Giant two-photon absorption of anatase TiO2 in Au/TiO2 core-shell nanoparticles

Lijie Wang; Tsz Him Chow; Malte Oppermann; Jianfang Wang; Majed Chergui

doi:10.1364/PRJ.487784

Abstract

We report on deep-to-near-UV transient absorption spectra of core-shell

Au / {SiO}_{2}

and

Au / {TiO}_{2}

nanoparticles (NPs) excited at the surface plasmon resonance of the Au core, and of UV-excited bare anatase

{TiO}_{2}

NPs. The bleaching of the first excitonic transition of anatase

{TiO}_{2}

\sim 3.8 eV

is a signature of the presence of electrons/holes in the conduction band (CB)/valence band (VB) of the material. We find that while in bare anatase

{TiO}_{2}

NPs, two-photon excitation does not occur up to the highest used fluences (

1.34 {mJ / cm}^{2}

), it takes place in the

{TiO}_{2}

shell at moderate fluences (

0.18 {mJ / cm}^{2}

) in

Au / {TiO}_{2}

core-shell NPs, as a result of an enhancement due to the plasmon resonance. We estimate the enhancement factor to be of the order of

\sim 10^{8} - 10^{9}

. Remarkably, we observe that the bleach of the 3.8 eV band of

{TiO}_{2}

lives significantly longer than in bare

{TiO}_{2}

, suggesting that the excess electrons/holes in the conduction/valence band are stored longer in this material.

1. INTRODUCTION

Transition metal oxides (TMOs) have been attracting growing interest as candidates for nonlinear (NL) optical materials, due to their high optical nonlinearities, ultrafast response times, and low absorption indices [1,2]. TMOs are also the prime candidates for transparent conductive oxides (TCOs), because of their large bandgap and transparent and conducting properties, whose films are utilized in flat panel displays [3], solar cells [4], and electroluminescent devices as transparent electrodes [5].

Being one of the most commonly investigated TMOs, ${TiO}_{2}$ in its various polymorphs (anatase, rutile, and amorphous) also possesses remarkable properties, such as the high stability of its excitonic transitions to perturbations such as defects, temperature [6], and excess charge doping [7]. For all optical switching applications, anatase ${TiO}_{2}$ thin films exhibit an interesting fast and reasonably large NL optical response at 800 nm [1]. It was, however, noted that the physical properties and, thus, the NL optical response of ${TiO}_{2}$ films are strongly dependent on the processing route [8], as well as on the inclusion of metallic nanoparticles (NPs) [9,10].

Measurements of the NL optical properties of ${TiO}_{2}$ polymer nanocomposites demonstrated negligible two-photon absorption (TPA) and a negative value of the NL refractive index [11]. This is in line with reports showing no TPA of anatase ${TiO}_{2}$ NPs in solution even up to fluences of $300 mJ / {cm}^{2}$ [12,13]. On the other hand, large second- and third-order optical nonlinearities of the $Au / {TiO}_{2}$ composite films were observed and attributed to the high density of Au particles in nonconductive films, and the strong local field enhancement caused by the localized surface plasmon resonance (LSPR) absorption [14]. In particular, it was reported that $Au / {TiO}_{2}$ composites can exhibit a self-defocusing and saturating nonlinearity at optical intensities up to $5.6 {GW / cm}^{2}$ , while measurements on a pure ${TiO}_{2}$ film did not show any nonlinearity [15].

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Stable core-shell $Au / {TiO}_{2}$ and $Ag / {TiO}_{2}$ NPs were reported to exhibit saturable absorption or optical limiting at 532 nm nanosecond-pulse excitation, depending on the applied laser fluence [16]. This behavior was explained in terms of the electronic Kerr nonlinearities and NL scattering in the NPs. The samples were subject to laser fluences up to $20 {J / cm}^{2}$ and powers up to $2.8 {GW / cm}^{2}$ , and showed no appreciable signs of laser-induced damage, making them promising candidates for high energy optical limiting [16]. It has also been noted that Au NP-doped multilayers such as $Au / {SiO}_{2}$ and $Au / {TiO}_{2}$ films can be used as optical filters, due to their high damage threshold and their larger third-order NL susceptibilities.

