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Numerical simulations and an analytic approach based on transmission line theory are used to design splitters for nano-plasmonic signal processing that allow to arbitrarily adjust the ratio of transmission from an input into two different output arms. By adjusting the geometrical parameters of the structure, either a high bandwidth or a sharp transmission resonance is obtained. Switching between the two arms can be achieved by modulating the effective refractive index of the waveguide. Employing the instantaneous Kerr effect, switching rates in the THz regime are potentially feasible. The suggested devices are of interest for future applications in nanoplasmonic information processing.
Plasmonic modes supported by noble-metal nanostructures offer strong subwavelength electric-field confinement and promise the realization of nanometer-scale integrated optical circuits with well-defined functionality. In order to measure the spectral and spatial response functions of such plasmonic elements, we combine a confocal microscope setup with spectral interferometry detection. The setup, data acquisition, and data evaluation are discussed in detail by means of exemplary experiments involving propagating plasmons transmitted through silver nanowires. By considering and experimentally calibrating any setup-inherent signal delay with an accuracy of 1 fs, we are able to extract correct timing information of propagating plasmons. The method can be applied, e.g., to determine the dispersion and group velocity of propagating plasmons in nanostructures, and can be extended towards the investigation of nonlinear phenomena.
We present polarimetry, i.e. the detection of optical rotation of light polarization, in a configuration suitable for femtosecond spectroscopy. The polarimeter is based on common-path optical heterodyne interferometry and provides fast and highly sensitive detection of rotatory power. Femtosecond pump and polarimeter probe beams are integrated into a recently developed accumulative technique that further enhances sensitivity with respect to single-pulse methods. The high speed of the polarimeter affords optical rotation detection during the pump-pulse illumination period of a few seconds. We illustrate the concept on the photodissociation of the enantiomers of methyl p-tolyl sulfoxide. The sensitivity of rotatory detection, i.e. the minimum rotation angle that can be measured, is determined experimentally including all noise sources to be 0.10 milli-degrees for a measurement time of only one second and an interaction length of 250 μm. The suitability of the presented setup for femtosecond studies is demonstrated in a non-resonant two-photon photodissociation experiment.
Optimal open-loop control, i.e. the application of an analytically derived control rule, is demonstrated for nanooptical excitations using polarization-shaped laser pulses. Optimal spatial near-field localization in gold nanoprisms and excitation switching is realized by applying a shift to the relative phase of the two polarization components. The achieved near-field switching confirms theoretical predictions, proves the applicability of predefined control rules in nanooptical light–matter interaction and reveals local mode interference to be an important control mechanism.
In the present contribution, we describe the synthesis of highly dispersed silver nanorods (NRs) of different aspect ratios using a chemical route. The shape and size of the synthesized NRs were characterized by Transmission Electron Microscopy (TEM) and UV-visible spectroscopy. Longitudinal and transverse absorptions bands confirm the rod type structure. The experimentally recorded UV-visible spectra of NRs solutions were fitted by using an expression of the extinction coefficient for rod like nano structures under the dipole approximation. Simulated and experimentally observed UV-visible spectra were compared to determine the aspect ratios (R) of NRs. The average values of R for NR1, NR2 and NR3 solutions are estimated to be 3.0 +/- 0.1, 1.8 +/- 0.1 and 1.2 +/- 0.1, respectively. These values are in good agreement with those obtained by TEM micrographs. The silver NRs of known aspect ratios are used to study antimicrobial activities against B. subtilis (gram positive) and E. coli (gram negative) microbes. We observed that the NRs of intermediate aspect ratio (R = 1.8) have greater antimicrobial effect against both, B. subtilis (gram positive) and E. coli (gram negative). The NRs of aspect ratio, R = 3.0 has better antimicrobial activities against gram positive than on the gram negative.
Ultraviolet irradiation of CO-releasing molecules (CORMs) in water eventually leads to the loss of several carbon monoxide ligands.We show for an exemplary manganese tricarbonyl CORM that only one ligand is photolyzed off on an ultrafast timescale and that some molecules may undergo geminate recombination.
We use pump-repump-probe transient absorption spectroscopy to investigate the role of higher-lying electronic states in the photochemistry of a molecular switch. Moreover, replacing the pump pulse by a pulse-shaper-generated phase-stable double pulse, triggered-exchange two-dimensional (TE2D) electronic spectroscopy is established in the visible regime.
