Direct observation of population inversion between Al+11 levels in a laser-produced plasma

Measured X-ray intensities of the resonance line series of Al⁺¹¹ in a laser-produced plasma shows population inversion between the n = 4,5 levels and the n =3 level at a plasma density N_e～10²⁰ cm^-3. The cooling of the expanding plasma leading to inversion is enhanced by a special target configuration. The gain coefficient in the 4 → 3 transition at 129.7 Å is estimated to be ～ 10 cm^-1, using both measured line intensities and numerical simulation.     

Design of a constant adiabaticity pulse for selective population inversion

A new adiabatic pulse for population inversion and the principles of its design are presented. The pulse shape is characterized by the combination of two constraints. (i) Adiabatic following of the central isochromat of the spectral region of interest occurs with constant, possibly small adiabaticity parameter; thereby, the center isochromat gets most efficiently inverted. (ii) Frequency and amplitude modulations obey the principle of offset-independent adiabaticity; thus, the inversion dynamics of the center isochromat is extended over the desired bandwidth. Selective population inversion can be achieved rather independently of spatial radio frequency field inhomogeneities and with significantly reduced peak RF amplitude in comparison with the well-known sech/tanh adiabatic pulse.     

Direct frequency comb measurements of absolute optical frequencies and population transfer dynamics

A phase-stabilized femtosecond laser comb is directly used for high-resolution spectroscopy and absolute optical frequency measurements of one- and two-photon transitions in laser-cooled 87Rb atoms. Absolute atomic transition frequencies, such as the 5S1/2 F=2-->7S1/2 F"=2 two-photon resonance measured at 788,794 768,921 (44) kHz, are determined without a priori knowledge about their values. Detailed dynamics of population transfer driven by a sequence of pulses are uncovered and taken into account for the measurement of the 5P states via resonantly enhanced two-photon transitions.     

Femtosecond population inversion and stimulated emission of dense Dirac fermions in graphene

We show that strongly photoexcited graphene monolayers with 35 fs pulses quasi-instantaneously build up a broadband, inverted Dirac fermion population. Optical gain emerges and directly manifests itself via a negative conductivity at the near-infrared region for the first 200 fs, where stimulated emission completely compensates absorption loss in the graphene layer. Our experiment-theory comparison with two distinct electron and hole chemical potentials reproduce absorption saturation and gain at 40 fs, revealing, particularly, the evolution of the transient state from a hot classical gas to a dense quantum fluid with increasing the photoexcitation.     

Calculation of accurate CO2 laser transition frequencies and their standard deviations

The paper is based on C¹²O₂ ¹⁶ rotational constants and line frequency data gained from laser frequency measurements performed recently. Equations for transition frequencies and their standard deviations have been derived. A list is presented containing accurate 10.4 and 9.4 μm line frequencies and their relative and absolute standard deviations; also wavenumbers and wavelengths are given for each line. For transitions usually observed in CO₂ lasers the relative standard deviations are on the order of a few megahertz, the absolute accuracy is about 25 MHz for the 10.4 μm band and about 18 MHz for the 9.4=μm band.     

Direct observation of the space-time transformation of a femtosecond laser pulse in fused quartz

The transformation of a femtosecond laser pulse propagating in fused quartz before and after its transition to a filamentation regime has been investigated by femtosecond time-resolved optical polarigraphy. The spatial periodicity of a light field along and across the propagation axis has been detected and its nature has been attributed to the interference of the conical and plane components of the wave packet of the filament. The “supraluminal” motions of the observed filament intensity maximum are due to the longitudinal transformation of the pulse profile.     

Direct observation of the second-order Stark effect of theX-B transition in molecular iodine

The second-order Stark shift of the components of the hyperfine structure of the transition¹ _g ⁺ ( = 0,j = 13, 15) ³ _ou ⁺ ( = 43,j = 12, 16) (of molecular iodine have been studied by means of saturated absorption spectroscopy in an external cell with the I₂ vapour located in an electric field. The anisotropic polarizabilities of the upper and lower levels together with the difference between the isotropic polarizabilities of the levels of the transition have been obtained.     

Direct observation of anharmonic coupling in the time domain with femtosecond stimulated Raman scattering

Off-resonant impulsive Raman excitation is used to initiate nonstationary nuclear motion in low-frequency vibrational modes of deuterio-chloroform. Femtosecond time-resolved stimulated Raman probing of the high-frequency C-D stretch reveals additional vibrational sidebands that arise as a result of anharmonic coupling between the impulsively excited low-frequency and the probed high-frequency vibrations. These experiments illustrate the detailed molecular information on anharmonic and reactive surfaces available from multidimensional femtosecond stimulated Raman techniques.     

Direct observation of the mott transition in an optically excited semiconductor quantum well

We have studied density-dependent time-resolved photoluminescence from a 80 A InGaAs/GaAs single quantum well excited by picosecond pulses. We succeed in giving evidence for the transition from an exciton-dominated population to an unbound electron-hole pair population as the pair density increases. For pair densities below this excitonic Mott transition we observe a spectrally separate emission from free electron-hole pairs in addition to excitonic luminescence, thereby proving the coexistence of both species. Exciton binding energy and band gap remain unchanged even near the upper bound of this coexistence region. Above the Mott density we observe a purely exponential high energy tail of the photoluminescence and a redshift of the band gap with pair density. The transition occurs gradually between 1 x 10(10) and 1 x 10(11) cm(-2) at the carrier temperatures of our experiment.     

