Pump Excitation Research Articles

Forward terahertz wave generation from liquid gallium in the non-relativistic regime

We characterize a terahertz (THz) source based on plasma in liquid gallium. The dependence of the emitted THz pulse energy on second-order phase, pump pulse energy, and polarization of the short laser pulse is demonstrated. Our study suggests that the THz emission mechanism is due to the ponderomotive force and is aided by a direct-field driven term. The proposed source and accompanying generation mechanism are studied under a non-relativistic regime ( 1 0 15 < I < 1 0 18 W / c m 2 ) for forward directed THz under a single pump excitation scheme.

Active control of plasmon-induced transparency based on a GaAs/Si heterojunction in the terahertz range

Active control optical terahertz modulator based on incorporating n-type GaAs film as the active media into plasmon-induced transparency metasurface, was investigated theoretically and experimentally. From the experimental results it could be found that the resonances transmission of the plasmon-induced transparency was optically modulated and the transmission modulation depth of 87% was achieved at 0.56 THz. This may be due to the fact that when the pump excitation power was tuned from 0 mW to 1000 mW, larger numbers of excited carriers were excited in the electronic “p-n" junction and diffused across the heterojunction to the n-type GaAs film. It increased the conductivity of the n-type GaAs film from 2.29 × 103 S/m to 1.56 × 104 S/m. As a results, the capacitive split gaps of the PIT split-ring resonators were short-circuited and the coupling strength between the resonators was decreased owing to the screening effect caused by the photo-excited carriers.

Mid-infrared emission properties of the Tm3+-doped sesquioxide crystals Y2O3, Lu2O3, Sc2O3 and mixed compounds (Y,Lu,Sc)2O3 around 1.5-, 2- and 2.3-μm

Tm3+-doped sesquioxides Y2O3, Lu2O3 and Sc2O3 are well known for their emissions around 2 μm (3F4 →3H6 transition). We study here their absorption and emission properties, both at 2 μm and at 1.5 μm (3H4 → 3F4) and 2.3 μm (3H4 → 3H5), by considering that the totality of the Tm3+ ions or only part of them are contributing to the line intensities. The study is also extended to the Tm3+-doped mixed compounds LuYO3, LuScO3 and YScO3. The results obtained in terms of radiative lifetimes and branching ratios, as derived from a Judd-Ofelt analysis of the absorption data, thus appear to be more consistent with the measured fluorescence decays, by assuming that only 75% of the Tm3+ ions, those sitting into C2 symmetry sites, are involved in the absorption and the emission spectra. The registered emission spectra are then calibrated in cross section unit and some of them compared between each other depending on the method (reciprocity, modified reciprocity or Füchtbauer method) which is used. Calculations are finally carried out to predict the cross section of important excited-state transitions which are involved in the two-step excitation pumping of the 3H4 emitting level around 1.05 μm (3F4 → 3F2,3) and 1.45 μm (3F4 → 3H4).

Modulating the Electron Affinity of Small Bipyridyl Molecules on Single Gold Nanoparticles for Plasmon-Driven Electron Transfer

Developing controlled platforms for plasmon-driven chemistry is of great importance in catalytic reactions at the nanoscale. We report anion radical formation for five bipyridyl complexes of varying degrees of electron affinity utilizing optically focused intraband (594 nm) and interband (532 nm) pump excitation of single gold nanoparticles. The surface-enhanced Raman scattering (SERS) of anion radicals for the five nonresonant adsorbed molecules, 2,2′-bipyridine (22BPY), 4,4′-bipyridine (44BPY), trans-1,2-bis(4-pyridyl)ethylene (BPE), 1,2-bis(4-pyridyl)acetylene (BPA), and 1,2-bis(4-pyridyl)ethane (BPEt), were detected using localized surface-plasmon resonance (LSPR) excitation with 785 nm. The electron affinity of the five bipyridyl complexes were determined using electrochemistry. Molecules with low electron affinity experienced higher instances of radical anion formation under a plasmon-coupled intraband electron transfer excitation (594 nm), whereas molecules with high electron affinity showed a preference for anion radical formation under direct interband electron transfer excitation (532 nm). The lowest unoccupied molecular orbital (LUMO) energy levels for low electron affinity surface-bound molecules (22BPY, BPEt) are on average ∼0.43 eV higher than high electron affinity surface-bound molecules (BPA, BPE, 44BPY), as calculated using time-dependent density functional theory, elucidating the importance of plasmon coupling to energy levels that facilitate charge transfer pathways. We also show the ability to “activate” high versus low electron affinity single nanoparticles with the choice of pump excitation wavelength. The findings show the complex interplay between molecular electron affinity, orbital overlap with the density of states of the plasmonic metal, and excitation energetics of the pump laser wavelength. Potential applications of this work include enhanced control over molecular scale catalysis, biosensor design, and solar energy capture.

