arXiv subjects

Ara Tonoyan

Publications and source records attributed to Ara Tonoyan.

13 recordsLinked to original sources

arXiv2025

Second-order correlations in directed emissions in sodium atoms

We report on measurements of second-order intensity correlations $g^{(2)}(τ)$ of infrared emission under bichromatic excitation at 589.2\,nm and 569.0\,nm of sodium atoms contained in a buffer-gas-free and uncoated 10-cm-long vapor cell. Directional emissions at $2.34\,μ$m in the forward direction and $2.21\,μ$m in both forward and backward directions under different experimental parameters are considered for this study. The measured values of $g^{(2)}(0)$ in all cases are found to exceed unity, while remaining significantly below the thermal light limit of 2. Cross-correlation measurements reveal that forward- and backward-propagating $2.21\,μ$m radiations are correlated. Oscillatory features in $g^{(2)}(τ)$ are observed over a broad range of excitation powers, and the dependence of the oscillation frequency on laser power can be attributed to AC Stark shifts, with contributions from hyperfine atomic structure in selected atomic velocity groups even in the presence of Doppler broadening. Our study establishes that the observed mid-infrared emission arises from a phase-matched, continuous-wave cooperative process that combines features of lasing and collective amplified spontaneous emission. The results highlight the buildup of long-range dipole coherence and velocity-selective coupling of atomic groups, which together govern the observed photon correlations and forward-backward emission symmetry. The demonstrated backward emission is of particular interest for applications in laser guidestar generation and mesospheric remote sensing, where understanding the statistical properties of the emitted light is essential for optimizing sodium-based light sources.

physics.atom-ph↗

arXiv2023

Observation of magnetically-induced transition intensity redistribution in the onset of the hyperfine Paschen-Back regime

The Zeeman effect is an important topic in atomic spectroscopy. The induced change in transition frequencies and amplitudes finds applications in the Earth-field-range magnetometry. At intermediate magnetic field amplitude $B\sim B_0 = A_\text{hfs}/μ_B$, where $A_\text{hfs}$ is the magnetic dipole constant of the ground state, and $μ_B$ is the Bohr magneton ($B_0\approx 1.7$ kG for Cs), the rigorous rule $ΔF = 0, \pm1$ is affected by the coupling between magnetic sub-levels induced by the field. Transitions satisfying $ΔF = \pm2$, referred to as magnetically-induced transitions, can be observed. Here, we show that a significant redistribution of the Cs $6\text{S}_{1/2}\rightarrow 6\text{P}_{3/2}$ magnetically-induced transition intensities occurs with increasing magnetic field. We observe that the strongest transition in the group $F_g=3\rightarrow F_e=5$ ($σ^+$ polarization) for $B 3 B_0$. On the other hand, the strongest transition in the group $F_g=2\rightarrow F_e=4$ ($σ^-$ polarization) remains so for all our measurements with magnetic fields up to 9 kG. These results are in agreement with a theoretical model. The model predicts that similar observations can be made for all alkali metals, including Na, K and Rb atoms. Our findings are important for magnetometers utilizing the Zeeman effect above Earth field, following the rapid development of micro-machined vapor-cell-based sensors.

physics.atom-ph↗

arXiv2023

Formation of narrow atomic lines of Rb in the UV region using a magnetic field

Magnetically induced (MI) transitions (F${}_{g}$ = 1 $\rightarrow$ F${}_{e}$= 3) of ${}^{87}$Rb D${}_{2}$ line are among the most promising atomic transitions for applications in laser physics. They reach their maximum intensity in the 0.2--2 kG magnetic field range and are more intense than many conventional atomic transitions. An important feature of MI transitions is their large frequency shift with respect to the unperturbed hyperfine transitions which reaches $\sim$12 GHz in magnetic fields of $\sim$ 3 kG, while they are formed on the high-frequency wing of the spectrum and do not overlap with other transitions. Some important peculiarities have been demonstrated for the MI 5S${}_{1/2}$$\rightarrow$ 5P${}_{3/2}$ transitions ($λ$=780 nm). Particularly, it was shown that using a nanocell with thickness $L= 100$~nm it is possible to realize 1 $μ$m-spatial resolution which is important when determining magnetic fields with strong spatial gradient (of $>$~3G /$μ$m). Earlier, our studies have been performed for 5S${}_{1/2}$ $\rightarrow$ $n$P${}_{3/2}$ transition with $n = 5$, while it is also theoretically shown to be promising for the transitions with $n = 6, 7, 8$ and $9$, corresponding to the transition wavelengths of 420.2~nm, 358.7~nm, 334.9~nm and 322.8~nm, respectively.

