Ja-Hon Lin

Two-photon resonance assisted huge nonlinear refraction and absorption in ZnO thin films

Optical nonlinearities of ZnO thin films, made by laser deposition, were investigated by the Z-scan method using a mode-locked femtosecond Ti:sapphire laser. The measured bound-electron nonlinear index of refraction γ and the two-photon absorption coefficient β at near-IR wavelengths show an enormous enhancement compared with measurements on bulk ZnO at 532 nm. The results reveal that two-photon resonance to the band edge and exciton energy level is responsible for the nonlinear absorption and that the free carrier induced the optical nonlinearity. With the excitation wavelength operated between 810 to 840 nm, a negative β value is measured due to the saturation of linear absorption of the defect states. Finally, we compared the values of β from the closed aperture Z-scan data (by considering the multi-photon absorption induced thermal nonlinearity) with those obtained from the open aperture Z-scan data. The results show that nonlinear refraction in the near-IR region is dominated by the bound-electron an...

Stable Q-switched mode-locked Nd3+:LuVO4 laser by Cr4+:YAG crystal

In use of saturable Cr(4+):YAG crystals, we have demonstrated the stable Q-switched and mode-locked (QML) operation of the Nd:LuVO(4) laser. The operation range of QML in use of the Cr(4+):YAG is larger than that use of SESAM and NLM. The obtained shape of Q-switched envelope, repetition rate and pulse energy are demonstrated to depend on the initial transmittance of the Cr(4+):YAG and reflectivity of the output coupler. Using R = 60% and T(0) = 40%, the highest pulse energy of 77 microJ of each Q-switched envelope, and the highest peak power about 200 kW of Q-switched mode-locked pulses can be obtained at 15 W pump power. It demonstrate the superior characteristic of the Nd:LuVO(4) crystal.

Near-resonant high order nonlinear absorption of ZnO thin films

Nonlinear optical absorption of ZnO thin film grown by the laser molecular beam epitaxy was investigated by the Z-scan method using a frequency-doubled femtosecond Ti:sapphire laser. Due to near-exciton resonance, an enormous enhancement of two photon absorption (TPA) is about 3000 cm/GW at 390 nm which is about 6.5 times larger than the previously reported at two-photon exciton resonance in near-IR regime and 710 times larger than that of ZnO bulk measured at 532 nm. Besides, the free-carrier absorption resulting from linear absorption and TPA becomes essential when the excitation approaches the exciton resonance.

Synthesis and Two‐Photon Absorption Property Characterizations of Small Dendritic Chromophores Containing Functionalized Quinoxaliniod Heterocycles

A series of star-shaped multi-polar chromophores (compounds 1-3) containing functionalized quinoxaline and quinoxalinoid (indenoquinoxaline and pyridopyrazine) units has been synthesized and characterized for their two-photon absorption (2PA) properties both in the femtosecond and the nanosecond time domain. Under our experimental conditions, these model fluorophores are found to manifest strong and wide-dispersed two-photon absorption in the near-infrared region. It is demonstrated that molecular structures with multi-branched π frameworks incorporating properly functionalized quinoxalinoid units would possess large molecular nonlinear absorptivities within the studied spectral range. Effective optical-power attenuation and stabilization behaviors in the nanosecond time domain of a selected representative dye molecule (i.e., compound 2) from this model compound set were also investigated and the results indicate that such structural motif could be a useful approach for the molecular design toward strong two-photon-absorbing material systems for quick-responsive and broadband optical-suppressing-related applications, particularly to confront long laser pulses.

Low threshold and high power output of a diode-pumped nonlinear mirror mode-locked Nd:GdVO4 laser

We demonstrated nonlinear mirror mode-locking of a diode-pumped Nd:GdVO4 laser by using a 10 mm long KTP crystal. The stable CW mode locking can be reached with the relative low threshold pump power as low as 2.3W. The highest output power of 2.65W at the 10 W pump powers are generated with the 121-MHz repetition rate of pulses. It confirms the excellent laser performance of the Nd:GdVO4 for high-peak power of the pulse operation.

