Showing papers in "Applied Physics B in 2005"
TL;DR: In this article, the working mechanisms of femtosecond laser nanoprocessing in biomaterials with oscillator pulses of 80-MHz repetition rate and with amplified pulses of 1-kHz repetition rate were investigated.
Abstract: We review recent advances in laser cell surgery, and investigate the working mechanisms of femtosecond laser nanoprocessing in biomaterials with oscillator pulses of 80-MHz repetition rate and with amplified pulses of 1-kHz repetition rate. Plasma formation in water, the evolution of the temperature distribution, thermoelastic stress generation, and stress-induced bubble formation are numerically simulated for NA=1.3, and the outcome is compared to experimental results. Mechanisms and the spatial resolution of femtosecond laser surgery are then compared to the features of continuous-wave (cw) microbeams. We find that free electrons are produced in a fairly large irradiance range below the optical breakdown threshold, with a deterministic relationship between free-electron density and irradiance. This provides a large ‘tuning range’ for the creation of spatially extremely confined chemical, thermal, and mechanical effects via free-electron generation. Dissection at 80-MHz repetition rate is performed in the low-density plasma regime at pulse energies well below the optical breakdown threshold and only slightly higher than used for nonlinear imaging. It is mediated by free-electron-induced chemical decomposition (bond breaking) in conjunction with multiphoton-induced chemistry, and hardly related to heating or thermoelastic stresses. When the energy is raised, accumulative heating occurs and long-lasting bubbles are produced by tissue dissociation into volatile fragments, which is usually unwanted. By contrast, dissection at 1-kHz repetition rate is performed using more than 10-fold larger pulse energies and relies on thermoelastically induced formation of minute transient cavities with lifetimes <100 ns. Both modes of femtosecond laser nanoprocessing can achieve a 2–3 fold better precision than cell surgery using cw irradiation, and enable manipulation at arbitrary locations.
1,226 citations
TL;DR: The use of coherent population trapping (CPT) in alkali-metal atoms in the implementation of atomic frequency standards is discussed in this article, where a review of field and laboratory implementations using these approaches is made.
Abstract: The paper gives an overview of the use of the coherent population trapping phenomenon (CPT) in alkali-metal atoms in the implementation of atomic frequency standards. Several avenues are examined. These include: the approach using a combination of the CPT phenomenon and the Ramsey separated interaction field technique on an atomic beam; the passive approach in a cell in which the microwave hyperfine resonance excited by the CPT phenomenon is detected directly on the transmitted radiation; the maser approach in which the same resonance is observed by means of stimulated emission in a microwave cavity-cell arrangement; and, finally, the proposed approach using pulses in a time sequence that implements the combined CPT–Ramsey separated interaction field technique in time rather than in space. A review of field and laboratory implementations using these approaches is made.
575 citations
TL;DR: In this article, the ab initio plane-wave pseudopotential energy method is combined with the anionic group theory to evaluate the degree of approximation of the group theory for nonlinear optical crystals.
Abstract: The developments of nonlinear optical (NLO) crystals in the last decade are reviewed from two aspects. In the theoretical part, on the ab initio plane-wave pseudopotential energy method, calculation of the optical responses, including the second harmonic generation coefficients, a computer-program package, and a real-space atom-cutting method are described and employed to evaluate the degree of approximation of the anionic group theory for nonlinear optical crystals. The ab initio method combined with the anionic group theory gives much more insight into the understanding of the relationship between the optical responses and microscopic structures of NLO crystals, borate-based NLO crystals in particular. In the experimental part, we describe how to use the anionic group theory method and an experimental evaluation system to develop a series of borate-based UV-NLO crystals such as KBe2BO3F2 (KBBF) and K2Al2B2O7 (KABO). The capability of these new UV-NLO crystals in producing deep-UV harmonic output is evaluated and determined. Finally, recent advances in the generation of deep- and vacuum-UV harmonic output with these new NLO crystals are reported.
