Doppler-free Two-photon Transitions Research Articles

5S−5D two-photon transition in rubidium vapor at high densities

The rubidium $5S\text{\ensuremath{-}}5D$ Doppler-free two-photon transition has been observed in a thermal vapor cell by detecting the infrared radiation emitted during the decay from the $5D$ excited level to the $5P$ intermediate level. Different to the usual detection scheme based on blue light emission this approach does not suffer from reabsorption and still works at high densities. This not only allows for a dramatically enhanced detectable emission rate but also for investigating collisional effects such as collisional broadening and collisional energy transfer. Furthermore, we observe a yet unknown temperature dependence of the resonance linewidth for transitions into the states of the $5{D}_{\text{5/2}}$ level. Consequences for the design of an optical Rb-frequency standard are discussed.

Absolute hyperfine energy levels and isotope shift of Rb 5S–6S two-photon transition

Using a Rb cell and a wavemeter, with a Doppler-free two-photon transition near 993 nm, we have measured the four hyperfine components (two isotopes, ΔF = 0) of the 5S–6S transition, finding the absolute energies of those transitions to a spectral resolution of 0.002 cm−1 = 60 MHz. We find the (87 minus 85) isotope shift of the 5S–6S transition to be +94(12) MHz. Using our single frequency cw laser with focused intensity up to 104 W cm−2 at the cell, we find no AC Stark or light shift of the lines at 6 MHz spectral resolution. We find no density shift of the lines at 6 MHz spectral resolution, for a range of Rb atom densities from 3 × 1011 to 5 × 1013 atoms/cm3.

Measurement of the lifetime of the 7sS1/22 state in atomic cesium using asynchronous gated detection

We report a measurement of the lifetime of the cesium $7s\,^2S_{1/2}$ state using time-correlated single-photon counting spectroscopy in a vapor cell. We excite the atoms using a Doppler-free two-photon transition from the $6s\,^2S_{1/2}$ ground state, and detect the 1.47$\mu$m photons from the spontaneous decay of the $7s\,^2S_{1/2}$ to the $6p\,^2P_{3/2}$ state. We use a gated single photon detector in an asynchronous mode, allowing us to capture the fluorescence profile for a window much larger than the detector gate length. Analysis of the exponential decay of the photon count yields a $7s\,^2S_{1/2}$ lifetime of 48.28$\pm$0.07ns, an uncertainty of 0.14%. These measurements provide sensitive tests of theoretical models of the Cs atom, which play a central role in parity violation measurements.

Specific mass shift measurements in rubidium by Doppler-free two-photon transitions

We measured the isotope shifts between Rb isotopes (86m, 86g, 81) and 87Rb for the 5S1/2 to 5D5/2 transition using the Doppler-free two-photon transition technique. By making a King plot, the difference between the specific mass shift constants of the 5S1/2 and 5D5/2 states is deduced to be 77(33) GHz amu. In addition, we measured the hyperfine constants of the 5D5/2 states in those three Rb isotopes, and we have determined an improved ground state hyperfine constant in 86mRb, A(5S1/2) = 563.04(5) MHz.

Hyperfine coupling constants of cesium 7D states using two-photon spectroscopy

Doppler-free two-photon transitions of cesium 6S1/2→7D3/2, 5/2 were observed in a thermal stabilized cell. A repeated spectrum of 75 MHz introduced from the side band of an electro-optical modulator served as a frequency marker to improve the accuracy of frequency measurement. The hyperfine magnetic dipole constant A and electrical quadrupole constant B of Cs 7D3/2, 5/2 can be derived from the splitting intervals of the observed spectra. The results are: A=7.36(07) MHz, B=−0.88(87) MHz for the 7D3/2 state, and A=−1.81(05) MHz, B=1.01(1.06) MHz for the 7D5/2 state. These coefficients are improvements for testing high-precision measurements and determining fundamental physical constants.

Foreign gas pressure broadening and shifts of the 2S–4S two-photon transition inlithium

We have observed broadening and shift of the 2S–4S Doppler-free two-photontransition of atomic Li by collisions with neon and argon buffer gases in aheat-pipe oven. Measured broadening and shift rates are presented and comparedwith theoretical predictions calculated in the impact approximation using threedifferent interaction potentials. A superposition of polarization and modifiedFermi potentials gives good agreement with the experimental data.

