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

MEMS/NEMS  Superconducting Quantum Circuits  Nanofabrication 

Micro/Nanomechanical Systems

Semiconductor nanomechanical devices enable the pursuit of new physical phenomenon that can only be observed in these dynamical systems to probe the fundamental nature of the world as well enabling the development of nanoscience and nanotechnology. These systems can be manipulated by electrical or optical means permitting applications such as ultra high sensitivity detectors for weak forces as well as mechanical logic to be developed.


Embedding a mechanical oscillator in a GaAs crystal allows the electro-to-mechanical transduction to be mediated via the piezoelectric effect. This piezoelectric transducer also enables the parametric excitation of the fundamental mechanical mode. Uniquely, the parametric resonance has only two stable phases of oscillation which can be used to encode bit information enabling the development of a mechanical computer. We also demonstrated the control of the Q-factor and self-excited oscillations in GaAs cantilevers, which are based on the piezoelectric effect induced by optical carrier excitation. This technique is applicable to improve sensitivity of the resonators and to develop self-actuators.


The opportunity exists to fabricate a new class of devices whose performance is superior to more conventional systems for example ultra sensitive force detectors (i.e. mass, spin, charge) and high speed opto-mechanical switches. Furthermore, by integrating multiple mechanical oscillators that are parametrically excited could enable a nanomechanical computer to be pioneered which could potentially have very low power consumption.

Recent Activities
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.
Hopf and period-doubling bifurcations in an electromechanical resonator
I. Mahboob, R. Dupuy, K. Nishiguchi, A. Fujiwara, and H. Yamaguchi, Appl. Phys. Lett. 109, 073101 (2016)  
An electromechanical resonator is developed in which the dissipation can be dynamically eliminated. The resultant motional dynamics captured by the Van der Pol equation of motion opens up the possibility of a Hopf bifurcation where the mechanical resonance loses stability when the dissipation is eliminated and period-doubling bifurcations when the dissipation becomes negative. In this latter regime, the mechanical spectral response is characterised by multi-stability spanning a bandwidth that is more than an order of magnitude wider than the intrinsic linewidth and it sustains a peak structure that can be tuned by the input used to dynamically manipulate the dissipation.
An electromechanical Ising Hamiltonian
I. Mahboob, H. Okamoto, and H. Yamaguchi , Sci. Adv. 2, e1600236 (2016)  
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.
Gate-controlled electromechanical backaction induced by a quantum dot
Y. Okazaki, I. Mahboob, K. Onomitsu, S. Sasaki, and H. Yamaguchi, Nature Communications 7, 11132 (2016)  
Semiconductor-based quantum structures integrated into mechanical resonators have emerged as a unique platform for generating entanglement between macroscopic phononic and mesocopic electronic degrees of freedom. A key challenge to realizing this is the ability to create and control the coupling between two vastly dissimilar systems. Here, such coupling is demonstrated in a hybrid device composed of a gate-defined quantum dot integrated into a piezoelectricity-based mechanical resonator enabling milli-Kelvin phonon states to be detected via charge fluctuations in the quantum dot. Conversely, the single electron transport in the quantum dot can induce a backaction onto the mechanics where appropriate bias of the quantum dot can enable damping and even current-driven amplification of the mechanical motion. Such electron transport induced control of the mechanical resonator dynamics paves the way towards a new class of hybrid semiconductor devices including a current injected phonon laser and an on-demand single phonon emitter.
Phonon propagation dynamics in band-engineered one-dimensional phononic crystal waveguides
D. Hatanaka, A. Dodel, I. Mahboob, K. Onomitsu, and H. Yamaguchi , New J. Phys. 17 113032 (2015)  
The phonon propagation dynamics in a phononic crystal waveguide, realized via a suspended one-dimensional membrane array with periodic air holes, is investigated as function of its geometry. The bandstructure of the phononic crystal waveguide can be engineered by modifying the characteristics of the phonon waves by varying the waveguide width and the pitch of the air holes. This enables the phonon transmission bands, the bandgaps, the velocity and the nonlinear dispersion in the phononic crystal to be controlled. Indeed the engineered bandstructure can even be tuned to sustain multiple phonon modes in a given branch which while being spectrally degenerate can be temporally resolved via their differing group velocities. Furthermore, the ability to tune the bandstructure and thus the nonlinear dispersion can be harnessed to efficiently activate nonlinear phenomena such as mechanical four wave mixing. This systematic study reveals the key geometric parameters that enable the phonon transport in phononic crystal waveguides to be fully controlled.
