Optical damage limit of efficient spintronic THz emitters.

THz pulses are generated from femtosecond pulse-excited ferromagnetic/nonmagnetic spintronic heterostructures via inverse spin Hall effect. The highest possible THz signal strength from spintronic THz emitters is limited by the optical damage threshold of the corresponding heterostructures at the excitation wavelength. For the thickness-optimized spintronic heterostructure, the THz generation efficiency does not saturate with the excitation fluence even up till the damage threshold. Bilayer (Fe, CoFeB)/(Pt, Ta)-based ferromagnetic/nonmagnetic (FM/NM) spintronic heterostructures have been studied for an optimized performance for THz generation when pumped by sub-50 fs amplified laser pulses at 800 nm. Among them, CoFeB/Pt is the best combination for an efficient THz source. The optimized FM/NM spintronic heterostructure having α-phase Ta as the nonmagnetic layer shows the highest damage threshold as compared to those with Pt, irrespective of their generation efficiency. The damage threshold of the Fe/Ta heterostructure on a quartz substrate is ∼85 GW/cm2.

Enhancement in optically induced ultrafast THz response of MoSe2MoS2 heterobilayer.

THz conductivity of large area MoS2 and MoSe2 monolayers as well as their vertical heterostructure, MoSe2MoS2 is measured in the 0.3-5 THz frequency range. Compared to the monolayers, the ultrafast THz reflectivity of the MoSe2MoS2 heterobilayer is enhanced many folds when optically excited above the direct band gap energies of the constituting monolayers. The free carriers generated in the heterobilayer evolve with the characteristic times found in each of the two monolayers. Surprisingly, the same enhancement is recorded in the ultrafst THz reflectivity of the heterobilayer when excited below the MoS2 bandgap energy. A mechanism accounting for these observations is proposed.

Temperature-Dependent Ultrafast Response and π-Plasmon Dynamics in Single-Walled Carbon Nanotubes.

Temperature-dependent femtosecond time-resolved carrier relaxation dynamics has been studied in thin films of single-walled carbon nanotubes. An early time evolution of the photoexcited relaxation shows evidence of superimposed transient bleaching and induced photo absorption of almost similar strengths, whereas at longer times it is governed by slow recovery of long-lived dark excitons. After about 3 ps, the signal is dictated by the slowest negative relaxation component attributed to the low-energy π-plasmons. An absorption trough near 500 fs in the ultrafast response evolves with the increasing sample temperature. This particular feature is masked by the reduced induced transmission at room temperature and above. We have estimated the electron-phonon coupling constant to be ∼0.86 from the linear temperature dependence of the slow relaxation time constant. More such studies can help advance the understanding of the intrinsic charge and energy loss mechanisms to improve the efficiency of the optoelectronic devices based on them.

Magnetic field-dependent resistance crossover and anomalous magnetoresistance in topological insulator Bi2Te3.

We report a metal-insulator like transition in single-crystalline 3D topological insulator Bi2Te3 at a temperature of 230 K in the presence of an external magnetic field applied normal to the surface. This transition becomes more prominent at larger magnetic field strength with the residual resistance value increasing linearly with the magnetic field. At low temperature, the magnetic field dependence of the magnetoresistance shows a transition from logarithmic to linear behavior and the onset magnetic field value for this transition decreases with increasing temperature. The logarithmic magnetoresistance indicates the weak anti-localization of the surface Dirac electrons while the high temperature behavior originates from the bulk carriers due to intrinsic impurities. At even higher temperatures beyond ∼230 K, a completely classical Lorentz model type quadratic behavior of the magnetoresistance is observed. We also show that the experimentally observed anomalies at ∼230 K in the magneto-transport properties do not originate from any stacking fault in Bi2Te3.