Nanostructures with Ge–Si quantum dots for infrared photodetectors

In this paper questions of optimization of growth conditions in the method of molecular beam epitaxy for creation of high-efficient quantum dot infrared photodetectors are considered. As a model material system for theoretical investigations, heterostructures with germanium-silicon quantum dots on the silicon surface are chosen. For calculations of the dependencies of quantum dots array parameters on synthesis conditions the kinetic model of growth of differently shaped quantum dots based on the general nucleation theory is proposed. The theory is improved by taking into account the change in free energy of nucleation of an island due to the formation of additional edges of islands and due to the dependence of surface energies of facets of quantum dots on the thickness of a 2D wetting layer during the Stranski–Krastanow growth. Calculations of noise and signal characteristics of infrared photodetectors based on heterostructures with quantum dots of germanium on silicon are done. Dark current in such structures caused by thermal emission and barrier tunneling of carriers, as well as detectivity of the photodetector in the approximation of limitation by generation-recombination noises are estimated. Moreover, the presence of dispersion of quantum dots by size is taken into account in the calculations of the generation-recombination noises. Results of calculations of the properties of structures with quantum dots and their dependencies on growth parameters, as well as the characteristics of quantum dot photodetectors are presented. Comparison of the estimated parameters of quantum dots ensembles and the characteristics of quantum dot photodetectors with experimental data is carried out.     

Two-Dimensional Materials of Group IVA: Latest Advances in Epitaxial Methods of Growth

Russian Physics Journal - Two-dimensional materials have become one of the central research topics of scientists around the world after the production of graphene – a monatomic layer of...     

Applications of ultrashort shaped pulses in microscopy and for controlling chemical reactions

This article presents a new perspective on laser control based on insights into the effect of spectral phase on nonlinear optical processes. Gaining this understanding requires the systematic evaluation of the molecular response as a function of a series of pre-defined accurately shaped laser pulses. The effort required is rewarded with robust, highly reproducible, results. This approach is illustrated by results on selective two-photon excitation microscopy of biological samples, where higher signal and less photobleaching damage are achieved by accurate phase measurement and elimination of high-order phase distortions from the ultrashort laser pulses. A similar systematic approach applied to laser control of gas phase chemical reactions reveals surprising general trends. Molecular fragmentation pattern is found to be dependent on phase shaping. Differently shaped pulses with similar pulse duration have been found to produce similar fragmentation patterns. This implies that any single parameter that is proportional to the pulse duration, such as second harmonic generation intensity, allows us to predict the molecular fragmentation pattern within the experimental noise. This finding, is illustrated here for a series of isomers. Bond selectivity, coherent photochemistry and their applications are discussed in light of results from these systematic studies.     

Isomeric identification by laser control mass spectrometry

The influence shaped femtosecond laser pulses have on molecular photofragmentation and ionization, coupled with the intrinsic sensitivity of mass spectrometry, results in a powerful tool for fast, accurate, reproducible and quantitative isomeric identification. Complex phase functions are introduced to enhance differences during the laser-molecule interactions, which depend on geometric structure, resulting in different fragmentation fingerprints. A full account is given on the setup and results leading to a technique that can be used to distinguish between compounds normally indistinguishable by conventional electron ionization mass spectrometry. We demonstrate geometric and structural isomer identification of cis-/trans-3-heptene, cis-/trans-4-methyl-2-pentene, o-/p-cresol and o-/p-xylene. For the positional isomers of xylene we present a complete dataset consisting of 1024 different phases to explore phase complexity. A selection of two phases from that data can then be used to achieve quantitative identification in mixtures of xylene isomers. Finally, we evaluate receiver operational curves obtained from our experimental data to demonstrate the reliability that can be achieved by femtosecond laser control mass spectrometry.     

Search space mapping: getting a picture of coherent laser control

Search space mapping is a method for quickly visualizing the experimental parameters that can affect the outcome of a coherent control experiment. We demonstrate experimental search space mapping for the selective fragmentation and ionization of para-nitrotoluene and show how this method allows us to gather information about the dominant trends behind our achieved control.     

Quantitative mass spectrometric identification of isomers applying coherent laser control

Mass spectrometry (MS) is one of the oldest and most trusted analytical methods for chemical identification. Advances in biology, such as metabolic analysis and proteomics, have fueled a growing number of refinements in this method. Unfortunately, isomers, for example, o- and p-xylene, are seldom identifiable by MS because they produce identical spectra. Time-consuming and less sensitive multidimensional methods are subsequently required for structural determination. The sensitivity of MS coupled with shaped femtosecond laser pulses that control molecular fragmentation and ionization results in a new, fast, and reproducible method for molecular identification which is used here to distinguish positional and geometric isomer compounds and quantify their relative concentration in mixtures.     

Coherent control improves biomedical imaging with ultrashort shaped pulses

Although ultrashort pulses are advantageous for multiphoton excitation microscopy, they can be difficult to manipulate and may cause increased sample damage when applied to biological tissue. Here we present a method based on coherent control that corrects phase distortions introduced by high numerical aperture (NA) microscope objectives, thereby achieving the full potential of ultrashort pulses. A number of useful phase functions are recommended to gain selectivity that is similar to that which can be achieved by tuning a longer laser pulse; however this one involves no moving parts and maintains perfect optimization. This capability is used to demonstrate functional imaging by selective two-photon excitation of a pH-sensitive chromophore. Finally, we show that phase functions can also be introduced to minimize multiphoton excitation damage, while maintaining a high efficiency of two-photon excitation.     

Multiphoton intrapulse interference. IV. Ultrashort laser pulse spectral phase characterization and compensation

We introduce a noninterferometric single beam method to characterize and compensate the spectral phase of ultrashort femtosecond pulses accurately. The method uses a pulse shaper that scans calibrated phase functions to determine the unknown spectral phase of a pulse. The pulse shaper can then be used to synthesize arbitrary phase femtosecond pulses or it can introduce a compensating spectral phase to obtain transform-limited pulses. This method is ideally suited for the generation of tailored spectral phase functions required for coherent control experiments.     

Binary phase shaping for selective single‐beam CARS spectroscopy and imaging of gas‐phase molecules

We report on mode‐selective single‐beam coherent anti‐Stokes Raman scattering spectroscopy of gas‐phase molecules. Binary phase shaping (BPS) is used to produce single‐mode excitation of O₂, N₂, and CO₂ vibrational modes in ambient air and gas‐phase mixtures, with high‐contrast rejection of off‐resonant Raman modes and efficient nonresonant‐background suppression. In particular, we demonstrate independent excitation of CO₂ Fermi dyads at ∼1280 and ∼1380 cm⁻¹ and apply BPS for high‐contrast imaging of CO₂ jet in ambient air. Copyright © 2010 John Wiley & Sons, Ltd.