Modulation Regime Research Articles

Optical dephasing in solution: A line shape and resonance light scattering study of azulene in isopentane and cyclohexane

Results of a line shape and resonance light scattering study of the S1←S0 and S2←S0 electronic transitions of azulene in isopentane and cyclohexane are reported. The results are analyzed using two different non-Markovian master equations that make different assumptions about the statistical properties of the bath. For both these origin transitions we find that the solution dynamics fall in the so-called intermediate modulation regime. If exponential decay is assumed for the bath correlation function we obtain a correlation time of the bath of 25 fs for the S1←S0 transition and of 13 fs for the S2←S0 transition at room temperature. From the frequency dependence of the ratio of fluorescence to Raman yields of the S1←S0 transition we calculate an excited state lifetime of 1.4±0.2 ps using the parameters of the bath derived from the line shape analysis, and irrespective of which master equation is used.

A Simple Explicit form of Finite Differences to Compute the Effective Gain in Degenerate Two-wave Mixing; Comparison with the Shooting Method

Abstract A numerical study is presented of the effective gain in degenerate two-wave mixing, for a reflection geometry in a photorefractive crystal. Apart from the so-called ‘shooting method’ we employ a simple explicit method of ‘finite differences’ to obtain numerical solutions of the nonlinear coupled-wave equations. A comparison between the two indicates that the computed results from the finite differences method are still in reasonable agreement with those obtained from the shooting method. At the same time the finite differences method is more straightforward and usually found to be faster than the shooting method for a small number of discrete points. The dependence of the effective gain on various parameters is shown at the saturation as well as non-saturation (i.e. below saturation) stage of the refractive-index modulation. A significant difference in gain is found to exist between these two regimes of modulation. Numerical results are compared and presented as graphs.

Dynamics of emission from an (InGa)AsP heterolaser with two-channel lateral confinement

An investigation was made of the dynamics of amplitude modulation of 1.3-μm radiation emitted by highly efficient low-threshold stripe heterolasers with lateral confinement due to two parallel epitaxial buried channels in a double heterostructure. The rise time of the transient characteristic was 0.1–0.6 ns, depending on the modulation regime. Single radiation pulses of duration less than 70 ps and regular trains of such pulses of frequency ~ 1 GHz were generated. The amplitude-frequency characteristics were determined under weak modulation conditions.

The ultimate frequency response and time evolution of resonant tunneling in double-barrier structures

A new approach to treating the problem of the frequency response of tunneling barriers is demonstrated. The Schrödinger equation for an incident monoenergetic electron is solved in the case of a double-barrier structure with an applied potential oscillating at an arbitrary frequency. The details of the tunneling in the transient stage and the following steady state, as well as the charging and discharging processes around the structure, are revealed in the high-frequency modulation regime. We find that the linear frequency response for a typical fast structure rolls off above 1.5 THz with a 1/f dependence. An enhancement in the nonlinear frequency response at 770 GHz is observed for a single electron tunneling. From the motion of the wave front in the structure, we find that the phase transit time across the structure ranges from 140 to 40 fs decreasing with frequency. Generation of the transmitted pulses with an energy spread about ℏω where ω is the angular modulation frequency is observed. This corresponds to an emission and absorption of a quantum of ℏω.

A digital system for generating dynamic sinusoidal gas concentration signals.

A computer-controlled gas-mixing system is presented. It is capable of mixing four gases, the concentration of three of which will follow a path to be determined by the user. For our purposes the output O2 fraction is maintained constant and the levels of Ar and N2O vary sinusoidally and independently, with periods between 0.25 and 30 min. A fourth gas, N2 is necessary to make the sum of the individual fractions 100%. The system uses banks of between one and four solenoid valves each linked via a sonic choke to a common mixing chamber. A regime of pulse frequency modulation is employed. All calculations and timing of valve switching are performed by a dedicated microcomputer built for the purpose. The device has been used to provide respiratory gas forcing functions for a program of research in respiratory monitoring.

Raman study of the ν(CO) mode of ethyl trifluoroacetate in solution. Reorientational and vibrational relaxation

AbstractThe reorientational and vibrational correlation functions of the carbonyl stretching mode of ethyl trifluoroacetate have been determined from its infrared and Raman band shapes in solution in hexane, CCl4, CS2 and CHCl3. Reorientational motion is less hindered in hexane and CCl4 than in CS2 or CHCl3. Kubo and Rothschild's and Oxtoby's stochastic theories have been applied to describe the experimental correlation functions. An intermediate modulation regime is observed in the solvents used. The fastest modulation corresponds to CS2. Fisher and Laubereau's isolated binary collision model for vibrational dephasing does not account for the observed vibrational widths.