343 citations
TL;DR: An external cavity (EC) quantum cascade laser (QCL) configuration with the thermoelectrically cooled gain medium fabricated using a bound-to-continuum design and operating in continuous wave at ∼5.2 μm is reported in this article.
Abstract: An external cavity (EC) quantum cascade laser (QCL) configuration with the thermoelectrically cooled gain medium fabricated using a bound-to-continuum design and operating in continuous wave at ∼5.2 μm is reported. The EC architecture employs a piezo-activated cavity mode tracking system for mode-hop free operation suitable for high resolution spectroscopic applications and multiple species trace-gas detection. The performance of the EC-QCL exhibits coarse single mode tuning over 35 cm-1 and a continuous mode-hop free fine tuning range of ∼1.2 cm-1.
254 citations
TL;DR: In this article, a cavity enhanced absorption spectroscopy (OF-CEAS) was proposed to record the transmission maxima of the successive TEMoo resonances of a high-finesse optical cavity when a Distributed Feedback Diode Laser is tuned across them.
Abstract: A new technique of cavity enhanced absorption spectroscopy is described. Molecular absorption spectra are obtained by recording the transmission maxima of the successive TEMoo resonances of a high-finesse optical cavity when a Distributed Feedback Diode Laser is tuned across them. A noisy cavity output is usually observed in such a measurement since the resonances are spectrally narrower than the laser. We show that a folded (V-shaped) cavity can be used to obtain selective optical feedback from the intracavity field which builds up at resonance. This induces laser linewidth reduction and frequency locking. The linewidth narrowing eliminates the noisy cavity output, and allows measuring the maximum mode transmissions accurately. The frequency locking permits the laser to scan stepwise through the successive cavity modes. Frequency tuning is thus tightly optimized for cavity mode injection. Our setup for this technique of Optical-Feedback Cavity-Enhanced Absorption Spectroscopy (OF-CEAS) includes a 50 cm folded cavity with finesse ∼20 000 (ringdown time ∼20 μs) and allows recording spectra of up to 200 cavity modes (2 cm−1) using 100 ms laser scans. We obtain a noise equivalent absorption coefficient of ∼5×10−10 cm−1 for 1 s averaging over scans, with a dynamic range of four orders of magnitude.
235 citations
TL;DR: In this paper, the plasmon polariton wavelength of arrays of silver nanoparticles was measured with increasing exposure to ambient laboratory air and it was shown that the resonance peak wavelength shifts 65 nm in 36 h (1.8 nm/h).
Abstract: Silver has useful surface-plasmon-resonance properties for many potential applications. However, chemical activity in silver nanoparticles exposed to laboratory air can make interpretation of optical scattering and extinction spectra problematic. We have measured the shift of the plasmon polariton wavelength of arrays of silver nanoparticles with increasing exposure to ambient laboratory air. The resonance peak wavelength shifts 65 nm in 36 h (1.8 nm/h). We show by scanning Auger spectroscopy that the shift is due to contamination from sulfur, most likely chemisorbed on the surface. The rate of corrosion product growth on the nanoparticles is estimated to be 3 nm per day, 7.5 times higher than that of bulk Ag under the same conditions.
204 citations
TL;DR: In this article, the optimal design of photonic band gaps for two-dimensional square lattices is considered and the level set method is used to represent the interface between two materials with two different dielectric constants.
Abstract: The optimal design of photonic band gaps for two-dimensional square lattices is considered. We use the level set method to represent the interface between two materials with two different dielectric constants. The interface is moved by a generalized gradient ascent method. The biggest gap of GaAs in air that we found is 0.4418 for TM (transverse magnetic field) and 0.2104 for TE (transverse electric field).
198 citations
TL;DR: This review summarizes these techniques of ferroelectric domain patterning and highlights systematically their strengths and weaknesses.
Abstract: In recent years ferroelectric domain patterning has become a popular topic of physical research because it enables photonic applications as well as data storage. For generation of tailored domain structures and for further understanding of ferroelectricity, a visualization of the domain patterns is required. A large number of imaging techniques have therefore been developed. This review summarizes these techniques and highlights systematically their strengths and weaknesses.
