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Journal of Modern Optics

Studies on rubidium 5S-5d two-photon absorption

Vinay Shukla, Sudip K. Nath, Vaishali Naik, Alok Chakrabarti, Ayan Ray

Published Mar 8, 2021• 5 citations
Publisher

Abstract

We studied non-resonant two photon absorption (NRTPA) 5S1/2→5D5/2 of hot Rb atom using a laser emitting at 778nm. The decay channel 5D5/2→6P3/2→5S1/2 is monitored to record the blue fluorescence. Two important aspects of blue fluorescence are explored: (i) photon number statistics under counter-propagating beam geometry, (ii) extraction of a workable discriminator signal by using modulation transfer for offset locking. The blue photon statistics histogram closely follows super–Poissonian distribution revealing the partially coherent or chaotic character under the limit of large photon counts. On the other hand modulation amplitude vs. peak height of 1-st derivative signal reveals that too small or too large modulation is detrimental for extracting discriminator signal. Under weak modulation, the offset stabilized laser attains square root of Allan variance σ(τ) ∼ 1.5 × 10−10 at integration time τ>10s. We propose generation of an offset stabilized chaotic blue light source by coupling (i) and (ii).

DOI
10.1080/09500340.2021.1894361

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American Journal of Physics•May 20, 2013•23 citations
We studied non-resonant two photon absorption (NRTPA) 5S1/2→5D5/2 of hot Rb atom using a laser emitting at 778nm. The decay channel 5D5/2→6P3/2→5S1/2 is monitored to record the blue fluorescence. Two important aspects of blue fluorescence are explored: (i) photon number statistics under counter-propagating beam geometry, (ii) extraction of a workable discriminator signal by using modulation transfer for offset locking. The blue photon statistics histogram closely follows super–Poissonian distribution revealing the partially coherent or chaotic character under the limit of large photon counts. On the other hand modulation amplitude vs. peak height of 1-st derivative signal reveals that too small or too large modulation is detrimental for extracting discriminator signal. Under weak modulation, the offset stabilized laser attains square root of Allan variance σ(τ) ∼ 1.5 × 10−10 at integration time τ>10s. We propose generation of an offset stabilized chaotic blue light source by coupling (i) and (ii).
In this work, we address the issue of the suitability of blue fluorescent light as a frequency offset reference during the ladder-level excitation 5S1/2→5P3/2→5D3/2 of Rb85 atoms. A simple pump–probe spectroscopy experiment is performed to generate non-degenerate blue light emission produced by the spontaneous decay route 5D3/2→6P3/2→5S1/2. By varying the relative intensity of the pump–probe combination, we can hover between regimes dominated by mechanisms such as double resonance optical pumping (DROP) and Autler–Townes splitting. The efficacy of the blue fluorescence (420 nm) as a frequency offset reference is pronounced under the DROP regime due to its narrow (∼natural) linewidth. To provide further insight into the stability performance, the pump laser connecting the 5P3/2→5D3/2 transition is stabilized to the peak of the blue light with the help of a probe stabilized to the saturation absorption signal of the 5S1/2→5P3/2 transition. For this purpose, we used the coherence-induced modulation transfer technique. The error frequency noise power spectra and the subsequent Allan variance calculation clearly show that probe fluctuations are transferred to the pump laser because of an existing two-photon coherent coupling.
In this experimental work, we report our findings about a transition between states of even parity 5S1/2→5D5/2 of Rb atoms under different conditions of transit time broadening. For this purpose, the blue fluorescence emanating from the associated decay 5D5/2→6P3/2→5S1/2 channel is monitored. Based on the relative magnification of the laser beam, which is used in excitation between levels of even parity, the system exhibits two photon absorption (TPA) through blue fluorescence peaks. The TPA profile appears as a Lorentzian peak on a Doppler background. The change in the laser beam diameter directly influences the relative contributions of Lorentzian and Doppler parts. A direct inference drawn from of this study may be utilized in optimizing the extraction of the first derivative spectrum corresponding to 5S→5D transitions.
In this report, we present an experimental investigation on atomic frequency offset locking (AFOL) of a laser under ladder ( ) level coupling scheme.The 5S 1/2 → 5P 3/2 → 5D 5/2 two-photon transition manifold of the rubidium (Rb) atom is chosen to demonstrate the performance of stabilization scheme in terms of frequency stability and tunability.The coherent pump-probe spectroscopy performed on the 5S → 5P → 5D levels results in signatures of two-photon absorption (TPA) and electromagnetically induced transparency (EIT).Here the pump laser is locked to the fringe of a scanning Fabry-Perot interferometer with the help of frequency modulation spectroscopy.The probe laser is subsequently stabilized on the resulting EIT (TPA) signals.It is found that the probe laser attains relative frequency stability σ (2, τ ) ∼ 1.4 × 10 -12 (2.0 × 10 -12 ) as a square root of Allan variance calculated from the frequency noise power density S ν ( f ) under closed-loop condition.Further, the current locking scheme has wide tuning range {Doppler width of 5S 1/2 (F = 2 → 5P 3/2 (F ) transition manifold.This particular tunable AFOL (TAFOL) scheme can provide large tuning range without compromising the frequency stability and such implementation of TAFOL may open new opportunities in research areas like metrology, ultra precision coherent spectroscopy, etc.
We investigate the excitation of the 5D_{5/2} level in Rb atoms using counter-propagating laser beams, which are nearly resonant to the one-photon 5S_{1/2} - 5P_{3/2} and 5P_{3/2} - 5D_{5/2} transitions, ensuring that a sum of the optical frequencies corresponds to the 5S_{1/2} - 5D_{5/2} transition. The excitation produced by two-photon and step-wise processes is detected via spontaneously emitted fluorescence at 420 nm arising from the 6P_{3/2} - 5S_{1/2} transition. The dependences of blue fluorescence intensity on atomic density and laser detuning from the intermediate 5P_{3/2} level have been investigated. The sensitivity of the frequency detuned bi-chromatic scheme for atom detection has been estimated. A novel method for sum frequency stabilization of two free-running lasers has been suggested and implemented using two-photon Doppler-free fluorescence and polarization resonances.
Abstract The coherent blue light generated in rubidium vapor due to the combined action of an ultrashort pulse train and a continuous wave diode laser is investigated. Each step of the two-photon transition 5S–5P 3 / 2 –5D is excited by one of the lasers, and the induced coherence between the 5S and 6P 3 / 2 states is responsible for generating the blue beam. Measurements of the excitation spectrum reveal the frequency comb structure and allow us to identify the resonant modes responsible for inducing the nonlinear process. Further, each resonant mode excites a different group of atoms, making the process selective in atomic velocity. The signal dependency on the atomic density is characterized by a sharp growth and a rapid saturation. We also show that for high intensity of the diode laser, the Stark shift at resonance causes the signal suppression observed at low atomic density.
We describe an experiment for generating and characterizing a beam of collimated blue light (CBL) in a rubidium vapor. Two low-power, grating-feedback diode lasers, operating at 780.2 nm (5S1/2→5P3/2) and 776.0 nm (5P3/2→5D5/2), respectively, provide step-wise excitation to the 5D excited state in rubidium. Under the right experimental conditions, cascade decay through the 6P excited state will yield a collimated blue (420-nm) beam of light with high temporal and spatial coherence. We investigate the production of a blue beam under a variety of experimental conditions and characterize the spatial coherence and spectral characteristics. This experiment provides advanced undergraduate students with a unique opportunity to investigate nonlinear optical phenomena in the laboratory and uses equipment that is commonly available in laboratories equipped to investigate diode-laser-based absorption spectroscopy in rubidium.