Role of Solvent in Electron-Phonon Relaxation Dynamics in Core-Shell Au-SiO2 Nanoparticles.

Relaxation dynamics of plasmons in Au-SiO2 core-shell nanoparticles have been followed by femtosecond pump-probe technique. The effect of excitation pump energy and surrounding medium on the time constants associated with the hot electron relaxation has been elucidated. A gradual increase in the electron-phonon relaxation time with pump energy is observed and can be attributed to the higher perturbation of the electron distribution in AuNPs at higher pump energy. Variation in time constants for the electron-phonon relaxation in different solvents is rationalized on the basis of their thermal conductivities, which govern the rate of dissipation of heat of photoexcited electrons in the nanoparticles. On the other hand, phonon-phonon relaxation is found to be much less effective than electron-phonon relaxation for the dissipation of energy of the excited electron and the time constants associated with it remain unaffected by thermal conductivity of the solvent.

Transient absorption spectroscopy to explore cellular pathways to photobiomodulation.

Photobiomodulation (PBM) describes the use of low irradiance light in the red to near-infrared wavelength range to stimulate biological effects in tissue, and many biological and spectroscopic techniques are used to study PBM. However, these techniques focus on the products or downstream effects rather than the electronic transitions that initiate the PBM processes. This study presents a novel approach to studying low irradiance light exposures on individual proteins and/or protein complexes by combining a continuous wave (CW) laser diode with femtosecond transient absorption spectroscopy (TAS), coined here as CW-TAS, and tests the system on reduced cytochrome c (Cyt c) for proof of principle. TAS was conducted using a 532-nm excitation pump beam and a 350-600nm supercontinuum probe. CW laser diodes with wavelengths of 450nm, 635nm, and 808nm were interchangeably fiber coupled into the HELIOS Fire. Samples of Cyt c were tested by TAS using a pump power of 15μW, both with and without CW exposure. CW exposures were carried out with irradiances of 1.60 and 3.20mW/cm2, except for 808nm, which was only tested at 1.60mW/cm2. Both kinetic and global analyses were performed on the TAS data and the time constants for sets with and without CW exposures were compared. The TAS data for Cyt c with the full dosage of CW exposures did not alter the TAS data distinguishably from the control data. No new electronic transient signals were observed beyond the background when testing Cyt c with the CW exposures. Kinetic analysis confirmed that existing transients did not deviate beyond uncertainty. Global time constants for Cyt c were calculated to be 0.25±0.03ps and 5.1±0.3ps for the control study, and the time constants for the CW exposed Cyt c were not significantly different. This study concludes that CW irradiation, at doses delivered, does not alter the transient absorption data of Cyt c. The CW-TAS method provides a new tool for studying PBM effects in other proteins and protein complexes that might respond to the CW wavelengths, such as Complex IV, in future studies.

S1-State Decay Dynamics of Benzenediols (Catechol, Resorcinol, and Hydroquinone) and Their 1:1 Water Clusters.

The S1-state decaying rates of the three different benzenediols, catechol, resorcinol, and hydroquinone, and their 1:1 water clusters have been state-specifically measured using the picosecond time-resolved parent ion transients obtained by the pump (excitation) and probe (ionization) scheme. The S1 lifetime of catechol is found to be short, giving τ ∼ 5.9 ps at the zero-point level. This is ascribed to the H-atom detachment from the free OH moiety of the molecule. Consistent with a previous report (J. Phys. Chem. Lett. 2013, 4, 3819-3823), the S1 lifetime gets lengthened with low-frequency vibrational mode excitations, giving τ ∼ 9.0 ps for the 116 cm-1 band. The S1 lifetimes at the additional vibronic modes of catechol are newly measured, showing the nonnegligible mode-dependent fluctuations of the tunneling rate. When catechol is complexed with water, the S1 lifetime is enormously increased to τ ∼ 1.80 ns at the zero-point level while it shows an unusual dip at the intermolecular stretching mode excitation (τ ∼ 1.03 ns at 146 cm-1). Otherwise, it is shortened monotonically with increasing the internal energy, giving τ ∼ 0.67 ns for the 856 cm-1 band. Two different asymmetric or symmetric conformers of resorcinol give the respective S1 lifetimes of 4.5 or 6.3 ns at their zero-point levels according to the estimation from our transients taken within the temporal window of 0-2.7 ns. When resorcinol is 1:1 complexed with H2O, the S1 decaying rate is slightly accelerated for both conformers. The S1 lifetimes of trans and cis forms of hydroquinone are measured to be more or less same, giving τ ∼ 2.8 ns at the zero-point level. When H2O is complexed with hydroquinone, the S1 decaying process is facilitated for both conformers, slightly more efficiently for the cis conformer.

