Doppler-free Resonances Research Articles

Laser frequency stabilization via bichromatic Doppler-free spectroscopy of an 87Rb D1 line.

We demonstrate a bichromatic Doppler-free spectroscopy of an 87RbD1 line by using a dual-frequency, counterpropagating laser field with orthogonal linear polarizations. A reversed Doppler-free resonance dip is observed in the dual-frequency scheme, and a significant improvement of frequency discrimination curve is acquired due to the coherent population trapping (CPT) effect. The influence of the static magnetic field and laser intensity on the spectroscopy is studied in both single- and dual-frequency schemes. After locking the laser frequency to the 87RbD1 line in the dual-frequency stabilization scheme, the beat note fractional frequency stability is at the level of 7×10-12 at 1 s integration time. This technique can be used in various applications, such as CPT atomic clocks, laser spectroscopy, quantum optics, and laser-cooling experiments.

First demonstration of Doppler-free 2-photon in-source laser spectroscopy at the ISOLDE-RILIS

Collinear Doppler-free 2-photon resonance ionization has been applied inside a hot cavity laser ion source environment at CERN-ISOLDE. An injection-seeded Ti:sapphire ring laser was used to generate light pulses with a Fourier-limited linewidth for high-resolution spectroscopy. Using a molybdenum foil as a reflective surface positioned at the end of the target transfer line, rubidium was successfully ionized inside the hot cavity. The results are presented alongside previously obtained data from measurements performed at the RISIKO mass separator at Mainz University, where collinear and perpendicular ionization geometries were tested inside an RFQ ion guide. This work is a pre-cursor to the application of the Doppler-free 2-photon in-source spectroscopy method at ISOLDE. This approach aims to take advantage of the unmatched sensitivity of in-source spectroscopy, without the disadvantage of Doppler broadening.

Long-term frequency stabilized and linewidth-narrowed cw-laser system for excitation of lithium Rydberg states

We transfer the frequency stability from a diode laser, which was locked to Doppler-free saturation absorption resonance in lithium vapor cell, to a tunable Ti-sapphire laser. We get the laser linewidth < 100 kHz/100 ms and absolute frequency stability ±0.5 MHz. The uv laser system which included the stabilized Ti-sapphire laser and frequency doubler has output optical beam with power near 100 mW and wavelength 350 nm. This uv laser system will be used for excitation and study of Rydberg states in lithium atoms.

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.

High-contrast sub-Doppler absorption spikes in a hot atomic vapor cell exposed to a dual-frequency laser field

The saturated absorption technique is an elegant method widely used in atomic and molecular physics for high-resolution spectroscopy, laser frequency standards and metrology purposes. We have recently discovered that a saturated absorption scheme with a dual-frequency laser can lead to a significant sign reversal of the usual Doppler-free dip, yielding a deep enhanced-absorption spike. In this paper, we report detailed experimental investigations of this phenomenon, together with a full in-depth theoretical description. It is shown that several physical effects can support or oppose the formation of the high-contrast central spike in the absorption profile. The physical conditions for which all these effects act constructively and result in very bright Doppler-free resonances are revealed. Apart from their theoretical interest, results obtained in this manuscript are of great interest for laser spectroscopy and laser frequency stabilization purposes, with applications in laser cooling, matter-wave sensors, atomic clocks or quantum optics.

High-precision, accurate optical frequency reference using a Fabry-Perót diode laser.

We show that the optical output of a temperature and current-tuned Fabry-Perót diode laser system, with no external optical feedback and in which the frequency is locked to Doppler-free hyperfine resonances of the 87Rb D2 line, can achieve high frequency stability and accuracy. Experimental results are presented for the spectral linewidth, frequency stability, and frequency accuracy of the source. Although our optical source is limited by a short-term spectral linewidth greater than 2 MHz, beat signal measurements from two such sources demonstrate a frequency stability of 1.1 kHz, or minimum Allan deviation of 4×10-12, at an integration time τ=15 s and with a frequency accuracy of 60 kHz at τ=300 s. We demonstrate the use of the optical source for the precision measurement of hyperfine level frequency spacings in the 5P3∕2 excited state of 87Rb and provide an accurate frequency scale for optical spectroscopy.

