Single-longitudinal mode Nd:YVO<Subscript>4</Subscript>/YVO<Subscript>4</Subscript>/KTP green solid state laser

We present the concept and practical realization of a single frequency, tuneable diode pumped Nd:YVO₄/YVO₄/KTP microchip laser operating at 532 nm. Theoretical analysis of the single mode operation of such a laser configuration is presented. The single frequency operation has been obtained in a birefringent filter, where an YVO₄ beam displacer acts as an ideal polarizer. Experimental results are in good agreement with theoretical analysis. We have obtained stable single frequency operation, tuneable over 0.6 nm in the spectral range around 1064 nm. The laser operated with output power up to 110 mW at 53 nm. The total optical efficiency (808 nm to 532 nm) was 14%.     

Compact soft x-ray microscope using a gas-discharge light source

We report on a soft x-ray microscope using a gas-discharge plasma with pseudo spark-like electrode geometry as a light source. The source produces a radiant intensity of 4 x 10(13) photons/(sr pulse) for the 2.88 nm emission line of helium-like nitrogen. At a demonstrated 1 kHz repetition rate a brilliance of 4.3 x 10(9) photons/(microm2 sr s) is obtained for the 2.88 nm line. Ray-tracing simulations show that, employing an adequate grazing incidence collector, a photon flux of 1 x 10(7) photons/(microm2 s) can be achieved with the current source. The applicability of the presented pinch plasma concept to soft x-ray microscopy is demonstrated in a proof-of-principle experiment.     

Tunable Supercontinuum Generated in a Yb~(3+)-Doped Microstructure Fiber Pumped by Ti:Sapphire Femtosecond Laser

We experimentally demonstrate that a tunable supercontinuum(SC) can be generated in a Yb3+-doped microstructure fiber by the concept of wavelength conversion with a Ti:sapphire femtosecond(fs) laser as the pump.Experimental results show that an emission light around 1040 nm in an anomalous dispersion region is first generated and amplified by fs pulses in the normal dispersion region. Then, SC spectra from 1100 to 1380 nm and 630 to 840 nm can be achieved by combined effects of higher-order soliton fission and Raman soliton self-frequency shift in the anomalous dispersion region and self-phase modulation, dispersive wave, and four-wave mixing in the normal dispersion region. It is also demonstrated that the 20 nm change of pump results in a 280 nm broadband shift of soliton and the further red-shift of soliton is limited by OH-absorption at 1380 nm.     

Coherent two-dimensional ultraviolet spectroscopy in fully noncollinear geometry

We introduce fully noncollinear coherent two-dimensional (2D) spectroscopy in the UV domain with an all-reflective and miniaturized setup design. Phase stability is achieved via pairwise beam manipulation, and the concept can be transferred to all wavelength regimes. Here we present results from an implementation that has been optimized for wavelengths between 250 and 375 nm. Interferometric measurements prove phase stability over several hours. We obtained 2D spectra of the nonpolar UV chromophore p-terphenyl in ethanol, excited with 50 fs pulses at 287 nm.     

Wavelength division (de)multiplexing based on dispersive self-imaging

We proposed and experimentally demonstrated wavelength division (de)multiplexers (WDMs) utilizing the wavelength dispersive nature of self-imaging multimode interferometers. Proof-of-principle devices fabricated on the silicon-on-insulator platform operated as 4-channel WDMs with a free spectral range of >90 nm, an averaging cross talk of <-20 dB for a 1 nm band, and an insertion loss of <2.0 dB. The potential for higher channel counts and smaller channel wavelength spacing was also predicted. This type of WDM is easy to design and fabricate. The underlying concept is applicable to all planar waveguide platforms.     

Bidimensional near-field sampling spectrometry

We report on a concept of compact optical Fourier-transform spectrometer based on bidimensional (2D) spatial sampling of a confined interferogram. The spectrometer consists of a nanostructured glass surface on which two light beams interfere in total internal reflection. Subwavelength spatial sampling of the interferogram near field is achieved by introducing a tilt angle between a 2D array of optical nanoantennas and the interferogram pattern. The intensity distribution of the scattered light is recorded on a 2D CCD camera, and a one-dimensional Fourier transform of the interferogram is used to recover the input light spectrum. Experimental results show a wide spectral bandwidth in the visible range, down to 380 nm, with spectral resolution of 1.6 nm around 780 nm.     

Angle of arrival estimation using spectral interferometry

We have developed a correlative signal processing concept based on a Mach-Zehnder interferometer and spatial-spectral (S2) materials that enables direct mapping of RF spectral phase as well as power spectral recording. This configuration can be used for precise frequency resolved time delay estimation between signals received by a phased antenna array system that in turn could be utilized to estimate the angle of arrival. We present an analytical theoretical model and a proof-of-principle demonstration of the concept of time difference of arrival estimation with a cryogenically cooled Tm:YAG crystal that operates on microwave signals modulated onto a stabilized optical carrier at 793 nm.     