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.
Dispersive and Dissipative Coupling in a Micromechanical Resonator Embedded with a Nanomechanical Resonator
I. Mahboob, N. Perrissin, K. Nishiguchi, D. Hatanaka, Y. Okazaki, A. Fujiwara, and H. Yamaguchi , Nano Lett. 15, 2312 (2015)  
A micromechanical resonator embedded with a nanomechanical resonator is developed whose dynamics can be captured by the coupled-Van der Pol縫uffing equations. Activating the nanomechanical resonator can dispersively shift the micromechanical resonance by more than 100 times its bandwidth and concurrently increase its energy dissipation rate to the point where it can even be deactivated. The coupled-Van der Pol縫uffing equations also suggest the possibility of self-oscillations. In the limit of strong excitation for the nanomechanical resonator, the dissipation in the micromechanical resonator can not only be reduced, resulting in a quality factor of >3ラ10^6, it can even be eliminated entirely resulting in the micromechanical resonator spontaneously vibrating.
Two-Mode Thermal-Noise Squeezing in an Electromechanical Resonator
I. Mahboob, H. Okamoto, K. Onomitsu, and H. Yamaguchi , Phys. Rev. Lett. 113, 167203 (2014)  
An electromechanical resonator is developed in which mechanical nonlinearities can be dynamically engineered to emulate the nondegenerate parametric down-conversion interaction. In this configuration, phonons are simultaneously generated in pairs in two macroscopic vibration modes, resulting in the amplification of their motion. In parallel, two-mode thermal squeezed states are also created, which exhibit fluctuations below the thermal motion of their constituent modes as well as harboring correlations between the modes that become almost perfect as their amplification is increased. The existence of correlations between two massive phonon ensembles paves the way towards an entangled macroscopic mechanical system at the single phonon level.
Rapid switching in high-Q mechanical resonators
H. Okamoto, I. Mahboob, K. Onomitsu, and H. Yamaguchi , Appl. Phys. Lett. 105, 083114 (2014)  
Sharp resonance spectra of high-Q micromechanical resonators are advantageous in their applications, such as highly precise sensors and narrow band-pass filters. However, the high-Q characteristics hinder quick repetitive operations of mechanical resonators because of their long ring-down time due to their slow energy relaxation. Here, we demonstrate a scheme to solve this trade-off problem in paired GaAs micromechanical resonators by using parametrically induced intermode coupling. The strong intermode coupling induced by the piezoelectric modulation of tension allows on-demand energy transfer between closely spaced mechanical modes of the resonator via coherent control of the coupling. This enables rapid switching of the vibration amplitude within the ring-down time, leading to quick repetitive operations in high-Q mechanical resonators.
Phonon waveguides for electromechanical circuits
D. Hatanaka, I. Mahboob, K. Onomitsu and H. Yamaguchi , Nature Nanotech. 9, 520 (2014)  
Nanoelectromechanical systems (NEMS), utilizing localized mechanical vibrations, have found application in sensors, signal processors and in the study of macroscopic quantum mechanics. The integration of multiple mechanical elements via electrical or optical means remains a challenge in the realization of NEMS circuits. Here, we develop a phonon waveguide using a one-dimensional array of suspended membranes that offers purely mechanical means to integrate isolated NEMS resonators. We demonstrate that the phonon waveguide can support and guide mechanical vibrations and that the periodic membrane arrangement also creates a phonon bandgap that enables control of the phonon propagation velocity. Furthermore, embedding a phonon cavity into the phonon waveguide allows mobile mechanical vibrations to be dynamically switched or transferred from the waveguide to the cavity, thereby illustrating the viability of waveguide睦esonator coupling. These highly functional traits of the phonon waveguide architecture exhibit all the components necessary to permit the realization of all-phononic NEMS circuits.
