Conditions For Population Inversion Research Articles

Lasing conditions for polarized medium and a free electron lasing using Maxwell’s equations

Maxwell's equations with complex wave number, conductivity and electric Permittivity for electrons moving in a resistive medium under the effect of internal magnetic field and external oscillating electric field has been used to study lasing conditions. When the wave number and the conductivity are considered the medium acts as a resistor and inductor. The amplification factor is directly proportional to number of free electrons beside the internal magnetic field. Amplification takes place when the number of free electrons is very large ad when the applied magnetic field force is larger than the resistive force. This amplification condition is suitable to describe free electron lasing conditions. The proportionality of the amplification factor to the magnetic flux density resembles that obtained by some researchers. When the electric permittivity is also considered as a complex quantity,the amplification factor consists of an additional term depending on the number density of polarized excited atoms. This means that lasing can take place in a polarized medium. In this case lasing conditions require existence of large number of polarized excited atoms. This is equivalent to the population inversion condition.

Design and simulation of an electrically pumped SPASER

In this investigation, a novel SPASER which uses a PIN junction to induce population inversion condition, is designed and simulated using SILVACO software. In the proposed structure, the active region is in the middle and different materials are used for p and n regions to form heterojunctions. This PIN junction is in contact with gold and is forward biased. Free space wavelength of oscillations is 1550 nm that is used in photonic devices, frequently. To form the resonance cavity, two high reflective mirrors are placed at the ends of the plasmonic waveguide. Applied forward voltage and absorption coefficient of the SPASER are 1.2V and −0.33 cm−1, respectively. Thus, the optical gain for a 50 μm cavity length is 1 cm−1. Moreover, the power consumption of the proposed device at these conditions is 1.2 mW. The output plasmonic power is 0.6 mW which yields 50% power efficiency.

Determination of metastable state lifetimes of a high-concentration erbium-doped fiber under population inversion conditions at 980 nm pump and 1.5 µm probe wavelengths

We report two-wavelength pump-probe measurements of the metastable state lifetime in a high-concentration erbium-doped fiber (EDF). The lifetimes under population inversion conditions are measured using a 980 nm pump laser and a 1.5 µm probe laser. The lifetime variation from the lifetime under low excitation conditions to that of a highly inverted population are calculated using a model derived from the rate equations. To confirm the validity of our method, we compare the results obtained with those for temporally resolved spontaneous emissions in the perpendicular direction and those measured using a 1.5 µm single-wavelength pump–probe technique in our previous work.

Continuous two-photon source using a single quantum dot in a photonic crystal cavity

We propose methods for the realization of a continuous two-photon source using a coherently driven quantum dot embedded inside a photonic crystal cavity. We analyze the steady state population in quantum dot energy levels and field inside the cavity mode. We find conditions for population inversion in coherently driven and incoherently driven quantum dots. We discuss the effect of phonon coupling using the recently developed polaron transformed master equation at low temperatures. We show that on increasing temperature, two-photon gain decreases and single photon gain increases, due to phonon-assisted cavity mode feeding. We also propose a scheme for generating a squeezed state of field using four wave mixing.

Heterostructure and Q-factor engineering for low-threshold and persistent nanowire lasing

Continuous room temperature nanowire lasing from silicon-integrated optoelectronic elements requires careful optimisation of both the lasing cavity Q-factor and population inversion conditions. We apply time-gated optical interferometry to the lasing emission from high-quality GaAsP/GaAs quantum well nanowire laser structures, revealing high Q-factors of 1250 ± 90 corresponding to end-facet reflectivities of R = 0.73 ± 0.02. By using optimised direct–indirect band alignment in the active region, we demonstrate a well-refilling mechanism providing a quasi-four-level system leading to multi-nanosecond lasing and record low room temperature lasing thresholds (~6 μJ cm−2 pulse−1) for III–V nanowire lasers. Our findings demonstrate a highly promising new route towards continuously operating silicon-integrated nanolaser elements.

Cooperative excitonic quantum ensemble in perovskite-assembly superlattice microcavities

Perovskites—compounds with the CaTiO3-type crystal structure—show outstanding performance in photovoltaics and multiparameter optical emitters due to their large oscillator strength, strong solar absorption, and excellent charge-transport properties. However, the ability to realize and control many-body quantum states in perovskites, which would extend their application from classical optoelectronic materials to ultrafast quantum operation, remains an open research topic. Here, we generate a cooperative quantum state of excitons in a quantum dot ensemble based on a lead halide perovskite, and we control the ultrafast radiation of excitonic quantum ensembles by introducing optical microcavites. The stimulated radiation of excitonic quantum ensemble in a superlattice microcavity is demonstrated to not be limited by the classical population-inversion condition, leading to a picosecond radiative duration time to dissipate all of the in-phase dipoles. Such a perovskite-assembly superlattice microcavity with a tunable radiation rate promises potential applications in ultrafast, photoelectric-compatible quantum processors.