It has been established that excitation of the LSPR band of Au NPs deposited on or embedded in ${TiO}_{2}$ films leads to injection of electrons from the Au NP into ${TiO}_{2}$ [17 –23]. Just as for dye-sensitized NPs, probing and time-resolving the electron injection from molecular dyes into ${TiO}_{2}$ was performed using visible-to-THz probes [18,21,23,24], X-ray absorption spectroscopy (XAS) at the Ti K-edge [12,25,26], and, more recently, X-ray photoelectron spectroscopy sensitive to the Au $4 f$ - and Ti $2 p$ -orbitals [27]. Overall, the optical-domain pump-probe studies concluded that electron injection is prompt, within the cross correlation of the experiments (typ. $< 200 fs$ ), and takes place with an efficiency reaching 40%–50% [18,23]. It should be stressed that these time-resolved experiments concerned Au NPs deposited on ${TiO}_{2}$ surfaces [18,24,27] or embedded in ${TiO}_{2}$ films [23], and were all carried out in the linear regime of pump-pulse intensity.

Thus, it appears that the Au NPs can influence the optical properties of the $Au / {TiO}_{2}$ system in different ways: electron injection and/or enhanced NL effects (e.g. TPA, saturable absorption) due to the presence of the plasmon field. To weigh these various contributions, it is important to have simultaneously spectroscopic fingerprints of the presence of electrons in the conduction band (CB) of ${TiO}_{2}$ and of the Au NP transitions, as well as their temporal evolutions. This has not been the case in any of the above-mentioned studies reporting TPA in $Au / {TiO}_{2}$ composite materials.

In recent years, Baldini et al. [6,13,28] carried out a thorough investigation of the spectroscopy and charge carrier dynamics of bare anatase ${TiO}_{2}$ NPs using pump-probe spectroscopy in the deep-ultraviolet (3.3–4.6 eV). In particular, they identified the first excitonic transition of the system at $\sim 3.8 eV$ and showed that it could be used as a marker of electron injection in dye-sensitized ${TiO}_{2}$ NPs, by detecting its bleaching due to the blocking of the transition that results from the excess of electrons/holes in the CB/valence band (VB). Further to this, Wang et al. [29 –31] used the same tools to investigate the spectroscopy and charge carrier cooling in Au NPs. The novelty of the latter work lies in the fact that they probed the region of interband transitions of the Au NPs for the first time. Thus, in the present case, the deep-UV probe offers the advantage that the characteristic transitions of both the Au core and the ${TiO}_{2}$ shell fall in the same spectral region, so that their behavior can be simultaneously monitored.

Here we investigate the response of $Au / {TiO}_{2}$ NPs in aqueous solution using pump-probe TA spectroscopy upon resonant excitation of the LSPR of the Au NP core in the visible, while the probing is done using the deep-UV broadband continuum. By detecting the excitonic transition of anatase ${TiO}_{2}$ when excessive electrons/holes are present in the CB/VB [13], we get insights into the excitation mechanism of the system. We compare these results with LSPR-excited core-shell $Au / {SiO}_{2}$ NPs and above the bandgap (BG) excited bare ${TiO}_{2}$ NPs. The BG of ${SiO}_{2}$ is too large (7.5 to 9.6 eV) for any electron injection from the excited Au core to take place, or it would require absorption by three to four visible photons, which is a very low-probability event. We observe that at pump energies resonant with the LSPR of the Au core, efficient TPA of anatase ${TiO}_{2}$ promotes electrons from the VB directly into the CB by simultaneously absorbing two green photons. Even at higher pump fluences, no TPA is observed in the case of bare ${TiO}_{2}$ . We also find that the excess electrons/holes generated in the CB/VB have a significantly longer lifetime than in bare ${TiO}_{2}$ , offering a strategy to store charges. The experimental setup and procedures are presented in the appendix.

