Probe Wavelength Research Articles

Part-per-billion level photothermal nitric oxide detection at 5.26 µm using antiresonant hollow-core fiber-based heterodyne interferometry.

In this work, I demonstrate a novel configuration of a photothermal gas sensor. Detection of nitric oxide at a wavelength of 5.26 µm was possible by constructing an absorption cell based on a self-fabricated antiresonant hollow core fiber characterized by low losses at both the pump and probe wavelengths. Proper design of the sensor allowed using the heterodyne interferometry-based signal readout of the refractive index modulation, which yielded a record noise equivalent absorption of 2.81×10-8 cm-1 for 100 s integration time for mid-infrared fiber-based gas sensors. The obtained results clearly demonstrate the full potential of using properly designed antiresonant hollow core fibers in combination with sensitive gas detection methods.

Photoexcited carrier dynamics of thin film Cd3As2 grown on a GaAs(111)B substrate by molecular beam epitaxy

The topological Dirac semimetal ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ has drawn great attention for its novel physics and technical applications in optoelectronic devices operating in the infrared and THz regimes. Exploring the photoexcited carrier dynamics in ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ is helpful to understand the transient carrier occupation and cooling processes that are closely associated with its unconventional band characteristic. Here, the photoexcited carrier dynamics in the ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ thin film epitaxially grown on the $\mathrm{GaAs}(111)B$ substrate was investigated by employing the time-resolved midinfrared transient reflectance measurements, along with a theoretical modeling concerning the electron-polar optical ($e$-PO) phonon interaction. The measured photoexcited electron relaxation time of ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ increases with the probe wavelength, and increases with temperature at temperatures higher than 30 K. Theoretical modeling under the framework of the two-temperature model can give qualitatively good description on both the temperature and probe wavelength dependence of the carrier relaxation time. Additionally, theoretical modeling shows that charge screening of the interaction of electrons with polar optical phonons can greatly reduce the hot carrier relaxation rate. The measured transient reflectance analyses indicate that the hot phonon effect plays a negligible role on the carrier relaxation in ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$. Our findings provide further insight into the photoexcited carrier dynamics in ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ and valuable reference for developing its high-performance infrared optoelectronic devices.

Ultrafast carrier dynamics in SnSe thin film studied by femtosecond transient absorption technique

The femtosecond transient absorption (TA) technique is used to study the ultrafast carrier dynamics of SnSe thin film prepared by pulsed laser deposition (PLD), which is basically characterized by X-ray diffraction (XRD). The XRD pattern shows pure single-phase of SnSe in term of the diffraction peak of crystalline direction (400), and the indirect bandgap can be estimated by Kubelka-Munk treatment in transmittance spectrum. The TA spectra of SnSe thin film are observed in picosecond timescale, corresponding to the process of carrier scattering, carrier recombination and lattice thermal effects. A triple exponential decay model, including fast (~2 ps), medium (~100 ps), and slow (ns) components, is well fitted to the TA dynamics of SnSe film. There are different dominant processes in the fast, medium and slow components, which are related to the carrier process of scattering, recombination and lattice diffusion, respectively. And these responses are modulated by single pulse energy, the carrier concentration and temperature can affect the microscopic relaxation process. The thickness of SnSe thin films also affect the ultrafast carrier dynamics. And we find there are differences in different probe wavelengths. Our results can improve the comprehension of the relaxation mechanism in SnSe thin film, and provide the important view about the ultrafast optical properties of this layered IV-VI chalcogenide.

Femtosecond Wavepacket Dynamics Reveals the Molecular Structures in the Excited (S1) and Cationic (D0) States.

