Jongseok Lim

Modeling sympathetic cooling of molecules by ultracold atoms.

We model sympathetic cooling of ground-state CaF molecules by ultracold Li and Rb atoms. The molecules are moving in a microwave trap, while the atoms are trapped magnetically. We calculate the differential elastic cross sections for CaF-Li and CaF-Rb collisions, using model Lennard-Jones potentials adjusted to give typical values for the s-wave scattering length. Together with trajectory calculations, these differential cross sections are used to simulate the cooling of the molecules, the heating of the atoms, and the loss of atoms from the trap. We show that a hard-sphere collision model based on an energy-dependent momentum transport cross section accurately predicts the molecule cooling rate but underestimates the rates of atom heating and loss. Our simulations suggest that Rb is a more effective coolant than Li for ground-state molecules, and that the cooling dynamics is less sensitive to the exact value of the s-wave scattering length when Rb is used. Using realistic experimental parameters, we find that molecules can be sympathetically cooled to 100μK in about 10 s. By applying evaporative cooling to the atoms, the cooling rate can be increased and the final temperature of the molecules can be reduced to 1 μK within 30 s.

Terahertz waves emitted from an optical fiber.

We report a simple method of creating terahertz waves by applying the photo-Dember effect in a (100)-oriented InAs film coated onto the 45-degree wedged-end facet of an optical fiber. The terahertz waves are generated by infrared pulses guided through the optical fiber which is nearly in contact with a sample and then measured by a conventional photo-conductive antenna detector. Using this alignment-free terahertz source, we performed proof-of-principle experiments of terahertz time-domain spectroscopy and near-field terahertz microscopy. We obtained a bandwidth of 2 THz and 180-microm spatial resolution. Using this method, the THz imaging resolution is expected to be reduced to the size of the optical fiber core. Applications of this device can be extended to sub-wavelength terahertz spectroscopic imaging, miniaturized terahertz system design, and remote sensing.

Ultrafast IR spectroscopic study of coherent phonons and dynamic spin–lattice coupling in multiferroic LuMnO3

The concurrent existence of ferroelectricity and magnetism within a single crystalline system characterizes the multiferroic materials discovered in recent years. To understand and develop the multiferroic phenomenon, we need to investigate the unusual coupling between spin and lattice degrees of freedom. Spins in multiferroics are expected to be elastically coupled to phonons. Therefore, the time-dependent study can be a crucial factor in understanding the coupled dynamics. Here, we report the observations of strong dynamic spin–lattice coupling in multiferroic LuMnO3. A coherent optical phonon of 3.6 THz and its temperature dependence is measured for the first time from our femtosecond IR pump and probe spectroscopy. Also, we observed a coherent acoustic phonon of 47 GHz similar to a previous report (Lim et al 2003 Appl. Phys. Lett. 83 4800). Temperature-dependent measurements show that both optical and acoustic phonons become significantly underdamped as temperature decreases to TN, and they disappear below TN. These observations reveal that phonons are coupled to spins by magneto-elastic coupling, and the disappearance of phonon modes at TN is consistent with the isostructural coupling scheme suggested by Lee et al (2008 Nature 451 805).

Ultrafast Ramsey interferometry to implement cold atomic qubit gates

Quantum computing is based on unitary operations in a two-level quantum system, a qubit, as the fundamental building block, and the ability to perform qubit operations in an amount of time that is considerably shorter than the coherence time is an essential requirement for quantum computation. Here, we present an experimental demonstration of arbitrary single-qubit SU(2) quantum gate operations achieved at a terahertz clock speed. Implemented by coherent control methods of tailored ultrafast laser interaction with cold rubidium atomic qubits, Bloch vector manipulation about all three rotational axes was successfully demonstrated. The dynamic evolution of the qubits was successfully measured by devised femtosecond Ramsey interferometry. We anticipate this demonstration to be a starting point to process quantum algorithm in a simplified manner by a programmed sequence of femtosecond laser pulses.

Ultrafast Rabi flopping in a three-level energy ladder

We demonstrate Rabi oscillation of a resonant two-photon transition in a three-level ladder-type quantum system of gaseous rubidium atoms induced by a single femtosecond laser pulse. For this, we shape the flat-top spatial profile of the laser pulse and perform the ultrafast population cycling of the atoms as a function of pulse energy. The experimental result confirms that the Rabi frequency of the transition from a ground state to a final state depends linearly on the pulse area, although the transition is a two-photon process.

Strong-field two-photon absorption in atomic cesium: an analytical control approach

We have considered an analytical control of two-photon absorption process of atoms in the strong-field interaction regime. The experiment was performed on gaseous cesium atoms strongly interacting with a shaped laser-pulse from a femtosecond laser amplifier and a programmable pulse-shaper. When this shaped laser-pulse transfers the atomic population from the 6s ground state to the 8s excited state, we have found that both positively- and negatively-chirped laser pulses, compared with a Gaussian pulse, enhance this excitation in the strong-field regime of laser-atom interaction. This unusual phenomena is explained because the temporal shape of the laser intensity compensates the effect of dynamic Stark shift for the two-photon resonant condition to be optimally maintained. We provide analytic calculations using the strong-field phase matching, which show good agreement with the experiment.

Strong-field quantum control of 2 + 1 photon absorption of atomic sodium

We demonstrate ultrafast coherent control of multiphoton absorption in a dynamically shifted energy level structure. In a three-level system that models optical interactions with sodium atoms, we control the quantum interference of sequential 2 + 1 photons and direct three-photon transitions. Dynamic change in energy levels predicts an enormous enhancement of |7p>-state excitation in the strong-field regime by a negatively chirped pulse. In addition, the |4s>-state excitation is enhanced symmetrically by nonzero linear chirp rates given as a function of laser peak intensity and laser detuning. Experiments performed by ultrafast shaped-pulse excitation of ground-state atomic sodium verifies the various strong-field contributions to |3s>-|7p> and |3s>-|4s> transitions. The result suggests that for systems of molecular level understanding adiabatic control approach with analytically shaped pulses becomes a more direct control than feedback-loop black-box approaches.

Young's experiment with a double slit of sub-wavelength dimensions.

We report that the interference pattern of Youngs double-slit experiment changes as a function of polarization in the sub-wavelength diffraction regime. Experiments carried out with terahertz time-domain spectroscopy reveal that diffracted waves from sub-wavelength-scale slits exhibit either positive or negative phase shift with respect to Gouy phase depending on the polarization. Theoretical explanation based on the induction of electric current and magnetic dipole in the vicinity of the slits shows an excellent agreement with the experimental results.

Optimal Pulse Shaping for Ultrafast Laser Interaction with Quantum Systems

Coherent control method steers a quantum system to a desirable final quantum state among a number of final states otherwise possible in a given light-matter interaction, by using a specially shaped light form programmed in its spectral and/or temporal domain. In this chapter, we briefly review a number of light-form shaping methods previously considered for coherent control of ultra-fast laser interaction with atoms, and provide their application examples along with their experimental demonstrations.

Coherent Control in 2D Fourier Transform Optical Spectroscopy

Using individually shaped three pulses in 2D Fourier transform optical spectroscopy, we retrieve and coherently control the amplitude and phase of the two-photon transition between 5P1/2 and 5P3/2 levels in atomic Rubidium.