Nanomechanics Research Group
NTT Basic Research Laboratories
3-1, Morinosato Wakamiya, Atsugi-shi, Kanagawa, 243-0198 Japan

Electromechanics  Optomechanics  Magnomechanics  Nonlinear mechanics  Phononic crystals and waveguides 

Recent Activities
Rare-Earth-Mediated Optomechanical System in the Reversed Dissipation Regime
R. Ohta, L. Herpin, V. M. Bastidas, T. Tawara, H. Yamaguchi, and H. Okamoto, Phys. Rev. Lett. 126, 047404 (2021)
Strain-mediated interaction between phonons and telecom photons is demonstrated using excited states of erbium ions embedded in a mechanical resonator. Owing to the extremely long-lived nature of rare-earth ions, the dissipation rate of the optical resonance falls below that of the mechanical one. Thus, a "reversed dissipation regime" is achieved in the optical frequency region. We experimentally demonstrate an optomechanical coupling rate g0=2 πx21.7Hz, and numerically reveal that the interaction causes stimulated excitation of erbium ions. Numerical analyses further indicate the possibility of g0 exceeding the dissipation rates of erbium and mechanical systems, thereby leading to single-photon strong coupling. This strain-mediated interaction, moreover, involves the spin degree of freedom, and has a potential to be extended to highly coherent opto-electro-mechanical hybrid systems in the reversed dissipation regime.
Near-field cavity optomechanical coupling in a compound semiconductor nanowire
M. Asano, G. Zhang, T. Tawara, H. Yamaguchi, and H. Okamoto, Commun. Phys. 3, 230 (2020)
A III-V compound semiconductor nanowire is an attractive material for a novel hybrid quantum interface that interconnects photons, electrons, and phonons through a wavelength-tunable quantum structure embedded in its free-standing structure. In such a nanomechanical element, however, a challenge is how to detect and manipulate a small number of phonons via its tiny mechanical motion. A solution would be to couple an optical cavity to a nanowire by introducing the ‘cavity optomechanics' framework, but the typical size difference between them becomes a barrier to achieving this. Here, we demonstrate near-field coupling of a silica microsphere cavity and an epitaxially grown InP/InAs free-standing nanowire. The evanescent optomechanical coupling enables not only fine probing of the nanowire’s mechanical motion by balanced homodyne interferometry but also tuning of the resonance frequency, linewidth, Duffing nonlinearity, and vibration axis in it. Combining this cavity optomechanics with epitaxial nanowire engineering opens the way to novel quantum metrology and information processing.
Dynamic control of the coupling between dark and bright excitons with vibrational strain
R. Ohta, H. Okamoto, T. Tawara, H. Gotoh, and H. Yamaguchi, Phys. Rev. Lett. 120, 267401 (2018)
We numerically and experimentally investigate strain-induced coupling between dark and bright excitons and its dynamic control using a gallium arsenide (GaAs) micromechanical resonator. Uniaxial strain induced by the mechanical resonance efficiently detunes the exciton energies and modulates the coupling strength via the deformation potential in GaAs. This allows optical access to the long-lived dark states without using any external electromagnetic field. This field-free approach could be expanded to a wide range of solid-state materials, leading to on-chip excitonic memories and circuits based on micromechanical resonators.
An opto-electro-mechanical system based on evanescently-coupled optical microbottle and electromechanical resonator
M. Asano, R. Ohta, T. Yamamoto, H. Okamoto, and H. Yamaguchi, Appl. Phys. Lett. 112, 201103 (2018)
Evanescent coupling between a high-Q silica optical microbottle and a GaAs electromechanical resonator is demonstrated. This coupling offers an opto-electro-mechanical system which possesses both cavity-enhanced optical sensitivity and electrical controllability of the mechanical motion. Cooling and heating of the mechanical mode are demonstrated based on optomechanical detection via the radiation pressure and electromechanical feedback via the piezoelectric effect. This evanescent approach allows for individual design of optical, mechanical, and electrical systems, which could lead to highly sensitive and functionalized opto-electro-mechanical systems.
Feedback control of multiple mechanical modes in coupled micromechanical resonators
R. Ohta, H. Okamoto, and H. Yamaguchi, Appl. Phys. Lett. 110, 053106 (2017)  
