Dipicolinic acid (DPA), bound to calcium (Ca), is a main component of bacterial endospores. Complexation of DPA with lanthanide ions, particularly Terbium (Tb), allows for rapid detection via monitoring the lanthanide luminescence, with applications spanning from cell imaging to contamination and biohazard detection, to sterilization control. Here we present time-resolved luminescence of the Tb-DPA complex upon UV excitation at 266 nm. Our measurements directly monitor the luminescence dynamics and speak for a rise of the luminescence on the ns time scale, which is orders of magnitude faster than previously reported, and raise questions about the details of the energy transfer process in this complex and the states involved. The results are relevant for the design of more sensitive detection schemes for Tb-DPA fluorescence, as well as for the design of novel Tb-based luminescence probes or novel fluorescence probes working as FRET acceptors of Tb energy.

This paper reports design, microfabrication and characterization of a new resonant piston mode thin-film piezo based micromirror, optimized for high-speed Fourier transform spectroscopy. The device is 5x5 mm in size and features an optical membrane of 3 mm, suspended by six individually controlled Archimedean springs. The mirror achieves a total vertical stroke of more than 250 μm at 3.6 kHz under atmospheric pressure, allowing for high-speed transient spectroscopic analysis. This to the best of our knowledge the first time that a piezo MEMS mirror was reported in a Fourier transform spectrometer.

A translatory MOEMS actuator is presented, which enables a precise out-of-plane translatory oscillation of a 5mm mirror with 700 μm large stroke at 267Hz, when driven at 4V in parametric resonance. Due to significant gas damping operation in vacuum is needed. The minimum requirements on vacuum pressure (pmax ≥ 3.21 Pa, Q ≥ 1177) were determined experimentally. Therefore, the MOEMS are permanently encapsulated by means of a wafer-level-vacuum package. The hermetic sealing of MEMS WLVP (stack of 4 wafer 6”) was realized by glass-frit bonding (i) to be compatible to MEMS process AME75 and (ii) to avoid any (vertical) TWI. The ductile glass frit bond layer allows hermetic sealing also on non-ideal wafer topologies with height differences of several 100 nm. But high process temperatures of 435°C are required. Despite the high process temperatures (430°C needed for glass frit bonding) a sufficient static mirror planarity of ≤ λ/10 was achieved. The paper will discuss details of VWLP development and MEMS system integration. The longterm stability of 0.1 Pa inner vacuum pressure was successfully tested to be < 10a using a Ne fine leakage test. For system integration into a miniaturized FT-NIR spectrometer selected MEMS with minimal tilt were used. The NIR-FTS achieved a spectral resolution of 8.3 cm-1 and SNR ≤ 8000 (with co-adding of 1000 spectra). The new translatory MEMS are very promising for compact FTS. The versatility and ruggedness of a MOEMS-FTS makes it ideal for process control in harsh environments (e.g. surveillance of fast chemical reactions).

We present a transient absorption setup based on a low-power laser system and a photonic crystal fiber for supercontinuum generation. The setup employs an ultrafast erbium-doped fiber laser system that emits at 775 nm with 80 MHz repetition rate which pumps a non-linear photonic crystal fiber that provides a supercontinuum in the NIR/VIS wavelength region for probing. By using an acousto optic modulator for pump-beam modulation and a lock-in amplifier we were able to achieve a detection-limit of 1 μOD. The setup reveals the potential of photonic crystal fibers as broadband sources in combination with low-power lasers.

Ultrafast fiber lasers represent an affordable source for performing non-linear spectroscopies, like transient absorption or coherent antistokes Raman scattering, and can advance these technologies towards commercial devices. We experimentally investigate the generation of white light in photonic crystal fibers at low pulse energies and high repetition rates using 775 nm pulses at 80 MHz from the second harmonic of an Er:fiber laser. Two different fibers were chosen based on non-linear beam-propagation simulations. The generated broadband light was characterized and compared in terms of spectral bandwidth, pulse duration and shot-to shot noise, showing good agreement with the simulations.

Proper optimization of a photonic structure for sensing applications is of extreme importance for integrated sensor design. Here we discuss on the definition of suitable parameters to determine the impact of photonic structure designs for evanescent-wave absorption sensors on the achievable resolution and sensitivity. In particular, we analyze the most widespread quantities used to classify photonic structures in the context of sensing, namely the evanescent-field ratio (or evanescent power factor) and the confinement factor Γ. We show that, somewhat counterintuitively, the confinement factor is the only parameter that can reliably describe the absorption of the evanescent-field in the surrounding medium, and, by quantifying the discrepancy between the two parameters for a set of realistic photonic structures, we demonstrate that using the evanescent-field ratio can lead to a wrong classification of the performance of different structures for absorption sensing. We finally discuss the most convenient simulation strategies to retrieve the confinement factor by FEM simulations.