173 citations
TL;DR: In this article, the 12 best NIR water transition line pairs for temperature measurements with a single DFB laser in flames are determined by systematic analysis of the HITRAN simulation of the water spectra in the 1-2-μm spectral region.
Abstract: The 12 best NIR water transition line pairs for temperature measurements with a single DFB laser in flames are determined by systematic analysis of the HITRAN simulation of the water spectra in the 1–2 μm spectral region. A specific line pair near 1.4 μm was targeted for non-intrusive measurements of gas temperature in combustion systems using a scanned-wavelength technique with wavelength modulation and 2f detection. This sensor uses a single diode laser (distributed-feedback), operating near 1.4 μm and is wavelength scanned over a pair of H2O absorption transitions (7154.354 cm-1 & 7153.748 cm-1) at a 2 kHz repetition rate. The wavelength is modulated (f=500 kHz) with modulation amplitude a=0.056 cm-1. Gas temperature is inferred from the ratio of the second harmonic signals of the two selected H2O transitions. The fiber-coupled-single-laser design makes the system compact, rugged, low cost and simple to assemble. As part of the sensor development effort, design rules were applied to optimize the line selection, and fundamental spectroscopic parameters of the selected transitions were determined via laboratory measurements including the temperature-dependent line strength, self-broadening coefficients, and air-broadening coefficients. The new sensor design includes considerations of hardware and software to enable fast data acquisition and analysis; a temperature readout rate of 2 kHz was demonstrated for measurements in a laboratory flame at atmospheric pressure. The combination of scanned-wavelength and wavelength-modulation minimizes interference from emission and beam steering, resulting in a robust temperature sensor that is promising for combustion control applications.
155 citations
TL;DR: In this paper, a trace gas sensor based on photoacoustic spectroscopy with a quantum cascade laser operating at 4.55 μm as an excitation source was developed for the detection of N2O and CO.
Abstract: A trace gas sensor based on quartz-enhanced photoacoustic spectroscopy with a quantum cascade laser operating at 4.55 μm as an excitation source was developed. The sensor performance was evaluated for the detection of N2O and CO. A noise-equivalent (1σ) sensitivity of 4 ppbv N2O with 3 s response time to (1-1/e) of the steady-state level was demonstrated. The influence of the relevant energy transfer processes on the detection limits was analyzed. Approaches to improve the current sensor performance are also discussed.
143 citations
TL;DR: In this article, the authors presented two novel semiconductor saturable absorber mirror (SESAM) designs which can exhibit more than ten times lower saturation fluence than classical SESAM devices.
Abstract: We present two novel semiconductor saturable absorber mirror (SESAM) designs which can exhibit more than ten times lower saturation fluence than classical SESAM devices. Design considerations and characterization data are presented. These devices are particularly suited for passively mode-locked lasers with ultra-high repetition rates.
TL;DR: In this paper, a new Yb-doped crystal Yb3+:Lu2SiO5 (Yb:LSO) under high power diode-pumping (15 W) was reported.
Abstract: We report here on the efficient laser action of a new Yb-doped crystal Yb3+:Lu2SiO5 (Yb:LSO) under high power diode-pumping (15 W). Its performances were compared to another Yb-doped crystal belonging to the oxyorthosilicate family: the Yb:YSO. For both crystals, more than 7 W of laser radiation around 1 μm was obtained under 14.4 W of incident pump power at 978 nm, leading to high optical conversions of more than 50%. Finally, both crystals demonstrate little sensitivity to pump wavelength drift and exhibit broad tunability at a multiwatt level (more than 4 W over 50 nm and 60 nm, respectively for Yb:LSO and Yb:YSO).
TL;DR: In this article, a semi-analytical extension of the classic volume-current method to low-index contrast systems was proposed, which corrected a longstanding limitation of such methods to low index contrast systems (the classic method may be off by an order of magnitude in high contrast systems).