High Accuracy Ultrafast Spatiotemporal Pump-Probe Measurement of Electrical Thermal Transport in Thin Film Gold.

A high resolution spatiotemporal ultrafast pump-probe system is developed to examine the interactions and transport phenomena between the electrical and the lattice thermal subsystems during ultrafast laser-matter interactions. This system incorporates an ultrafast pump-probe scheme with a stationary probe beam that interrogates the response to a spatial scanning pump beam, providing a full spatiotemporal mapping of a material's response due to an ultrafast pump excitation. The material's response, which is highly sensitive to its transport properties, is measured with a high spatial accuracy of up to ±10 nm and subpicosecond time resolution. Details of achieving this high spatial accuracy are described, and a study of the ultrafast transport processes in thin film gold is demonstrated. With the aid of transport and optical response models, the electrical thermal transport properties of gold and the electron-lattice coupling constant are simultaneously determined.

Exploring Ultrafast Photoswitching Pathways in RbMnFe Prussian Blue Analogue

AbstractWe study by femtosecond optical pump‐probe spectroscopy the photoinduced charge transfer (CT) in the RbMnFe Prussian blue analogue. Previous studies evidenced the local nature of the photoinduced MnIIIFeII → MnIIFeIII process, occurring within less than 1 ps. Here we show experimentally that two photoswitching pathways exist, depending on the excitation pump wavelength, which is confirmed by band structure calculations. Photoexcitation of α spins corresponds to the Mn(d‐d) band, which drives reverse Jahn–Teller distortion through the population of antibonding Mn‐N orbitals, and induces CT within ≈190 fs. The process launches coherent lattice torsion during the self‐trapping of the CT small‐polaron. Photoexcitation of β spins drives intervalence Fe→Mn CT towards non‐bonding states and results in a slower dynamic.

Exploring Ultrafast Photoswitching Pathways in RbMnFe Prussian Blue Analogue.

We study by femtosecond optical pump-probe spectroscopy the photoinduced charge transfer (CT) in the RbMnFe Prussian blue analogue. Previous studies evidenced the local nature of the photoinduced MnIII FeII → MnII FeIII process, occurring within less than 1 ps. Here we show experimentally that two photoswitching pathways exist, depending on the excitation pump wavelength, which is confirmed by band structure calculations. Photoexcitation of α spins corresponds to the Mn(d-d) band, which drives reverse Jahn-Teller distortion through the population of antibonding Mn-N orbitals, and induces CT within ≈190 fs. The process launches coherent lattice torsion during the self-trapping of the CT small-polaron. Photoexcitation of β spins drives intervalence Fe→Mn CT towards non-bonding states and results in a slower dynamic.

PL Spectra and Hot Charge Carrier Phenomena in an Undoped GaAs/AlGaAs Tunnel-Coupled Quantum Well

In this research, we experimentally investigated the photoluminescence (PL) spectra of an undoped GaAs/Al0.36Ga0.64As Tunnel-Coupled Quantum Well (TCQW) at 300 K and 77 K. At 300 K, PL spectra were broadened at various laser intensities due to the characteristic quantum-confinement electron levels in the active region of the TCQW during pumping excitation. At 77 K, at selected excitation intensities, the high-energy tails in the PL spectra of the studied structure corresponded with hot electron temperatures as functions of the energy of emitted photons. The average scattering energy rate of hot electrons in the system was also studied at a lattice temperature of 77 K. The average scattering energy rate of hot electrons obtained from this experimental study was in agreement with the calculated theoretical value.