Analysis of a Direct-Bandgap GeSn-Based MQW Transistor Laser for Mid-Infrared Applications

We propose the design and analysis of a group-IV material-based (Ge0.84Sn0.16/Si0.09Ge0.8Sn0.11) multiple quantum wells (MQW) transistor laser (TL) for mid-infrared applications. The base region incorporates Ge0.84Sn0.16/Si0.09Ge0.8Sn0.11 MQW structures pseudomorphically grown on silicon through GeSn virtual substrate, compatible with CMOS platforms for cost-effective integration of electronic and photonic circuits. With the introduction of a certain amount of tin (α-Sn) content (Sn > 6%) into germanium (Ge), GeSn alloy shows direct bandgap, thereby achieving the population inversion condition. In addition, the use of MQW structures such as the active region enhances gain due to quantum–confinement effects, and better carrier utilization over its bulk counterpart. A comparison is made between the theoretical finding for the proposed Ge0.84Sn0.16 MQW TL and the available experimental and theoretical data for currently employed InGaAs-based single quantum well (SQW) and MQW TLs. Estimated results show that a lower threshold base current of ∼ 2.65 mA and high modulation BW of ∼ 53 GHz can be achieved, which ensures the proposed GeSn-based MQW TL can be a good alternative for III–V-based TL.

Intersubband Transitions in Nonpolar GaN-based Resonant Phonon Depopulation Multiple-Quantum Wells for Terahertz Emissions

We investigate the polarization effect in intersubband transitions in polar and nonpolar GaN-based multiple-quantum well (MQW) structures for terahertz (THz) emissions by using systematic comparisons and design a nonpolar GaN/Al0.2Ga0.8N two-well-based MQW structure with an emitting photon of 7.27 THz (30.07 meV). Its lower energy separation (92.7 meV) matches the resonant phonon depopulation condition for better population inversion. It shows a lower threshold current density Jth at all temperatures (1.548 kA/cm2 at 90 K) and a higher output power of up to 86.1 mW at 5.8 K and 33.6 mW at 100 K. Our results for the polar GaN MQW are very close to the experimental data in the literature. We find that the Jth of the nonpolar GaN MQW increases more slowly than that of the polar GaN MQW as temperature increases, indicating the nonpolar GaN MQW may be a worth-trying direction for improving the operation temperature. These results can provide meaningful references for the design and fabrication of nonpolar GaN-based THz MQW or quantum cascade structures.

Conditions for Population of Energy Levels Inversion when Active Medium Based on Gadolinium Isotopes Gd155 and Gd156 Couple Neutron Pumping

The paper theoretically studies the possibility of energy transformation of fast and epithermal neutrons to energy of coherent photon radiation at the expense of a neutron pumping of the active medium formed by nucleus with longliving isomerous states. The channel of the nucleus formation in isomeric state as a daughter nucleus resulting from the nuclear reaction of neutron capture by a lighter nucleus is taken into consideration for the first time. Assessment of neutron flux spectrum parameters providing transition from the main state into one of the excited ones for the nuclei of isotopes54Xe130,10Ne22is made. It was shown that to transit the isotope nuclei into the excited state by forward neutron scattering on the nuclei it is necessary to “select” the isotopes not only with great specific energy of nucleons coupling but also with a small value of the neutron absorption cross section. Moreover, the paper performs the analysis of cross sections dependence of radiative neutron capture by the nuclei of gadolinium isotopes Gd155and Gd156. As a result, the speed of Gd156nuclei formation is stated to exceed the speed of their “burnup” in the neutron flux. It is provided by a unique combination of absorbing properties of two isotopes of gadolinium Gd155 and Gd156 in both thermal and resonance regions of neutron energy. We have formulated the conditions required for making isotope nuclei excited by forward neutron scattering on nuclei and for storing nuclei in excited states. The relation which allows estimating processes parameters of neutron capture by nuclei, formation and decay of nuclei isomeric states is obtained as a result of analytical solution of differential equations system of nuclide kinetics taking into account the decay of nuclei isomeric states. The paper makes the possibility analysis of neutron pumping of the participating medium created by the hafnium isotope nuclei. The properties of hafnium isotopes nuclei is found to do not allow providing conditions for population inversion of energy levels due to the formation of hafnium nuclei in isomeric state Hf178m2in the neutron flux. The paper shows the possibility of excess energy accumulation in the participating medium created by the nuclei of the pair of gadolinium isotopes Gd155and Gd156due to formation and storage of nuclei in isomeric state at radiative neutron capture by the nuclei of the stable isotope with a smaller mass. It is concluded that when the active medium created by gadolinium nuclei is pumped by neutrons with the flux density of the order of 1013cm-2·s-1, the condition of levels population inversion can be achieved in a few tens of seconds. The wave length of the radiation generated by the medium is 0.0006 nm. Sintered ceramics Gd2O3based on enriched in the 155-th isotope of gadolinium can be considered a possible active medium. Thus, there is a possibility of creation of the laser techniques of new generation with the parameters providing its application in pulse power engineering of the future.

