Pump-Probe Spectroscopy

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Pump-probe spectroscopy is a time-resolved technique used to study ultrafast processes in materials by using two laser pulses: a 'pump' pulse to excite the system and a 'probe' pulse to measure the resulting changes in the system's properties over time, allowing for the observation of dynamic phenomena at femtosecond timescales.

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Key research themes

1. How can ultrafast terahertz pump-probe spectroscopy characterize low-frequency vibrational and solvation dynamics in molecular condensed phases?

This research theme focuses on the development and application of femtosecond terahertz (far-infrared) pump-probe spectroscopy to directly probe low-frequency molecular motions, such as vibrational and solvent response dynamics, which play crucial roles in chemical and biological condensed phase processes. The methodology enables time-resolved measurement of transient vibrational spectra and relaxation mechanisms, offering insights into solvent rearrangement, charge transfer reactions, and phonon relaxation with ultrafast temporal resolution.

Femtosecond far-infrared pump-probe spectroscopy: a new tool for studying low-frequency vibrational dynamics in molecular condensed phases

by Klaas Wynne

2016

Key finding: The study presents a novel femtosecond terahertz pump-probe experimental setup using voltage-biased semiconductor wafers as intense terahertz pulse sources, enabling time-resolved probing of low-frequency solvent and lattice... Read more

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Ultrafast Dipole Solvation Measured in the Far Infrared

by Klaas Wynne

2016

Key finding: Using ultrafast visible/near-UV pump, terahertz probe spectroscopy, the authors reveal coherent beats and inhomogeneous solvent librational response arising from changes in dye molecular dipole moments, demonstrating a... Read more

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Study of Impulsive Stimulated Raman Scattering Effects Using the Femtosecond Pump–Probe Z-Scan Technique

by flaminia rondino

2023, Applied Sciences

Key finding: The research establishes that femtosecond pump-probe Z-scan methodology can effectively detect impulsive stimulated Raman scattering (ISRS) and Raman-induced Kerr nonlinearities in complex molecular systems, enabling... Read more

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Ultrabroadband Midinfrared Pump-Probe Spectroscopy Using Chirped-Pulse Up-conversion in Gases

by ㄊㄊ一

2016

Key finding: This paper demonstrates an ultrafast, ultrabroadband midinfrared pump-probe spectroscopy system using four-wave difference-frequency generation in gases combined with chirped-pulse up-conversion, achieving spectral coverage... Read more

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2. What are effective polarization-resolved pump-probe methods to measure anisotropic relaxation and excited state dynamics in biologically relevant molecules?

This theme investigates the use of polarization modulation in ultrafast pump-probe spectroscopy to resolve anisotropic relaxation and rotational diffusion processes in excited-state biologically relevant molecules such as NADH. Measuring absorption dichroism with improved signal-to-noise ratios under low-energy excitation conditions enables noninvasive probing of molecular orientation dynamics and local viscosity, holding significance for biochemical process understanding and intracellular environment characterization.

Anisotropic relaxation in NADH excited states studied by polarization-modulation pump–probe transient spectroscopy

by Yaroslav Beltukov

2021, Physical Chemistry Chemical Physics

Key finding: The study develops and applies a novel polarization-modulation pump-probe technique to measure excited state absorption dichroism of NADH in ethanol-water solutions with improved signal-to-noise ratio at low pump/probe... Read more

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3. How can multi-pulse pump–pump–probe and pump–repump–probe spectroscopy elucidate complex photoinduced charge transfer and vibrational relaxation pathways in molecular systems?

This theme explores advanced multi-pulse pump-probe variants such as pump–pump–probe and pump–repump–probe spectroscopy techniques to gain detailed insights into nonlinear excitation dynamics, charge accumulation processes in photocatalytic dyads, and vibrational overtone relaxations in hydrogen-bonded systems. These techniques enable selective excitation and temporal mapping of excited state populations and higher vibrational states, revealing pathways not visible in conventional pump-probe experiments and contributing to understanding energy transfer, catalysis, and anharmonic vibrational dynamics.

Highly Selective Relaxation of the OH Stretching Overtones in Isolated HDO Molecules Observed by Infrared Pump-Repump-Probe Spectroscopy

by Maximilian Bradler

2022, The journal of physical chemistry. A

Key finding: Utilizing a fifth-order infrared pump-repump-probe technique with independently tunable pump and repump pulses, the authors precisely measure the population lifetimes and relaxation pathways of OH stretching overtones in... Read more

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Photoinduced electron transfer in a molecular dyad by nanosecond pump—pump—probe spectroscopy

by Winfried Leibl

2024, Photochemical & Photobiological Sciences

Key finding: Nanosecond pump–pump–probe experiments on a porphyrin-ruthenium molecular dyad reveal mechanism and kinetics of successive photoinduced charge transfers leading to charge accumulation on the catalyst unit, critical for... Read more

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Remote Non-Invasive Fabry-Pérot Cavity Spectroscopy for Label-Free Sensing

by Almut Beige

2024, Sensors

Key finding: Although focused on sensing applications, this work demonstrates an innovative remote pump–probe concept using Fabry-Pérot cavities, where molecule-induced reflection back into the cavity modulates the interference conditions... Read more

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4. How can quantitative dielectric properties and electronic dynamics be extracted from conventional pump-probe spectroscopy to enhance interpretation of transient excited state phenomena?

This theme addresses the methodological challenges in interpreting optical pump-probe spectra obtained from differential transmission or reflection measurements, where combined contributions can obscure assigning spectral features. It focuses on novel model-independent analysis frameworks that transform raw pump-probe data into complex dielectric functions, enabling reliable quantification of underlying transient electronic states and disentanglement of absorption and refractive index changes, thus supporting quantitative photophysical insight into diverse materials including semiconductors and biological systems.

