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The current work focuses on the investigation of two functionalized naphthyridine derivatives, namely ODIN‐EtPh and ODIN‐But, to gain insights into the hydrogen bond‐assisted H‐aggregate formation and its impact on the optical properties of ODIN molecules. By employing a combination of X‐ray and electron crystallography, absorption and emission spectroscopy, time resolved fluorescence and ultrafast pump‐probe spectroscopy (visible and infrared) we unravel the correlation between the structure and light‐matter response, with a particular emphasis on the influence of the polarity of the surrounding environment. Our experimental results and simulations confirm that in polar and good hydrogen‐bond acceptor solvents (DMSO), the formation of dimers for ODIN derivatives is strongly inhibited. The presence of a phenyl group linked to the ureidic unit favors the folding of ODIN derivatives (forming an intramolecular hydrogen bond) leading to the stabilization of a charge‐transfer excited state which almost completely quenches its fluorescence emission. In solvents with a poor aptitude for forming hydrogen bonds, the formation of dimers is favored and gives rise to H aggregates, with a consequent considerable reduction in the fluorescence emission. The urea‐bound phenyl group furtherly stabilizes the dimers in chloroform. [ABSTRACT FROM AUTHOR]

Ultrafast pump-probe measurements are used to characterize various samples, such as biological cells, bulk, and thin-film structures. However, typical implementations of the pump-probe apparatus are either slow or complex and costly hindering wide deployment. Here we combine a single-cavity dual-comb laser with a simple experimental setup to obtain pump-probe measurements with ultra-high sensitivity, fast acquisition, and high timing precision over long optical delay scan ranges of 12.5 ns that would correspond to a mechanical delay of about 3.75 m. We employ digital signal balancing to obtain shot-noise-limited detection compatible with pump-probe microscopy deployment. Here we demonstrate ultrafast photoacoustics for thin-film sample characterization. We measured a tungsten layer thickness of (700 ± 4) Å with shot-noise-limited detection. Such single-cavity dual-comb lasers can be used for any pump-probe measurements and are especially well-suited for ultrafast photoacoustic studies such as involving ultrasonic echoes, Brillouin oscillations, surface acoustic waves and thermal dynamics.

The rapid growth of attention to tin sulfide (SnS) is due to its efficient application in the field of thermoelectricity, and its doping product SnS0.91Se0.09 can obtain a high average thermoelectric figure of merit (ZTave≈ 1.25). Although many efforts have been made to reveal the mechanisms associated with electron transport and heat conduction, the role of lattice vibration remains poorly understood. Here, we studied the coherent vibration dynamics in SnS0.91Se0.09 bulk single crystal by using the femtosecond two-color pump-probe optical reflectivity technique. The oscillation signal in the time domain of SnS0.91Se0.09 is considered to be an electron–phonon interaction. The coherent dynamics of longitudinal acoustic phonons in this coupling behavior is closely related to probe polarization and excitation power. The stability of oscillation frequency reveals the microscopic mechanism of phonon conduction, and further understands the lattice nonharmonic caused by electron–phonon interaction in light-induced.

Vanadium pentoxide can undergo a reversible phase transition by heating above 260 °C; its non-thermal phase transition, as well as ultrafast dynamical processes, is still not known. Here, femtosecond laser-induced phase transition properties in V2O5 thin films were first explored using femtosecond time-resolved pump–probe spectroscopy. The results show that the phase transient processes occur on a 10−15–10−13 temporal scale. The phase transition and recovery properties are dependent on both the substrates and pump laser energy densities. We propose the oxygen vacancies theory to explain the results, and we provide valuable insights into V2O5 films for potential applications.

