Laser-Induced Spectral Hole-Burning through a Broadband

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Laser-Induced Spectral Hole-Burning Through a Broadband Distribution of Au Nanorods Christopher J. DeSantis, Da Huang, Hui Zhang, Nathaniel John Hogan, Hangqi Zhao, Yifei Zhang, Alejandro Manjavacas, Yue Zhang, Wei-Shun Chang, Peter Nordlander, Stephan Link, and Naomi J. Halas J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 30 Sep 2015 Downloaded from http://pubs.acs.org on October 5, 2015

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Laser-Induced Spectral Hole-Burning Through a Broadband Distribution of Au Nanorods Christopher J. DeSantis†§, Da Huang†‡, Hui Zhang§, Nathaniel J. Hogan§, Hangqi Zhao§, Yifei Zhang‡, Alejandro Manjavacas§, Yue Zhang‡, Wei-Shun Chang‡, Peter Nordlander§, Stephan Link‡, and Naomi J. Halas§* AUTHOR ADDRESS ‡Department of Chemistry, and §Department of Electrical and Computer Engineering, Laboratory for Nanophotonics, Rice University, Houston, Texas 77005, United States

ABSTRACT:

Nanorods are amenable to laser-induced reshaping, a process that can

dramatically modify their shape and therefore their plasmonic properties. Here we show that when a broadband spectral distribution of nanorods is irradiated with a femtosecond-pulsed laser, an optical transmission window is formed in the extinction spectrum.

Surprisingly, the

transmission window that is created does not occur at the laser wavelength, but rather is consistently shifted to longer wavelengths, and the optical extinction on the short-wavelength side of the transmission window is increased by the hole-burning process. The laser irradiation results in a wavelength-dependent partial reshaping of the nanorods, creating a range of unusual nanoparticle morphologies. We develop a straightforward theoretical model that explains how the spectral position, depth and width of the laser-induced transmission window are controlled by laser irradiation conditions. This work serves as an initial example of laser-based processing of

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specially designed nanocomposite media to create new materials with “written-in” optical transmission characteristics.

KEYWORDS: Optical transmission window, localized surface plasmon resonance, laser reshaping, nanorods, band-pass filter

INTRODUCTION Optical transmission windows, essentially band-pass filters at optical frequencies, allow the transmission of light in specific wavelength ranges, while preventing the transmission of other wavelengths either through scattering or through absorption. The terrestrial atmosphere contains broad transmission windows that block high-energy photons, such as X-rays, and other specific wavelengths due to the fundamental excitations of atmospheric molecules, transmitting much of the visible region of the spectrum in an unimpeded manner.1,2 Complex biological fluids, such as human blood, have narrow optical transmission windows in the near-infrared region of the spectrum that have been exploited for a large range of biomedical photonics applications.3 To design and create optical transmission windows at specific frequencies, and with predetermined spectral widths, is a significant challenge. Plasmonic nanoparticles, with their strong size and geometry-dependent absorption and scattering cross sections due to their resonant response, provide highly promising building blocks for the development of media with these highly tailored optical properties.4–6 A particularly desirable capability would be to “write” a transmission window by physical means, at a wavelength of choice, rather than design a complex, passive mixture of nanoparticles with a pre-selected transmission window already in place.


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Here we show that an optical transmission window can be induced by laser irradiation in a broadband medium consisting of a specially designed distribution of nanorod absorbers. Following the design and synthesis of a nanorod suspension whose spectrally broad extinction spanned the visible and near-IR region of the spectrum, laser irradiation was used to induce an optical transmission window through photothermal reshaping of specific subpopulations of nanorods. Interestingly, the window minimum does not occur at the resonant laser wavelength, but instead shows a substantial redshift, which we attribute to the partial reshaping of the nanorods at and near the laser frequency. We examined how the characteristics of the laserinduced window are adjustable by controlling and varying laser power, resonance position, and irradiation time. Through theoretical modeling and timed irradiation studies, we developed a way to accurately predict the spectral position of the transmission window with respect to the resonant laser wavelength.