Molecular structures in the electronically excited (S1) and cationic (D0) states of 2-fluorothioanisole (2-FTA) have been precisely refined from the real-time dynamics of the femtosecond (fs) wavepacket prepared by the coherent excitation of the Franck-Condon active out-of-plane torsional modes in the S1 ← S0 transition at 285 nm. The simulation to reproduce the experiment in terms of the beating frequencies gives the nonplanar geometry of 2-FTA in S1, where the out-of-plane dihedral angle (φ) of the S-CH3 moiety is 51° with respect to the molecular plane. The behavior of the fs wavepacket in terms of the amplitudes and phases with the change of the probe (ionization) wavelength (λprobe = 300-330 nm) provides the otherwise veiled structure of the cationic D0 state. While the 2-FTA cation adopts the planar geometry (φ = 0°) at the global minimum, it is found to have a vertical minimum at φ ≈ 135° from the perspective of the D0 ← S1 vertical transition. Ab initio calculations support the experiment quite well although the simulation using the model potentials could improve the match with the experiment, giving the new interpretation for the previously disputed photoelectron spectroscopic results.

In-situ diagnostic of femtosecond laser probe pulses for high resolution ultrafast imaging

Ultrafast imaging is essential in physics and chemistry to investigate the femtosecond dynamics of nonuniform samples or of phenomena with strong spatial variations. It relies on observing the phenomena induced by an ultrashort laser pump pulse using an ultrashort probe pulse at a later time. Recent years have seen the emergence of very successful ultrafast imaging techniques of single non-reproducible events with extremely high frame rate, based on wavelength or spatial frequency encoding. However, further progress in ultrafast imaging towards high spatial resolution is hampered by the lack of characterization of weak probe beams. For pump–probe experiments realized within solids or liquids, because of the difference in group velocities between pump and probe, the determination of the absolute pump–probe delay depends on the sample position. In addition, pulse-front tilt is a widespread issue, unacceptable for ultrafast imaging, but which is conventionally very difficult to evaluate for the low-intensity probe pulses. Here we show that a pump-induced micro-grating generated from the electronic Kerr effect provides a detailed in-situ characterization of a weak probe pulse. It allows solving the two issues of absolute pump–probe delay determination and pulse-front tilt detection. Our approach is valid whatever the transparent medium with non-negligible Kerr index, whatever the probe pulse polarization and wavelength. Because it is nondestructive and fast to perform, this in-situ probe diagnostic can be repeated to calibrate experimental conditions, particularly in the case where complex wavelength, spatial frequency or polarization encoding is used. We anticipate that this technique will enable previously inaccessible spatiotemporal imaging in a number of fields of ultrafast science at the micro- and nanoscale.

Electron-phonon scattering in β -Ga2O3 studied by ultrafast transmission spectroscopy

Femtosecond pump-probe experiments in a transmission geometry were performed on Sn-doped n-type β-Ga2O3. With the pump and probe wavelengths below the bandgap, the differential transmission signal was determined by the free electron dynamics. Differential transmission decay times and their spectral dependence were used to evaluate electron-phonon scattering for polar optical (PO) and intervalley phonons. The obtained average electron-PO phonon scattering time is 4.5 ± 0.4 fs, while the electron scattering to and from the side valley is 80 ± 5 fs. The energy between the absolute and second lowest conduction band minima is estimated to be 2.6 ± 0.1 eV.

Temperature dependent electron–phonon coupling of Au resolved via lattice dynamics measured with sub-picosecond infrared pulses

The detailed understanding of energy transfer between hot electrons and lattice vibrations at non-cryogenic temperatures relies primarily upon the interpretation of ultrafast pump–probe experiments, where thermo-optical models provide insight into the relationship between optical response and temperature of the respective sub-systems; in one of the more studied materials, gold, the Drude model provides this relationship. In this work, we investigate the role of intra- and interband contributions applied to transient optical responses in ultrafast pump–probe experiments using both experiments and first-principle calculations, with probe wavelengths spanning from UV wavelengths into the infrared. We find that during conditions of electron–phonon equilibrium, the Drude model is not applicable to visible wavelengths due to interband transitions. Instead, at probe wavelengths far from these interband transitions (e.g., infrared wavelengths), the optical response is linearly proportional to the temperature of the phonon sub-system and is no longer obfuscated by Fermi-smearing, thus greatly simplifying the extraction of the electron–phonon coupling factor. Our intraband-probe measurements on the electron–phonon coupling factor of Au are in excellent agreement with analytical models and ab initio calculations; we observe a constant electron–phonon coupling factor up to electron temperatures of at least ∼2000 K.