A series of covalent ferrocene–BODIPY–fullerene triads with the ferrocene groups conjugated to the BODIPY π‐system and the fullerene acceptor linked at the boron hub by a common catecholpyrrolidine bridge were prepared and characterized by 1D and 2D NMR, UV/Vis, steady‐state fluorescence spectroscopy, high‐resolution mass spectrometry, and, for one of the derivatives, X‐ray crystallography. Redox processes of the new compounds were investigated by electrochemical (CV and DPV) methods and spectroelectrochemistry. DFT calculations indicate that the HOMO in all triads was delocalized between ferrocene and BODIPY π‐system, the LUMO was always fullerene‐centered, and the catechol‐centered occupied orbital was close in energy to the HOMO. TDDFT calculations were indicative of the low‐energy, low‐intensity charge‐transfer bands originated from the ferrocene–BODIPY core to fullerene excitation, which explained the similarity of the UV/Vis spectra of the ferrocene–BODIPY dyads and ferrocene–BODIPY–fullerene triads. Photophysical properties of the new triads as well as reference BODIPY–fullerene and ferrocene–BODIPY dyads were investigated by pump‐probe spectroscopy in the UV/Vis and NIR spectral regions following selective excitation of the BODIPY‐based antenna. Initial charge transfer from the ferrocene to the BODIPY core was shown to outcompete sub‐100 fs deactivation of the excited state mediated by the catechol bridge. However, no subsequent electron transfer to the fullerene acceptor was observed. The initial charge separated state relaxes by recombination with a time constant of 150–380 ps. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The photo-induced intramolecular charge transfer of the unsymmetrical PDI with Pt(II) and Pd(II) ions was slower than the symmetrical ones. • Photophysical properties of the triads were thoroughly studied and compared with the DFT calculations. • The intramolecular charge transfer rates can be controlled by the donor units on the symmetrical and the unsymmetrical PDI derivatives. • The 2 with the lowest band gap is the best candidate for the photosensitive material. Abstract To understand the influence of transition metal ion coordination on the properties and performance of the triads, the symmetric bridging ligand, 1,10-phenanthroline-perylene diimide-1,10-phenanthroline, 1,10-Phen-PDI-1,10-Phen (1) comprising four electron-donating 4-methoxyphenoxy bulky groups at bay -positions and its corresponding square-planar coordination compounds with dichloroplatinum(II), [{PtCl 2 } 2 - 1 ] (2) and palladium(II) [{PdCl 2 } 2 - 1 ] (3) were prepared in order to tune the photochemical and optical properties of these hybrid materials. These triads show strong electronic absorption bands attributed to the PDI and M(II)(1,10-Phen)Cl 2 moieties in DMSO. UV–vis absorption spectra of the compounds were calculated using Time-Dependent Density Functional Theory (TDDFT) for the ground state optimized structures in DCM. Current results indicate that 2 has the lowest HOMO-LUMO gap (2.29 eV in DCM) among the investigated molecules. The energy and charge transfer processes with tailoring molecular structures are one of the important strategies for the design of future functional triads based on donor and acceptor moieties for hybrids optoelectronic devices. Thus, we studied linear absorption, fluorescence, and ultrafast transient absorption spectra measurements for the triads in DCM to investigate the impact of different functionalization strategies on the optical characteristics, photo-stability, and photo-induced charge-transfer (CT) processes. The observed ultrafast intramolecular charge transfer from donor units to acceptor part of 1 – 3 is related to fluorescence quenching and faster singlet state decay on transient absorption measurements. The intramolecular charge transfer mechanism was also compared with the unsymmetrical counterparts that were investigated previously. Symmetrical compounds exhibit faster charge transfer in comparison with the unsymmetrical compounds. [ABSTRACT FROM AUTHOR]

