Large Population Inversion Research Articles

Experimental realization of wavelength multiplexed nonlinear upconversion in cesium atoms.

Blue-violet wavelength multiplexing offers the capability to enhance the operational capacity and efficiency in optical information processing, underwater communications, biology, and medicine. Here we present a specially designed atomic scheme of cesium for simultaneously generating two blue-violet lasers at 456 and 459 nm via four-wave-mixing (FWM)-based upconversion processes. We experimentally and theoretically show that a double-diamond-type coherent system pumped with 852 and 921 nm lasers is responsible for the efficient generation of the two blue-violet lasers. In this system, the large population inversions lead to self-amplified spontaneous emissions at 15.6 and 12.1µm, which, in turn, serve as the seeds for enhanced double FWM processes with dual-wavelength upconversions.

Pumping wavelength related population inversion in Nd:doped laser

Pumping wavelength related population inversion in an Nd:doped laser has been investigated, where the laser is regarded as two thermodynamic equilibrium systems: the upper state and the lower state. The population inversion limits at various pumping wavelengths and temperatures are compared. The effective metastable lifetime has been derived, where both pump and laser fields within the Nd:doped laser medium reduce the lifetime. The saturation of the lifetime is discussed. As a result, in-band pumping of the Nd:doped laser is good for improving the efficiency and achieving the narrow-linewidth of the laser wavelength due to less population inversion; off-band pumping is beneficial for extracting high pulse energy, thanks to large population inversion.

Nonresonant tunneling phonon depopulated GaN based terahertz quantum cascade structures

GaN based terahertz quantum cascade structures are theoretically studied. Since the Fröhlich interaction is ∼15 times higher in GaN than in GaAs, level broadening makes obtaining appreciable optical gain difficult even with a large population inversion. A density matrix Monte Carlo method is used to calculate the broadening of the optical gain spectra as a function of lattice temperature. We find by using a proposed method of nonresonant tunneling and electron-longitudinal-optical phonon scattering for depopulation of the lower lasing state, that it is possible to sufficiently isolate the upper lasing state and control the lower lasing state lifetime to obtain high optical gain in GaN. The results predict lasing out to 300 K which is significantly higher than for GaAs based structures.

Pumping-power-dependent photoluminescence angular distribution from an opal photonic crystal composed of monodisperse Eu^3+/SiO_2 core/shell nanospheres

High quality opal photonic crystals (PhCs) were successfully fabricated by self-assembling of monodisperse Eu(3+)/SiO(2) core/shell nanospheres. Angular resolved photoluminescence (PL) spectra of a PhC sample were measured with different pumping powers, and its PL emission strongly depended on spectroscopic position of the photonic stop band and the optical pumping power. Suppression of the PL occurred in the directions where the emission lines aligned with the center of the photonic stop band. Suppression and enhancement of the PL were observed at low- and high-pumping powers, respectively, in the directions where the emission lines were located at the edges of the photonic stop band. When pumping power exceeded 6 µJ/pulse, a super-linear dependence was found between the pumping power and PL intensity. The dramatic enhancement of PL was attributed to the amplification of spontaneous emission resulted from the creation of large population inversion and the slow group velocity of the emitted light inside the PhC. The opal PhC provided highly angular-selective quasi-monochromatic PL output, which can be useful for a variety of optical applications.

