Analysis of electromagnetic enhancement by a groove doublet in gold substrate.

A semianalytical Fabry-Perot model is presented to investigate the electromagnetic enhancement by double grooves in gold substrate. The influence of hybrid wave (HW) propagating along the metal surface on field enhancement is explored systematically with a HW model. Simulation results imply that the hybrid generated by one groove can affect the mode fields in the other groove after propagating along the metallic surface. Giant electromagnetic field intensity at resonance condition is found for the groove distance of 0.6λ with the contribution of surface waves, which is nearly independent of the incidence wavelength and groove width.

Impressive laser intensity increase at the trailing stage of femtosecond laser filamentation in air.

The longitudinal distribution of the laser peak intensity inside a half meter long femtosecond laser filament in air is studied by measuring the signal ratio of two nitrogen fluorescence lines, 391 nm and 337 nm. The experimental results reveal that laser peak intensity initially remains almost constant (~4.3 × 10(13) W/cm2) inside the filament. However, before the end of the filament, surprisingly the laser intensity undergoes dramatic increase. A maximum intensity as high as 2.8×10(14) W/cm2 could be reached. The experimental result is unexpected by the conventional intensity clamping scenario, according to which the laser peak intensity would feature low variation along a filament. The experimental result is then interpreted as being due to the generation of a short pulse at trailing stage of the filamentation with reduced diameter. This phenomenon might be of great interest owing to its potential application in high-order-harmonic generation and producing isolated single attosecond laser pulse through simple experimental approach.

Multiple filamentation generated by focusing femtosecond laser with axicon.

Multiple filamentation has been observed when focusing a femtosecond laser pulse into a methanol solution with an axicon. It is found that multiple long filaments are located on the central spot and ring structures of the quasi-Bessel beam created by the axicon. Since the quasi-Bessel profile is determined by the axicon properties, the axicon has been suggested as a simple optics to control multiple filaments.

Electromagnetic enhancement by a periodic array of nanogrooves in a metallic substrate.

We present comprehensive investigations of the electromagnetic enhancement by a periodic array of rectangular nanogrooves in a metallic substrate. The impacts of array parameters and of illumination conditions on the enhanced electric-field intensity are explored using fully vectorial methods. The calculations are performed mainly for gold and for the visible and infrared wavelengths. The fully vectorial results are reproduced and explained by a simple Fabry-Perot model. Compared with the case of a single groove, the electric-field enhancement of the groove array is found to be much higher by almost 1 order of magnitude, which is shown to be related to the excitation of surface plasmon polaritons with the aid of the model. Practical considerations of a finite groove number and of a finite groove length are also provided.

Electromagnetic enhancement by a single nano-groove in metallic substrate.

We propose systematic investigations of the electromagnetic enhancement by a single nano-groove in gold substrate. The impacts of the groove parameters and of the illumination conditions on the enhanced intensity are explored using a fully vectorial numerical method. The obtained data can be well predicted and explained by a simple Fabry-Perot model. By virtue of the semi-analytical model, we identify two main factors that enable giant electric-field enhancement in very narrow grooves: the Fabry-Perot resonance and the large wave impedance of the fundamental mode in the groove.

Portraying polarization state of terahertz pulse generated by a two-color laser field in air.

By using an electro-optic sampling technique, the instantaneous field vector of the terahertz (THz) pulse generated by mixing a near-IR femtosecond pulse and its second harmonic in air has been studied. The polarization of the investigated THz pulse is found to be predominantly linear and orthogonal to that of the second harmonic, revealing the major role of chi((3))(xyxy) during the four-wave mixing process.

Deconvolution methods for image deblurring in optical coherence tomography.

Two-dimensional deconvolution methods are proposed to deblur optical coherence tomography images. One employs a two-dimensional deconvolution with a matrix given by the product of the longitudinal and transversal point-spread functions as its kernel, which can be taken as the general point-spread function of an optical coherence tomography system. The other uses two one-dimensional deconvolutions with the longitudinal and transversal point-spread functions successively. It is shown that the two deconvolution methods can deblur the experimentally obtained optical coherence tomography images effectively.

Design of transmission blazed binary gratings for optical limiting with the form-birefringence theory.

The structure of transmission blazed binary gratings for optical limiting is designed with the form-birefringence theory. This kind of grating has subwavelength features, can imitate the transmission blazed grating effectively, and has higher efficiencies than a transmission blazed grating with a subwavelength structure. The diffraction efficiencies are calculated and analyzed. For the normal incident light with 10.6 microm wavelength, the transmissivities for the designed grating at 0 degrees deviation angle for TE and TM polarizations are 0.05% and 5.09%, respectively, which are basically identical to the results of the finite-difference time-domain method. The diffraction efficiencies of the first transmitted order for TE and TM polarizations are 93.95% and 83.88%, respectively.

