Large Transient Optical Modulation of EpsilonNear-Zero Colloidal Nanocrystals Benjamin T. Diroll,† Peijun Guo,‡ Robert P. H. Chang,‡ and Richard D. Schaller*,†,§ †
Center for Nanoscale Materials, Argonne National Laboratory, Lemont, Illinois 60439, United States Department of Materials Science and Engineering and §Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
‡
S Supporting Information *
ABSTRACT: Epsilon-near-zero materials may be synthesized as colloidal nanocrystals which display large magnitude subpicosecond switching of infrared localized surface plasmon resonances. Such nanocrystals offer a solution-processable, scalable source of tunable metamaterials compatible with arbitrary substrates. Under intraband excitation, these nanocrystals display a red-shift of the plasmon feature arising from the low electron heat capacities and conduction band nonparabolicity of the oxide. Under interband pumping, they show in an ultrafast blueshift of the plasmon resonance due to transient increases in the carrier density. Combined with their high-quality factor, large changes in relative transmittance (+86%) and index of refraction (+85%) at modest control fluences (2 ps) signal, which routinely appears in LSPR dynamics of noble metals and plasmonic chalcogenides with an intensity roughly 10−20% of the peak bleach feature, depending on the pump fluence.40−42 In switching applications, any signal at long time constrains the potential operation frequency. In particular, subpicosecond transients with negligible signal at longer times enables a high-fidelity switch at terahertz frequencies. Figure 2a shows that an expansion of the signal with 210 μJ/ cm2 incident power on 16.3%-doped ICO at longer delay times
Ce(Te) CL
dTe = −G(Te − TL) dt
dTL = G(Te − TL) + G L(TL − T0) dt
Intraband excitation raises the temperature of electrons and the lattice according to their specific heat capacities (Ce(Te) and CL, respectively). In contrast to earlier works,27,42 we compute the temperature-dependent electron heat capacity using the total energy density of the electron gas (see Supporting Information), which enables a more accurate description of transient optical changes, particularly at high electron temperatures. As described above, rapid heating of electrons within the nonparabolic conduction band of cadmium oxide generates a large ΔOD signal as the effective mass of electrons comprising the LSPR increases significantly, yielding a red-shifted LSPR feature. Figure 3b shows the experimentally measured transient dynamics of a spin-cast 10.2%-doped FICO film excited at the LSPR feature for several different pump powers overlaid in solid lines with the results of our TTM. The maximal bleach features of the transient signals displayed in Figure 3b show a relative increase in transmission of 87% and an absolute increase of 20.2%, from 23.3% (calculated from the static extinction spectrum) to 43.5% (maximum of the transient extinction spectrum) at the strongest measured bleach wavelength (2170 nm). As demonstrated by the inset in Figure 3b, the increase in ΔOD with pump fluence is linear, suggesting that the maximum modulation of transmittance can likely be further increased. The decay time, attributed to electron− phonon coupling, increases with fluence, but proportionally less than noble metals (see Supporting Information Figure S7).35 Complementing the large increase in transmittance is a large induced extinction to the red, representing a relative decrease of
Figure 2. (a) Comparison of the prompt signal, generated from pump scatter from a glass microscope slide (black) and the normalized bleach decay of a solution of 16.3%-doped ICO NCs. The thin red line indicates the signal level at long time, multiplied by 100. The plot is labeled with the processes typically attributed to the transient signal decay.42 Rise times and decay times of the bleach features induced by intraband pumping of ICO samples plotted against (b) the wavelength of the LSPR feature and (c) particle diameter measured from TEM. The dashed line in (b) represents the instrument response function (IRF).
