We address recent fiber-based femtosecond laser technology. Specifically, fiber-chirped pulse amplifier is discussed for
the enabling the concept of real-world applications. We review recent selected material applications demonstrating advantages of ultrafast dynamics of highly repetitive pulse train in nanoparticle generation in pulsed-laser deposition and reliable Si wafer singulation.
Chirped Pulse Amplification (CPA) is widely used for generating high-energy femtosecond pulses. This is most
commonly done with a solid-state Ti:Sapphire crystal through a free-space optical path. The high energy density in the
crystal and the precise optical path required with the use of bulk optics make it difficult to design a simple system with
good stability and beam quality over the environmental conditions typically encountered in the manufacturing
A CPA system using fiber architecture reduces the need for precise beam guiding since the light follows the fiber. The
pump energy is more evenly distributed along the length of the amplifier fiber, reducing the thermal dissipation that is
required (no water chiller is required) and improving the overall efficiency. The fiber architecture also produces a
superior quality beam that does not require great care to maintain.
IMRA's latest FCPA μJewel uses the inherent advantages of the FCPA architecture, along with extensive engineering, to
produce a compact and stable femtosecond fiber laser system. Its high repetition rate and stable performance enables
applications that were difficult to achieve previously.
This paper will review the general design architecture of the FCPA μJewel and discuss several applications.
High average power single-mode fiber lasers have attracted significant attention as alternatives to conventional solidstate lasers owing to their relative high brightness, compactness and robustness. Likewise the turn-key operation of industrially qualified ultrafast fiber oscillators is well established. In recent years the convergence of reliable ultrafast fiber oscillators, high brightness pump diodes and high power fiber amplifiers has enabled ultrafast fiber lasers to surpass ultrafast solid-state lasers in terms of average power. While fiber lasers have generally not been able to match the ultrashort pulse energies produced by solid-state lasers, careful management of nonlinearities can overcome the conventional B-integral limit of π thereby permitting stable operation of practical ultrafast fiber lasers with pulse energies approaching the milli-Joule level. Here we review modes of nonlinear propagation in fibers which have enabled increases in ultrashort pulse energies from nano-Joule to milli-Joule levels, namely: solitons, similaritons and cubicons. As an example of a practical high energy ultrafast fiber laser, we demonstrate a cubicon Yb fiber chirped pulse amplification system producing 550 fs pulses with 50 μJ at >15 W.
We report on the development of an apertureless scanning near-field optical microscope for characterization of dielectric properties of nano-structures at terahertz frequencies. A spatial resolution of ≈ 150 nm is achieved, which corresponds to a sub-wavelength factor of ≈1/1000. The imaging mechanism is due to a resonant coupling between light field and the tip-surface system. This allows for image contrasts which exceed those can be expected from Mie scattering by orders of magnitude. Terahertz images of organic and inorganic structures show that the apertureless terahertz microscopy gives insight into the dielectric properties on submicron scale.
We present an investigation of coherent mid-infrared pulse measurement system using free-space electro-optic sampling technique. A series of nonlinear materials are investigated for the nonresonant optical rectification and electro-optic sampling for the generation and detection, respectively. A sampling bandwidth up to 40 THz is achieved. For spectroscopic application we present THz field radiation by optically excited coherent phonons in the mid-infrared frequency range. We compare the THz radiation under extremely different excitation conditions. The comparison shows the THz radiation property of the resonantly driven phonons in the surface field layer. Furthermore we investigate the coherent reststrahl band reflectivity in the time domain. Unexpectedly strong oscillations are observed near the longitudinal-optical phonon frequency. This THz waveform is discussed with the reststrahl band dispersion of the reflectivity.
The dynamics of coherent plasmon-phonon oscillations in compound semiconductors are investigated using a pump-probe technique. It is shown that the optically excited coherent carriers drive the thermal carriers introduced by the doping in forming coherent plasma oscillations. By analyzing the coupled oscillations with different hole densities a strong influence of electron-hole scattering on the dephasing of coherent plasmons is found. For the near-band edge detection the reflectivity change by these oscillations is strongly enhanced by the Franz-Keldysh effect. Finally, the second harmonic of the coherent longitudinal optical phonon frequency is observed as a result of the dynamical Franz-Keldysh effect.
Time resolved measurements of the hot carrier relaxation in nP have been performed
at room temperature using four different femtosecond techniques. At carrier densities
between 1016cm3 and a few times 1018cm3 carrier-carrier scattering has been found to
dominate the initial relaxation ensuring the internal thermalization of electrons and holes on a
time scale of 1OO'2OO fs. At high excitation densities the subpicosecond cooling of the
electrons is found to be clearly slower than expected from simple calculations of the
e-LO-phonon interaction. The different relaxation behaviour in GaAs is attributed to strong
intervalley scattering mechanisms.