Brillouin Oscillations from Single Au Nanoplate Opto-Acoustic

Jun 26, 2017 - The increased damping is attributed to diffraction of the acoustic wave as ... frequency and spatial resolution in Brillouin oscillatio...
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Brillouin Oscillations from Single Au Nanoplate Opto-Acoustic Transducers Kuai Yu,*,§,† Tuphan Devkota,§,‡ Gary Beane,‡ Guo Ping Wang,*,† and Gregory V. Hartland*,‡ †

College of Electronic Science and Technology, Shenzhen University, Shenzhen 518060, P. R. China Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States

‡

S Supporting Information *

ABSTRACT: Brillouin oscillations, which are GHz frequency waves that arise from the interaction of light with acoustic waves, are experiencing increasing applications in biology and materials science. They provide information about the speed of sound and refractive index of the material they propagate in, and have recently been used in imaging applications. In the current study, Brillouin oscillations are observed through ultrafast transient reflectivity measurements using chemically synthesized Au nanoplates as opto-acoustic transducers. The Au nanoplates are semitransparent, which allows the Brillouin oscillations to be observed from material on both sides of the plate. The measured frequencies are consistent with the values expected from the speeds of sound in the different materials, however, the attenuation constants are much larger than those reported in previous studies. The increased damping is attributed to diffraction of the acoustic wave as it propagates away from the excitation region. This effect is more significant for experiments with high numerical aperture objectives. These results are important for understanding the relationship between frequency and spatial resolution in Brillouin oscillation microscopy. KEYWORDS: Brillouin oscillations, acoustic waves, ultrafast pump−probe spectroscopy, opto-acoustic transducer, sound velocity, Au nanoplate

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experiments, and in this case the frequencies provide information about the properties of the particles rather than the medium.27 Interestingly, the decay of both the Brillouin oscillations and the localized acoustic modes of metal nanoparticles have contributions from energy dissipation in the medium.28−33 In addition to spatial resolution, a critical issue for experiments where Brillouin oscillations are used for imaging applications is frequency resolution. The speeds of sound and refractive indexes of many soft matter systems are similar, which means that their Brillouin oscillation frequencies are also similar. Here we explore the spatial and frequency resolution issues through ultrafast transient reflectivity measurements on single Au nanoplates. These experiments were used to characterize acoustic wave generation and attenuation in several different environments. The Au nanoplates examined are partially transparent and provide the ability to study Brillouin oscillations from materials on both sides of the plate. The small size of the nanoplates examined in the experiments also makes them a model system for understanding localized phonon

pto-acoustic transducers that enable efficient coupling between photons and acoustic phonons are important in a number of emerging applications, including as the active elements in phonon lasers,1−3 and for imaging the mechanical properties of soft matter systems.4−11 In imaging experiments time domain measurements are typically used, where femtosecond laser pulses generate and detect the propagating acoustic waves.12−21 In these experiments the pump laser generates a transient stress in the sample, that launches acoustic waves into the surroundings. Reflection of the probe laser from the traveling wavefront creates an interference, which gives an oscillation in the transient reflectivity signal. The frequency of these so-called Brillouin oscillations depends on the refractive index and speed of sound of the material, thus, these measurements provide a way of differentiating different types of materials.12−19 The spatial resolution in imaging experiments depends on a number of factors that include the numerical aperture of the focusing lens and the size of the transducer, which is usually metal. Several groups have examined the effect of reducing the size of the transducer to the nanoscale region to improve imaging resolution.22−24 When nanoparticles are used a complicating issue for these experiments is that the ultrafast laser can also excite localized acoustic vibrations of the particles.25,26 These vibrations also give rise to oscillations in transient reflectivity © 2017 American Chemical Society