A multimode electromechanical parametric resonator array
I. Mahboob, M. Mounaix, K. Nishiguchi, A. Fujiwara, and H. Yamaguchi , Sci. Rep. 4, 4448 (2014)  
Electromechanical resonators have emerged as a versatile platform in which detectors with unprecedented sensitivities and quantum mechanics in a macroscopic context can be developed. These schemes invariably utilise a single resonator but increasingly the concept of an array of electromechanical resonators is promising a wealth of new possibilities. In spite of this, experimental realisations of such arrays have remained scarce due to the formidable challenges involved in their fabrication. In a variation to this approach, we identify 75 harmonic vibration modes in a single electromechanical resonator of which 7 can also be parametrically excited. The parametrically resonating modes exhibit vibrations with only 2 oscillation phases which are used to build a binary information array. We exploit this array to execute a mechanical byte memory, a shift-register and a controlled-NOT gate thus vividly illustrating the availability and functionality of an electromechanical resonator array by simply utilising higher order vibration modes.
Quantum point contact displacement transducer for a mechanical resonator at sub-Kelvin temperatures
Y. Okazaki, I. Mahboob, K. Onomitsu, S. Sasaki, and H. Yamaguchi , Appl. Phys. Lett. 103, 153105 (2013)  
Highly sensitive displacement transduction of a 1.67 MHz mechanical resonator with a quantum point contact (QPC) formed in a GaAs heterostructure is demonstrated. By positioning the QPC at the point of maximum mechanical strain on the resonator and operating at 80 mK, a displacement responsivity of 3.81 A/m is measured, which represents a two order of magnitude improvement on the previous QPC based devices. By further analyzing the QPC transport characteristics, a sub-Poisson-noise-limited displacement sensitivity of 25 fm/Hz^0.5 is determined which corresponds to a position resolution that is 23 times the standard quantum limit.
Multi-mode parametric coupling in an electromechanical resonator
I. Mahboob, V. Nier, K. Nishiguchi, A. Fujiwara, and H. Yamaguchi , Appl. Phys. Lett. 103, 192105 (2013)  
Parametric coupling between multiple vibration modes in an electromechanical resonator is demonstrated via a strain inducing piezoelectric pump which enables construction of a mechanical-vibration register. In particular, the coupling between the flexural and torsional vibration modes can exceed their intrinsic dissipation rates enabling operation deep into the strong-coupling regime. The dynamic nature of this parametric coupling also permits temporal manipulation of the mechanical-vibration register enabling both long-lived modes to be rapidly switched off and phonon populations to be coherently exchanged between modes.
Coherent phonon manipulation in coupled mechanical resonators
H. Okamoto, A. Gourgout, C-Y Chang, K. Onomitsu, I. Mahboob, E. Y. Chang, and H. Yamaguchi , Nature Physics 9, 480-484 (2013) 
Coupled nanomechanical resonators have recently attracted great attention for both practical applications and fundamental studies owing to their sensitive sympathetic oscillation dynamics. A challenge to the further development of this architecture is the coherent manipulation of the coupled oscillations. Here, we demonstrate strong dynamic coupling between two GaAs-based mechanical resonators by periodically modulating (pumping) the stress using a piezoelectric transducer. This strong coupling enables coherent transfer of phonon populations between the resonators, namely phonon Rabi oscillations. The nature of the dynamic coupling can also be tuned from a linear first-order interaction to a nonlinear higher-order process in which more than one pump phonon mediates the coherent oscillations (that is, multi-pump phonon mixing). This coherent manipulation is not only useful for controlling classical oscillations but can also be extended to the quantum regime, opening up the prospect of entangling two distinct macroscopic mechanical objects.
A phonon transistor in an electromechanical resonator array
D. Hatanaka, I. Mahboob, K. Onomitsu, and H. Yamaguchi , Appl. Phys. Lett. 102, 213102 (2013)  
An electromechanical resonator array is developed that consists of 5 mechanically coupled membranes. Mechanical excitation of the array results in 2 types of oscillations, an extended mechanical oscillation that propagates through all 5 membranes and a localized mechanical oscillation that is confined to just some of the membranes. The dynamic interaction of these 2 types of oscillations is used to implement a transistor in this phononic system.