Intense ultrashort laser–Xe cluster interaction

The last several years have witnessed a surge of activity involving the interaction of clusters with intense ultrashort pulse lasers. The interest in laser–cluster interaction has not been only of academic interest, but also because of the wide variety of potential applications. Clusters can be used as a compact source of X-rays, incoherent as well as coherent, and of fast ions capable of driving a fusion reaction in deuterium plasmas. In one set of xenon cluster experiments, in particular, amplification of ∼2.8 Å X-rays has been observed [28]. X-ray amplification in cluster media is a phenomenon of critical importance and may lead to applications such as EUV lithography, EUV and X-ray microscopy, X-ray tomography, and variety of applications in biology and material sciences. However, while amplification of ∼2.8 Å X-rays has been documented in experiments, the mechanism for producing it remains to be fully understood. In this talk, a xenon model of laser–cluster interaction dynamics is presented to shed light on the processes responsible for amplification. The focus of this research is on the feasibility of creating population inversions and gain in some of the inner-shell hole state transitions within the M-shell of highly ionized xenon. The model couples a molecular dynamics (MD) treatment of the explosively-driven, non-Maxwellian cluster expansion to a comprehensive multiphoton-radiative ionization dynamic (ID) model including single- and double-hole state production within the Co- and Fe-like ionization stages of xenon. The hole-state dynamics is self-consistently coupled to a detailed valence-state collisional-radiative dynamics of the Ni-, Co-, and Fe-like ionization stages of xenon. In addition, the model includes tunneling ionization rates that confirm an initial condition assumption that Ni-like ground states can be created almost instantaneously, on the order of a femtosecond or less, i.e., at laser intensities larger than 1019 W/cm2, all of the N-shell, n = 4 electrons are striped from a xenon atom in less than a femtosecond. Because of the abundance of these ground states, large numbers of n = 2, inner-shell hole states and large population inversions can be created when the Ni-like ground states are photo- or collisionally ionized. Once the M-shell is entered, tunneling ionization slows down as does collisional ionization due to the fall in ion density as the cluster expands. Moreover, as the cluster density goes down, our combined MD and ID calculations show that so do the calculated population inversions. Thus, our calculations do not support the initial experimental data interpretations in which the measured gains have been associated with double holes in more highly ionized stages of xenon (Xe32+, Xe34+, Xe35+, and Xe37+), which our calculations suggest would require laser intensities in excess of 1.5 × 1020 W/cm2, for a 248 nm, ∼250 fs laser pulse focused in a gas of xenon clusters. At laser intensities used in the experiment, such ionization stages would be reached, but only later in time when cluster densities have fallen by several orders of magnitude from their initial values to values where pumping rates are too low and gains cannot be generated.

High-gain all-fiber regenerative amplifier

An all-fiber regenerative amplifier at 1053 nm is demonstrated. The input signal pulse energy is 75 pJ in a 3.5-ns pulse at a 1-Hz repetition rate. At a low level of input pump power of 110 mW, the saturated output energy is 120 nJ with fluctuation less than 2% root mean square (RMS) even the fluctuation of the input pulse be about 15% (RMS). And the signal-to-noise ratio (SNR) is 66 dB. Maximum output energy of 780 nJ with a total gain of more than 40 dB is obtained at pump power of 130 mW. Scaling to higher pulse energy is constrained by stimulated Raman scattering. OCIS codes: 060.2320, 140.3280. doi: 10.3788/COL201210.S20610. As one of the most important components of a highpower laser system, the front end system seeds the subsequent laser system with pulses that have high stability, high beam quality, proper bandwidth, and high signalto-noise ratio (SNR). The output quality of front end determines the output quality of the whole laser system to a large extent. So developing a high-gain and high output stability amplifier with superior beam quality is attractive and important. Fiber optical amplifiers based on rare-earth-doped fibers have been investigated for decades [1 3] because of their inherent compactness, superior beam quality, and stability. Optical pulses with high repetition rate are routinely amplified by one or more staged single-pass rareearth-doped fiber amplifiers. However, the front end of high-power laser system runs in the state of extremely low repetition rate. The low-duty-cycle pulses leave large population inversion in the gain fiber for long durations between pulses, which can lead to serious broadband amplified spontaneous emission (ASE). The ASE competed for the unsaturated gain restricts the gain and degrade the SNR [4] .

