Phase-matched third-harmonic generation in dye solutions

Various dyes are investigated for their capability of generating 3^rd harmonic of Nd-laser light. The feasibility of achieving phase matching is demonstrated by direct measurements of the index of refraction. Third-harmonic conversion of hexamethylindocarbocyanine iodide, diethyloxacarbocyanine iodide and fuchsin red are compared. The highest conversion is obtained for hexamethylindocarbocyanine iodide (80 times higher than in fuchsin red). A further enhancement of the third harmonic by a factor of 70 is expected for the fundamental laser light at 1.15 μm.     

A combined harmonic phase and strain-encoded pulse sequence for measuring three-dimensional strain

Measurement of myocardial strain provides direct information about heart function that can be correlated with disease. We present an MRI pulse sequence that acquires in just six heartbeats both harmonic phase (HARP) and strain-encoded (SENC) images and provides dense measurements of radial, circumferential and longitudinal strains within a single short-axis slice. Normal volunteer data confirm the feasibility of this pulse sequence, and acquired data demonstrate the strain measurement reliability.     

Reconstruction of attosecond pulse trains using an adiabatic phase expansion

We propose a new method to reconstruct the electric field of attosecond pulse trains. The phase of the high-order harmonic emission electric field is Taylor expanded around the maximum of the laser pulse envelope in the time domain and around the central harmonic in the frequency domain. Experimental measurements allow us to determine the coefficients of this expansion and to characterize the radiation with attosecond accuracy over a femtosecond time scale. The method gives access to pulse-to-pulse variations along the train, including the timing, the chirp, and the attosecond carrier envelope phase.     

On the analysis of factors limiting third harmonic generation of Nd:YAlO<Subscript>3</Subscript> laser radiation in barium vapor

The efficiency of generation of the third harmonic (THG) of Nd:YAlO³ laser radiation in barium vapor is analyzed. The influence of the following factors is taken into consideration: the interference of transformed radiation in the case of THG via two channels ω+ω+ω→3ω and 2ω+2ω?ω →3ω, the Kerr effect, two-photon absorption, and the relationship between the energies of the first and second harmonics of radiation. It is shown that the Kerr effect, on the one hand, violates the phase matching conditions in the direct channel but, on the other hand, increases the conversion efficiency due to a defocusing action of the medium, which reduces the influence of the phase detuning between the pumping waves on the process of harmonic generation in the case of generation of difference frequencies. At the same time, the two-photon absorption leads to the depletion of one of the interacting waves (in this case, the second harmonic) and to the prevalence of third harmonic generation through the direct channel.     

Characterization of the third-harmonic radiation generated by intense laser self-formed filaments propagating in air

We perform laboratory experiments to study ultraviolet radiation generated by intense self-formed laser filaments produced by propagating high-power femtosecond laser pulses in air. The laser used in the experiment is a 0.5 TW Ti:sapphire system with the center wavelength at 800 nm. The observed ultraviolet emission occurs in the form of the third harmonic and frequency-upshifted radiation from the fundamental. We present direct characterization of the generated harmonic and frequency-upshifted radiation, including transverse imaging and spatially resolved spectral measurements.     

Aural harmonics: estimation consistency from tone-on-tone masking measurements

Amplitudes of inaudible "subjective" signals are inferred from tone-on-tone masking measurements. Several methodological issues are involved, e.g., the problems of how many and which measurements to include. To explore such sampling questions, sets of masked thresholds using a 250-Hz masker and a 500-Hz maskee are determined at various phase angles. These measurements are distributed into subsamples of (n=) 13, 9, 7, 5, or 4 each, and repeated estimates of the second aural harmonic inferred. For n greater than 4, these estimates averaged to within +/- 1 dB of one another,, although individual estimates vary considerably with the smaller n's (to 13 dB with n=4). Sets of five consecutive measurements also were analyzed when the masker : maskee phase angle was incremented in 90 degrees steps through two complete periods (of the maskee). Again, the reestimates for individuals can vary by at least 6 dB, but the averages over seven subjects remain within 2 dB of each other. Apparently, for n's of six or more, it matters little which masker: maskee phase angles are used to estimate the aural harmonic amplitudes.     