Abstract: We present predicted relative scattering losses from sidewall roughness in a strip waveguide compared to an identical waveguide surrounded by a photonic crystal with a complete or incomplete gap in both 2d and 3d. To do so, we develop a new semi-analytical extension of the classic “volume-current method” (Green’s functions with a Born approximation), correcting a longstanding limitation of such methods to low-index contrast systems (the classic method may be off by an order of magnitude in high-contrast systems). The resulting loss predictions show that even incomplete gap structures such as photonic-crystal slabs should, with proper design, be able to reduce losses by a factor of two compared to an identical strip waveguide; however, incautious design can lead to increased losses in the photonic-crystal system, a phenomena that we explain in terms of the band structure of the unperturbed crystal.
TL;DR: In this article, the spectral and polarization dependence of light scattering by individual nanoscopic metal wire tips is investigated using evanescent-wave exci- tation, and the results provide selection criteria for tips to be used in scattering-type near-field microscopy or for photoemission in inelastic tunneling spectroscopy.
Abstract: Linear light scattering by individual nanoscopic metal wire tips is investigated. Using evanescent-wave exci- tation, the spectral and polarization dependence of the emis- sion are addressed. Choosing gold and tungsten as represen- tative tip materials, intense scattering and a strongly plasmon- resonant behavior observed for gold contrasts a comparatively weak and spectrally flat response for tungsten. Spectral depen- dence and local-field enhancement are found to be sensitive to details of the structural parameters and can be described by a simple model. The results provide selection criteria for tips to be used in scattering-type near-field microscopy or for photoemission in inelastic tunneling spectroscopy.
TL;DR: In this paper, the authors show that the measured rollover is consistent with two-photon absorption only for short (femtosecond) pulses, while a stronger (yet unidentified) kind of nonlinear absorption is dominant for longer (picosecond), and discuss strategies to enhance or reduce this induced absorption.
Abstract: The reflectivity of a semiconductor saturable absorber mirror (SESAM) is generally expected to increase with increasing pulse energy. However, for higher pulse energies the reflectivity can decrease again; we call this a ‘roll-over’ of the nonlinear reflectivity curve caused by inverse saturable absorption. We show for several SESAMs that the measured roll-over is consistent with two-photon absorption only for short (femtosecond) pulses, while a stronger (yet unidentified) kind of nonlinear absorption is dominant for longer (picosecond) pulses. These inverse saturable absorption effects have important technological consequences, e.g. for the Q-switching dynamics of passively mode-locked lasers. A simple equation using only measurable SESAM parameters and including inverse saturable absorption is derived for the Q-switched mode-locking threshold. We present various data and discuss the sometimes detrimental effects of this roll-over for femtosecond high repetition rate lasers, as well as the potentially very useful consequences for passively mode-locked multi-GHz lasers. We also discuss strategies to enhance or reduce this induced absorption by using different SESAM designs or semiconductor materials.
TL;DR: In this paper, the authors analyzed the properties of magneto-optical defect states in two-dimensional photonic crystals and proposed a junction circulator structure with a magnetooptical cavity coupled to three waveguides.
Abstract: We analyze the properties of magneto-optical defect states in two-dimensional photonic crystals. With out-of-plane magnetization, the magneto-optical coupling splits doubly-degenerate TE states into two counter-rotating modes at different frequencies. The strength of magneto-optical coupling strongly depends on the spatial overlap of the cavity domain structures and the cross product of the modal fields. The transport property of the resultant nonreciprocal states is demonstrated in a junction circulator structure with a magneto-optical cavity coupled to three waveguides. By a proper matching of the magneto-optical frequency splitting with the cavity decay rate into the waveguide, ideal three-port circulator characteristics with complete isolation and transmission can be achieved, with an operational bandwidth proportional to the magneto-optical constant. The proposed optical circulator in a bismuth-iron-garnet/air photonic crystal is demonstrated with finite-difference time-domain calculations and is compared to an alternative implementation of silicon/air crystal infiltrated with a single bismuth-iron-garnet domain.