Ultrafast relaxation and absorption saturation at near exciton resonance in a thin ZnO epilayer

We observed ultrafast free exciton thermalization time of 700–900 fs and obtained the magnitude of maximal differential absorption to be 1.8×104 cm−1 with the pumping fluence of 10 μJ/cm2 by measuring transient differential transmission in a thin ZnO epitaxial layer at room temperature. The largest induced transparency occurs near exciton resonance associated with absorption saturation by comparing the excitation from the above band-gap to band-tail states. The pumping dependent transient absorption reveals transition of excitonic relaxation from exciton-phonon scattering to exciton-exciton scattering or to an electron-hole plasma.

Saturation of Large Nonresonant Third and Fifth Order Optical Nonlinearities from ZnSe Quantum Dots in Glass-Matrix Thin Films

Large nonresonant optical Kerr coefficient without apparent nonlinear absorption was obtained from ZnSe-doped glass thin films grown by pulsed laser deposition. A third nonlinear refractive index of +0.87 to 1.56 cm2/GW and fifth order of +17.2 cm4/GW2 were measured for a light beam having femtosecond pulsewidth with photon energy below one half bandgap using the Z-scan method. Besides the enhancement of the nonlinear refractive index results from the two-photon near resonance and quantum confinement effect, the intensity-induced saturation of nonlinear refractive index is explained by the quadratic optical Stark shift.

Manipulation of the resonance characteristics of random lasers from dye-doped polymer dispersed liquid crystals in capillary tubes

The resonance characteristics of random lasers from dye-doped polymer dispersed liquid crystals (DD-PDLCs) within capillary tubes were investigated. After adding a monomer (NOA65) into the liquid crystal mixtures, the emission spectra from the capillary tube revealed multiple emission spikes with narrow emission linewidth due to enhancement of the light scattering. Besides, the number of emission spikes, full width of the half maximum (FWHM), and lasing threshold from the DD-PDLCs were determined by the density and grain size of the polymer clusters within the PDLC mixtures through the alternation of the monomer concentration. Furthermore, the lasing performance of the DD-PDLCs in the capillary tube can be controlled by temperature. At low temperature, more emission spikes at long wavelengths were excited, and the laser revealed a relatively high Q-factor, accompanied with relatively low-threshold pump energy. It is because the increase in the birefringence of the liquid crystal molecules efficiently enhanced multiple recurrent light scattering.

Harmonic mode locking and multiple pulsing in a soft-aperture Kerr-lens mode-locked Ti:sapphire laser

We report the observation of harmonic mode locking and multiple pulse operations in a soft-aperture Kerr-lens mode-locked Ti:sapphire laser by varying the total intra-cavity dispersion. Second, third, and fourth order harmonic mode locking as well as multiple pulsing with interpulse separation ranging from femtosecond to nanosecond are observed. The laser is characterized in these regimes in terms of wavelength, bandwidth, pulsewidth, average and peak powers as functions of group velocity dispersion. By introducing a loss difference term into gain dynamic analysis, we conclude that gain depletion and recovery mechanisms are responsible for the underlying physics of current observations.

Switchable mode-locking states in an all-fiber all-normal-dispersion ytterbium-doped laser

Multi-operation state in an all-normal-dispersion ytterbium-doped fiber laser is obtained by introducing nonlinear polarization or hybrid mode-locking mechanism in the laser cavity. In addition to operating the laser at continuous-wave mode-locking, various operation states including continuous-wave, Q-switched mode-locking, pulse-splitting, and harmonic mode-locking can be achieved by tuning the laser pump power or an intracavity polarization controller. Insertion of a semiconductor saturable absorber mirror in the cavity helps to shorten the output pulsewidth of the hybrid mode-locked laser. When the pump power is rapidly adjusted, a strong asymmetric hysteresis behavior can be observed during the manipulation between different orders of harmonic mode-locking.