Yuri D. Glinka

The photoluminescence from hydrogen-related species in composites of SiO2 nanoparticles

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.

Ultrafast carrier dynamics in thin-films of the topological insulator Bi2Se3

Transient reflectivity measurements of thin films, ranging from 6 to 40 nm in thickness, of the topological insulator Bi2Se3 reveal a strong dependence of the carrier relaxation time on the film thickness. For thicker films, the relaxation dynamics are similar to those of bulk Bi2Se3, 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.

Resonance-type thickness dependence of optical second-harmonic generation in thin films of the topological insulatorBi2Se3

Optical second harmonic generation (SHG) has been measured for the first time in reflection from the nanometer-thick films (6 to 40 nm) of the topological insulator Bi2Se3 using 1.51 eV (820 nm) Ti:Sapphire laser photons and revealed a strong dependence of the integral SHG intensity on the film thickness. The integral SHG intensity was determined by area integration of the SHG rotational anisotropy patterns measured for different input-output light polarization geometries. A ~100-fold enhancement of the integral SHG intensity with decreasing film thickness has been suggested to result from the DC-electric-field-induced SHG (EFISHG) effects. Two sources of dynamically created DC electric field were proposed: (i) the capacitor-type DC electric field that gradually increases with decreasing film thickness from 40 to 6 nm due to a dynamical imbalance of photoexcited long-lived carriers between the opposite-surface Dirac surface states and (ii) an DC electric field associated with a nonlinearly excited Dirac plasmon, which is responsible for the resonant enhancement of the integral SHG intensity for the 10 nm thick film with a Lorentz-shaped resonance of ~1.6 nm full width at half maximum. Additionally to the general SHG enhancement trends with decreasing film thickness, a relative decrease of the out-of-plane contribution with respect to the in-plane contribution was observed. Using a theoretical treatment of the measured SHG rotational anisotropy patterns, this effect has been suggested to result from the joint contributions of the linear and quadratic DC electric field effects to the EFISHG response.

Luminescent observation of multiphoton ionization-fragmentation of chromate ions adsorbed on a disperse SiO2 surface

Direct evidence for the multiphoton ionization with subsequent fragmentation of the chromate ions (CrO42−)adsorbed on a disperse SiO2 surface is provided by luminescence measurements using the Nd-doped yttrium aluminum garnet pulsed laser as a source of excitation. Experiments were done with the fundamental laser frequency (λ=1.064 μm; τp=20 ns). The multiphoton absorption proceeds through an intermediate three-photon resonance state. The luminescence response results from recombination of photoelectrons with ionized CrO42− ions and ionized surface oxygendeficient centers. The dynamics of this process include autoionization, which is stimulated by intermolecular perturbation in overexcited states, fragmentation of chromate ions, and annealing of the oxygen-deficient centers.

Nonlinear optical observation of coherent acoustic Dirac plasmons in thin-film topological insulators

Low-energy collective electronic excitations exhibiting sound-like linear dispersion have been intensively studied both experimentally and theoretically for a long time. However, coherent acoustic plasmon modes appearing in time-domain measurements are rarely observed due to Landau damping by the single-particle continua. Here we report on the observation of coherent acoustic Dirac plasmon (CADP) modes excited in indirectly (electrostatically) opposite-surface coupled films of the topological insulator Bi2Se3. Using transient second-harmonic generation, a technique capable of independently monitoring the in-plane and out-of-plane electron dynamics in the films, the GHz-range oscillations were observed without corresponding oscillations in the transient reflectivity. These oscillations were assigned to the transverse magnetic and transverse electric guided CADP modes induced by the evanescent guided Lamb acoustic waves and remained Landau undamped due to fermion tunnelling between the opposite-surface Dirac states.

Non-local coherent coupling between excitons in a disordered quantum well

We investigated coherent coupling among multiple exciton resonances formed in a single disordered quantum well using the powerful electronic two-dimensional Fourier transform spectroscopy. Our experiment revealed clear signatures of non-local coherent coupling between the heavy-hole and light-hole excitons residing in regions that differ in thickness by one atomic layer. The experimental observation is qualitatively explained by spatial overlap between exciton linear response functions calculated within a single defect model.

Stimulated luminescence from chromate ions adsorbed on disperse silica surfaces under intense ultraviolet excitation

Luminescence measurements of chromate ions (CrO42−) adsorbed onto disperse silica (SiO2) surfaces under intense pulsed ultraviolet excitation (λexc=354.7 or 266 nm, τp=20 ns) were performed at room temperature. The luminescence spectra were collected for different values of the excitation intensity (I). The luminescence yield in the red band corresponding to emission from CrO42− ions varies linearly with I under 354.7 nm excitation. However, the intensity dependence of the luminescence yield under 266 nm excitation is characterized by a threshold (I∼0.6×106 W cm−2), after which the intensity dependence of the luminescence yield is described by a power law with index n=1.7 and then saturated at I∼1.0×106 W cm−2. This feature of the excitation dynamics has been attributed to stimulated emission and considered by means of a four-level scheme. The cross section of stimulated emission was estimated as ∼3.3×10−21 cm2.