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Overview

Since femtosecond laser thechnology is devepled, ultrafast science is one of the biggest issues in the modern science. In our lab, we research Ultrafast optical control including coherent quantum system control, control of THz wave for spectroscopy and imaging and femtosecond time resoloved control & measurment.

In coherent quantum control, we make better ultrafast pulse with pulse shaping technics to control atomic quantum system. we already have deveoped controlling 2 level system, 3 level ladder system, 3 level V-shaped system and so on. Controlling in more complicated quantum system or coherent control with terahertz wave are our future works.

Control of Terahertz wave is our big concern because of its unique property compared to other frequency region. Spetically, we are developing Terahertz controlling systems in THz-TDS(Terahertz time domain spectroscopy). In addiation, we are preparing to apply these controlling technics to our other researching area.

we also study ultrafast spectroscopy of material. To do this, we have developed femtosecond time resoloved control & measurment system. Currently we are interested in mutiferroic material such as LuMnO3.

Quantum Control

A shaped ultrafast pulse makes it possible to stir a quantum system and thus control a quantum process via light-matter interaction. In our setup, acousto-optic programmable dispersive fillter(Dazzler) is used to shape pulse.

Results

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.

27 April 2009 / Vol. 17, No. 9 / OPTICS EXPRESS 7648

Strong-Field two-photon transition by phase shaping
We demonstrate the ultrafast coherent control of a non-linear two-photon absorption in a dy- namically shifted energy level structure. We use a spectro-temporal laser pulse shaping that is programmed to preserve the resonant absorption condition during the intense laser field interaction. Experiments carried out in the strong-field regime of two-photon absorption in the ground state of atomic Cesium reveal that the analytically obtained o®set and curvature of a laser spectrum compensate the effect of both static and dynamic energy shifts of the given light-atom interaction.

PRA accepted Tuesday Jul 20, 2010

Coherent Control in 2D-FTES
We demonstrate the advantage of applying coherent control technique to 2D-FTES spectroscopy. By shaping individual pulses used in 2D-FTES on atomic model system, we selectively turn on and off specific couplings. This advanced 2D-FTES technique may be useful for probing time-dependent coupling paths among multilevel electronic energy states in complex systems

Control of THz wave

The significance in the electromagnetic waves in the recently available frequency range of 0.1 ~ 10 THz, or terahertz (THz) wave has been progressively increasing. For example, their applications in communications, material characterizations, biological and medical imaging, and precision spectroscopy of molecules are expected. Spetially, THz time domain spectroscopy (THz-TDS) has become generally adopted for the measurement of optical properties of materials in the THz frequency range. Our concern of THz-TDS is developing active control devices for imaging and spectroscopy.

Results

THz coherent optical computer
Single point terahertz imagery of 2D objects is demonstrated by exploiting the broadband nature of ultrafast terahertz wave in a coherent optical computing setup. In the devised imagery, a collimated terahertz beam is illuminated on an object and the scattered fields are measured through a spatial mask at the Fourier plane in a 4-f terahertz time-domain spectroscope. This arrangement allows conversion of radial spatial frequencies of the object to the temporal spectrum of the pulse. Hence, a 2D image stored in the terahertz waveforms can be readily obtained.

Optics Letters, Vol. 35, Issue 4, pp. 508-510 (2010)

single-pixel coherent diffraction imaging
We demonstrate single-pixel coherent diffraction imaging, whereby broadband terahertz (THz) waveforms passed through a slated phase retarder (SPR), diffracted from an object, were measured by a THz detector located in the far field. For 1D imaging, the fixed-location single-pixel broadband detector simultaneously measured all the spatial frequency components of the object because the frequency components of the source maintain a one-to-one correspondence with the object's spatial frequency. For 2D imaging, the angular position of the SPR enabled the diffracted THz wave to carry an angular projection image of the object. Thirty waveforms measured at different SPR orientations successfully reconstructed complex 2D images.

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 timedomain spectroscopy and near-field terahertz microscopy. We obtained a bandwidth of 2 THz and 180-mm 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.

21 June 2010 / Vol. 18, No. 13 / OPTICS EXPRESS 13693

Ultrafast control & measurment

Time-resloved pump-probe experiment is a useful tool in studying the dynamics related to electrons, phonons, and spin. The basic analogy of IR pump-probe method is simple that a powerful light pulse, usulally labeled the 'pump pulse' of 'excitation pulse', interacts with the sample and excites it into a non-equilibrium state. The sample thereafter relaxes towards a new equilibrium state. This process can be mapped by sending a second much weaker pulse, 'probe pulse', onto a sample. The 'probe pulse' detects a change of optical properties without disturbing the object under investigation.

Our research of ultrafast phenomena is investigations non-equilibrium states of electron, phonon, and spins in correlated electron materials, especillay we are interested in rare-earth manganites.

Results

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).

New Journal of Physics 12 (2010) 023017

Ultrafast near-infrared spectroscopic study of coherent phonons in the phase-separated manganite La1/4Pr3/8Ca3/8MnO3
We report the generation of coherent optical and acoustic phonons in mixed valent manganite La1/4Pr3/8Ca3/8MnO3 using femtosecond infrared pump-probe spectroscopy. Temperature-dependent measurements of the time-resolved optical reflectance, obtained over a range of 5–300 K, revealed that the energy of the photoexcited electrons dissipated during relaxation to acoustic phonons, in the high-temperature paramagnetic phase, and to optical phonons, in the low-temperature charge-ordering phase. Analysis of the temperaturedependent behavior reveals that the modal amplitudes of the coherent phonons appear strongly correlated with the charge-ordering phase.

Phys. Rev. B 81, 214416 (2010)

Strong spin-lattice coupling in multiferroic hexagonal manganite YMnO3 probed by ultrafast optical spectroscopy
We report the observation of spin-lattice coupling in multiferroic YMnO3 by femtosecond near-infrared pump and probe spectroscopy. A coherent 31~GHz acoustic phonon was detected above the magnetic ordering temperature, and a higher frequency coherent mode was observed in the anti-ferromagnetic phase. This temperature-dependent measurement demonstrates that the acoustic phonon excitation is coupled to spin ordering.

Appl. Phys. Lett. 97, 031914 (2010)