Time-domain analysis of phase-reversal travelling-wave integrated-optic modulators

The temporal response of periodic and nonperiodic phase-reversal integrated-optic travelling-wave modulators is derived for the lossless and lossy cases. Improved repetition rates are obtained by using nonperiodic structures. For all structures considered the highest usable repetition rate is much greater in the intensity modulation regime (using a Mach-Zehnder) than in the phase modulation regime.

The two-dimensional ANNNI model in the CVM approximation

The phase diagram of the two-dimensional ANNNI model is investigated by the cluster variation method (CVM). We confirm the stability of the disordered phase down toT=0 and the absence of a Lifshitz point at finite temperature forK<1/2, whereK the ratio of the second to the first neighbor pair interactions. Two different modulation regimes for the correlation functions of the disordered phase are separated by a “disorder” line along which theq vector of the susceptibility maximum undergoes a lock-in transition. The study in reciprocal space of the stability of the disordered phase allows us to define a critical line in the phase diagram along which theq vector characterizing the instability is incommensurate. Finally, we show the existence of another Lifshitz point forK tending to infinity.

Determination of the modulation regime for vibrational dephasing. Demonstration on the critical Raman broadening in nitrogen

We show that a joint analysis of spontaneous Raman spectra and time-resolved CARS experiments permits an unambiguous determination of the modulation speed leading to vibrational dephasing. For the case of nitrogen, we find in particular that the dephasing rate itself is enhanced when the critical point is approached. The modulation regime becomes intermediate between the fast and slow limits (homogeneous and inhomogeneous limits). The modulation speed, having a correlation time around 10 ps, is determined by the density fluctuations on the scale of the potential range.

Pulse-periodic and cw CO2lasers pumped with a self-sustained pulsed discharge for use in heat processing

A description is given of the construction and radiation characteristics of fast-flow pulse-periodic (with an output power of up to 3.5 kW) and cw (up to 1.8 kW) CO2 lasers excited by a self-sustained pulsed discharge. For the first time, a laser pulse duration of up to 5 msec was obtained and a cw modulated output regime was realized when the active medium was excited with short (1–3μsec) discharge pulses. Examples are given of the use of such lasers for heat processing.

E.P.R. Investigation of the incommensurate phase of ThBr4with the HELP of the Gd3+Vn.n.n. Br−

Abstract ThBr4 exhibits a commensurate incommensurate phase transition at TI = 95K From TI to 4K, the phase remains incommensurate (I). In a preliminary E. P.R. investigation we have shown the efficiency of Gd3+ centres to investigate the incommensurate phase, through fourth order crystal field terms (2). In the present work, we detail the properties of the particular probe which have been used, i.e. the Gd3+ ion substituted Th4+ and associated to a n.n.n. Br− vacancy. We focus our attention on the line shape for the ΔMS = 5/2 + 7/2 transition. Indeed, the line shape does not obey the usual model for a plane wave modulation of displacement field which would induce “edge singularities”. A recent investigation at 4.2K by luminescence on U4+ ions have evidenced phase pinning (3). With E. P.R. measurements, one is able to investigate the incommensurate phase in the whole temperature phase, and the plane wave modulation regime is not sufficient to interpretate the experimental results below TI - 7K.

Plane-wave and multisoliton lattice modulation regimes in incommensurate Rb2ZnCl4

The transition between the incommensurate plane-wave modulation regime and the multisoliton lattice modulation regime is studied by 87Rb quadrupole perturbed nuclear magnetic resonance in Rb2ZnCl4. Both phase and amplitude solitons are taken into account in comparing the experimental and theoretical lineshapes and determining the soliton density. Commensurate lines are clearly seen already in the middle of the incommensurate phase demonstrating that multisoliton lattice effects are important over a much wider temperature range than reported previously.

Contraction of the space of electron drift in low-frequency bunchers

In the paper we theoretically investigate the mechanism by which the space of drift electrons is contracted in klystron-type bunchers which modulate the electron beam in ion accelerators, in particular to excite an rf field. The contraction of the drift length is effected through a decrease in the mean beam velocity in the field of an electrostatic lens or a metal tube and is based on the fact that the relative motion of velocity-modulated electrons are independent of the mean velocity of the electron beam in a weak modulation regime.

