Theory and experiment of two-photon 'direct' interference for type-II spontaneous parametric down-conversion with an ultrafast pulse laser pump

Fourth-order spatial interference of entangled photon pairs generated in the process of spontaneous parametric down-conversion pumped by a femtosecond pulse laser has been performed for the first time. In theory, it takes into account the transverse correlation between the two photons and is used to calculate the dependence of the visibility of the interference pattern obtained in Young's double-slit experiment. In this experiment, a short focal length tens and two narrow band interference filters were adopted to eliminate the effects of the broadband pump laser and improve the visibility of the interference pattern under the condition of nearly collinear light and degenerate phase matching.

Depth dependence of the tetragonal distortion of a GaN layer on Si(111) studied by Rutherford backscattering/channeling

Rutherford backscattering and channeling have been used to characterize the structure of a GaN layer grown on a Si(111) substrate. The results show that a 1.26 mum GaN epitaxial layer with a rather abrupt interface and a good crystalline quality (chi(min)=3.4%) can be grown on a Si(111) substrate. Using the channeling angular scan around an off-normal <1 (2) over bar 13> axis in the {10 (1) over bar0} plane of the GaN layer, the tetragonal distortion e(T), which is caused by the elastic strain in the epilayer, can be determined. Moreover, the depth dependence of the e(T) can be obtained using this technique. A fully relaxed (e(T)=0) GaN layer for a thickness <2.8 mum is expected. (C) 2002 American Institute of Physics.

Spatial interference effect of two-photon in femtosecond-pulse pumped spontaneous parametric down-conversion

We have used the transverse correlated properties of the entangled photon pairs generated in the process of spontaneous parametric down-conversion, which is pumped by a femtosecond pulse laser, to perform Young's interference experiment. Unlike the case of a continuous wave laser pump, a broadband pulse laser pump can submerge an interference pattern. In order to obtain a high visibility interference pattern, we used a lens with a tunable focal length and two interference filters to eliminate the effects of the broadband pump laser. It is proven that the process of two-photon direct interference is a post-selection process.

Photoluminescence properties of a GaN0.015As0.985/GaAs single quantum well under short pulse excitation

Under short pulse laser excitation, we have observed an extra high-energy photoluminescence (PL) emission from GaNAs/GaAs single quantum wells (QWs). It dominates the PL spectra under high excitation and/or at high temperature. By measuring the PL dependence on both temperature and excitation power and by analyzing the time-resolved PL results, we have attributed the PL peak to the recombination of delocalized excitons in QWs. Furthermore, a competition process between localized and delocalized excitons is observed in the temperature-dependent PL spectra under the short pulse excitation. This competition is believed to be responsible for the temperature-induced S-shaped PL shift often observed in the disordered alloy semiconductor system under continuous-wave excitation. (C) 2001 American Institute of Physics.

Terahertz pulse generation with LT-GaAs photoconductive antenna

Low-temperature-grown GaAs (LT-GaAs) of 1-um thickness was grown at 250 degrees C on semi-insulating GaAs (001) substrate using EPI GEN-II solid-source MBE system. The sample was then in situ annealed for 10 min at 600 degrees C under As-rich condition. THz emitters were fabricated on this LTGaAs with three different photoconductive dipole antenna gaps of 1-mm, 3-mm, and 5-mm, respectively. The spectral bandwidth of 2.75 THz was obtaind with time domain spectroscopy. It is found that THz emission efficiency is increased with decreasing antenna gap. Two carrier lifetimes, 0.469 ps and 3.759 ps, were obtained with time-resolved transient reflection-type pump-probe spectroscopy.

Tandem electroabsorption modulators integrated with DFB laser by ultra-low-pressure selective-area-growth MOCVD for 10 GHz optical short pulse generation - art. no. 60200O

A novel device of tandem multiple quantum wells (MQWs) electroabsorption modulators (EAMs) monolithically integrated with DFB laser is fabricated by ultra-low-pressure (22 mbar) selective area guowth (SAG) MOCVD technique. Experimental results exhibit superior device characteristics with low threshold of 19 mX output light power of 4.5 mW and over 20 dB extinction ratio when coupled into a single mode Fiber. Moreover, over 10 GHz modulation bandwidth is developed with a driving voltage of 2 V. Using I this sinusoidal voltage driven integrated device, 10GHz repetition rate pulse with a width of 13.7 ps without any compression elements is obtained.