Extracting Quantitative Dielectric Properties from Pump-Probe Spectroscopy

by Aswathy V Girija

2021

Key finding: The authors develop a model-independent method to convert broadband differential transmission or reflection pump-probe spectra into transient changes in the complex dielectric function, enabling separation of absorptive and... Read more

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All papers in Pump-Probe Spectroscopy

Ultrafast Energy Transfer Between Molecular Assemblies and Surface Plasmons in Hybrid Nano-Materials in the Strong Coupling Regime

by Adi Salomon

2024, arXiv (Cornell University)

The nonlinear optical dynamics of nano-materials comprised of plasmons interacting with quantum emitters is investigated by a self-consistent model based on the coupled Maxwell-Liouville-von Neumann equations. It is shown that ultra-short... more

Figure 4b presents results of simulations at higher molecular density, clearly showing a collective molecular-plasmon mode” at 1.61 eV. This energy corresponds to a maximum in T and a minimum in R. Note that absorption now has two peaks, which match the positions of the minima of T and maxima of R. FIG. 4. Extinction spectra of the hybrid material depicted in the inset of panel (a). Both panels show absorption, A, (blue circles), transmission, 7, (green squares), and reflection, R, (red diamonds) as functions of the incident photon energy. Individual molecules are considered as two-level emitters with a doubly degenerate excited state, as shown in the inset of panel (b). Panel (a) shows spectra at a low molecular number density of 3x10” m™. Panel (b) shows the results of simulations at a higher density of 2.5x10"° m®. The slit array period is 410 nm. The other parameters are as in Fig. 2.

FIG. 1. A high intensity pump (blue) interacts with the system under consideration (see text). At time At after the start of the pump, a low intensity short probe (red) is applied, and its transmission (or absorption) is monitored The key challenge in modeling nonlinear dynamics using a pump-probe pulse sequence s to disentangle signals caused by the strong pump from observations by the weak probe. When a system comprised of optically coupled emitters is excited by a strong pulse, it *xhibits polarization oscillations lasting long after the pump is gone. Consequently, when he system is probed by a low intensity pulse, one observes an undesired high intensity signal at the pump frequency caused by induced polarization oscillations. | such oscillations may be filtered out so as not to interfere with the probe Experimentally, in the far field. n simulations, these unwanted oscillations must be handled carefully because they may nterfere with the signal produced by the probe.

FIG. 2. One-photon absorption for the 10 nm thick molecular layer depicted in the left inset as a function of the inci- dent photon energy calculated at three molecular number densities: blue circles corresponding to 10” m™°, green squares for 5x107 m™°, and red diamonds for 2.5x107° m*. The molecules are modeled as two-level emitters (right inset). The vertical dashed line shows the molecular transition energy of 1.61 eV. In these simulations the molecular transition dipole is 10 Debye, the radiationless lifetime of the excited state |2> is 1 ps, and the pure dephasing time is 100 fs.

FIG. 3. Transient absorption spectra of the pure molecular system as a function of the pump-probe delay (horizontal axis) and the incident photon energy (vertical axis) evaluated for 180 fs pump pulses. The horizontal dashed black line in each panel is at the molecular transition energy, 1.61 eV. Panel (a) shows the spectrum for a m-pulse (Ey = 2.075x10° V/m), panel (b) shows the data for a 2n-pulse (Ey = 4.150x10° V/m), panel (c) shows results for a 32-pulse (Eo = 6.225x10° V/m), and panel (d) is for a 4n-pulse (Ey = 8.300x10° V/m). In all cases the pump frequency is reso- nant with the molecular transition energy. Other parameters are the same as in Fig. 2. A pump-probe delay of 0 fs cor- responds to the probe applied at a time when the pump is still off (i.e. at the beginning of the leading edge of the pump; see Fig. 1 for details).

FIG. 5 (Color online) Transient spectroscopy calculations for a 15 fs pump pulse with a peak amplitude of 4x10° V/m centered at the molecular transition energy of 1.61 eV. Panel (a) shows the change in reflection AR (see Eq. (9) ) as a function of the pump-probe delay and incident photon energy . Panel (b) depicts one-dimensional cuts of AR at three

To demonstrate that the observed changes are caused by energy transfer between the upper/lower polaritons and the molecular layer, we plot AR in Fig. 5b at three energies as a function of the pump-probe delay. The period of oscillations is 3.75 fs. Interestingly, simulations performed at higher molecular concentrations or different array spacings show that these factors have a very minimal effect on the period of oscillations, which is fully determined by the local electric field amplitude and the strength of the molecular dipole. We also performed calculations using off-resonant pump excitation, as in Ref. [28]. The data obtained for the off-resonant case shows the same behavior — ultra-fast en- ergy oscillations between the upper/lower polaritons and the molecules. FIG. 6 (Color online) The change in reflection AR as a function of the pump-probe delay at the upper polariton energy for two pump amplitudes: red squares are for 2x10° V/m (peak amplitude) showing an oscillation period of 6 fs, and blue circles are for 4x10° V/m with a period of 3 fs. The pump is resonant with the molecular transition energy (1.61 eV), and its duration is 30 fs. The other parameters are as in the previous figures.

FIG. 7. Nonlinear dynamics during a 180 fs pump. The blue solid curve shows AR as a function of the pump-probe delay at the upper polariton energy. The dashed green curve presents the ensemble-averaged ground state population of the molecular system. (The population p;; is multiplied by 40 for clarity). Vertical red arrows indicate direct corre- spondence between oscillations in reflection and the ground state population. The pump amplitude is 4.3x10° V/m. The incident frequency is at the molecular transition energy of 1.61 eV. The other parameters are as in Fig. 2. awa For example, for a 30 fs pump with an amplitude of 2x10’ V/m and the molecular param- eters in Fig. 2, we have S = 1.74xn. Consequently, a two-level emitter exposed to such an excitation undergoes less than one Rabi cycle. (The population remaining in the ground state is 0.74 after the pump; i.e. the emitter is excited and then partially de-excited). Sim- nle calculations nerformed for 9 sinsole two-level emitter acree nerfectly with (11). If we now apply this simple model to estimate how many Rabi cycles the hybrid mate- rial undergoes, we find that the number of cycles predicted by (11) is always smaller than the actual number computed from the complete set of Maxwell-Liouville-von Neumann equations. The only parameter that is different in the complete model is the local electric