TL;DR: In this article, the effect of the frequency dependence of the effective mode area in photonic crystal fiber on supercontinuum generation was quantified through a modified optical shock term and identified the major consequence as a reduction in the extreme long-wavelength edge of the super-continuum spectrum.
Abstract: Numerical simulations are used to study the effect of the frequency dependence of the effective mode area in photonic crystal fiber on supercontinuum generation. We quantify how the frequency dependence of the effective area influences the propagation characteristics through a modified optical shock term and identify the major consequence as a reduction in the extreme long-wavelength edge of the supercontinuum spectrum. Our results show that, for the parameter regimes used in many previous supercontinuum generation experiments using near-infrared femtosecond pump sources around 800 nm, this effect would be expected to be negligible. However, for pumps in the 1000–1500 nm range, the inclusion of this effect would be expected to be crucial for accurate comparison of simulations with experiment.
TL;DR: In this paper, an experimental set-up to generate laser beams with locally varying polarization distribution is presented, and a radially polarized beam of high quality regarding intensity distribution, polarization and phase front distortion is generated.
Abstract: We present an experimental set-up to generate laser beams with locally varying polarization distribution. In a linear set-up, a radially polarized beam of high quality regarding intensity distribution, polarization and phase-front distortion is generated. This beam can be used for tight focusing. Further applications are discussed.
TL;DR: In this article, a field-deployable, pulsed quantum cascade laser spectrometer was proposed to measure the 13C/12C isotopic ratio in CO2 released from volcanic vents.
Abstract: We report the development of a field-deployable, pulsed quantum cascade laser spectrometer. The instrument is designed to measure the 13C/12C isotopic ratio in the CO2 released from volcanic vents. Specific 12CO2 and 13CO2 absorption lines were selected around 4.3 μm, where the P-branch of 12CO2 overlaps the R-branch of 13CO2 of the 0001–0000 vibrational transition. This particular selection makes the instrument insensitive to temperature variations. A dual-channel cell balances the two absorption signals. We provide details of the instrument design and a preliminary demonstration of its performance based on laboratory measurements of 16O12C16O and 16O12C18O.
TL;DR: In this article, a pulsed quantum cascade laser (QCL)-based spectroscopic trace gas sensor for subpart-per-million detection of nitric oxide (NO) and capable of monitoring other molecular species such as CO2, H2O, and NH3 in industrial combustion exhaust systems is reported.
Abstract: Development of a pulsed quantum cascade laser (QCL)-based spectroscopic trace-gas sensor for sub-part-per-million detection of nitric oxide (NO) and capable of monitoring other molecular species such as CO2, H2O, and NH3 in industrial combustion exhaust systems is reported. Rapid frequency modulation is applied to the QCL to minimize the influence of fluctuating non-selective absorption. A novel method utilizes only a few laser pulses within a single wavelength scan to probe an absorption spectrum at precisely selected optical frequencies. A high-temperature gas cell was used for laboratory evaluation of the NO sensor performance. A noise-equivalent sensitivity (1σ) of ∼ 100 ppb × m/\(\sqrt{Hz}\) at room temperature and ∼ 200 ppb × m/\(\sqrt{Hz}\) at 630 K was achieved by measuring the NO R(6.5) absorption doublet at 1900.075 cm−1.
TL;DR: In this paper, a few-cycle 0.2 TW optical parametric chirped pulse amplifier system operating at 20 Hz was characterized and a specially designed third-order correlator was used to characterize and optimize the contrast of the system.
Abstract: We report on the pulse contrast-ratio characterization of a few-cycle 0.2 TW optical parametric chirped pulse amplifier system operating at 20 Hz. A specially designed third-order correlator was used to characterize and optimize the contrast of the system. We demonstrate that the pulse contrast depends much more on the temporal overlap between the pump and the seed pulse than the shape of the amplified spectrum. The best amplified pulse contrast-ratio was 10-4 at Δt=±25 ps and >10-9 at Δt=±150 ps delays.