Phonon Lasing in an Electromechanical Resonator
I. Mahboob, K. Nishiguchi, A. Fujiwara, and H. Yamaguchi , Phys. Rev. Lett. 110, 127202 (2013) 
An electromechanical resonator harboring an atomlike spectrum of discrete mechanical vibrations, namely, phonon modes, has been developed. A purely mechanical three-mode system becomes available in the electromechanical atom in which the energy difference of the two higher modes is resonant with a long-lived lower mode. Our measurements reveal that even an incoherent input into the higher mode results in coherent emission in the lower mode that exhibits all the hallmarks of phonon lasing in a process that is reminiscent of Brillouin lasing.
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.
An electromechanical membrane resonators
D. Hatanaka, I. Mahboob, H. Okamoto, K. Onomitsu and H. Yamaguchi , Appl. Phys. Lett. 101, 063102 (2012) 
An electrically active membrane-based mechanical resonator was fabricated from a GaAs/AlGaAs heterostructure. The mechanical motion of the membrane was piezoelectrically excited and detected. The piezoelectric transducer could also excite a range of resonance modes in the membrane that were identified and mapped via optical interferometry. Furthermore, the various mode shapes combined with the piezoelectric transduction could be used to execute mechanical-logic-gates. The development of an electrically active membrane-based mechanical resonator paves the way towards responsive electromechanical detectors and highly functional opto-electro-mechanical systems.
Phonon-cavity electromechanics
I. Mahboob, K. Nishiguchi, H. Okamoto, and H. Yamaguchi , Nature Physics 8. 387-392 (2012) 
Photonic cavities have emerged as an indispensable tool to control and manipulate harmonic motion in opto/electromechanical systems. Invariably, in these systems a high-quality-factor photonic mode is parametrically coupled to a high-quality-factor mechanical oscillation mode. This entails the demanding challenges of either combining two physically distinct systems, or else optimizing the same nanostructure for both mechanical and optical properties. In contrast to these approaches, here we show that the cavity can be realized by the second oscillation mode of the same mechanical oscillator. A piezoelectric pump generates strain-induced parametric coupling between the first and the second mode at a rate that can exceed their intrinsic relaxation rate. This leads to a mechanically induced transparency in the second mode which plays the role of the phonon cavity, the emergence of parametric normal-mode splitting and the ability to cool the first mode. Thus, the mechanical oscillator can now be completely manipulated by a phonon cavity.
Motion detection of a micromechanical cantilever through magneto-piezovoltage in two-dimensional electron systems
H. Yamaguchi, H. Okamoto, S. Ishihara , and Y. Hirayama , Appl. Phys. Lett. 100, 012106 (2012) 
We study the strain-induced voltage generation, i.e., piezovoltage, in a two-dimensional electron system under a magnetic field at low temperature. We find its strong magnetic-field dependence, where the voltage increases up to several microvolts at the boundaries between localized and extended electronic states. The order of magnitude of the generated electrical power is comparable to that of the energy dissipation in mechanical vibration, indicating high-efficiency mechanical-to-electrical energy transduction.
Interconnect-free parallel logic circuits in a single mechanical resonator
I. Mahboob, E. Flurin, K. Nishiguchi, A. Fujiwara,and H. Yamaguchi, Nature Communications 2 (2011) 198 
In conventional computers, wiring between transistors is required to enable the execution of Boolean logic functions. This has resulted in processors in which billions of transistors are physically interconnected, which limits integration densities, gives rise to huge power consumption and restricts processing speeds. A method to eliminate wiring amongst transistors by condensing Boolean logic into a single active element is thus highly desirable. Here, we demonstrate a novel logic architecture using only a single electromechanical parametric resonator into which multiple channels of binary information are encoded as mechanical oscillations at different frequencies. The parametric resonator can mix these channels, resulting in new mechanical oscillation states that enable the construction of AND, OR and XOR logic gates as well as multibit logic circuits. Moreover, the mechanical logic gates and circuits can be executed simultaneously, giving rise to the prospect of a parallel logic processor in just a single mechanical resonator.
