The Microfabricated Alkali Vapor Cell with High Hermeticity for Chip-Scale Atomic Clock

Herein, a microfabricated millimeter-level vapor alkali cell with a high hermeticity is fabricated through a wet etching and single-chip anodic bonding process. The vapor cell, containing Rb and N2, was investigated in a coherent population trapping (CPT) setup for the application of a chip-scale atomic clock (CSAC). The contrast of CPT resonance is up to 1.1% within the only 1 mm length of light interacting with atom. The effects of some critical external parameters on the CPT resonance, such as laser intensity, cell temperature, and buffer gas pressure, are thoroughly studied and optimized. The improved microfabricated vapor cell also exhibited great potential for other chip-scale atomic devices.

Microfabrication of Alkali Vapor MEMS Cells for chip-scale atomic clock

The technology of MEMS atomic cells containing rubidium or caesium vapors in an atmosphere of neon buffer gas has been developed. Two-chamber silicon cells containing an optical cavity, shallow filtration channels and a technical container for a solid-state alkali source have been implemented in a single-step process of anisotropic wet chemical etching. To prevent significant undercutting of the filtration channels during etching of the through silicon cavities, the shapes of the compensating elements at the convex corners of the silicon nitride mask have been calculated and the composition of the silicon etchant has been experimentally found. The sealing of the cells has been carried out by silicon-glass anodic bonding at a temperature of 250 °C. For this purpose the LK5 glass which has an increased ionic conductivity in comparison with the conventional glass Borofloat 33 was used. The best microfabricated cells allowed us to obtain estimates of the relative instability of the coherent population trapping resonance frequency at the level of 5 · 10-11 at 1 s.

Method for stabilization of the microwave modulation index in order to suppress the light shift of the coherent population trapping resonances

For the resonance of coherent population trapping (CPT), we show that in the case of a spatially inhomogeneous light shift (for example, due to the Gaussian transversal profile of the light beam intensity), the zero position of the error signal, formed by the use of phase-jump technique, depends on the integration time of the spectroscopic signal. Basing on this effect, we propose two-loop method to stabilize the microwave power at the point where the light shift vanishes.

Temporal analog of Fabry-Pérot resonator via coherent population trapping

Ramsey spectroscopy via coherent population trapping (CPT) is essential in precision measurements. The conventional CPT-Ramsey fringes contain numbers of almost identical oscillations and so that it is difficult to identify the central fringe. Here we experimentally demonstrate a temporal analog of Fabry–Pérot resonator via double-Λ CPT of laser-cooled 87Rb atoms. By inserting a periodic CPT pulse train between the two CPT-Ramsey pulses, due to the constructive interference of spin coherence, the transmission spectrum appears as a comb of equidistant peaks in frequency domain and thus the central Ramsey fringe can be easily identified. From the five-level Bloch equations for our double-Λ system, we find that the multi-pulse CPT interference can be regarded as a temporal analog of Fabry–Pérot resonator. Because of the small amplitude difference between the two Landé g factors, each peak splits into two when the external magnetic field is not too weak. This splitting is exactly linear with the magnetic field strength and thus can be used for measuring a magnetic field without involving magneto-sensitive transitions.