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Femtosecond lasers can generate optical pulses with a duration on the order of 10-15 seconds. It¡¦s an important tool for the studying of ultrafast phenomena. In our lab, the operating femtosecond lasers include two home-built Cr:forsterite mode-locked lasers, three commercial Ti:sapphire mode-locked lasers, and one commercial optical parametric oscillator (OPO).
¡@¡@ The operation wavelengths of Ti:sapphire lasers range from 700 to 1000nm. The typical output power is 1.5W with a shortest pulse width of 30fs and a highest repetition rate of 2GHz. By frequency doubling with nonlinear crystals, blue or UV femtosecond pulses can be achieved from 350nm to 500nm. Combined with the OPO, the operation wavelengths of femtosecond pulses can be extend to 1 ~ 2mm.
¡@¡@ The home-built Cr:forsterite lasers operate around 1230nm with a 100fs pulse-width. At a 110MHz repetition rate, the output power could be as high as 300~500 mW. We successfully shrank the size of cavity through the use of double-chirped mirrors. Based on this system, Cr:forsterite lasers became more portable and could be widely applied to medical uses. In the future, our lab plans to build more portable Cr:forsterite femtosecond lasers with higher output power.
(2) Ultrafast laser fields diagnostics
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For ultrafast optical pulses, which are typically 100 femtosecond in duration, the variation of fields is faster than the speed of electronics. To obtain the complete field information, including amplitude and phase, we have developed a technique called "Triple-Optical Autocorrelation for Direct pulse shape measurement (TOAD)". From the measured triple-autocorrelation function by third-harmonic generation, we can analytically calculate the temporal shape of pulse intensity and uniquely determine the phase by a spectral measurement with Gerchberg-Saxton algorithm.
Figure 11¡G Third-harmonic-generation based triple-autocorrelation of laser pulses. According to the measured data, we can directly obtain the filed intensity in the time-domain without any iterative algorithm. (From "Triple-optical autocorrelation for direct optical pulse-shape measurement," Applied Physics Letters 81(8), pp. 1402-1404 (2002), http://link.aip.org/link/%3FAPPLAB/81/1402/1)
(3) Photonic-crystal-fiber-based ultrafast source
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Optical laser beams can be confined and guided within a small area by a photonic crystal fiber. When the optical intensity is getting higher and higher, the third order nonlinear effects such as self-phase modulation will appear. Balancing this effect with the dispersion of material, optical solitons can thus be formed, which can keep the pulse width unchanged for a long propagation length. In our lab, exploiting the self-frequency shift induced by optical soliton and Raman scattering effects, 1200nm~2200nm ultrafast sources can be obtained by the excitation of 1230nm femtosecond pulses from the Cr:forsterite laser. The wavelength can be tuned through the polarization of excitation, thus providing femtosecond sources with a broad tuning range. Under the funding of National Science Council and National Health Research Institute, our lab is now studying the ultrafast phenomenon of photonic crystals and planning to apply them to biology researches and optical control of pulse propagation.
(4) Ultrafast carrier dynamis of wide-bandgap light-emitting semiconductors
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The dynamic characteristics of carriers in semiconductors play an important role in the performance of the electronic and optoelectric devices. We use femtosecond-laser techniques to study the carrier dynamics within an ultrashort time domain, such as carrier-phonon interactions, radiative recombination processes from a signal quantum well, room-temperature exciton dynamics, and carrier dynamics in bandtail states. Besides wide-bandgap semiconductors such as GaN and ZnO, recently we also focus on the carrier dynamics in infrared materials such as InN.
Office : +886-02-3366-3700#319 ¡@Lab : +886-02-23659703¡@ Fax : +886-02-3366-3614
Location : Location : Room 319, EE Building II, No. 1 Sec. 4 Roosevelt Rd., Taipei 106, Taiwan