FIG. 9. Effect of the laser pulse shape on the change in reflectivity. Panel (a) illustrates the abrupt cutoff of the pump pulse and application of the probe pulse at time tcutog, Which is used as a control parameter. Panels (b)-(d) show AR as a function of the pump-probe delay at different photon energies. Black solid curves show results for a non-truncated pump, red dashed curves show data for fcutorr = 6.75 fs, blue solid curves with crosses are for teutore = 7.75 fs, and green dash-dotted lines are for fcutore = 8.75 fs. Panel (b) shows AR at the lower polariton energy of 1.59 eV, panel (c) shows AR at the molecular transition energy of 1.61 eV, and panel (d) presents AR at the upper polariton energy of 1.67 eV. Simulations are performed for a slit array with a period of 370 nm. The total pump pulse duration is 15 fs, the peak

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Chasing Vibro-Polariton Fingerprints in Infrared and Raman Spectra Using Surface Lattice Resonances on Extended Metasurfaces

by Francesco Verdelli

2023, Journal of Physical Chemistry C

DOI to the publisher's website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page... more

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Liquid-phase exfoliation of flaky graphite

by Екатерина Образцова

2023, Journal of Nanophotonics

The majority of currently available methods of graphene production have certain drawbacks limiting its scaling. Unlike the others, liquid-phase exfoliation of graphite is a promising technique for high-yield graphene production. In this... more

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Trion-Induced Negative Photoconductivity in MonolayerMoS2

by victor pilon

2023, Physical Review Letters

Optical excitation typically enhances electrical conduction and low-frequency radiation absorption in semiconductors. We have, however, observed a pronounced transient decrease of conductivity in doped monolayer molybdenum disulfide (MoS... more

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Charge Photogeneration in Few-Layer MoS2

by Nataša Vujičić

2022, Advanced Functional Materials

Figure 2. (a) cw photoinduced absorption spectrum of MoS2 in PMMA at room temperature for excitation at 3.1 eV. In-phase (blue) and quadrature (green) signal components are shown for a modulation frequency of 245 Hz. (b) Five Gaussian fits (red) whose sum (blue) fits the normalized in-phase spectrum (open squares).

Figure 1. (a) Absorption spectrum of the sample of MoS2 in PMMA. Inset shows a photograph (the dark area has a diameter of approximately 7-8 mm). (b) Raman spectra for two excitation wavelengths at 633 nm (red line) and 488 nm (blue line).

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Ultrafast Energy Transfer between Molecular Assemblies and Surface Plasmons in the Strong Coupling Regime

by Adi Salomon

2022, ACS Nano

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Field-induced charge separation dynamics in monolayer MoS2

by Dumitru Dumcenco

2022, 2D Materials

The absorption spectra of 2D semiconductors are dominated by excitons with binding energy of several hundreds of meV. Nevertheless, even single layers show an appreciable photovoltaic effect and work as the active material in high... more

the device. Scale bar is 10 um. (Cc) Raman modes of the single layer MoS2. (d) Transfer

contributions from injected charges. peaks and a generic exponential function to account for the background from various scattering

Figure 3. (a) Schematic illustration of the pump-probe microscope setup. (b) The measured fluence is kept much lower than the pump fluence field.!’ In order to keep re-excitation or de-excitation by the probe at negligible levels, the probe

Figure 4. (a) Spectral contributions of primary (top panel) and secondary (bottom panel) transitions and the PA feature below 1.9 eV due to free carriers. !*-!9:?!

evolution of the absorption feature, observed in the A7T/T signal, averaged over the energy interval

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On-Demand Coupling of Electrically Generated Excitons with Surface Plasmons via Voltage-Controlled Emission Zone Position

by Florentina Gannott

2022, ACS Photonics

The ability to confine and manipulate light below the diffraction limit is a major goal of future multifunctional optoelectronic/plasmonic systems. Here, we demonstrate the design and realization of a tunable and localized electrical... more

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Photo-Induced Bandgap Renormalization Governs the Ultrafast Response of Single-Layer MoS 2

by Eva Pogna

2022, ACS Nano

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Observation of rapid exciton-exciton annihilation in monolayer molybdenum disulfide

by Avetik Harutyunyan

2022, Nano letters

Monolayer MoS2 is a direct-gap two-dimensional semiconductor that exhibits strong electron-hole interactions, leading to the formation of stable excitons and trions. Here we report the existence of efficient exciton-exciton annihilation,... more

KEYWORDS: MoS,, transient absorption spectroscopy, exciton dynamics, exciton—exciton annihilation mechanism operative in highly excited semiconductors with free charge carriers. Exciton—exciton annihilation and Auger recombination are particularly significant in systems of reduced dimensionality, as observed in quantum wells,'?~*? quantum dots,” and carbon nanotubes.”* *° Exciton—exciton annihila- tion is expected to be efficient in an atomically thin 2D material like single-layer MoS, with strong many-body interactions.”” Mores of transition metal dichalcogenides (TMDs), including MoS), MoSe,, WS,, and WSe), have recently attracted much interest as a new class of atomically thin materials.'"* Like graphene, these materials are environ- mentally stable and can be prepared both by exfoliation of bulk crystals and by chemical synthesis. In contrast to graphene, however, these two-dimensional (2D) materials exhibit a significant band gap. In addition, in the single-layer limit the crystals are direct-gap semiconductors and support efficient light emission.*° These materials are also of particular interest because they display distinctive valley-selective optical proper- ties.”’~'' A consequence of the quantum confinement and reduced dielectric screening in atomically thin 2D materials is the role of strong Coulomb interactions between carriers.'*~'° This leads to tightly bound excitons upon photoexcitation of the material. Indeed, the strength of the Coulomb interaction in these materials is sufficient to support charged excitons (trions), even at room temperature, in doped material.'”"*

Figure 1. (a) Photoluminescence spectrum of monolayer MoS, at room temperature under cw laser excitation at a wavelength of 532 nm. The sideband of photoluminescence located at lower energy side is due to the trion emission. Hot photoluminescence from the higher-lying B exciton transition can be seen at around 2.0 eV. The optical image in the inset shows a monolayer sample exfoliated on quartz substrate. The scale bar is $ pm. (b) Transient reflectivity spectrum measured at a time delay of 0.2 ps for monolayer sample. For comparison, the absorption spectrum is shown for the unpumped sample. The dotted line represents the contribution to the absorption spectrum from the A exciton based on a fit to two Lorentzian peaks for the A and B excitons and a linear background.

