models from low-resolution coded phaseless measurements at any diffraction zone that in contrast to prior contributions, the attainable resolution of the image is determined by the resolution of the coded aperture. For the proposed models, the existence of a unique solution (up to a global unimodular constant) is guaranteed with high probability, which can be increased by designing the coded aperture. Therefore, a strategy that designs the spatial distribution of the coded aperture is developed. Additionally, a super-resolution phase retrieval algorithm that minimizes a smoothed nonconvex least-squares objective function is proposed. The method first approximates the image by a spectral algorithm, which is then refined based upon a sequence of alternate steps. Simulation results show that the proposed algorithm overcomes state-of-the-art methods in reconstructing the high-resolution image. In addition, the reconstruction quality using designed coded apertures is higher than that of the non-designed ensembles.

Anderson localizations and photonic band-tail states observed in compositionally disordered platform

Anderson localization in random structures is an intriguing physical phenomenon, for which experimental verifications are far behind theoretical predictions. We report the first experimental confirmations of photonic band-tail states and a complete transition of Anderson localization. An optically activated photonic crystal alloy platform enables the acquisition of extensive experimental data exclusively on pure eigenstates, revealing direct evidence of band-tail states and Anderson localization transition within the band-tail states. Analyses of both experimental and simulated data lead to a comprehensive picture of photon localization that is highly consistent with theories by Anderson and others. We believe that our results provide a strong experimental foundation upon which both the fundamental understandings and application possibility of Anderson localization can be promoted significantly.

General C-H Arylation Strategy for the Synthesis of Tunable Visible Light-Emitting Benzo[a]imidazo[2,1,5-c,d]indolizine Fluorophores

Herein we report the discovery of the benzo[a]imidazo[2,1,5-c,d]indolizine motif displaying tunable emission covering most of the visible spectrum. The polycyclic core is obtained from readily available amides via a chemoselective process involving Tf₂O-mediated amide cyclodehydration, followed by intramolecular C-H arylation. Additionally, these fluorescent heterocycles are easily functionalized using electrophilic reagents, enabling divergent access to varied substitution. The effects of said substitution on the compounds' photophysical properties were rationalized by density functional theory calculations. For some compounds, emission wavelengths are directly correlated to the substituent's Hammett constants. Easily introduced nonconjugated reactive functional groups allow the labeling of biomolecules without modification of emissive properties. This work provides a straightforward platform for the synthesis of new moderately bright fluorescent dyes remarkable for their chemical stability, predictability, and unusually high excitation-emission differential.

In vitro tracking and intracellular protein distribution in immunology

New imaging techniques have enabled major advances in understanding how immune reactions are initiated, coordinated and controlled. Imaging methods, which were previously mostly descriptive and supplementary to more quantitative approaches, have now reached sufficient precision and throughput that they are becoming integral to almost all aspects of immunology research. Imaging methodologies that increase the resolution and sensitivity of detection, alongside an ever-expanding range of fluorescent reporters of molecular and cellular activity, and vastly improved analysis methods, have all facilitated this transformation. In this review, we will discuss how advances in imaging are changing the way we view immune activation and control using T cells as the model immune system. We will describe how imaging has transformed our knowledge of molecular and signalling events in T-cell activation, and the impact of these molecular events on the behaviour of T cells.

Near-field spectral mapping of individual exciton complexes of monolayer WS2 correlated with local defects and charge population

Exciton transitions are mostly responsible for the optical properties of transition metal dichalcogenide monolayers (1L-TMDs). Extensive studies of optical and structural characterization indicated that the presence of local structural defects and charge population critically influence the exciton emissions of 1L-TMDs. However, due to large variations of sample and experimental conditions, the exact mechanism of the exciton emission influenced by local structural defects and charge population is not clearly understood. In this work by using near-field scanning optical imaging and spectroscopy, we directly visualized spatially- and spectrally-resolved emission profiles of excitons, trions and defect bound excitons in CVD-grown monolayer tungsten disulfide (1L-WS₂) with ∼70 nm spatial resolution. We found that exciton emission is spatially uniform while emission of trions and defect bound excitons was strongly modulated by the presence of structural features such as defects and wrinkles. We also visually observe a strong correlation between local charge accumulation and the trion formation upon increased photo-excitation.

