Deterministic Nonlinear Transformations of Phase Noise in Quantum-Limited Frequency Combs.

Optical phase noise of femtosecond lasers is analyzed over various steps of broadband nonlinear frequency conversion. The intrinsic phase jitter of our system originates from quantum statistics in the mode-locked oscillator. Supercontinuum generation by four-wave-mixing processes preserves a noise minimum at the optical carrier frequency. From there, a quadratic increase of the comb linewidth results with mutually anticorrelated phase fluctuations of both spectral wings. Passive phase locking by difference frequency generation strongly enhances the optical phase noise to a level equaling the carrier-envelope phase jitter of the fundamental comb. The same value results from quadratic extrapolation of the optical phase noise to radio frequencies. Our findings are consistent with a fully deterministic transformation of phase noise according to the elastic tape model.

Broadband interferometric subtraction of optical fields.

We present a Mach-Zehnder-like interferometer capable of simultaneous super-octave (950 - 2100 nm) destructive interference with an intensity extinction of 4 × 10-4. Achromatic nulling is achieved by unbalancing the number of Fresnel reflections off optically denser media in the two interferometer arms. With a methane gas sample in one interferometer arm, we isolate the coherent molecular vibrational emission from the broadband, impulsive excitation and quantitatively examine the potential improvement in detectable concentration, compared to direct transmission geometry. The novel concept will benefit sensing applications requiring high detection sensitivity and dynamic range, including time-domain and frequency-domain spectroscopy.

High-power frequency comb at 2 µm wavelength emitted by a Tm-doped fiber laser system.

We report on the generation of a high-power frequency comb in the 2 µm wavelength regime featuring high amplitude and phase stability with unprecedented laser parameters, combining 60 W of average power with <30 fs pulse duration. The key components of the system are a mode-locked Er:fiber laser, a coherence-preserving nonlinear broadening stage, and a high-power Tm-doped fiber chirped-pulse amplifier with subsequent nonlinear self-compression of the pulses. Phase locking of the system resulted in a phase noise of less than 320 mrad measured within the 10 Hz-30 MHz band and 30 mrad in the band from 10 Hz to 1 MHz.

Subcycle quantum electrodynamics.

Squeezed states of electromagnetic radiation have quantum fluctuations below those of the vacuum field. They offer a unique resource for quantum information systems and precision metrology, including gravitational wave detectors, which require unprecedented sensitivity. Since the first experiments on this non-classical form of light, quantum analysis has been based on homodyning techniques and photon correlation measurements. These methods currently function in the visible to near-infrared and microwave spectral ranges. They require a well-defined carrier frequency, and photons contained in a quantum state need to be absorbed or amplified. Quantum non-demolition experiments may be performed to avoid the influence of a measurement in one quadrature, but this procedure comes at the expense of increased uncertainty in another quadrature. Here we generate mid-infrared time-locked patterns of squeezed vacuum noise. After propagation through free space, the quantum fluctuations of the electric field are studied in the time domain using electro-optic sampling with few-femtosecond laser pulses. We directly compare the local noise amplitude to that of bare (that is, unperturbed) vacuum. Our nonlinear approach operates off resonance and, unlike homodyning or photon correlation techniques, without absorption or amplification of the field that is investigated. We find subcycle intervals with noise levels that are substantially less than the amplitude of the vacuum field. As a consequence, there are enhanced fluctuations in adjacent time intervals, owing to Heisenberg's uncertainty principle, which indicate generation of highly correlated quantum radiation. Together with efforts in the far infrared, this work enables the study of elementary quantum dynamics of light and matter in an energy range at the boundary between vacuum and thermal background conditions.

Detection of isocyanates and polychlorinated biphenyls using proton transfer reaction mass spectrometry.

RATIONALE: Isocyanates are highly reactive species widely used in industry. They can cause irritation of the eyes, trigger asthma, etc. Polychlorinated biphenyls (PCBs) were widely used in electrical equipments like capacitors and transformers in the last century and are still present in the environment today. PCBs are known to cause cancer and to affect the immune, reproductive, nervous and endocrine systems. Therefore, there is a need for a simple, rapid and reliable analytical method for the detection of traces of isocyanates and of PCBs. METHODS: The data presented in this paper were obtained using a proton transfer reaction (PTR) time-of-flight mass spectrometer and a high sensitivity PTR quadrupole mass spectrometer. We also utilized a recently developed direct aqueous injection (DAI) inlet system for proton transfer reaction mass spectrometry (PTR-MS) instruments that allows the analysis of trace compounds in liquids. RESULTS: We detected four isocyanates in the headspace above small sample quantities and investigated their fragmentation pathways to obtain a fundamental understanding of the processes involved in proton transfer reactions and also to determine the best operating conditions of the PTR-MS instruments. In addition, nine PCBs were unambiguously identified via their exact mass and isotopic distribution and detected in different concentration levels via direct injection of the liquid. CONCLUSIONS: Utilizing recent developments and improvements in PTR-MS, we can rapidly detect two important environmental pollutant compound classes (isocyanates and PCBs) at high accuracy and without any sample preparation. In this paper, we provide proof of the detection of traces of isocyanates and PCBs in air and also of PCBs in liquids. These results could be used for the development of a real-time monitoring device for industrial waste, polluted air or water quality surveillance.