Effect of pulse heating parameters on the microscale bubble dynamics at a microheater surface

We study the effects of pulse heating parameters on the micro bubble behavior of a platinum microheater (100 mu m x 20 mu m) immersed in a methanol pool. The experiment covers the heat fluxes of 10-37 MW/m(2) and pulse frequencies of 25-500 Hz. The boiling incipience is initiated at the superheat limit of methanol, corresponding to the homogeneous nucleation. Three types of micro boiling patterns are identified. The first type is named as the bubble explosion and regrowth, consisting of a violent explosive boiling and shrinking, followed by a slower bubble regrowth and subsequent shrinking, occurring at lower heat fluxes. The second type, named as the bubble breakup and attraction, consists of the violent explosive boiling, bubble breakup and emission, bubble attraction and coalescence process, occurring at higher heat fluxes than those of the first type. The third type, named as the bubble size oscillation and large bubble formation, involves the initial explosive boiling, followed by a short periodic bubble growth and shrinking. Then the bubble continues to increase its size, until a constant bubble size is reached which is larger than the microheater length. 

Ultra-broadband optical parametric chirped-pulse amplification by matching of both group-velocity and pulse-front

Ultra-broadband optical parametric chirped-pulse amplification is analyzed based the compensation of phase-mismatch, which is achieved by matching of both group-velocity and pulse-front between signal and idler by the combination of the noncollinear-phase-match and pulse-front-tilt. The results show exactly matching of both group-velocity and pulse-front is the important criterion for constructing an UBOPCPA. Its general model is developed, in which the group velocities, noncollinear angles. spatial walk-off angles, linear angular spectral dispersion coefficients and pulse-front tilted angles are suitably linked to each other. Finally, specific numerical calculations and simulations are presented for beta-barium borate OPCPA with type-1 noncollinear angularly dispersed geometry. (C) 2005 Elsevier B.V. All rights reserved.

Experimental and theoretical analysis of nondegenerate ultrabroadband chirped pulse optical parametric amplification

Experimental investigations of nondegenerate ultrabroadband chirped pulse optical parametric amplification have been carried out. The general mathematical expressions for evaluating parametric bandwidth, gain and gain bandwidth for arbitrary three-wave mixing parametric amplifiers are presented. In our experiments, a type-I noncollinear phase-matched optical parametric amplifier based on lithium triborate, which was pumped by a 5-ns second harmonic pulses from a Q-switched Nd:YAG operating at 10 Hz, seeded by a 14-fs Ti:sapphire laser at 800 nm, was presented. The 0.85 nJ energy of input chirped signal pulse with 57-FWHM has been amplified to 3.1 muJ at pump intensity 3 GW/cm(2), the corresponding parametric gain reached 3.6 x 10(3), the 53 nm-FWHM gain spectrum bandwidth of output signal has been obtained. The large gain and broad gain bandwidth, which have been confirmed experimentally, provide great potentials to amplify efficiently the broad bandwidth femtosecond light pulses to generate new extremes in power, intensity, and pulse duration using optical parametric chirped pulse amplifiers pumped by powerful nanosecond systems.

Picosecond ultra-wideband short pulse radiated by antenna arrays

This report presents the experiments to study the characteristics of the picosecond ultra-wideband pulses coherent radiation. The testing involves bow-tie horn antennas for both the transninting and receiving antenna. Sixteen channels of electrical pulses with 290 ps duration and jitter < 30 ps have been used. The antenna arrays with various frames of 4 x 1, 4 x 2, 4 x 3, 4 x 4 are employed to radiate the pulses. The receiving antenna measures the electrical field in different distance front the transmitting antennas arrant The results show that if the pulses are in coherent condition, the peak power pulse of output by antennas array with N elements are N-2 of that of the single element antenna. (c) 2007 Wiley Periodicals, Inc.

Generation of the high efficiency high peak-power femtosecond blue optical pulse

The analysis and calculation of the compensation for the phase mismatch of the frequency-doubling using the frequency space chirp introduced from prisms are made. The result shows that suitable lens can compensate the phase mismatch in a certain extent resulting from wide femtosecond spectrum when the spectrum is space chirped. By means of this method, the experiment of second harmonic generation is carried out using a home-made femtosecond KLM Ti:sapphire laser and BBO crystal. The conversion efficiency of SHG is 63 %. The average output power of blue light is 320 mW. The central wavelength is 420 nm. The spectrum bandwidth is 5.5 nm. It can sustain the pulse width of 33.6 fs. The tuning range of blue light is 404-420 nm,when the femtosecond Ti:sapphire optical pulse is tuned using the prisms in the cavity.

Pulse-front matched ultra-broadband optical parametric chirped pulse amplifier for sub-12 fs pulse generation

We propose a LBO-based ultra-broadband chirped pulse optical parametric amplifier employing pulse-front-matching to yield transform-limited sub-12-fs pulses. Measurement of the maximum possible gain bandwidth for the LBO-based OPCPA demonstrates more than 60nm gain bandwidth FWHM. For the generation of TL pulses by the use of this OPCPA, a suitable combination of OPCPA and PFM is first presented. The PFM pump geometry realizes tilt-free signal amplification, and permits this OPCPA to generate TL sub-12-fs pulses.