Saturation Spectroscopy Research Articles

Infrared laser locking to a rubidium saturated absorption spectrum via a photonic chip frequency doubler.

To extend the coherence of quantum transitions for laser locking, as well as increase the compactness and stability of the experimental setup, we propose to utilize photonic integrated resonators with high second-harmonic (SH) generation efficiencies as reliable frequency doublers that link the desired frequencies with the frequency references. In this Letter, a sufficiently strong SH signal up to microwatts was generated by a photonic integrated frequency doubler using a milliwatt infrared (IR) laser source. Furthermore, an increased SH generation bandwidth covering Rb85 and Rb87D2 transition lines, as well as saturated absorption spectroscopy, was demonstrated by tuning the pump power and chip temperature. Here we present, to the best of our knowledge, the first successful locking of an IR laser to Rb saturated absorption lines via a photonic chip frequency doubler.

Incoherent and coherent manipulation of surface plasmon hole burning

We theoretically investigate the spectral hole burning of surface plasmon polaritons (SPPs) at an interface between rubidium (87Rb) atomic medium and silver. A typical Lamb dip can be observed at a certain frequency where the absorption of SPPs is suddenly reduced. By applying Kerr-type nonlinearity to the 87Rb atomic gas, the slope of dispersion becomes more normal and steeper. Under the effect of Doppler broadening, a group index of at the SPPs hole burning region is obtained and is further enhanced to with the application of Kerr-nonlinearity. Similarly, the propagation length of SPPs is increased at the hole burning region and is further enhanced with Kerr-nonlinearity. Further, we show that this nonlinear effect results in the enhancement of the phase shift from 0.02 to 1.5 radian at the hole burning region. These results may find important applications in nano-photonics, optical tweezers, photovoltaic devices and sensing technology.

Cavity-enhanced saturation spectroscopy of molecules with sub-kHz accuracy

Saturation spectroscopy is frequently used to obtain sub-Doppler measurement of atomic and molecular transitions. Optical resonant cavities can be used to enhance the effective absorption path length, and the laser power inside the cavity as well to saturate very weak ro-vibrational transitions of molecules. Three different cavity-enhanced methods, cavity enhanced absorption spectroscopy, cavity ring-down spectroscopy, and noise-immune cavity enhanced optical heterodyne molecular spectroscopy (NICE-OHMS), were compared by measuring the Lamb dip of a C2H2 line at 1.4 µm using a cavity with a finesse of 120000. The center of the line was determined by different cavity-enhanced methods, each giving a sub-kHz (δν/ν≈10−12) statistical uncertainty. The sensitivity and precision of different methods were analyzed and compared. As demonstrated in this study, the NICE-OHMS method is the most sensitive one, but more investigation on the systematic uncertainty is necessary before its application in metrology studies toward a sub-kHz accuracy.

Saturation cavity ring-down spectrometry using a dynamical relaxation model

We propose a new simple approximate solution to the two-state rate equation model for analyzing decay signals of saturation cavity ring-down spectrometry in the adiabatic and low-saturation regime. It helps obtain baseline-immune Doppler-free spectra for hyperfine transitions and linear absorption coefficients of a gas in the saturation regime. To demonstrate it, a baseline-immune Lamb dip spectrum of the R1A2 transitions in the 2v2 + v3 band of methane was recorded. The line position was determined to be 6 076.108 457 7(11) cm-1, the relative uncertainty being 1.8 × 10-10.

Investigation of the two-color polarization spectroscopy between the excited states based on cesium atoms