The characterization of the CuInSe 2 (CIS), CuInGaSe (CIGS) and CuGaSe 2 (CGS) based semiconductor thin films are very important role for solar cell and various nonlinear optical applications. In this paper, the amorphous CuIn 0.7 Ga 0.3 (Se 1−x Te x ) 2 semiconductor thin films (0 ≤ x ≤ 1) were prepared with 60 nm thicknesses by using vacuum evaporation technique. The nonlinear absorption properties and ultrafast transient characteristics were investigated by using open aperture Z-scan and ultrafast pump-probe techniques. The energy bandgap values were calculated by using linear absorption spectra. The bandgap values are found to be varying from 0.67 eV to 1.25 eV for CuIn 0.7 Ga 0.3 Te 2 , CuIn 0.7 Ga 0.3 Se 1.6 Te 0.4 , CuIn 0.7 Ga 0.3 Se 0.4 Te 1.6 and CuIn 0.7 Ga 0.3 Se 2 thin films. The energy bandgap values decrease with increasing telluride (Te) doping ratio in mixed CuIn 0.7 Ga 0.3 (Se 1−x Te x ) 2 films. This affects nonlinear characteristics and ultrafast dynamics of amorphous thin films. Ultrafast pump-probe experiments indicated that decreasing of bandgap values with increasing the Te amount switches from the excited state absorption signals to ultrafast bleaching signals. Open aperture Z-scan experiments show that nonlinear absorption properties enhance with decreasing bandgaps values for 65 ps pulse duration at 1064 nm. Highest nonlinear absorption coefficient was found for CuIn 0.7 Ga 0.3 Te 2 thin film due to having the smallest energy bandgap. [ABSTRACT FROM AUTHOR]

The nonlinear absorption properties and ultrafast dynamics of pure and boron doped GaSe crystals have been studied by open aperture Z-scan and ultrafast pump probe spectroscopy techniques. All of the studied crystals showed nonlinear absorption under 100 fs pulse duration and 1200 nm wavelength excitations. Nonlinear absorption coefficients increase with increasing the doping ratio of boron atoms in crystals. These findings indicate that free carrier density increase with boron doping and this behavior leads to excited state absorption. Second harmonic generation signals of crystals were detected with the help of fiber optic spectrometer. The blue shift in the energy of the second harmonic generation signals was observed in boron doped crystals. Ultrafast pump probe experiments indicate that the excited state absorption signal with long lifetime observed for undoped GaSe crystal switches to bleach signal for boron doped GaSe crystals at 625 nm probe wavelength. The effects of increasing doping ratio were observed on ultrafast dynamics as a switching time changes. Our experimental results indicate that it is possible to control nonlinear absorption properties, frequency conversion and ultrafast dynamics of GaSe crystal by changing boron doping ratio. [ABSTRACT FROM AUTHOR]

The rapid growth of attention to tin sulfide (SnS) is due to its efficient application in the field of thermoelectricity, and its doping product SnS 0.91 Se 0.09 can obtain a high average thermoelectric figure of merit (Z T ave ≈ 1.25). Although many efforts have been made to reveal the mechanisms associated with electron transport and heat conduction, the role of lattice vibration remains poorly understood. Here, we studied the coherent vibration dynamics in SnS 0.91 Se 0.09 bulk single crystal by using the femtosecond two-color pump-probe optical reflectivity technique. The oscillation signal in the time domain of SnS 0.91 Se 0.09 is considered to be an electron–phonon interaction. The coherent dynamics of longitudinal acoustic phonons in this coupling behavior is closely related to probe polarization and excitation power. The stability of oscillation frequency reveals the microscopic mechanism of phonon conduction, and further understands the lattice nonharmonic caused by electron–phonon interaction in light-induced. • In this paper, we presented the generation and detection of coherent acoustic phonons in SnS 0.91 Se 0.09 bulk single crystal by ultrafast two-color pump-probe reflectivity method. • The experiment results show that the coherent phonons with an oscillation frequency of 39.2 GHz originate from the dominant mechanism of deformation potential coupling stress. • In addition, this work proposes the view of the unique carrier dynamics with optical anisotropic response in SnS 0.91 Se 0.09 coherent phonon oscillation. [ABSTRACT FROM AUTHOR]

