ABSTRACT
We perform an experimental parametric study of the chaos generated by a laser diode subjected to phase-conjugate feedback. In addition to the typical figure of merit, i.e., chaos bandwidth, the corresponding spectral flatness and permutation entropy at delay is analyzed. Our experimental observations reveal that the chaos can be generated with a bandwidth of ≈29 GHz, a spectral flatness up to 0.75, and a permutation entropy at delay of up to 0.99. These optimized performances are maintained over a large range of parameters and have not been achieved in the conventional optical feedback configuration. Interestingly, reducing the pump current reduces the chaos bandwidth while keeping the spectral flatness and the permutation entropy at delay the same as observed for increased pump current. Our experimental findings are consistent with the presented numerical simulations produced using the Lang-Kobayashi model.
ABSTRACT
An 852 nm semiconductor laser is experimentally subjected to phase-conjugate time-delayed feedback achieved through four-wave mixing in a photorefractive ($ {{\rm BaTiO}_{3}} $BaTiO3) crystal. Permutation entropy (PE) is used to uncover distinctive temporal signatures corresponding to the sub-harmonics of the round-trip time and the relaxation oscillations. Complex spatiotemporal outputs with high PE mostly upwards of $ \sim 0.85 $â¼0.85 and chaos bandwidth (BW) up to $ \sim 31\;{\rm GHz} $â¼31GHz are observed over feedback strengths up to 7%. The low-feedback region counterintuitively exhibits spatiotemporal reorganization, and the variation in the chaos BW is restricted within a small range of 1.66 GHz, marking the transition between the dynamics driven by the relaxation oscillations and the external cavity round-trip time. The immunity of the chaos BW and the complexity against such spatiotemporal reorganization show promise as an excellent candidate for secure communication applications.
ABSTRACT
We present a new example of a mononuclear iron(ii) complex exhibiting a correlated spin-crossover (SCO) transition and strong fluorescence, whose coordination sphere is saturated, for the first time, by six phosphorescent ligands. The interplay between SCO and light emission properties in the thermal region of the spin transition was investigated by means of magnetic, fluorescence, optical absorption and optical microscopy measurements on a single crystal. Overall, the results show an excellent correlation between fluorescence and magnetic data of the present gradual transition, indicating an extreme sensitivity of the optical activity of the ligand to the spin state of the active iron(ii) ions. These results open the way for conceiving new prototypes of pressure and temperature sensors based on this synergy between SCO and luminescence properties. In particular, the fact that cooperative SCO material is not a prerequisite for obtaining such synergetic effects, is useful for the design of thin films or nanoparticles, in which the cooperativity is reduced, for appropriate implementation in nanosized devices to enhance the sensing properties at the nanoscale.
ABSTRACT
We analyze experimentally and theoretically the chaotic dynamics generated by a laser diode subjected to phase-conjugate feedback. Phase conjugation is obtained from four-wave mixing in a BaTiO3 photorefractive crystal. We demonstrate that the chaos bandwidth first increases linearly with feedback ratio but then saturates to relatively high values. With a single optical feedback, a chaos bandwidth up to about 18 GHz is achieved, which is about five times as large as the free-running laser diode relaxation oscillation frequency. Numerical simulations confirm our experimental observations and unveil that the finite depth penetration into the crystal is responsible for the observed saturation.
ABSTRACT
Hybrid perovskite thin films have demonstrated impressive performance for solar energy conversion and optoelectronic applications. However, further progress will benefit from a better knowledge of the intrinsic photophysics of materials. Here, the temperature-dependent emission properties of CH3NH3PbI3 single crystals are investigated and compared to those of thin polycrystalline films by means of steady-state and time-resolved photoluminescence spectroscopy. Single crystals photoluminescence present a sharp excitonic emission at high energy, with full width at half maximum of only 5 meV, assigned to free excitonic recombination. We highlight a strong thermal broadening of the free excitonic emission, due to exciton-LO-phonon coupling. The emission turned to be very short-lived with a subnanosecond dynamics, mainly induced by the fast trapping of the excitons. The free excitonic emission is completely absent of the thin film spectra, which are dominated by trap state bands.
ABSTRACT
The orientation of the high-spin (HS)-low-spin (LS) macroscopic interface at the thermal transition of thin [{Fe(NCSe)(py)2}2(m-bpypz)] crystals is explained by considering the possible vanishing of the structural mismatch between the coexisting phases. The structural property which allows mismatch-free interfaces is characterized. The observed orientations of the interface and the tilt angle between the HS and LS domains are accurately reproduced by a two-dimensional continuous medium model, based on the structural data. Simulations using an atomistic electro-elastic model meet the predictions of the macroscopic analysis and provide information on the distribution of the elastic energy density in the biphasic state. The presence of mismatch-free domain structures can explain the exceptional resilience of these crystals upon repeated switching.
ABSTRACT
The colorimetric analysis of images recorded with an optical microscope during the onset of the spin crossover transformation allows monitoring separately the involved electronic and structural aspects, through the separation of resonant absorption and scattering effects. Complementary information can also be obtained by using the polarized modes of the microscope. These potentialities are illustrated by the observation of [Fe(ptz)(6)](BF(4))(2) single crystals during the onset of the thermal transitions in the 110-140 K range. We characterized the interplay between the electronic (HS <--> LS) and structural (order <--> disorder) transformations. Elastic stresses and mechanical effects (hopping, self-cleavage) generated by the volume change upon electronic transition are also illustrated, with their impact on the photoswitching properties of the crystals.
