Two-photon Raman Resonance Research Articles

Evaluating the single-shot photodegradation of optically-transparent bulk polymers under two-photon Raman-resonant laser irradiation in nanosecond regime (multiplex CARS test)

Laser-induced photodegradation of bulk optically-transparent polymers was probed under laser irradiation at two-photon Raman resonance. The photodegradation effect was inspected for organic polymers poly-(methyl methacrylate) and polystyrene by means of single-shot multiplex coherent anti-Stokes Raman scattering (CARS) spectroscopy performed with nanosecond-pulsed laser sources. Under such conditions of photoirradiation in the visible spectral range, the laser pulse intensity threshold for photodegradation of these polymers was determined.

Ramsey spectroscopy by direct use of resonant light on isotope atoms for single-photon detuning

We demonstrate Ramsey spectroscopy with cold 87Rb atoms via a two-photon Raman process. One laser beam has a cross-over resonant frequency on the 85Rb transition and the other beam has a 6.8 GHz shifted frequency. These two laser beams fulfill the two-photon Raman resonance condition, which involves a single-photon detuning of −2.6 GHz. By implementing these two lasers on cold 87Rb atoms, we demonstrate Ramsey spectroscopy with an interrogation time of the intermediate state by using π/2 Raman pulses. In our laser system, we can change the single-photon detuning to 1.2, 4.2 or −5.6 GHz by changing the 85Rb transition line used as a locking signal and an injected sideband. The laser system that directly uses resonant light on isotope atoms will be described in this paper.

Spectral narrowing of coherent population trapping resonance in laser-cooled and room-temperature atomic gas

We have investigated coherent population trapping (CPT) in laser-cooled as well as room-temperature (with and without buffer gas) rubidium atoms. The characteristic broad signal profile emerging from the two-photon Raman resonance for room-temperature atomic vapour is consistent with the theoretical calculation incorporating associated thermal averaging. The spectral width of the dark resonance obtained with cold atoms is found to be broadened, compared to room-temperature vapour cell, due to the feeble role played by thermal averaging, although the cold atomic sample significantly overcomes the limitation of the transit time broadening. An alternative way to improve transit time is to use a buffer gas, with which we demonstrate that the coherent population trapping signal width is reduced to < 540 Hz.

Polarization rotation under two-photon Raman resonance for magnetometry

The polarization rotation and coherent population trapping signal arising due to two photon process using linearly polarized light are found to be significantly enhanced for a Zeeman degenerate system. The zero crossing of the dispersive profile is found to be shifting proportional to the applied magnetic field, albeit the absorptive profile position remains invariant for a slightly imbalanced orthogonal circular polarization component. It provides an alternative method for precise measurement of vector magnetic field without requirement of a bias field. The use of polarization rotation signal for magnetic field measurement offers added advantage due to improved signal to noise ratio.

Broadband cavity electromagnetically induced transparency

Cavity electromagnetically induced transparency (EIT) is created in a three-level atomic system confined in a cavity and coupled to a free-space control laser and is manifested as a narrow transmission peak of a probe laser coupled into the cavity mode and tuned to the two-photon Raman resonance with the control laser. Cavity EIT can be observed with a control laser detuned from the atomic transition frequency in a range limited by the vacuum Rabi splitting of two cavity-atom normal modes. This leads to the broadband cavity EIT obtained in the coupled-cavity-atom system with a free-space, broadband control laser. We report an experimental observation of broadband cavity EIT in cold Rb atoms with a frequency-modulated control laser and discuss its application in multichannel and multifrequency light memory.

GIANT THIRD-ORDER KERR NONLINEARITIES AND SLOW OPTICAL SOLITONS IN DOUBLE QUANTUM-WELL

We show the formation of slow optical solitons in the asymmetric coupled double quantum wells (CQW) via a two-photon Raman resonance. With the consideration of real parameters in AlGaAs -based CQW, we indicate the possibility to have cancelation of the linear absorption, giant Kerr nonlinearities, and slow group velocity propagation of the weak probe pulse at the same one-photon detuning frequency around several THz .

Realization of universal quantum cloning with superconducting quantum-interference device qubits in a cavity

We propose a scheme to realize $1\to 2$ universal quantum cloning machine (UQCM) with superconducting quantum interference device (SQUID) qubits, embeded in a high-Q cavity. CNOT operations are derived to present our scheme, and the two-photon Raman resonance processes are used to increase the operation rate. Compared with previous works, our scheme has advantages in the experimental realization and further utilization.

Optical modulation transfer by molecular modulation

Modulation features in the frequency domain can be transferred from an amplitude-modulated laser field to a nonmodulated one by a strong Raman coupling, which is based on refractive index control with a strong Raman coherence established with two sufficiently intense laser fields slightly off the two-photon Raman resonance of a selected Raman mode. Numerical simulations show that modulation transfers between the driving lasers, such as transfers from ultraviolet to extreme ultraviolet and from infrared to middle infrared lasers, can be realized in gaseous Raman media. This provides an efficient way to extend the optical modulation range beyond the transparent ranges of commercial modulators. Modulation features can also be transferred from a low-intensity driving laser to a high-intensity one, which ensures a direct application to modulate lasers with high intensities beyond the damage thresholds of standard modulators.

