The ferro-rotational order 1-3 , whose order parameter (OP) is an axial vector invariant under both time reversal (TR) and spatial inversion (SI) operations, is the last remaining category of ferroics to be observed after the ferroelectric, ferromagnetic, and ferro-toroidal orders. This order has become increasingly popular in many new quantum materials, especially in complex oxides 1,3 , and is considered responsible for a number of novel phenomena such as polar vortices 4 , giant magnetoelectric coupling 5 , and type-II multiferroics 6 . However, physical properties of the ferrorotational order have been rarely studied either theoretically or experimentally. Here, using high sensitivity rotational anisotropy second harmonic generation (RA SHG), we have, for the first time, exploited the electric quadrupole (EQ) contribution to the SHG to directly couple to this centrosymmetric ferro-rotational order in an archetype of type-II multiferroics, RbFe(MoO4)2. Surprisingly, we have found that two types of domains with opposite ferro-rotational vectors emerge with distinct populations at the critical temperature Tc ~195 K and gradually evolve to reach an even ratio at lower temperatures. Moreover, we have identified the ferro-rotational order phase transition as weak first order, and have revealed its conjugate coupling field as a unique combination of the induced EQ SHG and the incident fundamental electric fields. Our results on physical properties of a ferro-rotational order provide crucial knowledge for understanding and searching for novel phases of matter built upon the ferro-rotational order. Further, these results open the possibility of revealing unconventional centrosymmetric orders and identifying their conjugate coupling fields with second order nonlinear optics.

An echelle spectrometer has been constructed, which is optimized for use with a charge-coupled array detector, with dispersion and resolution appropriate for elemental analysis and plasma diagnostics. Design considerations, characterization, software, and initial experimental data are reported.

Surface plasmon polaritons (SPPs) are surface charge density oscillations localized to a metal-dielectric interface. In addition to being considered as promising candidates for a variety of applications, structures that support SPPs, including metal-insulator-metal (MIM) multilayers, are of fundamental interest because of the variety of collective plasmonic modes they support. Previously, a particular class of “forbidden” plasmon polariton modes (PPMs) was proposed that includes plasmon polariton modes confined to a region of dispersion space not typically accessible to surface-constructed collective excitations. Specifically, for these modes, known as Guided Wave PPMs (GW-PPMs), due to the dielectric asymmetry of the central layer, the solution to the wave equation in the center insulator layer is oscillatory while remaining surface bound both to the supporting substrate and the exposed surface. These modes are supported by a simple physical structure that results from a minor symmetry modification of the traditional MIM structure, specifically the use of a central insulator layer with a higher refractive index than the supporting substrate. However, they display fundamental properties that are distinctly different from those of standard SPPs and from recently reported hybrid plasmonic modes. While GW-PPMs have been explored theoretically, they have not yet been realized experimentally. In this article, we present the first experimental demonstration of GW-PPMs. Specifically, we excite and detect GW-PPMs at visible frequencies and match model predictions to experimental results with remarkable accuracy using minimal parameter fitting. In addition to the experimental detection, we calculate and report on other interesting and relevant features of the detected modes, including the associated electric field profiles, confinement values, and propagation lengths, and discuss in terms of the applications-relevance of GW-PPMs.

Frequency combs, broadband light sources whose spectra consist of coherent, discrete modes, have become essential in many fields. Miniaturizing frequency combs would be a significant advance in these fields, enabling the deployment of frequency-comb based devices for diverse measurement and spectroscopy applications. We demonstrate diode-laser based frequency comb generators. These laser diodes are simple, electrically pumped, inexpensive and readily manufactured. Each chip contains several dozen diode-laser combs. We measure the time-domain output of a diode frequency comb to reveal the underlying frequency dynamics responsible for the comb spectrum, conduct dual comb spectroscopy of a molecular gas with two devices on the same chip, and demonstrate that these combs can be battery powered.

No abstract