Transient Population Inversion Research Articles

Wrinkled Graphene Structure and Localized Surface Plasmon Resonance Induced Stretchable White Random Lasers Based on GLN‐functionalized 2D WS2 Quantum Dots

AbstractThis study presents investigations into the fabrication, characterization, and performance analysis of stretchable white random lasers based on 2D glutamine(GLN)‐functionalized WS2 quantum dots (QDs) enhanced by the integration of Au nanoparticles (NPs) and a wrinkled graphene structure. Wrinkled graphene holds the potential for achieving transient population inversion through electron collisions. Incorporating Au NPs introduces localized surface plasmon resonance (LSPR), which enhances the light‐matter interaction, resulting in reduced lasing thresholds. The GLN‐functionalized WS2 QDs exhibit strong photoluminescence emission, and their integration with a wrinkled graphene structure and Au NPs creates a synergistic effect that enhances the emission efficiency and enables the realization of white random lasing. The extensive characterization and analysis of the emission spectra under different deformation ratios provide valuable insights into the tunability and reliability of these devices, as well as the importance of light trapping due to the wrinkled graphene structure. The findings of this study underscore the significant potential of LSPR and wrinkled graphene structure induced stretchable and white random lasers based on 2D GLN‐functionalized WS2 QDs. These lasers may have a promising application in the field of flexible and wearable photonics, which is a critical step towards the development of next‐generation optoelectronic devices with improved performance.

Transient nuclear inversion by x-ray free electron laser in a tapered x-ray waveguide

By restricting the spatial energy transmission of an electromagnetic wave, dielectric waveguides transmit light over long distances at sustained intensity. Waveguides have been used in the microwave and optical range to maintain strong signal intensities in connection with lasers, but guiding of intense short-wavelength radiation such as x-rays has proven more cumbersome. Here we investigate theoretically how tapered x-ray waveguides can focus and guide radiation from x-ray free electron lasers. Elliptical waveguides using a cladding material with high atomic number such as platinum can maintain an x-ray intensity up to three orders of magnitude larger than in free space. This feature can be used to significantly enhance resonant interactions of x-rays, for instance driving nuclear transitions up to transient nuclear population inversion. This could be the first breakthrough in nuclear state population control. Our results anticipate the important role of tapered x-ray waveguides in the emerging field of x-ray quantum optics with nuclear transitions.

Transient stimulated emission from multi-split-gated graphene structure

Mechanism of transient population inversion in graphene with multi-split (interdigitated) top-gate and grounded back gate is suggested and examined for the mid-infrared spectral region. Efficient stimulated emission after fast lateral spreading of carriers due to drift–diffusion processes is found for the case of a slow electron–hole recombination in the passive region. We show that with the large gate-to-graphene distance, the drift process always precedes the diffusion process, due to the ineffective screening of the inplane electric field by the gates. Conditions for lasing with a gain above 100 cm−1 are found for cases of single- and multi-layer graphene placed in the waveguide formed by the top and back gates. Both the waveguide losses and temperature effects are analysed.

Microscopic mechanism for transient population inversion and optical gain in graphene

Based on microscopic calculations, we predict a transient femtosecond population inversion in graphene suggesting graphene as a new active gain material covering a broad frequency range. In this paper, we microscopically shed light on the underlying elementary many-particle processes: Transient gain and population inversion occur due to an interplay of strong optical pumping and carrier cooling that fills states close to the Dirac point giving rise to a relaxation bottleneck. The subsequent femtosecond decay of the optical gain is mainly driven by Coulomb-induced Auger recombination. Our findings are in excellent agreement with recent experimental data.

Transient Population Inversion Induced and Probed by a Signal Pulse in a Continuous-Wave-Pumped Erbium-Doped Fiber Amplifier

Transient Population Inversion Induced and Probed by a Signal Pulse in a Continuous-Wave-Pumped Erbium-Doped Fiber Amplifier

Transient Population Inversion Induced and Probed by a Signal Pulse in a Continuous-Wave-Pumped Erbium-Doped Fiber Amplifier

We report on the transient population inversion in a continuous-wave (CW)-pumped erbium-doped fiber amplifier. The temporal profile of the population inversion of the erbium-doped fiber amplifier is probed by a signal pulse that induces a decrease in the population inversion. The pump and signal power dependences of the profile are measured. We analyze the dependences by applying an analytical formula describing the temporal evolution of the population inversion at the input end of the fiber.

