The propagation of light is usually interchangeable, which means that the trajectory of light traveling in one direction is the same as the trajectory of light traveling in the opposite direction. Breaking reciprocity allows light to travel in only one direction. Optical components that support this unidirectional optical flow, such as isolators and circulators, are integral building blocks in many modern laser and communication systems. Currently, it is almost entirely based on the magneto-optical effect, making the device bulky and difficult to integrate. Therefore, magnetic-free paths for non-reciprocal light propagation in many optical applications are in high demand, and now scientists have developed a new type of optical subsurface.

The propagation of light is usually interchangeable, which means that the trajectory of light traveling in one direction is the same as the trajectory of light traveling in the opposite direction. Breaking reciprocity allows light to travel in only one direction. Optical components that support this unidirectional optical flow, such as isolators and circulators, are integral building blocks in many modern laser and communication systems. Currently, it is almost entirely based on the magneto-optical effect, making the device bulky and difficult to integrate. Therefore, magnetic-free paths for non-reciprocal light propagation in many optical applications are in high demand, and now scientists have developed a new type of optical subsurface.

It spatially and temporally phase modulates the reflected light, resulting in different forward and backward propagation paths for the light. Nonreciprocal transport of light in free space is experimentally achieved using ultrathin components. "This is an optical quasi-surface with controllable ultrafast time-varying properties, capable of breaking optical reciprocity without giant magnets," said Xingjie Ni, assistant professor of Charles H. Fett in Penn State's Department of Electrical Engineering. The findings were published in the journal Light: Science and Applications. The ultrathin subsurface consists of a silver back reflector that supports the bulk silicon nanoantennas.

It has a large nonlinear Kerr index at the near-infrared wavelength of about 860 nm. Using heterodyne interference between two laser lines with similar frequencies, traveling-wave refractive index modulation is generated on the nanoantenna, resulting in ultrafast spatiotemporal phase modulation with a temporal modulation frequency of 2.8 THz. This dynamic modulation technique exhibits flexibility in tuning spatially and temporally modulated frequencies. In the subwavelength interaction range of 150 nm, the experiment achieves complete asymmetric reflection of forward and backward light transmission with a bandwidth of about 5.77 THz. Momentum shifts due to spatial phase gradients and frequency shifts due to temporal modulation are obtained from the spatiotemporal subsurface reflected light.

It exhibits asymmetric photon conversion between forward reflection and backward reflection. Furthermore, by exploiting the unidirectional momentum transfer provided by the subsurface geometry, selective photon conversion can be freely controlled by engineering an undesired output state, which is located in the forbidden region, i.e., the non-propagating region. This method shows flexibility in controlling light in momentum and energy space. It will provide a new platform for exploring interesting physics arising from time-varying material properties, and will open a new paradigm in the development of scalable, integratable, magnetless, non-reciprocal devices.

Creating materials with time-varying properties is critical to breaking the reciprocity that imposes fundamental constraints on wave propagation. However, achieving ultrafast temporal modulation in photonic systems is very challenging. Using the spatial and temporal phase manipulation provided by ultrathin nonlinear deformable surfaces, the new study experimentally demonstrates non-reciprocal light reflection at wavelengths around 860 nm. The subsurface undergoes traveling wave modulation on nonlinear Kerr building blocks, resulting in spatial phase gradients and multi-terahertz temporal phase jitter, resulting in unidirectional photon transitions in both momentum space and energy space. The new method highlights a potential means of fabricating miniaturized and integratable non-reciprocal optical components.

Accelerate to a similar strength, the required volume is only 1/10,000 of the ordinary accelerator.

This method does not simply gather the lasers in one place, but engraves the laser into a certain shape through special optical elements. Invent an optical element like an amphitheater with concentric steps of different radii. When using this element to focus the high-energy laser, different concentric circles will cause the laser to have different delays, which is equivalent to "imprinting" the laser pulse into a certain shape.

When the imprinted pulse enters the plasma, it creates a wake similar to the wake of a motorboat running on the water. The wake travels at the speed of light, and electrons are accelerated by the wake, just as a water skier rides a boat's wake. Electrons can accelerate beyond the speed of light and continue to accelerate, the researchers say.

While this research is still in the theoretical stage, researchers at the University of Rochester are planning to build EP-OPAL, the world's most powerful laser accelerator. By then, this device will be able to accelerate electrons to a level that cannot be achieved by current technology, providing a powerful tool for high-speed particle research.