We experimentally demonstrate that a metamaterial nanostructure can have a localized heating response owing to plasmonic resonances in the near-infrared wavelength range (from 1.5 to 2µm). With a broadband nanosecond-pulse light, the temperature of composing gold particles in the nanostructure can be easily increased to over 900K within only several nanoseconds, resulting in re-shaping of the particles. The photothermal effect is elaborated with finite-element based numerical simulations. The absorption resonance can in principle be tailored with a great freedom by choosing appropriate metamaterial parameters. The light-induced heating in an artificial metamaterial can be potentially used for all-optical acute temperature tuning in a micro-environment, which may open new frontiers especially in nanotechnology and biotechnology.
© 2011 OSA
A small-volume metal particle exhibits arguably the most efficient photothermal response when excited by a laser close to its plasmonic resonance frequency. Photothermal effects in plasmonic nanoparticles have already found applications in cancer therapy [1,2], optofluidic control , nano-welding [4,5], optical data storage , imaging , sensing , etc. A number of experimental studies on plasmonic-enhanced photothermal phenomena have been carried out especially in the last ten years or so [1–18]. A clear focus of the previous studies is on photothermal effect by metallic particles in various shapes (e.g. gold nano-spheres as in , nano-rods as in , and nano-shells as in ) dispersed in an aqueous environment. Shape engineering serves as an effective way to tailor the particle plasmonic resonance to a longer wavelength [19,20] which is often considered favourable in various applications from integrated photonics to biomedicine [21–25]. Despite theoretical claims [18–20], there are only very few experimental demonstrations of photothermal effect in a complex plasmonic system with a peak absorption wavelength beyond one micron [26,27]. Moreover, the aqueous environment in previous photothermal experiments [9–15], a natural consequence of the wet-chemical method deployed for fabricating the nanoparticles, in a way impairs the full technological implications of the demonstrated photothermal effect: despite the nanoscaled particles, their photothermal effect has to manifest through a bulky colloid (which is usually set in motion for avoiding aggregation of particles). Here we experimentally demonstrate photothermal effect of a metamaterial nanostructure with a peak absorption wavelength over 1.5µm. After illuminating the sample with a pulsed broadband source, we observe, most importantly, re-shaping of its top-layer gold particles from thin blocks to spherical domes. The experiment finding is theoretically elaborated through numerical calculations which take both electromagnetic (EM) scattering and heat transfer dynamics into account. Our demonstration of the device in an all-dry procedure can potentially bring such photothermal effect a leap forward to real applications in nanoscience and nanotechnology.
Figure 1 illustrates our experiment flow. First a gold nanostructure is fabricated by a standard nanofabrication procedure (Appendix A). A single unit of the structure consists of a layer of 40nm-thick gold particle and a 60nm-thick continuous gold film, separated by a 10nm-thick Al2O3 dielectric layer, shown in Fig. 1(A). A 150μm-thick SiO2 substrate is further beneath the structure (not shown). The gold nanoparticles have a rectangular shape of dimension 230 × 170nm2. A plasmonic resonance at NIR wavelength (λp) exists due to coupling between each gold particle and the bottom gold film . Such a subwavelength resonator unit repeats in a square lattice with a period of 310nm, forming a metamaterial film which efficiently absorbs incoming light at λp with minimum reflection and transmission (refer to Fig. 2(A) for the absorption spectrum of our fabricated nanostructure sample). Such artificial structures formed with lossy EM resonators were reported also in [29–32]. We irradiate (Fig. 1(B)) the fabricated gold nanostructure by a focused broadband white light. The setup is illustrated in Fig. 3 . A super-continuum white light source (NKT SuperK Compact) with a spectral range of 0.5-2.4µm, a repetition rate of 27 kHz, and an average output power of 101mW is utilized. The output fiber of the source is connected to a reflective collimator (Thorlabs RC08FC), generating a collimated beam with a diameter of 8.5 mm. The collimated beam is then attenuated by two circular variable BK7 neutral-density filters (Melles Griot CND-1-100.0M) and then focused by an aspherized achromatic lens (Edmund NT49-665, EFL = 5cm) onto the nanostructure. The time-averaged light power reaching the sample is about 2.3mW, with a beam diameter of 20μm. The exposure time is fixed at about 0.2 second. Behind the sample there are a 20 × long-working-distance objective (Mitutoyo 378-804-2, NA = 0.42, WD = 20mm) and a CCD camera (Micro Ocular MD300) connected to a computer, working as a microscope to keep track of the nanostructure position. Figure 1(C) shows the schematic picture of the sample after irradiation. The top gold particles are converted to spherical domes due to excessive heating and reshaping owing to surface tension in liquid phase.
