Anomalous Nonlinear Optical Selection Rules in Metallic Quantum Wells

Abstract

Intersubband transitions (ISBTs) in conduction-band quantum wells (QWs) have attracted tremendous attention for their high technological potential, ranging from quantum cascade lasers, quantum well infrared photodetectors, to various nonlinear optical elements. One of the main characteristics of using the ISBTs is their polarization selection rule, which forbids a normalincidence geometry. Here, it is shown that the ISBT selection rule is not strict on optical nonlinearities in metallic QWs (MQWs). The nonlinear process of second harmonic generation nearly follows the selection rule, while the optical Kerr nonlinear process severely deviates from it. The anomalous optical selection rules result from the non-negligible ultrafast electron-electron scattering in these plasmonic systems, and a coupled mode theory is provided to get a physical grasp of the problem. The flexible selection rule in MQWs could bring drastic improvements in efficiency and diversity of ISBT-based devices.

Nonlinear Computational Edge Detection Metalens

“…The metalens relies on the resonators‐on‐a‐mirror structure to tailor a phase profile and form an image of an object in its reflection mode. Because the TiN quantum wells possess giant Kerr nonlinearity, [ 38 ] the optical constants of the TiN/Al 2 O 3 multilayers and thus the resultant phase of the metalens becomes tunable with respect to the local illumination intensity. Therefore, this nonlinear metalens acts as not only an imaging element but also an image processing element, which transforms the image by superimposing an additional phase profile defined by the intensity distribution in space.…”

“…[ 41 ] The giant Kerr‐type nonlinearity in metallic quantum well system mainly stems from the intersubband transition and can be aligned at the desired wavelength by tuning the quantum well thickness. [ 38 ] In this work, 2.4 nm TiN quantum wells are chosen for the computational metalens, enabling intersubband transition enhanced giant Kerr nonlinearity with working wavelength also around 800 nm. The Z‐Scan experiment for the nonlinear property is described in Supporting information, Section S3.…”

Roadmap on electromagnetic metamaterials and metasurfaces

“…Recently, it has been shown that ultrathin metallic quantum well possesses exceptionally large nonlinearity at visible to near-IR frequency range. A nonlinear metasurface based on such material system will revolutionize the field of tunable computational metasurface, laying the cornerstone for nonlinear optical computations [177][178][179].…”

Effect of Large Lattice Mismatch on Superconductivity and Plasmonic Properties of Epitaxial NbTiN Films

“…3 Anomalous nonlinear optical selection rules that are very useful for many nonlinear optical elements have been observed in metallic quantum wells with an ultrathin plasmonic TiN layer sandwiched by Al 2 O 3 layers. 4 To construct single-crystalline rock-salt TMNs/metal−oxide heterostructures with superior performance, the lattice mismatch between these two functional layers is a nonneglectable factor. The difficulty of fabricating single-crystalline heterostructures with a lattice mismatch of over 5% can hinder the epitaxial integration of rock-salt TMNs with many functional oxides that span a wide spectrum of lattice mismatch.…”

“…For example, unexpected emergent interfacial room-temperature ferromagnetism has been found in CrN/Cr 2 O 3 superlattices . Anomalous nonlinear optical selection rules that are very useful for many nonlinear optical elements have been observed in metallic quantum wells with an ultrathin plasmonic TiN layer sandwiched by Al 2 O 3 layers …”