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Journal of the Optical Society of America B

Spin polarization of Doppler-broadened atoms by the broadband nanosecond and transform-limited picosecond laser pulses: case study for the muonium

Takashi Nakajima

Published Aug 16, 2012• 7 citations
Publisher

Abstract

We study the spin polarization of Doppler-broadened atoms by broadband nanosecond (ns) pulses, and compare the polarization efficiency with that by the transform-limited picosecond (ps) pulses that have the same spectral bandwidth. Specific calculations are performed for the case of muonium with a set of density matrix equations and with rate equations using the uncoupled basis states. We find that the polarization efficiency with the broadband ns laser pulses is rather good, although it is not as good as that with the transform-limited ps pulses. Our results imply that, depending on the available laser system, we can use either broadband ns or transform-limited ps laser pulses to polarize almost any Doppler-broadened atoms.

DOI
10.1364/josab.29.002420

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We study the spin polarization of Doppler-broadened atoms by broadband nanosecond (ns) pulses, and compare the polarization efficiency with that by the transform-limited picosecond (ps) pulses that have the same spectral bandwidth. Specific calculations are performed for the case of muonium with a set of density matrix equations and with rate equations using the uncoupled basis states. We find that the polarization efficiency with the broadband ns laser pulses is rather good, although it is not as good as that with the transform-limited ps pulses. Our results imply that, depending on the available laser system, we can use either broadband ns or transform-limited ps laser pulses to polarize almost any Doppler-broadened atoms.
We theoretically study the ionization efficiency of Doppler-broadened atoms through a one-photon resonant two-photon ionization process using nanosecond pump and ionizing laser pulses. Under the presence of significant (> tens of GHz) Doppler broadening, the bandwidth of the transform-limited nanosecond pump pulse is far smaller than the Doppler width, and the use of the broadband nanosecond pulse rather than the transform-limited nanosecond pulse seems to be much more efficient to pump and eventually ionize atoms. It turns out, however, that the transform-limited nanosecond pump pulse with sufficient high intensity can outperform the broadband nanosecond pump pulse in the high intensity regime for the ionizing pulse, at which the other broadening mechanisms because of the strong pump and fast ionization play more important roles than the bandwidth of the pump pulse. Using a set of density matrix equations, we present specific numerical results for muoniums (μ+e−) with enormous Doppler widths (80 and 230 GHz), which support the above argument.
The physics of short-pulse polarization spectroscopy (PS) and the diagnostic potential for quantitative measurements of species concentration are investigated by direct numerical integration (DNI) of the time-dependent density matrix equations for a multistate system. The effects of laser power, collision rates, and Doppler broadening on the short-pulse PS signal generation process are investigated by systematically varying these parameters in the numerical calculations. It is found that the use of a short-pulse laser (laser pulse width τL<characteristic collision time τC) significantly decreases the collision-rate dependence of the PS signal compared to the long-pulse laser case (τL>τC), even for a nonsaturating pump beam. For a saturating pump beam, the short-pulse PS signal is found to be nearly independent of collision rate. Increasing the collision rate by a factor of 100 (from 108 to 1010 s−1), the calculated PS signal strength decreases by only a factor of 2 for a 100-ps pump laser at high intensity. This insensitivity of the PS signal to the collision rate in the medium enhances greatly the potential for quantitative application of the technique for concentration measurements in reacting flows. The underlying physics of the short-pulse PS is explored by studying the effects of collision rate, Doppler broadening, and the pump laser intensity on the temporal profile of the Zeeman state populations and the coherences between the Zeeman states.
We present a scheme to control the spin-polarization of photoelectrons/photoions using short laser pulses. It is based on the pump-probe method. By exciting fine structure manifolds of a system by the short pump pulse with a sufficient bandwidth, a superposition of fine structure states is created. After the pump pulse, the coherently superposed excited state evolves in time under the field-free condition with a period determined by the inverse of energy difference between them. In terms of the spin state, this suggests that the different spin state evolves differently in time, leading to the time-varying spin-polarization. Therefore, varying the time delay between the pump and probe pulses leads to the control of spin states upon photoionization. Specific theoretical results are presented for two-valence-electron atoms, in particular for Mg, which demonstrate that, under certain conditions, not only the degree of spin-polarization but also its polarity can be manipulated through time delay. Furthermore, we propose a new technique to accelerate the coupling time by the introduction of a dressing laser with a few ns duration. Since the underline physics is rather general and transparent, the presented scheme may be potentially applied to nanostructures such as quantum wells and quantum dots.
Atoms in Rydberg states are an important building block for emerging quantum technologies. While excitation to Rydberg orbitals is typically achieved in more than tens of nanoseconds, the physical limit is in fact much faster, at the ten picoseconds level. Here, we tackle such ultrafast Rydberg excitation of a rubidium atom by designing a dedicated pulsed laser system generating 480 nm pulses of 10 ps duration. In particular, we improved upon our previous design by using an injection-seeded optical parametric amplifier (OPA) to obtain a stable pulsed energy, decreasing the fluctuation from 30 to 6%. We then succeeded in ultrafast excitation of Rydberg atoms with excitation probability of ∼90%, not limited anymore by energy fluctuation but rather by the atomic state preparation, addressable in future works. This achievement broadens the range of applications of Rydberg atoms.
We have studied laser optical pumping of Rb at densities up to 4\ifmmode\times\else\texttimes\fi{}1014{10}^{14}1014 atoms/cm3{\mathrm{cm}}^{3}cm3 and produced polarized 3He^{3}\mathrm{He}3He by spin exchange with Rb. Numerical calculations show that for a high-density Rb vapor GaAlAs diode laser arrays can provide results similar to those obtained with a broadband Kr+{\mathrm{Kr}}^{+}Kr+-ion--dye-laser system. We have confirmed these calculations with measurements using a 40-GHz full width at half maximum bandwidth dye laser and 100- and 500-mW GaAlAs diode laser arrays (Spectra Diode Labs Array Nos. SDL-2410C and SDL-2450C). We have optically pumped Rb of density [Rb]=4\ifmmode\times\else\texttimes\fi{}1014{10}^{14}1014 atoms/cm3{\mathrm{cm}}^{3}cm3, and we have measured up to 70% 3He^{3}\mathrm{He}3He polarization in a 4-cm3{\mathrm{cm}}^{3}cm3 volume with 600 mW from the dye laser. With the diode laser arrays, we have pumped with 30 and 300 mW with results comparable to those obtained with the dye laser. With a 1-W diode laser array, we anticipate 70--80 % 3He^{3}\mathrm{He}3He polarization in a 4-cm3{\mathrm{cm}}^{3}cm3 volume.
The uncoupled basis set is used to describe the interaction between the ultrashort pulses and the Na atom. Several polarization configurations of laser pulses have been investigated, which produce different effects on the two-step photoexcitation process. The study shows that the final states of the transitions driven by ultrashort laser pulses have a strong dependence on the laser polarization configuration. A comparison of the two different basis sets shows that the uncoupled basis set may provide more information than the coupled basis set, such as angular distribution of the atom in the final states of the transitions.