We present a fast and sensitive polarimeter combining common-path optical heterodyne interferometry and accumulative spectroscopy to detect rotatory power. The sensitivity of rotatory detection is determined to be 0.10 milli-degrees for a measurement time of only one second and an interaction length of 250 µm. Its suitability for femtosecond studies is demonstrated in a non-resonant two-photon photodissociation experiment.
In the present contribution, we describe the synthesis of highly dispersed silver nanorods (NRs) of different aspect ratios using a chemical route. The shape and size of the synthesized NRs were characterized by Transmission Electron Microscopy (TEM) and UV-visible spectroscopy. Longitudinal and transverse absorptions bands confirm the rod type structure. The experimentally recorded UV-visible spectra of NRs solutions were fitted by using an expression of the extinction coefficient for rod like nano structures under the dipole approximation. Simulated and experimentally observed UV-visible spectra were compared to determine the aspect ratios (R) of NRs. The average values of R for NR1, NR2 and NR3 solutions are estimated to be 3.0 ± 0.1, 1.8 ± 0.1 and 1.2 ± 0.1, respectively. These values are in good agreement with those obtained by TEM micrographs. The silver NRs of known aspect ratios are used to study antimicrobial activities against B. subtilis (gram positive) and E. coli (gram negative) microbes. We observed that the NRs of intermediate aspect ratio (R = 1.8) have greater antimicrobial effect against both, B. subtilis (gram positive) and E. coli (gram negative). The NRs of aspect ratio, R = 3.0 has better antimicrobial activities against gram positive than on the gram negative.
We present a study on the photoionisation of the cycloheptatrienyl (tropyl) radical, \(C_7H_7\), using tunable vacuum ultraviolet synchrotron radiation. Tropyl is generated by flash pyrolysis from bitropyl. Ions and electrons are detected in coincidence, permitting us to record mass-selected photoelectron spectra. The threshold photoelectron spectrum of tropyl, corresponding to the
\(X^{+1}A1’ ← X^2E_2”\) transition, reveals an ionisation energy of 6.23 ± 0.02 eV, in good agreement with Rydberg extrapolations, but slightly lower than the value derived from earlier photoelectron spectra. Several vibrations can be resolved and are reassigned to the C–C stretch mode \(ν_{16}^+\) and to a combination of \(ν_{16}^+\) with the ring breathing mode \(ν_2^+\). Above 10.55 eV dissociative photoionisation of tropyl is observed, leading to the formation of \(C_5H_5^+\) and \(C_2H_2\).
Three spectroscopic techniques are presented that provide simultaneous spatial and temporal resolution: modified confocal microscopy with heterodyne detection, space-time-resolved spectroscopy using coherent control concepts, and coherent two-dimensional nano-spectroscopy. Latest experimental results are discussed.
A comparative study is carried out on two spectroscopic techniques employed to detect ultrafast absorption changes in the mid-infrared spectral range, namely direct multichannel detection via HgCdTe (MCT) photodiode arrays and the newly established technique of chirped-pulse upconversion (CPU). Whereas both methods are meanwhile individually used in a routine manner, we directly juxtapose their applicability in femtosecond pump-probe experiments based on 1 kHz shot-to-shot data acquisition. Additionally, we examine different phase-matching conditions in the CPU scheme for a given mid-infrared spectrum, thereby simultaneously detecting signals which are separated by more than 200 cm−1.
We calculate two-dimensional (2D) spectra reflecting the time-dependent electronic predissociation of a diatomic molecule. The laser-excited electronic state is coupled non-adiabatically to a fragment channel, leading to the decay of the prepared quasi-bound states. This decay can be monitored by the three-pulse configuration employed in optical 2D spectroscopy. It is shown that in this way it is possible to state-selectively characterize the time-dependent population of resonance states with different lifetimes. A model of the NaI molecule serves as a numerical example.
Coherent two-dimensional electronic spectroscopy in the Soret band of a chiral porphyrin dimer
(2013)
Using coherent two-dimensional (2D) electronic spectroscopy in fully noncollinear geometry, we observe the excitonic coupling of β,β'-linked bis[tetraphenylporphyrinato-zinc(II)] on an ultrafast timescale in the excited state. The results for two states in the Soret band originating from an excitonic splitting are explained by population transfer with approximately 100 fs from the energetically higher to the lower excitonic state. This interpretation is consistent with exemplary calculations of 2D spectra for a model four-level system with coupling.
