Two-photon Resonance Frequency Research Articles

Enhancement of the two-photon blockade effect via Van der Waals interaction

We theoretically investigate the influence of the Van der Waals interaction on the two-photon blockade phenomenon with the corresponding photon correlation functions g(2)(0) > 1 and g(3)(0) < 1 in a two-atom cavity-QED system, where two three-level ladder-type atoms are coherently driven by a pumping field and a coupling field simultaneously. Choosing a specific frequency of the coupling field, we show the energy splitting phenomenon caused by electromagnetically induced transparency. Correspondingly, the two-photon blockade phenomenon can be achieved near the two-photon resonant frequency. Using the Van der Waals interaction between the Rydberg states of the two atoms, we also show that the two-photon blockade can be improved when two atoms radiate in-phase or out-of-phase. As a result, two photons leak from the cavity simultaneously, but the third photon is blockaded. These results presented in this study hold potential applications in manipulating photon states and generating nonclassical light sources.

Observation of a highly superluminal laser employing optically pumped Raman gain and depletion.

In this paper, we report a Raman laser which is extremely sensitive to a variation of the cavity length, using a scheme employing two stable isotopes of Rb. One isotope is used for producing a broad gain spectrum via the optically pumped Raman gain process, while the other is used for producing a narrow dip via the optically pumped Raman depletion process. By tuning the frequencies of the two Raman pumps, the center frequencies of the gain and dip can be aligned to the same frequency. This approach allows tuning of the gain and dip parameters independently over a broad range of operating conditions. With such a configuration, we can produce a negative dispersion around the two-photon resonance frequency in the vapor cell, which leads to a group index that is close to zero. By theoretically matching the experimental observations, we can infer that the sensitivity of such laser is enhanced by a factor of more than 2800, which is nearly a factor of three larger than the highest value reported previously using a different approach.

Direct frequency-comb spectroscopy of 6s2S1/2–8s2S1/2 transitions of atomic cesium

Direct frequency-comb spectroscopy is used to probe the absolute frequencies of the – two-photon transitions of atomic cesium in a hot vapor environment. By utilizing the coherent control method of temporally splitting the laser spectrum above and below the two-photon resonance frequency, Doppler-free absorption is built in two spatially distinct locations and imaged for high-precision spectroscopy. Theoretical analysis finds that these transition lines are measured with uncertainty below 5 × 10−10, mainly contributed from laser-induced AC Stark shift.

Generation and application of two-photon Raman laser for manipulation of internal state of Cs atom

Two-photon Raman process (TPRP) is an important technique in controlling the atomic internal states. It plays an important role in quantum manipulation and quantum information process. A reliable Raman laser for specific atom is the first step to demonstrate TPRP and quantum manipulation of an atom. In this paper, we theoretically analyze the two-photon Raman process regarding to Cesium “clock states” |6S1/2, F=4, mF=0 and |6S1/2, F=3, mF=0, and we obtain the dependences of the corresponding Rabi frequency on one-and two-photon detunings and one-photon Rabi frequencies in a realistic multi-level Cesium atom system. We find that to obtain an atom state flopping efficiency of 0.99 the Raman laser power fluctuation should be controlled to be smaller than 3.2%. We also report our simple experimental Raman laser system for TPRP of Cesium atom based on a fiber waveguide phase modulator. The phase modulator is driven by a 4.6 GHz microwave source and the two first-order sidebands with a frequency difference of 9.19 GHz are filtered out by a Fabry-Pérot cavity with a finesse of 48. After an amplitude-modulator-based intensity stabilization system, a total power of 73 μupW with a fluctuation of 2.2% within 90 min is obtained. By applying this Raman laser to a single Cesium atom trapped in a micrometer size far-off resonant trap (FORT), we obtain Raman spectra between Cesium “clock states” |6S1/2, F=4, mF=0 and |6S1/2, F=3, mF=0. The discrepancy between the two-photon resonance frequency and the defined clock frequency 9.192631770 GHz is due to the differential Stark shifts by FORT beam and Raman beams as well as the inaccuracy of the microwave source. By varying the Raman pulse length we also show the corresponding Rabi flopping with a rate of 153 kHz, which is consistent with the theoretical calculation. The obtained state transfer efficiency of 0.75 is much smaller than theoretical expectation 0.99, which is mainly limited by the state initialization efficiency. The Raman laser system reported in this paper is simple and reliable to realize and it provides a reliable method to manipulate the Cesium internal state. Moreover it could also be easily extended to other system for quantum manipulation of other species of atom.

