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The interaction of excited atoms and few-cycle laser pulses

View Article: PubMed Central - PubMed

ABSTRACT

This work describes the first observations of the ionisation of neon in a metastable atomic state utilising a strong-field, few-cycle light pulse. We compare the observations to theoretical predictions based on the Ammosov-Delone-Krainov (ADK) theory and a solution to the time-dependent Schrödinger equation (TDSE). The TDSE provides better agreement with the experimental data than the ADK theory. We optically pump the target atomic species and measure the ionisation rate as the a function of different steady-state populations in the fine structure of the target state which shows significant ionisation rate dependence on populations of spin-polarised states. The physical mechanism for this effect is unknown.

No MeSH data available.


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The panel on the left shows the optical pumping transitions for the 3P2 → 3D3 states with the associated magnetic substates, also shown are the decay rates for the individual sublevel transitions.The panel on the right displays the time evolution of the mj states of 3P2 neon being pumped with σ− polarised light tuned to the 3D3 → 3P2 transition. The intensity of the light is 20 times the saturation intensity of the transition. These results describe the system reaching steady state as described in the text, with 50% of the atoms in the displayed 3P2mj = −2 state. The remainder exist in the 3D3mj = −3 excited state, which is not displayed in the figure. When the atoms leave the pump beam they decay from the excited state into the mj = −2 state as described in the main text.
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f2: The panel on the left shows the optical pumping transitions for the 3P2 → 3D3 states with the associated magnetic substates, also shown are the decay rates for the individual sublevel transitions.The panel on the right displays the time evolution of the mj states of 3P2 neon being pumped with σ− polarised light tuned to the 3D3 → 3P2 transition. The intensity of the light is 20 times the saturation intensity of the transition. These results describe the system reaching steady state as described in the text, with 50% of the atoms in the displayed 3P2mj = −2 state. The remainder exist in the 3D3mj = −3 excited state, which is not displayed in the figure. When the atoms leave the pump beam they decay from the excited state into the mj = −2 state as described in the main text.

Mentions: The present work also utilised optical pumping of the target atom with another laser beam tuned to the cooling transition in order to spin-polarise the target atom. If the optical pumping laser light is circularly polarised, it acts on a target atom by causing many single-photon absorptions followed by relaxations due to spontaneous emission. The result of this process is that the atomic population is transferred into the largest mj = ±2 states (+2 for σ+ left hand circularly polarised light and −2 for σ− right hand circularly polarised light) after the interaction with the light beam29. Atoms with these magnetic projection quantum numbers have the maximum total angular momentum and are spin-polarised. The sublevel transitions and their associated decay probabilities are shown in Fig. 2. An additional laser beam was added after the optical collimator to facilitate the optical pumping. The laser beam interacted perpendicular to the atomic beam and was on resonance with the cooling transition used in the optical collimator. The laser beam was retro-reflected and the laser detuning is set to 0 MHz so that the net scattering force on the atoms in the atomic beam was zero30, thus ensuring that the trajectory of the atoms remained unaltered, which avoided a loss in ion yield signal. The polarisation state of this beam was altered using a quarter-wave plate and facilitated a polarisation change which changed the distribution of mj states. The optical pump beam has a measured power of 125 mW, across a collimated beam geometry with a 6.1 mm radius. This gives a pump intensity of 20 times the saturation intensity (4.22 mW/cm2) of the optical transition. We modelled the optical pumping process by numerically evaluating the optical Bloch equations (OBEs) in the rotating-wave approximation (RWA). The OBEs fully describe the evolution of the internal atomic states in the presence of an external field including the atomic state coherences and spontaneous decay. For example, Fig. 2 shows the evolution of a Ne 3P2 atoms pumped by σ+ light. The system reaches a steady state after approximately 1 μs with 50% of the atoms in the ground 3P2mj = 2 state and 50% of the atoms in the excited 3D2mj = 3 state. A fully polarised state is only reached after a period of relaxation where the system is allowed to evolve without the influence of the pump laser. This second step takes a further 80 ns, after which approximately 99% of the atoms are in the desired 3P2mj = 2 state. On average, an atom was under the influence of the optical pumping beam for 12 μs, which is more than sufficient to fully polarise the atomic beam. Between the optical pumping region and the interaction region (approximately 45 cm), there is a small residual magnetic field from the Earth, which could have induced a small depolarisation of the atoms31. However, our results show that the majority of atoms remain well polarised.