Figure 4(A) shows a top-view scanning electron microscopy (SEM) image of the illuminated sample. One sees clearly the reshaped particles (only the lower half of the melted pattern is shown). Outside the half-circular melted domain, the particles remain intact. The circular boundary separating the reshaped and original particles is caused by the Gaussian beam profile. Further increasing the incident power we observe, of less technological interest, fragmented gold particles in the beam center and even severely burnt sample (not shown). Fragmentation of gold particles due to laser pulse irradiation was previously thoroughly studied in . Figures 4(B) and 4(C) compare the detailed oblique SEM views of the particles before and after irradiation. The particles in thin blocks in Fig. 4(A) have grainy top surfaces and rough side edges due to, respectively, the electron beam evaporation method used for metal deposition and the lift-off patterning process. In contrast, the gold particles after irradiation have a much higher surface quality. The quality of gold domes can be further elucidated by examining the 3D atomic force microscopy images of the nanoparticles before and after irradiation, as shown in Fig. 5(A) and Fig. 5(B), respectively. The spherical particles have an average radius of 80nm (refer to Fig. 5(C)) and a height around 90nm; their contacting surface to the Al2O3 layer has a radius about 70nm.
The shape transformation and possibly some improvement in the inner structure of the gold nanoparticles should substantially influence the absorption characteristics of the metamaterial absorber. By using a homemade setup, we measure the absorbance (Appendix B) of the sample at both melted and un-melted regions with a 10° incidence angle. We point out that the absorber in such a configuration in general has an absorption spectrum insensitive to the incidence angle . The plane of incidence intersects with the metamaterial in the direction along which the particles have a smaller size. Two measurement results are given in Figs. 2(A) and 2(B) for both the transverse-magnetic (TM) and the transverse-electric (TE) polarizations. For the sample region with rectangular nanoparticles (Fig. 2(A)), the absorption peak differs for the two polarizations, at 1.58μm for TM and ~2µm for TE . For the region with dome-shaped nanoparticles (Fig. 2(B)), due to the higher symmetry of the unit cell, the absorption peaks for two polarizations almost overlap at 1.1μm. We carried out EM scattering simulations with structural parameters extracted from the experimental sample (Appendix C). The simulated absorption spectra for two sample regions are shown in Figs. 2(C) and 2(D). Overall, the experimental results agree well with simulated results. This study suggests that an absorption spectrum can serve as a second signature for such a photothermal fusion experiment. Here for the simulation of absorption by an unmelted sample region we used a constant much larger than its bulk value  to take into account the extra scattering caused by its rough surface, as a common practice in such EM simulations . However, for the simulation of absorption by a melted sample region we used a damping constant equal to its bulk value, owing to the higher-quality gold nanoparticles.
3. Numerical Confirmation
To unfold the mechanism behind the reshaping of the nanoparticles, we numerically simulate the photothermal heating process with the help of a finite-element numerical tool offered by the commercial COMSOL Multiphysics software . All input parameters are in accordance to those used in the experiment. In laser heating experiment, the plasmonic absorber is irradiated with a super-continuum light source with an average power of 2.3mW, a beam diameter of 20μm, and a repetition rate of27 kHz. Our simulation results are summarized in Fig. 6 . Figure 6(A) presents the spatial distribution of the top-layer nanoparticle temperature at various timings. The distribution in general inherits the Gaussian profile of the light beam. Notice that at time t = t0 + 1ns, where t0 is pulse delay for the heat source, gold nanoparticles at the beam centre exceeds 682K in temperature and are therefore experiencing melting. This portion of particles becomes at first larger, up to a circular region enclosed by r ≈ 5μm where r is the distance from the beam centre, and then smaller due to the dominance of heat diffusion. Notice that here the particle melting temperature, i.e. 682K, is interpreted according to the pattern of the melted sample as in Fig. 4(A), which roughly is a circle with a radius of 5μm. Outside the circle, the sample retains its structure after irradiation; just within the circle, the top-layer nanoparticles are expected to experience partial melting or surface melting; further to the centre of the circle, where temperature of gold particles reach 928K, complete melting and damage to the sample can happen. Figure 6(B) presents the transient temperature variations in both the top gold nanoparticle and the bottom gold film at r = 5µm, just within the reshaped region, subject to a time-dependent Gaussian heat source. It is seen that temperature in the gold particle reaches to its maximum at t0 + 1.5ns, i.e. 1.5ns after the peak heat source power occurs. The temperature drops slowly through heat diffusion, and back to 358K. A difference of ~50K between the particle and the film in their maximum achievable temperatures is clearly visible. The temperature distribution in a single unit of the sample at t = t0 + 1.5ns (as highlighted in Fig. 6(B)) is plotted in Fig. 6(C). One important observation is that the temperature has a huge spatial gradient of ~400K/200nm in z direction, due to the fact that the generated heat around the nanoparticle cannot be dissipated in a few nanoseconds. Although not shown explicitly, our calculation also reveals that the photothermal reshapings of the top nanoparticles can be induced with just one-pulse irradiance.
