Induced Transmission Changes Research Articles

Sub-diffraction resolution pump-probe microscopy with shot-noise limited sensitivity using laser diodes.

We demonstrate the use of intensity-modulated laser diodes to implement pump-probe microscopy and achieved sub-diffraction resolution imaging with shot-noise limited sensitivity with a scheme of balanced detection. This technique has several applications for various types of induced transmission change, including excited-state absorption, ground state absorption bleaching and stimulated emission. By using our technique, biological imaging of mouse T cells and the axons of neurons in the cerebral cortex was demonstrated.

UV laser induced transmission change of pure and doped silica glass

Pure and F, GeO2-doped silica glass cut from fiber preforms prepared by plasma assisted chemical vapor deposition(PCVD) were investigated by ultraviolet absorption spectroscopy. The ultraviolet absorption characteristics of these glasses were also studied after UV laser irradiation and heating treatment. It was found that absorption band near 240 nm assigned to GODC was found both in GeO2-doped and F-GeO2 co-doped silica glass, but absorption intensity of the latter was lower than that of the former. It’s because F can react with GODC and GeE’ simultaneously and reduce their concentration. After irradiation, UV absorption change of F-GeO2 co-doped silica glass was weaker than that of GeO2-doped silica glass,it thus can be concluded that introduction of F could depress the UV absorption of GeO2-doped silica core effectively.

Laser-based continuous-wave excitonic Lyman spectroscopy inCu2O

We developed continuous-wave, laser-based excitonic Lyman spectroscopy for sensitive and wide dynamic range detection of $1s$ paraexcitons in ${\mathrm{Cu}}_{2}\mathrm{O}$. A single-line tunable carbon dioxide laser was used as a probe light source. The minimum detectable induced transmission change was of the order of ${10}^{\ensuremath{-}5}$, allowing us to accurately measure the induced absorption spectra of paraexcitons in thermal equilibrium with the lattice in a low-density limit. Using this spectroscopy technique, we measured important basic parameters, such as the effective mass and lifetime of paraexcitons, that are crucial for discussing the possibility of reaching a Bose-Einstein condensation state with this elementary excitation in a solid.

Photoinduced redox-reactions and transmission changes in glasses doped with 4d- and 5d-ions

(Fluoride)phosphate and borosilicate glasses of high intrinsic transparency in the deep ultraviolet (UV), were doped with 50–5000ppm of the 4d- and 5d-ions Zr, Nb, Ta, Mo, or W. All of these ions absorb strongly in the UV. Samples plates were irradiated by UV lasers and the as a consequence generated various extrinsic and intrinsic defects were characterized by optical and EPR spectroscopy. The laser induced transmission changes depend not only on the glass matrix, but also on the valence of the dopants. Only fully oxidized d0-ions are observed in fluoroaluminate glasses. Laser irradiation photoreduces the d0-ions to extrinsic electron-centers (EC). Laser induced transmission changes extend from the UV up to 600nm in the visible. The dopants are easily reduced to lower valences in metaphosphate glasses. Extrinsic hole centers (HC) replace intrinsic HC in samples containing the reduced transition metal ions. The strong transmission changes seen below 300nm arise from intrinsic EC and extrinsic HC. The few remaining intrinsic HC (300–600nm) recombine rapidly with EC or transform into more stable extrinsic HC. Borosilicate glasses show the formation of intrinsic boron oxygen hole center in the EPR spectra and of intrinsic HC and EC in the optical spectra. The d1-ion Mo5+ is the only identified reduced dopant species in the borosilicate glasses. The band intensity of intrinsic EC in relation to intrinsic HC is correspondingly highest for the Mo-doped samples, in which extrinsic HC are generated.

Spin-dependent electron many-body effects in GaAs

Time- and polarization-resolved differential transmission measurements employing same and oppositely circularly polarized $150\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ optical pulses are used to investigate spin characteristics of conduction band electrons in bulk GaAs at $295\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Electrons and holes with densities in the $2\ifmmode\times\else\texttimes\fi{}{10}^{16}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}--{10}^{18}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ range are generated and probed with pulses whose center wavelength is between 865 and $775\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The transmissivity results can be explained in terms of the spin sensitivity of both phase-space filling and many-body effects (band-gap renormalization and screening of the Coulomb enhancement factor). For excitation and probing at $865\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, just above the band-gap edge, the transmissivity changes mainly reflect spin-dependent phase-space filling which is dominated by the electron Fermi factors. However, for $775\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ probing, the influence of many-body effects on the induced transmission change are comparable with those from reduced phase space filling, exposing the spin dependence of the many-body effects. If one does not take account of these spin-dependent effects one can misinterpret both the magnitude and time evolution of the electron spin polarization. For suitable measurements we find that the electron spin relaxation time is $130\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$.

