Mohan Wang

Photonic topological boundary pumping as a probe of 4D quantum Hall physics

Oded Zilberberg, Sheng Huang, Jonathan Guglielmon, Mohan Wang, Kevin Chen, Yaacov E. Kraus, † and Mikael C. Rechtsman Institute for Theoretical Physics, ETH Zurich, 8093 Zürich, Switzerland Department of Electrical and Computer Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA Department of Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, USA Department of Physics, Holon Institute of Technology, Holon 5810201, IsraelWhen a two-dimensional (2D) electron gas is placed in a perpendicular magnetic field, its in-plane transverse conductance becomes quantized; this is known as the quantum Hall effect. It arises from the non-trivial topology of the electronic band structure of the system, where an integer topological invariant (the first Chern number) leads to quantized Hall conductance. It has been shown theoretically that the quantum Hall effect can be generalized to four spatial dimensions, but so far this has not been realized experimentally because experimental systems are limited to three spatial dimensions. Here we use tunable 2D arrays of photonic waveguides to realize a dynamically generated four-dimensional (4D) quantum Hall system experimentally. The inter-waveguide separation in the array is constructed in such a way that the propagation of light through the device samples over momenta in two additional synthetic dimensions, thus realizing a 2D topological pump. As a result, the band structure has 4D topological invariants (known as second Chern numbers) that support a quantized bulk Hall response with 4D symmetry. In a finite-sized system, the 4D topological bulk response is carried by localized edge modes that cross the sample when the synthetic momenta are modulated. We observe this crossing directly through photon pumping of our system from edge to edge and corner to corner. These crossings are equivalent to charge pumping across a 4D system from one three-dimensional hypersurface to the spatially opposite one and from one 2D hyperedge to another. Our results provide a platform for the study of higher-dimensional topological physics.

Modified single crystal fibers for distributed sensing applications

Single crystal fibers like those made from sapphire are capable of operating at higher temperatures than conventional silica-glass-based fibers. This work aims to construct single-crystal optical fiber sensors capable of providing environmental data in combustion, high-temperature chemical processing, or power generation applications where temperatures exceed 1000 °C and standard silica fibers cease to provide useful information. Here, we explore the functionalization of single crystal fibers using methodologies intrinsic to the crystal growth process or with methods which do not severely reduce their operating temperature range. While operating a laser-heated pedestal growth system to produce single-crystal optical fibers from rod feedstock, we continuously vary parameters such as fiber diameter to produce novel single-crystal linear distributed-sensing devices. The spectral characteristics of those modified devices, along with sensing performance in a high-temperature harsh-environment are reported. Finally, a technique for increasing the intrinsic Rayleigh backscattering using femtosecond laser irradiation is discussed for temperature sensing applications.

Adaptive laser shock micro-forming for MEMS device applications

Laser shock micro-forming is a non-thermal laser forming method that uses laser-induced shockwave to modify surface properties and to adjust shapes and geometry of work pieces. The magnitude and spatial distribution of the laser-induced shockwaves depend on the energy profiles of the laser beam focused on sample surfaces. In this paper, we present an adaptive optical technique to engineer spatial profiles of laser beams to control the shapes, sizes, and locations of the laser-induced shockwaves and the resulting forming features. Using a spatial light modulator, this adaptive laser beam forming tool was used to process free-standing MEMS structures in aluminum, which has led to highly uniform forming features. Shockwave simultaneously excited by multiple laser beams generated by the spatial light modulator and its effects on the micro-forming process were also studied. The results presented in this paper show that the adaptive optics laser beam forming is an effective and flexible method to generate shockwave with various shapes and sizes of wavefront and at multiple locations for laser processing at microscales.

Discrimination of Temperature and Strain in Brillouin Optical Time Domain Analysis Using a Multicore Optical Fiber

Brillouin optical time domain analysis is the sensing of temperature and strain changes along an optical fiber by measuring the frequency shift changes of Brillouin backscattering. Because frequency shift changes are a linear combination of temperature and strain changes, their discrimination is a challenge. Here, a multicore optical fiber that has two cores is fabricated. The differences between the cores’ temperature and strain coefficients are such that temperature (strain) changes can be discriminated with error amplification factors of 4.57 °C/MHz (69.11 μϵ/MHz), which is 2.63 (3.67) times lower than previously demonstrated. As proof of principle, using the multicore optical fiber and a commercial Brillouin optical time domain analyzer, the temperature (strain) changes of a thermally expanding metal cylinder are discriminated with an error of 0.24% (3.7%).

Radiation resilient fiber Bragg grating sensors for sensing applications in nuclear reactor cores

This paper reports testing results of radiation resilient fiber Bragg grating (FBG) in radiation resistant fibers in the nuclear reactor core at MIT Research Reactor Lab. FBGs were fabricated by 140-fs ultrafast laser pulse using a phase mask approach. In-core test of fiber Bragg gratings was carried out in the core region of a 6-MW research reactor at temperature > 600°C and an average fast neutron (>1 MeV) flux >1×1014 n/s/cm2. First 100-day tests of FBG sensors shows less than 5 dB reduction in FBG peak strength after over 1×1020 n/cm2 of accumulated fast neutron dosage. To test temporal responses of FBG sensors, a number of reactor anomaly events were artificially created to abruptly change reactor power, temperature, and neutron flux over short periods of time. The thermal optical coefficients and temporal responses of FBG sensors are determined at different accumulated dosages of neutron flux. Results presented in this paper reveals that temperature-stable Type-II FBGs fabricated in radiation-hardened fibers could be used as sensors to perform in-pile measurements to improve safety and efficiency of existing and next generation nuclear reactors.

Dual-core optical fibers for simultaneous measurements of temperature and strain using Brillouin OTDA

We report a dual-core fiber for simultaneous sensing of strain and temperature using BOTDA. By adjusting dopant compositions, 37% difference in strain-optical coefficient was achieved between two cores to differentiate temperature and strain responses.

Adaptive laser beam forming for laser shock micro-forming for 3D MEMS devices fabrication

Laser shock micro-forming is a non-thermal laser forming method that use laser-induced shockwave to modify surface properties and to adjust shapes and geometry of work pieces. In this paper, we present an adaptive optical technique to engineer spatial profiles of the laser beam to exert precision control on the laser shock forming process for free-standing MEMS structures. Using a spatial light modulator, on-target laser energy profiles are engineered to control shape, size, and deformation magnitude, which has led to significant improvement of the laser shock processing outcome at micrometer scales. The results presented in this paper show that the adaptive-optics laser beam forming is an effective method to improve both quality and throughput of the laser forming process at micrometer scales.

Dual-Core Fiber Characterizations for Distributed Simultaneous Temperature and Strain Measurements Using Brillouin Optical Time Domain Analysis

This paper reports characterizations of dual-core optical fiber designed for simultaneous temperature and strain measurements using Brillouin optical time domain analysis