Self-consistent Maxwell-Bloch theory of quantum-dot-population switching in photonic crystals

We theoretically demonstrate the population switching of quantum dots (QD's), modeled as two-level atoms in idealized one-dimensional (1D) and two-dimensional (2D) photonic crystals (PC's) by self-consistent solution of the Maxwell-Bloch equations. In our semiclassical theory, energy states of the electron are quantized, and electron dynamics is described by the atomic Bloch equation, while electromagnetic waves satisfy the classical Maxwell equations. Near a waveguide cutoff in a photonic band gap, the local electromagnetic density of states (LDOS) and spontaneous emission rates exhibit abrupt changes with frequency, enabling large QD population inversion driven by both continuous and pulsed optical fields. We recapture and generalize this ultrafast population switching using the Maxwell-Bloch equations. Radiative emission from the QD is obtained directly from the surrounding PC geometry using finite-difference time-domain simulation of the electromagnetic field. The atomic Bloch equations provide a source term for the electromagnetic field. The total electromagnetic field, consisting of the external input and radiated field, drives the polarization components of the atomic Bloch vector. We also include a microscopic model for phonon dephasing of the atomic polarization and nonradiative decay caused by damped phonons. Our self-consistent theory captures stimulated emission and coherent feedback effects of the atomic Mollow sidebands, neglectedmore » in earlier treatments. This leads to remarkable high-contrast QD-population switching with relatively modest (factor of 10) jump discontinuities in the electromagnetic LDOS. Switching is demonstrated in three separate models of QD's placed (i) in the vicinity of a band edge of a 1D PC, (ii) near a cutoff frequency in a bimodal waveguide channel of a 2D PC, and (iii) in the vicinity of a localized defect mode side coupled to a single-mode waveguide channel in a 2D PC.« less

Continuous wave photonic crystal laser in ultraviolet range

It is shown that photonic band-gap (PBG) structures have great potential for the development of widely tunable continues wave and/or mode-locked ultrashort-pulse all-solid-stale lasers in UV and optical ranges. The basic idea is to decrease the laser threshold via inhibition of radiative decay of an upper laser level by embedding an active medium into spatial structure having PBG at the frequency of laser transition. This technique provides favorable conditions for coherent suppression of the excited state absorption crucial for short-wavelength solid-state lasing. It also resolves the laser gain dilemma, providing combination of high emission cross-section and large population inversion. Different designs of 2D and 3D photonic crystal laser are proposed.

Statistical model for energy-transfer-induced up-conversion inEr3+-doped glasses

We present a statistical model for studying excitation upconversion and migration caused by energy transfer between ${\mathrm{Er}}^{3+}$ ions in a glass matrix. Assuming a statistically uniform distribution of ions in the matrix we propose an approach for averaging the solution of the rate equations over possible ensemble configurations. The model provides a transcendental equation for the population inversion and the up-conversion rate in the steady-state case, and an integral equation for \ensuremath{\delta}-pulse excitation. Analytical expressions are given for the asymptotes at small and large population inversions, as well as at the initial time for the dynamic case. The model explains well the experimentally observed nonlinear dependence of the up-conversion rate versus population inversion, and the enhancement of the up-conversion caused by the migration.

Giant pulse lasing in three-level systems

We propose an alternative method to Q switching for generating giant pulses of laser light in three-level media. This mehod is based on the presence of an external coherent field driving one transition to allow the accumulation of a large population inversion in the other transition without laser oscillation even in a cavity with high-$Q$ factor. The switching off of the external coherent field causes the development of the giant pulse. Different time profiles for the switching off of the external field have been investigated.

Effects of spontaneous emission interference on population inversions of a V-type atom

The steady-state population behaviour in a laser driven V-type system has been analysed. The effects of spontaneous emission-induced coherence and of the relative phase of the two coherent driving fields are considered in detail. A large and unexpected population inversion is found on one of the optical transitions due to these coherent effects.