physics.atom-ph↗

arXiv2023

Formation of strongly shifted EIT resonances using "forbidden" transitions of Cesium

Atomic transitions satisfying $F_e - F_g = ΔF = \pm 2$ (where $F_e$ stands for excited and $F_g$ stands for ground state) of alkali atoms have zero probability in zero magnetic field (they are so-called "forbidden" transitions) but experience a large probabilty increase in an external magnetic field. These transitions are called magnetically induced (MI) transitions. In this paper, we use for the first time the $σ^+$ ($Δm_F~=~+1$) MI transitions $F_g = 3 \rightarrow F_e = 5$ of {Cesium} as probe radiation to form EIT resonances in strong magnetic fields (1 - 3 kG) while the coupling radiation frequency is resonant with $F_g=4\rightarrow F_e=5$ $σ^+$ transitions. The experiment is performed using a nanometric-thin cell filled with Cs vapor and a strong permanent magnet. The thickness of the vapor column is 852 nm, corresponding to the Cs $D_2$ line transition wavelength. Due to the large frequency shift slope of the MI transitions ($\sim$ 4 MHz/G), it is possible to form contrasted and strongly frequency-shifted EIT resonances. Particularly, a strong 12 GHz frequency shift is observed when applying an external magnetic field of $\sim$ 3 kG. Preliminary calculations performed considering Doppler-broadened three level systems in a nanocell are in reasonable agreement with the experimental measurements.

physics.atom-ph↗

arXiv2022

Photoionization cross sections of ultracold $^{88}$Sr in $^1$P$_1$ and $^3$S$_1$ states at 390 nm and the resulting blue-detuned magic wavelength optical lattice clock constraints

We present the measurements of the photoionisation cross sections of the excited $^1$P$_1$ and $^3$S$_1$ states of ultracold $^{88}$Sr atoms at 389.889 nm wavelength, which is the magic wavelength of the $^{1}$S$_{0}$-${}^{3}$P${}_{0}$ clock transition. The photoionisation cross section of the $^1$P$_1$ state is determined from the measured ionisation rates of $^{88}$Sr in the magneto-optical trap in the $^1$P$_1$ state to be 2.20(50)$\times$10$^{-20}$ m$^2$, while the photoionisation cross section of $^{88}$Sr in the $^3$S$_1$ state is inferred from the photoionisation-induced reduction in the number of atoms transferred through the $^3\text{S}_1$ state in an operating optical lattice clock to be $1.38(66)\times$10$^{-18}$ m$^2$. Furthermore, the resulting limitations of employing a blue-detuned magic wavelength optical lattice in strontium optical lattice clocks are evaluated. We estimated photoionisation induced loss rates of atoms at 389.889 nm wavelength under typical experimental conditions and made several suggestions on how to mitigate these losses. In particular, the large photoionisation induced losses for the $^3$S$_1$ state would make the use of the $^3$S$_1$ state in the optical cycle in a blue-detuned optical lattice unfeasible and would instead require the less commonly used $^3$D$_{1,2}$ states during the detection part of the optical clock cycle.

physics.atom-ph↗

arXiv2022

Coherent Optical Processes on Cs D$_2$ line Magnetically Induced Transitions

The increased spectral resolution allowed by the use of extremely thin vapor cells has led to the observation of interesting behaviour of alkali transitions when placed in a magnetic field. Particularly, transitions obeying an apparent $F_e-F_g\equivΔF =\pm2$ selection rule, referred to as magnetically-induced (MI) transitions, have their probabilities largely increase in the intermediate interaction regime while being null at zero and higher magnetic fields. With an 800 nm-thick Cs vapor cell placed in a field up to 1.5 kG, we show here that the generation of electromagnetically induced transparency (EIT), realized in $Λ$-systems involving $ΔF =- 2$ MI transitions, is only possible when both the coupling and probe beams are $σ^-$-circular polarized, demonstrating that EIT is affected by magnetic circular dichroism. A similar rule of thumb can be extrapolated for $ΔF =+2$ MI transitions and $σ^+$ polarization. Because of the high frequency shift slope (typ. 4 MHz/G), the generation of EIT resonances involving MI transitions is interesting, especially in the context of growing attention towards micro-machined alkali vapor cell sensors.