Applications of non-collinear second harmonic generation in one-dimensional periodically poled crystals

Whereas the commonly used collinear quasi-phase-matched interaction is usually suitable for phase matching only a single nonlinear process, non-collinear interactions offer rich variety of phase matching possibilities, employing different elements of the nonlinear tensor, and various orders of quasi phase matching. By studying the dependence of the second harmonic process on the angle and the phase matching order in a periodically poled LiNbO₃ crystal, we can non-destructively derive the duty cycle of the structure. The obtained duty cycle agrees well with surface scan measurements using an atomic force microscope. Furthermore, by proper selection of the non-collinear angle, we demonstrate simultaneous second harmonic generation of two cross polarized waves.     

Interferometric measurement of the atomic dipole phase for the two electronic quantum paths generating high-order harmonics

The first direct measurement of the atomic dipole phase in high-order harmonic generation obtained by an interferometric technique in the extreme ultraviolet (XUV) is reported. The effects associated with both the main quantum paths involved in the emission of a given plateau harmonic are investigated and compared with the expectations of the semi-classical model, which offers a very simple physical picture for the understanding of the involved electronic dynamics.     

High harmonic XUV spectral phase interferometry for direct electric-field reconstruction

We demonstrate the first experimental complete temporal characterization of high-harmonic XUV pulses by spectral phase interferometry, with an all-optical setup. This method allows us to perform single-shot measurements of the harmonic temporal profile and phase, revealing a remarkable shot-to-shot stability. We characterize harmonics generated in argon by a 50 fs 800 nm laser pulse. The 11th harmonic is found to be 22 fs long with a negative chirp rate of -4.8 x 10(27) s(-2). This duration can be reduced to 13 fs by modulating the polarization of the generating laser. The technique is easy to implement and could be routinely used in femtosecond XUV pump-probe experiments with harmonics.     

Single-shot measurement of carrier-envelope phase changes by spectral interferometry

We demonstrated single-shot measurements of spectral interference between a white-light continuum generated in a hollow-fiber and its second harmonic. The interference has information on the carrier-envelope phase of an input pulse to the fiber and the time delay of the blue wing of the continuum. By analyzing the observed spectral interference, we estimated shot-by-shot changes of the carrier-envelope phase. This method is useful for determining the carrier-envelope phase changes of a low-repetition-rate, high-intensity laser.     

Global optimization of high harmonic generation

We investigate the relevance of the absorption limit concept in the optimization of high harmonic generation. Thanks to the first direct observation of the coherence length of the process from high-contrast Maker fringes, we unravel experimental conditions for which the harmonic dipole response is enhanced when phase matching is realized within the absorption limit, leading to record conversion efficiencies in argon. Moreover, we show that harmonic generation in guided or freely propagating geometries are equivalent in the loose focusing regime. This analysis is generalized to other advanced phase-matching schemes, thereby predicting the possibility to boost the conversion efficiencies using light noble gases.     

Ionization phase-match gating for wavelength-tunable isolated attosecond pulse generation

High-order harmonic emission can be confined to the leading edge of an 800 nm driver laser pulse under moderately intense focusing conditions (7×10¹⁴ W/cm²) (Pfeifer et al. in Opt. Express 15:17120, 2007). Here, the experimentally observed curtailment of harmonic production on the leading edge of the driver pulse is shown to be controlled by an ionization-induced phase-matching condition. The transient plasma density inherent to the process of high-harmonic generation terminates the harmonic emission by an ultrafast loss of phase matching on the leading edge of the laser pulse. The analysis is supported by a reconstruction of the in situ intensity envelope of the driver pulse with attosecond temporal resolution, performed by measurements of the carrier-envelope phase dependence of individual half-cycle harmonic cutoffs. The method opens the way to wavelength-tunable isolated attosecond pulse generation.     

Pulse‐shape instabilities and their measurement

Multi‐shot pulse‐shape measurements of trains of ultrashort pulses with unstable pulse shapes are studied. Measurement techniques considered include spectral‐phase interferometry for direct electric‐field reconstruction (SPIDER), second harmonic generation frequency‐resolved optical gating (FROG), polarization gate FROG, and cross‐correlation FROG. An analytical calculation and simulations show that SPIDER cannot see unstable pulse‐shape components and only measures the coherent artifact. Further, the presence of this instability cannot be distinguished from benign misalignment effects in SPIDER. FROG methods yield a better, although necessarily rough, estimate of the pulse shape and also indicate instability by exhibiting disagreement between measured and retrieved traces. Only good agreement between measured and retrieved FROG traces or 100% SPIDER fringe visibility guarantees a stable pulse train.     

