A multiquantum well (MQW) material created by Seth Bank, a professor of electrical and computer engineering, and his group at The University of Texas at Austin shows potential for nonlinear photonics applications. These engineered semiconductor heterostructures caught the attention of Pernille Undrum Fathi, a Ph.D. candidate in Federico Capasso’s group at Harvard University, and colleagues because their electronic structure can be designed to create a tailor-made nonlinear response at desired wavelengths.
As with bulk materials, researchers are searching for a way to efficiently access nonlinear properties—and for these semiconductor heterostructures, the strongest nonlinear response is difficult to access from free space. Working with Marcus Ossiander, a professor at the Technical University of Graz in Austria, Fathi and colleagues developed a metasurface approach to access this nonlinear response.
“We were inspired to combine this material with metasurfaces because it enabled devices where we control the light-matter interaction at multiple levels—from how the light distributes and behaves within the material using metasurfaces to how it interacts with electrons through the design of coupled quantum wells,” she says. “In combination, it yields a huge, tunable, and free-space-accessible second-order optical nonlinearity.”
It starts with a MQW material
Fathi and colleagues’ work starts with the MQW material developed and grown by Seth Bank’s group. “They use molecular beam epitaxy to grow nanometer-scale layers of different semiconductor materials and create asymmetric coupled quantum wells,” she says. “By carefully choosing the composition and thickness of these layers, we can engineer the electronic energy levels of the material and its nonlinear optical properties.”
Established nonlinear quantum well approaches tend to rely on transitions between energy levels with the conduction band, which can enable extremely strong nonlinearities. “But the photon energies we can reach are limited by the conduction band offset of the materials, so this approach is predominantly used at mid-infrared wavelengths,” she says. “Instead, we use transitions between the valence and conduction bands to allow us to extend the concept to much higher photon energies—including near-infrared wavelengths used for telecommunications, with a route toward the visible part of the spectrum.”
Electron transitions within the material are governed by selection rules that determine which polarizations must be present. “For the interband transitions we use, the result is a material with very good nonlinear properties—but it’s difficult to access these properties when illuminating the material from free space,” says Fathi.
By combining it with metasurfaces, which are engineered arrays of nanostructures with sub-wavelength spacing, “we can tailor the electromagnetic field response so that the light is able to drive the interband transition within the material,” she says. “The metasurface introduces a resonant mode with high field intensity and the correct polarizations, which enables good access to the nonlinear properties of the material.”
How does the metasurface work?
The metasurface works like a grating coupler to a waveguide. “We pattern an array of nanoscale titanium dioxide (TiO2) pillars atop the nonlinear material,” Fathi explains. “This periodic structure redirects the incoming light into the high refractive index film, where it’s reflected at the interfaces due to total internal reflection. If the light accumulates the correct round trip phase it becomes coupled to a resonant guided mode, which allows the field to build up within the nonlinear material before being out-coupled by the same nanostructures.”
The resonant mode the researchers couple to has electric field components both parallel and perpendicular to the quantum well layers to allow it to access the nonlinear tensor element of their material, which requires orthogonally polarized field components. “In other words, the metasurface provides two key things: It converts the polarization of the incident fields to match what’s needed to access the nonlinearity, and it resonantly enhances the fields to make the nonlinear interaction stronger,” she says.
Working with a new material brings along uncertainty and, unlike off-the-shelf materials, it tends to lead to shifts in the optical response and larger mismatches between simulations and experiments. “The high quality factor of the resonances made resolving it in linear measurements difficult,” Fathi says. “For a long time, we were relying entirely on the nonlinear signals to validate our measurements against simulations.”
Experimental work comes with its own challenges. “It requires a range of different areas of expertise, as well as fabrication and characterization facilities, so our samples are well traveled,” she says. “The material is grown by Seth Bank’s group at The University of Texas at Austin, characterized by Xiaoqing Pan’s group at the University of California, Irvine, and substrate transferred by Igal Brener’s group at Sandia National Laboratories, before finally arriving at Harvard, where we fabricated the metasurface and characterized the material and combined the metasurface-heterostructure device. This is an interdisciplinary project combining materials science and semiconductor physics with nanophotonics. Bridging the gaps in understanding so everyone speaks the same language when codesigning components of the final device was a key part of this large collaboration.”