The interaction of excited atoms and few-cycle laser pulses
The panel on the left shows the optical pumping transitions for the 3P2 → 3D3 states with the associated magnetic substates, also shown are the decay rates for the individual sublevel transitions.The panel on the right displays the time evolution of the mj states of 3P2 neon being pumped with σ− polarised light tuned to the 3D3 → 3P2 transition. The intensity of the light is 20 times the saturation intensity of the transition. These results describe the system reaching steady state as described in the text, with 50% of the atoms in the displayed 3P2mj = −2 state. The remainder exist in the 3D3mj = −3 excited state, which is not displayed in the figure. When the atoms leave the pump beam they decay from the excited state into the mj = −2 state as described in the main text.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5035976&req=5

f2: The panel on the left shows the optical pumping transitions for the 3P2 → 3D3 states with the associated magnetic substates, also shown are the decay rates for the individual sublevel transitions.The panel on the right displays the time evolution of the mj states of 3P2 neon being pumped with σ− polarised light tuned to the 3D3 → 3P2 transition. The intensity of the light is 20 times the saturation intensity of the transition. These results describe the system reaching steady state as described in the text, with 50% of the atoms in the displayed 3P2mj = −2 state. The remainder exist in the 3D3mj = −3 excited state, which is not displayed in the figure. When the atoms leave the pump beam they decay from the excited state into the mj = −2 state as described in the main text.
Mentions: The present work also utilised optical pumping of the target atom with another laser beam tuned to the cooling transition in order to spin-polarise the target atom. If the optical pumping laser light is circularly polarised, it acts on a target atom by causing many single-photon absorptions followed by relaxations due to spontaneous emission. The result of this process is that the atomic population is transferred into the largest mj = ±2 states (+2 for σ+ left hand circularly polarised light and −2 for σ− right hand circularly polarised light) after the interaction with the light beam29. Atoms with these magnetic projection quantum numbers have the maximum total angular momentum and are spin-polarised. The sublevel transitions and their associated decay probabilities are shown in Fig. 2. An additional laser beam was added after the optical collimator to facilitate the optical pumping. The laser beam interacted perpendicular to the atomic beam and was on resonance with the cooling transition used in the optical collimator. The laser beam was retro-reflected and the laser detuning is set to 0 MHz so that the net scattering force on the atoms in the atomic beam was zero30, thus ensuring that the trajectory of the atoms remained unaltered, which avoided a loss in ion yield signal. The polarisation state of this beam was altered using a quarter-wave plate and facilitated a polarisation change which changed the distribution of mj states. The optical pump beam has a measured power of 125 mW, across a collimated beam geometry with a 6.1 mm radius. This gives a pump intensity of 20 times the saturation intensity (4.22 mW/cm2) of the optical transition. We modelled the optical pumping process by numerically evaluating the optical Bloch equations (OBEs) in the rotating-wave approximation (RWA). The OBEs fully describe the evolution of the internal atomic states in the presence of an external field including the atomic state coherences and spontaneous decay. For example, Fig. 2 shows the evolution of a Ne 3P2 atoms pumped by σ+ light. The system reaches a steady state after approximately 1 μs with 50% of the atoms in the ground 3P2mj = 2 state and 50% of the atoms in the excited 3D2mj = 3 state. A fully polarised state is only reached after a period of relaxation where the system is allowed to evolve without the influence of the pump laser. This second step takes a further 80 ns, after which approximately 99% of the atoms are in the desired 3P2mj = 2 state. On average, an atom was under the influence of the optical pumping beam for 12 μs, which is more than sufficient to fully polarise the atomic beam. Between the optical pumping region and the interaction region (approximately 45 cm), there is a small residual magnetic field from the Earth, which could have induced a small depolarisation of the atoms31. However, our results show that the majority of atoms remain well polarised.

View Article: PubMed Central - PubMed

ABSTRACT

This work describes the first observations of the ionisation of neon in a metastable atomic state utilising a strong-field, few-cycle light pulse. We compare the observations to theoretical predictions based on the Ammosov-Delone-Krainov (ADK) theory and a solution to the time-dependent Schrödinger equation (TDSE). The TDSE provides better agreement with the experimental data than the ADK theory. We optically pump the target atomic species and measure the ionisation rate as the a function of different steady-state populations in the fine structure of the target state which shows significant ionisation rate dependence on populations of spin-polarised states. The physical mechanism for this effect is unknown.

No MeSH data available.


Related in: MedlinePlus