Condition for Doppler-free three-photon resonance in a ladder-type atomic system.

We report the essential condition for three-photon electromagnetically induced absorption (TPEIA) in a Doppler-broadened ladder-type atomic system. When the two coupling lasers operate at different frequencies, we observed Doppler-free TPEIA resonance at a counterintuitive frequency, which is the almost half-frequency detuning of the frequency difference between the two coupling fields. Considering three-photon coherence in a Doppler-broadened ladder-type three-level atomic system, the TPEIA due to the atomic group of non-zero velocity was in good agreement with the calculated TPEIA spectrum under the one-photon resonance condition of all three optical fields. From the results, we found that an atomic group with a proper velocity for ladder-type TPEIA should satisfy the one-photon resonances of all three optical fields.

Relative amplitudes of external satellites of superfine-structure multiplets in the saturated absorption spectrum of SiF4

Variation of amplitudes of Doppler-free saturated absorption resonances as a result of changes in the gas pressure and power of light waves is studied theoretically and experimentally. The results of the investigation are used for the interpretation of weak satellites of superfine-structure multiplets in the spectrum related to tunnel transitions between energy states of a molecule corresponding to its rotation about equivalent symmetry axes. Relative amplitudes of satellites of the AFE and FEF multiplets of the superfine structure of the SiF4 molecule in the frequency tuning interval of a CO2 laser operating at the P(38) line of the 9.7-µm band are studied experimentally. It is confirmed that the variation of relative amplitudes of the satellites is caused mainly by the fact that the magnitudes of the self-induced transparency of the medium that are created by each light wave for itself are different for the main resonances of the multiplet and for their satellites. The discrepancy between the experimental and theoretical dependences of the relative amplitudes of the satellites upon variation of the gas pressure and power of light waves is analyzed. Based on the discovered discrepancy, it is suggested that molecular collisions with Bennett dip or peak transfer contribute to the formation of satellites of the FEF multiplet. Multiphoton processes participate in the formation of one of the multiplets of the FEF multiplet. It is suggested that the power of the light field partially lifts the ban on transitions participating in the formation of satellites of both studied multiplets. Processing of the experimental curves by the leastsquares method revealed spectrally unresolved satellites within the FEF multiplet, which represent crossover resonances between allowed and forbidden transitions. For these satellites, no additional dependence of the amplitude on the laser power or gas pressure was found. The dependence of the amplitude of these satellites is completely determined by different for the main resonances and satellites magnitudes of self-induced transparency of the medium experienced by each light wave.

Compact stabilized semiconductor laser for frequency metrology.

We report on the development of a frequency modulatable 795 nm semiconductor laser based on self-injection locking to a high-quality-factor whispering-gallery-mode microresonator. The laser is characterized by residual amplitude modulation below -80 dB and frequency noise better than 300 Hz/Hz(1/2) at offset frequencies ranging from 100 Hz to 10 MHz. The frequency modulation speed and span of the laser exceed 1 MHz and 4 GHz, respectively. Locking of the laser to the Doppler-free saturated absorption resonance of the (87)Rb D1 line is demonstrated and relative frequency stability better than 10(-12) is measured for integration time spanning from 1 s to 1 day. The architecture demonstrated in this study is suitable for the realization of frequency modulatable lasers at any wavelength.

Contrast Saturation Resonances in the Absorption Band of Rubidium Molecules

We study Doppler-free saturation resonances in the absorption band \( {{\mathrm{B}}^1}{\Pi_{\mathrm{u}}}-{{\mathrm{X}}^1}\Sigma_{{^{\mathrm{g}}}}^{+} \) of rubidium diatomic molecules in the frequency range near the D2 line of lithium atoms (671 nm). We observe contrast saturation resonances and record a variation in the laser light transmission of 4% due to optical saturation. The large optical nonlinearities in the molecular diatomic gas can be used for investigating the four-wave mixing and other nonlinear effects.

