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We report the study of ultrafast carrier dynamics of low temperature grown GaAs over wide spectral range (2.75 eV–0.88 eV) with above and below bandgap (E g) optical excitations using ultrafast transient absorption spectroscopy. The presence of defects and their subsequent effect on the pathway and lifetime of carrier relaxation with probing at different energy levels is presented. The carrier relaxation pathways and their corresponding models are predicted. It is observed that for pumping (2.58 eV) and probing above E g , the transient behavior is dominated by band filling while renormalization is in prominence when probed near to E g. For below E g pumping (0.88 eV) the optical behavior is mostly governed by defects. The calculated trap time is in the range of ~128 fs to ~807 fs while overall recombination time shows a wide variation from ~1.38 ps to ~1.25 ns depending on the position of traps/defects in the forbidden band. • Comprehensive analysis of transient behaviour of LT-GaAs is presented and models are proposed for the possible carrier dynamics as per rate equation model. • Below bandgap pumping gives a behaviour of trap/defect states. • For above band gap pumping, band filling effect dominates but near to band edge bandgap renormalisation overpowers bandfilling. • In LT-GaAs there are basically two trap bands. • Decay time is governed by position of trap state in forbidden gap. [ABSTRACT FROM AUTHOR]

We report here the two-dimensional layered material of MoS 2 /Bi 2 Se 3 vdW heterostructures formed by ultrasonication of different concentrations of exfoliated MoS 2 and Bi 2 Se 3 nanosheets. The vdW heterostructures show unique optical characteristics compared to individual exfoliated nanosheets. The enhanced lifetime of heterostructures explained the charge transfer with increasing efficacy for producing optoelectronic devices. Raman spectroscopy confirms the presence of intrinsic Raman active modes of Bi 2 Se 3 , MoS 2 , and respective modes of vdW heterostructures. TEM images show characteristic lattice planes in both materials and clear overlapping regimes, confirming successful heterojunction formation. Further, Zeta potential confirms the enhanced stability of the heterostructures formed while the significant PL quenching, strongly suggests the presence of charge transfer processes within the heterostructures. Ultrafast pump-probe spectroscopy further confirms the non-linear charge transfer and relaxation mechanism of the heterostructures. We employed Surface Xplorer and Glotaran software for model fitting and Glotaran's global fitting revealed unresolved charge carrier dynamics. [ABSTRACT FROM AUTHOR]

Laser-induced periodic surface structures are created by irradiating a Si(001) wafer with a femtosecond pulse laser, and then ultrafast transient absorption spectroscopy is carried out on these structures. The ultrafast charge carrier dynamics are reported on the sub-wavelength structuring obtained in a few microns up to hundreds of nm range, which are well below the diffraction limit and reveal the confinement effect. We observed a nearly three-fold increase in the lifetime of the generated hot carriers where several mechanisms and pathways alter the relaxation. The carrier recombination is observed to be slowed down as a result of the confinement due to structuring. This lifetime enhancement is investigated throughout a broad wavelength range of 400–1600 nm. By modulating the periodicity of the surface structures, which is implicitly dependent on the fluence and the number of laser pulses irradiation, it is possible to tune the relaxation lifetime of the material. [ABSTRACT FROM AUTHOR]

The study reports peculiar light absorption in LT-GaAs due to the prominence of arsenic vacancies (V As), interstitials (As i), and neutral arsenic anti-sites (As Ga 0) after UHV annealing under no Arsenic vapour pressure. We compared unannealed and annealed samples' transient absorption behaviour and vibrational modes through ultrafast transient absorption spectroscopy (UFTS) (over 2.75 eV–0.775 eV spectral range) and RAMAN spectroscopy, respectively. We found that the UHV heating of LT-GaAs has augmented the tensile strain in the film by incorporating split As i. However, there is overall defect mitigation, as evident from the reduced TO phonon peak (RAMAN) and ∼59 meV reduction in Fermi level (UFTS). Even with reduced defects, the UFTS data for the annealed sample shows a higher switching speed and responsivity (1.75 ps faster) in the NIR range and a higher photoconductive gain in the visible region. [ABSTRACT FROM AUTHOR]

The charge carrier and phonon dynamics are highly reliant on the material structural features, and understanding their dynamics help in interpreting the underlying phenomenological qualities of any material system. The phonon and carrier dynamics of Bi, Te, and BiTe 3 thin films formed on a Si (100) wafer through thermal evaporation are discussed in this article. The Field Emission Scanning Electron Microscopy (FESEM) and X-ray Diffraction (XRD) analysis are used to characterize the structural features of these films. The dynamics of charge carriers and phonons are further determined using femtosecond ultrafast transient absorption spectroscopy (UFTS). A comprehensive analysis of dynamics is carried out by performing multiple excitations probed in broad Near-Infrared (NIR) regimes. The current study would help collect the knowledge about the phonon and charge carrier dynamics of Bismuth, Tellurium, and Bismuth Telluride films for optical, spintronics, and terahertz applications. • Explored grain size and lattice strain-dependent phonon oscillations using ultrafast transient absorption spectroscopy. • As the grain size of thin films grows, so do the phonon oscillations. • The coherent optical phonons (COP) of Bismuth and Bismuth telluride oscillate at 2.31 THz and 1.1 THz, respectively. [ABSTRACT FROM AUTHOR]

In this article, we report the optical probing of SnTe (275 nm and 33 nm) and Indium-doped SnTe films (50 nm) which are deposited using thermal evaporation. XRD (X-ray diffraction) and Raman spectroscopy are used to characterize the films. Additionally, the optical characteristics of the material are examined using reflectance and PL (photoluminescence) spectroscopy. Simultaneously, the dynamics of charge carriers are studied using TRUS (Transient reflectance ultrafast spectroscopy). The TRUS probes the thin films in the visible to near-infrared range (400–1600 nm) with a probe latency of a few femtoseconds to 6 ns. The study demonstrates the presence of a resonant state in SnTe, which becomes more prominent and metastable with indium doping. Optical probing of SnTe (275 nm and 33 nm) and indium doped SnTe films (50 nm) which are deposited using thermal evaporation. The films are characterized through XRD (X-ray diffraction) and Raman spectroscopy. Further, the optical properties are investigated through reflectance spectroscopy and PL (photoluminescence) spectroscopy. The emission spectroscopy in the visible regime confirms the presence of 2.1 eV emission. The TRUS measurements confirm the confinement of the charge carriers at 33 nm SnTe film w. r.t 275 nm SnTe. The indium dopant films show the presence of a metastable state which is absent in SnTe. This state resembles the resonant state at 0.82 eV and eventually can be tuned to various optoelectronic applications. [ABSTRACT FROM AUTHOR]

Ultrafast transient absorption spectroscopy (UFTS) is a widely used technique to determine the change in optical parameters on laser irradiation and can highlight the pathways and the lifetime of decay of the charge carriers. Here, UFTS was used to detect the structural changes occurring in a MoO 3 /Ag heterostructure because of dewetting and dealloying. A comparative study of transient absorption behavior of samples exposed to different temperatures indicated a change in phase and crystallinity of the film. Later through GI-XRD, it was concluded that the MoO 3 film has become crystalline at 350 °C along α-(110) plane. A wide plasmonic band in UFTS data of dealloyed sample followed by FESEM showed randomly distributed Ag-Mo nanoporous clusters (315 nm - 6 μm). Tuning of surface plasmon resonance (~19 nm) was seen with etching time. This study proves UFTS's suitability for correlating the structural change in a film or heterostructure of films with spectroscopic results. [ABSTRACT FROM AUTHOR]