Use of proton transfer reaction time-of-flight mass spectrometry for the analytical detection of illicit and controlled prescription drugs at room temperature via direct headspace sampling.

The first reported use of proton transfer reaction time-of-flight mass spectrometry (PTR-TOF-MS) for the detection of a range of illicit and prescribed drugs is presented here. We describe the capabilities of PTR-TOF-MS to detect the following commonly used narcotics-ecstasy (N-methyl-3,4-methylenedioxyamphetamine), morphine, codeine, cocaine and heroin-by the direct sampling of the headspace above small solid quantities (approximately 50 mg) of the drugs placed in glass vials at room temperature, i.e. with no heating of the sample and no pre-concentration. We demonstrate in this paper the ability to identify the drugs, both illicit and prescribed, using PTR-TOF-MS through the accurate m/z assignment of the protonated parent molecule to the second decimal place. We have also included in this study measurements with an impure sample of heroin, containing typical substances found in "street" heroin, to illustrate the use of the technology for more "real-world" samples. Therefore, in a real-world complex chemical environment, a high level of confidence can be placed on the detection of drugs. Although the protonated parent is observed for all drugs, the reactant channel leading to this species is not the only one observed and neither is it necessarily the most dominant. Details on the observed fragmentation behaviour are discussed and compared to electrospray ionisation MS(n) studies available in the literature.

Proton transfer reaction mass spectrometry for the sensitive and rapid real-time detection of solid high explosives in air and water.

Relying on recent developments in proton transfer reaction mass spectrometry (PTR-MS), we demonstrate here the capability of detecting solid explosives in air and in water in real time. Two different proton transfer reaction mass spectrometers have been used in this study. One is the PTR-TOF 8000, which has an enhanced mass resolution (m/Δm up to 8,000) and high sensitivity (~50 cps/ppbv). The second is the high-sensitivity PTR-MS, which has an improved limit of detection of about several hundreds of parts per quadrillion by volume and is coupled with a direct aqueous injection device. These instruments have been successfully used to identify and monitor the solid explosive 2,4,6-trinitrotoluene (TNT) by analysing on the one hand the headspace above small quantities of samples at room temperature and from trace quantities not visible to the naked eye placed on surfaces (also demonstrating the usefulness of a simple pre-concentration and thermal desorption technique) and by analysing on the other hand trace compounds in water down to a level of about 100 pptw. The ability to identify even minute amounts of threat compounds, such as explosives, particularly within a complex chemical environment, is vital to the fight against crime and terrorism and is of paramount importance for the appraisal of the fate and harmful effects of TNT at marine ammunition dumping sites and the detection of buried antipersonnel and antitank landmines.

Dissociative electron attachment to pentaerythritol tetranitrate: significant fragmentation near 0 eV.

Gas phase dissociative electron attachment (DEA) measurements to pentaerythritol tetranitrate (PETN) are performed in a crossed electron-molecular beam experiment at high-energy resolution and high sensitivity. DEA is operative at very low energies close to approximately 0 eV showing unique features corresponding to a variety of fragment anions being formed. There is no evidence of the parent anion formation. The fragmentation yields are also observed for higher electron energies and are operative via several resonant features in the range of 0-12 eV. In contrast to nitroaromatic compounds, PETN decays more rapidly upon electron attachment and preferentially low-mass anions are formed. The dominant fragment ion formed through DEA is assigned to the nitrogen trioxide NO(3)(-) and represents about 80% of the total anion yield. Further intense ion signals are due to NO(2)(-) (11%) and O(-) (2.5%). The significant instability of PETN after attachment of an electron with virtually no kinetic energy confers a highly explosive nature to this compound.

Probing royal demolition explosive (1,3,5-trinitro-1,3,5-triazocyclohexane) by low-energy electrons: Strong dissociative electron attachment near 0 eV.

Low energy electron attachment to gas phase royal demolition explosive (RDX) (and RDX-A3) has been performed by means of a crossed electron-molecular beam experiment in an electron energy range from 0 to 14 eV with an energy resolution of approximately 70 meV. The most intense signals are observed at 102 and 46 amu and assigned to C(2)H(4)N(3)O(2) (-) and NO(2) (-), respectively. Anion efficiency curves of 16 anions have been measured. Product ions are observed mainly in the low energy region, near 0 eV arising from surprisingly complex reactions associated with multiple bond cleavages and structural and electronic rearrangement. The remarkable instability of RDX to electron attachment with virtually thermal electrons reflects the highly explosive nature of this compound. The present results are compared to other explosive aromatic nitrocompounds studied in our laboratory recently.