Resonant Band-Edge Emissive States in Strongly Confined CsPbBr3 Perovskite Nanoplatelets

Strongly confined, fully inorganic cesium lead halide perovskite nanocrystals are of great promise for light-emitting devices in the blue spectral range owing to their high photoluminescence quantum yields. A combination of broadband fluorescence up-conversion and transient absorption spectroscopies was used to study early exciton dynamics in quasi-1D CsPbBr3 nanoplatelets (3 × 4 × 23 nm3, NPls). This allowed to reveal emitting band-edge states in the NPls that form instantaneously upon photoexcitation and then relax within the first picosecond to lower energy confined hole states (CHSs). The influence of the pump excitation intensity on the latter process was further scrutinized. The band-edge population lifetime was found to increase with the rise of the photon fluence due to CHS filling. When the concentration of NPls in solution becomes very high, nanoparticles overlap, resulting in a decrease in their external absorption cross-section and an increase in the emission Stokes shift due to photon reabsorption.

Porosity inspection in directed energy deposition additive manufacturing based on transient thermoreflectance measurement

In additive manufacturing, material melting-solidification often introduces defects like porosity, lack-of-fusion, delamination, and crack. Such defects are detrimental to the mechanical properties and quality of the manufactured component. In this study, a transient thermoreflectance (TTR) technique using a femtosecond laser is used for porosity inspection in directed energy deposition (DED) additive manufacturing. First, a femtosecond laser TTR measurement system is developed for noncontact measurement of thermoreflectance from a deposited layer. Subsequently, porosity is inspected by comparing the thermoreflectance measured at different pump excitation modulation frequencies. Due to this variation in modulation frequency, the developed porosity inspection technique is rendered sensitive to porosity rather than thermal property variation. Owing to the noncontact nature and scanning capability of the proposed TTR technique, it can be readily applied to in-situ porosity monitoring during DED additive manufacturing. Validation tests were performed on Ti–6Al–4V samples additively manufactured with different printing parameters. The results highlight the feasibility of the proposed technique for in-situ monitoring of porosity in DED additive manufacturing.

Enhanced directional quantum emission by tunable topological doubly resonant cavities.

How to utilize topological microcavities to control quantum emission is one of the ongoing research topics in the optical community. In this work, we investigate the emission of quantum emitters in a doubly-resonant topological Tamm microcavity, which can simultaneously achieve dual resonances at two arbitrary wavelengths according to the needs of practical application. To achieve the enhancement of quantum emission in such cavities, we have exploited the tunable doubly-resonant modes, in which one of resonant modes corresponds to the pump laser wavelength and the other one is located at the emission wavelength of quantum emitters. Both theoretical and experimental results demonstrate that the pump excitation and emission efficiencies of quantum emitters are greatly enhanced. The main physical mechanism can be explained by the doubly-resonant cavity temporal coupled-mode theory. Furthermore, we observe the faster emission rate and the higher efficiency of unidirectional quantum emission, which have promising applications in optical detection, sensing, filtering, and light-emitting devices.

Chemical defects control the exciton lifetime in CVD grown, few-layer MoTe2

The dominant exciton decay mechanisms of chemical vapor deposition (CVD) grown, few-layer MoTe2 are investigated by measuring and statistically analyzing its ultrafast, optical transient reflectivity dynamics under different pump excitation energies, fluences and temperatures. Defect interaction controls the ultrafast exciton decay pathways. Weak intralayer bonding in few-layer, CVD MoTe2 causes it to have a greater defect density than in other mechanically exfoliated 2D transition metal dichalcogenides. Direct charge carrier occupation of trap states is thought to cause an additional transient feature on the A′ exciton which is not present in the absorption spectrum. Additionally, long-term ambient air exposure does not significantly affect few-layer MoTe2 exciton decay time and absorption peak shape, increasing this material’s suitability for many technological applications.

Transient absorption with a femtosecond tunable excitation pump reveals the emission kinetics of color centers in amorphous silica.

We report a set of femtosecond (fs) transient absorption (TA) measurements following the dynamics of the so-called nonbridging oxygen hole center in silica, a model color center in wide bandgap amorphous solids, characterized by a very large Stokes shift between the UV excitation and its associated red emission at 1.9 eV. The changes in the TA spectrum were probed in the UV-visible range at various delays after photoexcitation and analyzed as a function of the UV excitation energy, in single-photon absorption conditions. The combination of the experiments helps to clarify the defect photocycle, highlighting how TA measurements with tunable UV excitation could represent a powerful tool to investigate the dynamics of color centers embedded in transparent materials.

