Upper-state Population Research Articles

Synthesis of fast qudit gates by a train of coincident pulses

We propose an exact analytical method for the production of fast quantum gates in a system of d degenerate states, using a technique of a train of coincident pulses. It is an alternative to the adiabatic passage technique. This study exploits the Morris-Shore transformation and generalized quantum Householder reflection in which each of Householder reflection is implemented by n + m (n and m are arbitrary integers) sets of coincident pulses. Decoherence due to the population of the upper state is efficiently suppressed as the number of pulse sets (n and m ) increases. It is remarkable that the upper state population is damped considerably, even for a small number of pulse sets, despite the fact that all the fields applied were on resonance with their transitions. In this method, simple Gaussian pulses with minimal pulse areas were used, which is easy to achieve experimentally. As a case study to validate the method, we implement the quantum Fourier transform in qutrit and ququad by a proper pulse train.

Measurement of discharge driven HF chemical amplifier's integrated upper-state population density

The integrated small gain coefficient of discharge driven HF chemical gain module and the amplified spontaneous emission that propagated outside the gain module along the flow field are measured at different quantity of gas flow. And the integrated upper-state population density of the gain module is calculated based on the results of the integrated small gain coefficient and the amplified spontaneous emission that propagated outside the amplifier. The results of calculation show that the integrated upper state population density in discharge driven HF chemical gain module is about 1021/m3.

Amplified Spontaneous Emission in Discharge-Driven HF Chemical Lasers

An amplified spontaneous emission (ASE) calculation model is established for the discharge-driven CW HF chemical gain module. By using a small signal gain coefficient and upper state population density, the ASE propagated outside the amplifier is calculated, where after, it is measured by experiment. The result of theory and experiment shows that the experimental results are consistent with the calculations from the simplified model. The difference between them is less than 15%. This simple model can effectively calculate the ASE in the amplifier of master oscillator-power amplifier configuration.

Simulation and analytical study of amplified spontaneous emission in N2-lasers

By applying the geometrical model of the gain coefficient and population, we managed to describe the amplified spontaneous emission (ASE) in a N2-laser. For this purpose, and also for verifying the geometrical modeling we used the earliest available experimental data for the N2-laser output energy εASE versus the laser excitation length reported by Leonard for a N2-laser operating at low gas pressure (20Torr), with no preionization. The ASE output energy calculation was performed both numerically and analytically. Upon applying the full description of the gain coefficient and upper state population (with b-parameters≠0), the numerical calculations found to explain the experimental measurements of the εASE versus excitation length correctly. For the analytical approach, we used the approximate expression for the gain and population (with b-parameters=0) for simplifying the calculation. The results corresponding to both cases are introduced. Furthermore, the calculated gain coefficient for describing the Leonard's experiment has been compared with our recent gain measurements in a N2-laser with corona preionization, and higher operational gas pressure (∼100Torr). This observation is indicating that while the gain coefficients in both experiments are obeying the generalized formulation of the gain coefficient, but the gain profile for the higher gas pressure is slightly higher than the case when the laser is operating at a low gas pressure. That is, the laser geometry still has the dominant contribution for the laser gain coefficient.

Probing quantum superposition states with few-cycle laser pulses

The quantum dynamics of a two-level system illuminated by a few-cycle pulse with an adjustable carrier-envelope (C-E) phase is investigated theoretically. We consider the weak-field regime where tunneling processes and multiphoton ionization are negligible. It is shown that the upper state population exhibits a strong dependence on the C-E phase and on the time of arrival of the few-cycle pulse if the system is initially prepared in a coherent superposition state. We demonstrate that this effect can be employed to probe the coherence properties of the superposition state and allows one to determine the phase of the laser that prepares this state.

Modeling and optimizing of low-repetition-rate high-energy pulse amplification in high-concentration erbium-doped fiber amplifiers

Starting from the modeling of isolated ions and ion-pairs, a closed form rate and power evolution equations for pulse amplification in high-concentration erbium-doped fiber amplifiers (EDFAs) are constructed. According to the equations, the effects of ion-pairs on the performance of a high-concentration EDFA in steady state including upper-state population, ASE powers without input signal are analyzed numerically. Furthermore, the effects of ion-pairs on the dynamic characteristics of low-repetition-rate pulse amplification in the EDFA including the storied energy, output pulse energy and evolution of pulse waveform distortion are systematically studied by using the finite-difference method. The results show that the presence of the ion-pairs deteriorates amplifier performance, such as the upper-state population, ASE power, storied energy, output pulse energy, and saturated gain, etc. For the high-concentration EDFA, the optimum fiber length should be modified to achieve a better performance. The relations between the evolution of pulse waveform distortion or output pulse energy and the input pulse peak power are also discussed. The results can provide important guide for the design and optimization of the low-repetition-rate pulse amplification in high-concentration EDFAs.

