In a study that could open doors for new applications of photonics from molecular sensing to wireless communications, Rice University scientists have discovered a new method to tune the light-induced vibrations of nanoparticles through slight alterations to the surface to which the particles are attached. In a study published online this week in ("Tuning the acoustic frequency of a gold nanodisk through its adhesion layer"), researchers at Rice’s Laboratory for Nanophotonics (LANP) used ultrafast laser pulses to induce the atoms in gold nanodisks to vibrate. These vibrational patterns, known as acoustic phonons, have a characteristic frequency that relates directly to the size of the nanoparticle. The researchers found they could fine-tune the acoustic response of the particle by varying the thickness of the material to which the nanodisks were attached. 
Rice University researchers (clockwise from front) Man-Nung Su, Wei-Shun Chang and Fangfang Wen discovered a new method to tune the light-induced vibrations of nanoparticles through slight alterations to the surface to which they are attached. “Our results point toward a straightforward method for tuning the acoustic phonon frequency of a nanostructure in the gigahertz range by controlling the thickness of its adhesion layer,” said lead researcher Stephan Link, associate professor of chemistry and in electrical and computer engineering. Light has no mass, but each photon that strikes an object imparts a miniscule amount of mechanical motion, thanks to a phenomenon known as radiation pressure. A branch of physics known as optomechanics has developed over the past decade to study and exploit radiation pressure for applications like gravity wave detection and low-temperature generation. Link and colleagues at LANP specialize in another branch of science called plasmonics that is devoted to the study of light-activated nanostructures. Plasmons are waves of electrons that flow like a fluid across a metallic surface. When a light pulse of a specific wavelength strikes a metal particle like the puck-shaped gold nanodisks in the LANP experiments, the light energy is converted into plasmons. These plasmons slosh across the surface of the particle with a characteristic frequency, in much the same way that each phonon has a characteristic vibrational frequency. The study’s first author, Wei-Shun Chang, a postdoctoral researcher in Link’s lab, and graduate students Fangfang Wen and Man-Nung Su conducted a series of experiments that revealed a direct connection between the resonant frequencies of the plasmons and phonons in nanodisks that had been exposed to laser pulses. “Heating nanostructures with a short light pulse launches acoustic phonons that depend sensitively on the structure’s dimensions,” Link said. “Thanks to advanced lithographic techniques, experimentalists can engineer plasmonic nanostructures with great precision. Based on our results, it appears that plasmonic nanostructures may present an interesting alternative to conventional optomechanical oscillators.” Chang said plasmonics experts often rely on substrates when using electron-beam lithography to pattern plasmonic structures. For example, gold nanodisks like those used in the experiments will not stick to glass slides. But if a thin substrate of titanium or chromium is added to the glass, the disks will adhere and stay where they are placed. “The substrate layer affects the mechanical properties of the nanostructure, but many questions remain as to how it does this,” Chang said. “Our experiments explored how the thickness of the substrate impacted properties like adhesion and phononic frequency.” Link said the research was a collaborative effort involving research groups at Rice and the University of Melbourne in Victoria, Australia. “Wei-Shun and Man-Nung from my lab did the ultrafast spectroscopy,” Link said. “Fangfang, who is in Naomi Halas’ group here at Rice, made the nanodisks. John Sader at the University of Melbourne, and his postdoc Debadi Chakraborty calculated the acoustic modes, and Yue Zhang, a Rice graduate student from Peter Nordlander’s group at Rice simulated the optical/plasmonic properties. Bo Shuang of the Landes’ research group at Rice contributed to the analysis of the experimental data.”