2. RESULTS AND DISCUSSION

Figure 1 shows the absorption spectra of bare anatase ${TiO}_{2}$ and bare Au NPs, as well as core-shell $Au / {SiO}_{2}$ and $Au / {TiO}_{2}$ NPs; the inset zooms into the region of the LSPR band of $Au / {SiO}_{2}$ and $Au / {TiO}_{2}$ NPs. The bare anatase ${TiO}_{2}$ NP shows the well-known absorption with a wing extending down to $\sim 3 eV$ , due to scattering. The shoulder at $\sim 3.8 eV$ corresponds to the first excitonic transition of anatase ${TiO}_{2}$ , as previously discussed in detail [6]. The spectra of $\sim 25 - nm$ -diameter bare Au and $Au / {SiO}_{2}$ NPs are almost identical with both exhibiting the LSPR band at $\sim 2.35 eV$ , and at higher energies, the region of interband transitions, discussed in Refs. [29,30]. In the case of $Au / {TiO}_{2}$ core-shell NPs, the LSPR band shifts to the red by $\sim 0.37 eV$ and exhibits a red wing extending down to $\sim 1.5 eV$ spectrum. Its spectrum is similar to that of $\sim 10 - nm$ -diameter Au NPs embedded in a ${TiO}_{2}$ film [23]. The red shift of the LSPR in $Au / {TiO}_{2}$ compared to $Au / {SiO}_{2}$ is fully accounted for by the dielectric effects [23,32].

Steady-state absorption spectra of the samples investigated in this work. The two pink arrows show the LSPR peaks of Au/SiO2 and Au/TiO2 NPs, respectively, and the inset zooms into the LSPR part of the Au/SiO2 and Au/TiO2 NPs; all NPs are dispersed in aqueous solution.

Figure 1.Steady-state absorption spectra of the samples investigated in this work. The two pink arrows show the LSPR peaks of $Au / {SiO}_{2}$ and $Au / {TiO}_{2}$ NPs, respectively, and the inset zooms into the LSPR part of the $Au / {SiO}_{2}$ and $Au / {TiO}_{2}$ NPs; all NPs are dispersed in aqueous solution.

Figure 2 shows the time-energy deep-UV transient absorption (TA) maps and the corresponding TA spectra at different time delays of the three samples: bare anatase ${TiO}_{2}$ NPs excited at 4.0 eV above the BG, and $Au / {SiO}_{2}$ and $Au / {TiO}_{2}$ core-shell NPs excited into the Au LSPR band at 2.0–2.4 eV. The TA of bare anatase ${TiO}_{2}$ NPs [Fig. 2(a)] is identical to what was reported in Refs. [6,28]. The TA spectra of bare anatase ${TiO}_{2}$ at 1, 100, and 800 ps time delay [Fig. 2(b)] clearly exhibit a negative (bleach) signal over the entire probe range with a band centered at $\sim 3.83 eV$ , which represents the first excitonic transition of anatase ${TiO}_{2}$ [6]. The bleach signal is due to electrons/holes generated by BG excitation into the CB/VB of ${TiO}_{2}$ . Figure 2(c) shows the TA map for $Au / {SiO}_{2}$ NPs excited with a broadband pulse (2–2.4 eV) at the LSPR, and Fig. 2(d) shows the TA spectra. These results are close to those we recently reported for the case of bare Au NPs of identical size [29,30]. The TA maps and spectra for $Au / {SiO}_{2}$ NPs exhibit three main features: two negative ones at $\sim 3.8 eV$ and $\sim 4.4 eV$ , and a positive one at $\sim 3.4 eV$ . Briefly, they were identified in Refs. [29,30] using the band structure diagram of bulk Au, as interband transitions from the $5 d$ valence band to the $6 sp$ band above the Fermi level at the $X$ and $L$ symmetry points for the two negative ones, monitoring the population of electrons and holes, above and below the Fermi level, respectively. The positive band was identified as a transition between valence sub-bands (i.e. an intraband transition), monitoring the population of holes. The overall conclusion was that charge carrier cooling is slow ( $\sim 150 ps$ ) and commensurate with the time scale of thermal lattice cooling ( $\sim 250 ps$ ). The fluence dependence of the 3.4 and 3.8 eV bands at a time delay of 50 ps is shown in Fig. 3, and it clearly reflects a one-photon excitation process.

Figure 2.Time-energy TA maps and the corresponding spectral traces at 1, 100, and 800 ps. (a), (b) Bare anatase ${TiO}_{2}$ NPs upon above-BG excitation (4.0 eV); (c), (d) $Au / {SiO}_{2}$ NPs upon plasmon excitation (2.0–2.4 eV); (e), (f) $Au / {TiO}_{2}$ NPs upon plasmon excitation (2.0–2.4 eV) with a fluence of $\sim 330 μJ / {cm}^{2}$ ; all NPs are dispersed in aqueous solution.