Figure 2. (a) Transient reflectivity of monolayer MoS, at room temperature measured at two pump fluences of 22 and 3 yJ/cm” with a time scale of 600 ps. (b) Exciton dynamics at different pump fluences with a time scale of 45 ps. The right axis corresponds to photogenerated excitons. The solid lines represent a global fit to the exciton—exciton annihilation model as seen in eq 3, by convoluting with experimental instrumental response function. Figure 2a shows the transient absorption of monolayer MoS, for applied pump fluences of 22 and 3 yJ/cm*. The probe We present transient reflection/absorption data obtained for a probe photon energy of 1.88 eV, corresponding to measurements at the peak of the A exciton absorption. On the basis of the discussion above, we interpret the change in absorption as proportional to the density of A excitons in the MoS, monolayer. To gauge the pump-induced perturbation of the material, let us consider the conditions of Figure 1b with an applied pump fluence of 22 J/cm’. Taking into account the change in the applied field arising from the reflection from the substrate and the absorption of the MoS, monolayer at the pump wavelength (2.7%), this excitation corresponds to an absorbed fluence of 0.39 yJ/cm?. Assuming that each absorbed photon generates one A exciton, the resulting initial exciton density is 1.1 xX 10 cm™, that is, an average separation between excitons of 9.5 nm.

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Effects of neighboring transitions on the mechanisms of electromagnetically induced absorption and transparency in an open degenerate multilevel system

by Zeeshan Ali Safdar Jadoon

2022, Scientific Reports

In this study, optical Bloch equations with and without neighboring hyperfine states near the degenerate two-level system (DTLS) in the challenging case of 85Rb D2 transition, which involves the Doppler broadening effect, are solved. The... more

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Ultrafast Carrier Dynamics in Few Layer Colloidal Molybdenum Disulfide Probed by Broadband Transient Absorption Spectroscopy

by Pieter Geiregat

2021, The Journal of Physical Chemistry C

This research project has been funded by FWO-Vlaanderen.

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Strongly Coupled Coherent Phonons in Single-Layer MoS2

by Junjia Wang

2021, ACS Nano

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Giant Linear and Nonlinear Excitonic Responses in an Atomically Thin Indirect Semiconductor Nitrogen Phosphide

by Rekha Verma

2021, The Journal of Physical Chemistry C

Large linear and nonlinear optical responses can be obtained from two-dimensional semiconductors with an indirect electronic gap. Here, we demonstrate exceptionally large exciton-driven responses such as the fundamental exciton binding... more

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Transient absorption of transition metal dichalcogenide monolayers studied by a photodope-pump-probe technique

by Peymon Zereshki

2021, Physical Review B

We report three-pulse photodope-pump-probe measurements on photocarrier dynamics in semiconducting transition metal dichalcogenide monolayers of MoS 2 , WS 2 , MoSe 2 , and WSe 2. The samples are fabricated by metal-organic chemical vapor... more

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Field-induced charge separation dynamics in monolayer MoS2

by Eva Pogna

2021, 2D Materials

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Photoconductivity of solution-processed MoS2 films

by John Donegan

2021, Journal of Materials Chemistry C

Solution-exfoliated MoS 2 nano-platelets were formed into thin films by deposition onto a water surface followed by transfer to indium tin oxide coated glass. After drying, a gold electrode was evaporated on top to give a sandwich... more

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Active Control of Surface Plasmon–Emitter Strong Coupling

by Antti Moilanen

2021, ACS Photonics

Strong coupling between quantum emitters and surface plasmon polariton modes in metal nanostructures has been extensively studied in recent years. A natural direction of research and a prerequisite for many applications is the possibility... more

Figure 1: Spectral evolution of the photochromic molecules upon reversible switching. The photochromic molecule is indolinospiropyran (1’,3’-dihydro-1’,3’,3’- trimethyl-6-nitrospiro |2H-1-benzopyran-2,2’-(2H)-indole]). In its relaxed closed form, spiropyran (SP) is transparent in the visible region of the spectrum, whereas the open form merocyanine (MC) absorbs strongly at around 2.2 eV (560 nm). The switching from the SP to the MC form is perfofmed by UV light, and reversed by visible (green) light illumi- nation. (a) Absorption and (b) transmission spectra of the two forms of the photochromic molecules: spiropyran (dashed line) and merocyanine (solid line). (c) Spectral evolution of the transmission spectrum upon subsequent UV and visible light illumination. Reprinted with permission from Ref. 13. Copyright (2011) by the American Physical Society.

Figure 2: Dispersion of the surface-plasmon modes in a hole array before and after photoswitching. The photochromic molecules are in (a) spiropyran and in (b) merocyanine form, revealing splitting of 650 meV between the lower and upper dispersion branch after photoswitching. The vertical dashed line in (b) shows the exciton energy, whereas the curved dashed line in (b) indicates the parabolic surface-plasmon mode dispersion from (a). Reprinted with permission from Ref. 13. Copyright (2011) by the American Physical Society.