Received: April 18, 2017 Accepted: June 26, 2017 Published: June 26, 2017 8064

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ACS Nano sources.22−24 The nanoplates themselves show GHz acoustic vibrations, which can be suppressed through the judicious choice of the probe wavelength. The frequency and attenuation constants for the Brillouin oscillations were measured, and the results show that larger attenuation constants are observed when using high numerical aperture (NA) objectives. The larger attenuation is attributed to diffraction of the acoustic wave. This has consequences for trying to differentiate materials that have similar properties. The results from these experiments bring out a fundamental limitation for using high NA objectives and Brillouin oscillation measurements to image complex systems, specifically, that there is a trade-off between frequency and spatial resolution in Brillouin oscillation microscopy.

the two reflected beams. This creates an oscillation in the transient reflectivity signal of cos(kd) that depends on the path length difference d between the two beams and the wavevector k of the probe beam. Note that the Au nanoplates in our measurements are thin enough that the Brillouin oscillations can also be detected after the probe beam has passed through the plates.34 This situation is shown in Figure 1b. In this case the detected probe beam contains reflections from both Au interfaces as well as the strain wave. Figure 1c shows a more detailed view of the path length difference for the two reflected probe beams. At time t the acoustic wave has moved a distance vlt from the substrate where vl is the longitudinal sound velocity in the medium. This creates a path length difference of vlt(cos 2φ + 1)/cos φ = 2vlt cos φ where φ is the internal angle of incidence. The interference term is, thus, cos(2kvlt cos φ) = cos(2πf bt), where f b = 2vln cos φ/λpr is the Brillouin frequency, n is the refractive index of the medium and λpr is the probe wavelength.35,36 The Brillouin oscillations are damped by energy dissipation into the medium, and the coefficient of acoustic wave attenuation α can be evaluated as α = Γπ/vl, where Γ is the full-width at halfmaximum (fwhm) of the Brillouin peak.24,29,37 Typically time-resolved Brillouin oscillation experiments where a metal film is used as an opto-acoustic transducer are performed with metals that do not give strong transient reflectivity signals. This is not the case for gold. Figure 2a shows a transient reflectivity trace recorded at 530 nm from a single Au nanoplate on a glass substrate. The trace shows the typical response of gold nanostructures to ultrafast laser excitation: a fast decay due to electron−phonon coupling and a slower decay due to heat dissipation to the surroundings.25,38 An oscillating signal can also be seen in the first 100 ps of the trace. The modulated portion of the data after subtraction of the background, and the corresponding fast Fourier transform (FFT) are presented in Figure 2b and c, respectively. The frequency is fa = 64.9 GHz, which is assigned to the breathing mode of the Au nanoplates along the thickness dimension.39,40 The measured quality factor is Q = f/Γ = 7.2, which is relatively small compared to the breathing mode vibrations of Au nanoparticles and Au nanorods on glass substrates.31−33 This is attributed to the larger surface contact area for the nanoplates, which is expected to reduce the vibrational quality factors.26,41 The measured quality factor is consistent with previous studies for Au nanoplates on glass substrates,39 however, it is smaller than the quality factors measured for nanoplates suspended over a trench.40 For a nanoplate with a thickness h, the frequency of the fundamental thickness vibration is given by f = E /ρ /2h, where E = 115 GPa is the Young’s modulus for the [111] direction of gold (the direction of propagation of the acoustic wave for the thickness vibration of the gold nanoplates) and ρ is the density.40 From the measured vibrational frequency of fa = 64.9 GHz, we estimate a thickness of h = 18.8 nm. Figure 2d shows transient reflectivity measurements with a probe wavelength of 430 nm where the pump and probe beams are focused on the Au nanoplates through the glass-side of the sample with a 1.3 NA objective, as shown in Figure 1a. The transient reflectivity signal shows a lower frequency modulation, which is attributed to Brillouin oscillations generated by the ultrashort laser pulses.12−15,17−19 The 64.9 GHz acoustic mode is completely absent at this wavelength for the nanoplate in Figure 2. The acoustic vibrations of metal nanoparticles

RESULTS AND DISCUSSION Figure 1 shows a schematic diagram of the experiments. Ultrafast laser excitation of the Au nanoplates creates a

