Time‐Resolved Second Harmonic Generation Near‐Field Scanning Optical Microscopy

Abstract

Novel scanning probe microscopy (SPM) spectroscopic techniques are currently receiving much attention. The increasing number of spectroscopic techniques, when combined with the high spatial resolution that is attainable with SPM, makes it possible to probe a large range of material characteristics on the nanoscopic scale. The ability to probe samples on the nanoscale with femtosecond time resolution promises to increase our knowledge of nanoscale electronic devices and their dynamics.Several time-resolved spectroscopic probes have recently been demonstrated in combination with SPM techniques. Ambrose et al.[1] as well as Xie and Dunn [2] and Trautman and Macklin [3] have performed fluorescence lifetime measurements in combination with near-field scanning optical microscopy (NSOM), and found that the excited-state lifetimes of emissive dye molecules were sensitive to the proximity of a metal-coated NSOM probe. This change in lifetime was explained in terms of energy transfer between the dye and the metal coating of the probe.[4] Transient linear reflectivity, [5, 6] transient transmissivity, [7] and transient absorption NSOM techniques have also been demonstrated.[8±10] Shigekawa and co-workers have also demonstrated transient conductivity scanning tunneling microscopy (STM) on a GaAs sample using resonant, femtosecond-pulsewidth excitation. [11] Time-resolved second harmonic generation (TRSHG) has been utilized in far-field measurements of changes in molecular orientation at interfaces, [12] in dynamic studies of carrier lifetimes in semiconductors, [13] and as a probe of coherent phonon relaxation.[14] The magnitude of the second-order induced polarization, P (2) produced in a sample (the square of which is equal to the magnitude of the detectable SHG signal) is proportional to the elements of a nonlinear susceptibility tensor reducing agent. Because of the interference of TPA derivatives, our potential calibration with Fc/Fc couple may not be very accurate either, which leaves little room for accommodating the contradictory results.In summary, a stable and electrochemically reversible product can be voltammetrically detected after TPA is anodically oxidized in acetonitrile. Although it is not yet certain whether the observed redox wave is linked to the long-awaited reducing radical responsible for the generation of ECL excited state, the finding is an important clue to understanding the oxidation of TPA and ECL generation. With many follow-up reactions and short-lived intermediates, the anodic oxidation of aliphatic amines is very complicated. It is understandable that a convincing identification of the wave in question will need more experiments, which are under way in our laboratories and further results will be reported in due course.

Ultrafast Polarization Response of an Optically Trapped Single Ferroelectric Nanowire

“…48 We note that photoinduced changes in χ (2) in other materials have been attributed to carrier-induced modulations without associated structural modifications. 32,39,54 However, the 2−3 ps time scale observed here is inconsistent with an effect like this which would be expected to occur on instrument-limited time scales. Moreover, we also observe similar effects using pump and probe beams at 340 and 1030 nm, respectively, with 500 fs pulse width, indicating that the observed changes are not driven by nonthermal effects associated with nonequilibrium carrier densities but rather by the total energy absorbed by the nanowire (see Supporting Information).…”

“…Optical trapping enables manipulation of single nanostructures in three dimensions and characterization of their optical properties. − Nanowires with high aspect ratios tend to be aligned along the optical axis of the trapping beam in the trap. , Nonlinear second harmonic generation (SHG) techniques allow one to probe the structural symmetry of the nanowire and associated carrier relaxation dynamics , as encoded in the second-order nonlinear optical susceptibility (χ (2) ), sensitive to the deviation from centrosymmetry within the crystallographic unit cell, without being influenced by substrate effects. − Upon photoexcitation, incident photons are coupled into one end of a nanowire and waveguided along the nanowire length, with the fundamental generating the second harmonic along the length of the nanowire …”

Ultrafast Carrier Dynamics in Single ZnO Nanowire and Nanoribbon Lasers

“…The term η may also be a function of carrier density due to exciton interaction effects, as has been previously suggested; however, it is difficult to decouple this effect from a change in β. Previous studies on transient SHG using relatively low pump fluence have shown that the for a highly noncentrosymmetric material can be considerably lowered by pumping free carriers from the valence band to the conduction band . These excited carriers then no longer contribute to the intrinsically large χ (2) of the equilibrated material, probably because of the symmetry-breaking associated with excitation to further delocalized states.…”

“…Previous studies on transient SHG using relatively low pump fluence have shown that the χ eff (2) for a highly noncentrosymmetric material can be considerably lowered by pumping free carriers from the valence band to the conduction band. 22 These excited carriers then no longer contribute to the intrinsically large χ (2) of the equilibrated material, probably because of the symmetry-breaking associated with excitation to further delocalized states. As the carriers relax to the "ground-state" configuration, the χ (2) value recovers.…”

“…For example, the metal−insulator transitions in colloidal Ag nanoparticles and ultrafast phase transitions in GaAs have been observed by monitoring the bulk SHG response. In addition, previous studies of SHG microscopy have elucidated a variety of effects, − including changes in carrier dynamics associated with defects and interfaces.…”