Optical pumping

Optical pumping is a process in which light is used to raise (or "pump") electrons from a lower energy level in an atom or molecule to a higher one. It is commonly used in laser construction to pump the active laser medium so as to achieve population inversion. The technique was developed by the 1966 Nobel Prize winner Alfred Kastler in the early 1950s. Optical pumping is also used to cyclically pump electrons bound within an atom or molecule to a well-defined quantum state. For the simplest case of coherent two-level optical pumping of an atomic species containing a single outer-shell electron, this means that the electron is coherently pumped to a single hyperfine sublevel (labeled m_F!), which is defined by the polarization of the pump laser along with the quantum selection rules. Upon optical pumping, the atom is said to be oriented in a specific m_F! sublevel, however, due to the cyclic nature of optical pumping, the bound electron will actually be u

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Effective quality factor tuning mechanisms in micromechanical resonators

Azadeh Ansari, Luis Guillermo Villanueva Torrijo

Quality factor (Q) is an important property of micro- and nano-electromechanical (MEM/NEM) resonators that underlie timing references, frequency sources, atomic force microscopes, gyroscopes, and mass sensors. Various methods have been utilized to tune the effective quality factor of MEM/NEM resonators, including external proportional feedback control, optical pumping, mechanical pumping, thermal-piezoresistive pumping, and parametric pumping. This work reviews these mechanisms and compares the effective Q tuning using a position-proportional and a velocity-proportional force expression. We further clarify the relationship between the mechanical Q, the effective Q, and the thermomechanical noise of a resonator. We finally show that parametric pumping and thermal-piezoresistive pumping enhance the effective Q of a micromechanical resonator by experimentally studying the thermomechanical noise spectrum of a device subjected to both techniques. (C) 2018 Author(s).

2018

Nuclear polarization by optical pumping in InP:Fe above liquid nitrogen temperature

Jean-Philippe Ansermet, Murari Soundararajan, Dongyoung Yoon

Hyperpolarized nuclear spins are observed in optically pumped iron-doped InP from 70 K to 140 K. P-31 NMR was carried out at 9.28 T (159.8 MHz) during optical excitation with circularly polarized light, using a laser diode (lambda-830 nm) as a source. The enhancement of the nuclear spin polarization by optical pumping at 70 K is estimated to be about 34 for those nuclei in the region of the sample absorbing light. This enhancement decreases with increasing temperature. As the direction of the enhanced nuclear spin polarization is found parallel or antiparallel to the travelling direction of the sigma(+) or sigma(-), the contact hyperfine interaction is dominant compared to the dipolar hyperfine interaction. (C) 2015 Elsevier Inc. All rights reserved.

Academic Press Inc Elsevier Science2015

Electrical detection of spin hyperpolarization in InP

Jean-Philippe Ansermet, Christian Caspers

The electrical detection of surface spin polarization in Indium Phosphide (InP) is demonstrated. Using a planar four-terminal architecture on top of semi-insulating Fe: InP (001) wafers, optical orientation is separated from electrical detection. Spin filter tunnel contacts consisting of InP/oxide/Co reveal significant asymmetries in the differential resistance upon helicity change of the optical pumping. The iron-rich tunnel oxide provides the main spin selection mechanism. A reproducible helicity-dependent asymmetry as high as 18% could be observed at T = 55K and an external induction field mu H-0 = 1 T. At room temperature and zero external field, a helicity-dependent asymmetry of 6% suggests the stand-alone applicability of the device either as an electronic spin sensor or as an optical helicity sensor. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

Amer Inst Physics2014

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