Figure 3.Fluence dependence of the bands appearing in the plasmon-excited TA spectra of $Au / {SiO}_{2}$ NPs at 50 ps time delay, displayed as a log–log plot of TA intensity versus pump fluence: (a) 3.4 eV and (b) 3.8 eV.

Figures 2(e) and 2(f) show the TA maps and spectra for the $Au / {TiO}_{2}$ NPs excited by a 2.0–2.4 eV broadband pulse. Compared to Figs. 2(c) and 2(d), some notable differences but also similarities arise. First, the $\sim 3.4 eV$ positive signal (due to holes in the Au core) is shorter-lived in $Au / {TiO}_{2}$ . Second, a band shows up centered at $\sim 3.8 eV$ (the early time signal at this band is due to the Au NP, but it dramatically decays within $\sim 10 ps$ , as observed in the $Au / {SiO}_{2}$ case). Thereafter, the bleach signal persists up to the longest time of our temporal window. Figure 4 compares the 800 ps transients for the three samples. For the $Au / {SiO}_{2}$ sample, the signal has all but died away, except for the weak remnant of the positive feature $< 3.6 eV$ . However, the signal for bare anatase ${TiO}_{2}$ and $Au / {TiO}_{2}$ NPs exhibits the same bleach feature at 3.8–3.83 eV. Given that the equivalent band due to the Au core has fully decayed, we conclude that this is the bleach due to electrons/holes in the CB/VB of the ${TiO}_{2}$ shell in $Au / {TiO}_{2}$ NPs [13,28]. This is further supported by Fig. 5, which shows the time profiles of the 3.8 and 3.4 eV bands for both core-shell NPs. Finally, the features $> 4.1 eV$ in $Au / {SiO}_{2}$ do not show up in the case of $Au / {TiO}_{2}$ , due to the fact that the Au contribution to the transient is shaded by the strong absorption of ${TiO}_{2}$ in this region.

Figure 4.Comparison of the TA spectra of the three measured samples (rescaled for clarity). The blue solid line is the transient response of bare anatase ${TiO}_{2}$ at a delay time of 800 ps upon UV excitation (above-BG excitation, 4.0 eV). The orange and red lines represent the TA spectra of $Au / {SiO}_{2}$ and $Au / {TiO}_{2}$ at a time delay of 800 ps upon plasmon excitation (below-BG excitation, 2.0–2.4 eV), with a fluence of $\sim 330 μJ / {cm}^{2}$ .

Figure 5.Comparison of the TA time traces between $Au / {SiO}_{2}$ and $Au / {TiO}_{2}$ NPs at probe energies of (a), (b) 3.8 eV; (c), (d) 3.4 eV within (a), (c) 200 ps; (b), (d) 800 ps. The ${TiO}_{2}$ was excited above the BG, while the $Au / {SiO}_{2}$ and $Au / {TiO}_{2}$ were excited upon plasmon excitation, and all the samples are excited with a fluence of $\sim 330 μJ / {cm}^{2}$ . The blue and orange circles are experimental traces; the red and purple solid lines are the fitted time traces.

The main observation here is the long-lived bleach band in $Au / {TiO}_{2}$ NPs at the position ( $\sim 3.8 eV$ ) of the first excitonic transition of anatase ${TiO}_{2}$ . Excess electrons/holes in the CB/VB could either originate from the electrons transferred from the VB of ${TiO}_{2}$ or the injected electrons from Au NPs. The presence of the intraband transition of Au at $\sim 3.4 eV$ is indicative of holes in the Au NP core, due to electrons that have been removed from below the Fermi level upon LSPR excitation. They may end up in the CB of ${TiO}_{2}$ as expected in the scenario of plasmon-induced electron injection into the ${TiO}_{2}$ shell, typically within hundreds of femtoseconds [18,23]. However, the time scales of the band around $\sim 3.4 eV$ and $\sim 3.8 eV$ are not commensurate, implying that hole decay in the Au core is not due to a back-electron transfer from ${TiO}_{2}$ , but rather by the kinetics of charge carrier cooling just as in $Au / {SiO}_{2}$ NPs [29,30]. Of course, the decay time of the 3.4 eV band is shorter here, which may point to an influence of the ${TiO}_{2}$ shell. This is not surprising considering the way the latter affects the spectral features of the Au core (Fig. 1). In summary, it appears that the $\sim 3.8 eV$ feature consists of three contributions: one at very early times, due to charge injection from the Au core; the second at early to somewhat later times due to the response of the Au interband transitions (close to that of $Au / {SiO}_{2}$ NPs); and a third one that persists up to the longest time delay, which is due to the bleached ${TiO}_{2}$ excitonic band.