physics.atom-ph↗

arXiv2021

Strongest Magnetically Induced Transitions in Alkali Metal Atoms with nuclear spin $3/2$

The probabilities of atomic transitions $F_e - F_g = \pm 2$ between a ground $F_g$ and an excited $F_e$ level of $D_2$ line of any alkali metal atom are zero when no external magnetic field is applied. In an external magnetic field in the range $0.1 - 3$ kG, the probabilities of these transitions called magnetically induced (MI) are highly modified. For these MI transitions, we have previously exhibited the following rule: the probabilities of MI transitions with $ΔF = +2$ are maximal when using $σ^+$-polarized laser radiation, while the probabilities of MI transitions with $ΔF = -2$ are maximal when using $σ^-$-polarized laser radiation. This difference has been termed Type 1 Magnetically Induced Circular Dichroism (MCD1). It is demonstrated for the first time that for alkali atoms with a nuclear spin $I=3/2$ ($^{87}\text{Rb}$, $^{39}\text{K}$,$^{23}\text{Na}$, $^7\text{Li}$) in magnetic fields $> 100$ G, the probability of the strongest $σ^+$ MI transition of the group $F_g = 1 \rightarrow F_e = 3'$ (transition $\ket{1,-1}\rightarrow\ket{3',0'}$) is about 4 times higher than the probabilities of the strongest MI $σ^-$-transitions $\ket{1,-1}\rightarrow\ket{3',-2'}$ and $\ket{2,+1}\rightarrow \ket{0',0'}$. These properties make the $σ^+$ MI transition $\ket{1,-1}\rightarrow\ket{3',0'}$ an interesting candidate for the study of magneto-optical processes in strong magnetic fields.

physics.atom-ph↗

arXiv2020

Circular dichroism in atomic vapors: magnetically induced transitions responsible for two distinct behaviors

Atomic transitions of alkali metals for which the condition $F_e-F_g = \pm2$ is satisfied have null probability in a zero magnetic field, while a giant increase can occur when an external field is applied. Such transitions, often referred to as magnetically-induced (MI) transitions, have received interest because their high probabilities in wide ranges of external magnetic fields which, in some cases, are even higher than that of usual atomic transitions. Previously, the following rule was established: the intensities of MI transitions with $ΔF=\pm2$ are maximum when using respectively $σ^\pm$ radiation. Within the same ground state, the difference in intensity for $σ^+$ and $σ^-$ radiations can be significant, leading to magnetically induced circular dichroism (MCD), referred to as type-1. Here, we show that even among the strongest MI transitions, $i.e$ originating from different ground states for $σ^+$ and $σ^-$, the probability of MI transition with $ΔF = + 2$ is always greater, which leads to another type of MCD. Our experiments are performed with a Cs-filled nanocell, where the laser is tuned around the D$_2$ line; similar results are expected with other alkali metals. Theoretical calculations are in excellent agreement with the experimental measurements.

physics.atom-ph↗

arXiv2019

Dark resonance formation with magnetically-induced transitions: extension of spectral range and giant circular dichroism

Dark resonances were formed via electromagnetically induced transparency for the first time involving magnetically-induced $ΔF = \pm2$ atomic transitions of alkali metal atom, which are forbidden at zero magnetic field. The probability of these transitions undergoes rapid growth when $300 - 3000$~G magnetic field is applied, allowing formation of dark resonances, widely tunable in the GHz range. It is established that for $ΔF = +2$ ($ΔF =-2$) transition, the coupling laser tuned to $ΔF = +1$ ($ΔF =-1$) transition of the hyperfine $Λ$-system must be $σ^+$ ($σ^-$) polarized, manifesting anomalous circular dichroism.

physics.atom-ph↗

arXiv2018

Hyperfine Paschen-Back regime of Potassium D$_2$ line observed by Doppler-free spectroscopy

Selective reflection of a laser radiation from an interface formed by a dielectric window and a potassium atomic vapour confined in a nano-cell with $350~$nm gap thickness is implemented for the first time to study the atomic transitions of K D$_2$ line in external magnetic fields. In moderate $B$-fields, there are 44 individual Zeeman transitions which reduce to two groups (one formed by $σ^+$ the other one by $σ^-$ circularly-polarised light), each containing eight atomic transitions, as the magnetic field increases. Each of these groups contains one so-called "guiding" transition whose particularities are to have a probability (intensity) as well as a frequency shift slope (in MHz/G) that are constant in the whole range of $0 - 10~$kG magnetic fields. In the case of $π$-polarised laser radiation, among eight transitions two are forbidden at $B = 0$, yet their probabilities undergo a giant modification under the influence of a magnetic field. We demonstrate that for $B$-fields $> 165~$G a complete hyperfine Paschen-Back regime is observed. Other peculiarities of K D$_2$ line behaviour in magnetic field are also presented. We show a very good agreement between theoretical calculations and experiments. The recording of the hyperfine Paschen-Back regime of K D$_2$ line with high spectral resolution is demonstrated for the first time.

