Hajime Okamoto

Coherent phonon manipulation in coupled mechanical resonators

It is now shown that phonons can be coherently transferred between two nanomechanical resonators, it is now shown. The technique of controlling the coupling between nanoscale oscillators using a piezoelectric transducer is useful for manipulating classical oscillations, but if extended to the quantum regime it could also enable entanglement of macroscopic mechanical objects.

Optical Tuning of Coupled Micromechanical Resonators

Frequency tuning of two mechanically coupled microresonators by laser irradiation is demonstrated. The eigenfrequency of a doubly clamped GaAs beam shifts downward in proportion to laser power due to optically induced thermal stress, which modifies the spring constant of the resonator. This frequency tuning enables the control of the coupling efficiency and thus the realization of perfect coupling between the micromechanical resonators, i.e., purely symmetric and anti-symmetric coupled vibration. This optical tuning is a valuable method for the study of physics in coupled resonators as well as for expanding the applications of micromechanical resonators for sensors, filters, and logics.

Controllable coupling between flux qubit and nanomechanical resonator by magnetic field

We propose an active mechanism for coupling the quantized mode of a nanomechanical resonator to the persistent current in the loop of a superconducting Josephson junction (or phase slip) flux qubit. This coupling is independently controlled by an external coupling magnetic field. The whole system forms a novel solid-state cavity quantum electrodynamics (QED) architecture in the strong coupling limit. This architecture can be used to demonstrate quantum optics phenomena and coherently manipulate the qubit for quantum information processing. The coupling mechanism is applicable for more generalized situations where the superconducting Josephson junction system is a multi-level system. We also address the practical issues concerning experimental realization.

Coherent Control of Micro/Nanomechanical Oscillation Using Parametric Mode Mixing

We propose a novel concept for controlling high-Q micro/nanomechanical resonators. Parametric mode mixing transfers the mechanical oscillation from one mode to the other and it can enable rapid switching of mechanical oscillation between modes. This technique is the classical analog of the coherent operation used in quantum computation and it can be implemented using existing structures, allowing for high speed and flexible oscillation-amplitude control of micro/nano-mechanical resonator devices.

High-sensitivity charge detection using antisymmetric vibration in coupled micromechanical oscillators

High-sensitivity charge detection using antisymmetric vibration in two coupled GaAs oscillators is demonstrated. The antisymmetric mode under in-phase simultaneous driving of the two oscillators disappears with perfect frequency tuning. The piezoelectric stress induced by a small gate-voltage modulation breaks the balance of the two oscillators, leading to the re-emergence of the antisymmetric mode. Measurement of the amplitude change enables detection of the applied voltage or, equivalently, added charges. In contrast to the frequency-shift detection using a single oscillator, our method allows a large readout up to the strongly driven nonlinear response regime, providing the high room-temperature sensitivity of 147 e/Hz0.5.

Cavity-less on-chip optomechanics using excitonic transitions in semiconductor heterostructures.

The hybridization of semiconductor optoelectronic devices and nanomechanical resonators provides a new class of optomechanical systems in which mechanical motion can be coupled to light without any optical cavities. Such cavity-less optomechanical systems interconnect photons, phonons and electrons (holes) in a highly integrable platform, opening up the development of functional integrated nanomechanical devices. Here we report on a semiconductor modulation-doped heterostructure–cantilever hybrid system, which realizes efficient cavity-less optomechanical transduction through excitons. The opto-piezoelectric backaction from the bound electron–hole pairs enables us to probe excitonic transition simply with a sub-nanowatt power of light, realizing high-sensitivity optomechanical spectroscopy. Detuning the photon energy from the exciton resonance results in self-feedback cooling and amplification of the thermomechanical motion. This cavity-less on-chip coupling enables highly tunable and addressable control of nanomechanical resonators, allowing high-speed programmable manipulation of nanomechanical devices and sensor arrays.

Epitaxial Trilayer Graphene Mechanical Resonators Obtained by Electrochemical Etching Combined with Hydrogen Intercalation

We report on the mechanical resonance properties of trilayer graphene resonators created by controlling of the layer number. We epitaxially create bilayer graphene and an interfacial buffer layer on a SiC substrate. Using hydrogen intercalation combined with electrochemical etching, we break the Si–C bonds between the buffer layer and SiC substrate surface so that the bilayer graphene and buffer layer turn into three graphene layers. The successful creation of the trilayer graphene resonators is directly observed with a transmission electron microscope. By investigating the frequency shift induced by the laser irradiation, we estimate the thermal expansion coefficient. We find that a quality factor shows a typical temperature dependence of monolayer graphene and carbon-nanotube resonators with a doubly-clamped beam structure. This implies that there exists a general energy loss mechanism for both nanotubes and few-layer-graphene doubly clamped resonators.

Theoretical Prediction and Atomic Force Microscope Observations of the Protein Nanotube Consisting of Homo-L-Amino Acid Penta-Peptide Nanorings

An unusual penta-peptide nanotube was synthesized by a solid-phase method using Fmoc chemistry. This nanotube consists of the natural homo-L-amino acid sequence (cyclo[-(L-Gln)5]), which is different from the already-known dl-peptide nanotubes having an even number of residues. We also observed the morphology by atomic force microscope (AFM) and found meandering tubular structures on the substrate. This result is consistent with our ab initio energy calculations, which show that the penta-peptide nanorings stabilize by breaking the C5 symmetry and stack themselves to form a meandering nanotube through the inter-ring hydrogen bonds.

An electromechanical Ising Hamiltonian

The phonons localized in a mechanical resonator can be electrically manipulated to emulate the Ising Hamiltonian. Solving intractable mathematical problems in simulators composed of atoms, ions, photons, or electrons has recently emerged as a subject of intense interest. We extend this concept to phonons that are localized in spectrally pure resonances in an electromechanical system that enables their interactions to be exquisitely fashioned via electrical means. We harness this platform to emulate the Ising Hamiltonian whose spin 1/2 particles are replicated by the phase bistable vibrations from the parametric resonances of multiple modes. The coupling between the mechanical spins is created by generating two-mode squeezed states, which impart correlations between modes that can imitate a random, ferromagnetic state or an antiferromagnetic state on demand. These results suggest that an electromechanical simulator could be built for the Ising Hamiltonian in a nontrivial configuration, namely, for a large number of spins with multiple degrees of coupling.

IR Study on Stacking Manner of Peptide Nanorings in Peptide Nanotubes

We here report our theoretical as well as experimental studies on the stacking manner of peptide nanorings (PNRs) in peptide nanotubes (PNTs). We focus on the molecular vibrations of N–H and C=O stretching modes and discuss this subject via their infrared (IR) spectroscopy, because PNTs are formed by the inter-ring H bonds between the adjacent PNRs via –N–H...O=C–. Symmetry analysis based on group theory reveals that parallel stacking causes two IR active modes in these molecular vibrations while three modes are active in the antiparallel stacking. This difference in the number of IR active modes is determined only by the stacking manner and not by the number of amino acid residues composing the PNRs. By using two typical PNRs of cyclo[–(L-Gln–D-Ala)3] and cyclo[–(L-Gln–D-Ala)4], we further studied the favorable stacking manners of PNRs via IR observation. Our IR experiments as well as the ab initio energetics show that the former PNRs create a PNT by stacking themselves in parallel while the latter PNRs do so by stacking themselves in an antiparallel manner.