Spontaneous emission of a three-level lambda-type atom coupled to separate reservoirs

Considering the anisotropic dispersion model, the upper state population and spontaneous emission spectrum of three-level lambda-type atom with two transitions coupled to separate reservoirs are investigated using the resolvent operator. The upper state population reaches to a steady state value after a weak oscillation when the decay rate is zero and one transition frequency is inside the bandgap. The spectrum associated with each transition was given. Compared with results that were obtained by using isotropic dispersion model, the shape of spectrum changes significantly, and no dark line appears in the spectra.

Effects of the pulse area and pulse number on the population dynamics of atoms interacting with a train of ultrashort pulses

We theoretically investigate the effects of the pulse area and the pulse number in a train of ultrashort pulses with an ultrahigh repetition rate in terms of the population dynamics. Because of the interpulse interference the upper state population exhibits an oscillatory behavior as a function of the laser detuning. The pulse area and the pulse number significantly affect the periodic structure of population oscillation.

Decay properties of a two-level atom in a non-resonant damped cavity

The upper state population evolution and the spontaneous emission spectrum of a two-level atom in a non-resonant damped cavity are investigated by using the resolvent operator. The analytic expressions of the spectrum and the upper state population with strong and intermediate coupling are obtained. The influence of coupling intensity and detuning on atomic spectrum is discussed. We find that the detuning affects strongly the shape of spectra.

Decay properties of a three-level lambda-type atom in three-dimensional photonic crystals

For a three-level Λ-type atom embedded in three-dimensional photonic crystals (PCs), the upper state population and spontaneous emission spectra are investigated with both transitions coupled to the same modified reservoir. By using an anisotropic dispersion model the analytic expressions of the upper state population and spontaneous emission spectra were obtained. The upper state population reaches a steady state value with a slight oscillation, and the spectra show a non-Lorentzian for both transition frequencies inside the band gap.

Discrimination between thermal quenching of the fluorescence and Auger upconversion processes using thermal lens technique

In this work we used the Thermal Lens (TL) technique to discriminate two important processes responsible to reduce the upper-state population and fluorescence quantum efficiency (η) in Cr3+ doped colquiriite crystals: the thermal quenching of the fluorescence (TQF) and the Auger upconversion (ETU). We observed a nonlinear increase of the TL signal with laser power due to the decrease of η by ETU and/or TQF. The analysis of these curves allowed the determination of the thermal load, the increase of the crystal temperature as a function of the pump excitation and the discrimination between ETU and TQF processes.

Limitations of steady state solutions to a two-state model of population oscillations and hole burning

We consider a two-state system driven by an on-resonance, continuous wave pump laser and a much weaker pulsed probe laser that is slightly detuned from the pump laser frequency (usually this detuning is about ${\ensuremath{\omega}}_{p}\ensuremath{-}{\ensuremath{\omega}}_{P}=\ensuremath{\Delta}\ensuremath{\approx}1\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}$). The upper state population is assumed to be slowly decaying, but the off-diagonal element of the density matrix decays rapidly due to homogeneous broadening. This model has been solved by others in rare-earth-element-doped fibers and crystals in a usual steady state approximation for slow optical wave propagation. We show that in general the usual steady state approximation does not apply unless either $\ensuremath{\Delta}\ensuremath{\tau}⪢1$ or $(2S+1){\ensuremath{\gamma}}_{2}\ensuremath{\tau}⪢1$ where ${\ensuremath{\gamma}}_{2}$ is the decay rate of the excited state population, $\ensuremath{\tau}$ is the pulse length of the probe field, and $2S$ is the saturation parameter. Both conditions, however, are not satisfied in many population-oscillation- and corresponding group-velocity-reduction-related studies. Our theory and corresponding numerical simulations have indicated that for probe pulses that are much shorter than the lifetime of the upper state, there is no analytical theory for the amplitude, pulse shape, and group velocity of the probe field. In addition, there is no reason to assume that the group velocity remains small when ${\ensuremath{\gamma}}_{2}\ensuremath{\tau}⪡1$ and there is no reason to believe that many pulse length decays can be obtained for such short pulses.

Non-Markovian decay of a three-levelΛ-type atom in a photonic-band-gap reservoir

Considering a three-level {lambda}-type atom with both transitions coupled to a single photonic-band-gap reservoir, we investigate the upper-state population and spontaneous emission spectra by using the resolvent operator for the isotropic dispersion model, and give their analytical solutions. For the case of two transitions coupled to the same reservoir, the upper-state population always displays an oscillatory behavior and finally reaches a steady-state value. For atomic transition frequencies in the vicinity of the band-gap edge or inside the gap, the spontaneous emission spectra become strongly non-Lorentzian.

