Jakob Juul Larsen

Coherent anti-Stokes Raman scattering microscopy with a photonic crystal fiber based light source

Coherent anti-Stokes Raman scattering (CARS) microscopy is demonstrated using a light source consisting of the output from a photonic crystal fiber pumped by a standard Ti:sapphire oscillator and the fundamental oscillator beam.

Supercontinuum generation in a photonic crystal fiber with two zero dispersion wavelengths

We demonstrate supercontinuum generation in a highly nonlinear photonic crystal fiber with two closely lying zero dispersion wavelengths. The special dispersion of the fiber has a profound influence on the supercontinuum which is generated through self-phase modulation and phasematched four-wave mixing and not soliton fission as in the initial photonic crystal fibers. The supercontinuum has high spectral density and is extremely independent of the input pulse over a wide range of input pulse parameters. Simulations show that the supercontinuum can be compressed to ultrashort pulses.

Controlling the alignment of neutral molecules by a strong laser field

A strong nonresonant nanosecond laser pulse is used to align neutral iodine molecules. The technique, applicable to both polar and nonpolar molecules, relies on the interaction between the strong laser field and the induced dipole moment of the molecules. The degree of alignment is enhanced by lowering the initial rotational energy of the molecules or by increasing the laser intensity. The alignment is measured by photodissociating the molecules with a femtosecond laser pulse and detecting the direction of the photofragments by imaging techniques. The strongest degree of alignment observed is 〈cos2 θ〉=0.81.

ALIGNING MOLECULES WITH INTENSE NONRESONANT LASER FIELDS

Molecules in a seeded supersonic beam are aligned by the interaction between an intense nonresonant linearly polarized laser field and the molecular polarizability. We demonstrate the general applicability of the scheme by aligning I2, ICl, CS2, CH3I, and C6H5I molecules. The alignment is probed by mass selective two dimensional imaging of the photofragment ions produced by femtosecond laser pulses. Calculations on the degree of alignment of I2 are in good agreement with the experiments. We discuss some future applications of laser aligned molecules.

Continuous-wave wavelength conversion in a photonic crystal fiber with two zero-dispersion wavelengths.

We demonstrate continuous-wave wavelength conversion through four-wave mixing in an endlessly single mode photonic crystal fiber. Phasematching is possible at vanishing pump power in the anomalous dispersion regime between the two zero-dispersion wavelengths. By mixing appropriate pump and idler sources, signals in the range 500-650 nm are obtained in good accordance with calculated phasematching curves. The conversion efficiency from idler to signal power is currently limited to 0.3% by the low spectral density of the pump and idler sources at hand, but can be greatly enhanced by applying narrow line width lasers.

FEMTOSECOND PHOTODISSOCIATION DYNAMICS OF I2 STUDIED BY ION IMAGING

Ion imaging is employed to analyze the fragments from timed Coulomb explosion studies of femtosecond (fs) molecular dynamics. The technique provides high detection efficiency and direct recording of the two-dimensional velocity of all ionized fragments. We illustrate the approach by studying photodissociation of iodine initiated by a weak femtosecond pump pulse and probed by an intense, delayed femtosecond pulse. The ion images, recorded as a function of the pump-probe delay, reveal the evolution of the internuclear separation of the dissociating I2 molecule. The experimental results are in good agreement with quantum mechanical wave packet simulations. We discuss the perspectives for extending the studies to photochemical reactions of small polyatomic molecules.

The effect on wireless sensor communication when deployed in biomass.

Wireless sensor networks (WSN) have been studied in a variety of scenarios over recent years, but work has almost exclusively been done using air as the transmission media. In this article some of the challenges of deploying a WSN in a heterogeneous biomass, in this case silage, is handled. The dielectric constant of silage is measured using an open-ended coaxial probe. Results were successfully obtained in the frequency range from 400 MHz to 4 GHz, but large variations suggested that a larger probe should be used for more stable results. Furthermore, the detuning of helix and loop antennas and the transmission loss of the two types of antennas embedded in silage was measured. It was found that the loop antenna suffered less from detuning but was worse when transmitting. Lastly, it is suggested that taking the dielectric properties of silage into account during hardware development could result in much better achievable communication range.

Impact of loudspeaker nonlinear distortion on personal sound zones.

Personal sound zone systems aim at creating multiple listening zones within a shared space with minimum interference between zones, but the performance is often poorer than simulations predict and effects of nonlinear distortion are sometimes audible. This paper assesses the impact of nonlinear distortion on sound zones through simulations and measurements performed under anechoic conditions. Two sound zones, one bright and one dark, are created with acoustic contrast control using two loudspeaker arrays driven at 250 Hz. Nonlinear distortion is modelled using second or third order nonlinearities. Simulations show that nonlinear distortion degrades the acoustic contrast, which is confirmed by experimental measurements. The harmonic distortion is audible in the dark zone. Frequency resolved measurements reveal that harmonic distortion contributes to contrast loss, but nonlinear effects on the fundamental component are the main cause. Nonlinear distortion can be controlled through regularization of the loudspeaker control effort. Simulations and experiments show an optimum regularization range where contrast is optimized by balancing linear performance and nonlinear distortion.

A Multichannel, Low Noise Surface NMR Receiver System with Wireless Connections to Signal and Reference Coils

Summary Surface NMR holds great promise as a tool in groundwater measurements due to its unique direct sensitivity to water, but the method currently suffers from a number of drawbacks which limits its widespread applicability. Among these drawbacks are a low signal to noise ratio which limits the use of the method in many places of interest and a low production rate which makes the method costly in field campaigns. Hence there is a need for research further advancing the technology. In this paper we report on the development of a new multichannel, low noise surface NMR receiver system with wireless connections to reference coils. The receiver system works as a completely independent add-on to existing transmitter systems and consists of a number of independently operated data acquisition boxes connected with WiFi and synchronized by GPS. The internal electronic noise level of the system is 1.2 nV/sqrt(Hz). The timing jitter between data acquired in different boxes is less than 100 ns.

Removal of Co-frequency Powerline Harmonics from Multichannel Surface NMR Data

Powerline harmonics is often the dominant noise source in surface NMR data and can completely overwhelm the NMR signal. Several methods have been successfully implemented for removal of powerline harmonics including notch filtering, multichannel filtering and model-based subtraction. However, the performance of these methods can be problematic when one of the powerline harmonic components has a frequency close to or coincident with the Larmor frequency, referred to as a co-frequency harmonic. Removal of the co-frequency harmonic can distort the NMR signal causing erroneous estimates of water content and relaxation rates. To solve this problem we propose an extended method of multichannel model-based subtraction of powerline harmonics. In this method, the co-frequency component is modeled in the primary coil and a reference coil on noise-only data recorded immediately prior to the NMR excitation. The phase and amplitude relationships between the components measured in the two coils are calculated. The relationships are used to predict the co-frequency harmonic component parameters in the primary coil during the NMR decay based on NMR signal free, synchronously recorded reference coil data. The mathematical framework is developed and we give examples of the efficiency of the method based on synthetic NMR signals embedded in noise-only data.