Figure 1. Diagram of the experimental geometry for glass-side (a) and air-side (b) excitation of Au nanoplates. The pump pulses generate a thermal stress in the Au nanoplate, which launches strain waves in the surrounding media. The beams are shown at an angle of incidence for clarity, but the experiments were performed for normal incidence. Note that the reflected probe beam is transmitting through the thin Au nanoplate in geometry (b). (c) Diagram of the paths for the reflected probe beams. The difference in path length between the two beams is shown as the shaded lines. vlt is the distance traveled by the acoustic wave. φ is the internal angle of incidence for the probe beam.

transient stress that launches acoustic waves into the environment.12−15 The propagation of the acoustic waves can be monitored through the time-delayed probe beam. In our experimental configuration the pump and probe beams are focused through a microscope objective onto the surface of the Au nanoplates with a spread of angles determined by the NA of the objective. For glass side excitation the probe beam is reflected at the interface between the Au nanoplate and the substrate, see Figure 1a. The strain associated with the propagating acoustic waves creates a local refractive index variation, which also reflects the probe beam. The total detected light intensity contains an interference term between 8065

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Figure 2. Localized acoustic vibrations and Brillouin oscillations. The left panels (a) and (d) show the raw data, the middle panels (b) and (e) show the modulated portion of the data after subtraction of the background, and the right panels (c) and (f) are the Fourier transforms of the data. The corresponding Lorentz fits are given in red, along with the frequency and damping constant Γ. (a), (b), and (c) show the acoustic vibrations of Au nanoplates probed at 530 nm; (d), (e), and (f) show the Brillouin oscillations in the glass substrate probed at 430 nm.

Figure 3. Wavelength dependent Brillouin oscillations in glass. (a) Transient reflectivity data at different probe wavelengths, and (b) the corresponding Fourier spectra of the Brillouin oscillations. (c) Probe wavelength dependent Brillouin oscillation frequencies measured for several nanoplates, and a fit to the data based on f b = 2nvl/λpr.

determined in previous studies, which is α = 0.3 μm−1.24,29 The increased damping observed in our experiments is attributed to a geometrical effect from the high NA objective. An analogous spectral broadening has been observed in frequency domain experiments with high NA objectives,43 and was attributed to the range of angles created for the probe beam in combination with the angle dependence of the Brillouin oscillation frequency.36 As will be shown below, the effect is more complicated in time-resolved measurements. To confirm that the oscillations in Figure 2d are Brillouin oscillations, and not a different acoustic mode of the nanoplate, experiments were performed at different probe wavelengths. Figure 3a shows transient reflectivity measurements for probe wavelengths ranging from 420 to 440 nm for a single Au nanoplate, and the corresponding Fourier spectra are shown in Figure 3b. The data clearly shows that the oscillation frequency depends on the probe wavelength, as expected from the

appear in transient reflectivity experiments because they cause small changes in the size and shape of the particles, which shift the plasmon resonances.27,42 For the nanoplates the relevant resonance is the transverse plasmon mode, which occurs in the visible region of the spectrum.39 The results in Figure 2 show that tuning the probe laser away from the transverse plasmon mode into the near-UV effectively discriminates against the acoustic vibrations. Fourier transform of the oscillations in Figure 2e gives a frequency f b = 39.3 GHz with a damping constant of Γ = 2.6 GHz. For normal incidence the Brillouin oscillation frequency is f b = 2nvl/λpr. Using a refractive index of n = 1.46, we calculate a longitudinal speed of sound of vl = 5790 ± 30 m/s and a coefficient of acoustic wave attenuation of α = 1.4 ± 0.1 μm−1 for the nanoplate in Figure 2. While the speed of sound is consistent with the known speed of sound in glass, the coefficient for acoustic attenuation is larger than that 8066

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Figure 4. Brillouin oscillations detected from different sides of the Au nanoplates. (a) Time resolved changes in the reflectivity of the 430 nm probe under glass-side and air-side excitations. (b) Modulated portion of the signal after subtraction of the background signal. (c) Acoustic attenuation coefficient α in glass measured for glass side (black) and air side (red) excitation. (d) Phase difference between glass side and air side excitation, and the calculated Au nanoplate thickness.