Further insight into the origin of the population mechanisms in the CB of ${TiO}_{2}$ is given by the fluence dependence of the various bands at short (10 ps), intermediate (50 ps), and long (800 ps) time delays (see Appendix C). Figures 6(a)–6(c) show log–log plots of the signals of $Au / {TiO}_{2}$ at $\sim 3.8 eV$ at these time delays for pump fluences ranging from 0.16 to $0.98 {mJ / cm}^{2}$ . The slopes of the signal are $1.32 \pm 0.20$ , $1.55 \pm 0.66$ , and $1.83 \pm 0.21$ at 10, 50, and 800 ps time delay, respectively. While the latter points to a predominant two-photon process, the former two point to a regime intermediate between one- and two-photon absorption. Further, these slopes reflect an increasing weight of the TPA-induced signal as a function of time. This can be rationalized by noting that the $\sim 3.8 eV$ signal contains the contributions of the interband transition of the Au NP, the electron injected via LSPR excitation, and the excitonic transition of anatase ${TiO}_{2}$ . As already mentioned, the first contribution, which dies away in $\sim 250 ps$ , has a linear dependence on the pump fluence (Fig. 3). The second contribution, also linear with pump fluence, dies away much faster ( $< 50 ps$ ) according to Refs. [23,27]. Therefore, at 10 and 50 ps delays, it is present [as seen in Figs. 2(c) and 2(d)], while at 800 ps it has vanished. In the latter case, we are left with a predominant TPA contribution that involves the ${TiO}_{2}$ shell. This process is of course present from the earliest times onwards but is overlapped by the above one-photon contributions. Figure 7 shows the value of the $\sim 3.8 eV$ signal slope as a function of time. We assume a maximum value of 2 at infinite times, considering that all signals due to the Au core have died away. The biexponential fit of the data point yields values of $τ_{1} \approx 50 ps$ and $τ_{2} > 2 ns$ with uncertainties of a factor of 2. Bearing in mind the latter, it is fair to say that the first component decays on comparable time scales to its counterpart in $Au / {SiO}_{2}$ NPs, as is actually the case with the 3.4 eV band [Figs. 2(c) and 2(d)].

Figure 6.Fluence dependence of the 3.8 eV band of TA spectra of $Au / {TiO}_{2}$ NPs excited at the LSPR of the Au core, displayed as a log–log plot of TA intensity versus pump fluence, which varies in a range from 0.16 to $0.98 {mJ / cm}^{2}$ : (a) 10 ps, (b) 50 ps, and (c) 800 ps. At 800 ps, the positive contribution due to the Au has died away. The magenta dots are the experimental values, and the solid cyan lines represent their fits with a linear function.

Figure 7.Time dependence of the slopes of the fluence dependence of the $Au / {TiO}_{2}$ NPs at 3.8 eV. The solid line relates to the fit using the biexponential model with time constants of $τ_{1} = 49.5 ps$ and $τ_{2} > 2000 ps$ .