TL;DR: In this article, the authors describe the leakage of the core mode that leads to long-wavelength loss, influencing the operational bandwidth of these tapers, and explore these regimes as a potential basis for evanescent field sensing applications.
Abstract: Microstructured optical fibres (MOFs) have attracted much interest in recent times, due to their unique waveguiding properties that are vastly different from those of conventional step-index fibres. Tapering of these MOFs promises to significantly extend and enhance their capabilities. In this paper, we review the fabrication and characterisation techniques of these fibre tapers, and explore their fundamental waveguiding properties and potential applications. We fabricate photonic crystal fibre tapers without collapsing the air-holes, and confirm this with a non-invasive probing technique that enables the characterisation of the internal microstructure along the taper. We then describe the fundamental property of such tapers associated with the leakage of the core mode that leads to long-wavelength loss, influencing the operational bandwidth of these tapers. We also revisit the waveguiding properties in another form of tapered MOF photonic wires, which transition through waveguiding regimes associated with how strongly the mode is isolated from the external environment. We explore these regimes as a potential basis for evanescent field sensing applications, in which we can take advantage of air-hole collapse as an extra dimension to these photonic wires.
TL;DR: Gas phase O2 quenching of toluene laser-induced fluorescence (LIF) was studied at 300-650 K in a N2/O2 bath gas of 1-bar total pressure with O2 partial pressures.
Abstract: Gas-phase O2 quenching of toluene laser-induced fluorescence (LIF) is studied at 300-650 K in a N2/O2 bath gas of 1-bar total pressure with O2 partial pressures?400 mbar. With increasing vibrational excitation of the laser-excited toluene, intramol. decay becomes faster, resulting in a decreasing relative strength of collisional quenching by O2. Stern-Volmer plots are nonlinear for temps. >500 K in the case of 266-nm excitation and at all temps. for 248-nm excitation. This is attributed to the onset of internal conversion from specific vibrational levels. A photophys. model is developed that describes the exptl. data and predicts toluene LIF signal strengths for higher O partial pressures. For practical application, O2 quenching is not the dominant deexcitation process for engine-related temp. and pressure conditions, and application of the popular fuel-air ratio LIF (FARLIF) concept leads to erroneous signal interpretation.
TL;DR: In this paper, a broad-area vertical cavity surface emitting laser (VCSEL) with a saturable absorber was studied and the presence of cavity solitons was shown numerically.
Abstract: We study theoretically a broad-area vertical cavity surface emitting laser (VCSEL) with a saturable absorber. We show numerically the presence of cavity solitons in the system: they exist as solitary structures formed through a modulationally unstable homogeneous lasing state that coexists with a background with zero intensity. Such a peculiar scenario endows the solitons with unique properties compared to cavity solitons in most previously studied optical systems. In particular, these solitons do not as such rely on a proper phase of the addressing pulses to be either created or deleted. We show that exciting and deleting the solitons depend crucially on whether a threshold in the soliton peak has been reached.
TL;DR: In this article, the authors report on the generation and measurement of a > 108 contrast ratio between main pulse and amplified spontaneous emission (ASE) from a relativistic kHz chirped-pulse amplified laser.
Abstract: We report on the generation and measurement of a > 108 contrast ratio between main pulse and amplified spontaneous emission (ASE) from a relativistic kHz chirped-pulse amplified laser. We have enhanced the ASE contrast ratio as much as > 400 times by employing a pulse cleaner composed of a μJ preamplifier and a saturable absorber. A third-order cross-correlator with a dynamic range of > 109 and a scanning range of up to 4 ns has been developed for the contrast measurement. Detailed analysis of the cross-correlation trace shows that the random noise of spectral phase generates 20-ps pedestal structure starting from 10−6 level of the main pulse.
TL;DR: In this paper, the propagation of femtosecond terawatt laser pulses at reduced pressure (0.7 atm) was investigated experimentally, and it was shown that the nonlinear refractive index n2 is reduced by 30%, resulting in a slightly farther filamentation onset and a reduction of the filament number.