Figure 5. Au nanoplate opto-acoustic transducer in water. (a,b) Time resolved changes in the reflectivity at 430 nm probe in water and air, respectively. (c) The corresponding Fourier spectra of the Brillouin oscillations in panels (a,b). (d) Acoustic attenuation constants measured for water.

expression of the Brillouin oscillation frequency. Note that the excitation wavelength also changes in the experiments in Figure 3. However, the thermal stress created by the pump laser, which launches the acoustic waves into the environment, is only sensitive to the duration and intensity of the pump lasernot the wavelength. In contrast, the measured frequencies of the acoustic modes in single particle transient absorption measurements do not change with the pump and probe wavelengths.31 Figure 3c shows wavelength dependent data from several Au nanoplates. Fitting the data yields an acoustic wave velocity of vl = 5800 ± 50 m/s, assuming a refractive index n = 1.46 for glass. This value is in good agreement with the literature value for the speed of sound in glass of vl = 5790−5850 m/s.6,28

The chemically synthesized Au nanoplates used as optoacoustic transducers in our experiments are partially transparent. This makes it possible to observe Brillouin oscillations from the opposite side of the plate from the excitation and probe pulses. Figure 4a shows examples of Brillouin oscillations in the glass substrate detected after exciting from the glass-side and from the air-side (see Figure 1). The modulated portion of the data, after subtracting the background signal, is shown in Figure 4b. The corresponding Fourier spectra are shown in the Supporting Information, along with a statistical analysis of the Brillouin oscillation frequencies for different plates. This data shows that changing the excitation condition (glass side compared to air side) does not change the frequency. 8067

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Figure 6. Numerical aperture dependence of the Brillouin oscillation signal. (a) Time resolved changes in the reflectivity of the 430 nm probe collected with 0.25 and 1.2 NA objectives. (b) Oscillating portion of the data after subtraction of the electronic and thermal background response. (c) Fourier transform of the Brillouin oscillation data. (d) NA dependence of the attenuation coefficient α in glass. The blue line is a linear fit to the experimental data.

Figure 5a shows transient reflectivity data for Au nanoplates in water, with excitation from the water side. A trace recorded for the same nanoplate after removing the water is shown in Figure 5b. For the nanoplate in water, the Fourier spectrum in Figure 5c shows two peaks, one due to Brillouin oscillations in water ( f b = 9.3 GHz) and one due to glass (f b = 40 GHz). The glass frequency is observed because the nanoplate is thin enough that there is significant transmission of the probe beam through the nanoplate, see Figure 4. When the water is removed only the glass frequency is observed, proving that the 9.3 GHz peak is not an acoustic mode of the nanoplate. The value of f b = 9.3 GHz for water yields a longitudinal speed of sound of vl = 1510 m/s assuming n = 1.33, which is consistent with previous results.36 The measured attenuation coefficient for the Brillouin oscillations in water was α = 2.85 ± 0.25 μm−1 for a 1.2 NA microscope objective. Consistent with the experiments for glass in Figure 4, the measured attenuation constant is much larger than the literature value of ∼0.4 μm−1.36 The increase in the attenuation coefficients for the Brillouin oscillations in microscopy measurements is an important issue. Recent frequency domain light scattering experiments showed that the linewidth and frequency for the Brillouin peak depends on the NA of the collection optics.43 This effect was attributed to the angle dependence of the Brillouin frequency: high NA optics will give rise to a range of angles and, therefore, frequencies, which causes a broadening in spectral measurements.36,43 To investigate this effect, measurements of Brillouin oscillations in glass were performed for objectives with different NAs. Figure 6a,b shows representative transient reflectivity measurements recorded with 0.25 and 1.2 NA objectives. The corresponding Fourier spectra are presented in Figure 6c. The peak widths in the Fourier spectra are 1.2 and 2.4 GHz for the 0.25 and 1.2 NA objectives, respectively. Figure 6d shows a plot of the measured attenuation coefficient α versus the NA of the objective from a statistical analysis. Extrapolation of the data to a perfectly collimated beam (NA = 0) gives an attenuation