Expression of Cyclin D1 and P16 in Esophageal Squamous Cell Carcinoma

BACKGROUND Esophageal squamous cell carcinoma (ESCC) is one of the lethal cancers with a high incidence rate in Asia. Many genes including cyclin D1 and p16 play important role in its carcinogenesis. We aimed to analyze the expressions of cyclin D1 and p16 with the various clinicopathological characteristics of ESCC. METHODS We examined 30 biopsy samples of ESCC for cyclin D1 and p16 protein expressions using immunohistochemistry. Immunointensity was classified as no immunostaining (-), weakly immunostaining (+), weak immunostaining (++) and strongly positive immunostaining (+++). RESULTS Out of the 30 cases, positive expression of cyclin D1 was detected in 26 cases (86.7%). The percentage of tumors with invasion to the adventitia (88.2%), lymph node metastasis (87.5%), and tumors which were poorly differentiated (92.9%) were higher in cyclin D1 positive tumors than in the cyclin D1 negative tumors. However no significant association was found between cyclin D1 expression and the different clinicopathological parameters.There were 22 cases of ESCC (73.3 %) which showed negativity for p16. The percentage of tumors with invasion to the adventitia (82.4%) and poorly differentiated tumors (92.9%) were higher in the p16 negative tumors than in the p16 positive tumors. There was significant association between the histological grade and p16 expression (p=0.012). However, there were no significant association with regard to site, size and lymph node status of the tumors and p16 expression. CONCLUSION The study shows that alterations of cyclin D1 and p16 play an important role in ESCC. Loss of p16 expression was associated with poor differentiation.

Ultrafast Photodetection in the Quantum Wells of Single AlGaAs/GaAs-Based Nanowires

We investigate the ultrafast optoelectronic properties of single Al0.3Ga0.7As/GaAs core-shell nanowires. The nanowires contain GaAs-based quantum wells. For a resonant excitation of the quantum wells, we find a picosecond photocurrent which is consistent with an ultrafast lateral expansion of the photogenerated charge carriers. This Dember-effect does not occur for an excitation of the GaAs-based core of the nanowires. Instead, the core exhibits an ultrafast displacement current and a photothermoelectric current at the metal Schottky contacts. Our results uncover the optoelectronic dynamics in semiconductor core-shell nanowires comprising quantum wells, and they demonstrate the possibility to use the low-dimensional quantum well states therein for ultrafast photoswitches and photodetectors.

Single Molecules, Cells, and Super-Resolution Optics (Nobel Lecture)

The resolution of a microscope is determined by the diffraction limit in classical microscopy, whereby objects that are separated by half a wavelength can no longer be visually separated. To go below the diffraction limit required several tricks and discoveries. In his Nobel Lecture, E. Betzig describes the developments that have led to modern super high-resolution microscopy.

Conductive optical-fiber STM probe for local excitation and collection of cathodoluminescence at semiconductor surfaces

Luminescence imaging of semiconductor surfaces in nanometric resolution is a key to novel optoelectronic nano-devices, which requires local carrier excitation and local luminescence collection within the nanometric areas at the surfaces. However, there have not been a practical nanospectroscopies applicable to wide range of specimens. STM-cathodoluminescence (STM-CL) nanospectroscopy offers both high spatial resolution (of the order of 10 nm) and novel high carrier excitation power (up to ~1 mW), which enables local luminescence imaging of less-luminescent nano-structures. In this study, we advanced STM-CL technique by introducing a novel optical fiber probe with Cr thin film coating (Cr-FP), which was found to work as a STM probe, as an electron field-emitter for local carrier excitation, and as an alignment-free efficient local STM-CL collector which blinds luminescence after the minority carrier diffusion.