Tunable two-mode Cr2+ : ZnSe laser with a frequency-noise spectral density of 0.03 Hz Hz-1/2

An optically pumped cw laser on a Cr2+ : ZnSe crystal with a tunable (in the range of 2.3 — 2 .6 mm) wavelength, operating with generation of two axial modes, has been developed. It is shown that the minimum laser frequency-noise spectral density does not exceed 0.03 Hz Hz-1/2. Application of this laser in problems of Doppler and Doppler-free spectroscopy makes it possible to detect spectral absorption lines of gases with sensitivities of 5 × 10-12 and2 × 10-10 cm-1, respectively (averaging time τ = 1 s). Having stabilised this laser with respect to the Doppler-free resonances of saturated dispersion of methane molecule, one can obtain a short-term frequency stability of 10-15 – 10-16 (τ = 1 s).

Efficient reflection via four-wave mixing in a Doppler-free electromagnetically-induced-transparency gas system

We experimentally demonstrate the high-efficiency reflection of a probe field in \ensuremath{\Lambda}-type three-level atoms of cesium vapor driven by two counterpropagating coupling fields. More than 60$%$ of reflection efficiency is observed at the phase-matching angle. The underlying mechanism theoretically is investigated as the four-wave mixing is enhanced by the electromagnetically-induced transparency. Both of the two Doppler-free two-photon resonances (one for the probe and co-propagating fields, the other for the reflected and the counterpropagation fields) play an important role in satisfying the phase matching in the reflection direction. The phase compensation due to the anomalous dispersion and the decrease of effective absorption length in the atomic system allow the efficient reflection to be observed in a wide range of incident angles of the probe field and detunings of the coupling field.

Doppler-free two-photon resonances for atom detection and sum frequency stabilization

We investigate the excitation of the 5D5/2 level in Rb atoms using counter-propagating laser beams, which are nearly resonant to the one-photon 5S1/2 → 5P3/2 and 5P3/2 → 5D5/2 transitions, ensuring that a sum of the optical frequencies corresponds to the 5S1/2 → 5D5/2 transition. The excitation produced by two-photon and stepwise processes is detected via spontaneously emitted fluorescence at 420 nm arising from the 6P3/2 → 5S1/2 transition. The dependences of blue fluorescence intensity on atomic density and laser detuning from the intermediate 5P3/2 level have been investigated. The sensitivity of the frequency-detuned bi-chromatic scheme for atom detection has been estimated. A novel method for sum-frequency stabilization of two free-running lasers has been suggested and implemented using two-photon Doppler-free fluorescence and polarization resonances.

Narrow absorptive resonances in a four-level atomic system

We study the effect of a control beam on a Λ electromagnetically induced transparency (EIT) system in 87Rb. The control beam couples one ground state to another excited state forming a four-level -system. Phase coherent laser beams to drive the -system are produced using a double injection locking scheme. We show that the control beam can be used to Stark shift or split the EIT resonance. Finally, we show that, when the control beam is on resonance, one observes a Doppler-free and sub-natural absorptive resonance with a width of order 100 kHz. Crucially, this narrow absorptive resonance only occurs when atoms with a range of velocities are present, as is the case in a room-temperature vapour.

A compact laser head with high-frequency stability for Rb atomic clocks and optical instrumentation

We present a compact and frequency-stabilized laser head based on an extended-cavity diode laser. The laser head occupies a volume of 200cm3 and includes frequency stabilization to Doppler-free saturated absorption resonances on the hyperfine components of the Rb87D2 lines at 780 nm, obtained from a simple and compact spectroscopic setup using a 2cm3 vapor cell. The measured frequency stability is ⩽2×10−12 over integration times from 1 s to 1 day and shows the potential to reach 2×10−13 over 102−105 s. Compact laser sources with these performances are of great interest for applications in gas-cell atomic frequency standards, atomic magnetometers, interferometers and other instruments requiring stable and narrow-band optical sources.