Level magnetizabilities of the alkaline metal atoms

The Zeeman effects of Doppler-free two-photon transitions as induced between the ground and several low-quantum electronic states of Li, Na, K, Rb, and Cs have been recorded in fields up to 4.5 T. Adequate selection rules were chosen to suppress the dominating orbital and spin first-order frequency field splittings, and atomic magnetizabilities were then extracted from the higher-order effects. The values can be accounted for quantitatively using quantum defect single-electron hydrogen wave functions. Fine structure and hyperfine structure coefficients have been obtained from the zero-field spectra.

Collisional broadening ofRydberg atom transitions by rare gas perturbers

In this paper we present the results and discuss theapplicability of an impact theory of lineshapes, developed byHenry and Herman for the pressure-induced broadening andshifting of transitions from the ground state to various Rydbergstates of alkalis, to the situation in which the perturbingatoms are ground-state rare gas atoms. This theory, which hassuccessfully described the collisional broadening of Rydbergstates by alkali atoms, can without major alterations predictthe widths and shifts of Doppler-free two-photon transitions tonS and nD states of Rb and Na, 6⩽n⩽30, perturbedby He, Ne, Ar, Kr and Xe. The results in all cases showreasonable agreement with experiment, with the transitions to Na**(nD) states being somewhat less satisfactory than the others.

Doppler-free two-photon millimeter wave transitions in OCS and CHF3.

Doppler-free two-photon rotational transitions J = 13<--<--11 and J = 12<--<--10 of OCS and J = 8<--<--6 and J = 7<--<--5 of CHF (3) were detected in the frequency range 134-156 GHz, using a novel, highly sensitive intracavity-jet technique. The sub-Doppler narrowing of the observed peaks (down to 40 kHz full width at half maximum as compared to 300 kHz of the Doppler width) demonstrates the potential of this new technique for high precision millimeter wave spectroscopy. The possibilities of the further reduction of the two-photon absorption line widths are considered.

Continuous-wave Doppler cooling of hydrogen atoms with two-photon transitions

We propose and analyze the possibility of performing two-photon continuous-wave Doppler cooling of hydrogen atoms using the $1S\ensuremath{-}2S$ transition. ``Quenching'' of the $2S$ level (by coupling with the $2P$ state) is used to increase the cycling frequency, and to control the equilibrium temperature. Theoretical and numerical studies of the heating effect due to Doppler-free two-photon transitions evidence an increase of the temperature by a factor of 2. The equilibrium temperature decreases with the effective (quenching-dependent) width of the excited state and can thus be adjusted up to values close to the recoil temperature.

Adiabatic population transfer by acoustooptically modulated laser beams

We demonstrate coherent two-photon optical population transfer (STIRAP, J. Chem. Phys. 92, 5363 (1990)) between electronic states of atoms by use of two cw laser beams with high spectral resolution, which are modulated by an acoustooptical modulator (AOM) and provide a counterintuitive sequence of temporally delayed pulses of several hundred nanoseconds length. The efficiency of optical population transfer induced by this AOM-STIRAP method is shown in the lambda-type system of metastable Ne atoms, produced in a nozzle beam, where we compare directly spatially and phase separated pulses from cw laser beams (cw-STIRAP). As an application, efficient and selective transfer of population into a decaying electronically excited state of Na is demonstrated using the Doppler-free two-photon ladder-type transition 3S-3P-5S in a sodium cell. The experimental results are reproduced satisfactorily by use of a density matrix approach.

Second-order Zeeman effect in the 5 2S-3 2S and 4 2D-3 2S two-photon transitions of atomic sodium.

We have observed the second-order, field-induced Zeeman effects of the Doppler-free two-photon transitions 5${\mathrm{}}^{2}$S--3${\mathrm{}}^{2}$S and 4${\mathrm{}}^{2}$D--3${\mathrm{}}^{2}$S of atomic sodium using fields up to 5 T. Using a suitable experimental configuration it was possible to avoid the appearance of first-order effects in the high-field limit. The remaining small field-induced shifts are of the order of magnitude of a few linewidths. They yield the differences of the diamagnetic susceptibilities of the connected atomic states. The method is therefore capable of providing data related to atomic electronic structure. \textcopyright{} 1996 The American Physical Society.