is present but accounts for 2 ps) is very weak. Although changes in environment might be expected to alter this phonon−phonon relaxation time, we have not explored this phenomenon in this work. The advantages of ENZ cadmium oxide NCs for optical switching with minimal “leakage” signal are particularly apparent at modest control fluences: electron temperatures of up to 2000 K result in lattice heating of just ∼3 K. Although strong bleach features apparent in Figure 3b arise naturally from a microscopic picture in which intraband excitation easily heats electrons due to their low heat capacity, a far-field optical view also leads to similar conclusions. The ENZ region of a bulk material determines the LSPR wavelength of a resulting nanoparticle of the same, modulated by shape,
size, and medium effects. Doped cadmium oxide shows ENZ behavior in the near-IR as expected for a Drude metal with carrier density of 1019−1021 cm−3. The permittivity and index of refraction computed for the 10.2%-doped FICO sample are plotted in Figure 3d,e, respectively, based upon fitting of the room-temperature extinction spectrum and the subsequent modulation of the Drude function to reflect changes in the electron temperature with intraband pumping. The ENZ region, which signals a transition from dielectric (ε′ > 0) to metallic (ε′ < 0) behavior, for this sample is near 1300 nm at 300 K. As previously demonstrated with ITO and AZO films, ENZ materials can give rise to large optical modulation of transmission and reflectivity because absolute transient changes in real permittivity or refractive index are proportionately larger in the ENZ region.24,27 Using the known conduction band nonparabolicity of cadmium oxide, which provides electrontemperature dependence of the carrier effective mass, we determine the change in the dielectric function of doped cadmium oxide as a function of electron temperature. The real permittivity (Figure 3d) and index of refraction (Figure 3e) are shown for the peak electron temperatures achieved at different intraband excitation powers. The spectrally resolved value of Δn for cadmium oxide as a function of time for 4.7 mJ/cm2 excitation is shown in Figure 3f. The transient increase in electron temperature red-shifts the plasma frequency, which generates a large Δn in the ENZ region. The increase in n reaches 0.4 in absolute terms and up to 85% in relative terms without saturation of Δn (Supporting Information Figure S6). Not only do our measurements indicate large, ultrafast changes in transmittance under intraband excitation, they also suggest that colloidal NCs, especially printed in periodic arrays, are 10102
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Figure 4. (a) Map of the transient extinction signal (ΔOD) of 10.2% InF3-doped cadmium oxide NCs plotted versus wavelength and time for an interband pump power of 7.46 mJ/cm2 at 267 nm. (b) Transient extinction spectra collected at fixed delay time (500 fs) with increasing power using 267 nm pump pulses. (c) Time-resolved transient absorption dynamics at the specified pump powers normalized at the maximum in ΔOD. (d) Time-dependent transient extinction spectra of 10.2% InF3-doped cadmium oxide NCs at a fixed pump power of 10.3 mJ/cm2.
promising candidates for transient wavefront engineering and beam steering at low control fluences. Transient Optical Response with Interband Excitation. In addition to modulation of the transmittance with intraband excitation, the plasmon response of cadmium oxide NCs can also be optically controlled using interband excitation. In this case, ultraviolet 267 nm excitation is used to excite a colloidal solution of 10.2% InF3-doped NCs with the resulting typical transient extinction data shown in Figure 4a. This excitation is sufficiently high in energy (see Supporting Information Figure S10) that, unlike noble metals,46 specific isolated bands (e.g., d-band in gold or silver) are unlikely to be excited. Interband excitation generates an increase in carrier concentration (photodoping) as well as carrier heating. Carrier generation dominates carrier heating effects overall, which blueshifts (and slightly broadens) the LSPR owing to increased total dopant level as reflected in the NIR transient extinction spectra collected at a fixed delay time in Figure 4b. As the number of photoinduced carriers increases, the LSPR shifts increasingly to the blue. The observed blueshift, observed as the positive lobe of a derivative line shape in Figure 4b,d, at the maximal pump fluence of 8.4 mJ/cm2 corresponds to an increase of ∼1200 free electrons per NC, or equivalently 8 × 1020 cm−3, determined only by a change in the electron concentration, disregarding possible heating. (See also Supporting Information Figures S10 and S11.) Interestingly, in both absolute and relative terms, interband pumping results in smaller changes to the transient extinction signal than intraband excitation. At 8.4 mJ/cm2 with 267 nm excitation, at which the NCs have an extinction which is one-half the magnitude of LSPR maximum, the maximal change in transmission observed is a relative change of −9.4% and an absolute decrease of −6.2% at 1438 nm. Although interband excitation also heats the carriers in the NC, the time-domain response of the transient extinction signal
is slower for interband excitation (Figure 4c) than for intraband excitation, dominated by carrier recombination. This is confirmed by the similarity of decay dynamics of the photoinduced extinction of the plasmon and indirect band gap absorption in the visible47 (Supporting Information Figure S11). The dynamics upon interband pumping show little power dependence over nearly an order of magnitude in fluence consistent with previous literature on ITO nanorod arrays,48 with a decay constant of 1.5 ± 0.1 ps. As photoinduced carriers recombine, the transient-induced extinction shifts according to the new carrier density of the NCs. At the same time, recombination changes the carrier density of the NCs. Figure 4d shows the red-shift of the LSPR feature as the carrier density falls from the maximum value of 2.7 × 1021 cm−3 at the earliest time back to the steady-state value of 1.6 × 1021 when excited at 10.3 mJ/cm2.