There are two notable differences between the glass-side and air-side excitation experiments. First, the more subtle effect is that the acoustic attenuation constants (α = Γπ/vl) are smaller for air-side excitation. The acoustic attenuation coefficients determined for the different nanoplates are shown in Figure 4c. The average attenuation coefficients are α = 1.85 ± 0.35 μm−1 for probing from the glass side, and α = 1.18 ± 0.29 μm−1 for probing from the air side. Note that the reported errors are standard deviations. This difference is attributed to the geometrical effect noted above that higher NA objectives give larger attenuation coefficients. For air-side excitation the microscope objective is used without immersion oil, which reduces the effective NA for focusing and collecting the probe light. This leads to smaller attenuation constants compared to the experiments with glass-side excitation, which were performed with immersion oil. The second more noticeable difference between the glass side and air-side excitation experiments is the phase of the oscillations. A difference in phase can be clearly seen in the data in Figure 4b, and a statistical analysis of the change in phase for glass-side versus air-side excitation is shown in Figure 4d, where the magnitude of the change in phase for air side excitation compared to glass side excitation (|Δ⌀| = |⌀air − ⌀glass|) is presented. The change in phase arises because the probe beam has to travel through the Au nanoplate for air side excitation. Following the derivation in refs 18,44, it is straightforward to show that for normal incidence |Δ⌀| = 4πhnAu/λpr, where nAu is the refractive index of Au at the probe wavelength and h is the thickness of the Au nanoplate (see the Supporting Information for details). The Au nanoplate thicknesses calculated from the phase data are included in Figure 4d. The calculated thicknesses are between ∼10−20 nm, which are consistent with the thicknesses estimated from the acoustic vibration measurements (see Figure 1). To further investigate the attenuation constants for the Brillouin oscillations, experiments were performed for water. 8068

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ACS Nano coefficient α = 0.29 ± 0.04 μm−1 which is in good agreement with previous measurements in glass using Brillouin scattering spectra.29 There are several possible reasons for the increased attenuation observed in the experiments with high NA optics: (i) decoherence of the reflected probe beams;35 (ii) spectral broadening from the range of angles for the probe beam at the sample;36,43 and (iii) acoustic diffraction.35 In the following we consider each of these possibilities separately. First, decoherence of the reflected probe beams: the Brillouin oscillation signal arises from interference between the beams reflected from the substrate and the acoustic wave. These two pulses must overlap in time in order to interfere at the detector. For a pump−probe delay time of 1 ns the path length difference between the two reflected probe beams is 3 μm for water (vl ≈ 1500 m/s). This implies a time delay of 10 fs. For SiO2 (vl ≈ 6000 m/s) the time delay is 40 fs. Both these times are much shorter than the ca. 300 fs pulsewidth of the laser system, which means that decoherence should not be an issue for these experiments. The second possibility is spectral broadening from the range of angles for the probe beam created by the high NA objective.43 To analyze this effect we consider our pulse to be a Gaussian beam.45 At the focus a Gaussian beam is a plane wave, thus, there are no angle dependent effects in the interaction of the beam with the acoustic wave. The acoustic wave will only experience a significant range of angles when it moves outside the Rayleigh Range of the beam, which is z = πW20/λ where W0 is the beam waist.45 If we use W0 ≈ λ/2NA as an estimate of the beam waist, then the times for the acoustic pulse to move outside the Rayleigh Range for the 0.25 NA objective are 1.2 ns for SiO2 and 3.7 ns for water. For the 1.2 NA objective the times are 50 ps for SiO2 and 150 ps for water. Thus, spectral broadening could be an issue for the 1.2 NA objective, but not the 0.25 NA objective. Note that if this effect was present, we would expect a shift in the detected frequency with time during the experiments.43 However, Fourier transforming the data over different time windows shows that the frequency is constant in time, implying that spectral broadening is not significant. The final possibility considered is spreading of the acoustic wave due to diffraction, which will reduce the amplitude along any specific direction.35,45 To judge whether diffraction is an issue we consider the Fraunhofer distance for the acoustic wave: dF = W20/λac, where λac is the acoustic wavelength and we take the laser spot size at the sample to be equal to the size of the acoustic source. This is appropriate for excitation of metals where the charge carriers rapidly thermalize, leading to short thermal diffusion lengths for ultrafast excitation.35 For our reflection mode experiments, the acoustic wavelength is λac = λ/ 2n.10 For the 0.25 NA objective, the acoustic wave moves out of the near-field range (vlt > dF) in 1 ns for SiO2, and in 3.3 ns for water. Thus, we do not expect significant spreading of the acoustic wave in this case. However, for the 1.2 NA objective, the acoustic wave leaves the near-field range in 50 ps for SiO2, and in 140 ps for water. Thus, significant effects are expected from acoustic diffraction for the 1.2 NA objective. Because diffraction of the acoustic wave directly affects the amplitude of the signal, we believe that it is the most likely explanation for the increased attenuation in the high NA microscopy experiments. The data and analysis presented above clearly show that low angle collection conditions have to be used to accurately measure attenuation coefficients in time-resolved experiments.