Experimental determination of nanocomposite grating structures by light- and neutron-diffraction in the multi-wave-coupling regime

We experimentally demonstrate how to accurately retrieve the refractive index profile of photonic structures by standard diffraction experiments and use of the rigorous coupled-wave analysis in the multi-wave coupling regime, without the need for taking any auxiliary data. In particular, we show how the phases of the Fourier components of a periodic structure can be fully recovered by deliberately choosing a probe wavelength of the diffracting radiation much smaller than the lattice constant of the structure. In the course of our demonstration, we accurately determine the slight asymmetry of the structure of nanocomposite phase gratings by light and neutron diffraction measurements.

Ni-in-garnet geothermometry in mantle rocks: a high pressure experimental recalibration between 1100 and 1325\xa0\xb0C

The temperature-dependent exchange of Ni and Mg between garnet and olivine in mantle peridotite is an important geothermometer for determining temperature variations in the upper mantle and the diamond potential of kimberlites. Existing calibrations of the Ni-in-garnet geothermometer show considerable differences in estimated temperature above and below 1100 °C hindering its confident application. In this study, we present the results from new synthesis experiments conducted on a piston cylinder apparatus at 2.25–4.5 GPa and 1100–1325 °C. Our experimental approach was to equilibrate a Ni-free Cr-pyrope-rich garnet starting mixture made from sintered oxides with natural olivine capsules (Niolv ≅ 3000 ppm) to produce an experimental charge comprised entirely of peridotitic pyrope garnet with trace abundances of Ni (10–100 s of ppm). Experimental runs products were analysed by wave-length dispersive electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We use the partition coefficient for the distribution of Ni between our garnet experimental charge and the olivine capsule left( {{text{lnD}}_{{{text{grt}}/{text{olv}}}}^{{{text{Ni}}}} ; frac{{{text{Ni}}_{{{text{grt}}}} }}{{{text{Ni}}_{{{text{olv}}}} }}} right), the Ca mole fraction in garnet ({mathrm{X}}_{mathrm{grt}}^{mathrm{Ca}}; Ca/(Ca + Fe + Mg)), and the Cr mole fraction in garnet ({mathrm{X}}_{mathrm{grt}}^{mathrm{Cr}}; Cr/(Cr + Al)) to develop a new formulation of the Ni-in-garnet geothermometer that performs more reliably on experimental and natural datasets than existing calibrations. Our updated Ni-in-garnet geothermometer is defined here as:T left(^circ{rm C} right)=frac{-8254.568}{left(left( {mathrm{X}}_{mathrm{grt}}^{mathrm{Ca}} times 3.023 right)+left({mathrm{X}}_{mathrm{grt}}^{mathrm{Cr}} times 2.307 right)+left({mathrm{lnD}}_{frac{mathrm{grt}}{mathrm{olv}}}^{mathrm{Ni}} - 2.639 right)right)}-273pm 55where {mathrm{D}}_{mathrm{grt}/mathrm{olv}}^{mathrm{Ni}}= frac{{mathrm{Ni}}_{mathrm{grt}}}{{mathrm{Ni}}_{mathrm{olv}}}, Ni is in ppm, {mathrm{X}}_{mathrm{grt}}^{mathrm{Ca}} = Ca/(Ca + Fe + Mg) in garnet, and {mathrm{X}}_{mathrm{grt}}^{mathrm{Cr}}= Cr/(Cr + Al) in garnet. Our updated Ni-in-garnet geothermometer can be applied to garnet peridotite xenoliths or monomineralic garnet xenocrysts derived from disaggregation of a peridotite source. Our calibration can be used as a single grain geothermometer by assuming an average mantle olivine Ni concentration of 3000 ppm. To maximise the reliability of temperature estimates made from our Ni-in-garnet geothermometer, we provide users with a data quality protocol method which can be applied to all garnet EPMA and LA-ICP-MS analyses prior to Ni-in-garnet geothermometry. The temperature uncertainty of our updated calibration has been rigorously propagated by incorporating all analytical and experimental uncertainties. We have found that our Ni-in-garnet temperature estimates have a maximum associated uncertainty of ± 55 °C. The improved performance of our updated calibration is demonstrated through its application to previously published experimental datasets and on natural, well-characterised garnet peridotite xenoliths from a variety of published datasets, including the diamondiferous Diavik and Ekati kimberlite pipes from the Lac de Gras kimberlite field, Canada. Our new calibration better aligns temperature estimates using the Ni-in-garnet geothermometer with those estimated by the widely used (Nimis and Taylor, Contrib Mineral Petrol 139:541–554, 2000) enstatite-in-clinopyroxene geothermometer, and confirms an improvement in performance of the new calibration relative to existing versions of the Ni-in-garnet geothermometer.