We present a wafer-level vacuum-packaged (WLVP) translatory micro-electro-mechanical system (MEMS) actuator developed for a compact near-infrared-Fourier transform spectrometer (NIR-FTS) with 800–2500 nm spectral bandwidth and signal-nose-ratio (SNR) > 1000 in the smaller bandwidth range (1200–2500 nm) for 1 s measuring time. Although monolithic, highly miniaturized MEMS NIR-FTSs exist today, we follow a classical optical FT instrumentation using a resonant MEMS mirror of 5 mm diameter with precise out-of-plane translatory oscillation for optical path-length modulation. Compared to highly miniaturized MEMS NIR-FTS, the present concept features higher optical throughput and resolution, as well as mechanical robustness and insensitivity to vibration and mechanical shock, compared to conventional FTS mirror drives. The large-stroke MEMS design uses a fully symmetrical four-pantograph suspension, avoiding problems with tilting and parasitic modes. Due to significant gas damping, a permanent vacuum of ≤3.21 Pa is required. Therefore, an MEMS design with WLVP optimization for the NIR spectral range with minimized static and dynamic mirror deformation of ≤100 nm was developed. For hermetic sealing, glass-frit bonding at elevated process temperatures of 430–440 °C was used to ensure compatibility with a qualified MEMS processes. Finally, a WLVP MEMS with a vacuum pressure of ≤0.15 Pa and Q ≥ 38,600 was realized, resulting in a stroke of 700 µm at 267 Hz for driving at 4 V in parametric resonance. The long-term stability of the 0.2 Pa interior vacuum was successfully tested using a Ne fine-leakage test and resulted in an estimated lifetime of >10 years. This meets the requirements of a compact NIR-FTS.

The optical bandgap of anatase TiO2 nanoparticles is dominated by bulk absorption bands in the deep-ultraviolet due to strongly bound excitons. These spectral features can be utilized as a sensitive probe of carrier and lattice dynamics inside the TiO2 nanoparticles. Here, we implement ultrafast broadband spectroscopy tuned to the exciton resonances in order to track the electron cooling in the conduction band of bare anatase nanoparticles and monitor the electron injection dynamics from an external dye in the case of sensitized anatase nanoparticles.

The recent identification of strongly bound excitons in room temperature anatase TiO2 single crystals and nanoparticles underscores the importance of bulk many-body effects in samples used for applications. Here, for the first time, we unravel the interplay between many-body interactions and correlations in highly excited anatase TiO2 nanoparticles using ultrafast two-dimensional deep-ultraviolet spectroscopy. With this approach, under nonresonant excitation, we disentangle the optical nonlinearities contributing to the bleach of the lowest direct exciton peak. This allows us to clock the ultrafast time scale of the hot electron thermalization in the conduction band with unprecedented temporal resolution, which we determine to be <50 fs, due to the strong electron-phonon coupling in the material. Our findings call for the design of alternative resonant excitation schemes in photonics and nanotechnology.

A translatory MOEMS actuator with extraordinarily large stroke - especially developed for fast optical path-length modulation in miniaturized FT-spectrometers (FTS) designed for NIR spectral region (800 nm - 2500 nm) - is presented. A precise translational out-of-plane oscillation at 260 Hz with a stroke of up to 700 mu m and minimized dynamic mirror deformation of 80 nm is realized by means of an optimized MEMS design. The MOEMS device is driven electro-statically near resonance and is manufactured in a CMOS-compatible SOT process. Due to the significant viscous gas damping, dominated by the drag resistance of the comparatively large mirror plate with 5mm diameter, the resonant MEMS device has to operate under reduced pressure. A mirror stroke of 700 mu m at a driving voltage of 4V is achieved by hermetic encapsulation of the actuator at at a maximal pressure of 3.2 Pa. For FTS system integration the MOEMS actuator has been encapsulated in an optical vacuum wafer-level package (VWLP) to guarantee a long-term stable vacuum pressure of 0.1 Pa and lifetime t >= 10a.

