Doppler-free Saturation Spectroscopy Research Articles

Exploration of a vapor cell optical frequency standard scheme implemented using Doppler-free spectroscopy.

We implement a compact optical frequency standard scheme with laser frequency locked to the 5S1/2 (F = 2) - 6P3/2 (F' = 3) transition of the second excited state of 87Rb atoms in a 3 mm cubic glass cell, using a Doppler-free saturated absorption spectroscopy. The experimental results show the frequency stability at the level of 2.2 × 10-12 at 1 s. Furthermore, we conduct an experimental study on the effect of a repump laser on the frequency performance of the saturated absorption spectroscopy optical frequency standard, providing valuable experimental results with reference values for implementing this type of optical atomic clock.

Hyperfine structure and isotope shifts of xenon measured for near-infrared transitions with Doppler-free saturated absorption spectroscopy

Hyperfine structure and isotope shifts of Xe were investigated with Doppler-free saturated absorption spectroscopy for all transitions in the 820-841 nm spectral interval. The Xe samples were commercial gases of natural isotopic abundances. The measurements were performed by employing a widely tunable narrow line Ti:sapphire laser. The hyperfine structures for 129Xe and 131Xe were well resolved. For the transitions with the wavelengths in vacuum near 820.860 nm and 841.150 nm also isotope shifts of the even isotopes were clearly observed. The modeling of the shapes of the observed absorption lines qualitatively reproduces experimental spectral profiles. The measured absorption peaks of the hyperfine structure and isotope shifts of Xe isotopes present characteristic spectral fingerprints for their identification. Eventually, quantitative spectral analysis will provide isotopic abundances of natural, as well as radioactive Xe gas samples.

Saturated absorption spectroscopy of buffer-gas-cooled Barium monofluoride molecules

We report an experimental investigation on the Doppler-free saturated absorption spectroscopy of buffer-gas-cooled Barium monofluoride (BaF) molecules in a 4~K cryogenic cell. The obtained spectra with a resolution of 19~MHz, much smaller than previously observed in absorption spectroscopy, clearly resolve the hyperfine transitions. Moreover, we use these high-resolution spectra to fit the hyperfine splittings of excited $A(v=0)$ state and find the hyperfine splitting of the laser-cooling-relevant $A^2\Pi_{1/2}(v=0, J=1/2, +)$ state is about 18 MHz, much higher than the previous theoretically predicted value. This provides important missing information for laser cooling of BaF molecules.

Frequency of the ultranarrow S01−P23 transition in Sr87

We determine the frequency of the ultranarrow $^{87}\text{Sr}$ ${^{1}\text{S}_{0}} - {^{3}\text{P}_{2}}$ transition by spectroscopy of an ultracold gas. This transition is referenced to four molecular iodine lines that are observed by Doppler-free saturation spectroscopy of hot iodine vapor. The frequency differences between the Sr and the I$_2$ transitions are measured with an uncertainty of 250 kHz. The absolute frequency of the $^{87}\text{Sr}$ ${^{1}\text{S}_{0}} - {^{3}\text{P}_{2}}$ ($\text{F}'=7/2$) transition is 446648775(30) MHz and limited in accuracy by the iodine reference. This work prepares the use of the Sr ${^{1}\text{S}_{0}} - {^{3}\text{P}_{2}}$ transition for quantum simulation and computation.

Doppler-free, Stark broadened profiles at low plasma densities in helium

This work utilizes Doppler-free saturation spectroscopy to measure Doppler-free, Stark broadened spectral profiles for the π-polarization of the HeI 21P → 61D transition in a low density (ne=7×1010−2×1012 cm−3) helium plasma. The measurements were performed in an electron-cyclotron resonance discharge at 23 mTorr with electron density being diagnosed using a combination of Langmuir probe and microwave interferometry techniques. The Doppler-free profiles were observed to be nearly symmetric at ne < 1011 cm−3 but markedly asymmetric above this transition point. Electron density is extracted from the spectral data via fitting to a spectral model based on quasi-static Stark broadening. The fit results are compared to ne measurements obtained using a combination of Langmuir probe and microwave interferometry techniques. The fit and measured ne are shown to agree within 20% on average. Finally, the quasi-static model is shown to be valid in the low-density regime for 21P → 61D helium transition.