Imaging of intracellular singlet oxygen with bright BODIPY dyes

Singlet oxygen is a cytotoxic reactive species which is involved in the photodynamic therapy of cancer. It is also known to be produced endogenously in most eukaryotic cells and implicated in many biochemical processes, including apoptotic response. We now report that Bodipy based fluorescent dyes with singlet oxygen reactive modules, signal the intracellular generation of singlet oxygen through photosensitization. We believe long wavelength probes of singlet oxygen, based on this approach will be highly valuable.

Using computational chemistry to design pump-probe schemes for measuring nitrobenzene radical cation dynamics.

The electronic potential energy surfaces of the nitrobenzene cation obtained from time-dependent density functional theory and coupled cluster calculations are used to predict the most efficient excitation wavelength for femtosecond time-resolved mass spectrometry measurements. Both levels of theory identify a strongly-coupled transition from the ground state of the nitrobenzene cation with a geometry-dependent oscillator strength, reaching a maximum at 90° C-C-N-O dihedral angle with a corresponding energy gap of ∼2 eV. These results are consistent with the experimental observation in the nitrobenzene cation of a coherent superposition of vibrational states: a vibrational wave packet. Time-resolved measurements using a probe wavelength of 650 nm, nearly resonant with the strong transition, result in enhanced ion yield oscillation amplitudes as compared to excitation with the nonresonant 800 nm wavelength. Analogous behavior is found for the closely related molecules 2- and 4-nitrotoluene. These results demonstrate that computational chemistry can predict the best choice of probe wavelength in time-resolved measurements of vibrational coherent states in molecular cations.

Wigner distribution of self-amplified spontaneous emission free-electron laser pulses and extracting its autocorrelation.

The emerging concept of `beam by design' in free-electron laser (FEL) accelerator physics aims for accurate manipulation of the electron beam to tailor spectral and temporal properties of the radiation for specific experimental purposes, such as X-ray pump/X-ray probe and multiple wavelength experiments. `Beam by design' requires fast, efficient, and detailed feedback on the spectral and temporal properties of the generated X-ray radiation. Here a simple and cost-efficient method to extract information on the longitudinal Wigner distribution function of emitted FEL pulses is proposed. The method requires only an ensemble of measured FEL spectra and is rather robust with respect to accelerator fluctuations. The method is applied to both the simulated SASE spectra with known radiation properties as well as to the SASE spectra measured at the European XFEL revealing underlying non-linear chirp of the generated radiation. In the Appendices an intuitive understanding of time-frequency representations of chirped SASE radiation is provided.