Figure 3: Photochromic switching of strong coupling in a system supporting hybrid-plasmon waveguide modes (HPWMs). (a) Dispersion of the HPWMs in a nanodisk array upon photoswitching. The green markers indicate the energies of the bare HPWMs, and the red and blue markers show the resulting energies after photoswitching. The solid lines are obtained by fitting a two-coupled-oscillators model into the measured data. The largest reversible splitting observed was Q. = 572 meV. The horizontal dashed line indicates the molecular transition energy for the merocyanine form. (b) Transmission spectrum of the system in the initial state ("As-prepared"), after UV illumination, and after green light illumination. Reversibility of the Rabi splitting is clearly visible upon photo- switching. Reprinted with permission from Ref. 14. Copyright (2016) by the American Chemical Society.

Figure 5: Polarization-dependent control of the coupling on a single plasmonic dimer structure accompanied with J-aggregates. (a) Measured and (b) computed (FDTD) scattering spectra for the system with different polarization angles. Splitting tc lower (LB) and upper (UB) polariton bands becomes apparent as the polarization angle decreases (polarization changes from transverse to longitudinal). Also red shift of UB peak is visible upon changing the polarization. (c) Near-field profiles for the dimer with different polarization directions (white arrows). (d) Spectral position of the UB peak as a function of the polarization angle. (e) Scattering intensity of the LB peak as a function of polarization angle. The blue solid line in both (d) and (e) shows the computed polarization-dependence of the nearfield enhancement in the gap. Adapted from Ref. 26. Copyright (2013) by the American Chemical Society.

Figure 6 : Dispersion of the plasmon resonance modes in a hole array covered with J-aggregates. Calculated reflection spectrum under TE-polarized exciting field for (a) bare under T] array and (b) array covered with J-aggregates. (c) Measured reflection spectrum -illumination for the array covered with J-aggregates. Splitting of the dispersion into lower (LBP) and upper polariton band (UBP) is observed. (d)—(f) show the respective results for TM polarization. Adapted from Ref. 27. Copyright (2017) by the Optical Society of America.

Figure 7: Absorption spectra of the silver nanodisk array — MoS, monolayer hybrid system. (a) Absorption spectra of the nanodisk array fabricated on the MoS, monolayer as a function of pump power. The absorption spectra shows red shift of the peak from 720 nm to 695 nm, when the pump power is increased from 0 to 4 mW. Also the overall absorption intensity shows decrease upon increasing the pump power. (b) Absorption spectra of the nanodisk array remains unaltered upon increasing the pump power. The radii of the disks was 60 nm. Silicon dioxide (SiO) and silver films are used as the base substrate. Adapted from Ref. 33. Copyright (2016) by Wiley-VCH Verlag GmbH & Co. KGaA. switching between weak and strong coupling by simply changing the polarization of the

Figure 8: Electrical control of coupling in a silver nanodisk array on a MoS» monolayer. (a) Schematic of the system: MoS, monolayer and silver nanodisk array fabri- cated in a field-effect transistor (F] ET) configuration which allows for injecting and ejecting charge carriers to the monolayer by applying voltage on the electrical contacts. (b) Coupling strength (obtained by coupled oscillator model fitting) for excitons (A°) and trions (A7—) as a function of gate voltage. Adapted from Ref. 38. Copyright (2017) by the American Chemical Society.

Figure 9: Thermal control of the interaction between the WS, monolayer and a single gold nanorod Photoluminescence spectrum of (a) WS2 monolayer and (b) individual gold nanorod on a SiQg substrate for temperatures between 293-433 K. (c) Scattering spectra of a WS. monolayer — gold nanorod hybrid system. The exciton energy shows redshift (the gray arrow) upon increasing temperature which can be used for tuning the exciton and the nanorod energies in and out of resonance. At room temperature a Rabi splitting of about 110 meV is observed. Adapted from Ref. 41. Copyright (2017) by the American Chemical Society.

Figure 10: Chemical control of strong coupling between organic dye molecules and surface plasmon polaritons on gold thin film. Dispersion relations for different NO, gas concentrations: (a) 0 parts per million (ppm), (b) 0.3 ppm, (c) 4.6 ppm, and (d) 6 ppm. Upon increasing the concentration, Rabi splitting up to about 130 meV becomes apparent. Adapted from Ref. 62. Copyright (2011) by the American Chemical Society.

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Manipulation of exciton and trion quasiparticles in monolayer WS2 via charge transfer

by Sandwip Dey

2021, Applied Physics Letters

Charge doping in transition metal dichalcogenide is currently a subject of high importance for future electronic and optoelectronic applications. Here we demonstrate chemical doping in CVD grown monolayer (1L) of WS2 by a few commonly... more

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Imaging spin dynamics in monolayer WS2 by time-resolved Kerr rotation microscopy

by iwan martin

2021, 2D Materials

Monolayer transition metal dichalcogenides (TMD) have immense potential for future spintronic and valleytronic applications due to their 2D nature and long spin/valley lifetimes. We investigate the origin of these long-lived states in... more

Figure 1. TRKR on high quality CVD WS, monolayers. (a) (left) The atomic structure of WS); tungsten is the central blue atom, and the sulfur atoms are yellow. (Right) The schematic band structure of monolayer WS, at the K and K’ points. The spin-valley coupling allows one to selectively excite spins in either the K or K’ valley. (b) Optical micrograph of one of the triangular islands usec in this study. (c) Photoluminescence spectroscopy of five monolayer WS; flakes measured at 8 K are shown in red. In each spectra, the neutral exciton, X®, the trion, X~, and the defect, Lp, are labeled. The spectra are fit in the black curve, with the individual peak fits shown in green (neutral exciton), blue (trion), and purple (defect). (d) Diagram of the TRKR microscopy set-up. (e) Representative Kerr rotation as a function of pump-probe time delay for monolayer WS, (flake 5) at 8 K and zero magnetic field. The excitation and detection wavelength was 625 nm. The red curve is a bi-exponential fit yielding time constants of 320 ps and 5.4 ns. Inset: Kerr rotation at short time delays. An exponential fit to the fast decay (green curve) yields a time constant of 3.0 ps.

