Session R61: Nonequilibrium Laser Science

Live

Laser beam combining has attracted much attention recently for free space laser communication and high power laser applications. Most of the methods used required additional components in the beam’s path such as beam splitters, polarizers, diffraction gratings, or fiber optics combining. This reduces the power and affects the combined beam profile. In this study, we present a model for incoherently combining three laser beams using a system of two optical lenses to allow for longer overlap distance between the beams and minimum power loss. We have developed a ray tracing model based on ABCD theory and finite elements method to conduct ray tracing and spatially combine the three incoherent laser beams. The initial horizontal spread of the three beams was 21.4 mm with each beam measuring 11.2 mm in waist. Using our optimized system, we were able to combine 98% of the three beams at a distance of 9 m. The overlap of the combined beams only changed by 2% over a long distance range of 4 m (from 7 m to 11 m). This result did not require any additional optical adjustments or components. The method is currently in the process of implementation experimentally indoors under laboratory conditions and will be extended to about 1 km in outdoors environment.   

Live

GeSn alloys grown on Si are attractive a possible gain medium for Si-based infrared lasers. GeSn waveguides have recently been shown to exhibit lasing at low temperatures, but room temperature lasing has not been achieved. In this work, we analyze the experimental results of optical emission from n-type GeSn (5-6% Sn) waveguides under optical pumping at room temperature. The data indicate that lasing is not achieved but that optical gain leads to a nonlinear increase in output power with respect to pump power. We have developed a theoretical model for the optical gain in the GeSn material and the emission spectrum and power dependence of the waveguides that includes both spontaneous and stimulated emission. When compared to our experimental results, it shows that stimulated emission plays a role in the waveguide emission, and that the it was just below the lasing threshold. We validated this model by reproducing experimental data from the literature where lasing was observed at low temperature in undoped GeSn (12% Sn) waveguides, and our model’s predictions for the lasing threshold temperature and spectral properties of the emission agree with the experimental data.   

Live

We investigate the non-equilibrium indirect exchange interaction in irradiated, magnetically doped two-dimensional election systems. Our theory goes beyond the linear response regime corresponding to the conventional RKKY theory and is non-perturbative in the strengths of both the electron-impurity spin interaction and the driving light field. Using magnetically doped graphene as an example, we focus on the case of strong coupling between magnetic impurities and present analytic and numerical results of the time-averaged exchange coupling as a function of the driving light field and other system parameters. In particular, we discuss the effects of interplay between non-equilibrium Floquet driving and quantum interference effects due to higher-order electron-impurity spin scattering.   

Live

Periodic driving of quantum systems offers a new platform for controlling and engineering materials’ properties. We present a theory for the time-averaged polarization function of a monochromatically irradiated two-dimensional electron gas. Our theory shows that non-equilibrium Floquet excitations lead to the appearance of new non-analytic features in the polarization function, i.e., non-equilibrium Kohn anomalies. We obtain approximate analytic formulas for the polarization function and Kohn anomalies by approximating the Green’s functions in the Keldysh-Floquet formulation of the polarization function in terms of successive powers of light-matter coupling. We find that irradiation introduces considerable corrections to the n=0 equilibrium-like Kohn anomaly. Additionally, our results show that the Kohn anomalies associated with electrons traversing Floquet sidebands with |n|>0 are proportional to even powers of the light-matter coupling and that all the Kohn anomalies share the same non-analytic behavior. We discuss the consequences of these new Kohn anomalies on the screened Coulomb potential, Friedel oscillations, and the RKKY interaction.   

Live

The cubic nonlinear process of three-photon down-conversion is investigated. It is shown that the onset of parametric oscillations of the subharmonic signal is accompanied by phase tristability. An analytically solvable second-order oscillator model is derived for the signal. The stability of the ternary phase states is proven through a Lyapunov function and the influence of noise is investigated. The presence of phase tristability can be utilized for all-optical implementation of the Potts model for unconventional computing applications.   

Live

We offer a new perspective to study the interplay between intrinsic emittance and quantum efficiency (QE), and the challenges associated with tailoring the phase-space of electrons. This alternative photoemission mechanism, coined as Transient Work Function Gating (TWFG)[1], has the potential to circumvent the long-standing trade-off in photoemission between intrinsic emittance and QE. TWFG is theorized to be able to temporally and energetically control the carrier density in near-threshold photoemission using, in addition to a photoemission laser, both strong-field single-cycle[2] and multi-cycle[3] terahertz fields. We will examine TWFG in the production of electron beams for ultrafast electron sources, linear accelerators, and X-ray free-electron lasers.