Coherent vibrational dynamics of [Au25(SR)18]- nanoclusters

Coherent vibrational dynamics can be observed in atomically precise gold nanoclusters using femtosecond time-resolved pump-probe spectroscopy. It can not only reveal the coupling between electrons and vibrations, but also reflect the mechanical and electronic properties of metal nanoclusters, which holds potential applications in biological sensing and mass detection. Here, we investigated the coherent vibrational dynamics of [Au25(SR)18]− nanoclusters by ultrafast spectroscopy and revealed the origins of these coherent vibrations by analyzing their frequency, phase and probe wavelength distributions. Strong coherent oscillations with frequency of 40 cm−1 and 80 cm−1 can be reproduced in the excited state dynamics of [Au25(SR)18]−, which should originate from acoustic vibrations of the Au13 metal core. Phase analysis on the oscillations indicates that the 80 cm−1 mode should arise from the frequency modulation of the electronic states while the 40 cm−1 mode should originate from the amplitude modulation of the dynamic spectrum. Moreover, it is found that the vibration frequencies of [Au25(SR)18]− obtained in pump-probe measurements are independent of the surface ligands so that they are intrinsic properties of the metal core. These results are of great value to understand the electron-vibration coupling of metal nanoclusters.

Probing Delocalized Current Densities in Selenophene by Resonant X-ray Sum-Frequency Generation

Time-resolved, resonant X-ray sum-frequency generation in aligned selenophene molecules is calculated. A wave packet of valence-excited states, prepared by an extreme-ultraviolet pump pulse, is probed by two 12-keV X-ray probe pulses resonant with the Se core-excited states for variable time delays. At these hard-X-ray frequencies, the angström wavelength of the X-ray probe is comparable to the molecular size. We thus employ a nonlocal description of the light-matter interaction based on the minimal-coupling Hamiltonian. The wavevector-resolved resonant stimulated sum-frequency-generation signal, obtained by varying the propagation direction of hard-X-ray pulses, can thus directly monitor the transition current densities between core and ground/valence states. This is in contrast to off-resonant diffraction, which detects the transition charge densities.

Manipulating Exciton Transfer between Colloidal Quantum Dots and Graphene Oxide

Colloidal quantum dots and graphene oxide composite are promising semiconductor nanomaterials for optoelectronic devices due in part to their tunable light absorption and extraordinary carrier transport properties. We investigate the exciton interaction between them during laser irradiation by transient absorption spectroscopy. In the transient measurement, an exciton bleach buildup signal is exhibited before its recovery at the long probe wavelength. It can be attributed that the trapped electron of graphene oxide backward transfers to the conduction band of quantum dots. With the reduction degree of graphene oxide being increased by laser irradiation, backward electron transfer would take place at both longwave and shortwave, resulting in a broader and stronger exciton bleach feature. Subsequently, exciton bleach disappeared at longwave because of hole trapping by graphene oxide. Exciton transfer of quantum dots and graphene oxide composite has been efficiently manipulated by laser irradiation.

Flujometría por láser Doppler como método de análisis para la evaluación de la mucositis oral inducida por quimioterapia: Un estudio piloto

Background: Oral mucositis (OM) is an inflammation of the oral mucosa due to cancer therapy that compromises the patient’s quality of life. Laser Doppler flowmetry (LDF) is a non-invasive method to monitor microvascular blood flow (BF) in real-time. Purpose: Develop a method to evaluate BF in the genian region cheek in patients undergoing chemotherapy by LDF and compare the degrees of OM and pain with evaluation of BF. Material and methods: Evaluation of OM was performed using the World Health Organization (WHO) and Oral Mucositis Assessment Scale (OMAS) scales and the visual analog scale for pain evaluation. For flowmetry analysis, a laser Doppler flowmeter (moorVMSTM™, 780 nm wavelength and VP3 probe), fixed by an acrylic resin support was used; VP3 probe was positioned on the genian region and the patient’s head was stabilized with a neck pillow for an accurate measurement. The Wilcoxon test was used to compare the flowmetry results at the studied times. The Pearson correlation coefficient was used to evaluate relationships between BF and the WHO, OMAS and visual analog scales. Results: Eleven patients of both sexes, aged between 30 and 78 years, with OM were included. An increase in cutaneous BF was observed at the initial times of OM, with progressive reduction during the chemotherapy cycle. There was a statistical difference (p<0.05) between time point T0 (first consultation) and time point T6 (last consultation). Conclusion: The method developed in this pilot study is effective, reliable, and reproducible, and allows the evaluation of BF dynamics in the genian region using LDF of patients undergoing chemotherapy at risk of developing OM.

