Nishant Nookala

Gradient Nonlinear Pancharatnam-Berry Metasurfaces

We apply the Pancharatnam-Berry phase approach to plasmonic metasurfaces loaded by highly nonlinear multiquantum-well substrates, establishing a platform to control the nonlinear wave front at will based on giant localized nonlinear effects. We apply this approach to design flat nonlinear metasurfaces for efficient second-harmonic radiation, including beam steering, focusing, and polarization manipulation. Our findings open a new direction for nonlinear optics, in which phase matching issues are relaxed, and an unprecedented level of local wave front control is achieved over thin devices with giant nonlinear responses.

Ultrathin gradient nonlinear metasurface with a giant nonlinear response

Gradient metasurfaces have recently been demonstrated to provide control of the phase of scattered fields over subwavelength scales, enabling a broad range of linear optical components in a flat, ultrathin, integrable platform. Additionally, the development of nonlinearmetasurfaces has disrupted conventional nonlinear optical device design by relaxing phase matching constraints, reducing size and dimensionality, and providing record values of localized nonlinear responses. However, extending the “flat optics” paradigm to the nonlinear case faces important challenges, since we are required to simultaneously achieve efficient frequency conversion and sub-diffractive phase control. Here, we experimentally demonstrate continuous phase control of the giant nonlinear second harmonic optical response from metasurfaces tied to intersubband transitions in semiconductor multi-quantum wells, establishing an exciting path toward realizing the vision of flat, nonlinear optics.

Strain compensated superlattices on m-plane gallium nitride by ammonia molecular beam epitaxy

The results of tensile strained AlN/GaN, AlGaN/GaN, and compressive strained InGaN/GaN superlattices (SLs) grown by Ammonia MBE (NH3-MBE) are presented. A combination of atom probe tomography and high-resolution X-ray diffraction confirms that periodic heterostructures of high crystallographic quality are achieved. Strain induced misfit dislocations (MDs), however, are revealed by cathodoluminescence (CL) of the strained AlN/GaN, AlGaN/GaN, and InGaN/GaN structures. MDs in the active region of a device are a severe problem as they act as non-radiative charge recombination centers, affecting the reliability and efficiency of the device. Strain compensated SL structures are subsequently developed, composed of alternating layers of tensile strained AlGaN and compressively strained InGaN. CL reveals the absence of MDs in such structures, demonstrating that strain compensation offers a viable route towards MD free active regions in III-Nitride SL based devices.

Nonlinear optics with quantum-engineered intersubband metamaterials

Intersubband transitions in n-doped semiconductor heterostructures provide the possibility to quantum engineer one of the largest known nonlinear optical responses in condensed matter systems, limited however to electric field polarized normal to the semiconductor layers. Here we show that by coupling of electromagnetic modes in plasmonic metasurfaces with quantum-engineered intersubband transitions in semiconductor heterostructures one can create ultra-thin highlynonlinear metasurfaces for normal light incidence. Structures discussed here represent a novel kind of hybrid metalsemiconductor metamaterials in which exotic optical properties are produced by coupling electromagneticallyengineered modes in dielectric and plasmonic nanostructures with quantum-engineered intersubband transitions in semiconductor heterostructures. Record values of effective optical nonlinearities of over 400 nm/V are experimentally measured for metasurfaces optimized for efficient second harmonic generation at 9.7 μm pump wavelength under normal incidence.

Quantum Confinement in Oxide Heterostructures: Room-Temperature Intersubband Absorption in SrTiO3/LaAlO3 Multiple Quantum Wells

The Si-compatibility of perovskite heterostructures offers the intriguing possibility of producing oxide-based quantum well (QW) optoelectronic devices for use in Si photonics. While the SrTiO3/LaAlO3 (STO/LAO) system has been studied extensively in the hopes of using the interfacial two-dimensional electron gas in Si-integrated electronics, the potential to exploit its giant 2.4 eV conduction band offset in oxide-based QW optoelectronic devices has so far been largely ignored. Here, we demonstrate room-temperature intersubband absorption in STO/LAO QW heterostructures at energies on the order of hundreds of meV, including at energies approaching the critically important telecom wavelength of 1.55 μm. We demonstrate the ability to control the absorption energy by changing the width of the STO well layers by a single unit cell and present theory showing good agreement with experiment. A detailed structural and chemical analysis of the samples via scanning transmission electron microscopy and electron energy loss spectroscopy is presented. This work represents an important proof-of-concept for the use of transition metal oxide QWs in Si-compatible optoelectronic devices.