However, the broadening from the NA effect also directly impacts the accuracy of the frequency measurements and, therefore, the ability of Brillouin microscopy to identify different materials. Measurements for different nanoplates show that the Brillouin oscillation frequencies have a standard deviation of ca. 2% in microscopy experiments with high NA objectives (see the scatter plots in the Supporting Information). This level of accuracy is more than sufficient to differentiate water ( f b = 9.3 GHz) from polymer ( f b ≈ 18 GHz) and, indeed, should be sufficient for most applications in soft matter systems. For example, an accuracy of 2% in the Brilloiun frequency is good enough to image internal features in cells.10,11 Finally, several of the transient reflectivity traces show a lower frequency mode at 2−3 GHz, see Figures 2c and 5c for examples. These lower frequency oscillations are assigned to vibrations of the Au nanoparticles relative to the surface.46 Guillet et al. studied these modes for Au nanospheres on a SiO2 surface. They observed quality factors Q < 10, consistent with fast energy relaxation into the substrate.46 The measured quality factors for the Au nanoplates in our experiments are on the order of 1. However, this is due to the limited scanning range of the delay line in our experiments, rather than enhanced energy relaxation compared to ref 46.

CONCLUSIONS Ultrafast laser excitation of metal nanostructures provides an efficient way of generating acoustic waves in materials. The waves can be detected as GHz oscillations in reflectivity measurements. The frequency of these Brillouin oscillations depends on the refractive index and speed of sound of the medium, as well as the probe wavelength. Thus, in principle Brillouin oscillations measurements can be used in microscopy to identify different materials. In this article we used transient absorption microscopy in reflection mode to study Brillouin oscillations induced by excitation of chemically synthesized Au nanoplates. The nanoplates are semitransparent, which allows us to observe Brillouin oscillations from both sides of the plate. The measured frequencies were in good agreement with the values calculated from the speed of sound and refractive index of the medium, however, the acoustic attenuation constants were larger than expected. This is attributed to diffraction of the acoustic wave. The extra damping from acoustic diffraction also affects the accuracy of the frequency measurements, and we estimate a 2% standard deviation for experiments with 1.3 NA objectives. Less damping and, therefore, greater frequency accuracy can be obtained with lower NA objectives, suggesting a trade-off between spatial and frequency resolution. However, an accuracy of 2% should be sufficient for many applications in materials science and biological imaging.10,11 METHODS Au Nanoplate Synthesis. The synthesis procedure is the same as that described in ref 40. Briefly, all glassware was cleaned with aqua regia and rinsed with deionized water before use. Four milliliters of ethylene glycol was heated at 160 °C for 5 min in an oil bath, followed by addition of 4 mL of 20 mM cetyltrimethylammonium bromide (CTAB) and 270 mM poly(vinylpyrrolidone) (PVP, mol wt 40 000 g/ mol) while stirring. After temperature equilibration, 30 mM HAuCl4 in ethylene glycol was added and continuously stirred for 30 min. The solution became colorless, quickly turned brown, and finally a metallic gold color appeared, indicating the formation of Au nanoplates. The solution was brought to room temperature and the precipitated plates were washed with acetone, ethanol, and water to remove the excess 8069