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Figure 1. A. UV—visible spectra for the eleven nanorod component samples included in our nanorod-based broadband absorber. B. Extinction spectra for the eleven nanorod components at


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the relative concentrations required to generate a spectral plateau, the simulated linear combination of those spectra (solid line), and the experimental spectrum (dashed line) generated using the appropriate concentrations. C. Tabulated values of the fraction of each nanorod component of the mixture needed to generate the spectral plateau shown in B.

RESULTS & DISUSSION The design and assembly of the broadband absorber consisting of a distribution of nanorods is shown in Figure 1. First, we prepared a colloidal suspension of nanorods with a broadband spectral plateau between 600-1100 nm by mixing Au nanorod batches of eleven different sizes. The Au nanorod distribution was prepared using the Ag-assisted seed-mediated method.7,8 The optical extinction spectra of the individual components of the nanorod mixture used to generate the broadband absorber are shown in Figure 1A. Using a linear combination of normalized extinction spectra of the different nanorod components, we calculated the relative concentrations for each component that would result in a spectrum with nearly the same extinction intensity across the wavelength range from 600-1100 nm. This distribution yielded a relatively flat spectral plateau (Fig. 1B) with a small residual spectral maximum present at 520 nm, corresponding to the transverse plasmon mode of the nanorods. The experimental broadband spectrum obtained by combining nanorods in relative quantities according to the normalized, calculated values (Fig. 1C) was just slightly more uniform than the predicted spectrum (Fig. 1B). This is most likely due to slight discrepancies in lineshape for some of the nanorod component samples.


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Figure 2. A. UV—visible spectra of a mixture of nanorods before (blue) and after (red) irradiation with an 800 nm laser. B. Corresponding histograms of aspect ratio measurements for the nanorod mixture before (blue) and after (red) irradiation, indicating the extent to which reshaping has been induced. C. Sequential UV—visible spectra of a mixture of Au nanorods at specified time intervals during laser irradiation. D. Change in extinction versus time at 753 nm and 846 nm, corresponding to i and ii in C, respectively, as well as the change in peak width for the spectral window (indicated by iii in C) as a function of time.


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The nanorod mixture was irradiated with an unamplified 800 nm ultrafast laser (Chameleon Ultra II, Coherent) with an average power of 0.45 W, a pulsewidth of 150 femtoseconds, a pulse energy density of 0.91 mJ/cm2, a spot size of 28 µm, and a repetition rate of 80 MHz (Supporting Information Figure S1). The changes induced in the optical spectrum by laser irradiation and the corresponding modifications of the nanorod aspect ratios in the distribution are shown in Figure 2. Following laser irradiation, a clear decrease in extinction intensity for the broadband nanorod mixture between 800-870 nm was observed (Fig. 2A). The extinction at the dip minimum of 850 nm was 56% of the extinction of the spectral plateau, while the full width half minimum (FWHM) of the laser-induced window was 160 nm, far broader than the FWHM of the incident laser used for irradiation (7 nm). Statistical analysis of scanning electron microscope (SEM) images of the nanorod distributions before and after laser irradiation was performed (Fig. 2B). Histograms were obtained by measuring the length and width of 300 nanorods in SEM images obtained both before and after laser irradiation. This analysis revealed a decrease in the number of nanorods with an aspect ratio between 2.0-4.5 and an increase in the number of nanorods with an aspect below 2 and between 4.5-6.0 (Supporting Information Figure S2). Our theoretical calculations (using Gans theory) also indicate that a decrease in nanorods with aspect ratios between 2-4.5 would result in a transparency window near 800 nm (Supporting Information Figure S3). Although the transmission window depth induced in the extinction spectrum by laser irradiation is comparable to previous theoretical simulations of passively formed transmission windows obtained by combining specific nanorod mixtures,9 the width of the laser-induced window is substantially broader than what would be obtained by removal of a single component of the nanorod mixture (Supporting Information Figure S4). Upon closer