Standoff spectroscopy via remote generation of a backward-propagating laser beam.

In an earlier publication we demonstrated that by using pairs of pulses of different colors (e.g., red and blue) it is possible to excite a dilute ensemble of molecules such that lasing and/or gain-swept superradiance is realized in a direction toward the observer. This approach is a conceptual step toward spectroscopic probing at a distance, also known as standoff spectroscopy. In the present paper, we propose a related but simpler approach on the basis of the backward-directed lasing in optically excited dominant constituents of plain air, N(2) and O(2). This technique relies on the remote generation of a weakly ionized plasma channel through filamentation of an ultraintense femtosecond laser pulse. Subsequent application of an energetic nanosecond pulse or series of pulses boosts the plasma density in the seed channel via avalanche ionization. Depending on the spectral and temporal content of the driving pulses, a transient population inversion is established in either nitrogen- or oxygen-ionized molecules, thus enabling a transient gain for an optical field propagating toward the observer. This technique results in the generation of a strong, coherent, counterpropagating optical probe pulse. Such a probe, combined with a wavelength-tunable laser signal(s) propagating in the forward direction, provides a tool for various remote-sensing applications. The proposed technique can be enhanced by combining it with the gain-swept excitation approach as well as with beam shaping and adaptive optics techniques.

Ultrafast nonlinear optical response of photoexcited Ge/SiGe quantum wells: Evidence for a femtosecond transient population inversion

Femtosecond time-resolved pump-probe spectroscopy is used to investigate the ultrafast carrier dynamics of Ge/SiGe quantum wells grown on a Si substrate. Pronounced nonequilibrium effects in the relaxation dynamics of the optically injected carrier distributions are observed and analyzed using a microscopic many-body theory. Transient population inversion and optical gain is obtained on a femtosecond time scale for excitation at energies slightly above the lowest direct quantum-well transition.

Amplification of mid- and far-IR pulses in synchronously pumped low-dimensional heterostructures

The synchronous propagation of optical pulses and mid- and far-IR pulses in waveguide low-dimensional quantum-well semiconductor heterostructures is considered. It is shown that low-frequency radiation can be considerably amplified in these systems even at room temperature because a high-power high-frequency pulse produces transient population inversion at the corresponding low-frequency transition formed by the dimensional quantisation levels of the semiconductor. The optimal parameters of the pump optical pulse and heterostructure are determined at which mid- or far-IR picosecond pulses of power ≈100 mW or 0.1 mW, respectively, can be amplified by more than two orders of magnitude without considerable changes in their duration and shape.

Efficient amplification of mid- to far-infrared pulses due to optical pulse conversion in waveguiding quantum-well heterostructures

A method is suggested of efficient mid- to far-infrared (ir) pulse amplification via driving optical pulse energy conversion in waveguiding quantum well (QW) heterostructures at room temperature. It is based on the optical pulse creating transient population inversion at a long-wavelength transition in the three-level scheme formed by QW levels of dimensional quantization. As a result, efficient amplification of a weak mid- to far-ir pulse, propagating simultaneously with the driving one, becomes possible. A waveguide and QW heterostructure design that is optimal for optical-long-wavelength pulse conversion is proposed and shown to be much simpler than that used in quantum cascade lasers. For the typical input peak power of a picosecond driving pulse and mid- to far-ir pulse of $\ensuremath{\sim}100\phantom{\rule{0.3em}{0ex}}\mathrm{W}$ and $100\char21{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{mW}$, respectively, the present scheme is able to produce output ir pulses with peak powers of at least several tens of watts or several tens of milliwatts in the mid- or far-ir range, respectively, i.e., to convert an appreciable part of the optical pulse energy into the mid-ir signal.

Feasibility of achieving gain in transition to the ground state of C VI at 34 nm

We present numerical studies of recombination gain in the transition to the ground state of H-like C (2→1 transition at λ=3.4 nm). It is shown that high gain (up to about 180 cm−1) can be achieved using currently available, relatively compact, laser technology. The model includes the ionization of the plasma by an ultraintense, ultrashort laser pulse, followed by plasma expansion, cooling, and recombination. Transient population inversion is generated during the recombination process. We investigate the behavior of the gain with respect to different plasma parameters and pump pulse parameters and explain how the different properties of the plasma and the pump pulse affect the gain.

