Summary 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/cm 2 .
Spintronic heterostructures are considered to be the new generation THz sources for their capability in producing high power and broadband THz radiation. Here, we provide a brief review on the state-of-the-art in this field. The optically excited bi-and tri-layer combinations of ferromagnetic and nonmagnetic thin films have become increasingly popular. Towards optimizing the THz conversion efficiency and broadband gapless spectrum from these THz emitters, various control parameters need to be taken into consideration. The inverse spin Hall effect in the heavy metal layer of the heterostructure is primarily responsible for the generation of THz pulses. A few new results on iron, platinum and tantalum based heterostructures have also been reported here. It is observed that the Ta(2nm)/Fe(2nm)/Pt(2nm) tri-layer heterostructure generates ~40(250)% stronger THz signal as compared to the counterpart Fe(2nm)/Pt(2nm) (Fe(3nm)/Ta(2nm)) bi-layer heterostructure.
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.The family of two-dimensional semiconducting transition metal dichalcogenides (2D-TMDs) such as MoS2, WS2, MoSe2 and so on, has grown significantly [1] and it is likely to remain one of the leading topics in science for many years to come due to many facets of scientific findings and knowledge they can contribute to. Heterostructures of such 2D systems offer not only a way to study their electronic properties and interlayer interactions but also provide a rich playground to expand the physics of the constituting layers [2], eventually to open enormous possibilities of utilizing them for various technological applications [3,4]. Unlike conventional semiconductor heterostructures, van der Waals (vdW) 2D heterostructures having atomically sharp interfaces are relatively easy to make either via mechanical exfoliation from bulk crystals by scotch tape method [5] or via bottom-up approaches such as using chemical vapor deposition techniques [6][7][8], without considering the lattice mismatch between different 2D layers. Heterostructures formed by two TMD monolayers can exhibit type-I or type-II electronic band alignments [9,10], leading to the formation of interlayer radiative excitons where the bottom of the conduction band and the top of the valence band reside in different layers. In such a case, valley lifetimes become longer than that for intralayer excitons [11,12]. More importantly, the electronic band alignment provides spatial separation of electrons and holes after photoexcitation in the heterostructures, facilitating an important virtue for their applications in photovoltaic devices [13,14]. The interfacial electronic interactions and charge transfer/separation between constituting TMDs is one of the keys to determine the characteristics of such heterostructures and hence the performance of devices made from them.In a few reports on vertically stacked vdW heterostructures of TMDs, possible mechanisms for the charge transfer have been inferred from time-resolved optical absorption and photoluminescence studies [15][16][17][18][19][20]. Photoluminescence time-resolved spectroscopy has been used to study the fast interlayer energy transfer in MoSe2/WS2 heterostructures [21], while ultrafast time-resolved optical pump probe spectroscopy has been used to study possible charge transfer mechanisms, including the exciton localization [22], inte...
Monolayers of transition metal dichalcogenides are semiconducting materials which offer many prospects in optoelectronics. A monolayer of molybdenum disulfide (MoS 2 ) has a direct bandgap of 1.88 eV. Hence, when excited with optical photon energies below its bandgap, no photocarriers are generated and a monolayer of MoS 2 is not of much use in either photovoltaics or photodetection. Here, we demonstrate that large size MoS 2 monolayer sandwiched between two graphene layers makes this heterostructure optically active well below the band gap of MoS 2 . An ultrafast optical pump-THz probe experiment reveals in real-time, transfer of carriers between graphene and MoS 2 monolayer upon photoexcitation with photon energies down to 0.5 eV. It also helps to unravel an unprecedented enhancement in the broadband transient THz response of this tri-layer material system. We propose possible mechanism which can account for this phenomenon. Such specially designed heterostructures, which can be easily built around different transition metal dichalcogenide monolayers, will considerably broaden the scope for modern optoelectronic applications at THz bandwidth.
We report the temperature-dependent electrical transport and photoconductivity in carbon nanoparticle films. The electrical transport is dominated by thermally activated conduction at higher temperatures in the range of ∼350–285 K, whereas at lower temperatures <280 K, the conduction is mostly due to the hopping mechanism. A film of an n-type semiconductor with a carrier concentration of ∼1016 cm−3 is prepared by pulsed laser ablation in the scanning technique. The photoconductivity shows a persistent behavior that lasts for several hundreds of seconds on sub-bandgap laser excitations. A broad green luminescence spectrum suggests the presence of a large number of oxygenated-impurity states in the nanoparticles. An unusual behavior in the temperature-dependent photoluminescence is observed in which the photoluminescence intensity first increases up to ∼100 K with the increasing temperature followed by a continuous decrease at higher temperatures. The observed persistent nature of the photocurrent and anomalous temperature dependence in photoluminescence is attributed to the presence of a large number of trap states in the nanoparticles. Due to the ability to trap and retain charges within the disordered carbon nanoparticle films, it can be utilized in the memory applications.
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