physics.atom-ph↗

arXiv2017

Theoretical Investigation of $^{41}$K States Behavior under Strong Magnetic Field and $π$ Polarized Laser Field

Theoretically research of the behavior of $^{41}$K $D_1$ and $D_2$ lines for $π$ polarized resonant light is conducted in the presence of strong magnetic field when the total electronic angular momentum $J$ and nuclear spin $I$ are decoupled. We show that in the case of linear polarization and for D1 line there are two transitions, so called guiding transitions (GT) that maintain their probabilities and frequency slopes. In Hyperfine Paschen-Back (HPB) regime other transitions are coming together to those GTs, making 2 groups (4 in each). Each transition in the group has the same frequency slope and probability as the GT in their group. It is demonstrated that from 12 ($D_1$) and 20 ($D_2$) initially allowed Zeeman transitions (taking into account the selection rules) at low B-field, only 8 transitions in each $D$ line remain in absorption spectra at $B > 200$ G. A complete HPB regime for relatively low magnetic fields $B \sim 200$ G has been observed. This value is the smallest for all alkali metals.

physics.atom-ph↗

arXiv2017

Magnetic field--induced modification of selection rules for Rb D$_2$ line monitored by selective reflection from a vapor nanocell

Magnetic field-induced giant modification of the probabilities of five transitions of $5S_{1/2}, F_g=2 \rightarrow 5P_{3/2}, F_e=4$ of $^{85}$Rb and three transitions of $5S_{1/2}, F_g=1 \rightarrow 5P_{3/2}, F_e=3$ of $^{87}$Rb forbidden by selection rules for zero magnetic field has been observed experimentally and described theoretically for the first time. For the case of excitation with circularly-polarized ($σ^+$) laser radiation, the probability of $F_g=2, ~m_F=-2 \rightarrow F_e=4, ~m_F=-1$ transition becomes the largest among the seventeen transitions of $^{85}$Rb $F_g=2 \rightarrow F_e=1,2,3,4$ group, and the probability of $F_g=1,~m_F=-1 \rightarrow F_e=3,~m_F=0$ transition becomes the largest among the nine transitions of $^{87}$Rb $F_g=1 \rightarrow F_e=0,1,2,3$ group, in a wide range of magnetic field 200 -- 1000 G. Complete frequency separation of individual Zeeman components was obtained by implementation of derivative selective reflection technique with a 300 nm-thick nanocell filled with Rb, allowing formation of narrow optical resonances. Possible applications are addressed. The theoretical model is perfectly consistent with the experimental results.

physics.atom-ph↗

arXiv2016

Decoupling of hyperfine structure of Cs $D_1$ line in strong magnetic field studied by selective reflection from a nanocell

Decoupling of total electronic and nuclear spin moments of Cs atoms in external magnetic field for the case of atomic $D_1$ line, leading to onset of the hyperfine Paschen-Back regime has been studied theoretically and experimentally. Selective reflection of laser radiation from an interface of dielectric window and atomic vapor confined in a nanocell with 300 nm gap thickness was implemented for the experimental studies. The real time derivative of selective reflection signal with a frequency position coinciding with atomic transitions was used in measurements, providing $\sim$ 40 MHz spectral resolution and linearity of signal response in respect to transition probability. Behavior of 28 individual Zeeman transitions in a wide range of longitudinal magnetic field (0 - 6 kG) has been tracked under excitation of Cs vapor by a low-intensity $σ^+$- polarized cw laser radiation. For $B\ge 6~$kG, only 8 transitions with nearly equal probabilities and the same frequency slope remained in the spectrum, which is a manifestation of the hyperfine Paschen-Back regime. The obtained experimental results are consistent with numerical modeling. Due to small divergence of selective reflection signal, as well as sub-wavelength thickness and sub-Doppler spectral linewidth inherent to nanocell, the employed technique can be used for distant remote sensing of magnetic field with high spatial and $B$-field resolution.

physics.atom-ph↗