Study of exciton-polariton spin dynamics

We study the relaxation dynamics of spin-polarized exciton-polaritons using a technique in a pump-probe configuration: According to their total angular momentum, exciton-polaritons can be excited by circularly polarized light of either σ+ or σ− helicities. In addition, a bound state of two excitons may exist to which the transition is only allowed when using two photons of opposite helicities. After excitation by a pump pulse of σ+ helicity, additional polaritons from a probe pulse of σ− helicity may be absorbed to excite biexcitons, while absorption of those with σ+ helicity becomes possible only after a spin-flip process. It is characterized by a time-constant τs, which can be determined by a time-resolved study of the induced transmission changes at the exciton–biexciton transition. We demonstrate this method using bulk CuCl, in which the biexciton binding energy is large and in which effects due to polariton propagation can be nicely evidenced. We thus identify a spectral region where the spin-flip process is dominating, which is situated just below the polariton bottleneck.

Femtosecond intersubband dynamics of electrons in AlGaN/GaN-based high-electron-mobility transistors

Ultrafast electron dynamics in the inversion layer of an AlGaN/GaN transistor is studied in pump–probe experiments with 50 fs mid-infrared pulses. Two-colour pump–probe measurements show an instantaneous transmission increase for all spectral positions of the probe, which demonstrates that homogeneous broadening is an important contribution to the total linewidth in this material. We observe the maximum of the induced transmission change around 5 µm. This large Stokes shift might be caused by the extremely large electron–LO-phonon scattering rate.

Large electrically induced transmission changes of GaAs/AlGaAs quantum-cascade structures

Using a tunable midinfrared light source we study optical transmission changes of an electrically driven GaAs/AlGaAs quantum-cascade structure without resonator. For forward bias, we observe a transmission increase in the spectral range around the electroluminescence maximum which is due to resonant optical amplification. The observed transmission increase is enhanced up to 20-fold with respect to a bare active region by the internal field enhancement in the quantum-cascade structure and by interference effects in the midinfrared beam. Model calculations account quantitatively for this behavior.

Mid-IR studies of third-order nonlinear optical susceptibilities with picosecond pulses

The extension of infrared double resonance spectroscopy to picosecond studies of both real and imaginary part of third-order optical susceptibility χ (3) (−ω; ω′, −ω′, ω) by the help of a complete polarization analysis is reported. Using an intrinsic intensity measurement via induced transmission changes in systems with saturable absorptions very reliable values for nonlinear refractive coefficients in the mid IR are obtained. In time resolved investigations different axes of molecular reorientation can be observed in one experiment. Experimental results on liquid samples of CS 2 and CHBr 3 are presented and discussed.

Ultrafast Ligth Induced Transmission Changes and Laser Emission of Semiconductor Quantum Dots

AbstractTransitions between energy levels of carriers spatially confined within CdSe nanocrystals are observed as bleaching bands in time‐resolved nonlinear transmission spectra. Gain and laser generation at the transition between the lowest levels of size quantization are achieved.

Two-photon absorption induced transmission changes in ZnSe interference filters

We observe two-photon absorption (2PA) of picosecond, 0.6-μm, 0.3-nJ pulses in a 0.4-μm film of ZnSe. We use high-frequency-modulation phase-sensitive detection to be near the shot-noise limit combined with a low-frequency pulse-delay modulation to discriminate against large thermal nohlinearities. This dual-modulation technique, combined with the field enhancement produced by placing the film in a resonant cavity, permits observation of 2PA and other fast nonlinearities by using pulses having peak powers of the order of 1 W.

Influence of the Membrane Potential on the Intracellular Light Induced Ca2+ -Concentration Change of the Limulus Ventral Photoreceptor Monitored by Arsenazo III under Voltage Clamp Conditions

Abstract The light induced transmission change (Arsenazo signal) of an Arsenazo III injected ventral photoreceptor cell of Limulus polyphemus was studied under voltage clamp. The transmission change which represents a change of free intracellular calcium ion concentration, [Ca2+]i, was investigated for its dependence upon membrane voltage. The peak amplitude of the Arsenazo signal decreases in a linear fashion with the clamp voltage in the examined voltage range (from -80 to + 40 mV). In low Ca2+ saline ([Ca2+]e = 250 μᴍ) this decrease in the amplitude of the Arsenazo signal was more pronounced, while in saline with increased Ca2+ ([Ca2+]e = 40, 50 and 100 mᴍ), there is almost no change of the Arsenazo signal with varied membrane voltage. The recovery of the Arsenazo signal (i.e. recovery of the transmission back to the value before the light flash) is faster during hyperpolarization, this recovery being slowed down when the cell is depolarized. From these experiments it is concluded that a substantial part of the Arsenazo signal is due to a light induced influx of Ca2+ from the extracellular space across the cell membrane into the cytoplasma. Conceivably the Ca2+ could pass through light activated Na+ channels. Subsequently the increased intracellular Ca2+ is lowered to the preillumination level, by a membrane voltage dependent mechanism possibly an Na+-Ca2+ exchange. The data do not exclude the possibility that a part of the Ca2+ responsible for the Arsenazo signal is released from intracellular stores.