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inspection, we observed that the laser-induced evolution of the transmission window is quite complex (Fig. 2C). The laser-induced reshaping process results in: (i) an increased absorption on the short-wavelength side of the laser wavelength, (ii) a transmission decrease that, even at its earliest stages, is substantially redshifted from the laser wavelength, and whose minimum shifts with increasing laser irradiation time, and (iii) a FWHM of the laser-induced transmission window that increases by almost a factor of two following its initial appearance, accompanied by a substantial reshaping of the transmission window at later irradiation times. This evolution suggests that laser-induced spectral hole-burning under these conditions is a complex process. Processes (i) and (ii), the increase in extinction at 753 nm and the decrease in extinction at 846 nm, were observed within 5 minutes of irradiation (Fig. 2C). The timescale of the entire reshaping process observed here is controlled by sample mixing, since each nanorod in the 0.5 ml solution must move into the 28 µm radius of the beam spot in order to the reshape. Measurements past 3 hours are close to the asymptote of the curves, indicating a depletion of nanorods available for reshaping. Even at this longer timescale for laser irradiation, the absorbance minimum is still nonzero, indicating that the nanorods are generally undergoing partial reshaping but not fully reshaping into spheres, which would remove their extinction entirely from this region of wavelengths. To corroborate the partial-reshaping mechanism, we increased the laser power to 27.1 mJ/cm2 and found that the extinction intensity for wavelengths greater than 800 nm drops to zero, consistent with a full reshaping of nanorods to spheres due to melting (Supporting Information Figure S5).10–12 Thus, the conditions for generating an optical transparency window as shown in Figure 2 are most likely inducing photothermal reshaping, not complete melting, of the resonant nanorod constituents.


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Figure 3. UV—visible spectra (A, C, E, G) of Au nanorod solutions before and during laser irradiation at 800 nm and corresponding histogram measurements (B, D, F, H, where blue is before and red is after) for samples with initial longitudinal resonances at (A-B) 740 nm, (C-D) 814 nm, (E-F) 900 nm, and (G-H) 1020 nm. Arrows indicate the change in extinction profile over time.


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To investigate how detuning of the nanorod plasmon resonance from the irradiation wavelength affects spectral hole-burning, we irradiated four individual solutions of nanorods with longitudinal plasmon resonances at 740 nm, 814 nm, 900 nm, and 1051 nm, spanning the wavelength region corresponding to the irradiation laser frequency and the spectral hole-burning response (Figure 3, Supporting Information Figure S6). For each of these measurements, the identical irradiation and stirring conditions as those of the laser-induced hole burning experiments (Figure 2) were used. Nanorods with their plasmon resonance at 740 nm (Fig. 3A) showed no change in extinction spectrum upon irradiation, in agreement with the unmodified aspect ratio observed from SEM statistics before and after laser treatment (Fig. 3B). Previously reported irradiation experiments where nanorods of the same aspect ratio as those used here showed substantial reshaping. However, those experiments were performed under very different illumination conditions.13 The 814 nm nanorod resonant sample showed substantial modification in its optical spectrum upon laser irradiation, where the extinction spectrum was both blueshifted and broadened substantially (Fig. 3C). This corresponded to a decreased aspect ratio for the illuminated nanorods (Fig. 3D). The extinction spectrum of the 900 nm resonant nanorods was modified most dramatically: upon irradiation, the spectrum partially blueshifted, widened broadly, and appeared to show two distinct spectral features following laser irradiation (Fig. 3E). The histogram of nanorod aspect ratio in this case showed the appearance of two distinct aspect ratio maxima of nanorods following illumination. This bimodal distribution of nanorod aspect ratios was also observed for irradiated nanorods with a longitudinal plasmon resonance redshifted with respect to an amplified femtosecond laser source.14 For this case (Fig. 3E), where the nanorod plasmon resonance is substantially detuned from the laser wavelength, it is possible that the lower thermal stability of longer aspect ratio nanorods relative to shorter aspect ratio


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nanorods may result in a greater likelihood of reshaping. This result may also indicate the extent of the detuning range for the nanorod resonance with respect to the laser wavelength beyond which reshaping will no longer occur under these conditions. For both the 814 nm and 900 nm resonant cases, the reshaped nanorod histogram peaks at an aspect ratio of ~3.4, which corresponds to a plasmon resonance sufficiently blueshifted from the laser wavelength so that further reshaping will no longer occur.