Modulation on Optical Limiting and Two-Photon Absorption Behavior of a Molecular Medium by Two-Color Ultrashort Laser Pulses

The dynamical behavior of two-color ultrashort laser pulse propagating in a molecular medium 4,4'-bis(di-n -butylamino)stilbene (BDBAS) has been studied by employing an iterative predictor-corrector finite-difference time domain (FDTD) tech- nique to solve the Maxwell-Bloch equations. The results indicate that the spectrum of electric field shows oscillation feature around the two-photon resonant frequency when the delay time exists due to the interaction between two sub-pulses. The opti- cal limiting window turns narrower and the saturation value of output intensity gets larger with the presence of delay time compared with no delay time case. The dynam- ical TPA cross section is smaller with the increase of delay time. The studies suggest a method to modulate the nonlinear optical properties of the medium by controlling the delay time of the two-color pulse. PACS: 33.80.-b, 82.50.Pt, 31.15.A-

Frequency Matching in Light-Storage Spectroscopy of Atomic Raman Transitions

We investigate the storage of light in an atomic sample with a Lambda-type coupling scheme driven by optical fields at variable two-photon detuning. In the presence of electromagnetically induced transparency (EIT), light is stored and retrieved from the sample by dynamically varying the group velocity. It is found that for any two-photon detuning of the input light pulse within the EIT transparency window, the carrier frequency of the retrieved light pulse matches the two-photon resonance frequency with the atomic ground state transition and the control field. This effect which is not based on spectral filtering is investigated both theoretically and experimentally. It can be used for high-speed precision measurements of the two-photon resonance as employed, e.g., in optical magnetometry.

Detuned slow light in the Doppler broadened multi-level D2 line of Rubidium

We observed a detuned slow light phenomenon based on electromagnetically induced transparency in (87)Rb D2 line composed of multiple excited-hyperfine states within a Doppler-broadened linewidth. The results show that the maximum group delay of a probe occurs at off-detuned two-photon resonance frequency. The observed detuned group delay is analyzed with numerical calculations for a probe pulse interacting with the neighboring excited-states-modified Doppler broadening atoms for a fixed coupling field. The experimental results are in good agreement with the numerical calculations.

Coherent control calculations for helium atom in short and intensive XUV laser pulses

Coherent control calculations are presented for helium. With the help of a genetic algorithm (GA) phase-modulated extreme ultra violet (XUV) laser pulses were controlled to maximize or minimize the non-resonant two-photon 1s1s → 1s3s excitation. Linearly polarized laser pulses were chosen at the frequency of 0.4 a.u. with a duration of about 15 fs and 1014 Wcm-2 peak intensity. We verified the theory that an antisymmetric spectral phase around the two-photon resonance frequency maximizes the transition probability. Dark pulses which do not excite the system at all have however symmetric spectral phases around the two-photon resonance frequency.

Femtosecond-pulse two-photon resonant difference-frequency mixing in gases: A technique for tunable vacuum-ultraviolet femtosecond-pulse generation and a spectroscopic tool for studying atoms in strong laser fields

Two-photon resonant and near-resonant four-wave difference-frequency mixing in gases in the interaction regime when laser pulse durations are comparable to or shorter than the medium polarization relaxation time ${T}_{2}^{\ensuremath{'}}$ is investigated. The results of experimental studies of the process in Ar and Kr using pump pulses from the ArF-excimer laser are presented demonstrating a generation of tunable short pulse radiation in the range 102--124 nm. The results are discussed in terms of a theoretical model based on a self-consistent solution of the Bloch equations for the atomic transitions and the Maxwell equations for the fields. This enables one to interpret specific nonstationary resonant and quasiresonant phenomena involved in the frequency conversion process. It is shown that the femtosecond-pulse four-wave frequency-mixing technique with probe pulses significantly shorter than the pump pulses makes it possible to study the coherent dynamics of an atomic transition exposed to an intense field. Using atomic Kr as the nonlinear medium, coherent Rabi oscillations and the subsequent phase relaxation of excitation were observed under the condition of two-photon interaction of Kr with femtosecond 193-nm laser pulses. The obtained information is important for controlling and optimizing processes of two-photon resonant frequency conversion and for time-resolved studies of Rydberg states in atoms and molecules.