Distinct, but not completely separate spatial transport routes in the nuclear pore complex

The nuclear pore complex (NPC), which provides the permeable and selective transport path between the nucleus and cytoplasm of eukaryotic cells, allows both the passive diffusion of small molecules in a signal-independent manner and the transport receptor-facilitated translocation of cargo molecules in a signal-dependent manner. However, the spatial and functional relationships between these two transport pathways, which represent critical information for unraveling the fundamental nucleocytoplasmic transport mechanism, remain in dispute. The direct experimental examination of passive and facilitated transport with a high spatiotemporal resolution under real-time trafficking conditions in native NPCs is still difficult. To address this issue and further define these transport mechanisms, we recently developed single-point edge-excitation sub-diffraction (SPEED) microscopy and a deconvolution algorithm to directly map both passive and facilitated transport routes in three dimensions (3D) in native NPCs. Our findings revealed that passive and facilitated transport occur through spatially distinct transport routes. Signal-independent small molecules exhibit a high probability of passively diffusing through an axial central viscous channel, while transport receptors and their cargo complexes preferentially travel through the periphery, around this central channel, after interacting with phenylalanine-glycine (FG) filaments. Strikingly, these two distinct transport zones are not completely separate either spatially or functionally. Instead, their conformations are closely correlated and simultaneously regulated. In this review, we will specifically highlight a detailed procedure for 3D mapping of passive and facilitated transport routes, demonstrate the correlation between these two distinct pathways, and finally, speculate regarding the regulation of the transport pathways driven by the conformational changes of FG filaments in NPCs.

Scanning near-field optical coherent anti-Stokes Raman microscopy (SNOM-CARS) with femtosecond laser pulses in vibrational and electronic resonance

Accessing ultrafast photoinduced molecular dynamics on a femtosecond time-scale with vibrational selectivity and at the same time sub-diffraction limited spatial resolution would help to gain important information about ultrafast processes in nanostructures. While nonlinear Raman techniques have been used to obtain highly resolved images in combination with near-field microscopy, the use of femtosecond laser pulses in electronic resonance still constitutes a big challenge. Here, we present our first results on coherent anti-Stokes Raman scattering (fs-CARS) with femtosecond laser pulses detected in the near-field using scanning near-field optical microscopy (SNOM). We demonstrate that highly spatially resolved images can be obtained from poly(3-hexylthiophene) (P3HT) nano-structures where the fs-CARS process was in resonance with the P3HT absorption and with characteristic P3HT vibrational modes without destruction of the samples. Sub-diffraction limited lateral resolution is achieved. Especially the height resolution clearly surpasses that obtained with standard microCARS. These results will be the basis for future investigations of mode-selective dynamics in the near field.

Single-molecule studies of nucleocytoplasmic transport: from one dimension to three dimensions

In eukaryotic cells, the bidirectional trafficking of proteins and genetic materials across the double-membrane nuclear envelope is mediated by nuclear pore complexes (NPCs). A highly selective barrier formed by the phenylalanine-glycine (FG)-nucleoporin (Nup) in the NPC allows for two transport modes: passive diffusion and transport receptor-facilitated translocation. Strict regulation of nucleocytoplasmic transport is crucial for cell survival, differentiation, growth and other essential activities. However, due to the limited knowledge of the native configuration of the FG-Nup barrier and the interactions between the transiting molecules and the barrier in the NPC, the precise nucleocytoplasmic transport mechanism remains unresolved. To refine the transport mechanism, single-molecule fluorescence microscopy methods have been employed to obtain the transport kinetics of individual fluorescent molecules through the NPC and to map the interactions between transiting molecules and the FG-Nup barrier. Important characteristics of nucleocytoplasmic transport, such as transport time, transport efficiency and spatial distribution of single transiting molecules in the NPC, have been obtained that could not be measured by either ensemble average methods or conventional electron microscopy. In this critical review, we discuss the development of various single-molecule techniques and their application to nucleocytoplasmic transport in vitro and in vivo. In particular, we highlight a recent advance from one-dimensional to three-dimensional single-molecule characterization of transport through the NPC and present a comprehensive understanding of the nucleocytoplasmic transport mechanism obtained by this new technical development (105 references).

