We report on experimental preparation of the second-harmonic-wave laser and the single-mode squeezed vacuum state of 795 nm (rubidium atom D1 line) with periodically-poled KTiOPO4 (PPKTP) bulk crystals. By using a four-mirror bow-tie type ring doubling cavity we achieved ~111 mW of continuous-wave single-frequency ultra-violet (UV) laser radiation at 397.5 nm with ~191 mW of 795 nm fundamental-wave laser input. The corresponding doubling efficiency is 58.1%. To our knowledge, this is the highest doubling efficiency at 795 nm so far. Employing the 397.5 nm UV laser as a pump source of an optical parametric oscillator (OPO) with a PPKTP crystal, we achieved 5.6 dB of 795 nm single-mode squeezed vacuum output at analyzing frequency of 2 MHz. To our knowledge, this is the highest squeezing level of 795 nm single-mode squeezed vacuum so far. We analyzed the pump power dependence of the squeezing level, and concluded that UV laser induced losses of PPKTP crystal are main limiting factors for further improving the squeezing level. The generated 795 nm vacuum squeezing has huge potential applications in quantum memory and ultra-precision measurement with rubidium atoms.
The narrow electromagnetically-induced transparency (EIT) resonance peaks are observed with two low-power counter-propagating diode lasers in cesium (Cs) 6S1/2 - 6P1/2 - 8S1/2 ladder-type atomic system. To precisely determine the centers of resonance peaks, multiple background-free EIT signals are achieved using a novel scanning scheme in which the coupling laser driving Cs 6P1/2 - 8S1/2 transition is scanned and the probe laser driving Cs 6S1/2 – 6P1/2 is frequency locked. A temperature-stabilized fiber-pigtailed waveguide-type phase electro-optical modulator (EOM) and a stable confocal Fabry-Perot cavity are used as a precise frequency marker to measure the hyperfine splitting of Cs 8S1/2 state. The impact of the external magnetic field on the measurement is also investigated. Furthermore, the hyperfine structure constant (here it is the hyperfine magnetic dipole constant, A) of Cs 8S1/2 state is determined to be A = 219.06 MHz ± 0.12 MHz based on the measured hyperfine splitting (Δhfs = 876.24 MHz ± 0.50 MHz).
Optical dipole traps (ODT) with far-off-resonance laser are important tools in more and more present cold-atom
experiments, which allow confinement of laser-cooled atoms with a long storage time. Particularly, the magic
wavelength ODT can cancel the position-dependent spatially inhomogeneous light shift of desired atomic transition,
which is introduced by the ODT laser beam. Now it plays an important role in the state-insensitive quantum engineering
and the atomic optical clock. To verify the magic wavelength or the magic wavelength combination for D2 line transition
of cesium (Cs) and rubidium (Rb) atoms, we calculated and analyzed the light shift of the 133Cs 6S1/2 - 6P3/2 transition for
a monochromatic ODT, and also the 87Rb 5S1/2 - 5P3/2 transition for a dichromatic ODT with a laser frequency ratio of
2:1. Also a dichromatic magic-wavelength ODT laser system for 87Rb atoms is proposed and experimentally realized by
employing the quasi-phase-matched frequency doubling technique with telecom laser and fiber amplifier.
Two schemes of Doppler-free high-resolution velocity-selective optical-pumping atomic spectroscopy, named
single-resonance optical pumping (SROP) and double-resonance optical pumping (DROP), are performed and
characterized with room-temperature cesium vapor cells. Due to velocity-selective optical pumping from one hyperfine
fold of ground state to another via one-photon excitation in SROP or cascade two-photon excitation in DROP and decay
processes thereafter, the atomic population variation of one hyperfine fold of ground state is indicated by SROP and
DROP spectra by using of the transmission of the probe laser which is usually frequency locked to a cycling hyperfine
transition. As a result, SROP and DROP spectra often have flat background and higher signal-to-noise ratio. Therefore,
SROP and DROP spectra are very useful for measurement of the dressed-state splitting of ground state with an alkali
atomic vapor cell, precise measurement of hyperfine splitting of alkali atomic excited states, frequency references for
laser frequency stabilization, two-color MOT, and so on.
The spectra of cesium 6P3/2 - 8S1/2 excited-state transition have been obtained using double resonance optical-pumping
(DROP) technique in a room-temperature vapor cell, and have shown a much better signal-to-noise ratio (SNR)
compared with that using the traditional optical-optical double resonance (OODR) method. Furthermore, the line-width
of DROP spectra is obviously narrowed by electromagnetically-induced transparency (EIT) effect in cesium 6S1/2 F=4 -
6P3/2 F'=5 - 8S1/2 F''=4 transitions. Finally, such DROP spectrum of 6P3/2 F'=5 - 8S1/2 F''=4 transition with a high SNR
and a narrow line-width is applied into frequency stabilization of a 795 nm external-cavity diode laser, and the residual
frequency fluctuation is ~ 600 kHz within 500 s.
We demonstrate the spectra of 87Rb 5S1/2 - 5P3/2 - 4D3/2 transitions by utilizing the double-resonance optical-pumping
(DROP) and optical-optical double-resonance (OODR) techniques, respectively. The DROP spectrum, compared with
the traditional OODR spectrum, show a much better signal-to-noise ratio (SNR). Paying special attention to the influence
of alignment of lasers where the coupling and probe beams are counter-propagation and co-propagation on DROP
spectrum, so as to further narrow the spectral width by means of electromagnetically induced transparency (EIT). When
-the frequency of 1.5μm fiber-pigtailed butterfly-type distributed-feedback (DFB) diode laser is stabilized to the DROP
spectrum of 87Rb 5P3/2 - 4D3/2 transition, the preliminary result of residual frequency jitter after stabilization is ~ ±1.3
MHz within 60 s.
In our experiment, a polarization-maintaining (PM) fiber-pigtailed butterfly-sealed 1560nm distributed-feedback (DFB)
laser diode is amplified by a 5-Watt EDFA, then a multiple-period PPLN crystal (1mm×10mm×20mm) and a
single-period PPKTP crystal (1mm×2mm×30mm) are utilized to perform SHG via single pass configuration. The second
harmonic power of ~ 239 mW@780 nm for PPLN and ~ 210 mW@780 nm for PPKTP are obtained with ~5W@1560
nm laser input, corresponding to SHG efficiency of ~ 5.2% for PPLN and ~ 4.4% for PPKTP, respectively. Finally the
1560 nm laser diode's frequency is locked to rubidium absorption line via SHG and rubidium absorption spectroscopy,
the laser frequency drift for free-running case is ~ 56 MHz in 30 s, the residual frequency after being locked drift is ~ ±
3.5 MHz.
Single cesium atom prepared in a large-magnetic-gradient magneto-optical trap (MOT) has been efficiently loaded into a
microscopic far-off-resonance optical trap (FORT, or optical tweezer), and the atom can be transferred back and forth
between two traps with high efficiency. The intensity noise spectra of tweezer laser are measured and the heating
mechanisms in optical tweezer are analyzed. To prolong the lifetime of single atom trapped in optical tweezer, laser
cooling technique is utilized to decrease atom's kinetic energy, and the effective temperature of single atom in tweezer is
estimated by the release-and-recapture (R&R) method. Thanks to laser cooling, typical lifetime of ~ 130.6 ± 1.8 s for
single atom in tweezer is obtained. These works provides a good starting point for coherent manipulation of single atom.
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