-Wei Chen

Electrically controllable laser based on cholesteric liquid crystal with negative dielectric anisotropy

This work examines a planar cholesteric liquid crystal (CLC) cell with a negative dielectric anisotropy, doped with laser dye, as an electrically tunable one-dimensional photonic crystal laser device. The lasing wavelength is demonstrated to be tunable by applying a voltage. Additionally, lasing can be switched on and off changing the frequency of the applied voltage. Wavelength tuning caused by the shift of the reflection band of CLC is attributed to the electrohydrodynamical effect in the negative dielectric cell.

Mass transport following impulsive optical excitation

Transition from reverse-saturable absorption to saturable absorption of the chloroaluminum phthalocyanine solution excited by a giant laser pulse is ascribed not just to the saturation of excited state absorption, but also to the outward migration of the solute molecules at the laser beam center. While the saturation of excited state absorption occurs within a single picosecond laser pulse, the beam center population decrease is sustained much longer than the pulse duration. We distinguish these two mechanisms with the Z-scan technique, utilizing picosecond pulses with pulse-to-pulse separations ranging from 0.1 to 5.0 s.

Negative nonlinear refraction obtained with ultrashort laser pulses.

We present nonlinear refraction results for liquids methanol and acetic acid obtained with the Z-scan technique and 28 femtosecond (fs) 800 nm laser pulses. In contrast to the positive lensing effect obtained previously with picosecond and nanosecond laser pulses, a negative lensing effect is observed. The associated mechanism features the third-order polarization arising from the nonlinear response of the molecular skeletal motion that is driven into resonance through its electrostatic coupling to the valence electron cloud distorted by the fs laser field.

Solute migration caused by excited state absorptions

Using the Z-scan technique, we find that migration of chloroaluminum phthalocyanine in liquid ethanol can be induced by the absorption of a 19 ps laser pulse with energy exceeding a threshold but not by that of a 2.8 ns pulse depositing more energy at the solute molecules. Considering each solute molecule as an oscillator confined within a potential well, we explain, in accordance with the five-energy-band model, that solute molecules excited by a 19 ps pulse retain more translational excess energy to overcome the potential well barrier compared with those excited by a 2.8 ns pulse of equal energy. Therefore, they are more likely to migrate out of the laser beam center, weakening the solutions absorption that we detect in the Z-scan measurements. Furthermore, we theoretically infer that the 19 ps pulse-induced solute migration tends to be nonquasistatic and experimentally verify that it cannot be attributed to the Soret effect, a quasistatic process.

Short-pulse-induced solute migration in an organic solution at two temperatures

Using the Z-scan technique, we verify the threshold energy for a 19 ps laser to induce outward solute migration decreases with the increase of the solution temperature, well agreeing with our proposed potential well model.

Thermal lensing effect of methanol studied with femtosecond laser pulses

Using the Z-scan technique with an 800-nm 28 femtosecond laser running at 82-MHz, we verify thermal lensing effect accumulated across pulses in a transparent (both linear and nonlinear) liquid: methanol.

Study of Negative Lensing Effect in Transparent Molecular Liquids by Femtosecond Laser Pulses

A negative lensing effect is observed in transparent liquids C₂H₄Cl₂ and C₂H₄Br₂ using the Z-scan technique and 28 femtosecond titanium: sapphire laser pulses. Two possible mechanisms, Raman induced Kerr effect and thermal lensing effect, are invoked to account for the sign change of nonlinear refraction.

A controllable double-well magneto-optical trap for Rb and Cs atoms

We experimentally demonstrate a novel scheme to simultaneously confine two atomic species of (87)Rb and (133)Cs with adjustable spatial separation by a controllable double-well magneto-optic trap. Using a single-loop wire and a magnetic bias field, the two clouds, each containing more than 1 x 10(6) atoms, are spatially separated above and below the wire center of the double-well MOT. The cloud interdistance can be controlled by independently varying the wire current and external bias field. This allows to load the double-well magnetic trap, and to study the dynamics of cold collisions between two-species atoms.

Negative nonlinear refraction of simple molecular liquids studied with a femtosecond laser pulse

We observed a negative lensing effect in transparent molecular liquids (methanol and acetic acid) with 28 femtosecond laser pulses. In this paper, we propose two possible mechanisms to explain why the sign is opposite to that observed with picosecond and nanosecond laser pulses.

Momentum conservation in coherent transient optical Kerr effect

Using a femtosecond (fs) laser we have studied the momentum conservation of Raman-induced Kerr effect (RIKE) in CHBr/sub 3/ liquid, where two intramolecular vibrations are Raman-excited. It is learned that in order to observe a given vibration in RIKE the fs pump laser must be focused to converge at a sufficiently large angle.