Measuring the Twisting Force of Light

Measurement of the twisting force, or torque, generated by light on a silicon chip holds promise for applications such as miniaturized gyroscopes and sensors to measure magnetic field, which can have significant industrial and consumer impact. (Source: Col. of Science and Engineering, U Minnesota)

Measurement of the twisting force, or torque, generated by light on a silicon chip holds promise for applications such as miniaturized gyroscopes and sensors to measure magnetic field, which can have significant industrial and consumer impact. (Source: Col. of Science and Engineering, U Minnesota)

Researchers in the Univer­sity of Minnesota’s College of Science and Engi­neering have measured the twisting force, or torque, generated by light on a silicon chip. Their work holds promise for appli­cations such as minia­turized gyroscopes and torsional sensors to measure magnetic field, which can have significant industrial and consumer impact. Torque, in the context of light, stems from the spin angular momentum of photons, and its measurement is mechanical proof of the quantum nature of light. Although such measure­ments have been performed in much larger scale systems, the latest results were achieved within a micro­meter-sized waveguide and demonstrated the use of optical torque to induce rotational motion in a micro scale mechanical device.

With circular polari­zation, the electric field of light rotates in a circle because of which the photons have spin angular momentum. Theory suggests  that such spin angular momentum will lead to a mechanical torque on the objects that interact with the circularly polarized light. While optical forces such as radiation pressure have been studied and harnessed for a while, angular momentum and the force it induces, optical torque, have remained relatively unexplored. Polari­zation of light plays a critical  role in optical communi­cation. Each time the state of polarization changes, photons exchange angular momentum with the device thereby inducing an optical torque. Measurement and exploi­tation of angular momentum and the resulting optical torque could give scientists new insights into controlling and mani­pulating light for new technologies.

Mo Li and his team fabricated an integrated opto­mechanical device on a silicon chip, with the core element of the device being a waveguide, measuring only 400 nm wide and 340 nm high, suspended like a string from the substrate. The rect­angular cross-section of the waveguide causes the light with horizontal polari­zation to travel slower than light with vertical polari­zation. Such an effect is called bire­fringence, and in this particular case is caused by the geometry of the wave­guide rather than the material of the waveguide.

The waveguide works in the same way as a wave plate to change the polari­zation state of light. When circularly-polarized light is sent into such a waveguide, its polari­zation state continues to change as it propagates in the waveguide and consequently, the photons exchange spin angular momentum with the waveguide. “Controlling polari­zation is critical for modern optical communi­cation. We know from theory that when polarization is changed in an optical fiber or a silicon waveguide, a torque is applied on them,” said Huan Li. “The mechanical effect is that the waveguide is  twisted [by light] by a very tiny amount that has not been previously measured.”

To measure this twisting caused by light, a small silicon beam inscribed with a high quality optical cavity is attached to the waveguide. This provides high measure­ment sensi­tivity to the rotation of the beam and the waveguide. The silicon beam is like the board of a seesaw and the waveguide is like the shaft in the center. When light twists the shaft, the latter rotates and the seesaw tilts, and this is  detected by the optical cavity. By changing the polari­zation of input light perio­dically, Mo Li’s team observed that the nano beam rotated perio­dically as well, revealing the optical torque applied on the wave­guide. “From the measure­ment results, we were able to calculate the spin angular momentum carried by a single photon, which equals to the fun­damental Planck constant multiplied by a factor that can be controlled by the waveguide geometry,” said Li He. “Our experiment reveals the quantum mechanical property of light on a chip.”

For Mo Li and his team, it is exciting that their experiment provides the first unambi­guous measurement of the spin angular momentum of photons and the optical torque generated in an integrated photonic device. The result of their experiment also demonstrates that optical torque is influenced by the geometric bire­fringence, in addition to the material of the wave­guide. Also, since the angular momentum of photons is inde­pendent of the frequency of light, the effect of optical torque is the same over the spectral band. (Source: U Minnesota)

Reference: Li He et al.: Optomechanical measurement of photon spin angular momentum and optical torque in integrated photonic devices, Sci. Adv. 2, e1600485 (2016); DOI: 10.1126/sciadv.1600485

Link: Dept. of Electrical and Computer Engineering, University of Minnesota, Minneapolis, USA

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