Realization of Stable Narrow Linewidth Dual-Wavelength Lasing in an Erbium-Doped Fibre Laser by Cleaving the Wavelength-Selective Filter Spectrum

We propose and demonstrate a new concept of stable narrow-line-width and close wavelength spacing dualwavelength lasing in an Er-doped fibre ring laser （EDFRL） by cleaving the spectrum with a wavelength-selective component in the EDFRL. A fibre loop mirror （FLM） combining with a polarization controller （PC） acts as the cleaver. The cleaver can produce a fine pectinate spectrum. By adjusting the PC, the fine pectinate spectrum can be so changeable that cleaving the spectrum of a fibre Bragg grating （FBG） into two parts. As a result, we obtain the dual-wavelength fibre lasering with a bandwidth of only 0.03nm and a wavelength spacing of only 0.07nm. Furthermore, the laser can also perform stable switchable single wavelength or stable different-bandwidth dual-wavelength by carefully adjusting the PC at room temperature.     

Enhanced light confinement in a near-field optical probe with a triangular aperture

We present a probe concept for scanning near-field optical microscopy combining the excellent background suppression of aperture probes with the superior light confinement of apertureless probes. A triangular aperture at the tip of a tetrahedral waveguide (full taper angle approximately 90 degrees ) shows a strong field enhancement at only one rim when illuminated with light of suitable polarization. Compared to a circular aperture of equivalent size, the resolution capability is doubled without loss of brightness. For a approximately 60 nm sized triangular aperture, we measured an optical resolution <40 nm and a transmission of approximately 10(-4).     

Development of a Fourier-transform waveguide spectrometer for space applications

We describe the development of a waveguide Fourier-transform spectrometer for space-borne high-resolution sensing. A prototype device is designed to monitor the water vapor absorption band near 1,364?nm with a resolution of 0.05?nm. It has no moving parts and is based on a unique concept of arrayed interferometers implemented in silicon-on-insulator planar waveguide chip. The optical input is formed by many independent waveguides, providing a significantly increased light gathering capability (étendue) compared to single-waveguide input configurations. Enhancements of the spectrometer capabilities are achieved by stacking planar waveguide layers and by using surface gratings to couple light into the waveguides.     

Integrated mechano-optical hydrogen gas sensor using cantilever bending readout with a Si3N4 grated waveguide

We demonstrate a proof of concept of a novel and compact integrated mechano-optical sensor for H(2) detection based on a microcantilever suspended above a Si(3)N(4) grated waveguide. The fabricated devices are mechanically and optically modeled and characterized. Sensing operation of the sensor is demonstrated with 1% H(2) in N(2). The error in detection of the cantilever bending induced by absorption of H(2) is estimated to be approximately 10 nm. Significantly improved sensitivity (down to ～33 pm) is expected for reduced initial bending of the microcantilever. The simulation and experimental results are in good agreement and provide a good guideline for further optimization of the sensor.     

Few-optical-cycle pulses in the near-infrared from a noncollinear optical parametric amplifier

We extend the concept of broadband phase matching in a noncollinear optical parametric amplifier (NOPA) to the near-IR. In an 800 nm pumped NOPA using periodically poled stoichiometric lithium tantalate, we amplify a spectrum spanning the 1.1-1.7 microm range and corresponding to two optical cycles of the carrier wavelength. A limited portion of the spectrum is compressed by a prism pair down to 16 fs.     

Widely tunable diffraction limited 1000 mW external cavity diode laser in Littman/Metcalf configuration for cavity ring-down spectroscopy

We report on recent progress on external cavity diode lasers (ECDL) using a new concept of a Littman/Metcalf configuration. Within this concept one facet of the diode laser chip is used for coupling to a high quality Littman/Metcalf resonator whereas the other side of the diode laser chip emits the output beam. The alignment of the external resonator is independent from the alignment of the output beam and there is no need for any compromise in the alignment. This results in an improved behavior of the external resonator with the benefit of a drastic increase in power and single mode tuning.We investigated this light source for high resolution spectroscopy in the field of cw-cavity ring-down spectroscopy (CRDS). The monitoring of environmental and medical gases from vehicles or human breath requires a suitable radiation source in the mid-infrared (MIR) between 3 and 5 μm that is frequency stable and can be widely tuned. Since this wavelength cannot be reached via direct emitting room temperature semiconductor lasers, additional techniques like difference frequency generation (DFG) are essential. Tunable difference frequency generation relies on high power, small linewidth, fast tunable, robust laser diode sources with excellent beam quality.With our new compact, alignment-insensitive and robust ECDL concept, we achieved an output power of 1000 mW and an almost Gaussian shaped beam quality (M²<1.2). The coupling efficiency for optical waveguides as well as single mode fibers exceeds 70%. The wavelength is widely tunable within the tuning range of 20 nm via remote control. This laser system operates longitudinally in single mode with a mode-hop free tuning range of more than 150 GHz without current compensation and a side-mode-suppression better than 50 dB. This concept is currently realized within the wavelength regime between 750 and 1080 nm.Our high powered Littman/Metcalf laser system was part of a MIR-light source which utilizes DFG in periodically poled lithium niobate (PPLN) crystals. At the wavelength of 3.3 μm we were able to achieve a high-resolution absorption spectrum of water with four different isotoplogues of H₂O components. This application clearly demonstrates the suitability of this laser for high-precision measurements. PACS 07.57.Ty; 42.55.Px; 42.62.Fi     