Calcium regulation of nucleocytoplasmic transport

Bidirectional trafficking of macromolecules between the cytoplasm and the nucleus is mediated by the nuclear pore complexes (NPCs) embedded in the nuclear envelope (NE) of eukaryotic cell. The NPC functions as the sole pathway to allow for the passive diffusion of small molecules and the facilitated translocation of larger molecules. Evidence shows that these two transport modes and the conformation of NPC can be regulated by calcium stored in the lumen of nuclear envelope and endoplasmic reticulum. However, the mechanism of calcium regulation remains poorly understood. In this review, we integrate data on the observations of calciumregulated structure and function of the NPC over the past years. Furthermore, we highlight challenges in the measurements of dynamic conformational changes and transient transport kinetics in the NPC. Finally, an innovative imaging approach, single-molecule superresolution fluorescence microscopy, is introduced and expected to provide more insights into the mechanism of calcium-regulated nucleocytoplasmic transport.

Imaging on the Nanoscale: Super-Resolution Fluorescence Microscopy

Although the resolution of a light microscope is fundamentally limited by diffraction to about half of the wavelength of light, in recent years several techniques have been developed that can overcome this limitation in fluorescence microscopy, allowing imaging with nanometre scale resolution. Many of these techniques are based on photoswitchable molecules that can switch between a bright, fluorescent and a dark, nonfluorescent state. Some of these techniques, as well as their limitations, are discussed.

Superresolution imaging using single-molecule localization

Superresolution imaging is a rapidly emerging new field of microscopy that dramatically improves the spatial resolution of light microscopy by over an order of magnitude (approximately 10-20-nm resolution), allowing biological processes to be described at the molecular scale. Here, we discuss a form of superresolution microscopy based on the controlled activation and sampling of sparse subsets of photoconvertible fluorescent molecules. In this single-molecule-based imaging approach, a wide variety of probes have proved valuable, ranging from genetically encodable photoactivatable fluorescent proteins to photoswitchable cyanine dyes. These have been used in diverse applications of superresolution imaging: from three-dimensional, multicolor molecule localization to tracking of nanometric structures and molecules in living cells. Single-molecule-based superresolution imaging thus offers exciting possibilities for obtaining molecular-scale information on biological events occurring at variable timescales.

Indirect excitons in elevated traps

We report on the study of indirect excitons in elevated traps. The transition from a normal to elevated trap results in the appearance of narrow lines in the emission spectrum. The density, temperature, and voltage dependences indicate that these lines correspond to the emission of individual states of indirect excitons in a disorder potential in the elevated trap.

Anisotropy contrast in phonon-enhanced apertureless near-field microscopy using a free-electron laser

We demonstrate the imaging of ferroelectric domains in BaTiO3, using an infrared-emitting free-electron laser as a tunable optical source for scattering scanning near-field optical microscopy and spectroscopy. When the laser is tuned into the spectral vicinity of a phonon resonance, ferroelectric domains can be resolved due to the anisotropy of the dielectric properties of the material. Slight detuning of the wavelength gives rise to a contrast reversal clearly evidencing the resonant character of the excitation. The near-field domain contrast shows that the orientation of the dielectric tensor with respect to the sample surface has a clear influence on the near-field signal.