Finally, the fact that three contributions are at play is visible from Fig. 8, which shows the change of profile of the TA spectra at 10 ps time delay for increasing fluences under visible (2.0–2.4 eV) excitation. Whilst for $Au / {SiO}_{2}$ the TA profile does not change with pump fluence [Fig. 8(b)], for $Au / {TiO}_{2}$ clear changes are observed with the growth of a blue shifted bleach band implying that the TPA-induced ${TiO}_{2}$ excitonic bleach is taking over the overall bleach contribution with increasing pump power [Fig. 8(c)]. Note that for bare ${TiO}_{2}$ , there is no effect on the TA spectra over the same range of fluences, implying no signal indicative of charge carriers in the VB/CB. To further make the point about the two-photon process causing the bleach of the 3.8 eV band, Fig. 9 shows the log–log plots of the signal due to the $\sim 3.4 eV$ band of $Au / {TiO}_{2}$ NPs at 10 ps and 50 ps, respectively. Its slope is close to 1 (bearing in mind the large error bars), which is reasonable since this band is due to holes in the Au core caused by the LSPR excitation [29]. Note that the 3.4 eV band is at the limit of our detection, and therefore its signal at 50 ps is weak, which explains the relatively large uncertainty of its slope.

Figure 8.TA spectra at 10 ps of (a) bare ${TiO}_{2}$ NPs, (b) $Au / {SiO}_{2}$ , and (c) $Au / {TiO}_{2}$ . All samples are excited at the plasmon resonance (2.0–2.4 eV) for different values of the fluence.

Figure 9.Fluence dependence of the $Au / {TiO}_{2}$ NPs at 3.4 eV, displayed as a log–log plot of TA intensity versus pump fluence, which varies in a range from 0.16 to $0.98 {mJ / cm}^{2}$ : (a) at 10 ps and (b) at 50 ps.

Coming back to the $\sim 3.8 eV$ signal, one may ask whether the early time one-photon absorption signal could be due to injection of electrons from the Au core [18,21,33], which then decay on a $\sim 250 ps$ time scale. This might be excluded because: first, it is not supported by the shape of the transients in Fig. 8(b); second, its decay is comparable to what was reported for the same band in $Au / {SiO}_{2}$ NPs [29,30]; and, third, as was shown by Ratchford et al. [23] in the case of Au NPs embedded in ${TiO}_{2}$ films, the injected electrons have a fast decay ( $\sim 2 ps$ ) followed by a long-lived (on the window of 20 ps of their time scan) weak component. We can therefore conclude that the early time signal is largely due to the response of the Au core. This does not exclude a ${TiO}_{2}$ response due to injected electrons, but their contribution is short-lived. The bleach signal at 800 ps time delay, when the signal of the Au core has died away, is therefore due to the ${TiO}_{2}$ excitonic bleach only. Its slope of 1 $. 83 \pm 0.21$ reflects the fact that the electrons/holes in the CB/VB of ${TiO}_{2}$ have been generated via a TPA process. Again, we stress that the same range of fluences does not show any signal in bare ${TiO}_{2}$ NPs, which would be due to electrons/holes in the CB/VB [Fig. 8(a)]. This is in line with previous studies, which used fluences up to $200 {mJ / cm}^{2}$ of a 10 ps/532 nm laser [12], i.e., two orders of magnitude larger than in the present case, showing no evidence of two-photon excitation. In Ref. [13], femtosecond pulse excitation at either 400 nm or 550 nm with fluences up to $53 {mJ / cm}^{2}$ also showed no sign of multiphoton excitation. On the other hand, in the case of dye-sensitized ${TiO}_{2}$ NP, the fluence dependence of the $\sim 3.8 eV$ band due to injected electrons remained in the linear regime up to $336 μJ / {cm}^{2}$ using 550 nm excitation. These observations lead us to conclude that the occurrence of multiphoton excitation of ${TiO}_{2}$ in $Au / {TiO}_{2}$ core-shell NPs is due to the presence of the Au core and is mediated by the resonance excitation of its LSPR.

We estimated the enhancement factor of the TPA of ${TiO}_{2}$ in $Au / {TiO}_{2}$ NPs, as described in Appendix D. Based on our experimental TA intensity (we took an average $Δ A$ value of $\sim 0.3$ at 3.8 eV, over the 10–200 ps range), from Fig. 2(e), we calculate that in $Au / {TiO}_{2}$ NPs, the enhancement factor of the third-order NL two-photon process is $\sim 10^{8} - 10^{9}$ . This is consistent with the results of surface-enhanced Raman scattering (SERS) [34,35] utilizing LSPR of nanostructured metallic NPs (e.g. Au, Ag, Cu) to amplify Raman signals of molecules at their surface with enhancement factors of $10^{6} - 10^{11}$ . It is also reported that the quantum yield of fluorescence in Au nanorods compared to a bulk metal surface is enhanced by a factor of more than 1 million [36,37]. In addition, simulations show that the enhancement of the molecular TPA cross section by the LSPR may reach many orders of magnitude, depending on the distance of the NPs and the ${| E |}^{4}$ local field [38,39].

