ABSTRACT
We develop a theory of refractive index tuning for symmetry-protected optical bound states (SP-BICs) in high-contrast gratings (HCGs). A compact analytical formula for tuning sensitivity is derived and verified numerically. We also discover a new type of SP-BIC in HCGs that has an accidental nature with a spectral singularity, which is explained in terms of hybridization and strong coupling among the odd- and even-symmetric waveguide-array modes. Our work elucidates the physics of tuning SP-BICs in HCGs and significantly simplifies their design and optimization for dynamic applications in light modulation, tunable filtering, and sensing.
ABSTRACT
Laser direct writing is an attractive method for patterning 2D materials without contamination. Literature shows that the ultrafast ablation threshold of graphene across substrates varies by an order of magnitude. Some attribute it to the thermal coupling to the substrates, but it remains by and large an open question. For the first time the effect of substrates on the femtosecond ablation of 2D materials is studied using MoS2 as an example. We show unambiguously that femtosecond ablation of MoS2 is an adiabatic process with negligible heat transfer to the substrates. The observed threshold variation is due to the etalon effect which was not identified before for the laser ablation of 2D materials. Subsequently, an intrinsic ablation threshold is proposed as a true threshold parameter for 2D materials. Additionally, we demonstrate for the first time femtosecond laser patterning of monolayer MoS2 with sub-micron resolution and mm/s speed. Moreover, engineered substrates are shown to enhance the ablation efficiency, enabling patterning with low-power ultrafast oscillators. Finally, a zero-thickness approximation is introduced to predict the field enhancement with simple analytical expressions. Our work clarifies the role of substrates on ablation and firmly establishes ultrafast laser ablation as a viable route to pattern 2D materials.
ABSTRACT
We develop a theory for Fano resonance tuning in dual-mode high-contrast gratings (HCGs). Compact analytical formulas of tuning sensitivity are derived and verified numerically, and are in good agreement with reported experiments. We show that the resonance tuning in HCGs, containing cooperative contribution from two propagating modes, is fundamentally different from that in single-mode microresonators. Our theory reveals the important role of the higher-order mode, which can possess large modal dispersion, especially in the long-wavelength limit beyond the cutoff of slab waveguides, to enable large tuning sensitivity. Our findings will simplify the design and optimization of active and passive tuning in HCG resonators.
ABSTRACT
Ultrashort bunches of electrons, emitted from solid surfaces through excitation by ultrashort laser pulses, are an essential ingredient in advanced X-ray sources, and ultrafast electron diffraction and spectroscopy. Multiphoton photoemission using a noble metal as the photocathode material is typically used but more brightness is desired. Artificially structured metal photocathodes have been shown to enhance optical absorption via surface plasmon resonance but such an approach severely reduces the damage threshold in addition to requiring state-of-the-art facilities for photocathode fabrication. Here, we report ultrafast photoelectron emission from sidewalls of aligned single-wall carbon nanotubes. We utilized strong exciton resonances inherent in this prototypical one-dimensional material, and its excellent thermal conductivity and mechanical rigidity leading to a high damage threshold. We obtained unambiguous evidence for resonance-enhanced multiphoton photoemission processes with definite power-law behaviors. In addition, we observed strong polarization dependence and ultrashort photoelectron response time, both of which can be quantitatively explained by our model. These results firmly establish aligned single-wall carbon nanotube films as novel and promising ultrafast photocathode material.
ABSTRACT
Thermal refraction focusing in planar index-antiguided lasers is investigated both theoretically and experimentally. An analytical model based on zero-field approximation is presented for treating the combined effects of index antiguiding and thermal focusing. At very low pumping power, the mode is antiguided by the amplifier boundary, whereas at high pumping power it narrows due to thermal focusing. Theoretical results are in reasonable agreement with experimental data.
ABSTRACT
A model is reported that describes a bidirectional homogeneously broadened index-antiguided (IAG) slab laser having arbitrary single-pass gain and distributed losses. Maximum extraction efficiency and corresponding optimum output coupling are determined for various values of unsaturated gain and loss per pass. A method is proposed to determine the intrinsic laser parameters from output power measurements.
ABSTRACT
We investigate numerically and analytically the effects of gain saturation on the propagation of the fundamental mode in a gain-guided index-antiguided slab waveguide. The propagating mode adapts to gain saturation by becoming less confined, while at the same time its peak intensity increases more slowly. At steady state, both the mode shape and the power remain constant.
ABSTRACT
We theoretically analyze gain guiding in large-core Bragg fibers, to be used for large-mode-area laser amplifiers with single-transverse-mode operation. The signal is gain-guided in a low-index core, whereas the pump is guided by the photonic bandgap of the Bragg cladding to achieve good confinement. The high-index layers in the Bragg cladding are half-wave thick at the signal wavelength in order to eliminate Bragg reflection, reducing the Bragg fiber effectively to a step-index fiber for gain guiding.
Subject(s)
Amplifiers, Electronic , Computer-Aided Design , Fiber Optic Technology/instrumentation , Optical Devices , Refractometry/instrumentation , Equipment Design , Equipment Failure AnalysisABSTRACT
A new optically pumped waveguide amplifier with ultra-large mode area is proposed. This amplifier is based on gain guiding in a transverse grating waveguide in which the pump is confined by the photonic bandgap while the signal is guided by optical gain. Characteristics of the propagating modes of the waveguide amplifier are analyzed theoretically using the transfer matrix method, indicating robust single-transverse-mode operation with large modal gain.
ABSTRACT
Tungsten nanogratings with sub-100nm linewidths and subwavelength periods are fabricated by laser-induced chemical vapor deposition using a single 400 nm femtosecond pulsed laser beam without any beam shaping. Combining advantages of parallel and direct-write processing, this method can produce various nanograting structures on a wide range of substrates in a single step.
