Determination of hot carrier energy distributions from inversion of ultrafast pump-probe reflectivity measurements

Heilpern, Tal; Manjare, Manoj; Govorov, Alexander O.; Wiederrecht, Gary P.; Gray, Stephen K.; Harutyunyan, Hayk

doi:10.1038/s41467-018-04289-3

Cited by 80 publications

(77 citation statements)

References 31 publications

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“…Now we compare our model predictions for τ E of Au, Al, and Pt with experiment. While a variety of experimental studies are sensitive to the cooling rates of photoexcited electrons 47 , interpretation of such experiments is not straightforward 18,37,53 . Time-resolved measurements of changes in optical properties, e.g., time-domain thermoreflectance or time-domain transient absorption, are common methods for studying nonequilibrium electron dynamics 1,26,33,36,47,54 .…”

Section: Discussionmentioning

confidence: 99%

Parametric dependence of hot electron relaxation timescales on electron-electron and electron-phonon interaction strengths

Wilson

Coh

2020

Commun Phys

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Understanding how photoexcited electron dynamics depend on electron-electron (e-e) and electron-phonon (e-p) interaction strengths is important for many fields, e.g. ultrafast magnetism, photocatalysis, plasmonics, and others. Here, we report simple expressions that capture the interplay of e-e and e-p interactions on electron distribution relaxation times. We observe a dependence of the dynamics on e-e and e-p interaction strengths that is universal to most metals and is also counterintuitive. While only e-p interactions reduce the total energy stored by excited electrons, the time for energy to leave the electronic subsystem also depends on e-e interaction strengths because e-e interactions increase the number of electrons emitting phonons. The effect of e-e interactions on energy-relaxation is largest in metals with strong e-p interactions. Finally, the time high energy electron states remain occupied depends only on the strength of e-e interactions, even if e-p scattering rates are much greater than e-e scattering rates.

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Section: Discussionmentioning

confidence: 99%

Parametric dependence of hot electron relaxation timescales on electron-electron and electron-phonon interaction strengths

Wilson

Coh

2020

Commun Phys

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“…The sub‐picosecond spectral changes of TiN and ZrN result principally from the carrier plasma cooling through coupling to phonons. The opposing redshift of TiN and blueshift of ZrN are most likely due to distinct band structure and oscillator strength of specific transitions close to the Fermi level, which are not experimentally known, [ 83 ] with a potential secondary contribution from residual nonthermal energy distribution of electrons which occurs in the first 100 fs following excitation. [ 84 ]…”

Section: Overview Of Transient Reflectivitymentioning

confidence: 99%

Broadband Ultrafast Dynamics of Refractory Metals: TiN and ZrN

Diroll

Saha

Shalaev

et al. 2020

Advanced Optical Materials

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Recent work on transition metal nitrides has highlighted their optical properties as alternatives to noble metals employed in plasmonics such as gold (Au). [1-9] Titanium nitride (TiN), zirconium nitride (ZrN), and other plasmonic nitrides such as hafnium nitride (HfN) and tantalum nitride (TaN) are particularly attractive due to their high melting points that bolster stability at higher ambient temperatures [10-12] and/or under higher laser irradiation intensities, [13-16] in addition to their mechanical hardness [17,18] and complementary metal-oxide-semiconductor compatibility. [19-21] Recent work has demonstrated that TiN shows strong local heating compared to Au, [22-24] which may be exploited for photothermal therapy, [25,26] shape-memory effects, [27] thermochromic windows, [28] photoreactions, [29-32] heat transducers or thermophotovoltaic materials, [22,33-37] or photodetection. [38] Implicit in these observations and devices are very different optical responses of metallic nitrides compared to gold-the most similar classical plasmonic material-particularly with regard to the dissipation of heat. Transient reflection spectroscopy offers a window into the dynamics of heating in metal nitrides upon photoexcitation. Previous pump-probe spectroscopy on titanium nitride and other refractory metals has mainly consisted of single pump and probe wavelengths. [39-42] While some broadband transient absorption studies have been performed on these materials, [43] the epsilon-near-zero (ENZ) window where the real part of the dielectric permittivity crosses zero, has thus far remained unexplored. Importantly, based upon earlier spectroscopic investigations of noble metals and heavily doped semiconductors, the largest changes of transient reflectivity are anticipated at ENZ regions. [44-47] Furthermore, earlier works yield contradictory conclusions from ultrafast pump-probe studies. Early data, in which sub-picosecond dynamics were not observed, suggested that electron-phonon coupling in TiN is weak, [39] with electron equilibration with the lattice requiring tens or hundreds of picoseconds in comparison to ≈1 ps in gold. [48-51] Subsequent experiments with high pump fluences were able to observe the fast dynamics, conveying strong electron-phonon coupling resulting in sub-picosecond equilibration of electrons and the lattice. [42] Transition metal nitrides have recently gained attention in the fields of plasmonics, plasmon-enhanced photocatalysis, photothermal applications, and nonlinear optics because of their suitable optical properties, refractory nature, and large laser damage thresholds. This work reports comparative studies of the transient response of films of titanium nitride (TiN), zirconium nitride (ZrN), and gold (Au) under femtosecond excitation. Broadband transient optical characterization helps to adjudicate earlier, somewhat inconsistent reports regarding hot electron lifetimes based upon single wavelength measurements. These pump-probe experiments show sub-picosecond transient dynamics only within the e...