[1] Carbajo, Sergio. "Transient Work Function Gating: A New Photoemission Regime." Journal of Applied Physics 128, 023102 (2020);
[2] Wu, Xiaojun, et al. "Terahertz generation in lithium niobate driven by Ti: sapphire laser pulses and its limitations." Optics letters 39.18 (2014): 5403-5406.
[3] Carbajo, Sergio, et al. "Efficient narrowband terahertz generation in cryogenically cooled periodically poled lithium niobate." Optics letters 40.24 (2015): 5762-5765.   

Live

With the advent of augmented brightness x-ray free electron lasers (XFELs), such as the LCLS-II, there are ever-increasing requirements on increasing the brightness of electron sources. One avenue to increase the brightness of an XFEL is through reduction of electron beam emittance and energy spread via photoinjector laser shaping[1]. We present a novel method for the simultaneous upconversion and shaping ultraviolet pulses via sum frequency generation. We demonstrate the mixing of two broadband infrared pulses that are efficiently upconverted to narrowband pulses with flat temporal profile as demanded by LCLS-II photoinjector requirements[2]. Additionally, we discuss laser-based spatio-temporal shaping and conditioning of electron beam phase space in order to selectively promote XFEL operational modes.
1. Penco, G., et al. "Experimental demonstration of electron longitudinal-phase-space linearization by shaping the photoinjector laser pulse." Physical Review Letters 112.4 (2014): 044801.
2. Gilevich, Sasha, et al. "The LCLS-II Photo-Injector Drive Laser System." 2020 Conference on Lasers and Electro-Optics (CLEO). IEEE, 2020.   

Live

Two-Dimensional (2D) transition metal dichalcogenides (TMDs) have recently been studied as promising materials with tunable optoelectronic properties. Here we used scanning probe microscopic techniques and optical spectroscopy to observe strong coupling in hybrid systems using gold nano-particles (AuNPs) in 2D materials such as MoS2 and WS2 nano-layer materials and lateral heterostructures. Surface morphology and contact potential difference have been measured using atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM) providing correlated nanoscale information. Tip-enhanced PL (TEPL) maps of the aggregated AuNPs showed large Rabi splitting under ambient conditions. Our result may be used in various applications such as nano-optoelectronics, quantum computers and biosensing.   

On Demand

The orbital angular momentum (OAM), both integer and fractional, carried by vortex light beams holds a great promise for forefront development of multi-dimensional high capacity spin-OAM optical communication and quantum information–processing technologies. While integrated vortex beam generators have been previously demonstrated in different optical settings, dynamical and ultrafast control of (fractional) OAM laser emission with low-power control, suitable for high-speed optical communication and computing, remains challenging. By harnessing the properties of total angular momentum conservation, spin-orbit interaction, optically controlled non-Hermitian symmetry breaking and fast transient optical gain dynamics, we demonstrate an on-chip integrated (fractional) OAM-tunable vortex microlaser, providing ultrafast reconfigurable chiral light emission with desired topological charge at a single telecom wavelength. Our work provides a route for the development of the next generation of multi-dimensional high capacity information system in both classical and quantum regimes.   

On Demand

We demonstrate that free-standing multi-walled CNTs exhibit extraordinary nonlinear THz responses when the CNTs are optically excited. Strong THz pulses induce nonlinear absorption in unexcited CNTs due to the carrier generation via interband tunneling and impact ionization, where the field-induced absorption monotonically increases as the field gets stronger. The THz transmission of optically excited CNTs is, however, not monotonically decreasing as the field strength increases. The trend is reversed in the intermediate range of the THz field strength between 550 and 650 kV/cm, where the CNTs become more insulating. The transition is abrupt and the change in transmission is very large. There are two possible underlying mechanisms. One is that the field-induced band bending gives rise to a misalignment between the conduction and valence bands, preventing interband tunneling in this intermediate range, while the strong THz fields enhance the carrier-carrier scattering and lower the carrier mobility. Field-enhanced charge recombination is also possible, where strong electric fields mediate the Coulomb interaction between electronhole pairs and enhance charge recombination under non-equilibrium conditions.