Standing-wave spectrometry in silicon nano-waveguides using reflection-based near-field scanning optical microscopy

Utilizing reflection-based near-field scanning optical microscopy(NSOM) to image and analyze standing-wave patterns, we present a characterization technique potentially suitable for complex photonic integrated circuits. By raster scanning along the axis of a straight nano-waveguide in tapping mode and sweeping wavelength, detailed information of propagating waves in that waveguide has been extracted from analyses in both space and wavelength domains. Our technique needs no special steps for phase stabilization, thus allowing long-duration and environment-insensitive measurements. As a proof-of-concept test, in a silicon single-mode waveguide with a few of etched holes, the locations and reflection strengths of the inner defects have been quantified. The measurement uncertainty of the reflection amplitude is less than 25% at current stage. Our technique paves the way for non-destructively diagnosing photonic circuits on a chip with sub-wavelength spatial resolution and detailed information extraction.     

Open-loop electrooptic sampling for real-time analysis and near-field imaging of ultrafast electronic devices

In this paper, we present an open-loop electrooptic sampling system for real-time characterization and near-field scanning of ultrafast electronic devices. The system capabilities such as picosecond time resolution as well as 210 GHz of measurement bandwidth are verified with measurement of a CMOS nonlinear transmission line and an ultimate bandwidth of 230 GHz has been achieved with a post-process algorithm. The noise of the system is quantified and imaging over a broad range of frequencies for an on-chip antenna is demonstrated.     

Adaptive spectrum sampling in pulsed laser spectrometry

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 51, No. 6, pp. 973–980, December, 1989.     

Bidimensional planar micro-optics for optochemical absorbance sensing

A new approach for developing optochemical absorbance sensors is presented. The method is based on a planar micro-optic circuit in which an optochemically active membrane that responds to selective compounds is deposited in the device, yielding a part of the guiding planar structure. In this way the optical field is confined in the direction transverse to the substrate and controlled in the lateral direction by means of planar micro-optics components. High sensitivity of the device can be easily obtained because of the relatively long light paths through the membrane, and the response time is low because the analyte has to diffuse through a several-micrometer-thin membrane. Experimental results of measurements of the concentration of potassium are also presented to verify the possibilities of these devices as specific absorbance sensors.     

Bidimensional antiferromagnetism in layered NH4FeF4

Summary The compound NH₄FeF₄, characterized by layers of (FeF₆)³⁻ octahedra alternating with NH ₄ ⁺ layers, has been investigated by means of M?ssbauer spectroscopy. Experimental data, which evidentiate an antiferromagnetic order, are compared with the molecular-field model for anisotropic media. Magnetization is found orthogonal to the octahedra layers and the octahedra axes are found to be tilted by 17° with respect to the magnetization direction.

---

Riassunto Il composto NH₄FeF₄, caratterizzato da strati di ottaedri (FeF₆)³⁻ alternati con strati NH ₄ ⁺ , è stato studiato con la spettroscopia M?ssbauer. I dati sperimentali, che evidenziano un ordine antiferromagnetico, sono stati interpretati col modello del campo molecolare per mezzi anisotropi. Si è trovato che la magnetizzazione è ortogonale agli strati ottaedrici e che gli assi ottaedrici sono inclinati di 17° rispetto alla direzione di magnetizzazione.

---

Резюме С помощью м?ссбауэровской спектроскопии исследуется соединение NH₄FeF₄, характеризующееся слоями (FeF₆)³⁻, которые чередуются с NH ₄ ⁺ слоями. Экспериментальные данные, которые подтверждают антиферромагнитную упоряденность, сравниваются с моделью молекулярного поля для анизотропной среды. Обнаружено, что намагниченность ортогональна октаэдрическим слоям, а октаэдрические оси наклонены под углом 17° к направлению намагниченности.