Interpretation of transient V scheme amplification without inversion

Amplification without inversion (AWI) in a resonant V-type system is studied using a dressed basis which reduces the coherent interaction of both involved optical transitions to that of a single two-level system. The transient AWI recently reported through a numerical analysis of the density matrix equations of the V-system in the bare atom basis (Zhu Y 1996 Phys. Rev. A 53 2742) is damped Rabi nutation with transient population inversion in this dressed basis. Although the transient AWI in the bare state basis occurs both with and without an incoherent pump field, the integrated probe gain is positive only in the first case.

Soft-x-ray lasing to the ground states in low-charged oxygen ions.

Optimal schemes for the creation of a transient population inversion between the ground and excited states of an ion in a subpicosecond-pulse laser-produced plasma are discussed. First results on experimental investigations of possible amplification in the soft-x-ray spectral region on transitions to the gorund states in O ii, O iii, C iii, and N iii ions are reported. Ions are produced due to optical field ionization of ${\mathrm{O}}_{2}$, ${\mathrm{CO}}_{2}$, and ${\mathrm{N}}_{2}$ gases in an intense Ti:sapphire laser field. Amplificaiton on the 2p3s${(}^{3}$P)-2${\mathit{p}}^{2}$${(}^{3}$P) transition (\ensuremath{\lambda}=374.12 \AA{}) in O iii ions and on the 2${\mathit{p}}^{2}$3s${(}^{2}$P)-2${\mathit{p}}^{3}$${(}^{2}$D) transition (\ensuremath{\lambda}=616.56 \AA{}) in O ii ions has been observed.

Nonlinear behaviour of atomic fluorescence in mercury vapours following double-resonance laser excitation

When mercury atoms, present with a buffer gas (N2 or Ar) in a quartz cell or in a graphite furnace are excited into the 73S1 level by means of two pulsed dye lasers tuned at 253.652 nm (61So → 63P1) and 435.835 nm (63P1 → 73S1), a laser-like, collimated emission is observed along the axis of the cell at the green mercury line (73S1 → 63P2), 546.074 nm). This radiation exhibits a pronounced non-linear dependence upon the number density of the mercury atoms in the cell. This behaviour is interpreted as being due to amplified spontaneous emission (ASE), which occurs as a result of the transient population inversion between the 7s3S1 and 6p3P 2 o levels. The parameters governing the population inversion can be deduced by applying a rate equation analysis to the atomic system. A time-resolved observation of both ASE and spontaneous fluorescence signals confirms the totally different characteristics of the two emission processes.

Population inversion on the resonance line of strontium by using cascading superfluorescences in a three-level system

The observation of a transient population inversion in the resonance line of strontium is reported. Basically the process involves a two-photon excitation of the upper state in a three-level system, including the resonant level as the intermediate one. Both transitions involved emit superfluorescent pulses revealing the achievement of population inversions. Characteristics of the process are investigated in time and frequency domains.

Population inversion of Cd<sup>+</sup>excited states in the plasma of cadmium vapor vacuum arc

Population densities of excited states in a cadmium vacuum arc plasma were investigated spectroscopically and their absolute values were calculated using absolute measurements of the spectral lines' intensity emitted from the plasma. A transient population inversion was observed among the ng^{2}G(n = 6-11) group of levels and between the upper and lower levels of the transitions: 6g^{2}G_{9/2} \rightarrow 4f^{2}F_{7/2}, 10g^{2}G_{9/2} \rightarrow 4f^{2}F_{7/2}, 11g^{2}G \rightarrow 4f^{2}F_{7/2} , whose maximal estimated one-pass gain values are 1.75, 1.002, and 1.004, respectively.

Transient population inversion at 18.2 nm in a laser produced CVI plasma

Population inversion at 18.2 nm in plasma produced by 2J, 150 ps pulse irradiation of solid carbon targets is deduced from time and space resolved CV and CVI resonance emission spectra obtained with a VUV streak camera.

Enhanced light emission from a pulsed discharge in hydrogen

Stimulated emission by traveling wave amplification is observed on a low pressure (5 × 10 −2 torr), periodically pulsed capacitive discharge in hydrogen. Transient population inversion and anisotropic emission in the visible spectral region (H α , H β , H γ ) occur during a state of convective instability along the direction of propagation.