PubMed Health⌕ Search

Biomedical subjects

Qihuang Gong

Publications and source records attributed to Qihuang Gong.

10 recordsLinked to original sources
PubMed2006

Encrypted holographic memory using an encoded reference wave.

A secure holographic memory system is proposed by use of an encoded reference beam. The reference beam is encrypted by a fiber-optic faceplate, which serves as a phase mask. There are seven keys in the system including the position and direction of the fiber bundle and the direction of the incident beam. The experiment shows that the total key length is larger than 1.8 x 1019. The method can be used directly in a shift-multiplexing system with high selectivity.

Journal Article↗
PubMed2006

All-optical tunable photonic bandgap microcavities with a femtosecond time response.

An all-optical tunable photonic bandgap microcavity made from a two-dimensional polystyrene photonic crystal is fabricated by focused ion beam etching. The pump and probe scheme is adopted to measure tunability based on the femtosecond optical Kerr effect. An ultrafast response time of less than 120 fs is achieved for the tunable photonic bandgap microcavity. The microcavity resonant wavelength shifts 3.1 nm under excitation of 9.4 GW/cm2 pump intensity, which is in agreement with the theoretical prediction.

Journal Article↗
PubMed2006

Dynamic alignment of C2H4 investigated by using two linearly polarized femtosecond laser pulses.

We have studied multielectron ionization and Coulomb explosion of C2H4 irradiated by 110 fs, 800 nm laser pulses at an intensity of approximately 10(15) W/cm2. Strong anisotropic angular distributions were observed for the atomic ions Cn+(n = 1-3). Based on the results of two crossed linearly polarized laser pulses, we conclude that such anisotropic angular distributions result from dynamic alignment, in which the rising edge of the laser pulses aligns the neutral C2H4 molecules along the laser polarization direction. The angular distribution of the exploding fragments, therefore, reflects the degree of the alignment of molecules before ionization. Using the same femtosecond laser with intensity below the ionization threshold, the alignment of C2H4 molecules was also observed.

Journal Article↗
PubMed2006

Field-induced alignment of oxygen and nitrogen by intense femtosecond laser pulses.

Field-induced alignment of O2 and N2 was experimentally studied with laser intensities varying from 10(13) to 10(15) W/cm2. When the laser intensity was below the ionization threshold for these molecules, the interaction between the induced dipole moment of molecules and the laser electric field aligned the molecules along the laser polarization direction. After extinction of the exciting laser, the transient alignment revived periodically. Thus macroscopic ensembles of highly aligned O2 and N2 molecules were obtained under field-free conditions. When the laser intensity exceeded the ionization threshold for these molecules, multielectron ionization and Coulomb explosion occurred. Using two linearly polarized laser pulses with crossed polarization, we demonstrated that the rising edge of the laser pulse aligned the molecules along the laser polarization direction prior to ionization, which resulted in strong anisotropic angular distributions of exploding fragments. These results suggest that the degree of alignment should be taken into account when qualitatively comparing the ion yield of these molecules with their companion atoms.

Journal Article↗
PubMed2006

Spatiotemporal transformation of a focused femtosecond pulse in the absence of self-focusing.

The spatiotemporal transformation of a focused femtosecond pulse in fused silica at power much less than the critical power for self-focusing was numerically studied. We found that the pulse experiences unusual spatiotemporal reshaping, such as refocusing of the near-axis portion of the defocused pulse tail, even when the self-focusing effect was not taken into the calculation. Further studies revealed that a tubelike spatiotemporal distribution of the plasma is formed during pulse propagation and that it plays a decisive role in the transformations of the pulse. In addition, the tubelike plasma was found to exist universally under different external focusing conditions if the input pulse had sufficient power.

Journal Article↗
PubMed2006

Ultrafast tunable filter in two-dimensional organic photonic crystal.

A tunable filter made from a two-dimensional polystyrene photonic crystal is fabricated by focused ion-beam etching. The pump and probe scheme is adopted to measure tunability based on the picosecond optical Kerr effect. The response time of the tunable filter, approximately 10 ps, is within the measurement resolution. The maximum shift of optical channels is estimated to be 6 nm under excitation of 15.9 GW/cm2 pump intensity, which is in agreement with the theoretical prediction.

Journal Article↗
PubMed2005

Shift multiplexing by planar waveguide referencing.

We present a new method with which to implement shift multiplexing by planar waveguide referencing. In this method, a planar waveguide is used to steer the reference beam, and we implement shift multiplexing by shifting the recording medium. A spatial selectivity as high as 1.1 microm is obtained. By using waveguide referencing we can make a compact and simple holographic system.

Journal Article↗
PubMed2005

Measurement of the collision time of dense electronic plasma induced by a femtosecond laser in fused silica.

Electronic plasma induced by a focused femtosecond pulse (130 fs, 800 nm) in fused silica was investigated by use of pump-probe technology. Pump and probe shadow imaging and interferometric fringe imaging were combined to determine electronic collision time tau in the conduction band, and tau was measured to be 1.7 fs at an electron density near 5 x 10(19) cm(-3). The lifetime of the electronic plasma is also measured to be approximately 170 fs by use of the time-resolved shadow imaging technique.

Journal Article↗
PubMed2004

Fabrication of beam shapers in the bulk of fused silica by femtosecond laser pulses.

We reported a new approach to the fabrication of three-dimensional refractive-index-modified microstructures inside transparent materials by combining two-dimensional writing by scanning the focus of the femtosecond laser pulse and by forming the long filament in the third dimension. In this way, embedded diffractive beam shapers of grid, square, and ring gratings were obtained in the bulk of fused silica by use of a femtosecond laser with a wavelength of 810 nm and a repetition rate of 1 KHz. These structures and their refractive efficiencies were optimized by selection of the appropriate fabrication parameters, including the pulse energy, grating period, scanning speed, and scanning repetition. The good performance of these devices indicates that, owing to its simple and flexible method for fabricating complex phase elements inside transparent materials, this technique has potential applications to integrated optics.

Journal Article↗