High resolution magnetic field measurements in hydrogen and helium plasmas using active laser spectroscopy.

Passive spectroscopic measurements of Zeeman splitting have been used reliably to measure magnetic fields in plasmas for decades. However, a requirement is that the field magnitude must be sufficiently strong to be resolved over Doppler and instrument broadening (typically >10 000 G). A diagnostic for measuring magnetic fields spectroscopically well below this limit (>20 G) with high sensitivity has been developed at the Oak Ridge National Laboratory. The diagnostic relies on measuring a high resolution spectral profile using Doppler-free saturation spectroscopy (DFSS) and then fitting the spectrum to a quantum mechanical model. DFSS is an active, laser based technique that greatly reduces the influence of Doppler broadening and eliminates instrument broadening. To date, the diagnostic has been successfully employed to measure the magnetic field in magnetized (550-900 G), low-temperature (5-10 eV), low-density (1010-1012 cm-3), hydrogen and helium plasmas in the 5-200 mTorr pressure range using a low power (25 mW) diode laser. Implementing an approximate crossover resonance model, the measurements are shown to be accurate within 5 G for helium and 83 G for hydrogen. The accuracy in hydrogen can be improved to 39 G if the crossover resonances are neglected. A more robust crossover model can decrease this error to <1 G.

Doppler-free spectroscopy with a terahertz quantum-cascade laser.

We report on the Doppler-free saturation spectroscopy of a molecular transition at 3.3 THz based on a quantum-cascade laser and an absorption cell in a collinear pump-probe configuration. A Lamb dip with a sub-Doppler linewidth of 170 kHz is observed for a rotational transition of HDO. We found that a certain level of external optical feedback is tolerable as long as the free spectral range of the external cavity is large compared to the width of the absorption line.

Saturation spectroscopy of an optically opaque argon plasma

A pure argon (Ar) plasma formed by a capacitively coupled radio-frequency discharge was analyzed using Doppler-free saturation spectroscopy. The expected line shape was a characteristic of sub-Doppler spectra in the presence of velocity-changing collisions, a narrow Lorentzian centered on a Doppler pedestal, but the observed line shapes contain a multi-peak structure, attributed to opacity of the medium. Laser absorption and inter-modulated fluorescence spectroscopy measurements were made to validate opacity as a driving factor of the observed line shapes. Spectral line shapes are further complicated by the spatial dependence of the pump laser, probe laser and of the absorbing medium, as well as the large absorbance of the transition under investigation. A numerical line shape was derived by accounting for the spatial variation of the pump and probe with a saturated line shape obtained from the rate equations for an equivalent two-level system. This simulated line shape shows good qualitative agreement with the trends observed in the data.

Frequency Measurement of Tellurium Lines Near Calcium

We report measurement of frequencies of tellurium lines with respect to atomic calcium employing a 423 nm commercial violet laser diode placed in an extended cavity. Doppler-free saturated absorption spectroscopy of molecular tellurium is performed on three lines, which are lower and higher in energy to the calcium 4s2 1S0–4s4p 1P1 transition. The measured frequency difference of 717(13) MHz between the strong absorption line, #1508 in the tellurium atlas and the calcium transition is in agreement within 10% of the values available from literature. Wavenumbers of two new tellurium lines are derived from the measurement. In addition, spectroscopy and laser cooling and trapping of francium (n = 7) on the ns 2S1/2–(n + 1)p 2P3/2 transition and spectroscopy of Te2 lines near francium could be performed employing the 423 nm diode laser.

Applications of Doppler-free saturation spectroscopy for edge physics studies (invited).

Doppler-free saturation spectroscopy provides a very powerful method to obtain detailed information about the electronic structure of the atom through measurement of the spectral line profile. This is achieved through a significant decrease in the Doppler broadening and essentially an elimination of the instrument broadening inherent to passive spectroscopic techniques. In this paper we present the technique and associated physics of Doppler-free saturation spectroscopy in addition to how one selects the appropriate transition. Simulations of Hδ spectra are presented to illustrate the increased sensitivity to both electric field and electron density measurements.