Towards an accurate frequency standard at λ778 nm using a laser diode stabilized on a hyperfine component of the Doppler-free two-photon transitions in rubidium

Two systems are built in which a commercially available GaAlAs laser diode is stabilized on a hyperfine component of the 5S 1 2 −5D ( 3 2 , 5 2 ) two-photon transitions in rubidium at λ=778 nm ( v=385 THz). Some preliminary metrological results are presented. The frequency repeatability has been found to be of 200 Hz (5.2 parts in 10 13) and a short-term frequency stability of 3×10 -13/ τ /gt up to 2000 s is currently obtained.

Line shape of a Doppler-free two-photon transition in a metastable neon beam.

We present a model of the line shape of a Doppler-free two-photon transition in a metastable neon beam. This model considers the ac Stark effect, the Gaussian profile of the laser beam, and the depletion, or bleaching, of the metastable state as the atoms cross the laser. The model fit the experimental data well and was recently used to verify the time-dilation effect at an accuracy of a few ppm.

Two-step Optical Excitation for Doppler Linewidth Reduction and Motion Study of Ions Stored in a Paul Trap

Abstract Atomic thorium ions were stored in an r.f. quadrupole trap and excited by tunable laser light. One-step and two-step excitation schemes were applied; a four to eightfold reduction of the Doppler linewidth is obtained for the two-step scheme with essentially no loss in signal strength. The Doppler-free two-photon transition was also observed. The lineshapes were calculated using a simple model; the observed profiles were well reproduced proving that the velocity and acceleration features of stored ions are properly accounted for. A further reduction of the Doppler linewidth is expected for the two-step method if a short lived intermediate level is used.

1S-3S and 1S-3D doppler-free two-photon transition in hydrogen and deuterium

We have observed the 1S-3S and 1S-3D Doppler-free two-photon transitions in hydrogen and deuterium using an intense monochromatic light source at 2050 å. The experiment is done in a cell in an afterglow discharge. We present the experimental results and discuss some possible future applications.

High-precision measurements of Doppler-free two-photon transitions in Rb: new values for proposed dye-laser reference wavelengths

We have made new measurements of the 85Rb 5s–ns Doppler-free two-photon transitions (n = 14–50) with estimated accuracy of better than 1 part in 108. The wavelength of our dye laser was measured by using photographic Fabry–Perot interferometry. Our results differ systematically by as much as 7 parts in 108 from the previous measurements of Lee et al. [ Opt. Lett.3, 141– 143 ( 1978)], who proposed that these two-photon transitions be used as reference lines for dye-laser wavelength calibration. The observed discrepancies cannot be explained by known systematic shifts of the Rb levels in the thermionic diode detectors employed in both experiments. We have confirmed the accuracy of our interferometric wavelengths by measuring Doppler-free saturated absorption lines in 127I2. These results suggest that recently reported Rydberg-state measurements in Li, K, Rb, and Cs, which were calibrated with respect to the previous measurements of the 85Rb 5s–ns transitions, may require revision.

Doppler-free two-photon absorption and dispersion spectroscopy with a Fabry-Perot cell

Infrared spectroscopy has been performed using a CO 2 laser and a Fabry-Perot resonator filled with SF 6 as absorber. Transmitted as well as reflected power displays sharp Doppler-free two-photon (DFTP) transitions of the 0–2v 3 SF 6 band. With the experimental set-up described, the DFTP dispersion can be easily observed. Owing to the saturability of the SF 6 absorber the Fabry-Perot resonances display dispersive bistability.

Remarks on E1-E2 and E1-M1 two-photon transitions

We investigate the possibility to observe Doppler-free two-photon transitions in the case where one of the transitions is an electric dipole transition and the other one is a magnetic dipole or an electric quadrupole transition. We show that it is not possible to obtain Doppler-free lines by reflecting the beam on itself. On the other hand, such an effect should be observable if the two counterpropagating beams have different polarizations.

Doppler-free two-photon excitation of nitric oxide with frequency-stabilized cw dye laser radiation

Doppler-free two-photon transitions are induced in nitric oxide with blue, narrowband, tunable radiation generated by a frequency-stabilized cw dye ring laser. The recorded two-photon spectra provide a precise measurement of the spin-rotation splitting of the excited rotational levels (A 2∑ +, ν' = 0) N' = 1 to N' = 19. The splitting constant γ is determined to be γ = 80.35(15) MHz.