The world's first realization of an optomechanical device with extremely low optical energy loss
- Progress toward the creation of smaller and more efficient optical amplification devices -

Researchers at Nippon Telegraph and Telephone Corporation (NTT; Head office: Chiyoda-ku, Tokyo; President & CEO: Jun Sawada) have sucessfully realized an optomechanical device (*3) with extremely low light energy loss by embedding rare-earth light-emmiting centers (*2) inside a microscopic mechanical oscillator (*1).

In conventional optomechanical devices, light couples to sound where the relaxation time (*4) of that light is usually much shorter than the corresponding relaxation time of the mechanical oscillator. In this regime it is possible to control and manipulate the mechanical oscillations using optical resonances. For technological applications, however, it is often desirable to achieve the opposite situation, where one uses the mechanical oscillations to control the light. Despite numerous experimental efforts, it has remained technologically challenging to acheive.

In our work, we have designed and experimentally realized a novel optomechanical device by coupling rare-earth emmiting centers to a mechanical resonator. Our device works in an unconventional limit: the relaxation time of the rare-earth emmiting centers is extremely long in comparison to the relaxation time of the mechanical motion. This is known as the reverse dissipation regime. Previous theoretical works have shown that in this regime it is possible to control light using the mechanical oscillation and to even to amplify that light using the optomechanical elements. We envision that our experimental results will lead to the development of compact, highly-efficient, and energy-saving optical devices such as on-chip optical amplifiers using small optomechanical elements with large non-linearities.

The results were published in "Physical Review Letters" on January 29, 2021.

1. Background and summary of the results

Micro-mechanical oscillators in semiconductor chips are used in diverse technological applications including high-sensitivity sensors and high-frequency filters to mention but a few. Among micro-mechanical oscillators, MEMS (Micro - Electro - Mechanical - Systems) are of utmost importance for technological applications because they can be controlled using electrical signals. Recently, there has been an increasing interest (both experimental and theoretical) of systems where light couples to sound, a research field known as optomechanics. Most optomechanical devices consist of a mechanical oscillator coupled to an optical resonator in which light is spatially confined by mirrors / holes, or an optical resonance in which light is selectively absorbed.
The behavior of such optomechanical devices is determined by the ratio between the relaxation time of the light compared to the mechanical oscillation. Specifically, a physical system with a short relaxation time can be used to control another physical system with long relaxation time. In conventional optomechanical devices, the relaxation time of the light is extremely short compared to that of the mechanical oscillation. Thus, one can exploit this feature to control mechanical oscillations using light. On the other hand, the technological challenge has been to control light using mechanical oscillations (sound). In our work we have overcome this difficulty for the first time. We have fabricated a new optomechanical device in which the ratio between the relaxation time of the mechanical oscillation and light is reversed. We achieved this by embedding rare-earth emmiting centers with exceptionally long relaxation time into the mechanical oscillator. This will open a new avenue of research as one can use those mechanical oscillations to amplify and modulate optical resonances in the light-emitting centers. This has been extremely difficult to achieve in the past with other platforms.

2. Outline of the experiment

The mechanical oscillator used in the experiment (Fig. 1) was fabricated by milling YSO crystals (*5) containing Erbium atoms (a rare-earth element). By placing the mechanical oscillator on top of a piezoelectric actuator, it was possible to electrically induce vertical oscillations and make the mechanical oscillator resonate at a natural frequency. This resonance allows one to induce a local strain inside the mechanical oscillator. By constructing an experimental system that can measure light absorption and emission depending on that strain, we successfully observed a strain-mediated interaction between the rare-earth emmiting centers and the mechanical oscillator (Fig. 2). Subsequent measurements of the energy loss confirmed that the relaxation time of the rare-earth emmiting centers is exceptionally long compared to that of the mechanical oscillator (Fig. 3).

3. Key technologies

⁽¹⁾To efficiently couple light to a mechanical oscillator, light must be confined tightly in space for a long time. In conventional optomechanical devices, optical resonators (*6) and quantum dots in semiconductors (*7) were used as mechanisms to spatially confine that light. However, the optical relaxation time in those devices is short (on the order of few nanoseconds). This is extremely fast compared to the mechanical oscillators relaxation time. In our work, we have successfully fabricated a novel optomechanical device in which light remains confined in a tight space for time scales much longer than the relaxation time of the mechanical oscillator. This achievement was possible by embedding rare-earth emmiting centers whose relaxation time was several milliseconds into the mechanical oscillator. This not only improves the efficiency of coupling between the light field and mechanical oscillator, but also opens the possibility to amplify and manipulate light using those mechanical oscillations (this has been difficult to achieve with the conventional devices).

⁽¹⁾The relaxation rate of these rare earth atoms embedded in solids is extremely long. Furthermore, crystals such as YSO containing rare earth elements are hard materials meaning it has been difficult to fabricate mechanical structures with them. We have overcome this difficulty by performing oblique milling using ion beams (*8). For the first time in the literature we have successfully fabricated a mechanical oscillator coupled to rare-earth emmiting centers. By cooling the sample to liquid helium temperature (4K), we have quantitatively determined the interaction strength between the mechanical oscillator and the long-lived rare-earth emmiting centers.

4. Future development

Our work will open new avenues of research in this field. In the near future, we can use our optomechanical device that we have fabricated to demonstrate optical amplification and manipulate that light using mechanical oscillations. We will optimize the device structure so that the amplified light can be efficiently extracted. Further, while at present we have only demonstrated its operation at liquid helium temperatures, we aim our device to operate at both liquid nitrogen (77 K) and room (298 K) temperatures with the goal of using it within the framework of IOWN. We also aim to create energy-efficient optical devices that combine elastic waves and sound waves by increasing the interaction strength between the light and mechanical oscillator.

Paper publication information

"Rare-earth-mediated opto-mechanical system in the reversed displacement regime", Ryuichi Ohta, Loïc Herpin, Victor M. Bastidas, Takehiko Tawara, Hiroshi Yamaguchi, and Hajime Okamoto, Physical Review Letters 126, 047404 (2021)

Glossary

Figure. 1: Schematic diagram of a mechanical oscillator including a light-emitting center formed from a rare earth element. The area below and beside the mechanical oscillator was removed by oblique milling using ion beams. This produced a mechanical oscillator with an inverted triangle cross-section. The transducer is 160 microns long, 14 microns wide, and 7 microns thick. The mechanical oscillator is mounted on a piezoelectric actuator which can induce a mechanical resonance when an AC voltage is applied to the actuator. Further, a large ensemble of rare-earth emmiting centers is embedded inside the mechanical oscillator. These emitters are affected by strain caused by a mechanical resonance. Hence by driving the rare-earth emmiting centers using a laser, it is possible to investigate the optical absorption characteristics depending on the strain caused by mechanical oscillation.

Figure 2: Illustration of the modulation of the absorption wavelength of the light-emitting centers due to periodic motion of the mechanical oscillator. The wavelength of absorbed light by rare-earth emmiting centers changes sinusoidally due to the distortion (displacement) caused by the mechanical oscillator (dotted lines indicate changes in central wavelength). The modulation of absorption wavelength is the result of the strain-mediated interaction between the mechanical oscillator and the rare-earth emmiting centers.

Figure 3: Plot showing the energy decay of rare-earth light-emitting centers and the mechanical oscillator. Remarkably, the energy of the mechanical oscilator dissipates at a higher rate than that of the light. This confirms the realization of the new optomechanical device in which the relation of the energy relaxation is reversed from the conventional optomechanical approach.