Improved measurement of the hyperfine structure of the laser cooling level $$4f^{12}(^3H_6)5d_{5/2}6s^2$$ 4 f 12 ( 3 H 6 ) 5 d 5 / 2 6 s 2 $$(J=9/2)$$ ( J = 9 / 2 ) in $${}^{169}_{\,\,69}{{\mathrm {Tm}}}$$ 69 169 Tm

We report on the improved measurement of the hyperfine structure of $4f^{12}(^3 H_6)5d_{5/2}6s^2$ $(J=9/2)$ excited state in Tm-169 which is involved in the second-stage laser cooling of Tm. To measure the absolute value of the hyperfine splitting interval we used Doppler-free frequency modulation saturated absorption spectroscopy of Tm atoms in a vapor cell. The sign of the hyperfine constant was determined independently by spectroscopy of laser cooled Tm atoms. The hyperfine constant of the level equals $A_J=-422.112(32)$ MHz that corresponds to the energy difference between two hyperfine sublevels of $-2110.56(16)$~MHz. In relation to the saturated absorption measurement we quantitatively treat contributions of various mechanisms into the line broadening and shift. We consider power broadening in the case when Zeeman sublevels of atomic levels are taken into account. We also discuss the line broadening due to frequency modulation and relative intensities of transitions in saturated-absorption experiments.

A simplified scheme for generating narrow-band mid-ultraviolet laser radiation.

We report on the development and characterization of continuous, narrow-band, and tunable laser systems that use direct second-harmonic generation from blue and green diode lasers with an output power level of up to 11.1 mW in the mid-ultraviolet. One of our laser systems was tuned to the mercury 6(1)S0 → 6(3)P1 intercombination line at 253.7 nm. We could perform Doppler-free saturation spectroscopy on this line and were able to lock our laser to the transition frequency on long time scales.

Interference Cancellation for Hollow-Core Fiber Reference Cells

Doppler-free saturated absorption spectroscopy of gases in hollow-core fiber (HCF)-based cells can be used for realizing new compact, robust, and portable frequency standards. In this paper, methods for cancelling interferences resulting from the optical connections between standard fiber and HCF and other factors such as varying coupling to HCF modes are investigated. Laser power modulation with simultaneous detection of ac and dc signal is used to separate saturated absorption from interferences. In addition, a technique of two piezoelectric stack actuators stretching the fiber at different locations is described. The presented experimental results demonstrate that 99% interference attenuation is readily attainable with the techniques. Frequency comb-referenced measurement of saturated acetylene absorption features near $1.54~\mu \text{m}$ , with fiber length and power modulation, is presented. For comparison, measurements and results of the same acetylene fiber, using a system having pump and probe beams at mutually shifted frequencies, are also given.

Measurement of the 21P1→31D2transition frequency in4He

We report a frequency measurement of the $2\phantom{\rule{0.16em}{0ex}}{}^{1}{P}_{1}\ensuremath{\rightarrow}3\phantom{\rule{0.16em}{0ex}}{}^{1}{D}_{2}$ transition in ${}^{4}$He at 668 nm. Doppler-free saturation spectroscopy is performed in an rf discharged sealed-off cell using an external cavity diode laser stabilized by an optical frequency comb. The measured absolute frequency is 448 791 399.113 (268) MHz with a relative uncertainty of $6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$. Our result is more precise than the previous measurement by one order of magnitude. The ionization energy of the $2\phantom{\rule{0.16em}{0ex}}{}^{1}{P}_{1}$ state using this result and the theoretical value of the $3\phantom{\rule{0.16em}{0ex}}{}^{1}{D}_{2}$ state is derived to be 814 709 148.133 (269) MHz, in agreement with our previous determinations derived from other precisely measured transitions. We also derive the ionization energy of the $2\phantom{\rule{0.16em}{0ex}}{}^{1}{S}_{0}$ state and the energy separation between $3\phantom{\rule{0.16em}{0ex}}{}^{1}{D}_{2}$ and $3\phantom{\rule{0.16em}{0ex}}{}^{3}{D}_{1}$ states using this result and other transitions. These determinations should be essential for comparison with the theoretical calculations.