Observation of a three-photon electromagnetically induced transparency in hot atomic vapor

We demonstrate experimentally the effect of a three-photon electromagnetically induced transparency in hot ${}^{85}\mathrm{Rb}$ atomic vapor driven by two coherent electromagnetic fields. The narrow transparency window is observed on the background of high-contrast Doppler-free subnatural absorption resonance. Our analytical calculations and numerical simulations are in good agreement with the experimental data. Both the transparency and the absorption resonances can find applications in high-precision metrology.

Unexpected Doppler-free resonance in generalized double dark states

Doppler-free resonances have been observed in Rb atomic vapor coherently driven by two strong-coupling fields in an intrinsically non-Doppler-free geometry. A four-level theoretical model explains the experimental results. The explanation of the physics is based on the interplay between coherences generated in a four-level system.

Self-aligned extended-cavity diode laser stabilized by the Zeeman effect on the cesium D_2 line

An extended-cavity diode laser at 852 nm has been built especially for the purpose of cooling and probing cesium atoms. It is a compact, self-aligned, and continuously tunable laser source having a 100-kHz linewidth and 60-mW output power. The electronic control of the laser frequency by the piezodriven external reflector covers a 4.5-kHz bandwidth, allowing full compensation of acoustic frequency noise without any adverse effect on the laser intensity noise. We locked this laser to Doppler-free resonances on the cesium D(2) line by using the Zeeman modulation technique, resulting in the frequency and the intensity of the laser beam being unmodulated. We also tuned the locked laser frequency over a span of 120 MHz by using the dc Zeeman effect to shift the F = 4-F' = 5 reference transition.

Four-wave mixing under conditions of Doppler-free resonance induced by strong radiation

The possibilities of enhancing the nonlinear optical response of a gaseous medium and the radiation conversion efficiency in four-wave mixing processes through the use of atomic coherence effects and the induced elimination of the Doppler broadening of resonances are investigated. Numerical illustrations of the effects are given for experiments in progress.

Doppler-free optical-optical double resonance polarization spectroscopy of the Cs2 (2)3Π+u, (2)3Π1u, C 1Πu, and 1u[(2)3Σu++(2)3Πu] states

High resolution spectra of the (2)3Πu[Ω=0+ and 1](v,J)←X 1Σg+(v″,J″) transitions of the Cs2 molecule in the 15 400−16 000 cm−1 region are measured by the technique of Doppler-free optical-optical double resonance polarization spectroscopy (OODRPS). Molecular constants of the (2)3Π0+u(v=0−18) and (2)3Π1u(v=0−13) levels are determined, and potential energy curves are constructed by the RKR method. The RKR potentials are found to be in very good agreement with the ab initio potentials computed by Spies. OODRPS spectra of the C 1Πu(v′=19−21,J′)←X 1Σg+ transitions are measured in order to expand the data on the C 1Πu state. We reconsider the perturbation of the C 1Πu(v′⩾13,J′) levels, and the perturbing state is identified as the 1u[(2)3Σu++(2)3Πu] state. The molecular constants of the perturbing state are determined to be Te=16 332.3 cm−1, ωe=21.9 cm−1, and Be=0.007 94 cm−1, and the potential is constructed by the RKR method. Energy shifts of both e and f levels of the C 1Πu state centered at the same J and the hyperfine splittings are identified as the results of spin-orbit coupling between the C 1Πu and 1u[(2)3Σu++(2)3Πu] states. Another type of energy shifts observed in only f level of the C 1Πu state is identified as the results of the L-uncoupling interaction between the (2)3Σu+[0u−(c)] state and the C 1Πu state which is mixed with the (2)3Πu[1u(a)] state by spin-orbit coupling. Line broadenings observed in the C 1Πu(v′⩾16) levels are identified as originating from the predissociation, which arises from interactions with the repulsive part of the 1u(a) state which has the potential hill with a peak of 16 420 cm−1 at about 6.6 Å.