- Researchers develop a system for converting one wavelength of light into another, even if they don’t have same phase or momentum.
- It relies on a metasurface made of silicon nanostructures, integrated into lithium niobate waveguide.
- It efficiently works over a wide bandwidth range.
Building integrated photonic circuits, which use light instead of electron to carry and send data, is a quite complex task. It involves many difficult challenges, and one of them is controlling the momentum of light.
Different light colors travel at different speeds through a medium. To make the light switch between colors, it must have the same phase or momentum. In fact, several instruments have been built to match phase or momentum of the light at different points throughout an integrated circuit.
This time, researchers at Harvard University and Columbia University have developed a system that can convert one wavelength of light into another even if they don’t have same phase or momentum.
The Color Changing Converter
In order to make the wavelength conversion process effective and efficient, it must be designed to match phase, and it only operates at a single wavelength. However, the new device doesn’t need to fulfill the phase or momentum-matching requirement, and it can convert light in a wide range of colors.
The working of device relies on a metasurface, which is made of silicon nanostructures, integrated into lithium niobate waveguide. The light interacts with these nanostructures while passing through a waveguide. The nanostructures receive optical signal, changes its phase and re-emit it back into the waveguide.
Schematic of an integrated nonlinear photonic device
Since the light interacts with the metasurface while being confined inside a waveguide, one can take advantage of both the long interaction distance and phase control from the metasurface.
Along with controlling and converting the polarization and mode of a guided wave, the team showed that they could increase the frequency of a wavelength by 100 percent, changing infrared colors to red, with higher efficiency over a wide bandwidth range.
How this integrated metasurface is different than other momentum-matching approaches, you asked? Well, it offers unidirectional optical phase to attach optical energy from one to another color module while preventing the inverse process. This is kind of essential for realizing broadband nonlinear conversion.
Scanning electron microscope image of a fabricated device
More specifically, the team showed unidirectional effective wavevector provided by gradient metasurfaces enables one-way transmission of optical power from the pump to second harmonic signal. It represents an eccentric scheme of momentum-matching nonlinear generation, where the nonlinear generation efficiency is not sensitive to the instrument geometry and variation of pump frequency.
They demonstrated robust and efficient second harmonic generation, where second harmonic signal monotonically rises over many coherence lengths inside lithium niobate waveguides patterned with gradient metasurfaces. One more thing, efficient second harmonic generations are observed over broad wavelengths (1580 to 1650 nanometers).
Drawback and Future Work
One limitation of the current technique is the difficulty in collecting efficient light from the generated higher order waveguide modes. However, this issue could be solved by utilizing a single antenna array to transform the second harmonic light into one particular mode, and extracting the rest of the pump light in an adjacent waveguide to repeat the procedure.
The research team plans to demonstrate similar devices for realizing other useful functionalities, like optical modulation.