Figure 2. Spatially-resolved images of TRKR. Scanning a WS, island beneath the overlapped pump and probe beams at a fixed time delay produces a high-resolution spatial map of spin density. The series of images are snapshots of the spin density at time delays of 80, 250, 600, 2000, 4000, and 11 000 ps. The images reveal a complex spatial dependence and time evolution of the spins, with regions of high and low spin density in close proximity.

Figure 3. Kerr rotation and spectrally resolved photoluminescence of WS, flakes. (a) TRKR maps for each of the five different flakes used in this study are shown in row (a). The maps for flakes 1—4 are ata pump-probe time delay of 100 ps. The map for flake 5 is ata pump-probe time delay of 80 ps. (b) Maps of the full spectrum integrated PL for each of the five flakes are shown in row (b). At each point on the flake, the PL spectra was integrated over the entire emission range. (c) Maps of the exciton peak amplitude from fitting the PL spectrum are shown in row (c) for each flake. When compared with the TRKR maps, it is clear that the exciton PL is correlated with the TRKR signal. (d) Maps of the trion peak amplitude from fiting the PL spectrum are shown in row (d). For flake 4, there was no resolvable trion peak. (e) Maps of the defect peak amplitude from fitting the PL spectrum are shown in row (e). For flake 5, the defect peak could not be fit with a single Lorentzian. All data was taken at 8 K.

Figure 4. Bright and dark trion states in WS. (a) A singlet trion optically excited in the K valley. The electron in the lower valence band is due to n-type doping of the material. (b) The singlet trion can become a dark trion by momentum scattering of the spin up electron in the K valley to the spin up band of the K’ valley. (c) Another possibility for the singlet trion to convert to a dark trion is though a spin flip to the lower conduction band in the K valley. (d) A triplet trion. The electron and hole are optically excited in the K valley, and the electron in the K’ valley is due to n-type doping. (e) The triplet trion can convert to a dark trion through momentum scattering of the electron in the K valley to the lower conduction band of the K’ valley. (f) Another possibility for the triplet trion to convert to a dark trion is through a spin flip from the upper conduction band of the K valley to the lower conduction band of the K valley.

Figure 5. Spin—orbit stabilized spins in WS». (a) Normalized Kerr rotation as a function of time delay for different Bex. The non-precessing decay curves are characteristic of a spin—orbit-stabilized system. (b) Spin lifetime as a function of B.., obtained by fitting the TRKR data in (a). The spin lifetime is robust to an external magnetic field up to 700 mT. (c) Detailed time delay scans of Kerr rotation for different B..:. Curves are zoomed in and offset to show a small oscillatory component of the overall TRKR signal (<3%). (d) and (e) Larmor precession frequency and dephasing rate, respectively, of the oscillatory component. Both exhibit a linear dependence on B,x,, which indicate spin dynamics like those of materials without spin-stabilization due to strong spin-orbit coupling.

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Multiphoton nonlinear frequency mixing effects on the coherent electromagnetically induced absorption spectra of 85 Rb atoms under a longitudinal magnetic field: Theory and experiment

by Zeeshan Ali Safdar Jadoon

2020, Physical Review A (PRA)

Multiphoton nonlinear frequency mixing effects on coherent electromagnetically induced absorption spectra of 85 Rb atoms using two orthogonal linear polarizations of strong-coupling and weak probe beams are investigated theoretically and... more

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Contrasting hydration dynamics in DME and DMSO aqueous solutions: A combined optical pump-probe and GHz-THz dielectric relaxation investigation

by Debasish Das Mahanta and

2020, Journal of Molecular Liquids

Addition of co-solvents could introduce perturbation in the H-bond dynamics of water and the extent of perturbation is mostly dependent on the H-bond formation affinity of the respective co-solvent. In this contribution we have studied... more

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Femtosecond spectroscopy on MoS 2 flakes from liquid exfoliation: surfactant independent exciton dynamics

by victor vm

2017

Ionic surfactants, which are widely used to stabilize nanomaterials in dispersions, can drastically alter the nanomaterial’s photophysical properties. Here, we use femtosecond optical spectroscopy to study the dynamics of excitons and... more

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Exciton and charge carrier dynamics in few-layer WS 2

by victor vm and

2017

Semiconducting transition metal dichalcogenides (TMDs) have been applied as the active layer in photodetectors and solar cells, displaying substantial charge photogeneration yields. However, their large exciton binding energy, which... more

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TERS at work: 2D materials, from graphene to 2D semiconductors

by Damien ESCHIMESE

2017

We report results of TERS characterization of graphene oxide and the 2D semiconductors, MoS2 and WS2. The gap mode TERS signal of these 2D materials becomes dramatically enhanced over wrinkles and creases, as well as over nanopatterns... more

This approach was applied for TERS characterization of monolayer flakes of WS, deposited on gold. After transfer t
gold substrate, some wrinkles appeared in WS flakes, which provided us with a possibility to compare the behavior
the wrinkles in TMDCs and graphene oxide. In order to make the comparison complete, we also imprinted a line
pattern in the flake in the same way as it was done in case of patterning of graphene oxide.

Results of nanoscale AFM-Raman characterization of patterned and wrinkled flake of WS» are presented in Fig.6. |
expected, both the folds in WS, flake and the circular pattern imprinted demonstrated showed a significantly increas
photoluminescence and, to some extent, TERS signal, though in this specific case the TERS lines were not very stror
Also, the photoluminescence signal from the pattern was not as strong as of the wrinkles, and not as uniform as t
response from the patterns imprinted in graphene oxide. Nonetheless, this set of experiments confirmed earl:
observations that wrinkles and artificial patterns in 2D materials may possess a number of interesting properties differs
from the adjacent flat material.
Fig.6. a)-Topography image of a portion of WS; flake featuring few folds and a circular pattern imprinted with a diamond probe; b)-
Corresponding TEPL/TERS map; c)- typical spectra taken from the flat portion of the flake (blue spectrum), imprinted pattern (red)
and the wrinkle ( green).