     

Heat Flux for a Relativistic Dilute Bidimensional Gas

Relativistic kinetic theory predicts substantial modifications to the dissipation mechanisms of a dilute gas. For the heat flux, these include (in the absence of external forces) a correction to the thermal conductivity and the appearance of a new, purely relativistic, term proportional to the density gradient. In this work we obtain such constitutive equation for the particular case of a bidimensional gas. The calculation is based on the Chapman–Enskog solution to the relativistic Boltzmann equation and yields analytical expressions for the corresponding transport coefficients, which are evaluated for the particular case of hard disks. These results will be useful for numerical simulations and may be applied to bidimensional non-dense materials.     

THz near-field imaging

We present first results of near-field imaging with ultrashort, broadband far-infrared pulses. By focusing the radiation into a tapered metal tip with a small exit aperture and scanning a sample in the near field of this aperture, sub-wavelength spatial resolution better than λ/4 is demonstrated.     

光存储技术中超分辨近场结构研究进展

超分辨近场结构(Super-RENS)是近场光学存储中最具有潜力和应用前景的方案之一。介绍了Super-RENS从基本类型到第三代Super-RENS的发展历程,简述了掩膜材料的研究进展以及Super-RENS在不同记录系统中的应用。展望了它的发展前景。     

Cloaked near-field scanning optical microscope tip for noninvasive near-field imaging

Near-field imaging is a well-established technique in biomedical measurements, since closer to the detail of interest it is possible to resolve subwavelength details otherwise unresolved by regular lenses. A near-field scanning optical microscope (NSOM) tip may indeed overcome the resolution limits of far-field optics, but its proximity inherently perturbs the measurement. Here, we apply the recent concept of a "cloaked sensor" to an NSOM device in collection mode, showing theoretically how a proper plasmonic cover applied to an NSOM tip may drastically improve its overall measurement capabilities.     

Scanning near-field optical microscopy

Scanning Near-field Optical Microscopy (SNOM) allows the investigation of optical properties on subwavelength scales. During the past few years, more and more attention has been given to this technique that shows enormous potential for imaging, sensing and modification at near-molecular resolution. This article describes the technique and reviews recent progress in the field.     

Apertureless near-field laser nanotechnology

We consider apertureless near-field optics that provides subwavelength resolution. We study the enhancement of the electromagnetic field near nanospheres and under the tip of a scanning probe microscope using the finite difference time-domain (FDTD) method. We discuss the mechanisms of field enhancement connected with the system geometry (“lightning rod effect”) and resonance excitation of local plasmon eigenmodes for different materials of the tip and various geometrical parameters of the system. We describe the possible applications in nano-optics and nanotechnology. We present the experimental achievements in apertureless near-field nanolithography.     

Hybrid near-field acoustic holography

Hybrid near-field acoustical holography (NAH) is developed for reconstructing acoustic radiation from an arbitrary object in a cost-effective manner. This hybrid NAH is derived from a modified Helmholtz equation least squares (HELS) formula that expands the acoustic pressure in terms of outgoing and incoming waves. The expansion coefficients are determined by solving an overdetermined linear system of equations obtained by matching the assumed-form solution to measured acoustic pressures through the least squares. Measurements are taken over a conformal surface around a source at close range so that the evanescent waves can be captured. Next, the modified HELS is utilized to regenerate as much acoustic pressures on the conformal surface as necessary and take them as input to the Helmholtz integral formulation implemented numerically by boundary element method (BEM). The acoustic pressures and normal velocities on the source surface are reconstructed by using a modified Tikhnov regularization (TR) with its regularization parameter determined by generalized cross validation (GCV) method. Results demonstrate that this hybrid NAH combines the advantages of HELS and inverse BEM. This is because a majority of the input data are regenerated but not measured, thus the efficiency of reconstruction is greatly enhanced. Meanwhile, the accuracy of reconstruction is ensured by the Helmholtz integral theory and modified TR together with GCV method, provided that HELS converges fast enough on the measurement surface. Numerical examples of reconstructing acoustic quantities on the surface of a simplified engine block are demonstrated. [Work supported by NSF.]     

