Detection of monolayer gratings of adsorbates by linear optical diffractions

We report the detection of linear optical diffraction of a He-Ne laser probe beam from a monolayer grating of molecular adsorbates up to the fifth order with no electronic or electromagnetic enhancement. The advantages of using linear diffraction to probe surface diffusion of adsorbates are discussed.     

Scanning plasmon near-field microscopy on dye clusters

A scanning plasmon near-field microscope has been used to image dye clusters on a silver surface. This demonstrates the potential of optical near-field microscopy for the investigation of surfaces and non-conducting adsorbate layers. Images recorded in parallel with the conventional scanning tunneling microscope show the underlying conductive substrate.     

Improved resolution in VUV isochromat spectroscopy

The energy resolution of a VUV isochromat spectrometer employing the traditional energy selective Geiger counter can be significantly improved. The variance of the optical resolution function with the usual CaF₂ entrance window is (240 meV)². With SrF₂ we obtain (113 meV)² at room temperature and (73 meV)² at 70°C. A direct confirmation of these data, which were derived from a moment analysis of the threshold behaviour of an Au isochromat is provided by a measurement of image potential states at Cu(001).     

Study of relaxation of molecular iodine by the method of saturated absorption spectroscopy

The paper describes detailed experimental studies of the width, shape and shifts of saturated absorption resonances at the X(v=0, j=13)B(v=43, j=12) transition of molecular iodine. Use of the geometry of oppositely travelling and unidirectional waves in the experiment and study of the luminescence decay under pulsed excitation allowed estimation of the cross-sections of elastic and inelastic scattering of I₂ molecules on the upper and lower levels of transition. The observed nonlinear dependence of width and shift of the saturation resonance on iodine pressure is explained by the influence of elastic collisions with no phase mismatch.     

Resonance lonization mass spectroscopy with a pulsed thermal atomic beam

Resonance ionization mass spectroscopy (RIMS) and pulsed-laser induced desorption (PLID) have been combined for ultrasensitive detection and spectroscopy of very small samples of refractive elements. The method has been tested and applied to laser spectroscopy of 5×10⁹ atoms (1.5 pg) of¹⁹⁵Au (T _1/2= 183d) implanted at the ISOLDE online mass separator with 60 keV into graphite. A pulsed thermal atomic beam was formed by laser desorption with a 10 ns NdYag laser pulse. Subsequently the atoms were photoionized in a three-colour, three-step resonant excitation to an autoionizing state. The selectivity was enhanced by a time-of-flight measurement of the photo ions. In resonance, one ion was detected per 10⁵ atoms implanted resulting in a gain in detection efficiency by three orders of magnitude in comparison to the use of a continuous atomic beam. In the course of the experiments several unknown autoionizing states were found, and the lifetime of the 6d ² D ^3/2 state of gold was determined to be=10.7(6) ns.     

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.     

Spectroscopy of Te2 with modulation transfer: Reference lines for precision spectroscopy in Yb+ at 467 nm

Modulation-transfer spectroscopy is used to observe resonances in ¹³⁰Te₂ near 467 nm. A signal-to-noise ratio of 500 is obtained with 1 mW laser power and with a time constant of 10 ms; the lineshapes are investigated in detail for different demodulation phases. The signal can be used to servolock a blue dye ring laser at 467 nm on a ¹³⁰Te₂ resonance which serves as reference line for precision spectroscopy on single, trapped Yb⁺ ions at 467 nm.     

Modulated optical reflectance measurements on amorphous silicon layers and detection of residual defects

Modulated optical reflectance measurements on amorphous silicon layers are presented and a simple theoretical model, which is in good agreement with the experiment, is proposed. Further, the correlation between defects remaining after recrystallization of the amorphous layers and the measured modulated optical reflectance is established. This measurement technique turns out to be useful for characterizing amorphous Si layers produced by ion implantation, for controlling the recrystallization of such layers, and for detecting residual defects.     

Ellipsometric and reflectance studies of GaAs/AlAs superlattices

Multiple-angle-of-incidence ellipsometry at 632.8 nm and optical reflectance spectra are used to study GaAs/AlAs superlattices on GaAs substrates. The diagnostic potential of the two methods are compared to each other. Precise values of the total film thicknesses are obtained from the ellipsometric data. Significant disagreement between the measured and nominal thicknesses based on RHEED controlled MBE growth conditions is observed. Nonuniformity of a superlattice film is identified with the spatial resolution of about 1 mm². The thickness variations are accompanied by pronounced variations of the optical constants.     

Study of Ge x Si1−x /Si superlattices by ellipsometry

Multiple-angle-of-incidence (MAI) ellipsometry is used to study Ge _x Si_1–x /Si superlattices fabricated by a RRH/VLP-CVD system. Based on the fundamental ellipsometric equations and properties of semiconductor superlattices (SL), various parameters of SL structure, such as the sublayer thickness, complex refractive index and composition, are achieved precisely and simultaneously. The results obtained from ellipsometry are well consistent with those from growth conditions and other characterization techniques.     

Surface sum-frequency mixing for auto- and cross-correlation of ultrashort UV and IR pulses

Surface sum-frequency mixing is shown to be a valuable tool in determining the width and shape of ultrashort ultraviolet and infrared laser pulses. The method is demonstrated to measure UV pulses of 300 fs and IR pulses of about 2.9 ps duration. In cross-correlation set-ups the method is inherently background-free, but background-free arrangements can also be realized for autocorrelation measurements, useful especially in the ultraviolet. In the present experiments on GaAs(111) and Pd(111) a spectral range from 3.3 m to below 200 nm is covered, which certainly can be extended to longer as well as to shorter wavelengths. Due to the small source volume phase-matching is not required.     

