Properties Of Gain Media Research Articles

The Impact of the Gain Medium Properties and the Resonator Morphology on the Whispering Gallery Mode Spectrum of Polystyrene Microspheres Coated with AgInS2/ZnS Quantum Dots

The modulation of fluorophore emission by whispering gallery modes (WGMs) in active spherical resonators holds great potential for advanced photonic devices. In the present work, we fabricate and systematically study a series of active WGM resonators based on polystyrene microspheres and heavy metal-free ternary AgInS2/ZnS quantum dots with broad and tunable photoluminescence bands coated on their surface as a gain medium. The variation in the mean size and emission wavelength of the quantum dots allowed us to establish a correlation between their characteristics and the WGM properties, including the resonance peaks intensity profile and the Q-factors. Other aspects such as coupling efficiency, polarization-dependent mode damping, and peak broadening are also discussed. Furthermore, we applied statistical analysis on multiple individual resonators within the batch to calculate the standard deviation of the resonator size and ellipticity. Our study highlights that both the cavity and the gain medium influence the WGM parameters in active spherical resonators.

Frequency‐Modulated Combs via Field‐Enhancing Tapered Waveguides

AbstractFrequency‐modulated (FM) combs feature flat intensity spectra with a linear frequency chirp, useful for metrology and sensing applications. Generating FM combs in semiconductor lasers generally requires a fast saturable gain, usually limited by the intrinsic gain medium properties. Here, it is shown how a spatial modulation of the laser gain medium can enhance the gain saturation dynamics and nonlinearities to generate self‐starting FM combs. This is demonstrated with tapered planarized terahertz (THz) quantum cascade lasers (QCLs). While simple ridge THz QCLs typically generate combs presenting a mixture of amplitude and frequency modulation, the on‐chip field enhancement resulting from extreme spatial confinement leads to an ultrafast saturable gain regime, generating a pure FM comb with a flatter intensity spectrum and a clear linear frequency chirp. The observed linear frequency chirp is reproduced using a spatially inhomogeneous mean‐field theory model, which confirms the crucial role of field enhancement. In addition, the modified spatial temperature distribution within the waveguide results in an improved high‐temperature comb operation, up to a heat sink temperature of 115 K, with comb bandwidths of 600 GHz at 90 K. The spatial inhomogeneity leads as well to very intense radio frequency (RF) beatnotes up to ‐30 dBm and facilitates dynamic switching between various harmonic states in the same device.

Homogeneous enhancement of near-fields in all-dielectric metasurfaces with cluster-based unit cells.

To construct a dielectric analog of a spaser, we study several configurations of cluster-based unit cells for an all-dielectric metasurface characterized by resonant conditions of the trapped mode excitation. Excitation of the trapped mode is realized by performing either specific displacement of particles in the cluster or perturbation of the equidistantly spaced particles by off-centered holes. The latter approach is more advantageous for enhancement of the electric near-field with homogeneous distribution in-plane of the structure and strong field localization outside the high-refractive-index dielectric particles. This feature opens prospects for realization of subwavelength flat lasing structures based on strong near-field interaction with substances exhibiting nonlinear characteristics and properties of gain media.

Universal Quasi-Level Parameter for the Characterization of Laser Operation

Impact Statement: The classification of lasers in three- or four-level schemes is of fundamental importance for describing their operational characteristics. From solid state and fiber lasers, it is well known that the observed characteristics does not coincide with the number of physical levels, and typically even intermediate behavior is observed, which can be described by a continuous quasi-level parameter. Here we generalize the quasi-level concept so that it can be applied to a wide range of laser media. Abstract: A parameter is proposed which classifies the laser operating characteristics according to the quasi-level terminology, i.e., as intermediate behavior between that of an ideal two- and three-level or three- and four-level laser scheme. Since the quasi-level parameter is purely based on a generic rate equation description of the laser, no inherent assumptions about gain medium properties or the pumping process are required. The validity of the quasi-level parameter is verified for various prototypical laser schemes. As a specific example of a nonideal laser, the operating behavior of an experimental quantum cascade laser is classified, which constitutes a quite generic laser type since the active region properties can be custom-engineered by quantum design.

