The SH-generating quantum well layer (pink) and the beam-shaping split-ring metamaterial resonators (the “C”-shaped structures). The phase-coherent emission from individual resonators opens the door for many beam-shaping applications, some of which are presented in the insets. Each device type can be achieved via proper design of individual metamaterial resonators and their arrangement on the surface.

The SH-generating quantum well layer (pink) and the beam-shaping split-ring metamaterial resonators (the “C”-shaped structures). The phase-coherent emission from individual resonators opens the door for many beam-shaping applications, some of which are presented in the insets. Each device type can be achieved via proper design of individual metamaterial resonators and their arrangement on the surface.

Plasmonic phased-array sources* have been the subject of much interest recently. They can perform a many useful optical-beam manipulations such as beam steering, beam splitting, polarization manipulation, and angular momentum generation (see illustration). Usually, these sources operate through simple linear scattering of an incident laser beam. In a paper in Nature Communications, our research team demonstrates a new, nonlinear phased-array source at infrared frequencies that uses nanocavities coupled to highly nonlinear semiconductor heterostructures.

This is the first time that metamaterial nanocavities coupled to semiconductors have been used to generate light at new wavelengths and manipulate the resulting beams. The electric field of the incident fundamental beam drives a nonlinear polarization in the vicinity of the gap of the split-ring resonator. The nonlinear polarization then acts as a source that “feeds” the resonators at the second harmonic (SH) frequency.

Our team’s phased-array source concept is a single active layer (call it a ‘metasurface’) complemented with a semiconductor heterostructure. Each split-ring nanocavity resonator on the metasurface can act as a point source for a higher-order nanocavity emission. Each nanocavity resonator can be individually tailored with a specific optical phase: to manipulate angular momentum, polarization, spin, etc. A benefit of this approach is that the desired output beam is at a different frequency (wavelength) than the pump—and residual unscattered pump radiation can easily be removed, something that is not possible with simple, linear phased arrays.

*   Phased antenna arrays comprise ensembles of subwavelength sources, each radiating with a definite phase relationship relative to the other array elements.

   Omri Wolf, Salvatore Campione, Sheng Liu, and Igal Brener (all in Sandia’s Applied Photonic Microsystems Dept.); Ting Luk (in Sandia’s Nanostructure Physics Dept.); Emil Kadlec and Eric Shaner (both in in Sandia’s Laser, Optics & Remote Sensing Dept.); John Klem (in Sandia’s RF/Optoelectronics Dept.); Michael Sinclair (in Sandia’s Electronic, Optical, & Nano Materials Dept.); Alexander Benz (no longer at Sandia); and Arvind Ravikumar (Electrical Engineering Dept., Princeton Univ.).

   A semiconductor heterostructure is a stack of very thin (a few nanometers) semiconducting layers having different properties (in this case, bandgaps and doping). When properly designed, these layers can exhibit quantum phenomena due to electrons being confined to specific layers; this subclass of heterostructures is known as quantum wells (QWs) due to the shape of the electronic potential. QWs can be designed to have a multitude of interesting properties. In this work, we design them to support very large second-order optical nonlinearities.

This technology delivers second harmonic generation in a device that is extremely thin, which could lead to a whole new line of very compact infrared sensors and devices such as tunable filters, lenses, polarization devices, beam splitters/steerers, angular-momentum generators, detectors, and/or modulators (and also to advancements in quantum information science and quantum computing). As an example, our team demonstrated two-second harmonic phased-array sources that perform two optical functions at the second harmonic wavelength (~5 µm): a beam splitter and a polarizing beam splitter. Our metamaterial nanocavities coupled to highly nonlinear semiconductor heterostructures enhance second harmonic generation by orders of magnitude. Arrays of these coupled systems act like a phased array second harmonic source. Proper design of the nanocavity and nonlinear heterostructure will enable such phased arrays to span most of the infrared spectrum.

Our approach to phased-array sources at mid-infrared wavelengths extends our team’s past work on metamaterial nanocavities coupled to semiconductors by now including the optical nonlinearities of the quantum well (QW) to create a phase-locked, localized feed to resonators in the array. We also showed that we could manipulate the phase of the radiated second harmonic by changing the design of the metamaterial nanocavity. By spatially varying the nanocavity shape/orientation in the array, we are able to spatially vary the phase and manipulate the second harmonic beam.

  • We fabricated doubly resonant metamaterial arrays on top of a semiconductor heterostructure designed to have a large second-order nonlinearity (arising from intersubband transitions
[ISTs] in the QW).
  • Through measurements of the second harmonic radiation far-field pattern, we proved that the metamaterial resonators were emitting in a phase-coherent manner.
  • We varied the phase across the array at the second harmonic wavelength to create new functionality.
  • Metamaterial nanocavities were grown on top of an indium-gallium-arsenide/aluminum-indium-gallium-arsenide QW stack. The separations of subbands 1 → 2 and 2 → 3 are resonant with the fundamental beam, thereby causing the 3 → 1 separation to be resonant with the second harmonic.

    Metamaterial nanocavities were grown on top of an indium-gallium-arsenide/aluminum-indium-gallium-arsenide QW stack. The separations of subbands 1 → 2 and 2 → 3 are resonant with the fundamental beam, thereby causing the 3 → 1 separation to be resonant with the second harmonic.

    Exploiting the phase coherence of the second harmonic radiation, we designed and fabricated arrays with multiple resonators per unit cell. By adjusting the relative phase of the resonators within the unit cell we were able to experimentally demonstrate a polarization beam splitter combine with a source. For one polarization, a single output beam is generated at the second harmonic frequency in the broadside direction; and for an orthogonal polarization, two output lobes are produced at a predetermined angle.

    Exploiting the phase coherence of the second harmonic radiation, we designed and fabricated arrays with multiple resonators per unit cell. By adjusting the relative phase of the resonators within the unit cell we were able to experimentally demonstrate a polarization beam splitter combine with a source. For one polarization, a single output beam is generated at the second harmonic frequency in the broadside direction; and for an orthogonal polarization, two output lobes are produced at a predetermined angle.

    The nanocavities were designed to be resonant at both the fundamental and second harmonic frequencies. The second harmonic polarization depends quadratically on the fundamental E-field (left), so that it becomes symmetric with respect to the gap and can couple to the second harmonic resonance (right).

    The nanocavities were designed to be resonant at both the fundamental and second harmonic frequencies. The second harmonic polarization depends quadratically on the fundamental E-field (left), so that it becomes symmetric with respect to the gap and can couple to the second harmonic resonance (right).

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    Our preliminary results show that this design can be readily transferred to different wavelengths by changing the QW materials (e.g., III-nitrides have ISTs at near-infrared frequencies while most III–V heterostructures support ISTs in the terahertz range) and the nanocavity design.

    Although our example focuses on second harmonic generation, the new concept is general and can be applied to other types of nonlinear frequency generation. For example, large resonant third-order nonlinear susceptibilities have also been demonstrated in QWs and designing a triply resonant cavity is, in principle, achievable. We expect that our structure will serve as a model system for studying resonant nonlinearities in strongly coupled systems.

    This research was supported by the Department of Energy’s Office of Science Basic Energy Sciences (BES) Materials Sciences and Engineering (MSE) Division. All the work was done at Sandia: material growth was done at the Microsystems and Engineering Sciences Applications (MESA) Complex. Processing was done at MESA and the Center for Integrated Nanotechnologies (CINT) and made use of the extensive III-V semiconductor epitaxial growth, nanofabrication techniques, optical characterization, and modeling infrastructure at CINT.