Subwavelength-resolution near-field Raman spectroscopy

The resolution capabilities of near-field Raman spectroscopy based on a giant enhancement of the electric field near a nanosized metal probe are studied. As a test sample, bundles of single-walled carbon nanotubes deposited on glass substrates are used. It is shown that this method ensures a subwavelength spatial resolution of about 50 nm and demonstrates a Raman scattering enhancement of the order of 10⁴.     

High-sensitivity near-field laser microscopy

We propose a new method of laser spectroscopy, which combines high subwavelength space resolution, high sensitivity, and spectral resolution. The method is based on using a fiber laser near the generation threshold instead of a fiber optical near-field microscope. The near-field subwavelength aperture in the “active” fiber is employed for probing. Absorption on the objects under consideration (atoms, molecules, nanostructure, etc.) leads to the failure of oscillations in the fiber laser. We make a computer simulation of the system under consideration and analyze the method’s sensitivity.     

Scanning near-field acoustic microscopy

Scanning near-field acoustic microscopy (SNAM) is a new method for imaging the topography of nonconducting surfaces at a potential lateral resolution in the sub-micron range. The basic element of this method is a distance sensor consisting of a sharply pointed vibrating tip, which is part of a high-Q quartz resonator driven at its resonance frequency. The decrease of the resonance frequency or of the amplitude of vibration when an object comes into the proximity of the tip serves as the important signal. The dependence of this signal on pressure and composition of the coupling gas shows that the hydrodynamic forces in the gas are responsible for the coupling between object and tip. The sensor is incorporated into a scanning device. Well-resolved line scans of a grating of 8 m periodicity, a lateral resolution of 3 m and a vertical resolution of 5nm have been achieved in our first experiments.     

Pseudopulse near-field subsurface tomography

Decisive success has been achieved in developing the subsurface near-field scanning tomography that overcomes the Rayleigh diffraction limit of a resolution. It is related to the transformation of the multifrequency inverse scattering problem to that for a complex-valued synthesized pulse (pseudopulse). It leads to the integral equation that has maxima in the depth dependence of its kernel and, hence, to the much better depth resolution of tomography. Moreover, the noise related to surface scattering is mainly suppressed in such an approach. This idea is realized here in the microwave subsurface tomography of 3D inhomogeneous dielectric structures. For homogeneous dielectric targets, this approach is applied to obtain holography images of their shape.     

Subsurface near-field scanning tomography

The scanning tomography method is developed for electromagnetic sounding of a 3D structure of an inhomogeneous dielectric half-space. It is shown that known methods of physical diagnostics are suitable for this tomography with the depth of analysis from nanometers at optical frequencies up to several kilometers at ultralow frequencies. The areas of application include nanophysics, biological and medical diagnostics, subsurface remote sensing in geophysics and geology, etc. This approach is realized in the microwave scanning tomography of living tissues where a subwavelength resolution is achieved.     

Dual mode near-field scanning optical microscopy for near-field imaging of surface plasmon polariton

In this paper, we theoretically and experimentally demonstrate that metal coated apertured probes are efficient near-field probes on surfaces with high reflectivity for the scattering as well as for the collection mode near-field scanning optical microscopy (NSOM). We show that a blunt apertured metal coated tip is very effective in suppressing image dipoles which affect strongly the signals scattered from frequently used sharp metal tips or gold nanoparticle attached probes. By using a simultaneous collection and scattering mode (dual mode) NSOM we measure the near-field images of surface plasmon polariton (SPP) launched from a slit. The collection mode measures propagating SPP along lateral distance in a long scan range with high signal-to-noise ratio, and the scattering mode measures the polarization resolved near-field of SPP. Comparisons of the measured data obtained in the dual mode enable to easily characterize SPP and to separate the measured near-field into the propagating SPP and the directly transmitted light.