The extinction spectrum for nanorods with their

longitudinal plasmon resonance at 1020 nm was virtually unchanged upon laser irradiation (Fig. 3G), with only minimal changes observed in the SEM histogram (Fig. 3H).

Figure 4. TEM images of nanorods before (A, D, G, J) and after (B-C, E-F, H-I, K) irradiation with an 800 nm laser with an initial resonance of 740 nm (A-C), d-f), 814 nm (D-F), 900 nm (GI), and 1020 nm (J-K).


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A closer look at the four cases of nanorod reshaping shown in Figure 3 reveals some unusual and interesting features of the irradiated nanorods (Fig. 4). For the nanorod sample whose resonance was at 740 nm, blueshifted with respect to the laser frequency (Fig. 4A), we observe that, although the aspect ratio of the nanorods is essentially unchanged by irradiation, other, more subtle changes in nanorod morphology occur. In this case we see a consistent “roughening” of the nanorod morphology on a length scale far smaller than the dimensions of the nanorod (Fig. 4B, C). This morphological change may be a reshaping mediated by the chemical constituents at the nanorod surface or near-field mediated ablation.15–17 The 814 nm resonant nanorods (Fig. 4D) show a more dramatic reshaping under laser irradiation (Fig. 4E, F). For this sample, we observed some fragmentation (Fig. 4E), although the majority of the nanorods do not fragment as the pulse energy density of 2.03 mJ/cm2 is far less than the 560 mJ/cm2 needed for extensive fragmentation.18 In addition, we see the appearance of sickle-shaped nanoparticle morphologies following laser irradiation, often spanned by what appears to be a single domain boundary (Fig. 4F), a morphology predicted in low-temperature simulations and previously observed under other irradiation conditions.14,19–21 For redshifted nanorods with a 900 nm resonant plasmon mode, yet another unusual morphology begins to appear consistently. In addition to the nanorod aspect ratio of the non-irradiated sample (Fig. 4G), a cohort of larger aspect ratio nanorods is induced by laser irradiation that is substantially narrower (Fig. 4H,I) than the non-irradiated nanorods, consistent with the observed increase in aspect ratio for the mixture of nanorods (Figure 2B and Figure S2). For the 1020 nm resonant nanorod sample, no variation in nanoparticle morphology was observable when comparing images before (Fig. 4J) or after (Fig. 4K) laser irradiation.


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Figure 5. a) Simulated extinction spectrum of sixteen samples of Au nanorods at different aspect

ratios before (top) and after (bottom) 800 nm laser irradiation. Blue lines indicate the linear combination of the sixteen samples. b) Plot of energy density versus aspect ratio for q, the reshaping energy density (solid line) and η, the absorbed energy density (dashed lines) of the


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sixteen simulated samples (inset: reshaping energy density versus aspect ratio of the sixteen nanorod samples plotted to better show its dependence on nanorod aspect ratio). c) Simulated extinction spectrum before (blue) and after (red) laser irradiation at 800 nm.

The primary characteristic of laser-induced spectral hole burning with nanorods is the substantial redshift of the transmission window relative to the laser wavelength. Based on the trends observed from the size and extinction spectrum measurements of the monodisperse samples (Fig. 3), we attribute this redshift to two primary factors. First, if the Au nanorods absorb sufficient energy to undergo reshaping, their aspect ratios will decrease, blue-shifting their longitudinal extinction maxima (Figure 3C,E).13,14,22 Second, because nanorods with higher aspect ratios have more surface energy than those with lower aspect ratios, higher aspect ratio nanorods should have a relatively higher susceptibility of reshaping during laser irradiation (Figure 3E).23–25 To understand the first factor, we employed Gans theory to simulate the extinction spectra of sixteen nanorod samples which, when combined, form a spectral plateau analogous to the assembled broadband nanorod distribution (Figure 5A, top). Then, we determined which nanorods would absorb sufficient energy from the laser at 800 nm to reshape (Figure 5B). To do so, we first calculated the absorbed energy density, η, for each nanorod component in our distribution. The energy density η is given by:

