All-optical modulation based on electromagnetically induced transparency

We numerically investigate the implementation of all-optical absorption modulation of electromagnetic pulses by a medium that exhibits electromagnetically induced transparency. The quantum system is modelled as a three-level Λ-type system that interacts with two electromagnetic pulses, a probe pulse and a coupling pulse. The dynamics of the system is described by the coupled Maxwell-density matrix equations, and we explore the dependence of the optical modulation efficiency on the parameters of the system.     

Measurement of Small One-Bond Proton–Carbon Residual Dipolar Coupling Constants in Partially Oriented C Natural Abundance Oligosaccharide Samples: Analysis of Heteronuclear JCH-Modulated Spectra with the BIRD Inversion Pulse

Two 2D J-modulated HSQC-based experiments were designed for precise determination of small residual dipolar one-bond carbon–proton coupling constants in ¹³C natural abundance carbohydrates. Crucial to the precision of a few hundredths of Hz achieved by these methods was the use of long modulation intervals and BIRD pulses, which acted as semiselective inversion pulses. The BIRD pulses eliminated effective evolution of all but ¹J_CH couplings, resulting in signal modulation that can be described by simple modulation functions. A thorough analysis of such modulation functions for a typical four-spin carbohydrate spin system was performed for both experiments. The results showed that the evolution of the ¹H–¹H and long-range ¹H–¹³C couplings during the BIRD pulses did not necessitate the introduction of more complicated modulation functions. The effects of pulse imperfections were also inspected. While weakly coupled spin systems can be analyzed by simple fitting of cross peak intensities, in strongly coupled spin systems the evolution of the density matrix needs to be considered in order to analyse data accurately. However, if strong coupling effects are modest the errors in coupling constants determined by the “weak coupling” analysis are of similar magnitudes in oriented and isotropic samples and are partially cancelled during dipolar coupling calculation. Simple criteria have been established as to when the strong coupling treatment needs to be invoked.     

Comparison of the Effects of Different <Superscript>19</Superscript>F <Emphasis Type="Italic">π</Emphasis> Pulses on the Sensitivity and Phaseability of the <Superscript>19</Superscript>F-<Superscript>13</Superscript>C HSQC

The ¹⁹F-¹³C heteronuclear single quantum coherence (HSQC) experiment is vital for the structural elucidation of polyfluorinated organic species, yet its sensitivity and phaseability are limited by difficulties in uniform excitation of the widely disperse ¹⁹F spectral window. Adiabatic pulses of different types have previously been employed to generate effective π pulses for inversion and refocussing, but a systematic comparison of various adiabatic and other inversion pulses has not been published. In this work, it was observed that the use of a broadband inversion pulse (BIP) during the t ₁ evolution period resulted in properly phaseable spectra for experiments optimized to detect ¹ J _CF, in contrast to CHIRP or WURST adiabatic pulses. For the INEPT and reverse-INEPT transfer segments of the HSQC, optimal sensitivity for resonances distant from the transmitter frequency was afforded by optimized universal rotation (BURBOP) or CHIRP pulses. In HSQC experiments with delays optimized for two-bond correlations, only the use of BURBOP pulses in INEPT and reverse-INEPT sequences afforded spectra cleanly phaseable across the F ₂ and F ₁ spectral windows. This observation is supported by off-resonance pulsed field gradient spin-echo experiments.     

Enhancement of resonant bleaching of <Emphasis Type="Italic">J</Emphasis>-aggregates upon lengthening of an exciting radiation

Thin films of J-aggregates of a new amphiphilic thiacarbocyanine dye of the benthiazole series are prepared and the nonlinear optical response of molecular J-aggregates is studied for femto-and nanosecond exciting radiation pulses. It is found that the nonlinear optical response of J-aggregates exhibits substantial enhancement upon an increase in the pulse duration, which cannot be described by the saturation effect in the model of a two-level system. This effect is considered using a three-level model taking into account the formation of self-trapped exciton states in molecular J-aggregates.     

Electron-positron pair production by electromagnetic pulses

Electron-positron pair production in vacuum by a single focused laser pulse and by two counter-propagating colliding focused pulses is analyzed. A focused laser pulse is described using a realistic three-dimensional model based on an exact solution of Maxwell’s equations. In particular, this model reproduces an important property of focused beams, namely, the existence of two types of waves with a transverse electric or magnetic vector (e-or h-polarized wave, respectively). The dependence of the number of produced pairs on the radiation intensity and focusing parameter is studied. It has been shown that the number of pairs produced in the field of a single e-polarized pulse is many orders of magnitude larger than that for an h-polarized pulse. The pulse-intensity dependence of the number of pairs produced by a single pulse is so sharp that the total energy of pairs produced by the e-polarized pulse with intensity near the intensity I _S = 4.65 × 10²⁹ W/cm² characteristic of QED is comparable with the energy of the pulse itself. This circumstance imposes a natural physical bound on the maximum attainable intensity of a laser pulse. For the case of two colliding circularly polarized pulses, it is shown that pair production becomes experimentally observable when the intensity of each beam is I ～ 10²⁶ W/cm², which is one to two orders of magnitude lower than that for a single pulse.     