Very compact tunable solid-state laser utilizing a thin-film organic semiconductor

Optically pumped organic semiconductor lasers are fabricated by evaporation of a thin film of tris(8-hydroxyquinoline) aluminum (Alq(3)) molecularly doped with a laser dye on top of a polyester substrate with an embossed grating structure. We achieve low-threshold, longitudinally monomode distributed-feedback laser operation. By varying the film thickness of the organic semiconductor film, we can tune the wavelength of the surface-emitting laser over 44 nm. The low laser threshold allows the use of a very compact all-solid-state pump laser ( approximately 10 cm long). This concept opens up a way to obtain inexpensive lasers that are tunable over the whole visible range.     

Discriminability of tryptophan containing dipeptides using quantum control

We show that the coherent manipulation of molecular wavepackets in the excited states of trp-containing dipeptides allows efficient discrimination among them. Optimal dynamic discrimination fails, however, for some dipeptide couples. When considering the limited spectral resources at play (3 nm bandwidth at 266 nm), we discuss the concept of discriminability, which appears uncorrelated to both static spectra and relaxation lifetimes.     

Microtags with 150-nm line gratings fabricated by use of extreme-ultraviolet lithography

The microtag concept is an anticounterfeiting and security measure. Microtags are computer-generated holograms (CGH's) consisting of 150-nm lines arranged to form 300-nm-period gratings. The microtags that we describe were designed for readout at 442nm . The smallest microtag measures 56micromx80 microm when viewed at normal incidence. The CGH design process uses a modified iterative Fourier-transform algorithm to create either phase-only or phase-and-amplitude microtags. We also report on a simple and compact readout system for recording the diffraction pattern formed by a microtag. The measured diffraction patterns agree very well with predictions.     

Dominoes in carbon nanotubes

We demonstrate by molecular dynamics simulations that the domino process can be developed in single-walled carbon nanotubes (SWCNTs). Once a section of a SWCNT with an appropriate diameter (>3.5 nm) is collapsed, the successive collapse of the neighboring portions can generate a domino wave along the longitudinal direction of the tube. The wave is driven by van der Waals potential energy and its natural speed can be up to 1 km/s. Molecules inside the SWCNT can be accelerated by the domino wave and finally shot out. The finding shows for the first time that a SWCNT can be an energy supplier, which provides opportunities for designing new concept (domino-driven) nanoelectromechanical system devices.     

Spatially selective melting and evaporation of nanosized gold particles

We have developed an atomic force microscope-tip-based concept to pattern metallic nanoparticles on substrates. This new process has the potential to control the assembly of nanometer sized particles by combining their unique optical and thermophysical properties and is a flexible and low energy method of patterning at the nanoscale. The proof of concept is detailed by preliminary experimental work showing selective melting and evaporation of groups of 50 and 100 nm gold spherical particles.     

Laser Shock Processing of 6061-T6 Al alloy with 1064 nm and 532 nm wavelengths

Laser Shock Processing (LSP) has been proposed as a competitive alternative technology to classical treatments for improving fatigue and wear resistance of metals. We present a configuration and results in the LSP concept for metal surface treatments in underwater laser irradiation at 532 nm and 1064 nm. The purpose of the work is to compare the effect of both wavelengths on the same material. A convergent lens is used to deliver 1.2 J/pulse (1064 nm) and 0.9 J/pulse (532 nm) in a 8 ns laser FWHM pulse produced by 10 Hz Q-switched Nd:YAG laser with spots of a 1.5 mm in diameter moving forward along the work piece. A LSP configuration with experimental results using a pulse density of 2500 pulses/cm² and 5000 pulses/cm² in 6061-T6 aluminum samples are presented. High level compressive residual stresses are produced using both wavelengths. It has been shown that surface residual stress level is comparable to that achieved by conventional shot peening, but with greater depths. This method can be applied to surface treatment of final metal products.     

Localization and waveguiding of surface plasmon polaritons in random nanostructures

We propose to use channels in strongly scattering nonabsorbing random media for guiding electromagnetic waves, and demonstrate this concept using near-field microscopy of surface plasmon polaritons (SPP's) propagating along the gold film surface covered with randomly located scatterers. In the wavelength range of 725-765 nm, we observe complete inhibition of the SPP propagation inside the random structures composed of approximately 50-nm-wide gold bumps and their clusters with the density of 50 microm(-2), as well as well-defined SPP guiding along corrugation-free 2- and 4-microm-wide channels.