Laser linewidth effects in quantum state discrimination by electromagnetically induced transparency

We discuss the use of electromagnetically modified absorption to achieve selective excitation in atoms: that is, the laser excitation of one transition while avoiding simultaneously exciting another transition whose frequency is the same as or close to that of the first. The selectivity which can be achieved in the presence of coherent population trapping (CPT) is limited by the decoherence rate of the dark state. We present exact analytical expressions for this effect, and also physical models and approximate expressions which give useful insights into the phenomena. When the laser frequencies are near-resonant with the single-photon atomic transitions, CPT is essential for achieving discrimination. When the laser frequencies are far detuned, the ``bright'' two-photon Raman resonance is important for achieving selective excitation, while the ``dark'' resonance (CPT) need not be. The application to laser cooling of a trapped atom is also discussed.

Experimental implementation of a four-level N-type scheme for the observation of electromagnetically induced transparency

A nondegenerate four-level N-type scheme was experimentally implemented to observe electromagnetically induced transparency (EIT) at the 87Rb D 2 line. Radiations of two independent external-cavity semiconductor lasers were used in the experiment, the current of one of them being modulated at a frequency equal to the hyperfine-splitting frequency of the excited 5P 3/2 level. In this case, apart from the main EIT dip corresponding to the two-photon Raman resonance in a three-level L-scheme, additional dips detuned from the main dip by a frequency equal to the frequency of the HF generator were observed in the absorption spectrum. These dips were due to an increase in the medium transparency at frequencies corresponding to the three-photon Raman resonances in four-level N-type schemes. The resonance shapes are analyzed as functions of generator frequency and magnetic field.

Nonadiabatic interaction effects on population transfer inH2by stimulated Raman transition with partially overlapping laser pulses

We have theoretically investigated the population transfer in a four-level ${\mathrm{H}}_{2}$ system by stimulated Raman transition from the ground $X{}^{1}{\ensuremath{\Sigma}}_{g}^{+}({\ensuremath{\nu}}_{g}{=0,J}_{g}=0)$ level to higher rovibrational levels $({\ensuremath{\nu}}_{f}{,J}_{f})$ of the $X{}^{1}{\ensuremath{\Sigma}}_{g}^{+}$ state via the excited intermediate $B{}^{1}{\ensuremath{\Sigma}}_{u}^{+}({\ensuremath{\nu}}_{i}{=14,J}_{i}=1)$ and $C{}^{1}{\ensuremath{\Pi}}_{u}^{+}({\ensuremath{\nu}}_{i}{=3,J}_{i}=1)$ levels coupled with each other by nonadiabatic interaction, using time-dependent overlapping pump and Stokes laser fields. The density-matrix treatment, which permits the convenient inclusion of the spontaneous emissions from the intermediate levels, has been employed to describe the dynamics of the two-photon Raman resonance process. The present study performs the calculations of final populations (after both the pulses are over) of the ground and terminal levels for Q-branch ${(J}_{f}=0)$ fundamental $({\ensuremath{\nu}}_{f}=1)$ and first overtone $({\ensuremath{\nu}}_{f}=2)$ transitions and the S-branch ${(J}_{f}=2)$ fundamental $({\ensuremath{\nu}}_{f}=1)$ transition as a function of time delay between the two pulses for the cases of on-resonance as well as off-resonance excitations in a wide range $(2\ifmmode\times\else\texttimes\fi{}{10}^{5}--2\ifmmode\times\else\texttimes\fi{}{10}^{7}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2})$ of peak intensities ${I}_{P}^{0}$ ${(I}_{S}^{0})$ of the pump (Stokes) fields. Both fields are assumed to have the same temporal shape, duration, peak intensities, and linear parallel polarizations. The accurate values of spontaneous radiative relaxation rates of the intermediate levels to the initial and final levels, taking into account their J and M dependence, are explicitly included in our calculations. The pulse width (full width at half maximum) ${\ensuremath{\tau}}_{p}$ is taken as 170 ns so that total spontaneous decay can occur during the pulse duration. The transfer efficiency is found to be very sensitive to the peak intensities of the laser pulses in each case of transition considered. Special attention is paid to the effects of the nonadiabatic (NA) interaction between $B(14,1)$ and $C(3,1)$ levels on population transfer efficiency. Calculations are also done in some particular cases using the adiabatic Born-Oppenheimer (ABO) approximation. The results with ABO approximation are found to differ remarkably from those obtained including NA interaction. Our calculations for the four-level ${\mathrm{H}}_{2}$ system reveal that almost complete population is transferred in counterintuitive pulse order for both on-resonance and off-resonance excitations with intermediate and high values of ${I}_{P}^{0}$ ${(I}_{S}^{0}).$ For intuitive pulse sequence also a large population transfer is achieved for on-resonance excitation at intermediate values of ${I}_{P}^{0}$ ${(I}_{S}^{0})$ and for off-resonance excitation at intermediate and high values of ${I}_{P}^{0}$ ${(I}_{S}^{0}).$

Population transfer through the continuum using laser-controlled Stark shifts

We show, through analysis and numerical modeling of hydrogen and sodium, that it should be possible to transfer atomic population, with high probability, using a delayed-pulse two-photon Raman transition in which intermediate states are in the ionization continuum. We propose using auxiliary laser pulses to overcome dynamic Stark shifts that would otherwise prevent population transfer. We find that, even without such auxiliary pulses, population transfer can be improved by tuning the carrier frequencies away from the two-photon Raman resonance condition. Our computations for hydrogen predict nearly 85 percent population transfer.