The pulse shape of a passively Q-switched microchip laser

The shape of the intensity pulse of a passively Q-switched microchip laser is investigated numerically and analytically. Our analysis is motivated by independent microchip laser experiments exhibiting nearly symmetric pulses in the case of a semiconductor saturable absorber and asymmetric pulses in the case of a solid state saturable absorber. Asymptotic methods are used to determine limiting behaviors of the pulse shape for both symmetric and asymmetric pulses. In the first case, we determine a sech² solution parametrized by one parameter which can be determined by solving two coupled nonlinear algebraic equations. In the second case, the pulse solution is decomposed into two distinct approximations exhibiting different amplitude and time scales properties. We review earlier approximations of the repetition rate and the pulse width. Received 2 August 1999     

Numerical simulation of optical nutations and a precursor pulse in electromagnetically induced transparency

The results of numerical simulation of the electromagnetically induced transparency phenomenon in the ?? scheme of degenerate quantum transitions J = 0 ?? J = 1 ?? J = 2 with Doppler broadening of spectral lines are presented for the cases of continuous and abrupt rises in the leading edge of the input probe radiation pulse. For circularly polarized radiations, it has been shown that the deviation from the condition of adiabatic sequence can lead to a separation of the probe pulse of the adiabaton into a train of subpulses. In the case of linear polarization of the input probe field and the circular polarization of the input control field, the probe radiation pulse is split into two pulses with opposite circular polarizations and a multispike structure of the envelopes. In the case of an abrupt rise in the leading edge of the input probe pulse, regardless of the peak intensity of the latter, the precursor pulse arises at a probe radiation frequency and the velocity of its propagation coincides with the speed of light in vacuum.     

Probeheads and instrumentation for pulse EPR and ENDOR spectroscopy with chirped radio frequency pulses and magnetic field steps

Probeheads and instrumentation for modern X-band pulse EPR and ENDOR experiments with chirped radio-frequency pulses and rapidB ₀-field pulses are described. The resonant frequency, the quality factor and, for the first time, the response of a pulse ENDOR resonator structure to a microwave pulse in the subnanosecond time scale have been calculated. The performance of the probeheads for time-domain chirp ENDOR and electron Zeeman-resolved EPR is demonstrated.     

Effects of imperfect pulses on dynamical decoupling using quantum trajectory method

The dynamical decoupling(DD) method is widely adopted to preserve coherence in different quantum systems. In the case of ideal pulses, its effects on the suppression of noise can be analytically described by the mathematical form of filter function. However, in practical experiments, the unavoidable pulse errors limit the efficiency of DD. In this paper,we study the effects of imperfect pulses on DD efficiency based on quantum trajectories. By directly generating a pseudo noise sequence correlated in time, we can explore the performance of DD with different pulse errors in the typical noise environment. It shows that, for the typical 1/f noise environment, the phase error of operational pulses severely affects the performance of noise suppression, while the detuning and intensity errors have less influence. Also, we get the thresholds of these errors for efficient DD under the given experimental conditions. Our method can be widely applied to guide practical DD experimental implementation.     

Optimum conditions for superradiant damping

The conditions are investigated under which cooperative or superradiant effects are greatest in an inhomogeneously broadened atomic system that is excited by a coherent light pulse. The coupled non-linear atomic equations of motion are solved numerically for excitation pulses of various areas. It is shown that the absolute intensity of the response decreases strongly with decreasing pulse are below π, but that the relative superradiant contribution increases with decreasing pulse area. The reasons for this are discussed, and it is suggested that excitation by a pulse in the neighborhood of π/2 may represent an optimum compromise for the observation of superradiance.     

Integrable models for the dynamics of a longitudinal-transverse acoustic wave in a crystal with paramagnetic impurities

We theoretically study the evolution of longitudinal-transverse acoustic pulses propagating parallel to an external magnetic field in a system of resonant paramagnetic impurities with an effective spin S=1/2. For equal group velocities of the longitudinal and transverse waves, the pulse dynamics is shown to be described by evolution equations. In limiting cases, these equations reduce to equations integrable in terms of the inverse scattering transform method (ISTM). For the most general integrable system of equations that describes the dynamics of acoustic pulses outside the scope of the slow-envelope approximation, we derive the corresponding ISTM equations. These equations are used to find a soliton solution and a self-similar solution. The latter describes the leading edge of the packet of acoustic pulses generated when the initial unstable state of a spin system decays. Analysis of our solutions and models indicates that the presence of a longitudinal acoustic wave leads not only to a change in the amplitude and phase of the transverse wave but also to a qualitatively new dynamics of sound in such a medium.     