Two-color polarization spectroscopy (TCPS) of cesium 6S<sub>1/2</sub>-6P<sub>3/2</sub>-8S<sub>1/2</sub> (852.3 nm + 794.6 nm) ladder-type system in a room-temperature vapor cell are investigated. The frequency of 852.3 nm laser used as a pump beam is locked on one of the hyperfine transitions between the ground state 6S<sub>1/2</sub> and excited state 6P<sub>3/2</sub> by the saturated absorption spectroscopy technique, which can populate some atoms on the 6P<sub>3/2</sub> excited state and induce anisotropy in the atomic medium. The frequency of 794.6 nm laser serving as a probe beam is scanned across the whole 6P<sub>3/2</sub>→8S<sub>1/2</sub> transition to ascertain this anisotropy, and thus the TCPS is obtained. In experiment, we measure and analyse the influence of frequency detuning of 852.3 nm pump laser on TCPS, and especially reveal that some of hyperfine energy levels of intermediate excited state 6P<sub>3/2</sub>, which has no direct interaction with the 852.3 nm pump laser, are also populated by a small fraction of atoms with a specific speed in the direction of pump laser beam due to Doppler effect, so they also have contribution to the TCPS when the 794.6 nm probe laser is scanned to the resonance transition line between the 6P<sub>3/2</sub> and 8S<sub>1/2</sub> states after the Doppler frequency shift has been considered. In addition, we prove that the atomic coherence like electromagnetically induced transparency effect obviously results in a narrower line width of TCPS in the case of counter-propagating experimental configuration than that in the case of pump beam co-propagating with the probe beam in the Cs vapor cell. Finally, we apply the TCPS with dispersive shaped feature to frequency stabilization with no modulation, and the frequency fluctuations of 794.6 nm laser are ～0.5 MHz and ～9.2 MHz for the frequency-locking and free running in ～225 s, respectively. The above research work is expected to play a role in precisely measuring the atomic energy level structure and its related hyperfine structure constant (magnetic dipole and electric quadrupole coupling constants), and also in stabilizing the laser frequency to the excited state transition especially for the optical fiber communication, two-color laser cooling/trapping neutral atoms, optical filter, etc.

Tunable single-mode lasing in a single semiconductor microrod.

Developing micro/nanoscale wire lasers with single-mode operation and lasing wavelength modulation is essential for realizing their practical applications such as optical communication and saturated spectroscopy. We demonstrated, to the best of our knowledge, the first tunable single-mode microrod laser without complicated micro/nano-manipulation and without additional environmental requirement. In this letter, we realized the wavelength modulation in a single semiconductor microrod simply and directly by changing the axial location of the active region, owing that the active region position plays a key role in determining the lasing mode of microrod lasers. Based on this feature, we proposed a pair of asymmetrical distributed Bragg reflectors (DBRs) with specific spectral selectivity to be induced in a GaN microrod to realize tunable single-mode lasing in a single semiconductor microrod. By using this method, lasing wavelength can be modulated from 369.5 to 375.7 nm flexibly and repeatedly in a 45 μm GaN microrod with the change of the excitation source position. This approach demonstrates a big application potential in numerous fields consisting of optical telecommunication and environmental monitoring.

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.

Watt-level single-frequency tapered amplifier laser using a narrowband interference filter.

We demonstrate a tunable external cavity tapered amplifier laser (ECTAL) using a narrowband interference filter as the wavelength discriminator. The laser is tunable over a wavelength range from 1006 to 1031nm with an output power of ∼1 W. The amplified stimulated emission of the laser system is suppressed to better than 32dB. The laser is applied to study the saturation spectroscopy on the R(39) 57-0 line of iodine molecule, which, to our best knowledge, is the first measurement of this line close to the dissociation limit. The linewidth of the a1 component is ∼2 MHz at the iodine vapor pressure of ∼11 Pa, and the pressure-broadening coefficient is ∼156 kHz/Pa. This laser system is also used for the injection seeding of a 1030nm disk laser to perform hyperfine spectroscopy of muonic hydrogen. To reach a satisfactory condition for disk laser use, the ECTAL is successfully stabilized to the iodine Doppler-free spectroscopy of the P(26) 43-0 line near 515nm, with continuous locking over 48h.