Abstract: The propagation of femtosecond terawatt laser pulses at reduced pressure (0.7 atm) is investigated experimentally. In such conditions, the non-linear refractive index n2 is reduced by 30%, resulting in a slightly farther filamentation onset and a reduction of the filament number. However, the filamentation process, especially the filament length, is not qualitatively affected. We also show that drizzle does not prevent the filaments from forming and propagating.
TL;DR: In this article, a collinear double-pulse laser-induced plasma was characterised by means of a spectrally and time resolved imaging technique, where the beams of two Q-switched Nd:YAG lasers were focused on a brass target in a vacuum chamber to form the plasma.
Abstract: A collinear double pulse laser-induced plasma was characterised by means of a spectrally and time resolved imaging technique. The beams of two Q-switched Nd:YAG lasers were focused on a brass target in a vacuum chamber to form the plasma. The plume emission intensity and spatial distribution were recorded with temporal resolution using an intensified CCD. Using a set of interference filters, we collected images of the emission from the major target components as well as from oxygen. Both the laser inter-pulse separation (in the range between 0 and 10 μ s) and the ambient air pressure value (in the range between 105 and 10 Pa) were varied during the experiment. At atmospheric pressure, an enhancement of the line emission from the target elements was observed for delayed laser pulses compared to coincident pulses. However, this enhancement effect tends to fall at low pressure values, and a decrease of the signal is observed for pressures under about 104 Pa. Moreover, it was observed that the evaluation of the enhancement factor strongly depends on the detector field of view. The propagation of the emitting plume was also studied at several pressures and inter-pulse delays.
TL;DR: In this paper, a cavity-enhanced spectrometer was developed for detection of exhaled nitric oxide in human breath using a thermoelectrically cooled, pulsed, quantum cascade laser coupled to a high-finesse cavity.
Abstract: A cavity-enhanced spectrometer is developed for detection of exhaled nitric oxide in human breath. A thermoelectrically cooled, pulsed, quantum cascade laser, coupled to a high-finesse cavity, is used for trace-gas measurements. The trace-gas analyzer operates at 5.2 microns and utilizes integrated cavity output spectroscopy. Effective optical path lengths of 1.5 km are achieved in a 50-cm-length cell with a sample volume of 60 mL. The instrument is also capable of simultaneously measuringCO2 concentration in exhaled breath. Measurements were performed on human breath samples as well as simulated breath samples. Here we report a detection limit of ≤ 1 ppbv in 4 s for NO in human breath samples.
TL;DR: In this article, the damping of Rabi oscillations in quantum dots as well as the renormalization of the carrier-light coupling due to the interaction with longitudinal acoustic phonons are studied as a function of temperature and laser pulse parameters.
Abstract: The damping of Rabi oscillations in quantum dots as well as the renormalization of the carrier-light coupling, due to the interaction with longitudinal acoustic phonons are studied as a function of temperature and laser pulse parameters. Numerical results are obtained by using a correlation expansion within the density matrix theory. The observed features like a non-monotonous dependence of the damping on the pulse duration are characteristic for the strongly non-Markovian nature of the phonon coupling in these systems. The results can be well interpreted on the level of a perturbation expansion in the carrier-phonon interaction.
TL;DR: In this article, a method for controlling randomly generated multiple filaments during the propagation of femtosecond laser pulses in optical media is proposed, which is based on introducing a periodic amplitude modulation of the transverse beam profile.
Abstract: We propose a new method for controlling randomly generated multiple filaments during the propagation of femtosecond laser pulses in optical media. The method is based on introducing a periodic amplitude modulation of the transverse beam profile. It is shown both experimentally and numerically that the introduction of a periodic mesh into a propagation path of a femtosecond near-infrared laser pulse leads to a deterministic spatial distribution of multiple filaments in the presence of initial random fluctuations. As a result, the number of filaments is increased as compared to the random case.