Multiphoton-pumped UV-Vis transient absorption spectroscopy of 2D materials: basic concepts and recent applications.

2D materials are considered a key element in the development of next-generation electronics (nanoelectronics) due to their extreme thickness in the nanometer range and unique physical properties. The ultrafast dynamics of photoexcited carriers in such materials are strongly influenced by their interfaces, since the thickness of 2D materials is much smaller than the typical depth of light penetration into their bulk counterparts and the mean free path of photoexcited carriers. The resulting collisions of photoexcited carriers with interfacial potential barriers of 2D materials in the presence of a strong laser field significantly alter the overall dynamics of photoexcitation, allowing laser light to be directly absorbed by carriers in the conduction/valence band through the inverse bremsstrahlung mechanism.

Characterization of charge-carrier dynamics at the BiSe/MgFinterface by multiphoton pumped UV-Vis transient absorption spectroscopy.

Separate relaxation dynamics of electrons and holes in experiments on optical pumping-probing of semiconductors is rarely observed due to their overlap. Here we report the separate relaxation dynamics of long-lived (∼200s) holes observed at room temperature in a 10 nm thick film of the 3D topological insulator (TI) BiSecoated with a 10 nm thick MgFlayer using transient absorption spectroscopy in the UV-Vis region. The ultraslow hole dynamics was observed by applying resonant pumping of massless Dirac fermions and bound valence electrons in BiSeat a certain wavelength sufficient for their multiphoton photoemission and subsequent trapping at the BiSe/MgFinterface.

Two-photon IR pumped UV-Vis transient absorption spectroscopy of Dirac fermions in the topological insulator BiSe.

It is often taken for granted that in pump-probe experiments on the topological insulator (TI) BiSeusing IR pumping with a commercial Ti:sapphire laser [∼800 nm (1.55 eV photon energy)], the electrons are excited in the one-photon absorption regime, even when pumped with absorbed fluences in the mJ cmrange. Here, using UV-Vis transient absorption (TA) spectroscopy, we show that even at low-power Infrared (IR) pumping with absorbed fluences in theJ cmrange, the TA spectra of the TI BiSeextend across a part of the UV and the entire visible region.

Coherent surface-to-bulk vibrational coupling in the 2D topologically trivial insulator BiSe monitored by ultrafast transient absorption spectroscopy.

Ultrafast carrier relaxation in the 2D topological insulator (TI) BiSe [gapped Dirac surface states (SS)] and how it inherits ultrafast relaxation in the 3D TI BiSe (gapless Dirac SS) remains a challenge for developing new optoelectronic devices based on these materials. Here ultrashort (~ 100 fs) pumping pulses of ~ 340 nm wavelength (~ 3.65 eV photon energy) were applied to study ultrafast electron relaxation in the 2D TI BiSe films with a thickness of 2 and 5 quintuple layers (~ 2 and ~ 5 nm, respectively) using transient absorption (TA) spectroscopy in the ultraviolet-visible spectral region (1.

Dynamic Opening of a Gap in Dirac Surface States of the Thin-Film 3D Topological Insulator BiSe Driven by the Dynamic Rashba Effect.

Optical control of Dirac surface states (SS) in topological insulators (TI) remains one of the most challenging problems governing their potential applications in novel electronic and spintronic devices. Here, using visible-range transient absorption spectroscopy exploiting ∼340 nm (∼3.65 eV) pumping, we provide evidence for dynamic opening of a gap in the Dirac SS of the thin-film 3D TI BiSe, which has been induced by the dynamic Rashba effect occurring in the film bulk with increasing optical pumping power (photoexcited carrier density).