Simultaneous control of multiple mechanical modes is demonstrated in AlGaAs/GaAs resonators by an optomechanical active feedback due to the photothermal stress. Four mechanical modes can be amplified with a single feedback loop, which is formed by a combination of an optical detector, an electrical delay line, and an optomechanical feedback source. The feedback polarities are tailored through the electric delay line, which enables individual control of the linewidths of each mechanical mode. Linewidth narrowing and damping control of multiple mechanical modes will be used for improving the detection sensitivity of mechanical sensor arrays and for controlling their ring-down speed.
Cavity-less on-chip optomechanics using excitonic transitions in semiconductor heterostructures
H. Okamoto, T. Watanabe, R. Ohta, K. Onomitsu, H. Gotoh, T. Sogawa, and H. Yamaguchi, Nature Communications 6, 8478 (2015)  
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.
Optically induced strong intermodal coupling in mechanical resonators at room temperature
R. Ohta, H. Okamoto, R. Hey, K. J. Friedland, and H. Yamaguchi , Appl. Phys. Lett. 107, 091906 (2015)  
Strong parametric mode coupling in mechanical resonators is demonstrated at room temperature by using the photothermal effect in thin membrane structures. Thanks to the large stress modulation by laser irradiation, the coupling rate of the mechanical modes, defined as half of the mode splitting, reaches 2.94 kHz, which is an order of magnitude larger than electrically induced mode coupling. This large coupling rate exceeds the damping rates of the mechanical resonators and results in the strong coupling regime, which is a signature of coherent mode interaction. Room-temperature coherent mode coupling will enable us to manipulate mechanical motion at practical operation temperatures and provides a wide variety of applications of integrated mechanical systems.
Optomechanical photoabsorption spectroscopy of exciton states in GaAs
T. Watanabe, H. Okamoto, K. Onomitsu, H. Gotoh, T. Sogawa and H. Yamaguchi , Appl. Phys. Lett. 101, 082107 (2012)  
We demonstrate a scheme for the photoabsorption spectroscopy of semiconductors via mechanical vibration characteristics. The thermal vibration of an AlGaAs/GaAs heterostructure-based cantilever sensitively reflects the photoabsorption properties of GaAs because of the optically induced piezoelectric effect. The Q factor and the peak amplitude of mechanical vibration are strongly enhanced near the exciton-related absorption peaks of GaAs at 10K, showing good agreement with reported photoluminescence spectra.
Vibration Amplification, Damping, and Self-Oscillations in Micromechanical Resonators Induced by Optomechanical Coupling through Carrier Excitation
H. Okamoto, D. Ito, K. Onomitsu, H. Sanada, H. Gotoh, T. Sogawa, and H. Yamaguchi, Phys. Rev. Lett. 106 (2011) 036801 
Carrier-induced dynamic backaction in micromechanical resonators is demonstrated. Thermal vibration of an n-GaAs=i-GaAs bilayer cantilever is amplified by optical band-gap excitation, and for the excitation power above a critical value, self-oscillations are induced. These phenomena are found in the [110]- oriented cantilever, whereas the damping (deamplification) is observed in the [-110] orientation. This optomechanical coupling does not require any optical cavities but is instead based on the piezoelectric effect that is generated by photoinduced carriers.
Optical Tuning of Coupled Micromechanical Resonators
H. Okamoto, T. Kamada, K. Onomitsu, I. Mahboob,and H. Yamaguchi, Applied Physics Express 2 (2009) 062202
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.
Controlling quality factor in micromechanical resonators by carrier excitation
H. Okamoto, D. Ito, K. Onomitsu, T. Sogawa, and H. Yamaguchi, Applied Physics Express 2, 035001 (2008)
The quality factor (Q-factor) of GaAs microcantilevers consisting of Si-doped and undoped GaAs layers can be controlled by tuning the wavelength of the incident laser used for carrier excitation. With laser irradiation to [110]-oriented cantilevers at near-absorption-edge wavelengths, the Q-factor increases with increasing the laser power, whereas shorter-wavelength irradiation decreases the Q-factor. We observed the opposite laser power dependence for [-110]-oriented cantilevers. These results suggest the Q-control is due to the piezoelectric stress generated by the photovoltaic effect.

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