Multiphoton-process-induced coherence effects in a dissipative quantum system

The work is aimed at modeling coherence effects in a dissipative quantum system under the semi-classical density matrix formalism. A number of multiphoton coherence effects have been shown to be feasible by controlling the incoherent pumping mechanism in a generic four-level model. The specialty of the model is in finding inversionless gain without any restrictions being imposed on the decay rates of the driven transitions. Many of the coherence effects, such as absorption cancellation, inversionless gain at the line center, and sidebands of an absorptive line shape accompanied by dispersive switching, are explained using the same model under different parametric conditions. Further, it is shown that modulation in the coherence effects can effectively be controlled by the two- and three-photon excitation mechanisms with the proper choice of the rates of incoherent pumping under a stimulated Raman process. Different conditions of population inversion are found for both coherent and incoherent control of stimulated Raman resonance. In addition to Raman gain, gain without inversion in any standard state basis is found to be achieved in our present model. The proposed scheme can have potential application in controlling the properties of an optical field on propagation through rare earth ion-doped fiber material or other solid-state, as well as atomic, media.

Stimulated emission on impurity – band optical transitions in semiconductors

This paper examines conditions for population inversion and amplification in the terahertz range using impurity – band electron transitions in semiconductors and semiconductor structures. Our estimates indicate that stimulated emission on such transitions under optical excitation of impurities can be obtained in a semiconductor with a sufficiently high doping level if electron heating is restricted. At a laser pump power density near (photon energy of ), the gain in n-GaAs may exceed the loss by provided the electron gas temperature does not exceed . We analyse the influence of the carrier effective mass and doping compensation on the gain coefficient and briefly discuss the use of resonance tunnelling for obtaining stimulated emission on impurity – band transitions in quantum cascade heterostructures.

A numerical demonstration of far field photon intensity enhancement without stimulated emission

A laser is conventionally generated by pumping molecules to a population inversion condition followed by stimulated emission to produce coherent, monochromatic, high intensity beams of light. Herein, we numerically show an alternative way to achieve far field photon intensity enhancement without stimulated emission. The intensity enhancement is achieved by taking advantage of the physical cross section variation of the diffracted light in a two dimensional silver nanoparticle array and the excitation of the quadrupole mode of the metal nanoparticles. The results have potential applications in the development of high intensity light sources for nanodevices.

Population inversion study of GaAs/AlGaAs three-quantum-well quantum cascade structures

A population inversion study of GaAs/AlxGa1-xAs three-quantum-well quantum cascade structures is presented. We derive the population inversion condition (PIC) of the active region (AR) and discuss the PICs on different structures by changing structural parameters such as the widths of quantum wells or barriers in the AR. For some instances, the PIC can be simplified and is proportional to the spontaneous emission lifetime between the second and the first excited states, whereas some other instances imply that the PIC is proportional to the state lifetime of the second excited state.

Graphene Terahertz Lasers: Injection versus Optical Pumping

ABSTRACTIn this paper we demonstrate that graphene is one of the best materials for new types of terahertz lasers as optical and/or injection pumping of graphene can exhibit negative-dynamic conductivity in the terahertz spectral range. We analyze the formation of nonequilibrium states in optically pumped graphene layers and in forward-biased graphene structures with lateral p-i-n junctions and consider the conditions of population inversion and lasing. The latter provides a significant advantage of the injection pumping in realization of graphene terahertz lasers. We benchmark graphene as a prospective material for injection-type terahertz lasers.

Theory research of negative dynamic conductivity in electrically pumped multiple graphene layer structures with split gates

The negative dynamic conductivity of graphene in THz range makes it to be a promise medium in THz radiation and amplification. This paper proposes electrically pumped multiple graphene layer structures with split gates, sets up the theory model of electrically induced n-i-p junction, calculates the ac conductivity associated with the interband and intraband transitions under the conditions of population inversion, discusses the bias voltage, gate voltage, number of graphene layers and the momentum relaxation time dependences of ac conductivity. It is shown that the real part of dynamic conductivity within terahertz range can be negative in certain conditions, namely, interband radiation is greater than the intraband absorption, which demonstrates the feasibility of taking electrically pumped multiple graphene layer structures with split gates as an active medium in radiating terahertz coherent source.