𝜂𝜂 =

𝐼𝐼 𝐶𝐶 𝑉𝑉𝑉𝑉

where I is the laser fluence, C is the longitudinal absorption cross section at 800 nm of each nanorod aspect ratio represented, V is the volume of each nanorod component in this distribution, and ρ is the density of Au. If η is larger than the reshaping energy density q, which


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is some fraction of the bulk melting energy density, then the nanorod at that aspect ratio will reshape. We approximate q as an analytical function of aspect ratio (Fig. 5B, inset), which follows the same sigmoidal trend obtained as when irradiating Au nanorods on a substrate with a single laser pulse.24 We fix the average of q over our range of aspect ratios to be approximately 40% of the bulk melting energy density, as suggested by experiments.26 Setting the laser fluence to be the same as the experiments at 0.91 mJ/cm2, we calculate η for each aspect ratio, using Gans theory. For the aspect ratio range of 3.7-5.0, η is greater than q and reshaping should occur (Fig. 5B). Since the nanorods in our experiments undergo more than 10000 pulses per rod, we expect the nanorod solution to reach a steady-state equilibrium. For this reason, the six samples within the range that is expected to reshape were set to an aspect ratio of 3.7 after reshaping (Figure 5A, bottom) corresponding to a 740 nm resonance and consistent with the monodisperse sample measurements shown in Figure 3. Linearly combining the spectra of the simulated samples after the reshaping process at the same proportions of the spectral plateau produces an optical window that is redshifted with respect to the laser-resonant wavelength (Fig. 5C). On the short wavelength side of the optical window an absorption maximum appears, which is due to the increased 3.7 aspect ratio nanorod component induced by laser irradiation. The simulated window is in good agreement with experiment (Figure 2) and demonstrates the predictability of the location of the optical transparency window under long irradiation times. We used these theoretical guidelines to predict a transmission window at ~1050 nm when irradiating with a laser at 1000 nm, demonstrating the modifiability of the window minima position (Supporting Information S7). A general theory under weak laser irradiation process, either for short irradiation times or single-pulse irradiation, is provided in the Supporting Information (Figure


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S8). We note, however, that these models are only generalizable under conditions in which the vast majority of nanorods reshape and do not fracture or agglomerate. CONCLUSIONS In summary, we have developed a method to “write” an optical transmission window in a broadband distribution of nanorods by laser irradiation. The relative ease and programmability of selecting a window at any wavelength of choice makes this approach practical, manufacturable, and widely applicable. We have developed a theoretical model that captures the characteristics of this process, and that could be used to predict spectral hole-burning properties at other wavelengths for a nanorod-constituent broadband absorber starting material.

This

combination of laser-induced reshaping of tailored nanoparticle distributions may, quite generally, open the door to simple, scalable methods for the lineshape engineering of complex optical materials with properties not found in natural media.

AUTHOR INFORMATION Corresponding Author Naomi J. Halas, [email protected]; Stephan Link, [email protected].

Author Contributions †These authors contributed equally. Funding Sources


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This work was supported by the Army Research Office under grant W911NF-12-1-0407 and by the Robert A. Welch Foundation under grants C-1220 (N.J.H), C-1222 (P.N.) and C-1664 (S.L).

ACKNOWLEDGMENT We thank the Weisman laboratory for access to their UV—Visible spectrophotometer and Chao Zhang for assistance in laser use.

SUPPORTING INFORMATION AVAILABLE Synthesis procedures, spectral plateau generation, photothermal reshaping set-up, characterization details, additional images and histogram measurements are available free of charge via the Internet at http://pubs.acs.org.

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Laser-Induced Spectral Hole-Burning through a Broadband

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