We investigate the ultrafast transient absorption response of tetrakis(μ-pyrophosphito)diplatinate(II), [Pt2(μ-P2O5H2)4]4- [hereafter abbreviated Pt(pop)], in acetonitrile upon excitation of its lowest singlet 1A2u state. Compared with previously reported solvents [van der Veen RM, Cannizzo A, van Mourik F, Vlček A, Jr, Chergui M (2011) J Am Chem Soc 133:305-315], a significant shortening of the intersystem crossing (ISC) time (<1 ps) from the lowest singlet to the lowest triplet state is found, allowing for a transfer of vibrational coherence, observed in the course of an ISC in a polyatomic molecule in solution. Density functional theory (DFT) quantum mechanical/molecular mechanical (QM/MM) simulations of Pt(pop) in acetonitrile and ethanol show that high-lying, mostly triplet, states are strongly mixed and shifted to lower energies due to interactions with the solvent, providing an intermediate state (or manifold of states) for the ISC. This suggests that the larger the solvation energies of the intermediate state(s), the shorter the ISC time. Because the latter is smaller than the pure dephasing time of the vibrational wave packet, coherence is conserved during the spin transition. These results underscore the crucial role of the solvent in directing pathways of intramolecular energy flow.

Ultrafast interfacial electron transfer in sensitized solar cells has mostly been probed by visible-to-terahertz radiation, which is sensitive to the free carriers in the conduction band of the semiconductor substrate. Here, we demonstrate the use of deep-ultraviolet continuum pulses to probe the interfacial electron transfer, by detecting a specific excitonic transition in both N719-sensitized anatase TiO2 and wurtzite ZnO nanoparticles. Our results are compared to those obtained on bare nanoparticles upon above-gap excitation. We show that the signal upon electron injection from the N719 dye into TiO2 is dominated by long-range Coulomb screening of the final states of the excitonic transitions, whereas in sensitized ZnO it is dominated by phase-space filling. The present approach offers a possible route to detecting interfacial electron transfer in a broad class of systems, including other transition metal oxides or sensitizers.

Ferrous iron(II) hexacyanide in aqueous solutions is known to undergo photoionization and photoaquation reactions depending on the excitation wavelength. To investigate this wavelength dependence, we implemented ultrafast two-dimensional UV transient absorption spectroscopy, covering a range from 280 to 370 nm in both excitation and probing, along with UV pump/visible probe or time-resolved infrared (TRIR) transient absorption spectroscopy and density functional theory (DFT) calculations. As far as photoaquation is concerned, we find that excitation of the molecule leads to ultrafast intramolecular relaxation to the lowest triplet state of the [Fe(CN)6]4- complex, followed by its dissociation into CN- and [Fe(CN)5]3- fragments and partial geminate recombination, all within <0.5 ps. The subsequent time evolution is associated with the [Fe(CN)5]3- fragment going from a triplet square pyramidal geometry, to the lowest triplet trigonal bipyramidal state in 3-4 ps. This is the precursor to aquation, which occurs in ∼20 ps in H2O and D2O solutions, forming the [Fe(CN)5(H2O/D2O)]3- species, although some aquation also occurs during the 3-4 ps time scale. The aquated complex is observed to be stable up to the microsecond time scale. For excitation below 310 nm, the dominant channel is photooxidation with a minor aquation channel. The photoaquation reaction shows no excitation wavelength dependence up to 310 nm, that is, it reflects a Kasha Rule behavior. In contrast, the photooxidation yield increases with decreasing excitation wavelength. The various intermediates that appear in the TRIR experiments are identified with the help of DFT calculations. These results provide a clear example of the energy dependence of various reactive pathways and of the role of spin-states in the reactivity of metal complexes.

The recent unravelling of strongly bound excitons in anatase TiO$_2$ nanoparticles at room temperature has shed light on the importance of many-body effects in samples used for applications. Here, we demonstrate the interplay between many-body interactions and correlations in highly-excited anatase TiO$_2$ nanoparticles by means of ultrafast two-dimensional deep-ultraviolet spectroscopy. We observe that the exciton optical nonlinearities upon non-resonant excitation are dominated by phase-space filling and long-range Coulomb screening, reflecting the dynamics of the photoexcited electron density. By tracking the renormalization of the exciton feature over time, we unveil the timescales involved in the intraband hot electron thermalization. We observe that the strong electron-phonon coupling results in an ultrafast dissipation of the electronic excess energy within 50 fs. Our findings open new perspectives in the design of alternative resonant excitation schemes to be tested in photocatalysis and nanotechnology.

We report the results of ultrafast transient absorption studies of tetrakis(μ-pyrophosphito)diplatinate(II), [Pt2(μ-P2O5H2)4]4− (Pt(pop)) and its perfluoroborated derivative [Pt2(μ-P2O5(BF2)4]4− (Pt(pop-BF2)) in water and acetonitrile upon excitation of high lying (<300nm) UV absorption bands. We observe an ultrafast relaxation channel from high lying states to the lowest triplet state that partly (Pt(pop) in H2O, Pt(pop-BF2)) or fully (Pt(pop) in MeCN) bypasses the lowest singlet excited state. As a consequence, vibrational wave packets are detected in the lowest triplet state and/or the lowest excited singlet of both complexes, even though the electronic relaxation cascade spans ca. 2 and 1.3eV, respectively. In the case of Pt(pop-BF2), coherent wave packets generated by optical excitation of the lowest singlet 1A2u state also are reported. Overall, the reported dephasing times of the Pt-Pt oscillator in the ground, singlet and triplet states do not depend much on the solvent or the molecular structure.

