Flat grating lens utilizing widely variable transmission-phase via guided-modes.

We experimentally demonstrate a polarization-independent flat grating lens in the near-infrared region. The grating lens consists of ridges in the square lattice arrangement, and the ridge dimensions are gradually changed to distribute a phase map with focusing ability. It is well known that guided modes in gratings offer unity-reflection at a resonance, and therefore the transmission phase is widely varied around the resonance. We employ such transmission phase behavior and show that high transmittance is obtained in each unit cell for wide variation range of the transmission phase at the operation wavelength by sharpening the resonance. This enables us to accomplish a highly efficient transmissive grating lens.

Experimental investigation of double-groove grating satisfying total internal reflection condition.

We experimentally demonstrate a TiO(2) double-groove grating coupler with two different groove widths on a SiO(2) substrate in the visible region. Tolerance investigations based on Bloch-mode profiles in the grating and coupling strengths between the Bloch modes and diffraction orders reveal that the transmission performance is robust when one of the paired ridges is narrow enough (60 nm and less) considering a typical nanofabrication accuracy. Moreover, the ridge shape affects weakly the transmission performance due to the non-resonance operation of our dielectric device. Such tolerance investigations together with current nanofabrication technology enable us to accomplish a 70% efficiency for coupling the normal incident light into the + 1st order transmission diffraction satisfying the total internal reflection condition at a 640 nm wavelength of operation.

Wavefront control by stacked metal-dielectric hole array with variable hole shapes.

A stacked metal-dielectric hole array (SHA) containing rectangular holes whose shape gradually varies in-plane is proposed as a means of achieving wavefront control. The dependence of the transmitted phase on the frequency can be tuned by the hole shape, in particular the length of the sides that are normal to the incident polarization. The combination of periodic holes along the polarization direction and the gradual change in hole shape normal to the polarization direction produce an inclined wavefront for 1-dimensional beam steering. An in-plane phase difference of 0.6π using an SHA with a thickness of one-sixth of the wavelength has been experimentally demonstrated.

Transmission phase control by stacked metal-dielectric hole array with two-dimensional geometric design.

Transmission phase control is experimentally demonstrated using stacked metal-dielectric hole arrays with a two-dimensional geometric design. The transmission phase varies drastically with small frequency shifts due to structural resonances. Laterally propagating surface plasmon polaritons excited by the periodic hole array roughly determine the resonance frequency, whereas localized resonances in each hole determine the dispersion. The transmission phase at various frequencies is directly evaluated using interferometric microscopy, and the formation of an inclined wavefront is demonstrated using a beam steering element in which the hole shapes gradually change in-plane from square to circular.

Arm-edge conditions in plasmonic folded dipole nanoantennas.

Silver folded dipoles consisting of two parallel nanowires may operate as efficient transmitting and receiving nanoantennas in the optical domain in both cases of silver-nanowire-terminated arm-edges and open-terminated arm-edges, in contrast to their conventional radio-frequency (RF) counterparts that only operate efficiently when they are short-wire-terminated arm-edges. The mode decomposition analysis with the equivalent circuit reveals that the difference of the wave numbers between the common and the differential modes allows this feature for the optical folded dipole nanoantenna under both arm-edge conditions. The analysis also estimates the efficiency of the folded dipoles via the equivalent radius of nanowires for the common mode. These folded nanostructures may exhibit the enhanced efficiency with the maintained radiation patterns, where the efficiency of folded dipoles is the same as that of the effective single dipole at resonance.

Role of propagating modes in a double-groove grating with a +1st-order diffraction angle larger than the substrate-air critical angle.

Here we show, analytically and numerically, that in a TiO(2) double-groove grating with two different groove widths per period attached on the SiO(2) substrate, the normally incident light couples to the +1st-order transmission with 96.9% efficiency and with a 50° diffraction angle that is larger than the SiO(2)-air interface critical angle. Modal analysis reveals that three propagating modes for the +1st diffraction order reach the grating back end in phase, while the corresponding propagating modes for the -1st and zeroth orders are added destructively at the grating end. Four optical devices based on this grating characteristic are numerically demonstrated.