This paper reports on a method for improving the optical efficiency of micromachined reflectors integrated in rubidium vapor cells. A hybrid bulk micromachining and multilayer thin film process is used to form the reflectors on angled sidewalls, which redirect laser light through the vapor cell and back toward the plane of the source with reduced optical power loss. The optical return efficiency of two paired dielectric reflectors is shown to be improved by as much as eight times over silicon reflectors alone. The D1absorption line in a one cubic millimeter miniature alkali 87 Kb vapor cell by use of two integrated thin film reflectors is experimentally demonstrated.
We investigate the architecture of microfabricated vapor cells with reflective sidewalls for applications in chip scale atomic sensors. The optical configuration in operation is suitable for both one-beam and two-beam (pump & probe) schemes. In the miniaturized vapor cells, the laser beam is reflected twice by the aluminum reflectors on the wet etched 54.7° sidewalls to prolong the optical length significantly, thus resulting in a return reflectance that is three times that of bare silicon sidewalls. To avoid limitations faced in the fabrication process, a simpler, more universal and less constrained fabrication process of microfabricated vapor cells for chip scale atomic sensors with uncompromised performance is implemented, which also decreases the fabrication costs and procedures. Characterization measurements show that with effective sidewall reflectors, mm³ level volume and feasible hermeticity, the elongated miniature vapor cells demonstrate a linear absorption contrast improvement by 10 times over the conventional micro-electro-mechanical system (MEMS) vapor cells at ~50 °C in the rubidium D1 absorption spectroscopy experiments. At the operating temperature of ~90 °C for chip scale atomic sensors, a 50% linear absorption contrast enhancement is obtained with the reflective cell architecture. This leads to a potential improvement in the clock stability and magnetometer sensitivity. Besides, the coherent population trapping spectroscopy is applied to characterize the microfabricated vacuum cells with 46.3 kHz linewidth in the through cell configuration, demonstrating the effectiveness in chip scale atomic sensors.
Selective-reflection ellipsometry from miniaturized rubidium vapor cells is investigated numerically and experimentally as a phase-sensitive extension of conventional selective-reflection spectroscopy. While most previous studies of selective reflection have mainly focused on scalar reflectivity and line-shape analysis, the present work analyzes the reflected p- and s-polarized fields through the ellipsometric parameters
Ψ and
Δ . The reflected field from a finite rubidium vapor layer is calculated using a Fourier-series expansion of the coupled field–polarization equations for vapor-layer thicknesses ranging from
L=λ/2 to
L=9λ/4 , where
λ=795nm . The numerical results show that wavelength-scale changes in the vapor-layer thickness modify the selective-reflection spectrum and produce thickness-dependent resonant features. The calculated
Ψ and
Δ spectra exhibit inverse-like behavior at complementary incidence angles, including
30∘,60∘ and
37∘,53∘ . This behavior is interpreted as the combined result of angle-dependent optical path, wavelength-scale phase accumulation, Fabry–Pérot-type thin-layer interference, and resonant near-surface atomic polarization associated with selective reflection. Experimentally, the ellipsometric parameters are extracted from p- and s-polarized reflection measurements performed on a fabricated
50μm rubidium microcell. The experimental spectra support the physical observability of the complementary-angle ellipsometric response in a real miniaturized vapor-cell geometry. These results show that selective-reflection ellipsometry can provide a useful phase-sensitive approach for studying resonant optical responses in confined atomic vapor layers and offers a useful platform for compact spectroscopy, laser stabilization, and atom–surface interaction studies.
Hot alkali metal vapors enclosed in sub-micron spectroscopic cells provide an ideal system for fundamental studies of the atom-wall and atom-light interactions at nanoscale. Here, we propose a novel approach for calculating the eigenmodes of a thin-vapor-layer beyond the limitations of the first-order perturbation theory in optical density for the case of quenching of atomic polarization upon collisions of atoms with dielectric walls. We show that higher-order optical density corrections lead to a remarkable density-dependent blue shift and deformation of the spectral line shapes of reflection, transmission, and absorption. We also demonstrate that the eigenmodes of the thin-vapor-layer can be calculated independently of the choice of optical boundary conditions. This greatly extends the applicability of the constructed theory for the development of miniature atomic sensors.
This paper reports on an original architecture of microfabricated alkali vapor cell designed for miniature atomic clocks. The cell combines diffraction gratings with anisotropically etched single-crystalline silicon sidewalls to route a normally-incident beam in a cavity oriented along the substrate plane. Gratings have been specifically designed to diffract circularly polarized light in the first order, the latter having an angle of diffraction matching the (111) sidewalls orientation. Then, the length of the cavity where light interacts with alkali atoms can be extended. We demonstrate that a longer cell allows to reduce the beam diameter, while preserving the clock performances. As the cavity depth and the beam diameter are reduced, collimation can be performed in a tighter space. This solution relaxes the constraints on the device packaging and is suitable for wafer-level assembly. Several cells have been fabricated and characterized in a clock setup using coherent population trapping spectroscopy. The measured signals exhibit null power linewidths down to 2.23 kHz and high transmission contrasts up to 17%. A high contrast-to-linewidth ratio is found at a linewidth of 4.17 kHz and a contrast of 5.2% in a 7-mm-long cell despite a beam diameter reduced to 600 μm.