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Rice University researchers (clockwise from front) Man-Nung Su, Wei-Shun Chang and Fangfang Wen discovered a new method to tune the light-induced vibrations of nanoparticles through slight alterations to the surface to which they are attached. “Our results point toward a straightforward method for tuning the acoustic phonon frequency of a nanostructure in the gigahertz range by controlling the thickness of its adhesion layer,” said lead researcher Stephan Link, associate professor of chemistry and in electrical and computer engineering. Light has no mass, but each photon that strikes an object imparts a miniscule amount of mechanical motion, thanks to a phenomenon known as radiation pressure. A branch of physics known as optomechanics has developed over the past decade to study and exploit radiation pressure for applications like gravity wave detection and low-temperature generation. Link and colleagues at LANP specialize in another branch of science called plasmonics that is devoted to the study of light-activated nanostructures. Plasmons are waves of electrons that flow like a fluid across a metallic surface. When a light pulse of a specific wavelength strikes a metal particle like the puck-shaped gold nanodisks in the LANP experiments, the light energy is converted into plasmons. These plasmons slosh across the surface of the particle with a characteristic frequency, in much the same way that each phonon has a characteristic vibrational frequency. The study’s first author, Wei-Shun Chang, a postdoctoral researcher in Link’s lab, and graduate students Fangfang Wen and Man-Nung Su conducted a series of experiments that revealed a direct connection between the resonant frequencies of the plasmons and phonons in nanodisks that had been exposed to laser pulses. “Heating nanostructures with a short light pulse launches acoustic phonons that depend sensitively on the structure’s dimensions,” Link said. “Thanks to advanced lithographic techniques, experimentalists can engineer plasmonic nanostructures with great precision. Based on our results, it appears that plasmonic nanostructures may present an interesting alternative to conventional optomechanical oscillators.” Chang said plasmonics experts often rely on substrates when using electron-beam lithography to pattern plasmonic structures. For example, gold nanodisks like those used in the experiments will not stick to glass slides. But if a thin substrate of titanium or chromium is added to the glass, the disks will adhere and stay where they are placed. “The substrate layer affects the mechanical properties of the nanostructure, but many questions remain as to how it does this,” Chang said. “Our experiments explored how the thickness of the substrate impacted properties like adhesion and phononic frequency.” Link said the research was a collaborative effort involving research groups at Rice and the University of Melbourne in Victoria, Australia. “Wei-Shun and Man-Nung from my lab did the ultrafast spectroscopy,” Link said. “Fangfang, who is in Naomi Halas’ group here at Rice, made the nanodisks. John Sader at the University of Melbourne, and his postdoc Debadi Chakraborty calculated the acoustic modes, and Yue Zhang, a Rice graduate student from Peter Nordlander’s group at Rice simulated the optical/plasmonic properties. Bo Shuang of the Landes’ research group at Rice contributed to the analysis of the experimental data.”
Magnetic nanoparticles arranged in arrays put a twist on light: depending on the distance between the nanoparticles, one frequency of light (visible to the human eye by its colour) resonates in one direction; in the other direction, light (induced by quantum effects in the magnetic material) is enhanced at a different wavelength. The coupling between light and magnetization in ferromagnetic materials arises from quantum mechanical interactions. These interactions result in magneto-optical effects that modify the properties, such as the polarization axis or intensity of the light. Interactions between light and matter are enhanced at the nanoscale. This is a key motivation in the field of plasmonics, which studies light interacting with metal nanostructures. A nano-sized, metallic nanoparticle behaves very much like an antenna for visible wavelengths; such antennas are familiar to us in numerous everyday devices that operate on much longer radio- and micro-waves. The researchers took advantage of a phenomenon known as surface lattice resonances in which all the nanoparticles, the little antennas, radiate in unison in an array. The key to this is to assemble the magnetic nanoantennas on a length scale that matches the wavelength of the incoming light. In periodic arrays, nanoparticles interact strongly with each other, giving rise to collective oscillations. Such behavior has been previously reported in noble metal nanoparticles and researched extensively at Aalto University in the Quantum Dynamics (QD) research group. Now, a collaborative effort between QD and the Nanomagnetism and Spintronics (NanoSpin) group shows that such collective oscillations can also be observed in magnetic materials. The surface lattice resonances enhance the light polarization change in ferromagnetic materials, the so-called magneto-optical Kerr effect. - A key finding of the study was that the frequency that is the colour of light, for which this happens can be made different from the frequency where the purely optical effect is strongest. The separation of magneto-optical and optical signals was achieved by choosing a different distance between the nanoparticles in the two directions of the array, explains Professor Törmä. Using magnetic materials was not an obvious choice. So far, optical activity in ferromagnetic materials has been limited by their high resistance, which makes it impossible to observe the impressive plasmon resonances seen in noble metals. However, by ordering the nanoparticles in arrays and taking advantage of collective resonances, this problem can be mitigated. This result opens an important new direction in the research field that focuses on the coupling of light and magnetization at the nanoscale, says Professor Sebastiaan van Dijken. The benefits of collaboration between research groups – those working in different fields – was essential for the success of the project. The authors stress that this kind of project would not have been possible to achieve without extensive knowledge in both optics and magnetism at the nanoscale. Their innovative work has created the groundwork for further explorations and has the potential to advance applications beyond fundamental physics. The joint team used the nanofabrication facilities in the Micronova cleanroom as well as the electron microscopy tools available in the Nanomicroscopy Center.