Investigation of the Lamb Dip in the Near IR Spectra of Water Containing DNA

Investigation of the Lamb Dip in the Near IR Spectra of Water Containing DNA

Intensity of a crossover signal relative to Lamb dips observed in Stark spectroscopy of methane

We have observed twelve triplets of two Lamb dips and a crossover signal in Stark spectroscopy of the ν3 band of methane using a comb-referenced sub-Doppler resolution spectrometer. The crossover signal consists of two crossover resonances in the Λ- and V-type three-level systems, which overlap each other in frequency under the first-order Stark shifts (Tyurikov et al., 1997). The intensity of the crossover signal relative to the Lamb dips agrees well with a simple calculation based on steady-state solutions of rate equations in weak saturation using the observed intensity ratio of the Lamb dips in the Λ-type three-level system.

Two-tone frequency modulation saturation spectroscopy for tunable frequency offset locking of a single laser

The two-tone frequency modulation spectroscopy (TTFMS) is widely used for gas trace detection with its low noise absorption signal. In this paper we propose and implement the TTFMS scheme for frequency offset locking a single diode laser to an atomic resonance line by using its dispersion signal. The TTFMS theory is first discussed under the assumption that the intermediate modulation frequency is comparable to the linewidth of the absorption feature, and the dependence of the TTFMS absorption and dispersion features on the intermediate modulation frequency and modulation index are investigated theoretically. Based on a fiber-coupled electro-optic modulator (EOM) with two-tone modulation, we experimentally demonstrated the performance of the frequency offset locking method. The result shows a short-term frequency stability of the frequency offset locked laser reached around 1.1 × 10−11, with an averaging time of 2 s. This method can find wide applications in fields requiring widely tunable frequency offset locking a single laser to the atomic resonance line, like precision spectroscopy and Raman optics for atom interferometers.

Rovibrational fine structure and transition dipole moment of CF3H by frequency-comb-assisted saturated spectroscopy at 8.6 µm

We report on rovibrational fine structure spectroscopy of room-temperature trifluoromethane with a saturated absorption method, using a quantum cascade laser at around 8.6 µm phase-locked to a self-referenced mid-infrared frequency comb synthesizer. The absolute line-center frequencies of 36 lines have been measured by means of the wavelength modulation method with a best fractional uncertainty of ∼3×10−9 in a single acquisition scan from 34.7426 to 34.7443 THz. In addition, an extensive investigation of the measured Lamb-dip profiles using both direct pump-probe spectroscopy method and wavelength modulation technique has been performed for two selected lines, rR40(64) and rR36(38), belonging to the υ5 vibrational band, as a function of either the gas pressure or the pump beam power. This approach allowed the measurement of the main spectroscopic parameters, such as saturation intensity, transition electric dipole moment, as well as self-pressure broadening coefficient.

Linewidth in saturated absorption spectroscopy for two-level atoms: an empirical formula.

We present an empirical formula for linewidth in saturated absorption spectroscopy for two-level atoms with cycling transition lines, taking the coherence term in the Doppler-broadened limit into account. The full width at half-maximum is obtained as (1+(1+as0)b)γt, where s0 is the on-resonance saturation parameter, γt is the transverse decay rate, and a and b are the parameters that depend on γt. We find that as γt increases, a and b approach 1 and 1/2, respectively. These are the typical values in the case without the coherence term.

Exercise At Simulated Altitude Increases Gastrointestinal Barrier Damage And Promotes Leukocyte Activation