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“…An interesting method to study the dynamics of hot carriers are ultrafast pump-probe reflectivity measurements. 53 Photo-generated carriers introduce refractive index changes that result in a reflectance variation. Figure 7c shows the estimated electron occupancy generated on an illuminated 30 nm gold film.…”

Section: Manipulating Hot-carrier Relaxationmentioning

confidence: 99%

“…(d) Experimental differential reflectance of a gold-clad silicon disk. Figures adapted with permission from American Chemical Society and Nature Publishing Group: (a-b)55 (c)53 (d)54…”

mentioning

confidence: 99%

From Optical to Chemical Hot Spots in Plasmonics

et al. 2019

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In recent years, the possibility to induce chemical transformations by using tunable plasmonic modes has opened the question of whether we can control or create chemical hot spots in these systems. This can be rationalized as the reactive analogue of the well-established concept of optical hot spots, which have drawn a great deal of attention to plasmonic nanostructures for their ability to circumvent the far-field diffraction limit of conventional optical elements. The parameters that determine the reactivity of plasmonic systems are deeply influenced by the dynamics and interplay of photons, plasmon-polaritons, carriers, phonons and molecular states. Although optical hot spots can be mainly defined by the geometry and permittivity of the nanostructures, the degrees of freedom influencing their photocatalytic properties appear to be much more numerous. These degrees of freedom can affect the reaction rates, the product selectivity or the spatial localization of a chemical reaction. In this Account, we show how the control of chemical hot spots can be achieved by carefully tuning the parameters that influence the cascade of events following the optical excitation of plasmonic modes in nanostructures. We discuss here a series of single photocatalyst techniques and ideas of how plasmonic nanoscale reactivity can be spatially mapped and imaged, and how the lifetime of hot and thermalized carriers can be altered by trap states on semiconductors and by metal-semiconductor interfaces. In addition, the tailored generation of non-thermal phonons in metallic nanostructures and their dissipation is shown as a promise to understand and exploit thermal photocatalysis at the nanoscale. Surpassing or enhancing each of these energy channels should enable to engineer solar nanometric photocatalysts. Nevertheless, the ultimate capability of a plasmonic photocatalyst to trigger a chemical reaction is correlated to its ability to navigate through, or even modify, the potential energy surface of a given chemical reaction. Here we reunite both worlds, the plasmonic photocatalysts and the molecular ones, identifying different energy transfer pathways and their influence on selectivity and efficiency of chemical reactions. We foresee that the migration from optical to chemical hot spots will greatly assist the understanding of ongoing plasmonic chemistry.

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Determination of hot carrier energy distributions from inversion of ultrafast pump-probe reflectivity measurements

Cited by 80 publications

References 31 publications

Parametric dependence of hot electron relaxation timescales on electron-electron and electron-phonon interaction strengths

Parametric dependence of hot electron relaxation timescales on electron-electron and electron-phonon interaction strengths

Broadband Ultrafast Dynamics of Refractory Metals: TiN and ZrN

From Optical to Chemical Hot Spots in Plasmonics

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