Anatase TiO2 is among the most studied materials for light-energy conversion applications, but the nature of its fundamental charge excitations is still unknown. Yet it is crucial to establish whether light absorption creates uncorrelated electron-hole pairs or bound excitons and, in the latter case, to determine their character. Here, by combining steady-state angle-resolved photoemission spectroscopy and spectroscopic ellipsometry with state-of-the-art ab initio calculations, we demonstrate that the direct optical gap of single crystals is dominated by a strongly bound exciton rising over the continuum of indirect interband transitions. This exciton possesses an intermediate character between the Wannier-Mott and Frenkel regimes and displays a peculiar two-dimensional wavefunction in the three-dimensional lattice. The nature of the higher-energy excitations is also identified. The universal validity of our results is confirmed up to room temperature by observing the same elementary excitations in defect-rich samples (doped single crystals and nanoparticles) via ultrafast two-dimensional deep-ultraviolet spectroscopy.Here the authors combine steady-state angle-resolved photoemission spectroscopy, ellipsometry and ultrafast two-dimensional ultraviolet spectroscopy to examine the role of many-body correlations in anatase TiO2, revealing the existence of strongly bound excitons in single crystals and nanoparticles.

Significance We demonstrate the occurrence of tryptophan (Trp) to heme electron transfer (ET) in ferrous myoglobins by ultrafast UV spectroscopy. The ET gives rise to the theoretically predicted, low-valence Fe(II)(porph ●− ) anion radical, which we observe for the first time to our knowledge under physiological conditions. These results highlight the generality of Trp–porphyrin electron transfer events in heme proteins and question the systematic use of Trp fluorescence in FRET studies of protein dynamics.

It is known that excitation by visible light of the singlet metal-to-ligand charge-transfer ( 1 MLCT) states of Fe (II) complexes leads to population of the lowest-lying high-spin quintet state ( 5 T) with unity quantum yield. Here we investigate this so-called spin crossover (SCO) transition in aqueous iron (II) tris(bipyridine). We use pump–probe transient absorption spectroscopy with a high time resolution of <60 fs in the ultraviolet probe range, in which the 5 T state absorbs, and of <40 fs in the visible probe range, in which both the hot MLCT state and the 5 T state absorb. Our results show that the 5 T state is impulsively populated in less than 50 fs, which is the time we measured for the depopulation of the MCLT manifold. We propose that non-totally-symmetric modes mediate the process, possibly high-frequency modes of the bipyridine (bpy) ligand. These results show that even though the SCO process in Fe (II) complexes represents a strongly spin-forbidden (Δ S = 2) two-electron transition, spin flipping occurs at near subvibrational times and is intertwined with the electron and structural dynamics of the system.

We report on a study of the early relaxation processes of met-Myoglobin in aqueous solution, using a combination of ultrafast broadband fluorescence detection and transient absorption with a broad UV-visible continuum probe at different pump energies. Reconstruction of the spectra of the transient species unravels the details of the haem photocycle in the absence of photolysis. Besides identifying a branching in the ultrafast relaxation of the haem, we show clear evidence for an electronic character of the intermediates, contrary to the commonly accepted idea that the early time relaxation of the haem is only due to cooling. The decay back to the ground state proceeds partially as a cascade through iron spin states, which seems to be a general characteristic of haem systems.

Tryptophan is commonly used to study protein structure and dynamics, such as protein folding, as a donor in fluorescence resonant energy transfer (FRET) studies. By using ultra-broadband ultrafast two-dimensional (2D) spectroscopy in the ultraviolet (UV) and transient absorption in the visible range, we have disentangled the excited state decay pathways of the tryptophan amino acid residues in ferric myoglobins (MbCN and metMb). Whereas the more distant tryptophan (Trp(7)) relaxes by energy transfer to the heme, Trp(14) excitation predominantly decays by electron transfer to the heme. The excited Trp(14)→heme electron transfer occurs in <40 picoseconds with a quantum yield of more than 60%, over an edge-to-edge distance below ~10 angstroms, outcompeting the FRET process. Our results raise the question of whether such electron transfer pathways occur in a larger class of proteins.