Anomalous resonance fluorescence and dressed-state inversion by squeezed light in a Fabry-Pérot microcavity

It has been suggested that phased atomic decay in a squeezed vacuum could be detected in the fluorescence spectrum emitted from a driven two-level atom in a cavity. Recently, the existence of other very distinctive features in the fluorescence spectra arising from the nonclassical features of the squeezed vacuum has been reported. In this paper, we investigate the possibility of experimental observation of these spectra. The main obstacle to the experimentalist is ensuring an effective squeezed-vacuum-atom coupling. To overcome this problem we propose the use of a Fabry-P\'erot microcavity. The analysis involves a consideration of the three-dimensional nature of the electromagnetic field, and the possibility of a mismatch between the squeezed and cavity modes. The problem of squeezing bandwidths is also addressed. We show that under experimentally realistic circumstances many of the spectral anomalies predicted in free space also occur in this environment. In addition, we report large population inversions in the dressed states of the two-level atom.

A double electron capture by alpha particles in collisions with hydrogen molecules in low temperature plasma

Spectroscopic study has been made of double electron capture process by alpha particles, He 2+, when helium plasmas contact with cold hydrogen molecules. Fast enhancements of He I n = 2, 4 and 5 as well as of He I n = 3 level populations due to the selective double electron capture have been found at the early period of hydrogen gas injection into the helium plasmas. Large population inversion resulting from these enhancements has been also observed between n = 2 and 3 levels. Numerical calculation based on a collisional-radiative model has revealed the enhancement mechanism.

Creation of population inversion by Fano interference in three-level cascade-type system

We demonstrate how Fano interference produces a large population inversion in a three-level cascade-type system involving an autoionizing state. We show that when an intense monochromatic field driving the lower transition of the atom by a two-photon resonant interaction creates a pair of dressed states which then are coupled by a second coherent field to ionization continuum and an autoionizing state, the Fano interference prevents the photoionization from the upper dressed state whereas the second state is quickly exhausted, thus making possible the coherent generation of short-wavelength light free of phase-matching condition. We discuss the incoherent processes which can destroy the Fano interference.

An experimental study of stationary population inversion in a cold recombining expanding helium plasma jet

A stationary cold recombining helium plasma is generated in an arc-heated magnetically trapped expanding plasma jet that was developed in the authors' laboratory. The plasma can be generated very stably and spectroscopic analysis is accomplished with sufficient accuracy. Population densities of the He I levels whose principal quantum numbers are 3-6 and 73D, 83D, 93D levels are observed spectroscopically. When the plasma comes downstream, the electron temperature and density become low and population inversion is obtained between some level pairs, such as 41S-31P and 43P-3 3D. The degree of inversion is insufficient to be used for lasing. The electron temperature as found by Boltzmann plots of the higher states drops to about 0.2 eV in the downstream expansion. The electrons are cooled by contact with cold residual gas. The magnetic confinement of the plasma jet is adequate to cause large population inversions without decreasing the electron density. The experimental results are explained in terms of a calculation based on a collisional radiative model including an optical thickness. The mechanisms of formation of population inversions are discussed in terms of atomic processes.

Resonance fluorescence of a two-level atom in an off-resonance squeezed vacuum

The authors present analytical and numerical results for resonance fluorescence produced by a two-level atom damped by a broad-band squeezed vacuum. Previous theoretical analyses of this model have assumed that the laser frequency and the carrier frequency of the squeezed vacuum are equal. The authors introduce a non-zero detuning between the laser frequency and the carrier frequency while maintaining resonance between the laser frequency and atomic resonance. In the absence of the driving field there is a squeezing threshold: above threshold the decay rate of the atomic polarization can be reduced, and below threshold a squeezing-induced two peak spectrum and frequency shift, which is analogous to the Bloch-Siegert shift, are predicted. In the presence of the driving field, the spectrum is asymmetric for Rabi frequencies smaller or comparable to the detuning of the squeezed vacuum. A symmetric spectrum is obtained for large Rabi frequencies. An interpretation is predicted by means of a dressed-state analysis. The authors find that for not too large Rabi frequencies a remarkably large population inversion can be induced in the dressed states, even at exact resonance of the driving field, provided that the laser frequency and carrier frequency are detuned. Moreover, for detection times larger than the inverse detuning, the spectrum is insensitive to the phase between the driving field and the squeezed vacuum, in marked contrast to the case of no detuning.

