Loss-induced suppression and revival of lasing Peng, B.; Özdemir, Ş. K.; Rotter, S. ...
Science (American Association for the Advancement of Science),
10/2014, Letnik:
346, Številka:
6207
Journal Article
Recenzirano
Controlling and reversing the effects of loss are major challenges in optical systems. For lasers, losses need to be overcome by a sufficient amount of gain to reach the lasing threshold. In this ...work, we show how to turn losses into gain by steering the parameters of a system to the vicinity of an exceptional point (EP), which occurs when the eigenvalues and the corresponding eigenstates of a system coalesce. In our system of coupled microresonators, EPs are manifested as the loss-induced suppression and revival of lasing. Below a critical value, adding loss annihilates an existing Raman laser. Beyond this critical threshold, lasing recovers despite the increasing loss, in stark contrast to what would be expected from conventional laser theory. Our results exemplify the counterintuitive features of EPs and present an innovative method for reversing the effect of loss.
When two resonant modes in a system with gain or loss coalesce in both their resonance position and their width, a so-called exceptional point occurs, which acts as a source of non-trivial physics in ...a diverse range of systems. Lasers provide a natural setting to study such non-Hermitian degeneracies, as they feature resonant modes and a gain material as their basic constituents. Here we show that exceptional points can be conveniently induced in a photonic molecule laser by a suitable variation of the applied pump. Using a pair of coupled microdisk quantum cascade lasers, we demonstrate that in the vicinity of these exceptional points the coupled laser shows a characteristic reversal of its pump dependence, including a strongly decreasing intensity of the emitted laser light for increasing pump power.
Neutron impact ionization of helium Feist, J; Liertzer, M; Nagele, S ...
Journal of physics. Conference series,
01/2012, Letnik:
388, Številka:
7
Journal Article
Recenzirano
Odprti dostop
Neutron impact on atoms can be used to study correlated electron dynamics. Through a collision with the nucleus, the neutron effects a true many-body transition by giving a sudden momentum boost to ...all electrons at the same time. We demonstrate this concept for helium atoms, where neutron impact leads to excitation of highly correlated doubly excited states that are not easily accessible otherwise.
We study the crossover between the diffusive and quasi-ballistic regimes of random lasers. In particular, we compare incoherent models based on the diffusion equation and the radiative transfer ...equation (RTE), which neglect all wave effects, with a coherent wave model for the random laser threshold. We show that both the incoherent and the coherent models predict qualitatively similar thresholds, with a smooth transition from a diffuse to a quasi-ballistic regime. The shape of the intensity distribution in the sample as predicted by the RTE model at threshold is also in good agreement with the coherent model. The approximate incoherent models thus provide useful analytical predictions for the threshold of random lasers as well as the shape of the random laser modes at threshold.
We explore excitation and ionization by neutron impact as a novel tool for the investigation of electron-electron correlations in helium. We present single and double ionization spectra calculated in ...accurate numerical ab-initio simulations for incoming neutrons with kinetic energies of up to 150 keV. The resulting electron spectra are found to be fundamentally different from photoioniza- tion or charged particle impact due to the intrinsic many-body character of the interaction. In particular, doubly excited resonances that are strongly suppressed in electron or photon impact become prominent. The ratio of double to single ionization is found to differ significantly from those of photon and charged particle impact.
Controlling and reversing the effects of loss are major challenges in optical systems. For lasers losses need to be overcome by a sufficient amount of gain to reach the lasing threshold. We show how ...to turn losses into gain by steering the parameters of a system to the vicinity of an exceptional point (EP), which occurs when the eigenvalues and the corresponding eigenstates of a system coalesce. In our system of coupled microresonators, EPs are manifested as the loss-induced suppression and revival of lasing. Below a critical value, adding loss annihilates an existing Raman laser. Beyond this critical threshold, lasing recovers despite the increasing loss, in stark contrast to what would be expected from conventional laser theory. Our results exemplify the counterintuitive features of EPs and present an innovative method for reversing the effect of loss.
Summary form only given. We present experimental results on coupled pairs of microdisk quantum cascade lasers (QCLs) emitting in the terahertz (THz) spectral region. This resonator type allows to ...control and to strongly confine the optical mode within the cavity. The coupling is based on the evanescent field of whispering gallery modes (WGMs) formed in each disk. Theoretical calculations predict that counterintuitive effects like pump induced ceasing of lasing due to an exceptional point can be expected in this system. Since QCLs are electrically pumped devices the gain and loss properties of each coupled disk can be individually adjusted electrically, making them ideal candidates for the investigation of these effects. The THz spectral region is chosen for the experiments providing long range evanescent fields which are essential for efficient coupling. A further advantage is that devices with large dimensions can be fabricated which still operate in the single mode emission regime in order to avoid undesired mode competition effects. We realized devices which make use of a double-metal cavity, where the active region is in between 2 metal layers acting simultaneously as electrical contact and waveguide layer. In order to observe the predicted effect additional losses have to be introduced to the lasers. Therefore we reduced the thickness of the waveguide and implemented absorption layers. The disks are fabricated in close vicinity in order to enhance the coupling of the exponentially decaying evanescent field.
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