Transient reflectivity measurements of thin films, ranging from 6 to 40 nm in thickness, of the topological insulator Bi 2 Se 3 revealed a strong dependence of the carrier relaxation time on the film thickness. For thicker films the relaxation dynamics are similar to those of bulk Bi 2 Se 3 , where the contribution of the bulk insulating phase dominates over that of the surface metallic phase. The carrier relaxation time shortens with decreasing film thickness, reaching values comparable to those of noble metals. This effect may result from the hybridization of Dirac cone states at the opposite surfaces for the thinnest films.Topological insulators (TIs) are novel electronic materials that have an insulator-type band gap in the bulk (for Bi 2 Se 3 E g ~ 0.3 eV) but have protected gapless conducting phase on their surface due to the combination of spin-orbit interactions and time-reversal symmetry. 1,2 The most effective experimental methods currently used to monitor metallic two-dimensional (2D) Dirac surface states (SS) of TIs are angle-resolved photoemission spectroscopy (ARPES) and time-resolved ARPES (TrARPES). 1-7 These techniques are equally sensitive to SS and the bulk atoms residing in the close proximity to the surface as a consequence of the extremely small penetration depth (a few nm) of incident energetic photons used for photoemission, combined with the limited escape depth of the electrons (also a few nm). Finite-size effects have also been studied for thin Bi 2 Se 3 films of only a few nm thick and a crossover of the three-dimensional (3D) TI Bi 2 Se 3 to the 2D limit (gapped SS) has been observed when the thickness is below six quintuple layers (~ 6 nm). 8 Reaching a similar sensitivity to SS using traditional optical pump-probe techniques (like transient reflectivity (TR)/transmission), which use less energetic photons in the visible/infrared range, seems problematic since the absorption length of the laser light normally used for these measurements (a few tens of nm) significantly exceeds the range where the effect of SS can actually be monitored. As a result, for bulk single crystals of Bi 2 Se 3 the transient optical response is dominated by the bulk contribution. To overcome the problem one can use SS/surface sensitive methods. An example of this approach has recently been demonstrated by illuminating Bi 2 Se 3 with circularly polarized near-infrared light. 9 The resulting photocurrent which reverses its direction with a reversal of the helicity of the light unambiguously proves the SS origin of the optical response. Another surface sensitive technique exploits the centrosymmetric nature of TI's, which governs exclusively the surface-related response which results in an optical second harmonic generation (SHG) process. 10,11 In this Letter we report on a new way to distinguish between the contributions from the TI (Bi 2 Se 3 ) bulk 3D states and the 2D gapless SS, which is based on differences in the carrier relaxation rates for the insulating and metallic phases. We demonstrate that the car...

The photoluminescence ͑PL͒ from 7-and 15-nm silica (SiO 2 ) nanoparticles induced both by ArF laser light ͓ exc ϭ193 nm (6.4 eV), L ϭ15 ns͔ and by Nd:YAG ͑yttrium-aluminum-garnet͒ laser light ͓ exc ϭ266 nm (4.66 eV), L ϭ8 ns͔ was studied. The laser light intensity dependencies of the PL yields reveal the two-photon ͑TP͒ process of the PL excitation in the case of ArF laser light. The PL results from the radiative relaxation of self-trapped excitons ͑STEthe blue band͒, also from the surface hydrogen-related species ͑the green band͒, and the bulk nonbridging oxygen hole centers ͑NBOHC'sthe red band͒ excited by a radiationless relaxation of TP-produced free excitons ͑FE's͒. The main point is focused on the effect of the nanoparticle surface condition on the FE dynamics. The dynamics includes either an elastic scattering or quenching by the nanoparticle boundary, the laser heating of FE's up to energies in excess of the STE barrier, the FE energy transfer to the surface and bulk NBOHC's and hydrogen-related centers, the saturation of the FE density, and the biexciton process in the formation of Frenkel defects with their subsequent transformation into NBOHC's.

Measurements of photoluminescence (PL) from composites of silica nanoparticles (the primary particle size 7 and 15 nm) as a function of heat treatment temperature show that the PL results from hydrogen-related species and thermally produced structural defects. The PL was induced by an ArF or Nd:YAG (yttrium–aluminum–garnet) laser (λexc=193 or 266 nm). The green PL exhibits a progression with spacings of about Δν=630 cm−1 assigned to the bending vibration of ≡Si–H on the surface of particles. The spacings increase up to Δν=1200 cm−1 when ≡Si–H and nonbridging oxygen (≡Si–O•) form interfacial water species.

Transient reflectivity (TR) from thin films (6 -40 nm thick) of the topological insulator Bi 2 Se 3 reveal ultrafast carrier dynamics, which suggest the existence of both radiative and non-radiative recombination between electrons residing in the upper cone of initially unoccupied high energy Dirac surface states (SS) and holes residing in the lower cone of occupied low energy Dirac SS. The modeling of measured TR traces allowed us to conclude that recombination is induced by the depletion of bulk electrons in films below ~20 nm thick due to the charge captured on the surface defects. We predict that such recombination processes can be observed using time-resolved photoluminescence techniques.Thin films of topological insulators (TIs) are threedimensional (3D) materials that are insulating in the bulk (bandgap of Bi 2 Se 3 , for example, E g ~ 0.3 eV), but conductive at the surfaces due to two-dimensional (2D) Dirac surface states (SS) caused by the combination of strong spin-orbit interaction and time-reversal symmetry.