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ACS Nano *E-mail: [email protected]. *E-mail: [email protected].

surfactants. Although the majority of the sample was made up of anisotropic hexagonal plates, some spheres were also present. These were removed by centrifuging the solution. The Au nanoplates were spin-coated on a clean glass coverslip (Electron Microscopy Services, No 1 glass coverslips, catalog number 63765-01) for optical measurements. The sample was then mounted on a piezoelectric stage in an inverted optical microscope. For the current experiments we only examined the smaller nanoplates in the sample. It was found that these plates give much stronger Brillouin oscillation signals, possibly due to better contact with the substrate. These nanoplates are thin (ca. 20 nm width) with smooth surfaces, which makes them semitransparent in the optical measurements. Brillouin Oscillation Detection. The Brillouin oscillations were studied using single-particle pump−probe spectroscopy.30−33,40,42,47,48 The instrument for these experiments is based on a Coherent Chameleon Ultra-II Ti:sapphire oscillator (80 MHz repetition rate, ca. 0.3 ps pulse width). For the majority of the experiments the output of the oscillator was tuned to 860 nm and split with a 80/20 beamsplitter. The weaker portion was used as the pump beam and was chopped by an acousto-optical modulator (IntraAction) at 400 kHz, triggered by the internal function generator of a lock-in amplifier (Stanford Research Systems SR844). The stronger portion of the laser output was frequency doubled with a BBO crystal to provide 430 nm probe pulses. The pump and probe beams were spatially overlapped with a dichroic beamsplitter and focused onto a single Au nanoplate with a microscope objective from either the glass side or the air side of the sample, see Figure 1. Note that the two beams were both expanded before entering the microscope to overfill the back aperture of the objective lens to realize its full numerical aperture (NA). Different microscope objectives with NAs ranging from 0.25 to 1.3 were used to characterize the Brillouin oscillations (the highest NA oil objective was used for most of the experiments). Typical powers were 2 mW for the pump and 200 μW for the probe. The experiments were performed in reflection mode, with a Hamamatsu C5331-11 avalanche photodiode (APD) to detect the probe. A Thorlabs DDS600 linear translation stage was used to control the time delay between the pump and probe beams. Transient reflectivity traces were recorded by monitoring the signal from the APD with the lock-in amplifier, with a time constant of 100 ms. Acoustic Vibration Detection. Besides acoustic wave generation in the medium, the pump laser can also excite the acoustic vibrations of the Au nanoplates.39,40 These vibrations cause periodic changes of the size and shape of the nanoparticle, which shift the plasmon resonance.27 This shift can be detected by tuning the probe wavelength close to the plasmon resonance.31,32 The vibrational modes of the Au nanoplates were excited with the pulse train at 720 nm from the Ti:sapphire laser and detected by probing at 530 nm instead of in the near-UV region.40 The 530 nm probe pulses were obtained from a Coherent Mira Optical Parametric Oscillator (OPO). Because the signal from the plasmon resonance of the nanoplates is very strong, the Brillouin oscillations are not detected at the 530 nm probe experiments. However, the acoustic vibrations from the nanoplates are occasionally seen in the 430 nm probe experiments.