Fig.1 Graphene oxide -CNT sample deposited on gold. Combined TERS map (D band-green, G band-blue, 2D band-red), left;
Current map recorded concurrently with the TERS map (right) and the two maps overlaid showing clear increase of conductivity over
the wrinkles.
of a dielectric graphene oxide should only increase. The sample presented in Fig.1, had both the graphene oxide flakes
and carbon nanotubes (CNTs) co-deposited on the same gold coated glass substrate. A small bias of 50mV was used for
the conductivity measurements that were carried out during the contact mode portion of the TERS mapping of the
sample, which was concurrent with the collection of Raman data. Our original assumption that increased conductivity in

graphene oxide flakes was due to the underlying CNTs, was discarded based on the analysis of Raman spectra of
features that demonstrated increased conductivity.

Wrinkles in graphene oxide sheets could be clearly differentiated from carbon nanotubes based on their nanoscale
Raman spectra as shown in Fig.2: Raman spectra from CNTs had both an intense 2D band and a narrow G band.
Wrinkles in graphene oxide, on the contrary, did not show significant 2D band and their G peak was broader and blue
shifted (higher energy) compared to the G peak of the CNTs.
Fig.2 GO-CNT sample deposited on gold, same flake as in Fig.1. Combined TERS map and characteristic spectra showing clear
differentiation between the Raman spectra of CNTs (intense 2D band, narrow red-shifted G band) and GO (broad blue-shifted G band,
no significant 2D band intensity).

Fig.3 Contact potential difference (surface potential) image of the graphene oxide flakes on gold taken in the dark and
under illumination with 638 nm laser. Corresponding TERS map (intensity of the D band) and typical spectra from the
flake (red) and adjacent location on gold substrate.
This interesting behavior of the wrinkles in graphene oxide sheets led us to to investigate whether other electric
properties of graphene oxide, such as its surface potential, change as a result of laser illumination. Since in the course of
TERS measurements the focused laser beam and the apex of the TERS probe are colocalized with about 50 nm
precision, a direct comparison of the surface potential in the dark and under illumination was fairly straightforward. As
expected, the surface potential of graphene oxide flakes did change under illumination with red laser (50 wW, 638 nm
which corresponds to approximately 10*W/cm’ power density). As can be seen from Fig.3, the value of the surface
potential of graphene oxide flakes changes by about 100mV when illuminated.

Fig.4. a)-TERS map (D-band) of a linear and 2D patterns imprinted into a single layer flake of graphene oxide on gold; b)-
corresponding topography image with natural edges highlighted with white dotted line; c)-typical TERS spectra from the 2D
pattern (green), natural edge (red) and flat area (blue). Scale bar in a) and b) is 200nm.

Fig.5. a)-TERS map of a few-layer thick flake of MoS2 exfoliated on gold, 408 cm” peak ( green) and 465 cm’ ( blue), scale bar is
500nm; b)- representative TERS spectra from a small flake (red) and from inner area of a large flake; c-corresponding topography
image; d) section analysis showing that the thinner part of the flake was 3 layer thick.

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Controlling Plasmon-Induced Resonance Energy Transfer and Hot Electron Injection Processes in Metal@TiO 2 Core–Shell Nanoparticles

by Brandon Yost and

2017, The Journal of Physical Chemistry C

Plasmonic metals can excite charge carriers in semiconductors through plasmon-induced resonance energy transfer (PIRET) and hot electron injection processes. Transient absorption spectroscopy reveals that the presence of plasmon-induced... more

@ INTRODUCTION In the case of hot electron injection’ ~° or PIRET,”* ° the plasmon can interact with the semiconductor in the near-field, improving both the above-band-edge and the below-band-edge photoconversion. That is, the plasmon can induce charge separation in the semiconductor where the plasmonic metal spectrally absorbs the incident light, not just where the semiconductor can absorb light. Great progress has been made in theoretically and experimentally understanding how hot electron transfer and PIRET can independently enhance photoconversion near/below the band edge.'4-78 However, it still remains unknown how these two enhancement mecha- nisms can coexist or compete and what parameters control their presence. Without this knowledge, plasmonic metal—semi- conductor heterojunction design will remain limited to optimizing spectral properties of metals instead of applying the best enhancement mechanism for a given semiconductor’s weaknesses. ‘Department of Physics and Astronomy and “Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, West Virginia 26506, United States When light is incident on a metal nanoparticle with frequency matching the plasmon resonance, the free conduction electrons oscillate collectively, leading to an intense local electromagnetic (EM) field. The energy stored in the local EM field can be reradiated as scatter, or the oscillations can dephase into a hot electron distribution, leading to light absorption. The spectral position of the localized surface plasmon resonance (LSPR) and the balance of scattering versus absorption can be tuned by adjusting the nanoparticle size, shape, or by selection of material.'"* Owing to these unique properties, plasmonic metals are capable of enhancing photoconversion in metal— semiconductor heterojunction-based photovoltaics and photo- catalysts.*~* In metal—semiconductor heterojunctions, the plasmon is able to induce charge separation in the semi- conductor via three mechanisms:* (i) photonic enhance- ment,’~'° (ii) direct electron transfer (DET) also known as hot electron injection, ~~ and plasmon-induced resonance energy transfer (PIRET).”’~*8

Figure la and Figure S1 show a schematic of the samples and the TEM images, respectively. As seen in the extinction spectra Figure 1. Core@shell metal nanoparticles for controlling plasmonic enhancement. (a) Schematic of metal core@TiO, shell nanoparticles. (b) Extinction of each metal core@TiO, shell nanoparticle as well as TiO, alone. The cutoff region for significant spectral overlap is marked. The existence or lack of spectral overlap or insulating SiO, barrier allows the different plasmonic enhancement mechanisms to be controlled.

Figure 2. Determining plasmonic enhancement mechanism by transient absorption spectroscopy. (a) After excitation of the plasmon by the pump laser the plasmon can enhance carrier creation in the TiO, by hot electron injection or PIRET. (b) The complex equilibration present in trap states after excitation allows separation of plasmonic enhancement from excitation of the semiconductor or metal alone. The region where excited population in the TiO, was extracted is marked, with the negative change in transmission from the background signal (Bkg. Signal Negative) and positive change in transmission from plasmonic enhancement (Enh. Signal Positive) labeled.

