Session N04: Optics and Photonics

Time-Resolved Spectroscopy of Multifunctional Materials

The ability to control and manipulate coherent vibrational degrees of freedom, i.e. Coherent Phonons

(CPs) on a nanoscale length and picosecond time scale promises to be crucial in the fields of quantum

phononics.  In this talk, we focus on magneto-optical measurements at high magnetic fields of two multifunction material systems for developing concepts for new device functionalities: Multiferroic BaTiO₃-BiFeO3 films and nanorods, and GaMnAs, a ferromagnetic semiconductor.

BaTiO₃-BiFeO₃ is a multiferroic material with strong magneto-electric (ME) properties. Specifically, by detecting coherent phonons (CP) we provided detailed spectroscopy information on coherent states in this material system. Our results provide new insights into the ME signatures in these heterostructures.

Additionally, we present our experimental and theoretical studies on the generation and propagation of longitudinal coherent acoustic phonons by ultrafast laser pulses in a ferromagnetic GaMnAs film at high external magnetic fields. Our observation provides new information, reporting an extremely large increase in the amplitude of the CP when an external magnetic field is applied. The ability to control and manipulate coherent vibrational degrees of freedom on a nanoscale length and picosecond time scale promises to be revolutionary in the fields of quantum phononics.

    

Mapping photophysics properties of 3D and 2D-layered hybrid perovskites for energy-conversion applications

Hybrid organic-inorganic perovskites have emerged as attractive solution-processed semiconductors for energy-conversion applications. With solar cells reaching 26 percent certified efficiency, they are now the most efficient thin-film photovoltaic material, and are also touted for other technologies such as light-emitting diodes, nanoscale lasers and detectors.  

In this talk, we discuss the fundamental photophysical processes that have enabled these materials to be such efficient light absorbers and charge collectors. At present, a major obstacle to further improve performance in compact devices is to pinpoint the role of heterogeneity in modifying carrier dynamics. Here we describe our efforts at the University of Alabama at Birmingham to determine the role of microstructure in dictating the photophysics and optoelectronic properties of hybrid perovskites on femtosecond timescale and at nanometer length scale. First, using pump-probe broadband microscopy, we probe the ultrafast dynamics of carriers in large-grain thin films, highlighting the detrimental effects of boundaries and interfaces. Second, using cathodoluminescence-enabled electron microscopy coupled with machine learning techniques, we discuss a method to quantify nanoscale heterogeneity within a single large-grain thin film of hybrid perovskite,  that is with spatial resolution well below the optical diffraction limit. This combined methodology allows us to disentangle complex hyperspectral datasets and create maps of various photophysical processes in hybrid perovskites, such as photon recycling and band-edge emission.   

Angle-resolved cathodoluminescence polarimetry of hybrid organic-inorganic lead halide perovskites

Hybrid organic-inorganic lead halide perovskites have garnered a great deal of interest for cutting edge opto-electronics and it is crucial to understand how microstructure modifies light emission processes. While much work is focused on the spectral content and the quantum efficiencies, little is known about the emission directionality and the polarization of the emitted photons. Here we use angle-resolved cathodoluminescence microscopy, having spatial resolution well below the optical diffraction limit, to probe the directionality and full polarization state of emitted photons. Furthermore, we investigated the impact of a boundary on the degree of polarization and emission directionality.

    

Mechanism for Conductivity of AlGaN Semiconductors Determined from EPR Spectra

Al_xGa_1-xN is an ultrawide band gap (5-6 eV) semiconductor with applications in deep UV LEDs and high-power electronics. In order to enable function and improve performance, UV LEDs need a highly conducting layer which can be achieved with heavily Si-doped Al_xGa_1-xN. The purpose of this research is to develop an understanding of the mechanism for conductivity in Al_xGa_1-xN, x > 0.6, by using Electron Paramagnetic Resonance (EPR) spectroscopy. To achieve this, the line width of the EPR spectra and the electron spin densities were measured in doped Al_xGa_1-xN, with Si concentrations between 10¹⁸-10²⁰ cm^-3.  

In Al_xGa_1-xN:Si, Si acts as a shallow, neutral donor with an EPR g value of about 1.98. The temperature dependence of the donor EPR line width and concentration indicate band formation at lower temperatures (< 40 K) as well as the existence of the structurally relaxed, EPR silent, negative charge state of the donor at higher Al concentrations. In the talk, the analysis of the temperature dependence leading to impurity band formation, as well as the role of EPR in the measurements made will be discussed.   

Probing exciton dynamics in hybrid perovskites via femtosecond interferometric techniques

Using femtosecond microscopy and interferometric frequency-resolved autocorrelation (IFRAC), we study fundamental light excitations, i.e., excitons, in hybrid organic—inorganic perovskites, novel solution-processed semiconductors for optoelectronic devices. Here, using two identical phase-locked sub-50 fs pulses that are temporally delayed, we excite our sample with micron spatial resolution to study the ultrafast emission properties at both grain interiors and boundaries. By analyzing the interference fringes of the resulting IFRAC trace we can draw conclusions regarding coherent and incoherent emission processes as modified by grain micro-structuring. Our results provide insights to understanding how microstructure modifies the presence of excitons and their stability, critical in dictating device performance.

    

Unsupervised learning of nanophotonic optical responses via embedded spectra in t-distributed stochastic neighbor embedding methods

Improving and designing new nanophotonic systems require a deep understanding of what structural parameters can be tuned in a given environment to modify their optical properties. Traditionally, this process relied on linear or mean squared error measures which are not always suitable for highly similar spectra. However, recent progress has been made for such dataset by employing graphical networks to learn underlying non-linear structures. Here  we apply a novel mapping method of embedded space via injection of additional synthetic spectra to our complex original dataset of Mie scattering spectra and quantify the performance of our approximated model. We also discuss how the underlying latent space can be interpreted as a series of Markov processes which map the resonance shifts.