ORCID

Kuai Yu: 0000-0001-6138-0367 Gary Beane: 0000-0001-5312-0477 Gregory V. Hartland: 0000-0002-8650-6891 Author Contributions §

KY and TD contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the US National Science Foundation through grant CHE-1502848 (T.D., G.B., G.H.). K.Y. acknowledges financial support from the National Natural Science Foundation of China (NSFC) (Grant Nos 11274247, 11574218) and Natural Science Foundation of SZU (Grant No 2017005). REFERENCES (1) Eichenfield, M.; Chan, J.; Camacho, R. M.; Vahala, K. J.; Painter, O. Optomechanical Crystals. Nature 2009, 462, 78−82. (2) Mahboob, I.; Nishiguchi, K.; Fujiwara, A.; Yamaguchi, H. Phonon Lasing in an Electromechanical Resonator. Phys. Rev. Lett. 2013, 110, 127202. (3) Behunin, R. O.; Kharel, P.; Renninger, W. H.; Rakich, P. T. Engineering Dissipation with Phononic Spectral Hole Burning. Nat. Mater. 2016, 16, 315−321. (4) Scarcelli, G.; Yun, S. H. Confocal Brillouin Microscopy for ThreeDimensional Mechanical Imaging. Nat. Photonics 2008, 2, 39−43. (5) Audoin, B.; Rossignol, C.; Chigarev, N.; Ducousso, M.; Forget, G.; Guillemot, F.; Durrieu, M. C. Picosecond Acoustics in Vegetal Cells: Non-Invasive in vitro Measurements at a Sub-Cell Scale. Ultrasonics 2010, 50, 202−207. (6) Dehoux, T.; Audoin, B. Non-Invasive Optoacoustic Probing of the Density and Stiffness of Single Biological Cells. J. Appl. Phys. 2012, 112, 124702. (7) Gadalla, A.; Dehoux, T.; Audoin, B. Transverse Mechanical Properties of Cell Walls of Single Living Plant Cells Probed by LaserGenerated Acoustic Waves. Planta 2014, 239, 1129−1137. (8) Dehoux, T.; Abi Ghanem, M.; Zouani, O. F.; Ducousso, M.; Chigarev, N.; Rossignol, C.; Tsapis, N.; Durrieu, M.-C.; Audoin, B. Probing Single-Cell Mechanics with Picosecond Ultrasonics. Ultrasonics 2015, 56, 160−171. (9) Danworaphong, S.; Tomoda, M.; Matsumoto, Y.; Matsuda, O.; Ohashi, T.; Watanabe, H.; Nagayama, M.; Gohara, K.; Otsuka, P. H.; Wright, O. B. Three-Dimensional Imaging of Biological Cells with Picosecond Ultrasonics. Appl. Phys. Lett. 2015, 106, 163701. (10) Meng, Z.; Traverso, A. J.; Ballmann, C. W.; Troyanova-Wood, M. A.; Yakovlev, V. V. Seeing Cells in a New Light: a Renaissance of Brillouin Spectroscopy. Adv. Opt. Photonics 2016, 8, 300−327. (11) Pérez-Cota, F.; Smith, R. J.; Moradi, E.; Marques, L.; Webb, K. F.; Clark, M. High Resolution 3D Imaging of Living Cells with SubOptical Wavelength Phonons. Sci. Rep. 2016, 6, 39326. (12) Thomsen, C.; Strait, J.; Vardeny, Z.; Maris, H. J.; Tauc, J.; Hauser, J. J. Coherent Phonon Generation and Detection by Picosecond Light Pulses. Phys. Rev. Lett. 1984, 53, 989−992. (13) Wright, O. B. Ultrafast Nonequilibrium Stress Generation in Gold and Silver. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 49, 9985−9988. (14) Devos, A.; Poinsotte, F.; Groenen, J.; Dehaese, O.; Bertru, N.; Ponchet, A. Strong Generation of Coherent Acoustic Phonons in Semiconductor Quantum Dots. Phys. Rev. Lett. 2007, 98, 207402. (15) Mante, P. A.; Devos, A.; Le Louarn, A. Generation of Terahertz Acoustic Waves in Semiconductor Quantum Dots Using Femtosecond

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b02703. Control experiments for the effect of the environment on the Brillouin oscillations, statistical analysis of the frequency measurements for different nanoplates, and derivation of the phase difference for the air-side and glass-side excitation experiments (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. 8070

DOI: 10.1021/acsnano.7b02703 ACS Nano 2017, 11, 8064−8071

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DOI: 10.1021/acsnano.7b02703 ACS Nano 2017, 11, 8064−8071