CONCLUSIONS In conclusion, we have demonstrated that colloidal NCs composed of heavily doped oxides with ENZ wavelengths tunable throughout the IR are excellent candidates for narrowband all-optical switching and beam steering. The optical response of tunable IR LSPRs on colloidal NCs can be red-shifted under intraband excitation and blue-shifted under interband excitation. Conduction band nonparabolicity and low electron heat capacity engineer a large nonlinear optical response, reaching a relative transmittance modulation and increase of index of refraction of more than 80% with subpicosecond recovery times and negligible lattice heating, enabling terahertz-frequency switching speeds. By exploiting colloidal dispersibility and low-temperature deposition, heavily doped oxide NCs are a promising, scalable source of metamaterials for optical switching and information processing technologies compatible other optoelectronic devices. 10103
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EXPERIMENTAL SECTION Materials. Cadmium acetylacetonate (99.9%), indium acetate (99.99%), indium fluoride (99.9%), tin(IV) acetate, octadecene (90%), and oleic acid (90%) were purchased from Sigma-Aldrich and used as delivered. All solvents were ACS grade or higher. Synthesis of Colloidal Nanocrystals (NCs). Synthesis of doped cadmium oxide NCs was performed using slightly modified literature protocols.14−16,19 Briefly, for the synthesis of indium-doped cadmium oxide, 1 mmol total metal content of cadmium acetylacetonate and indium acetate (1−20 mol % indium) was combined in a 50 mL threeneck flask with 5 mmol oleic acid and 25 mL of ODE. The mixture was heated under vacuum (∼1 Torr) to 125 °C and held for 1 h, becoming a homogeneous solution. Then, the solution was heated under nitrogen to reflux, ca. 316 °C, and held until the clear solution changed to greenish-brown, indicating the nucleation of particles. Then, the solution was heated further for 10 min, and cooled to room temperature. Purification was performed by precipitation three times with isopropanol and redispersion of the NCs in hexanes, which was centrifuged to remove any residual metal precipitate. For Sn-, In-, and F-doped samples, the dopant precursor was changed to tin(IV) acetate and InF3, respectively, and the amount of oleic acid used was 3.2 mmol. Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). ICP-OES elemental analysis was performed using an Agilent 700 Series ICP-OES. Samples were dissolved in pure nitric acid and then diluted with distilled water to 0.5−50 ppm metals. Steady-State Spectroscopy. Extinction spectra were collected using cuvettes or thin films on a Nicolet 6700 FT-IR or a PerkinElmer Lambda 950 spectrophotometer. Transient Extinction Spectroscopy. Mid-IR transient extinction spectra were collected by using the output of two OPAs, each of which took part of a split fundamental beam from a Ti:sapphire laser. The first OPA served as the pump source, and the second OPA was used to generate a MIR probe. Visible and NIR TA experiments were performed using white light generation through a CaF2 and sapphire crystal, respectively. The excitation source for UV-pumping experiments was a frequency tripled 267 nm beam. Modeling. A complete detail of modeling can be found in the Supporting Information file.
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b05116. Details of modeling as well as additional experimental data (PDF)
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was performed, in part, at the Center for Nanoscale Materials, a U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences User Facility under contract no. DE-AC02-06CH11357. P.G. and R.P.H.C. were funded by the MRSEC program (NSF DMR-1121262) at Northwestern University. REFERENCES (1) Naik, G. V.; Shalaev, V. M.; Boltasseva, A. Alternative Plasmonic Materials: Beyond Gold and Silver. Adv. Mater. 2013, 25, 3264−3294. 10104
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