Spectroscopic diagnostics of the vibrational population in the ground state of and molecules

A diagnostic method has been evaluated for measuring the relative vibrational ground-state population of molecular hydrogen and deuterium. It is based on the analysis of the diagonal Fulcher bands and the Franck-Condon principle of excitation. The validity of the underlying assumptions was verified by experiments in microwave discharges and the method is recommended for application in divertor plasmas in controlled fusion experiments. By attributing a vibrational temperature to the ground-state electronic level and assuming population via the Franck-Condon principle, the upper Fulcher state vibrational distribution can be derived theoretically with as parameter. Comparison with experimentally derived upper-state population gives the corresponding of the ground state. The Franck-Condon factors for the and transitions have been calculated for both hydrogen and deuterium from molecular constants using the FCFRKR code. The method has been applied to low pressure /He and /He microwave plasmas, showing good agreement of experimentally and theoretically derived upper Fulcher state vibrational distributions. The vibrational temperatures range from 3200 K to 6800 K for and 2600 K to 4000 K for . depending on molecular density, pressure and electron temperature, but indicating nearly the same vibrational population for and for comparable plasma conditions.

Spectroscopic properties and energy transfers in Tm3+ singly- and doubly-doped glasses

Spectroscopic measurements and analysis of energy transfer processes for Tm 3+ singly- and Tm 3+ Ho 3+ doubly-doped glasses pumped with 790 nm diode laser have been performed. The emission cross-sections and the net gain have been determined from the measured absorption spectra for both the ions on the basis of principle of reciprocity. Tm 3+ singly-doped fluorozircoaluminate glass, which has a higher fluorescence efficiency at 1.82 μm and a longer upper state lifetime, is a material suitable for Tm laser compared with oxide glasses. The quantum efficiency of Ho 3+ fluorescence of 5 I 7  5 I 8 at 2.05 μm for the Tm 3+ Ho 3+ doubly-doped glass is dominated by three energy transfer processes: Tm 3+ Tm 3+ cross-relaxation, net Tm 3+ Ho 3+ energy transfer, and energy transfer upconversion. The first process allows pumping efficiency to approach 2, and the second one contributes to population of the upper 5 I 7 laser level, while the final process gives rise to a sublinear increase in upper state population with pump power. Optimum doping levels for laser performance are easily predicted based on spectroscopic measurements and are found to be 2–8Tm 3+ (10 20 /cm 3 ) for Tm 3+ singly-doped and 2–8Tm 3+ /0.3–1Ho 3+ (10 20 /cm 3 ) for Tm 3+ /Ho 3+ doubly-doped systems, respectively.

A complete treatment of a steady-state four-level model for the interpretation of OH laser-induced fluorescence measurements in atmospheric pressure flames

In order to obtain OH concentration measurements in atmospheric pressure flames from saturated fluorescence signal, we have developed a stationary four-level model. A general relationship between the measured upper state population and the total number density has been derived. The effects of the imbalance between the rotational transfer rates in the upper and ground states is analyzed. We show that in the case of comparable rotational rates, the results of our treatment are similar to the ones obtained with the balanced cross rate model. A set of complementary data which should facilitate absolute OH concentration measurements is also pointed out.

Spectroscopy and diode laser-pumped operation of Tm,Ho:YAG

Spectroscopic measurements and analysis of diode laser-pumped operation of Tm,Ho:YAG at 2.1 mu m at room temperature have been performed. The peak effective stimulated emission cross section is measured to be 9*10/sup -21/ cm/sup 2/ at 2.091 mu m and the upper state lifetime is 8.5 ms. Under diode laser pumping, thresholds of 4.4 mW of absorbed power and slope efficiency of 19% have been demonstrated. Calculations of threshold power are performed based on the spectroscopic measurements. An energy transfer upconversion process is identified which leads to a sublinear rise in upper-state population with pump power.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Resonant laser excitation of uranium with a pulsed laser

The authors consider the resonant excitation of the 3868 cm-1 J=3 to 26558 cm-1 J=2 transition in uranium. An experiment has already been done by Scarl et al. (1981) which measures the upper-state population as a function of laser intensity. The authors re-analyse these data taking proper account of the laser's temporal pulse shape. The effect of the laser bandwidth is also included in a novel way. They find a fit to the data which gives different values for the physical parameters from those found by Scarl et al. and draw some general conclusions for experiments of this type.

Approximate analytical relations for radiative recombination of Br-atoms by free-bound transitions

Simple analytical expressions are derived for the emission intensity produced by the recombination of Br-atoms by free-bound transitions for optically thin media and for arbitrary optical depth. It is shown that the radiative free-bound transitions can formally be described by an effective upper-state population which is proportional to the collision-rate density at a relative velocity corresponding to the wavenumber of the transition considered. An analytical expression is given for the rate coefficient of the two-body radiative recombination, which is valid within 5% for temperatures up to 2500°K.

Observation of atomic coherence in laser-excited uranium vapor

For the pair of /sup 238/U levels at 3868 and 26 558 cm/sup -1/, the upper-state population was measured as a function of the irradiance of a resonant exciting pulse of fixed duration. The observed damped Rabi flopping showed that the two-level system was partially coherently excited. The results were compared with the prediction of an exact solution of the time-dependent Schroedinger equation for two-level systems excited by a detuned rectangular pulse. The Schroedinger equation solution, after averaging over laser irradiance fluctuations, agrees well with the data.