The effect of hard mode locking in a dye laser

The influence of the relaxation time of a saturable absorber on the dynamics of a quasi-continuous ring laser is investigated. It is shown that in a dye laser with a fast absorber, bistability arises; thus two multimode regimes coexist. The regime of shallow stochastic modulation is established spontaneously. Transition to the mode-locking regime accompanied by a sharp increase in the number of generated modes is feasible only under the action of an initiating pulse (hard mode locking). The analytical criterion for hard locking has been obtained. If the criterion is not satisfied, the only stable regime of a laser is mode locking. In this case the transient process is the longer the smaller the relaxation time of a saturable absorber.

Development of the dimple instability on liquid 4He

Electrons on the surface of liquid 4He have been used to study the transition from a homogeneous two-dimensional charge distribution to the modulated charge density regime where the so-called dimple crystal is formed. The observation of hysteresis effects suggests that the transition is discontinuous.

Observation of a localised oscillatory mode of optical emission from an air plasma

The expansion of plasma in air in a point-to-plane geometry under a modulated high-voltage pulse regime has been studied. Photographs show distinctive patterns in the spatial development of the discharges, and photomultiplier traces reveal a localised oscillatory mode of optical emission.

On the expansion of gaseous plasmas

The expansions of plasma in neon plus helium under a modulated high voltage pulse regime has been studied. Photomultiplier studies of the striated appearance show that the bright regions are developed in sequence.

The vibrational and rotational relaxation of iodine in solution by resonance Raman lineshape measurement

The isotropic linewidths of a series of overtones in the resonance Raman spectrum of a dilute solution of I2 and CCl4 were measured. The widths enabled the mechanism responsible for the vibrational dephasing to be identified as a vibrational frequency modulation. The transition between the fast and slow modulation regimes could be followed, by observing the dependence of overtone width on overtone order, and a characteristic time and mean square amplitude ascribed to the frequency modulation process. The anisotropic linewidths for the I2 anisotropic spectra in a series of solvents, chosen for the wide range of their viscosity, were also measured. The reorientation time was found to vary linearly with solvent viscosity.

Infrared spectra of chemisorbed CO on Rh/γ–Al2O3: Site distributions and molecular mobility

Chemisorption of CO on a 0.92 wt. % Rh/γ-Al2O3 catalyst gave three species of adsorbed CO molecules: Species RhCO, at about 2000 cm−1; species Rh(CO)2, showing a doublet with peak frequencies at 2100 and 2030 cm−1; and a bridged species Rh2CO, leading to a very broad band peaked between 1850 and 1900 cm−1. The thermostability of these adsorbed species is in the sequence Rh2CO≳RhCO≳Rh(CO)2. Only the doublet band of Rh(CO)2 and the very broad band of the bridged species Rh2CO were found on well dispersed Rh metal in its δ phase. The band contours of the RhCO and Rh(CO)2 species were Fourier transformed into the time domain. The result showed that the configuration RhCO was spread over an approximately four times wider distribution of adsorption sites (surface–adsorbed CO interactions) than that of the Rh(CO)2 species. Furthermore, it was shown that the CO molecules adsorbed as RhCO species are ’’floppy,’’ undergoing some type of fast (probably librational) motion with a characteristic time of 0.1×10−12 sec. On the contrary, the CO molecules adsorbed as RH(CO)2 obey a slow modulation regime, with a characteristic time of about 3×10−12 sec. It is pointed out that this greatly different behavior may be of significance with regard to the selectivity and activity of the different catalytic sites as well as to the directional properties of the metal–carbon bond orbitals.

Optimization and comparison of the characteristics of optical interference discriminators

The laser frequency can be stabilized with aid of the an external highly stable Fabry–Perot interferometer. It is proposed that light reflected from this interferometer, rather than the transmitted light, be recorded in the stabilization procedure. It is shown that this improves the sensitivity by a factor of 4–6 and increases the signal/noise ratio by at least 10 dB. In the modulation regime when the modulation amplitude is small and the predominantly reflection variant is adopted, the ratio of the signal to the shot noise of a photodetector can reach (other conditions being constant) a value of 20–30 dB. A calculation is made of the optimal parameters of both variants of a discriminator which has the same length, same optical power, and same absorption in the mirrors (assumed to be small).