Ultraweak-absorption microscopy of a single semiconductor quantum dot in the midinfrared range

We show that we can measure the room temperature ultraweak absorption of a single buried semiconductor quantum dot. This is achieved by monitoring the deformation field induced by the absorption of midinfrared laser pulses and locally detected with an atomic force microscope tip. The absorption is spectrally and spatially resolved around lambda approximately 10 microm wavelength with 60 nm lateral resolution (lambda/150). The electronic S-D intersublevel absorption of a single quantum dot is identified around 120 meV and exhibits a homogeneous linewidth of approximately 10 meV at room temperature.

Transition from two-dimensional to three-dimensional quantum confinement in semiconductor quantum wires/quantum dots

We report the photoluminescence (PL) and polarization-resolved PL characteristics of a novel GaAs/AlGaAs quantum wire/dot semiconductor system, realized by metalorganic vapor-phase epitaxy of site-controlled, self-assembled nanostructures in inverted tetrahedral pyramids. By systematically changing the length of the quantum wires, we implement a continuous transition between the regimes of two-dimensional and three-dimensional quantum confinement. The two main evidences for this transition are observed experimentally and confirmed theoretically: (i) strongly blue-shifted ground-state emission, accompanied by increase separation of ground and excited transition energies; and (ii) change in the orientation of the main axis of linear polarization of the photoluminescence, from parallel to perpendicular with respect to the wire axis. This latter effect, whose origin is shown to be purely due to quantum confinement and valence band mixing, sets in at wire lengths of only approximately 30 nm.

Scanning near-field optical coherent spectroscopy of single molecules at 1.4 K

We present scanning near-field extinction spectra of single molecules embedded in a solid matrix. By varying the tip-molecule separation, we modify the line shape of the spectra, demonstrating the coherent nature of the interaction between the incident laser light and the excited state of the molecule. We compare the measured data with the outcome of numerical calculations and find a very good agreement.

Imaging Intracellular Fluorescent Proteins at Nanometer Resolution

We introduce a method for optically imaging intracellular proteins at nanometer spatial resolution. Numerous sparse subsets of photoactivatable fluorescent protein molecules were activated, localized (to approximately 2 to 25 nanometers), and then bleached. The aggregate position information from all subsets was then assembled into a superresolution image. We used this method--termed photoactivated localization microscopy--to image specific target proteins in thin sections of lysosomes and mitochondria; in fixed whole cells, we imaged vinculin at focal adhesions, actin within a lamellipodium, and the distribution of the retroviral protein Gag at the plasma membrane.

Dark-state luminescence of macroatoms at the near field

We theoretically analyze the optical near-field response of a semiconductor macroatom induced by local monolayer fluctuations in the thickness of a semiconductor quantum well, where the large active volume results in a strong enhancement of the light-matter coupling. We find that in the near-field regime bright and dark excitonic states become mixed, opening new channels for the coupling to the electromagnetic field. As a consequence, ultranarrow luminescence lines appear in the simulated two-photon experiments, corresponding to very long lived excitonic states, which undergo Stark shift and Rabi splitting at relatively small field intensities.

Polariton condensation with localized excitons and propagating photons

We estimate the condensation temperature for microcavity polaritons, allowing for their internal structure. We consider polaritons formed from localized excitons in a planar microcavity, using a generalized Dicke model. At low densities, we find a condensation temperature T(c) proportional, rho, as expected for a gas of structureless polaritons. However, as T(c) becomes of the order of the Rabi splitting, the structure of the polaritons becomes relevant, and the condensation temperature is that of a BCS-like mean-field theory. We also calculate the excitation spectrum, which is related to observable quantities such as the luminescence and absorption spectra.

Ultrafast near-field spectroscopy of single semiconductor quantum dots

Excitonic and spin excitations of single semiconductor quantum dots (QDs) currently attract attention as possible candidates for solid-state-based implementations of quantum logic devices. Due to their rather short decoherence times in the picosecond to nanosecond range, such implementations rely on using ultrafast optical pulses to probe and control coherent polarizations. We combine ultrafast spectroscopy and near-field microscopy to probe the nonlinear optical response of a single QD on a femtosecond time-scale. Transient reflectivity spectra show pronounced oscillations around the QD exciton line. These oscillations reflect phase-disturbing Coulomb interactions between the excitonic QD polarization and continuum excitations. The results show that although semiconductor QDs resemble in many respects atomic systems, Coulomb many-body interactions can contribute significantly to their optical nonlinearities on ultrashort time-scales.