Quasi-cw laser action through pulsed laser pumping of metastable states in dense vapors

A new scheme is proposed for exciting laser action employing resonant pumping of metastable states together with collisional energy transfer in hot and dense vapors. The specific case of Ca is presented here as an example. Large population inversions and gain on Ca 4s5s 3S 1−4s4p 3P 2,1,0 transitions at 6162.17 Å, 6122.22 Å and 6102.72 Å are reported. A unique feature here is that these population inversions are maintained over long periods of time after the exciting laser pulse, extending from 90 μs in the case of J=0 state up to 250 μs in the case of J=1 state. This offers an unusual possibility of making quasi-continuous laser sources from short pulse excitation.

Spatial bunching of atoms in the standing light wave

The scattering of atoms on the standing wave is considered. A phenomenon of the atomic bunching along the standing wave is investigated. The multiple time scale perturbation method is used to obtain analytical results for the atomic space distribution. These results are in good agreement with the exact numerical evaluation. The simple expressions for the evolution of the atomic populations of the upper and lower levels are given and the possibility of creation of a large population inversion is reported.

State selective laser photodissociation of Tl 2 at 337.1 nm by optogalvanic spectroscopy

This paper reports the first time achievement of laser photolysis of thallium dimers by single photon absorption of N 2-laser light at 337.1 nm resulting in a large population inversion of thallium 7 2S 1 2 excited state with respect to 6 2P 3 2 state. The photodissociation nitrogen laser pulse is spatially and temporally overlapped with the tunable dye laser pulse that is used for confirming the production of selectively excited thallium atoms. The dye laser excites the thallium atoms from 7 2S 1 2 state to high lying Rydberg states that collisionally ionize giving an ion-current signal which is subsequently processed by a box-car average/integrator and recorded on the chart recorder. The photodissociation of Tl 2 to Tl(7 2S 1 2 ) state demonstrates the existence of the molecular dissociative state 1g that is correlated with Tl 7 2S 1 2 +6 2P 1 2 states. A complete absence of 6 2P 3 2 state population among the photolysis products indicates a 100% prompt population inversion between 7 2S 1 2 and 6 2P 3 2 atomic states.

Laser gain on 3p-3d and 3s-3p transitions and x-ray line ratios for the nitrogen isoelectronic sequence

The populations of the 72 levels belonging to the 2s22p3, 2s2p4, 2p5, 2s22p23s, 2s22p23p, and 2s22p23d configurations of the N i isoelectronic sequence have been calculated for the ions Ar xii, Ti xvi, Fe xx, Zn xxiv, and Kr xxx and for electron densities up to 1024 cm−3. Electron collisional monopole excitation from the 2s22p3 ground configuration to the 2s22p23p configuration contributes significantly to the populations of the excited levels and results in relatively large 2s22p23p populations. The population inversions and laser gain for transitions between the 2s22p23s and 2s22p23p configurations are calculated. It is also found that large population inversions and gain occur between levels in the 2s22p23p configuration and levels in the 2s22p23d configuration that cannot decay to the ground configuration by an electric dipole transition. For these potential laser transitions of the type 2s22p23p-2s22p23d, the reabsorption of the radiation from the lower laser level is less of a problem than for the 2s22p23s-2s22p23p laser transitions. The 2s22p23p 4P5/2-2s22p23d 4D7/2 and 2s22p23p 2F7/2-2s22p23d 2G9/2 transitions represent promising transitions for overcoming some of the problems of opacity in dense plasmas. At high electron densities where the excited levels become collisionally mixed, the intensities of the x-ray transitions from the 2s22p23p levels to the 2s2p4 levels decrease relative to the x-ray transitions from the 2s22p23s and 2s22p23d levels. The density dependence of these x-ray line ratios is presented.