Tunable polarization beam splitter and broadband optical power sensor using hybrid microsphere resonators.

Based on silica microsphere resonators embedded with iron oxide nanoparticles, we proposed and fabricated an all-optical and continuously tunable polarization beam splitter (PBS), and a broadband optical power sensor (OPS) with high sensitivity. The PBS is realized since the effective refractive indexes of the transverse-electric and transverse-magnetic polarization modes in the microsphere resonator are different. Due to the excellent photothermal effect of iron oxide nanoparticles, we realized the all-optical and continuously tunable PBS based on the hybrid microsphere resonator. A maximum polarization splitting ratio of 20 dB and a tuning range of 5 nm are achieved. Based on this mechanism, the hybrid microsphere resonator can also be used as a broadband OPS. The sensitivity of the OPS is 0.487 nm/mW, 0.477 nm/mW, and 0.398 nm/mW when the probe wavelength is 690 nm, 980 nm, and 1550 nm, respectively. With such good performances, the tunable PBS and the broadband OPS have great potential in applications such as optical routers, switches and filters.

Gas sensing with mode-phase-difference photothermal spectroscopy assisted by a long period grating in a dual-mode negative-curvature hollow-core optical fiber.

We demonstrate sensitive gas detection with mode-phase-difference photothermal spectroscopy assisted by a long period grating (LPG) inscribed on a dual-mode negative-curvature hollow-core fiber (NC-HCF). The LPG is inscribed using a pulsed CO2 laser, which enables pump propagation in the fundamental LP01 mode to achieve maximum photothermal phase modulation while exciting both the LP01 and LP11 modes at the probe wavelength to form a dual-mode interferometer for detection of the phase difference. With a 1533 nm pump and a 1620 nm probe, a noise equivalent concentration of ∼2.2 ppb acetylene is achieved with an 85-cm-long NC-HCF gas cell and 1 s lock-in time constant.

Ultrafast dynamics and short-lived carriers in Cu nitride and oxynitride layers

Ultrafast carrier dynamics in copper nitride (Cu3N) have been investigated using femtosecond pump-probe spectroscopy. Cu3N films were prepared on fused SiO2 substrates coated with thin Cu layers under NH3:O2 flow. The obtained differential transmission data revealed discrete maxima in the spectra at different probing energies. Lower energy maxima are attributed to direct bandgap transitions, while transient transmission peaks at higher energies are due to probing defect state transitions in the material. These states originate from crystalline structure defects and are energetically located close to the conduction and valence bands. The recombination times extracted from the data were of the order of few picoseconds which raises the question of how suitable this material is for energy conversion applications. Ultrafast pump-probe spectra obtained at a low temperature of 78 K revealed almost no shifting of the transition energies compared to room temperature spectra, although small differences in decay times were observed due to the absence of thermal effects. Measurements using longer probing wavelengths in the infrared region indicated state filling of defect states within the bandgap for wavelengths between 800 nm and 1400 nm while probing at longer wavelengths (1400–1600 nm) revealed initial free-carrier absorption and re-excitation of carriers to higher energy defect states above the conduction band. In addition, copper oxynitride layers were produced and studied in a similar fashion to investigate the effects of oxygen on the band structure and recombination times.

Nonlinear absorption and the ultrafast dynamic process of Au-Ag nanoshuttles

Nonlinear optical absorption of Au-Ag nanoshuttles (NSs) was studied using an open-aperture Z-scan experiment with a 532 nm nanosecond laser at different energies. It was found that, when the laser energy is relatively low, the Au-Ag NSs exhibit saturated absorption (SA). When the laser energy is high, a conversion from SA to reverse saturated absorption (RSA) occurs. The ultrafast dynamic process of Au-Ag NSs was also investigated by using a femtosecond pump-probe technique. It is found that the process contains a fast and slow decay component that depends strongly on the laser intensity. Furthermore, when the probe wavelength is far away from the plasma resonance peak, the decay shows modulation due to the vibration mode of the coherent excitation.