Near-field optical mapping of exciton wave functions in a GaAs quantum dot

Near-field photoluminescence imaging spectroscopy of naturally occurring GaAs quantum dots (QDs) is presented. We successfully mapped out center-of -mass wave functions of an exciton confined in a GaAs QD in real space due to the enhancement of spatial resolution up to 30 nm. As a consequence, we discovered that the spatial profile of the exciton emission, which reflects the shape of a monolayer-high island, differs from that of biexciton emission, due to different distributions of the polarization field for the exciton and biexciton recombinations. This novel technique can be extensively applied to wave function engineering in the design and the fabrication of quantum devices.

Long-range spatial correlations in the exciton energy distribution in GaAs/AlGaAs quantum wells

Variations in the width of a quantum well (QW) are known to be a source of broadening of the exciton line. Using low temperature near-field optical microscopy, we have exploited the dependence of exciton energy on well width to show that in GaAs QWs, these seemingly random well-width fluctuations actually exhibit well-defined order-strong long-range correlations appearing laterally, in the plane of the QW, as well as vertically, between QWs grown one on top of the other. We show that these fluctuations are correlated with the commonly found mound structure on the surface. This is an intrinsic property of molecular beam epitaxial growth.

Coherent nonlinear optical response of single quantum dots studied by ultrafast near-field spectroscopy

The nonlinear response of single GaAs quantum dots is studied in femtosecond near-field pump-probe experiments. At negative time delays, transient reflectivity spectra show pronounced oscillatory structure around the quantum dot exciton line, providing the first evidence for a perturbed free induction decay of the excitonic polarization. Phase-disturbing Coulomb interactions between the excitonic polarization and continuum excitations dominate the optical nonlinearity on ultrafast time scales. A theoretical analysis based on the semiconductor Bloch equations accounts for this behavior.

Near-field coherent spectroscopy and microscopy of a quantum dot system

We combined coherent nonlinear optical spectroscopy with nano-electron volt energy resolution and low-temperature near-field microscopy with subwavelength resolution (<lambda/2) to provide direct and local access to the excitonic dipole in a semiconductor nanostructure quantum system. Our technique allows the ability to address, excite, and probe single eigenstates of solid-state quantum systems with spectral and spatial selectivity while simultaneously providing a measurement of all the various time scales of the excitation including state relaxation and decoherence rates. In analogy to scanning tunneling microscopy measurements, we can now map the optical local density of states of a disordered nanostructure. These measurements lay the groundwork for studying and exploiting spatial and temporal coherence in the nanoscopic regime of solid-state systems.

Quantum mechanical repulsion of exciton levels in a disordered quantum well

Spatially resolved photoluminescence spectra of a single quantum well are recorded by near-field spectroscopy. A set of over four hundred spectra displaying sharp emission lines from localized excitons is subject to a statistical analysis of the two-energy autocorrelation function. An accurate comparison with a quantum theory of the exciton center-of-mass motion in a two-dimensional spatially correlated disordered potential reveals clear signatures of quantum mechanical energy level repulsion, giving the spatial and energetic correlations of excitons in disordered quantum systems.

