We suggest and demonstrate the concept of optical programmable metamaterials which can configure the device’s electromagnetic parameters by the programmable optical stimuli. In such metamaterials, the optical stimuli produced by a FPGA controlled light emitting diode array can switch or combine the resonance modes which are coupled in. As an example, an optical programmable metamaterial terahertz absorber is proposed. Each cell of the absorber integrates four meta-rings (asymmetric 1/4 rings) with photo-resistors connecting the critical gaps. The principle and design of the metamaterials are illustrated and the simulation results demonstrate the functionalities for programming the metamaterial absorber to change its bandwidth and resonance frequency.
Long-period fiber gratings (LPFGs) have found wide applications in optical communications and smart sensing. Among the various methods for fabrication of LPFGs, infrared femtosecond laser writing is one of the most attractive methods in terms of flexibility, being applicable to both photo sensitive and insensitive fibers, and no needs for any pre-designed masks. Although fabricating LPFGs by infrared femtosecond laser has been demonstrated for more than ten years, the reported maximum rejection band depth of LPFGs fabricated with this method for standard telecommunication fiber (SMF-28) without hydrogen loading is no more than 10 dB, and in the meanwhile its out-of-band loss is rather high. In this report, LPFGs with major resonant attenuation over -16 dB near 1300 nm are produced in standard single mode fibers with two dimension visional femtosecond laser manufacturing system. The contrast of the resonant rejection band resulting from core-cladding mode coupling can be significantly increased by applying a proper amount of axial stress along fiber during laser writing. With higher laser energy irradiation, LPFGs of multiresonance peaks plus larger out-of-band loss are frequently made. Such out-of-band loss is mainly caused by Mie scattering, and it can be restrained by properly selecting grating duty cycle.
Shadowgraphs of dynamic processes outside and inside the target during the intense femtosecond laser ablation of silica
glass at different energy fluences are recorded. Two material ejections outside the target and two corresponding stress
waves inside the target are observed. In particular, a third stress wave can be observed at energy fluence as high as 40
J/cm<sup>2</sup>. The pressure, the temperature, the free electron density, and the ionic components at the laser pulse end are
estimated, based on which the mechanical reaction of the laser heated material is investigated. According to our analysis,
the first wave is a thermoelastic wave, while the second and the third may be generated subsequently by the mechanical
expansions. Besides, the velocities of the stress waves are deduced from the time-resolved shadowgraphs, and it is found
that the first stress wave propagates with a velocity greater than the sound velocity, while the second stress wave
propagates with a velocity less than the sound velocity. Therefore, the first wave is a supersonic shockwave with a high
stress magnitude, while the second may be the plastic stress wave or subsonic shockwave with a lower stress magnitude.
Further more, the temporal evolution the second stress wave is investigated, and its velocity is found to increases
gradually at large delay times. According to the extrapolation curve, however, it is speculated that the velocity decreases
from a high value initially, which could be due to the interaction between the first and second stress waves at small delay
times. These results can provide a further support to the theory of highpressure shock phenomena in femtosecond laser
Laser pulses with different pulse durations between 50 fs and 12 ps are used to ablate different types of solid targets.
With the help of time-resolved shadowgraphy, the ultrafast dynamics associated with femtosecond laser ablation of solid
materials is experimentally investigated based on the commonly used pump-probe technique. It is revealed that both
photothermal and photomechanical mechanisms exist in the ablation processes for laser fluence far above the target
ablation threshold. From the recorded shadowgraphs, it is revealed that the material ejection due to femtosecond laser
ablation of solid materials has a typical velocity of 10<sup>5</sup> - 10<sup>4</sup> m/s. Such high jet velocities can lead to specific impulse of
10<sup>4</sup> - 10<sup>3</sup> s, which is much higher than the upper limit of the specific impulse of chemical propulsion (500 s). The
so-called ablative laser propulsion with high specific impulse can be thus realized. Momentum coupling coefficient is
determined through using a homemade torsion pendulum with a minimum measurable momentum of ~2x10<sup>-9</sup> N·s. The
dependence of the ultrashort laser ablation generated momentum is investigated on laser energy fluence and pulse width.
The main findings include: 1) As the pulse width increases, the laser generated momentum first increases rapidly and
then remains almost constant; 2) For 50 fs pulses, optimal laser fluence exists that leads to the maximum momentum for
aluminum and copper targets. At the optimal laser fluence, it is mostly the photomechanical mechanism that is
responsible for material removal, and in this case the ablated material also has relatively lower jet velocity.
Fundamental principles and advantages of the laser induced air ionization microcopy (LIAIM) and laser induced breakdown microscopy (LIBM) are introduced. Potential applications of these two new types of nonlinear imaging methods using ultrashort laser pulses in imaging both dielectric materials and bio-samples are demonstrated with some representative experimental results. Effects of different laser pulse widths on the discrimination power for laser-written microstructures inside transparent materials and the elemental composition are also investigated. It is shown that femtosecond laser induced ionization probe detects the variation of elemental composition of the sample materials with relatively higher contrast ratio, whereas the ionization probe generated by picosecond laser pulses is more sensitive to the material density or structural change. These observations can be well explained by the different roles of multi-photon ionization and avalanche ionization involved in material breakdown.
The dynamic process of 800 nm high fluence femtosecond laser ablation of aluminum is revealed by ultrafast time-resolved shadowgraphs. Both single and multiple laser pulses at perpendicular or oblique incidence are employed in the experiment. It is demonstrated that femtosecond laser ablation of aluminum with a fluence of 40 J/cm<sup>2</sup> is a complex process involving both photothermal and photomechanical mechanisms. For a single ablation pulse, the propagation direction of the ejected material remains normal to the target surface regardless of the incidence angle of the laser. For
the multiple pulses ablation with an oblique incidence angle, the propagation direction of the ejected material deviates
from the normal of the target surface gradually as the number of ablation pulses increases due to the topographic change
of the ablated region.
The light filament generated by focusing 50 fs laser pulses with a single pulse energy of 0.83 mJ using a 111 cm focal
length lens in air is used to propel micro glass beads. It is found that the propulsion efficiencies at various longitudinal
positions of the filament did not follow the filament's plasma distribution along the laser propagation direction. However,
the variation of the propulsion distances agrees well with the evolution of the measured ablation rates at different locations along the filament. It hints that it is the ablative material removal that gives the main contribution to the propulsion of the micro beads. Further numerical simulations taking into account both the nonlinear propagation of the femtosecond laser pulse and the ablation dynamics confirm our interpretation.