An improved rapid method for heteronuclear two-dimensional Jspectroscopy

Improved two-dimensional heteronuclear J spectroscopy pulse sequences with fast restoration of the z magnetization, multiple refocusing, and acquisition of both halves of the spin echos are described. The experimental results show that these methods can effectively enhance the signals of the 2-D J spectra.     

Optical generation of spin coherence in single-charged (In,Ga)As/GaAs self-assembled quantum dots

The generation of electron spin coherence has been studied in n-modulation-doped (In,Ga)As/GaAs self-assembled quantum dots (QDs) which contain on average a single electron per dot. The doping has been confirmed by pump–probe Faraday rotation experiments in a magnetic field parallel to the heterostructure growth direction. For studying spin coherence, the magnetic field was rotated by 90° to the Voigt geometry, and the precession of the electron spin about the field was monitored. The coherence is generated by resonant excitation of the QDs with circularly polarized laser pulses, creating a coherent superposition of an electron, and a trion state. The efficiency of the generation can be controlled by the pulse intensity, being most efficient for (2n+1)π pulses.     

Single and Multiple-Selective Excitation Combined with Pulsed Field Gradients

Applications of selective, multiselective, and semiselective pulses with pulsed field gradients are described. The use of multiple-selective excitation and PFGs for coherence selection in the selective one-dimensional experiments results in spectra devoid of artifacts and with remarkable solvent suppression. Multiple-selective excitation is also employed in an experiment called Multigate, a variant of the well-known WATERGATE experiment, in order to achieve multiple solvent signal suppression. Finally, new pulse sequences are shown for recording pure absorption ω₁semiselective PFG NOESY, ROESY, and TOCSY experiments. The merits and limitations of these experiments are discussed.     

Quantum-classical simulation of the photoisomerization during the transformation of ultrashort light pulses by a molecule

The dynamics of photoisomerization of a model molecule during its transformation of ultrashort (with a duration much shorter than the lifetime of the resonant excited electronic state) light pulses is simulated numerically. The two-level electronic subsystem of the molecule is described using the quantum theory, while the nuclear subsystem (taking into account the two isomeric states of the molecule) and the radiation field are described using the classical theory. The ranges of the carrier frequency, the peak intensity, and the durations of nπ sinusoidal pulses (n = 1–10) irradiation with which results in the photoisomerization of molecules of the type under study (for example, cyanine dyes) are determined from the analysis of solutions to self-consistent equations that describe the motion of the “isomerization oscillator” and the time evolution of the population amplitude of the resonant electronic state of the molecule. Each of these non-overlapping ranges corresponds to a particular value of n. Bifurcation values of the above parameters of the light pulse are boundaries of these ranges.     

Improved J-Compensated Apt Experiments

The derivation and investigation of two new J-compensated attached-proton-test experiments, CAPT2 and CAPT3, are presented. These methods incorporate fewer pulses than CAPT and are shown to be more effective over a wider range of ¹J_CH than spin flip, APT, and CAPT for CH, CH₂, and CH₃ spin systems. In addition, the magnitude of the flip angle of the initial pulse which creates transverse carbon magnetization is unrestricted in CAPT2 and CAPT3. The compensated CAPT3 sequence, which is patterned after the 90°_x90°_y90°_x composite pulse, is found to be excellent for routine use in ¹³C spectroscopy.     

Solvent Suppression in High-Resolution 1H NMR Spectroscopy Using Conventional and Phase Alternated Continuous Wave Free Precession

Solvent suppression is frequently mandatory in ¹H high-resolution nuclear magnetic resonance (NMR), especially for those experiments designed for non-deuterated solvent, normally used in protein and in vivo analysis, and also in liquid chromatography-NMR. Here, simple pulse sequences, which are based on continuous wave free precession (CWFP), consisting of a train of pulses separated by a time interval $ T_{\text{p}} \ll T_{2}^{*} $ , is applied to suppress one or more solvent signals in ¹H high-resolution NMR experiments, because of its dependency on the offset frequency. The conventional CWFP pulse sequence, that uses pulses with the same phase and duration, introduces some phase anomaly in the Fourier-transformed spectrum. This problem is minimized when the pulses are applied with phase alternation by π/2 in relation to the preceding pulse. Some problems with signal intensity can also be minimized using a shorter pulse width. Both CWFP and phase alternated CWFP can be easily used to suppress two solvent signals simultaneously, just using the correct T _p value, that must be equal to the inverse of frequency difference (?ν) between both signals to be suppressed. After modifications, we could introduce the CWFP train into 2D routine pulse sequences.