Ultralow noise and supermode suppression in an actively mode-locked external-cavity semiconductor diode ring laser

We report what is to our knowledge the lowest phase and amplitude noise characteristics achieved to date in a 10-GHz pulse train produced by the active harmonic mode locking of an external-cavity semiconductor diode laser. Supermode noise has also been suppressed below -140 dBc/Hz by use of a high-finesse fiber Fabry-Perot etalon as an intracavity filter. Novel noise sideband measurements that extend to the Nyquist offset frequency suggest a significant advantage in using harmonic (rather than fundamental) mode locking to produce ultralow-noise pulse trains, owing to the relationship between the noise roll-off frequency and the fundamental cavity frequency.     

Signatures of resonance superfluidity in a quantum Fermi gas

We predict a direct and observable signature of the superfluid phase in a quantum Fermi gas, in a temperature regime already accessible in current experiments. We apply the theory of resonance superfluidity to a gas confined in a harmonic potential and demonstrate that a significant increase in density will be observed in the vicinity of the trap center.     

Twist-grain-boundary structure in the B4 phase of a bent-core molecular system identified by second harmonic generation circular dichroism measurement

Second harmonic generation circular dichroism (SHG CD) measurements are performed on the B4 phase of a bent-core molecular system. Numerical analysis of SHG CD incorporating magnetic-dipole as well as electric-dipole interaction shows that the B4 phase is in a twist-grain-boundary structure with the helical axis along the bent direction of the molecules. The result is extremely important in the sense that achiral molecules are spontaneously optically resolved, i.e., deracemization, the chiral domains of which give rise to huge chiral nonlinear optical effect.     

On the correspondence of semiclassical and quantum phases in cyclic evolutions

Based on the exactly solvable case of a harmonic oscillator, we show that the direct correspondence between the Bohr-Sommerfeld phase of semiclassical quantum mechanics and the topological phase of Aharonov and Anandan is restricted to the case of a coherent state. For other Gaussian wave packets the geometric quantum phase strongly depends on the amount of squeezing.We dedicate this paper to our friend Professor Asim O. Barut—a great scientist, an extraordinary human being, and a man who enjoys the whole world as his home.     

Coherence and the statistics of the phase difference between partially polarized electromagnetic waves

Coherence between two vectorial harmonic light vibrations is analyzed in terms of the statistics of their phase difference. This provides a natural and simple extension of second-order coherence to cover more complicate situations. In particular this assigns large coherence to quantum light states providing the most accurate interferometric measurements allowed by the quantum theory, even if they are incoherent according to the standard second-order approach.     

Transition de phase dans le nitrate de calcium Ca(NO3)2

The phase transition in calcium nitrate at 14° C has been investigated. Results of second harmonic generation (S.H.G.), dielectric measurements and other investigations are presented. S.H.G. experiments indicate absence of inversion center below 14° C. The phase transition is clearly distinguished by the disappearance of the SHG signal at 14° C. Dielectric measurements show a sharp maximum of ε′ at 14° C. Ferroelectric hysteresis loop is not observed below this temperature. Physical properties of calcium nitrate and cadmium nitrate are also compared.     

Solid-state carrier-envelope phase stabilization via quantum interference control of injected photocurrents

We demonstrate carrier-envelope phase stabilization of a mode-locked Ti:sapphire laser by use of quantum interference control of injected photocurrents in a semiconductor. No harmonic generation is required for this stabilization technique. Instead, interference between coexisting single- and two-photon absorption pathways in the semiconductor provides a phase comparison between different spectral components. The phase comparison, and the detection of the photocurrent that it produces, both occur within a single low-temperature-grown gallium arsenide sample. The carrier-envelope offset beat note fidelity is 30 dB in a 10-kHz resolution bandwidth. The out-of-loop phase-noise level is essentially identical to the best previous measurements with the standard self-referencing technique.