100-kHz shot-to-shot broadband data acquisition for high-repetition-rate pump–probe spectroscopy
(2014)
Shot-to-shot broadband detection is common in ultrafast pump–probe spectroscopy. Taking advantage of the intensity correlation of subsequent laser pulses improves the signal-to-noise ratio. Finite data readout times of CCD chips in the employed spectrometer and the maximum available speed of mechanical pump-beam choppers typically limit this approach to lasers with repetition rates of a few kHz. For high-repetition (≥ 100 kHz) systems, one typically averages over a larger number of laser shots leading to inferior signal-to-noise ratios or longer measurement times. Here we demonstrate broadband shot-to-shot detection in transient absorption spectroscopy with a 100-kHz femtosecond laser system. This is made possible using a home-built high-speed chopper with external laser synchronization and a fast CCD line camera. Shot-to-shot detection can reduce the data acquisition time by two orders of magnitude compared to few-kHz lasers while keeping the same signal-to-noise ratio.
Shaping and spatiotemporal characterization of sub-10-fs pulses focused by a high-NA objective
(2014)
We describe a setup consisting of a 4 f pulse shaper and a microscope with a high-NA objective lens and discuss the spects most relevant for an undistorted spatiotemporal profile of the focused beam. We demonstrate shaper-assisted pulse compression in focus to a sub-10-fs duration using phase-resolved interferometric spectral modulation (PRISM). We introduce a nanostructure-based method for sub-diffraction spatiotemporal characterization of strongly focused pulses. The distortions caused by optical aberrations and space–time coupling from the shaper can be reduced by careful setup design and alignment to about 10 nm in space and 1 fs in time.
Theory predicts peculiar features for excited-state dynamics in one dimension (1D) that are difficult to be observed experimentally. Single-walled carbon nanotubes (SWNTs) are an excellent approximation to 1D quantum confinement, due to their very high aspect ratio and low density of defects. Here we use ultrafast optical spectroscopy to probe photogenerated charge-carriers in (6,5) semiconducting SWNTs. We identify the transient energy shift of the highly polarizable S\(_{33}\) transition as a sensitive fingerprint of charge-carriers in SWNTs. By measuring the coherent phonon amplitude profile we obtain a precise estimate of the Stark-shift and discuss the binding energy of the S\(_{33}\) excitonic transition. From this, we infer that charge-carriers are formed instantaneously (<50 fs) even upon pumping the first exciton, S\(_{11}\). The decay of the photogenerated charge-carrier population is well described by a model for geminate recombination in 1D.
We employ transient absorption from the deep-UV to the visible region and fluorescence upconversion to investigate the photoinduced excited-state intramolecular proton-transfer dynamics in a biologically relevant drug molecule, 2-acetylindan-1,3-dione. The molecule is a ß-diketone which in the electronic ground state exists as exocyclic enol with an intramolecular H-bond. Upon electronic excitation at 300 nm, the first excited state of the exocyclic enol is initially populated, followed by ultrafast proton transfer (≈160 fs) to form the vibrationally hot endocyclic enol. Subsequently, solvent-induced vibrational relaxation takes place (≈10 ps) followed by decay (≈390 ps) to the corresponding ground state.
Photochemical reactions in solution often proceed via competing reaction pathways
comprising intermediates that capture a solvent molecule. A disclosure of the underlying
reaction mechanisms is challenging due to the rapid nature of these processes and the
intricate identification of how many solvent molecules are involved. Here combining
broadband femtosecond transient absorption and quantum mechanics/molecular mechanics
simulations, we show for one of the most reactive species, diphenylcarbene, that the
decision-maker is not the nearest solvent molecule but its neighbour. The hydrogen bonding
dynamics determine which reaction channels are accessible in binary solvent mixtures at
room temperature. In-depth analysis of the amount of nascent intermediates corroborates
the importance of a hydrogen-bonded complex with a protic solvent molecule, in striking
analogy to complexes found at cryogenic temperatures. Our results show that adjacent
solvent molecules take the role of key abettors rather than bystanders for the fate of the
reactive intermediate.