PURPOSE: This study tested whether altitude-associated ischemic stress damages the gastrointestinal barrier, activates leukocytes, and promotes inflammation. METHODS: Subjects (N = 5) completed two 60 min bouts of matched-workload treadmill exercise (65% VO2max). One under control conditions (Normoxia, FIO2 = 20.9%) and the other at ~4000 m of simulated altitude (Hypoxia, FIO2 = 13.5%). Pulse oximetry was used to measure peripheral oxygen saturation (SpO2) and near-infrared spectroscopy was used to measure absolute tissue saturation (StO2) at 5 min intervals throughout exercise. Fatty acid-binding protein (I-FABP), markers of leukocyte activation (CD14, ICAM-1, IL-8, MCP-1, MPO), and cytokines (TNFα, IL-1β, IL-6, IL-10, IL-12) were measured in plasma samples that were collected Pre, Post, 1hr-Post, and 4hr-Post exercise. Data were analyzed with 2-Way (Condition x Time) RM ANOVAs with significance set at p ≤ 0.05. Post hocs (Newman-Keuls) were run where appropriate. RESULTS: Significant reductions in SpO2 and StO2 were shown during exercise at simulated altitude [(SpO2: Hypoxia = 79 ± 1% vs Normoxia = 94 ± 0.5%, p = 0.03) (StO2: Hypoxia = 61 ± 2 vs Normoxia = 69 ± 2, p < 0.01)]. A significant interaction effect was shown for I-FABP (p = 0.05), with post hoc analysis indicating I-FABP increased more from Pre to Post in Hypoxia (112%) than in Normoxia (30%). IL-8 increased more from Pre to Post (60%) and 1hr-Post (83%) in Hypoxia than in Normoxia (33% & 57%, respectively). Significant main effects were also shown for IL-6, ICAM-1, CD14, and MCP-1. All were higher in Hypoxia (p ≤ 0.05). MPO increased at Post in Normoxia (121%, p = 0.05) but did not increase until 1hr-Post in Hypoxia (129%, p = 0.02). CONCLUSIONS: Preliminary data suggest exercise at altitude may increase gastrointestinal barrier damage and leukocyte activation, as indicated by higher levels of I-FABP, IL-8, and MCP-1. Increased CD14 and ICAM-1 suggest TLR4-mediated inflammatory signaling may also be elevated, but the delayed increase in MPO following exercise at altitude warrants further investigation.

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.

Frequency metrology of the acetylene lines near 789 nm from lamb-dip measurements

Lamb-dips of the ro-vibrational lines of 12C2H2 near 789 nm were recorded using cavity ring-down saturation spectroscopy. Calibrated by an optical frequency comb, frequencies of 45 acetylene lines were determined with an accuracy of 1.1×10−7 cm−1 (δν/ν=8×10−12), which is over two orders of magnitude more accurate than previous Doppler-limited studies. An averaged shift of about 0.01 cm−1 were found by comparing the upper energies obtained in this work to those recently presented by Chubb et al. from a MARVEL analysis.

Lamb dip CRDS of highly saturated transitions of water near 1.4 μm.

Doppler-free saturated-absorption Lamb dips were measured at sub-Pa pressures on rovibrational lines of H216O near 7180 cm-1, using optical feedback frequency stabilized cavity ring-down spectroscopy. The saturation of the considered lines is so high that at the early stage of the ring down, the cavity loss rate remains unaffected by the absorption. By referencing the laser source to an optical frequency comb, transition frequencies are determined down to 100 Hz precision and kHz accuracy. The developed setup allows resolving highly K-type blended doublets separated by about 10 MHz (to be compared to a HWHM Doppler width on the order of 300 MHz). A comparison with the most recent spectroscopic databases is discussed. The determined K-type splittings are found to be very well predicted by the most recent variational calculations.

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.

Saturation spectroscopy of calcium atomic vapor in hot quartz cells with cold windows

Saturation spectroscopy of calcium atomic vapor was performed in hot quartz cells with cold windows. The Doppler-free absorption resonances with spectral width near 50 MHz were observed. For these experiments and future applications long-lived quartz cells with buffer gas were designed and made. A cooling laser for calcium magneto-optical trap will be frequency locked to the saturation resonances in the long-lived cells.

Iodine-stabilized single-frequency green InGaN diode laser.

A 520-nm InGaN diode laser can emit a milliwatt-level, single-frequency laser beam when the applied current slightly exceeds the lasing threshold. The laser frequency was less sensitive to diode temperature and could be finely tuned by adjusting the applied current. Laser frequency was stabilized onto a hyperfine component in an iodine transition through the saturated absorption spectroscopy. The uncertainty of frequency stabilization was approximately 8×10-9 at a 10-s integration time. This compact laser system can replace the conventional green diode-pumped solid-state laser and applied as a frequency reference. A single longitudinal mode operational region with diode temperature, current, and output power was investigated.