Femto-second pump-probe and micro-Raman spectroscopy (RS) measurements have been made to identify optical phonons in Ge 2 Sb 2 Te 5 /InAs(111) and an InAs(111) substrate. A theory of transient stimulated Raman scattering (TSRS) incorporating the Raman tensor predicts which phonon modes may be observed in transient reflectance (R) and anisotropic reflectance (AR) pump-probe measurements, and how their amplitudes depend upon angles φ and θ that describe the orientation of the pump and probe beam electric fields within the sample plane. AR measurements of an InAs(111) substrate revealed the 6.5 THz T 2 transverse optical phonon with amplitude proportional to sin(2(θ − φ)), as expected for both TSRS and the specular optical Kerr effect (SOKE), confirming that TSRS and SOKE are equivalent descriptions of the same phenomenon. The AR responses of Ge 2 Sb 2 Te 5 /InAs(111) revealed a single coherent optical phonon (COP) mode at about 3.4 THz with sin(2(θ − φ)) amplitude variation that confirms the T 2 -like character of the mode and hence the underlying cubic structure of the epilayer. This mode was also observed in the R measurement for one sample, with amplitude independent of φ and θ as predicted by TSRS theory. Both R and AR signals were heavily damped, which is attributed to dephasing of T 2 x , T 2 y and T 2 z modes that become non-degenerate due to structural distortions. RS measurements revealed three modes for Ge 2 Sb 2 Te 5 /InAs(111) and three modes for InAs(111). Taken together the TSRS and RS measurements provide rich information about optical phonons in the phase change material Ge 2 Sb 2 Te 5 and the InAs(111) surface. • Coherent optical phonons are observed in Ge 2 Sb 2 Te 5 /InAs(111) thin films. • Coherent optical phonons are excited by transient stimulated Raman scattering. • Optical polarisation dependence of coherent phonons confirms cubic crystal structure. • Damping of phonon oscillations is due to dephasing of 3 nearly degenerate modes. [ABSTRACT FROM AUTHOR]

Abstract: The interaction of excitons and other photoluminescent centers in semiconductors with plasmons represents the coupling of the fundamental one-particle, electron-hole excitation with the fundamental many-particle excitation in metals. We describe recent photoluminescence and pump-probe experiments that illustrate both the energetics and the dynamics of this interaction, in a model material incorporating ZnO films separated from a nanostructured plasmonic metal substrate by a variable-thickness spacer layer. We find evidence for different coupling mechanisms for the band-edge exciton and donor–acceptor pair defect luminescence, and discuss the competing roles of localized surface-plasmon resonances and propagating surface-plasmon polaritons. We also present first femtosecond pump-probe lifetime measurements for the band-edge exciton with and without the presence of nearby metal nanostructures. [Copyright &y& Elsevier]

Vanadium pentoxide can undergo a reversible phase transition by heating above 260 °C; its non-thermal phase transition, as well as ultrafast dynamical processes, is still not known. Here, femtosecond laser-induced phase transition properties in V2O5 thin films were first explored using femtosecond time-resolved pump–probe spectroscopy. The results show that the phase transient processes occur on a 10−15–10−13 temporal scale. The phase transition and recovery properties are dependent on both the substrates and pump laser energy densities. We propose the oxygen vacancies theory to explain the results, and we provide valuable insights into V2O5 films for potential applications. [ABSTRACT FROM AUTHOR]

Carrier-phonon interactions play a key role in low-dimensional semiconductors which have attracted great interest due to their rich physics and the potential for tailoring the electron mobility or thermal conductivity for device applications. In this work, we present new insights into the interaction between photo-induced carriers and lattice phonons in a representative quasi-layered material without a well-defined van der Waals gap, Bi 2 O 2 Se, through ultrafast time-resolved optical pump–probe measurements. Results reveal that in the nonlinear excitation regime, the phonon-assisted carrier relaxation in Bi 2 O 2 Se nanoplates follows a bi-exponential decay process and exhibits pump fluence-dependent relaxation times. This two-stage relaxation can be qualitatively explained by a phenomenological three-temperature model that considers energy exchange between three coupled subsystems (electrons, in-plane and out-of-plane phonons). This work expands the current understanding of carrier dynamics in Bi 2 O 2 Se and may pave the way for enhancing the performance of related optoelectronic applications. [ABSTRACT FROM AUTHOR]