Population inversion by chirped pulses

In this paper, we analyze the condition for complete population inversion by a chirped pulse over a finite duration. The nonadiabatic transition probability is mapped in the two-dimensional parameter space of coupling strength and detuning amplitude. Asymptotic forms of the probability are derived by the interference of nonadiabatic transitions for sinusoidal and triangular pulses. The qualitative difference between the maps for the two types of pulses is accounted for. The map is used for the design of stable inversion pulses under specific accuracy thresholds.

Upconversion excitations in Pr3+-doped BaY2F8 crystal

We report the orange-to-blue and infrared-(IR)-to-blue wavelengths upconversion luminescence in Pr3+:BaY2F8 crystals. Mechanism of the orange light upconversion into blue 3P0 state emission was confirmed to be energy transfer between two Pr3+ ions in the 1D2 state. IR-to-blue upconversion has only been observed under two different color IR pumping. The first resonant step was the 3H4→1G4 ground state absorption transition, and the second resonant transition was the excited state absorption from the 1G4 to 1I6 and 3PJ levels. A comparison of the efficiency of the IR-to-blue upconversion in several praseodymium activated host is presented and discussed. A model of the IR pumped upconversion praseodymium blue laser is presented and the population inversion conditions are calculated.

Static and Dynamic Response Analysis of Raman Injection Quantum Cascade Laser Using Circuit Level Modeling

In this paper a new approach for modeling of Raman quantum cascade lasers (QCLs) is presented which integrates electrical, optical and Raman effects. The equivalent circuit model of laser by employing simplified two-level rate equations of pump and Raman regions is developed. The total equivalent circuit is composed of pump laser circuit coupled to Raman laser circuit which models the behaviors of each component and whole device. By using the presented model the static and dynamic performances of laser for different parameters are investigated. The results of simulation show that the main characteristics of laser such as threshold current, quantum efficiency, output power and modulation response are affected by photon lifetime and variation of population inversion conditions. Static results of model agree favorably with experimental data from devices reported in the literatures.

Concentration dependence of the Er3+ visible and infrared luminescence in Y2−xErxO3 thin films on Si

Y 2 − x Er x O 3 thin films, with x varying between 0 and 0.72, have been successfully grown on crystalline silicon (c-Si) substrates by radio-frequency magnetron cosputtering of Y2O3 and Er2O3 targets. As-deposited films are polycrystalline, showing the body-centered cubic structure of Y2O3, and show only a slight lattice parameter contraction when x is increased, owing to the insertion of Er ions. All the films exhibit intense Er-related optical emission at room temperature both in the visible and infrared regions. By studying the optical properties for different excitation conditions and for different Er contents, all the mechanisms (i.e., cross relaxations, up-conversions, and energy transfers to impurities) responsible for the photoluminescence (PL) emission have been identified, and the existence of two different well-defined Er concentration regimes has been demonstrated. In the low concentration regime (x up to 0.05, Er-doped regime), the visible PL emission reaches its highest intensity, owing to the influence of up-conversions, thus giving the possibility of using Y2−xErxO3 films as an up-converting layer in the rear of silicon solar cells. However, most of the excited Er ions populate the first two excited levels I411/2 and I413/2, and above a certain excitation flux a population inversion condition between the former and the latter is achieved, opening the route for the realization of amplifiers at 2.75 μm. Instead, in the high concentration regime (Er-compound regime), an increase in the nonradiative decay rates is observed, owing to the occurrence of cross relaxations or energy transfers to impurities. As a consequence, the PL emission at 1.54 μm becomes the most intense, thus determining possible applications for Y2−xErxO3 as an infrared emitting material.

Optically active Er3+ centers in Si/Si1 − x Ge x structures under optical pumping

The Si/Si1 − xGex:Er/Si heterostructures, which are of interest for laser applications, have been investigated. The types of optically active Er3+ centers making a dominant contribution to the photoluminescence response of the structures studied are analyzed and their relationship with the parameters of the Si1 − xGex:Er heterolayer and the post-growth annealing conditions is shown. On the basis of the PL kinetic analysis of the structures with an isolated type of optically active Er3+ centers, it is concluded that population inversion of Er3+ ion states can be obtained under optical pumping and the effect of nonradiative recombination channels in Si1 − xGex:Er heterolayers on the excitation efficiency of Er centers and the conditions for population inversion is shown.