This huge enhancement of the TPA cross section in the case of core-shell $Au / {TiO}_{2}$ NPs should be contrasted to the linear dependence reported in the case of Au NP charge injection into anatase ${TiO}_{2}$ [17 –23]. This suggests that the architecture of NPs deposited on the surface of ${TiO}_{2}$ does not favor LSPR enhancement of TPA, which is likely caused by the limited contact interface with the NP in this architecture [18,24,27]. The case of Au NP embedded in the ${TiO}_{2}$ film [23] is close to the core-shell architecture (although the Au NPs are densely stacked), but the upper fluence used in their work is close to the lowest in ours. It is also interesting to note that while on the short time scale ( $< 20 ps$ ), the decay of the signal due to injected electrons is faster in the $Au / {TiO}_{2}$ , but leaves a weak long component, we observe that for the long time case, the excitonic bleach survives for significantly longer time than in bare ${TiO}_{2}$ NPs [Fig. 5(b)]. The short time decay is probably due to injected electrons that go back to the Au NP, as also suggested by Borgwardt et al. [27], while the long-time component in their work may correspond to the one we observe here.

3. CONCLUSION

This paper reports on two remarkable observations: first, TPA in ${TiO}_{2}$ occurs, and is initiated already at mild fluences; and, second, a longer-lived excitonic bleach of ${TiO}_{2}$ compared to the bare NPs, implying that the presence of the Au core “blocks” the electron–hole recombination in the ${TiO}_{2}$ shell. It is not clear what mechanism could be behind this behavior, as the charge carrier dynamics in the Au core is over, while the bleach of the excitonic band goes on up to the time limit of our detection. Nevertheless, that electrons/holes in the CB/VB are present for a long time deserves further investigation as it opens perspectives in terms of charge storage in a TMO, with possible applications in the field of TCO and photocatalysis.

Acknowledgment

Acknowledgment. The authors thank Dr. L. Mewes, Dr. M. Puppin, and B. Bauer for helpful discussions. L.W. acknowledges support from the China Scholarship Council (CSC).

APPENDIX A: SAMPLE PREPARATION

1 M = 1 m o l / L

{NH}_{3} \cdot H_{2} O

Au / {TiO}_{2}

Au / {SiO}_{2}

Figure 10.Transmission electron microscopy images of (a) pure Au NPs, (b) $Au / {SiO}_{2}$ NPs, and (c) $Au / {TiO}_{2}$ NPs. Below is a diagram of the ultrafast broadband deep-to-near-UV spectroscopy setup, detailing the broadband pump-probe experiment after the noncollinear optical parametric amplifier. The blue dashed box shows the schematic of the achromatic doubling, adapted from Ref. [43]: BS, beam splitter; M, mirror; MM, multilayer mirror; CM, chirped mirror; SM, spherical mirror; PM, off-axis parabolic mirror; P, prism; L, lens; F, multimode fiber.

APPENDIX B: EXPERIMENTAL PROCEDURES AND METHODS

< 20 fs

E = h c / λ

G (t) = \frac{1}{σ \sqrt{2 π}} {\exp (- \frac{t^{2}}{2 σ^{2}})}^{},

APPENDIX C: FLUENCE-DEPENDENCE MEASUREMENT AND CORRECTION OF THE EXCITATION INTENSITY

The fluence-dependence experiments were performed on the same deep-UV TA setup, measuring the TA at specific time delays between pump and probe, in a range of pump fluences.

\sim 9.9 ps

F

Au / {SiO}_{2}

APPENDIX D: ESTIMATION OF THE TPA ENHANCEMENT FACTOR

\frac{d I_{0}}{d z} = - β I_{0}^{2},

I

Δ A = A - A_{0} = l g \frac{I_{0}}{I} = l g \frac{1 + β l I_{0}}{1} .

β

Hence, System.Xml.XmlElementSystem.Xml.XmlElementSystem.Xml.XmlElementSystem.Xml.XmlElement

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