MEMS enables lasing in an ultrasonic vibration
MEMS technology has enabled researchers to develop a device where two oscillation states instead of two electronic states allow a material to play the same role as a laser
Tokyo based Nippon Telegraph and Telephone Corporation (NTT) has managed to fabricate a novel ultrasonic oscillator by applying a principle analogous to an optical laser, to a microelectromechanical system (MEMS).The frequency purity of the output vibration is less than 1 part in1,000,000.
A laser is a device that spontaneously generates high purity light utilising the amplification properties of stimulated emission of radiation. It is widely used as a highly purified light source.
Applying the microfabrication technology used for making semiconductor integrated circuits, novel fine structure devices functionalised for tiny motions have recently been developed by researchers at NTT.
This result was recently published in the American science journal Physical Review Letters, demonstrating a micromechanical device working on a totally novel mechanism.
Such devices, when combined with electrical functions, are referred to as MEMS. When these devices are further miniaturised down to the nanometre scale, they are called nanoelectromechanical systems, or NEMS.
Usually, lasers are composed from atomic gasses or semiconductor crystals that have at least two different energy states that are placed inside a light confinement structure, referred to as a "cavity". Now NTT have fabricated a "SASER" device, where two oscillation states were used instead of two electronic states, which were confined within the beam enabling the beam structure to play the same role as a laser.
One of the main challenges was to mimic a quartz crystal oscillator, a device that uses a mechanical resonance to create an electrical signal with high frequency stability. Thanks to its very high frequency purity, it is widely and indispensably used in telecommunication and information processing equipment.
However, there is an enormous demand for higher operation frequencies in ever smaller packages in order to develop even faster and more integrated communication networks and infrastructures. NTT laboratories have been engaged in developing such applications for MEMS and its miniaturised counterpart NEMS.
In this work, they have applied the operating principle of an optical laser to a micromechanical oscillator and have succeeded in observing a highly stable ultrasonic oscillation. At present, this is a "proof of principle" experiment but could be further miniaturised. This would enable the development of a higher frequency, and higher precision semiconductor on-chip oscillator than a quartz crystal oscillator.
The oscillator structure
The essential part of the oscillator is a tiny 250μm long, 85μm wide, and 1.4μm thick bar usually called a "beam," as shown in Figure 1 below.
Figure 1. Device structure. SEM micrograph of the fabricated device. The colours are artificially added. Orange indicates Au (gold) electrodes and blue indicates the conductive semiconductor. The beam oscillator shown in the figure vibrates along the surface normal. When a 70 Hz wide unstable oscillation was applied from the lower electrode, a highly purified oscillation with a frequency fluctuation of only 80 mHz was generated and observed from the upper electrode.
By applying an operating protocol similar to that of an optical laser to this structure, NTT succeeded in generating an ultrasonic vibration with a frequency fluctuation less than 1 part in 1,000,000.
The type of device that generates a highly stable ultrasonic oscillation using a similar principle to that of lasers is called a SASER, in analogy to a laser. NTT says the all-electrical operation of the SASER using MEMS was demonstrated for the first time in the world.
In an optical laser, a photon is emitted from an atom when it relaxes from a high-energy state to a low-energy one, as shown in Figure 2(a). In the newly developed SASER device, the role of the atom can be similarly played by the beam structure itself through the use of a high energy oscillation state and low energy one (see Figure 2(b)).
Figure 2. The mechanism to generate highly stable ultrasonic oscillation. 2(a) In a laser, photons are emitted when the electronic state of an atom changes from a high energy state to a low energy state. The emitted light is confined by two mirrors referred to as cavity to generate high-purity light. 2(b) In a SASER, when the beam oscillation changes from a high-energy oscillation state to low-energy oscillation state, it can also generate a high-purity ultrasonic vibration
Precise control of the oscillation states using piezoelectricity
The oscillation states could be electrically optimised by using piezoelectricity in order to effectively induce the ultrasonic wave. Piezoelectricity occurs when a voltage is applied to a special type of material to induce a mechanical expansion or a compression.
The frequency purity was demonstrated by confirming the generation of a highly frequency-stable output oscillation. In contrast to the frequency fluctuation of 70 Hz in the input oscillation, the fluctuation in the output oscillation was only 80 mHz, being just one part in 2,000,000 of the oscillation frequency, as illustrated in Figure 3 below.
Figure 3: The frequency spectrum generated by the SASER. The output spectrum obtained in the experiments. No signal was observed when the input oscillation voltage was less than 58 mV. However, an oscillation was observed immediately when the input voltage became higher than 58 mV. The above data was obtained when the input voltage is 58.5 mV, showing a small frequency fluctuation of only 80 mHz.
What's more, the output oscillation was only observed when the input voltage was larger than a specific value, that is, it showed the so called threshold characteristic commonly observed in optical lasers. These observations proved that a similar operating principle to that of laser was realised in the ultrasonic vibration using the beam structure.
Future developments
In this experiment, operation with a similar stability in the oscillation to that of a quartz crystal oscillator was confirmed at low temperature (2.0K) at a frequency of 174 kHz. NTT now plans to further miniaturise and optimise this structure and use more suitable materials in order to realise operation frequencies higher than 1 GHz at room temperature.
A part of this study was supported by Japan Society for the Promotion of Science and the work is further described in the paper, "Phonon Lasing in an Electromechanical Resonator," by Mahboob,K. et al in Physical Review Letters, published on 18th March 2013.
DOI: 10.1103/PhysRevLett.110.127202