An optical ring resonator designed for frequency tripling of cw dye laser radiation in metal vapors

This paper describes an optical ring resonator designed for frequency tripling of cw dye laser radiation in metal vapors. The five-mirror resonator contains a metal vapor cell and a prism which separates the laser radiation from the generated vacuum ultraviolet. A laser power of 800 mW (of 576 nm radiation) generated an intracavity power of 8.7 W. The two-photon resonant frequency tripling of this radiation produced in Sr vapor 1.9 × 10 10 photons s -1 (20.3 nW) of 192 nm VUV radiation. Compared to previous investigations of this third harmonic generation the intracavity tripling improves the efficiency by almost two orders of magnitude.

Generation of UV coherent radiation around 200 nm by two-photon resonant four-wave mixing in Mg vapor

Tuning the sum of a dye laser frequency ω L and its second harmonic ω DL to approach the two-photon resonance frequency of the 3s4d 1D or 3s5s 1S state of magnesium, strong uv coherent radiations at 202.6 nm, 199.2 nm and 211.4 nm have been generated effectively by four-wave mixing. Comparison was made between two-photon resonance with different frequencies, and with the same frequency. The factors limiting uv generation were studied.

Stimulated hyper Raman emission and super-fluorescent emission from sodium vapour

Stimulated hyper Raman scattering at 2.3 μm due to the 4d-4p transition of Na atoms and super-fluorescent emission in the wavelength range from 7680 Å due to the A 1Σ + u-X 1Σ + g transition of Na 2 molecules were observed when sodium vapour was optically pumped by a N 2-laser-pumped dye laser radiation with its frequency around the two-photon resonance of the 3s-4d transition of Na atoms. The former was as high as 64% in photon conversion efficiency and its frequency tunability was 26 cm -1. The pump frequency increase to maximal Raman output was approximately 17 276.6 cm -1, which was higher than the 3s-4d two-photon resonance frequency (17 274.4 cm -1). The latter was continuously observed over the pump wavelength range from 5740 Å to 5840 Å except the region in which Raman emission was produced. The pulse shape was composed of double peaks; each of them being 5 ns in FWHM and separated by 30 ns. Stimulated hyper Raman emission and super-fluorescent emission have shown different optimum temperatures of Na vapour.

Tunable VUV radiation generated by two-photon resonant frequency mixing in xenon

Efficient two-photon resonant frequency conversion of powerful pulsed dye laser radiation generates in Xe intense VUV light at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\omega_{VUV} = 2\omega_{UV} - \omega_{D}</tex> . The frequency ω <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">UV</inf> is tuned to the 5p-6p [ <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\frac{5}{2},2</tex> ] two-photon resonance ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\lambda_{UV} = 255.9</tex> nm). For dye laser radiation ω <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</inf> at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\lambda_{D} = 300-760</tex> nm ω <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">VUV</inf> is tunable in the spectral range of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\lambda_{VUV} = 155-220</tex> nm. The resonantly enhanced conversion efficiency is typically 0.05-0.2 percent; the generated VUV pulse power <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P_{VUV} = 0.5-1</tex> kW. The experimental results provide detailed information on conversion limiting saturation phenomena and their dependence on the input power and the detuning of ω <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">UV</inf> from the two-photon resonance.

Two-photon resonance fluorescence

The excitation spectrum of a two-level atom interacting with a strong electromagnetic field is considered when the atomic transition frequency is nearly twice the frequency of the laser field. The spectral function consists of four Lorentzian lines describing: the central line peaked near the two-photon resonance, two sidebands peaked at the high and low frequency regions respectively and the one-photon frequency mode. In the limit of high photon densities, the low frequency photon mode is induced by the pump field which removes the singularity occurring at the two-photon resonance frequency.