Doppler-free frequency-modulation spectroscopy of atomic erbium in a hollow-cathode discharge cell

The erbium atomic system is a promising candidate for an atomic Bose-Einstein condensate of atoms with a non-vanishing orbital angular momentum ($L \neq 0$) of the electronic ground state. In this paper we report on the frequency stabilization of a blue external cavity diode laser system on the 400.91 $nm$ laser cooling transition of atomic erbium. Doppler-free saturation spectroscopy is applied within a hollow cathode discharge tube to the corresponding electronic transition of several of the erbium isotopes. Using the technique of frequency modulation spectroscopy, a zero-crossing error signal is produced to lock the diode laser frequency on the atomic erbium resonance. The latter is taken as a reference laser to which a second main laser system, used for laser cooling of atomic erbium, is frequency stabilized.

Hyperfine structure of some near-infrared Xe I and Xe II lines

This work reports on the experimental determination of the hyperfine splitting of the Xe I lines at 828.01 nm and 834.68 nm and the Xe II line at 834.72 nm. Measurements were performed by means of Doppler-free saturation spectroscopy in a low-pressure radio-frequency discharge. The absolute wavelength of all hyperfine components is obtained by way of a high-precision wavemeter backed-up with the absorption spectrum of the NO 2 molecule. We provide an accurate estimate of hyperfine constants for the lower level of the Xe II transition at 834.72 nm. The two Xe I transition outcomes of our experimental study are compared with data available in the literature.

Isotope shift and hyperfine splitting of the4s→5ptransition in potassium

We have investigated $4s\;^2S_{1/2} \rightarrow 5p\;^2P_{1/2}$ transition ($D_1$ line) of the potassium isotopes $^{39}$K, $^{40}$K, and $^{41}$K using Doppler-free laser saturation spectroscopy. Our measurements reveal the hyperfine splitting of the $5p\;^2P_{1/2}$ state of $^{40}$K and we have determined the specific mass shift and the nuclear field shift constants for the blue $D_1$ line.

Spectroscopic study of the cycling transition4s[3/2]2–4p[5/2]3at 811.8 nm inAr39: Hyperfine structure and isotope shift

Doppler-free saturated absorption spectroscopy is performed on an enriched radioactive $^{39}\mathrm{Ar}$ sample. The spectrum of the $3{s}^{2}3{p}^{5}4s[3/2]{}_{2}$--$3{s}^{2}3{p}^{5}4p[5/2]{}_{3}$ cycling transition at 811.8 nm is recorded, and its isotope shift between $^{39}\mathrm{Ar}$ and $^{40}\mathrm{Ar}$ is derived. The hyperfine coupling constants $A$ and $B$ for both the $4s[3/2]{}_{2}$ and $4p[5/2]{}_{3}$ energy levels in $^{39}\mathrm{Ar}$ are also determined. The results partially disagree with a recently published measurement of the same transition. Based on earlier measurements as well as the current work, the isotope shift and hyperfine structure of the corresponding transition in $^{37}\mathrm{Ar}$ are also calculated. These spectroscopic data are essential for the realization of laser trapping and cooling of $^{37,39}\mathrm{Ar}$.

Green-pumped cw singly resonant optical parametric oscillator based on MgO:PPLN with frequency stabilization to an atomic resonance

We report on a green-pumped continuous-wave singly resonant optical parametric oscillator (cw SRO) based on MgO-doped periodically poled LiNbO3. Operating the SRO at crystal temperatures between 40.0°C to 80.0°C an idler wavelength range of 1406–1451 nm can be accessed. The system provides stable single-frequency idler radiation of more than 300 mW at a pump power of 2 W while featuring a low threshold (<1.2 W). Above a pump power of 2.1 W we observe multimode operation, which is similar to the behavior reported for infrared-pumped SROs. To show the applicability of the device we demonstrate Doppler-free saturation spectroscopy of the cesium D2 line using the signal wave and frequency stabilization to a crossover resonance of the D2 transition.

Doppler-free laser spectroscopy of buffer-gas-cooled molecular radicals

We demonstrate Doppler-free saturated absorption spectroscopy of cold molecular radicals formed by laser ablation inside a cryogenic buffer gas cell. By lowering the temperature, congested regions of the spectrum can be simplified, and by using different temperatures for different regions of the spectrum a wide range of rotational states can be studied optimally. We use the technique to study the optical spectrum of YbF radicals with a resolution of 30 MHz, measuring the magnetic hyperfine parameters of the electronic ground state. The method is suitable for high-resolution spectroscopy of a great variety of molecules at controlled temperature and pressure, and is particularly well suited to those that are difficult to produce in the gas phase.