Propagation theories of arbitrary-order correlations of partially coherent electromagnetic fields based on separated-coordinate mode decomposition.

Theories to calculate the propagation of arbitrary-order correlations of stationary or nonstationary partially coherent electromagnetic fields are proposed. The theories are based on separated-coordinate mode decomposition, and can make the well-developed propagation theories of fully coherent electromagnetic fields applicable to partially coherent electromagnetic fields governed by linear Maxwell equations. The validity of the theories is illustrated by an example.

Illumination system using LED sources for pocket-size projectors.

An illumination system using LED sources for pocket-size projectors is designed. Its color gamut is much larger than the sRGB standard. The maximum theoretical efficiency of this system is 29.98% by non-imaging theory-based calculations. To analyze the system model, 1.5 x 10(6) rays are traced by using the LightTools software. The total light flux that illuminates the digital micromirror device is 44 lm and the American National Standards Institute 13-point uniformity on the surface is 91.55%, -91.15% with a system efficiency of 28.3%. The calculations are in good agreement with the simulation results. The illumination system can be used for personal pocket-size projectors providing a 15-20 in. (38-51 cm) color display with the brightness comparable with a laptop.

Multi-dimensional observation of white-light filaments generated by femtosecond laser pulses in condensed medium.

The generation dynamics of white-light filaments in a transparent condensed medium is investigated experimentally in both spatiotemporal and frequency domains. We visualize the creation of multiple refocusing filaments with gradual increase of laser power by characterizing their spatial distributions of plasma luminescence and corresponding far-field emission patterns. The spatial fusion of multiple refocusing filaments into a single relatively long plasma channel is found to take place at higher laser powers. The measurement of supercontinua indicates that filamentation of ultrashort laser pulses in dispersive bulk media is closely connected with subsequent temporal events of pulse single splitting, then multiple splittings and finally coalescence. In addition, typical interference patterns of far-field emission are observed in the experiment, which reveals the spatial coherent properties among transverse filaments that are produced by a single laser beam.

Propagation theories of partially coherent electromagnetic fields based on coherent or separated-coordinate mode decomposition.

Propagation theories of partially coherent electromagnetic fields based on coherent mode decomposition or separated-coordinate mode decomposition are proposed. With the proposed propagation theories, various powerful theories for the propagation of fully coherent electromagnetic fields can be used for the propagation of partially coherent electromagnetic fields. The proposed theories are applicable to any propagation problem of partially coherent electromagnetic fields governed by linear Maxwell equations. Some examples are provided to illustrate the validity of the proposed theories.

Optical superresolution of focused partially spatially coherent laser beams.

We report design theories of a diffractive superresolution element (DSE) to implement optical superresolution of focused partially spatially coherent laser beams. The design problem of the DSE can be transformed into a problem of linear programming to obtain a globally optimal solution. By using the design theories, some fundamental limits of optical superresolution of focused partially spatially coherent laser beams are proposed, and several design examples are provided. As expected, both the fundamental limits and the design examples show that worse spatial coherence will cause worse superresolution performance. The design theories provide a design approach with partially coherent beams and may be useful for other design problems under partially coherent illumination.

Pulsed digital holography system recording ultrafast process of the femtosecond order.

We report, for the first time to our knowledge, a pulsed digital microholographic system with spatial angular multiplexing for recording the ultrafast process of the femtosecond order. The optimized design of the two sets of subpulse-train generators in this system makes it possible to implement a digital holographic recording with spatial angular multiplexing of a frame interval of the femtosecond order, while keeping the incident angle of the object beams unchanged. Three pairs of amplitude and phase images from the same view angle digitally reconstructed by the system demonstrated the ultrafast dynamic process of laser-induced ionization of ambient air at a wavelength of 800 nm, with a time resolution of 50 fs and a frame interval of 300 fs.

Pulse compression beyond the Fourier-transform limit.

Theories of a technique for compression of an ultrashort femtosecond laser pulse beyond its Fourier-transform limit based on the pulse shaping technique are proposed. The technique is called superresolution in time domain (STD). Global optimization theories for the design of a mask to modulate the spectrum of an input pulse to obtain a STD output pulse are proposed. Several design examples illustrate the feasibility of STD. Some fundamental limits of STD are also provided.