A 405 nm LDs crystallization method of a-Si has been applied to the processing of bottom gate (BG) type
microcystalline (μc-) Si TFT for the first time. We have successfully demonstrated superior I-V characteristics of BG μc-
Si TFTs. In order to verify the validity of our process, we performed a heat flow simulation and compared commercially
available lasers having wavelengths of 405, 445 and 532 nm. The simulation explained well the experimental results and
showed that the wavelength is a crucial factor on uniformity, energy efficiency, and process margin and the 405 nm gave
the best results among the three wavelengths.
Short-pulse lasers of femtosecond and picosecond durations are used in ultrahigh precision processing of optical, electronic, and micro-mechanical devices. In this paper we discuss the fabrication of high quality photonic crystal band-pass filters with femtosecond laser pulses. We also discuss the parallel processing of inkjet nozzles with a picosecond laser. A laser milling algorithm and a high accuracy beam scanner allows the formation of precisely shaped holes as inkjet nozzles, and an efficient diffractive beam splitter allows the simultaneous drilling of many hundreds of holes for low cost manufacturing.
Ultrafast lasers are a class of laser that produce pulse widths of picoseconds (10-12 sec) and femtoseconds (10-15 sec). They can achieve extremely high peak power with low pulse energies. The most important characteristics of ultrafast laser-matter interaction are precise ablation threshold and absence of heat diffusion into the material during laser irradiation, both due to the shortness of the laser pulse. One of the advantages of applying ultrafast lasers in materials processing is the versatility of these lasers in processes where material removal is required. As the laser pulse width decreases from milliseconds through microseconds to nanoseconds and picoseconds, the material removal mechanism transitions from melt expulsion to direct ablative removal. This process is similar in many different solid materials, regardless of the material composition. In this paper a number of ultrafast laser machining examples in a variety of materials will be presented to illustrate this point. Precise ablation threshold and little or no heat-affected zone combine to yield high quality drilling and cutting in these cases.
The technique of chirped pulse amplification makes possible the investigation of dielectric breakdown as a function of pulse duration over a continuous regime of pulse duration from the 100 ps to the tens of fs and for material with bandgap varying from the single eV to several eV. This study leads to the surprising result that even for 100 fs pulses the dielectric breakdown is dominated by impact ionization even in the case of single photon absorption. For larger bandgap the seeding of the avalanche is produced by multiphoton ionization. This makes possible for the first time the direct determination of the electron ionization rate in important electronic materials like silicon as a function of the electric field up to 2 108 V/an which is impossible by conventional electronic means.
We evaluated in vivo wound healing responses to plasma- mediated ablation in skin as a function of laser pulsewidth and energy. Experiments utilized a regeneratively amplified Ti:Sapphire laser operating at 800 nm with pulsewidths varied from 7 ns to 100 fs. Skin incisions were created in mice by tightly focusing the laser beam on the tissue surface. Incisions of equal depth were compared at time points ranging from 6 hours to 3 weeks using standard histologic methods. Incision depth was proportional to pulse energy at each pulsewidth. Fluence threshold dependence on laser pulsewidth agreed with those predicted by ex vivo testing. Histologic analysis revealed minimal adjacent tissue damage at pulsewidths less than a few picoseconds and energies near the fluence threshold. Longer pulsewidths and higher fluence levels were associated with more significant collateral effects. These in vivo results suggest collateral tissue damage and secondary effects may be minimized by controlling laser pulsewidth and energy.
We investigated plasma-mediated surface ablation in corneal tissue using picosecond and femtosecond laser pulses in order to achieve high precision, non-thermal tissue removal with a non-ultraviolet laser source. Experiments utilized three laser systems, a regeneratively amplified Ti:sapphire laser, a synchronously amplified dye laser, and a regeneratively amplified picosecond Nd:YLF laser. Tissue ablation was performed by tightly focusing the laser beam on the tissue surface. Ablation thresholds were determined by monitoring the plasma spark, as well as the tissue surface. Tissue ablations were then analyzed by standard histologic methods and scanning electron microscopy. We observed a decrease in the ablation fluence threshold as the pulse duration is shortened from 200 ps to approximately 140 fs, in agreement with our theoretical predictions. Using identical pulse energies, the femtosecond laser pulses ablated tissue at higher efficiencies than the picosecond laser, with an approximately two-fold improvement in the etch depth curve. Histologic analysis reveal minimal adjacent tissue damage at either pulse duration. Femtosecond laser pulses may offer advantages that make them ideal tools for high precision tissue ablation.
A diode-pumped Nd:fluorophosphate regenerative amplifier was developed. Chirped seed pulses were amplified from between 54 (mu) J at 100 Hz to 5 (mu) J at 10 kHz and compressed to 850 femtoseconds. The effects of continuous-wave pumping on the pulse energy at high repetition rates are also presented.
Single-shot laser induced breakdown, in wide band gap materials such as SiO2 and MgF2, has been studied over almost 5 orders of magnitude in duration from 150 fs to 7 ns. A Ti:sapphire chirped pulse amplification system was used in this experiement, so the pulse duration could be continuously adjusted without changing any other parameters. The damage threshold was detected by looking at the plasma formation and the change of material transmission coefficient. The avalanche mechanism was found to dominate over the entire pulse-width range even for 150 fs pulses where we would expect multi-photon processes to take over. A strong departure from the conventional fluence threshold scaling law is observed for pulses shorter than 10 ps, where beyond this point the fluence threshold increases. Also, it is observed for the first time that for short pulses the damage threshold becomes very accurate and less statistical than that for longer pulses.
Short-pulse high-intensity laser-plasma interactions are investigated numerically with a fluid code and experimentally with optical and x-ray diagnostics. The emitted x-ray spectrum is characterized in the vicinity of the water window for laser intensities of 4 X 1017 W/cm2 and pulsewidths of 400 fs. Optimization of the x-ray pulsewidth using these results is also discussed.
With a time-resolved pump-probe setup, the dynamical interactions of intense 400-fs laser pulses with a solid target are studied with a time resolution of about 250 fs. The motion of the plasma critical surface is measured by the means of the doppler shift of the probe laser. It is found that when the average electron quiver energy (mv2os/2) becomes comparable to the electron thermal energy (kTe), the ponderomotive force of the high-intensity laser significantly reduces the thermal expansion of the laser-plasma.
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