The data corresponding to this analysis is shown in Figure 3. The pump pulse had a fluence of ~8 mJ/cm* for each Figure 3. Control of plasmonic enhancement mechanism. Data points are taken from transient absorption measurements; if the signal at a given excitation wavelength was not above background, the data are shown as open symbols. The theoretical enhancement predicted for PIRET and hot electrons are shown as filled curves. (a) In Ag@TiO, contact and spectral overlap exists between metal and semiconductor, and PIRET and hot electron injection are both measured. (b) Addition of a SiO, barrier to Ag@TiO, eliminates hot electron injection. (c) Switching the metal core to Au eliminates spectral overlap and PIRET. (d) Inserting a SiO, barrier in Au@TiO, eliminates both hot electron injection and PIRET despite strong light absorption by plasmonic Au. The transient absorption percentage is scaled to correct for incident power fluctuations at each wavelength. The simulated data are fit to the experimental data in amplitude but not spectral dependence.

Figure 4. Variables that control the possible plasmonic enhancement mechanisms for extending photoconversion. Connecting the line between the properties of the given core@shell nanoparticle gives the corresponding enhancement mechanisms present. The metal@core—shell nanoparticles confirmed that the plasmonic enhancement mechanism can be engineered by the metal—semiconductor spectral properties and relative prox- imity. The sample space covered is summarized in Figure 4.

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Exploring the Optical Nonlinearities of Plasmon-Exciton Hybrid Resonances in Coupled Colloidal Nanostructures

by Yury Rakovich and

2016, The Journal of Physical Chemistry C

Strong coupling of plasmons and excitons can form hybrid states, the so-called " plexcitons ". Although plasmons have a low quality factor, the exceptionally high coupling strength with molecular aggregates, in particular J-aggregates,... more

Figure 1. (a) Scheme of hybridization between the longitudinal plasmon of an Au-NR and an exciton on J-aggregated dye molecules: one state at higher (UR) and one at lower energy (LR) are formed. (b) Extinction spectra of colloidal gold nanorods with different aspect ratios (top), J- aggregated dye molecules (middle), and coupled hybrid particles (bottom). Dashed magenta and purple curves follow the spectral positions of UR and LR. Dotted black curves are modeled extinction spectra. (c) Dispersion relations of UR (magenta squares) and LR (purple squares), theoretical values (lines), exciton energy and plasmon resonance of bare Au-NRs (gray and black dashed lines). (d) fwhm of UR and LR versus longitudinal plasmon energy of the Au-NR with theoretical values (black circles). The dashed curves illustrate the fwhm of the uncoupled exciton (gray) and of the plasmon (black).

The primed variables were used as fitting parameters and the other quantities were determined from the linear extinction spectra. Typical boundaries during the fitting procedure were f; —02<fj/<f,E-O1<E<E +01, andy, <7, <y,+ 0.2. For each fit, we made certain that the initial boundary conditions did not affect the result. Mathematica 10 was used to reproduce the transient absorption spectra. Two functions were used for fitting bare Au NRs and the hybrid particles:

Figure 2. (a) Transient absorption (TA) spectra for bare Au-NRs: Experimental data (solid curves) and theoretical approach (dotted curves) at different times after excitation. (b) Normalized relaxation dynamics of TA at the spectral position of the longitudinal plasmon (1.85 eV). (c) TA spectra and (d) normalized relaxation at 2.1 eV of J-aggregated dye molecules.

Figure 3. (a) Experimental (solid) and modeled (dotted) TA spectra for coupled hybrid particles with aspect ratios AR = 2.0 (blue) and AR = 4.0 (red) excited at 2.44 eV (S10 nm). (b) Relaxation dynamics at the spectral position of UR (magenta circles) and LR (purple circles) for hybrid particles with short AR = 2.0 and (c) long AR = 4.0 Au-NRs. (d) Scheme of transversal and longitudinal plasmon resonances (green and red arrows) and the dipole moment of the dye molecules (orange arrow). The pump pulse with 2.44 eV mainly excites the transverse plasmon. (e) Dependence of 7, (initial relaxation time obtained from a biexponential decay model) for AOD at the spectral position of LR (purple) and UR (magenta) of hybrid particles with different longitudinal plasmon energies of the Au-NRs. The dashed curve represents t, for the dye and the arrows mark the samples plotted in (a).

Figure 4. (a) TA spectra for coupled hybrid particles with aspect ratio AR = 2.0 at 200 fs (top) and 1 ps (bottom) after excitation with a photon energy of 2.18 eV (S70 nm); experimental (solid) and modeled (dotted) data. Inset: Extinction spectrum of particles. (b) Normalized relaxation dynamics at the spectral position of UR (magenta circes) and LR (purple circles) in the subpicosecond and (c) picosecond regime after excitation. (d) Scheme for excitation at 2.18 eV ($70 nm). The UR hybrid resonance comprising the coupled longitudinal plasmon and the exciton on the dye (red and orange arrows) is excited directly. (e) Fitting parameter AE for UR and LR: The spectral position of UR (magenta) shifts to lower energies and vice versa for LR (purple).

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Pump-Probe Spectroscopy

Key research themes

1. How can ultrafast terahertz pump-probe spectroscopy characterize low-frequency vibrational and solvation dynamics in molecular condensed phases?

2. What are effective polarization-resolved pump-probe methods to measure anisotropic relaxation and excited state dynamics in biologically relevant molecules?

3. How can multi-pulse pump–pump–probe and pump–repump–probe spectroscopy elucidate complex photoinduced charge transfer and vibrational relaxation pathways in molecular systems?

4. How can quantitative dielectric properties and electronic dynamics be extracted from conventional pump-probe spectroscopy to enhance interpretation of transient excited state phenomena?

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