Investigations of influences of spontaneous emission properties on surface plasmon polariton amplifications with the rate-equation theory

The influences of the position-related spontaneous emission properties of gain media on gain and amplified spontaneous emission (ASE) noise characteristics of surface plasmon polariton (SPP) amplification are investigated with rate-equation theory in this paper. It is found that both the spontaneous emission rate and the probability of spontaneous emission into the SPP mode of the gain media in an SPP waveguide are position related. Based on this, a rate-equation model considering those spontaneous emission properties is presented. With this model, the influences of the position-related spontaneous emission properties on the gain and ASE noise characteristics of a single-mode dielectric-loaded SPP amplifier are discussed. The results show that the spontaneous emission rate of gain media is enhanced near the metal interface and the probability of spontaneous emission into the SPP mode decreases near both the metal and air interfaces of the amplifier. This gives rise to reduced gain and low ASE output power. Moreover, the noise property of the amplifier may be improved because of the great decrease in the output ASE power.

Optimum design considerations for CW intra-cavity second harmonic generation in end-pumped solid-state lasers

An understanding of performance of a laser system in terms of design parameters is of the great importance to the engineer who is designing a laser system. This manuscript describes a theoretical report on the optimum design key parameters of an intracavity frequency doubled end-pumped solid-state laser based on the combination of space-dependent rate equations and nonlinear coupled-wave analysis of second-harmonic generation. The theoretical framework along with the results presenting the effect of various parameters such as pump beam characteristics, gain medium and nonlinear crystal properties to optimize the SHG efficiency has been investigated. By considering the optimized root mean square of pump beam radius as the effective value of pump spot size in the gain medium and perfect phase matching between fundamental and second harmonic waves,optimum nonlinear crystal's length, optimum mode size and second harmonic output power are derived as a function of pump-beam characteristics, and properties of the nonlinear and gain medium crystals. The results show that the optimum nonlinear crystal's length depends not only on the nonlinear coefficient and spot radius of laser beam but also on the pump beam properties and gain medium characteristic. In addition, the effect of pump and gain medium properties on the optimal fundamental beam by taking into account thermal aberration are investigated. The obtained results give useful guidelines for choosing the suitable active and nonlinear crystals for a given pump source and allow the straightforward evaluation of laser parameters for efficient intracavity doubling in end-pumped solid-state lasers.

Efficient design considerations for end-pumped solid-state-lasers

The temperature distribution in the gain medium and key design parameters for an ideal-four level end-pumped solid-state laser have been analyzed depending on the crystal's length, absorption coefficient, and pump beam M 2 factor. The optimum key design parameters and thermal focal lens are obtained by minimizing the root mean square of pump beam radius in the laser crystal. It is found that the focal thermal lens and key design parameters are dependent on the gain medium characteristics and pump beam properties as well. By considering the Poisson equation in cylindrical coordinate and Top-Hat pumping profile, an analytical formula has been derived to introduce the thermal focal length in the end-pumped lasers. A formula is also presented to relate the requirements of pump source to the gain medium properties for working laser at the design point.

Angular effects and beam quality in optical parametric amplification

Advances in optical parametric devices, in particular those requiring high conversion efficiency, rely on pump laser and gain medium properties. We describe and theoretically model the source of dephasing due to angular deviation from ideal phase matching in optical parametric amplification. Real laser beams have angular content, which is described by their spatial frequency spectrum. Such beams cannot be treated as single plane waves in nonlinear interactions. Our mathematical model is based on a plane wave decomposition of Gaussian and top-hat beams into their components in spatial frequencies. Several popular nonlinear materials (beta-barium borate, lithium borate, and potassium dihydrogen phosphate) are examined for phase matching angles and dephasing is rigorously calculated. The impact of the beam angular content on small signal gain and on conversion efficiency in the strongly depleted regime is evaluated numerically. In addition, a criterion is formulated for beam quality tolerance in optical parametric amplifiers, for critical and noncritical phase matching. The impact of initial conditions in optical parametric amplification is considered. Our calculations are intended primarily for devices pumped with long (nanosecond) pulses.

Many-body Coulomb effects in room-temperature II–VI quantum well semiconductor lasers

This letter investigates the gain medium properties in II–VI blue-green quantum well semiconductor lasers, including band structure and carrier interactions in the electron-hole plasma are found to be significantly more important than in infrared III–V lasers. In particular, the interband Coulombic enhancement of the optical transitions, together with a band gap renormalization result in an increase in gain and a reduction in the antiguiding or linewidth enhancement factor.