Three-dimensional analysis of light propagation through uncoated near-field fibre probes

The near-field emission from uncoated tapered fibre probes is investigated for different probe geometries. The three-dimensional model calculations are based on Maxwell's curl equations and describe the propagation of a 10 fs optical pulse (lambda = 805 nm) through tapers of different lengths and different diameters of the taper exit. The numerical evaluation is done with a finite difference time domain code. Two tapers with cone angles of 50 degrees, with taper lengths of 1.5 microm and 1.0 microm and exit diameters of 100 nm and 520 nm, respectively, are considered. We find that without sample the short taper with large exit diameter optimizes both light transmission and spatial resolution. In the presence of a sample with a high dielectric constant, however, the spatial near-field distribution changes drastically for both taper geometries. We find a pronounced increase in spatial resolution, down to about 250 nm inside the medium. This collimation of the near-field distribution arises from interferences between emitted and reflected light from the sample surface and from a collimation effect that the field experiences in the high-index semiconductor material. The combination of high spatial resolution and transmission and collection efficiencies makes such probes interesting for spectroscopic investigations, as demonstrated by recent experiments.

Ultra stable tuning fork sensor for low-temperature near-field spectroscopy

We report on a distance control system for low-temperature scanning near-field optical microscopy, based on quartz tuning fork as shear force sensor. By means of a particular tuning fork-optical fiber configuration, the sensor is electrically dithered by an applied alternate voltage, without any supplementary driving piezo, as done so far. The sensitivity in the approach direction is 0.2nm, and quality factors up to 2850 have been reached. No electronic components are needed close to the sensor, allowing to employ it in a liquid He environment. The system is extremely compact and allows for several hours of stability at 5 K.

A storage Dewar near-field scanning optical microscope

A near-field scanning optical microscope for operation within a storage Dewar is described. It was designed for studies of opaque samples and operates in the collection mode. Illumination can be either through the tip or from the side via a separate fiber. Scans can be begun within 2 h after start of cooldown. Its rigid design allows high resolution and long scans with no additional vibration isolation. To illustrate its performance, measurements of photoluminescence in GaAs/AlGaAs heterostructures are presented. The signal and noise levels for the two illumination modes are examined.

Microcavities combining a semiconductor with a fused-silica microsphere

We report studies of a novel microcavity system that combines a semiconductor quantum well with a fused-silica microsphere. We show that excitonic photoluminescence from the quantum well couples efficiently into whispering-gallery modes. Using a resonant light-scattering technique, we demonstrate that the Q factor of the combined system exceeds 10(5) . Our studies also point to the necessity of using semiconductor nanostructures with a capping layer no more than a few nanometers thick.

Scanning near-field fluorescence resonance energy transfer microscopy

A new microscopic technique is demonstrated that combines attributes from both near-field scanning optical microscopy (NSOM) and fluorescence resonance energy transfer (FRET). The method relies on attaching the acceptor dye of a FRET pair to the end of a near-field fiber optic probe. Light exiting the NSOM probe, which is nonresonant with the acceptor dye, excites the donor dye introduced into a sample. As the tip approaches the sample containing the donor dye, energy transfer from the excited donor to the tip-bound acceptor produces a red-shifted fluorescence. By monitoring this red-shifted acceptor emission, a dramatic reduction in the sample volume probed by the uncoated NSOM tip is observed. This technique is demonstrated by imaging the fluorescence from a multilayer film created using the Langmuir-Blodgett (LB) technique. The film consists of L-alpha-dipalmitoylphosphatidylcholine (DPPC) monolayers containing the donor dye, fluorescein, separated by a spacer group of three arachidic acid layers. A DPPC monolayer containing the acceptor dye, rhodamine, was also transferred onto an NSOM tip using the LB technique. Using this modified probe, fluorescence images of the multilayer film reveal distinct differences between images collected monitoring either the donor or acceptor emission. The latter results from energy transfer from the sample to the NSOM probe. This method is shown to provide enhanced depth sensitivity in fluorescence measurements, which may be particularly informative in studies on thick specimens such as cells. The technique also provides a mechanism for obtaining high spatial resolution without the need for a metal coating around the NSOM probe and should work equally well with nonwaveguide probes such as atomic force microscopy tips. This may lead to dramatically improved spatial resolution in fluorescence imaging.