What happens when you turn back time? Imagine a movie rewinding, where all actions reverse their original trajectory from the end to the beginning. If this phenomenon holds true, we call it "time reversal symmetry". "Time reversal symmetry" is always accompanied by another concept, "reciprocity". In simple terms, a physical process is said to have reciprocity if it is equivalent to its inverse process, but if it is not, it is called nonreciprocity.
Most of the common physical phenomena in life have "time-reversal symmetry". For example, two people standing on either side of a mirror, if you can see each other's eyes, then the other person can see your eyes at the same time. In optics, we call this the principle of optical path reversibility: that is, light always travels along the same path, whether from the forward or reverse direction.
But in some scenarios, this two-way reciprocal propagation of light can cause many problems. For example, if the light emitted from the laser is partially reflected back into the resonant cavity of the laser along the original path, it will affect the laser stability, and if the power is too high, it may even damage the components in the laser.
This is where "non-reciprocal devices" are needed to break the reversible nature of light propagation by blocking the backward propagation of light so that it only travels in a single direction. This device is named "optical isolator" because it blocks the light propagation in one direction. In the field of integrated optics, non-reciprocal devices, especially optical isolators, are the most technically challenging functional devices to implement. Recently, Hao Tian from Purdue University and Junqiu Liu from Swiss Federal Institute of Technology Lausanne have realized an on-chip integrated optical isolator by combining ultra-low loss integrated optics and microelectromechanical systems (MEMS). Opto-isolators are difficult to "integrate" and encounter bottlenecks
Traditionally, optoisolators use a special optical material - magneto-optical material - to realize the unidirectional propagation of light. The basic principle is the Faraday effect: under an applied magnetic field, the direction of polarization of light is deflected after passing through the magneto-optical material. The direction of this deflection is reversed in the forward and reverse directions of light propagation. We can use this property to allow only one direction of deflection to pass through, thus blocking the light of the other direction of deflection. This technique has been commercialized and is widely used in today's free-space optical systems.
However, optoisolators made using this technique have some drawbacks, such as their large size. Both the magneto-optical material and the magnet used to generate the magnetic field are not compatible with the current semiconductor mass integration process, so the optical isolators made by magneto-optical material cannot be effectively integrated on a single small chip. The size, weight, power consumption and cost of the device are factors that must be considered if we want to make a large optical system in the laboratory into a commercial product that is actually put into use in our lives. In this regard, it is especially critical if a magnetic field-independent optical isolator can be implemented on a chip. With the gradual maturation of nanofabrication processes, scientists have been able to integrate semiconductor lasers, optical modulators, low-loss optical waveguides, and optical detectors on a chip. The integrated optical isolator has become a bottleneck that prevents the integration of all optical components on the same chip. And this has gradually become a hot research topic in recent years. Scientists have proposed a variety of different solutions, but all have their own advantages and disadvantages. The reciprocity of optics is generally applicable to linear optical systems that do not vary with time. So some scholars proposed to exploit the nonlinear optical phenomena. With a special optical design, one can control the intensity of light in a resonant cavity according to the direction of light incidence, thus producing different propagation paths. However, this method greatly depends on the intensity of light incidence, and this nonreciprocal property disappears when the light intensity is above or below a certain range. This limits the range of applications for which such devices can be used, because in practice we cannot predict the intensity of the reflected light in advance. Combining micro-electro-mechanical (MEMS) technology to integrate optical isolators on photonic chips
In 2009, Professor Shanhui Fan and Dr. Zongfu Yu, Chinese electrical engineering experts, proposed space-time modulating of photons and gave rigorous theoretical proofs, which provided a solid theoretical basis for breaking optical reciprocity and opened up new directions. Based on this theory, Hao Tian and Junqiu Liu experimentally realized the unidirectional transmission of light and obtained the same results as predicted by the theory. They exploited the laws of energy and momentum conservation in the elastic scattering of photons and phonons. The modulation in time satisfies the energy conservation requirement, while the modulation in space is done to satisfy the momentum conservation. The momentum is a vector with direction, and the design can make the momentum of phonon point to a specific direction, then the light scattering will only occur in the direction along the momentum, but not in the other direction, which is also the "phase matching" condition. By controlling the scattering of light in a specific direction, we can control light propagation in only one direction.
As one of the most promising silicon-based materials, silicon nitride (Si3N4) has rich optical properties, especially ultra-low optical loss and ultra-wide spectral transparency range (from ultraviolet to mid-infrared), which make it suitable for many applications in thin-film optics, micro and nano-planar optics, and nonlinear integrated optics. However, no researcher has yet applied it to the development of "non-reciprocal devices". Combining the idea of "spatio-temporal modulation", Hao Tian and Junqiu Liu's team has realized the first optical isolator on silicon nitride. Taking advantage of the low waveguide loss of silicon nitride, the team achieved a significant reduction in signal loss of the optoisolator - at a level of 0.1dB, far below the 1dB level of current commercial optoisolators, i.e., the optical loss was reduced from 20% to 2.3%.
They have integrated three aluminum nitride (AlN) bulk acoustic wave resonators equally spaced on a microring resonator made of silicon nitride. Aluminum nitride (AlN) material, a highly efficient and mature piezoelectric material, has been widely used in wireless communications, and AlN-based bulk acoustic wave filters are used in almost every cell phone chip to receive wireless signals. The piezoelectric material is a material that can generate an electric field due to mechanical deformation, and the gas stove that can be found in every household uses the piezoelectric effect: by twisting the switch, the electric current generated by the pressure can help the gas stove to ignite a blue flame quickly. Tian Hao and Liu Junqiu are using aluminum nitride to excite sound waves. When the sound waves pass through the optical micro-ring resonant cavity, the pressure generated by the sound waves will change the refractive index of the optical waveguide, and also change the shape of the waveguide. These two effects together have a modulating effect on the optical waveguide, and the frequency of the modulation is the frequency of the acoustic wave vibration, which is generally a few gigahertz (GHz).
The lack of electro-optical effects in silicon nitride makes it difficult for people to modulate silicon nitride waveguides quickly and efficiently, and efficient modulation is the key to making the whole optical isolator possible. Last year, the team solved this problem when they achieved modulation of the gigahertz frequency of a silicon nitride optical resonant cavity using excited acoustic waves (the results were published in Nature Communications 2020); immediately after, they applied this technique to high-speed acousto-optical modulation of an integrated optical frequency comb (the results were published in Nature 2020). Unlike last year's work, this time the team improved the modulation efficiency by a factor of 100 by etching the underlying silicon substrate and binding the acoustic waves to a 5-micron-thick silicon oxide film, thereby greatly increasing the energy density of the acoustic waves. This also helps us to reduce the power consumption of the device.
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