Direct observation of the alpha-epsilon transition in shock-compressed iron via nanosecond x-ray diffraction

In situ x-ray diffraction studies of iron under shock conditions confirm unambiguously a phase change from the bcc (alpha) to hcp (epsilon) structure. Previous identification of this transition in shock-loaded iron has been inferred from the correlation between shock-wave-profile analyses and static high-pressure x-ray measurements. This correlation is intrinsically limited because dynamic loading can markedly affect the structural modifications of solids. The in situ measurements are consistent with a uniaxial collapse along the [001] direction and shuffling of alternate (110) planes of atoms, and are in good agreement with large-scale nonequilibrium molecular dynamics simulations.     

Direct observation of medium-range crystalline order in granular liquids near the glass transition

Collective behavior of driven granular matter is often strikingly analogous to that of thermal systems. Here we use a vibrated quasi-two-dimensional granular matter as a model system and investigate the mechanism of the liquid-glass transition. We demonstrate by direct observation the existence of long-lived medium-range crystalline order, which is found to be closely related to both dynamic heterogeneity and slow dynamics. Our findings are remarkably similar to recent numerical results on model thermal liquids and thus open an intriguing possibility of understanding the dynamic arrest in both thermal and athermal systems in a unified manner.     

Ultrafast inversion of population in the molecule vibrational mode pumped by a highpower picosecond IR laser pulse

A picosecond IR laser pulse, pumping multiphoton resonance, can excite a molecule from the ground state to a certain high-lying vibrational level, all the other levels being unpopulated by the end of the pulse. The neighbouring multiphoton resonances do not overlap, though the power broadening required may be much greater than the intervals between them. Complete selective population of a specific vibrational level can be achieved only under certain conditions of laser pulse amplitude and duration. Based on this effect, a novel method for investigating vibrational energy intermolecular and intramolecular relaxation is proposed. The possibility of mode-selective multiphoton excitation of polyatomic molecules is discussed.     

Direct observation of the transition from free to constrained single-segment motion in entangled polymer melts

We report a direct determination of the time dependent mean-squared segment displacement of a polymer chain in the melt covering the transition from free to constraint Rouse relaxation along the virtual tube of the reptation model. This has been achieved by a neutron spin-echo (NSE) measurement of the segmental self-correlation function as conveyed by the spin-incoherent scattering from two fully protonated polymer melts, polyethylene and polyethylene propylene. Within the scenario of de Gennes reptation model a transition of the time dependence of segmental mean-squared displacements from proportional, variant t(1/2) to proportional, variant t(1/4) is expected and clearly corroborated by the incoherent NSE results.     

Direct observation of the multisheet Fermi surface in the strongly correlated transition metal compound ZrZn2

The existence of flat areas of a Fermi surface (FS), predicted by electronic structure calculations and used in models of both magnetically mediated and phonon-mediated Fulde-Ferrell-Larkin-Ovchinnikov superconducting states, is reported in the paramagnetic phase of the ferromagnetic superconductor ZrZn2 using positron annihilation. The strongly mass-renormalized FS sheet, dominating the Fermi level density of states, is seen for the first time. The delocalization of the magnetization is studied using measured and calculated magnetic Compton profiles.     

Experimental investigation of the pulse inversion technique for imaging ultrasound contrast agents

The application of ultrasound contrast agents aims to detect low velocity blood flow in the microcirculation. To enhance discrimination between tissue and blood containing the contrast agent, harmonic imaging is used. Harmonic imaging requires the application of narrow-band signals and is obscured by high levels of native harmonics generated in an intervening medium. To improve discrimination between contrast agent and native harmonics, a pulse inversion technique has been proposed. Pulse inversion allows wide-band signals, thus preserving the axial resolution. The present study examines the interference of native harmonics and discusses the practical difficulties of wide-band pulse inversion measurements of harmonics by a single transducer. Native harmonics are not eliminated by pulse inversion. Furthermore, only even harmonics remain and are amplified by 6 dB, alleviating the requirement for selective filtering. Finally, it is shown that the contaminating third harmonic contained in the square wave activation signal leaks through in the emitted signal. The spectral location of the contaminating third harmonic is governed by the transducer spectral characteristics while the location of the native and contrast agent second harmonics is not. Thus the contaminating third harmonic and the native and contrast agent second harmonics may overlap and interfere. Optimal discrimination requires a balance between maximal sensitivity for the second harmonic at reception and minimal interference from the contaminating third harmonic.     

Direct measurement of transition frequencies in isolated pHe+ atoms, and new CPT-violation limits on the antiproton charge and mass

A radio frequency quadrupole decelerator and achromatic momentum analyzer were used to decelerate antiprotons and produce p4He+ and p3He+ atoms in ultra-low-density targets, where collision-induced shifts of the atomic transition frequencies were negligible. The frequencies at near-vacuo conditions were measured by laser spectroscopy to fractional precisions of (6-19) x 10(-8). By comparing these with QED calculations and the antiproton cyclotron frequency, we set a new limit of 1 x 10(-8) on possible differences between the antiproton and proton charges and masses.     

Pressure dependence of the Raman frequencies for the translational mode in ammonia solid II

We study here the translational mode of the ammonia solid II near the melting point by calculating its Raman frequencies as a function of pressure for the fixed temperatures of 230.4, 263.4 and 297.5 K. We perform this calculation of the Raman frequencies using the volume data by means of our Grüneisen relation. The Raman frequency shifts as the volume changes with the pressure, exhibit an anomalous behaviour near the melting point in the ammonia solid II.This anomalous behaviour can be examined experimentally on the basis of our calculations given in this study.