Nd3+/Yb3+ co-doped mid-infrared luminescence fluorobromide glass with energy transfer and zero-thermal-quenching IR emission

Nd3+/Yb3+ co-doped fluorobromide glass samples were prepared by melt quenching. The mid-infrared (MIR) luminescence of the Nd3+/Yb3+ co-doped fluorobromide glass was investigated by Br-doping reduces the phonon state density of the matrix. The 3.9 μm MIR luminescence of the samples excited at 793 and 980 nm pump excitation was investigated in detail. There is an effective mutual energy transfer process between Nd3+ and Yb3+. It is proved under 793 nm excitation that the luminescence of Nd3+ at 3.9 μm is reduced by effective energy transfer from Nd3+:2H11/2 → Yb3+:2F5/2. At the same time, it is proved that the effective energy transfer from Yb3+:2F5/2 → Nd3+:2H11/2 under the excitation of 980 nm enhances the luminescence of Nd3+ at 3.9 μm. In addition, it is found that the samples still have good infrared (IR) luminescent properties when the temperature changes. The emission cross-sectional area and the absorption cross-sectional area are σem (3.87 × 10−20 cm2) and σabs (4.25 × 10−20 cm2). The fluorescence decay characteristics of the sample at 3.9 μm at the 2H11/2 level were investigated and the fluorescence lifetime was calculated. The gain performance of the sample was calculated and analyzed, which can reach 4.25 × 10−20 cm2. Those results prove that Nd3+/Yb3+ co-doped fluorobromide glass is the potential mid-infrared laser gain material.

Optical Gain in Semiconducting Polymer Nano and Mesoparticles.

The presence of excited-states and charge-separated species was identified through UV and visible laser pump and visible/near-infrared probe femtosecond transient absorption spectroscopy in spin coated films of poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) nanoparticles and mesoparticles. Optical gain in the mesoparticle films is observed after excitation at both 400 and 610 nm. In the mesoparticle film, charge generation after UV excitation appears after around 50 ps, but little is observed after visible pump excitation. In the nanoparticle film, as for a uniform film of the pure polymer, charge formation was efficiently induced by UV excitation pump, while excitation of the low energetic absorption states (at 610 nm) induces in the nanoparticle film a large optical gain region reducing the charge formation efficiency. It is proposed that the different intermolecular interactions and molecular order within the nanoparticles and mesoparticles are responsible for their markedly different photophysical behavior. These results therefore demonstrate the possibility of a hitherto unexplored route to stimulated emission in a conjugated polymer that has relatively undemanding film preparation requirements.

Elliptical micropillar cavity design for highly efficient polarized emission of single photons

We present a numerical investigation of the performance of the micropillar cavity single-photon source featuring an elliptical cross section. While ellipticity serves to establish polarization control of the emitted photons, it introduces a trade-off due to reduced coupling to a Gaussian profile in the far field and the need for significantly increased pumping power when a resonant excitation pumping scheme is implemented. Starting from a circular micropillar, we identify optimum geometrical parameters leading to a maximum polarized coupling efficiency of 0.90 to a Gaussian profile in the far field combined with a modest relative increase in the pump power of 16 dB.

Ultrafast intraband Auger process in self-doped colloidal quantum dots

Summary Investigating the separate dynamics of electrons and holes has been challenging, although it is critical for the fundamental understanding of semiconducting nanomaterials. n-Type self-doped colloidal quantum dots (CQDs) with excess electrons occupying the low-lying state in the conduction band (CB) have attracted a great deal of attention because of not only their potential applications to infrared optoelectronics but also their intrinsic system that offers a platform for investigating electron dynamics without elusive contributions from holes in the valence band. Here, we show an unprecedented ultrafast intraband Auger process, electron relaxation between spin-orbit coupling states, and exciton-to-ligand vibrational energy transfer process that all occur exclusively in the CB of the self-doped β-HgS CQDs. The electron dynamics obtained by femtosecond mid-infrared spectroscopy will pave the way for further understanding of the blinking phenomenon, disproportionate charging in light-emitting diodes, and hot electron dynamics in higher quantum states coupled to surface states of CQDs.