Measurement of the scalar third-order electric polarizability of the Cs ground state using coherent-population-trapping spectroscopy in Ramsey geometry

The ac-Stark shift induced by blackbody radiation is a major source of systematic uncertainty in present-day cesium microwave frequency standards. The shift is parametrized in terms of a third-order electric polarizability α(3)0 that can be inferred from the static electric-field displacement of the clock transition resonance. In this paper, we report on an all-optical coherent-population-trapping pump-probe experiment measuring the differential polarizability Δα(3)0=α(3)0(F=4)−α(3)0(F=3) on a thermal Cs atomic beam, from which we infer α(3)0(F=4)=2.023(6)stat(9)systHz/(kV/cm)2, which corresponds to a scalar Stark shift parameter ks=−2.312(7)stat(10)systHz/(kV/cm)2. The result agrees within two standard deviations with a recent measurement in an atomic fountain, and rules out another recent result obtained in a Cs vapor cell

CPT-pump-probe measurement of the Cs clock transition DC Stark shift

We report progress in measuring the third order electric polarizability of the Cs ground states using a Ramsey pump-probe experiment on coherent population trapped (CPT) atoms in a thermal atomic beam. We give a short description of the apparatus as well as the Fourier transform method used to monitor the phase and frequency of the Ramsey signal. Analysis of a typical data set is shown, proving the consistency of the method. We show that the motional magnetic field phase shift can be used to test the reliability of the electric field modeling. Finally, we give a preliminary value for the Cs ground state polarizability and compare it to previous published values of the DC Stark shift

Defect detection in monocrystalline silicon wafers using high frequency guided waves

Monocrystalline silicon wafers are employed in the photovoltaic industry for the manufacture of solar panels with high conversion efficiency. Micro-cracks can be induced in the thin wafer surfaces during the cutting process. High frequency guided waves are considered for the testing of the wafers and the nondestructive characterization of the micro-cracks. Experimentally selective excitation of the fundamental Lamb wave modes was achieved using a custom-made angle beam transducer and holder to achieve a controlled contact pressure. The out-of-plane component of the guided wave propagation was measured using a noncontact laser interferometer, scanned parallel to the specimen surface using a positioning system. The material anisotropy of the monocrystalline silicon leads to variations of the guided ultrasonic wave characteristics depending on the propagation direction relative to the monocrystalline silicon orientation. In non-principal directions of the crystal, wave beam skewing occurs due to material anisotropy. Artificial defects were introduced in the wafers using a micro indenter with varying force. The defects were characterized from microscopy images to measure the indent depth and combined crack lengths. The scattering of the A0 Lamb wave mode was measured experimentally. The scattered wave field showed high amplitude peaks close to the defect location and an interference pattern indicative of a scattered wave, but was found to be not symmetric to the defect and crystallographic orientations. Characteristics of the scattered wave amplitudes were correlated to the defect size and the detection sensitivity discussed

High frequency guided wave propagation in monocrystalline silicon wafers

Monocrystalline silicon wafers are widely used in the photovoltaic industry for solar panels with high conversion efficiency. The cutting process can introduce micro-cracks in the thin wafers and lead to varying thickness. High frequency guided ultrasonic waves are considered for the structural monitoring of the wafers. The anisotropy of the monocrystalline silicon leads to variations of the wave characteristics, depending on the propagation direction relative to the crystal orientation. Full three-dimensional Finite Element simulations of the guided wave propagation were conducted to visualize and quantify these effects for a line source. The phase velocity (slowness) and skew angle of the two fundamental Lamb wave modes (first anti-symmetric mode A0 and first symmetric mode S0) for varying propagation directions relative to the crystal orientation were measured experimentally. Selective mode excitation was achieved using a contact piezoelectric transducer with a custom-made wedge and holder to achieve a controlled contact pressure. The out-of-plane component of the guided wave propagation was measured using a noncontact laser interferometer. Good agreement was found with the simulation results and theoretical predictions based on nominal material properties of the silicon wafer

High-Frequency Guided Wave Propagation and Scattering in Silicon Wafers

Abstract Thin monocrystalline silicon wafers are employed for the manufacturing of solar cells with high conversion efficiency. Micro-cracks can be induced by the wafer cutting process, leading to breakage of the fragile wafers. High-frequency guided waves allow for the monitoring of wafers and detection and characterization of surface defects. The material anisotropy of the monocrystalline silicon leads to variations of the guided wave characteristics, depending on the guided wave mode and propagation direction relative to the crystal orientation. Selective excitation of the first antisymmetric A0 wave mode at 5 MHz center frequency was achieved experimentally using a custom-made wedge transducer. Strong wave pulses with limited beam skewing and widening were measured using noncontact laser interferometer measurements. This allowed the accurate characterization of the Lamb wave propagation and scattering at small artificial surface defects with a size of less than 100 µm. The surface extent of the defects of varying size was characterized using an optical microscope. The scattered guided wave field was evaluated, and characteristic parameters were extracted and correlated with the defect size, allowing in principle detection of small defects. Further investigations are required to explain the systematic asymmetry of the guided wave field in the vicinity of the indents.</jats:p