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.
High-sensitivity charge detection using antisymmetric vibration in coupled micromechanical oscillators
H. Okamoto, N. Kitajima, K. Onomitsu,1 R. Kometani, S. Warisawa, Sunao Ishihara, and H. Yamaguchi, Appl. Phys. Lett. 98 (2011) 014103 
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.
A symmetry-breaking electromechanical detector
I. Mahboob, C. Froitier, and H. Yamaguchi, APPLIED PHYSICS LETTERS 96, 213103 (2010)
The dynamical double well potential underpinning the stable oscillation phases in an electromechanical parametric resonator is manipulated via a secondary field excitation applied at the natural frequency of the oscillator. This enables symmetry to be lifted in the dynamical potential well and results in the parametric resonator oscillating with a preferred phase. The ability to break symmetry in the dynamical double well potential permits the realization of a symmetry-breaking detector which can resolve resonance frequency (f0) shifts of δf0/f0 〜 10?7 in a single-shot measurement.
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.
Room temperature piezoelectric displacement detection via a silicon field effect transistor
I. Mahboob, K. Nishiguchi, A. Fujiwara, and H. Yamaguchi, Appl. Phys. Lett. 95, 233102 (2009)
An electromechanical oscillator embedded with a two dimensional electron gas is capacitively coupled to a silicon field effect transistor Si-FET. The piezovoltage induced by the mechanical motion modulates the current passing through the Si-FET enabling the electromechanical oscillator’s position to be monitored. When the Si-FET is biased at its optimal point, the motion induced piezovoltage can be amplified resulting in a displacement sensitivity of 6 10?12 mHz?1/2 for a 131 kHz GaAs resonator which is among the highest recorded for an all-electrical room temperature detection scheme.
Bit storage and bit flip operations in an electromechanical oscillator
I. Mahboob and H. Yamaguchi, Nature Nanotechnology 3, 275 (2008)
The Parametron was first proposed as a logic-processing system almost 50 years ago. In this approach the two stable phases of an excited harmonic oscillator provide the basis for logic operations. Computer architectures based on LC oscillators were developed for this approach, but high power consumption and difficulties with integration meant that the Parametron was rendered obsolete by the transistor. Here we propose an approach to mechanical logic based on nanoelectromechanical systems that is a variation on the Parametron architecture and, as a first step towards a possible nanomechanical computer, we demonstrate both bit storage and bit flip operations.
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.
Improved resonance characteristics of GaAs beam resonators by epitaxially induced strain
Applied Physics Letters 92 (25), 251913 (2008)
Micromechanical-beam resonators were fabricated using a strained GaAs film grown on relaxed In0.1Ga0.9As/In0.1Al0.9As buffer layers. The natural frequency of the fundamental mode was increased 2.5-4 times by applying tensile strain, showing good agreement with the model calculation assuming strain of 0.35% along the beam. In addition, the Q factor of 19,000 was obtained for the best sample, which is one order of magnitude higher than that for the unstrained resonator. This technique can be widely applied for improving the performance of resonator-based micro-/nanoelectromechanical devices.
Motion detection of a micromechanical resonator embedded in a d.c. SQUID
S. Etaki, M. Poot, I. Mahboob, K. Onomitsu, H. Yamaguchi and H. S. J. Van Der Zant, Nature Physics 4, 785 (2008)
Superconducting quantum interference devices (SQUIDs) are the most sensitive detectors of magnetic flux and are also used as quantum two-level systems (qubits). Recent proposals have explored a novel class of devices that incorporate micromechanical resonators into SQUIDs to achieve controlled entanglement of the resonator ground state and a qubit as well as permitting cooling and squeezing of the resonator modes and enabling quantum-limited position detection. In spite of these intriguing possibilities, no experimental realization of an on-chip, coupled mechanical-resonator-SQUID system has yet been achieved. Here, we demonstrate sensitive detection of the position of a 2MHz flexural resonator that is embedded into the loop of a d.c. SQUID. We measure the resonator's thermal motion at millikelvin temperatures, achieving an amplifier-limited displacement sensitivity of 10 fm/Hz0.5 and a position resolution that is 36 times the quantum limit.

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