Flat nonlinear optics: metasurfaces for efficient frequency mixing

Gradient metasurfaces, or ultrathin optical components with engineered transverse impedance gradients along the surface, are able to locally control the phase and amplitude of the scattered fields over subwavelength scales, enabling a broad range of linear components in a flat, integrable platform1–4. On the contrary, due to the weakness of their nonlinear optical responses, conventional nonlinear optical components are inherently bulky, with stringent requirements associated with phase matching and poor control over the phase and amplitude of the generated beam. Nonlinear metasurfaces have been recently proposed to enable frequency conversion in thin films without phase-matching constraints and subwavelength control of the local nonlinear phase5–8. However, the associated optical nonlinearities are far too small to produce significant nonlinear conversion efficiency and compete with conventional nonlinear components for pump intensities below the materials damage threshold. Here, we report multi-quantum-well based gradient nonlinear metasurfaces with second-order nonlinear susceptibility over 106 pm/V for second harmonic generation at a fundamental pump wavelength of 10 μm, 5-6 orders of magnitude larger than traditional crystals. Further, we demonstrate the efficacy of this approach to designing metasurfaces optimized for frequency conversion over a large range of wavelengths, by reporting multi-quantum-well and metasurface structures optimized for a pump wavelength of 6.7 μm. Finally, we demonstrate how the phase of this nonlinearly generated light can be locally controlled well below the diffraction limit using the Pancharatnam-Berry phase approach5,7,9, opening a new paradigm for ultrathin, flat nonlinear optical components.

Ultrathin nonlinear metasurfaces

We present a novel class of ultrathin metasurfaces operating in a nonlinear regime, simultaneously providing generation efficiencies that are many orders of magnitude larger than in other nonlinear setups, and, at the same time, capable of controlling the local phase of the nonlinear signal with high precision and subwavelength resolution. The key to achieving such outstanding performance is combining a strong local field enhancement and polarization selectivity of plasmonic nano-antennas with extremely high nonlinearity of multi-quantum well semiconductor stacks. In this work, we discuss the operation principles of such metasurfaces and provide experimental and numerical results. We also show how a savvy application of Lorentz reciprocity principle allows for fast and efficient analysis and modeling of such metasurfaces consisting of thousands of elements.

Ultrathin gradient nonlinear metasurfaces with giant nonlinear response(Conference Presentation)

Extending the ‘flat optics’ paradigm to the nonlinear optics faces important challenges, since, for any practical situation, we are required to simultaneously achieve sub-diffraction phase control and efficient frequency conversion in metasurfaces of sub-wavelength thickness. Here, we experimentally demonstrate giant nonlinear response and continuous phase control of the giant nonlinear response in metasurfaces based on plasmonic nanoresonators coupled to intersubband transitions in semiconductor multi-quantum wells. Over 0.075% of second-harmonic power conversion efficiency is achieved experimentally in a 400-nm-thick metasurface using 10 microns wavelength pump with 20 kW/cm2 intensity.

Giant nonlinear response of polaritonic metasurfaces coupled to intersubband transitions

We report highly-nonlinear metasurfaces based on combining electromagnetically-engineered plasmonic nanoresonators with quantum-engineered intersubband nonlinearities. Experimentally, effective nonlinear susceptibility over 480 nm/V was measured for second-harmonic generation at normal incidence.

Highly-nonlinear quantum-engineered polaritonic metasurfaces

Intersubband transitions in n-doped semiconductor heterostructures allow one to quantum-engineer one of the largest known nonlinear response in condensed matter systems but only for the electric field polarized normal to semiconductor layer. By coupling of a quantum-engineered multi-quantum-well semiconductor layer with electromagnetically-engineered plasmonic elements we may produce ultrathin metasurfaces with giant nonlinear response. Here we experimentally demonstrate metasurfaces designed for second harmonic generation at λ≈9.9 μm with a record-high nonlinear response for condensed-matter systems in infrared/visible spectral range, up to 1.17×106 pm/V. The practical impact of the nonlinear metasurfaces proposed here may be extended to a variety of fields, including THz generation and detection, phase conjugation, and other nonlinear optical processes.