In conclusion, our experimental and simulated results indicate that an artificially engineered metamaterial nanostructure can exhibit strong photothermal effects owing to plasmonic resonance at near-infrared wavelength. The drastic temperature elevation (as manifested in experiment) and the nanosecond response time (as analyzed in simulation) can be potentially exploited for temperature-related applications, especially in biotechnology potentially. At the same time, our observation of the gold nanoparticles reshaping promises a new route for fabrication of dome-shaped metallic nanoparticles and even other-shaped metallic components for improved plasmonic and metamaterial devices [36,37].
Appendix A: Sample Preparation
The silica substrate is first covered with a 60nm-thick gold film and then a 10nm-thick alumina film using electron-beam (E-beam) evaporation. A very low deposition rate of 0.5Å/s is used during the deposition of the layers in order to obtain smooth films. The substrate is then covered by a positive resist (ZEP 520A, R&D center, Special Materials Division, Japan) and rectangular nanoparticle structures are defined in the resist by E-beam lithography (Raith 150, Raith GmbH). Usually, E-beam lithography of such a high-density structure is challenging due to scattering of electrons within the resist, which can destroy the designed structure and cause the resist to collapse. In our fabrication, the best structures are obtained with a 25kV acceleration voltage and a very low beam current of 25pA. Using these settings, the scattering of electrons during E-beam writing is reduced. A 4nm-thick titanium layer and a 40nm-thick gold layer are then deposited by E-beam evaporation on the sample. The deposition rate for both the Titanium and gold layers is again at 0.5Å/s. A lift-off process is used to produce rectangular gold nanoparticles from the gold film. The Titanium layer functions as an adhesion layer so that the gold nanoparticles do not easily fall off. The fabricated metamaterial absorber sample has an area of 100×100μm2.
Appendix B: Transmission and Reflection Spectra Measurement
The transmission and reflection spectra in the case of oblique incidence are obtained by a homemade setup. Refer to Fig. 7 for the measurement setup. The light source (same as that in the photothermal experiment) first passes through a pinhole with a diameter of 600μm (Edmund NT56-288), which acts as an attenuator, then a linear polarizer (Thorlabs LPNIR100-NP), and finally is focused by an aspherized achromatic doublet. When reaching the nanostructure, the light beam has a diameter less than 50µm. The average beam power is kept less than 30µW to avoid melting of the gold particles. A 20× objective and a CCD are placed behind the sample, which play the same roles as they do in the photothermal experiment. The reflected light from the metamaterial absorber, after being focused by an achromatic doublet (Thorlabs AC254-045-C-ML, EFL=4.5cm) is collected by a multimode fiber (Thorlabs M31L03), which is connected to an optical spectrum analyzer (OSA, Agilent 86142B). The reflectance spectrum is normalized by the reflectance of a gold film. To measure the transmission spectrum, the objective as well as the CCD, which were placed behind the sample, is then replaced with the doublet and multi-mode fiber receiver. The transmission spectra are recorded by OSA connected to the fiber receiver.
Appendix C: EM Scattering Simulations
Numerical simulations are performed with the commercial COMSOL Multiphysics (Version 3.5a) using a 3D finite-element method. The permittivity of gold is given by the Drude model with the plasma frequency ωp= and the collision frequency γ=. This Drude model agrees well with the experimental values  in the concerned wavelength range of 0.5~2.4µm. The refractive index of Al2O3 is chosen as a fixed value of 1.75 over the wavelength range. All materials are assumed to be non-magnetic (µ=µ 0). Fine mesh is imposed on spatial regions where strong inhomogeneity exists.
This work is supported by the Swedish Foundation for Strategic Research (SSF) and the Swedish Research Council (VR).
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