Sensitive detection of methane with a 1.65 μm diode laser by photoacoustic and absorption spectroscopy

Received: 18 September 1997/Revised version: 11 November 1997     

Internal reflection mode scanning near-field optical microscopy with the tetrahedral tip on metallic samples

An internal reflection mode is introduced for scanning near-field optical microscopy (SNOM) with the tetrahedral tip. A beam of light is coupled into the tip and the light specularly reflected out of the tip is detected as a photosignal for SNOM. An auxiliary STM mode is used to control the distance during the scanning process and to record the topography of the sample simultaneously with the SNOM image. Images were obtained of different metallic samples which show a contrast in the order of 10% of the total reflected photosignal. In images of metallic samples an inverted contrast is consistently obtained compared to images previously obtained of comparable samples in a transmission mode. The contrast shows a pronounced dependence on the polarization of the incident beam with respect to the orientation of the edges of the tip. In the case of gold surfaces, the photosignal as a function of distance between the tip and the surface shows a pronounced peak in the near-field range of 0–20 nm which is tentatively attributed to the excitation of surface plasmons on the gold surface. The pronounced near-field effects and the strong contrast in the near-field images and the resolution well below 50 nm are an indication of a highly efficient coupling of the incident beam to a local excitation of the tip apex which is essential for the function of the tip as a probe for SNOM. Received: 17 May 1999 / Accepted: 18 May 1999 / Published online: 21 October 1999     

Frequency measurement of130Te2 resonances near 467 nm

Precision spectroscopy on molecular tellurium is performed by measuring the frequency difference between the observed lines and an eigenfrequency of a high-finesse cavity mode. The mode frequency is derived from a measurement of the cavity's free spectral range taking into account the cavity dispersion due to phase shifts in the dielectric mirror coatings. The experimental technique is based on dual frequency modulation and is applied to determine the transition wavenumbers of several lines in¹³⁰Te₂ near 467 nm with an uncertainty of 2 × 10^–8.     

Generation,stabilization, and amplification of subpicosecond pulses

A sync-pumped cw dye laser system has been used to produce subpicosecond pulses. Pulses as short as 0.7 ps, assuming a single-sided exponential pulse shape, were observed. A set of experiments was performed to investigate the origin and effects of noise in the sync-pumped system. A digital and an analog feedback loop have been designed to optimize the pulse width. The noise has been lowered by 10 dB for frequencies up to 10 kHz; long-term drift is also controlled by this method. A four-stage dye laser amplifier, pumped by a Nd:YAG laser which operates at a 10-Hz repetition rate, is synchronized electronically to the dye-laser picosecond pulses. A gain of 3×10⁶ has been achieved.This work was supported by the Joint Services Electronics Program     

An all-solid-state tunable 214.5-nm continuous-wave light source by using two-stage frequency doubling of a diode laser

3 and β-BaB₂O₄ crystals. The power of the generated 214.5-nm light amounts to more than 100 μW. This light source will be used for laser cooling of Cd⁺ ions. Received: 4 August 1997/Revised version: 28 October 1997     

Direct observation of the second-order Stark effect of theX-B transition in molecular iodine

The second-order Stark shift of the components of the hyperfine structure of the transition¹ _g ⁺ ( = 0,j = 13, 15) ³ _ou ⁺ ( = 43,j = 12, 16) (of molecular iodine have been studied by means of saturated absorption spectroscopy in an external cell with the I₂ vapour located in an electric field. The anisotropic polarizabilities of the upper and lower levels together with the difference between the isotropic polarizabilities of the levels of the transition have been obtained.     

On the theory of TE-polarized waves guided by a nonlinear three-layer structure

TE-polarized electromagnetic waves guided by a three-layer structure consisting of a film surrounded by semi-infinite media (all media are characterized by a Kerrlike dielectric function) are investigated by expressing the solution of the field equations in terms of Weierstrass' elliptic functions. Evaluation leads to a universal dispersion relation and its solutions and a universal expression for the power flow. Numerical results are presented for the effective wave number as a function of the intensity of the electric field at the lower surface of the nonlinear film, for various profiles of the field intensity, and for the power flow as a function of the effective wave number.     

Saturated structured confocal microscopy with theoretically unlimited resolution

The resolution of fluorescence microscopes is limited by diffraction, which determines the extension of their point spread functions. We propose and study numerically a simple method, based on a combination of subtraction microscopy with regular and annular excitation beams, which permits to double the resolution compared to wide field microscopy. When combined with the fluorescence saturation phenomenon, this approach would be able to deliver a resolution of a few tens of nanometers.     

One atom in an optical cavity: Spatial resolution beyond the standard diffraction limit

The position of a slow atom passing through a standing-wave light field in an ultrahigh-finesse optical resonator can be measured by observing either the intensity of the light transmitted through the cavity or its phase. Apart from the periodicity of the standing wave, both techniques allow to determine the position of the particle with a resolution much better than the standard classical diffraction limit /2. Position measurements with uncertainty </20 seem to be possible with all-optical techniques.These notes were prepared to celebrate H. Walther's 60th birthday and to honour his pioneering contributions to some of the most lively fields of quantum optics