Richard J. Blaikie

Super-resolution imaging through a planar silver layer

It has been proposed that a planar silver layer could be used to project a super-resolution image in the near field when illuminated near its plasma frequency [J. B. Pendry, Phys. Rev. Lett. 86, 3966 (2000)]. This has been investigated experimentally using a modified form of conformal-mask photolithography, where dielectric spacers and silver layers are coated onto a tungsten-on-glass mask. We report here on the experimental confirmation that super-resolution imaging can be achieved using a 50-nm thick planar silver layer as a near-field lens at wavelengths around 365 nm. Gratings with periods down to 145 nm have been resolved, which agrees well with our finite-difference time domain (FDTD) simulations.

Performance enhancement of spectral-amplitude-coding optical CDMA using pulse-position modulation

Spectral-amplitude-coding optical code-division multiple-access (OCDMA) systems are limited by interference between incoherent sources. A detailed analysis of this limit for a system with a balanced receiver is presented. Additional pulse-position modulation (PPM) coding is proposed as a method to improve the system performance beyond this limit. A simple and robust PPM decoding structure is proposed, and the performance analysis of the whole PPM-OCDMA system is presented. The interference-limited performance of the PPM-OCDMA system is found to be superior to that of the original system when the number of simultaneous users is of the order of the PPM word length or larger. In particular, for a PPM word length of two, an increase in spectral efficiency of up to 100% is possible with no change in the signaling rate, data rate, or bit-error rate (BER).

Sub-diffraction-limited patterning using evanescent near-field optical lithography

Patterning at resolution below the diffraction limit for projection optical lithography has been demonstrated using evanescent near-field optical lithography with broadband illumination (365–600 nm). Linewidths of 50 nm and gratings with 140 nm period have been achieved. Ultrathin photoresist layers in conjunction with conformable photomasks are employed and a reactive ion etching process using SF6 has been developed to transfer the patterns to a depth of more than 100 nm into silicon. Full electromagnetic field simulations of the exposure process show that a high contrast image is present within the resist layer, and that the exposure is dominated by one polarization for the grating structures studied.

Submicron imaging with a planar silver lens

Optical imaging through a thin planar silver layer has been achieved by utilizing near-field lithography techniques. A 120 nm thick silver lens that was placed 60 nm below a patterned mask, imaged the mask’s features onto a photosensitive material located 60 nm below the silver. The entire structure was exposed from above with a mercury lamp. Features sizes as small as 350 nm (at a 700 nm period) were imaged onto the photosensitive material, demonstrating the lensing ability of the planar silver slab.

Imaging through planar silver lenses in the optical near field

Near-field imaging through planar silver lenses has been demonstrated using a modified conformal-mask optical lithography arrangement. Dense feature resolution down to 250 nm (on a 500 nm period) has been achieved in 50 nm thick photoresist on silicon using broadband illumination from a mercury lamp. Finite difference time domain simulations have been performed to show the resolution improvements that can be expected for imaging through such silver lenses compared with near-field proximity imaging. The resolution enhancements that are predicted are in good agreement with the experimental results, and the conditions by which sub-diffraction-limited resolution may be achieved are given.

Evanescent interferometric lithography.

Simulation results are presented to illustrate the main features of what we believe is a new photolithographic technique, evanescent interferometric lithography (EIL). The technique exploits interference between resonantly enhanced, evanescently decaying diffracted orders to create a frequency-doubled intensity pattern in the near field of a metallic diffraction grating. It is shown that the intensity in a gratings near field can be enhanced significantly compared with conventional interferometric lithography. Contrast in the interference pattern is also increased, owing to a reduction in the zeroth-order transmission near resonance. The patterns depth of field reduces as the wavelength is increased beyond cutoff of the first-order diffracted components, and results are presented showing the trade-offs that can be made between depth of field and intensity enhancement. Examples are given for a 270-nm-period grating embedded in material with refractive index n = 1.6 and illuminated with wavelengths near 450 nm. Under these conditions it is predicted that high-intensity, high-contrast patterns with 135-nm period can be formed in photoresists more than 50 nm thick.

Subwavelength optical imaging of evanescent fields using reflections from plasmonic slabs

Reflection can significantly improve the quality of subwavelength near-field images, which is explained by appropriate interference between forward and reflected waves. Plasmonic slabs may form approximate super-mirrors. This paper develops general theory in both spectral and spatial representations that allows the reflector position and permittivity to be determined for optimum image uniformity. This elucidates previous observations and predicts behaviour for some other interesting regimes, including interferometric lithography.

Calculated and measured transmittance of a tunable metallic photonic crystal filter for terahertz frequencies

A tunable metallic photonic crystal filter with a mechanical tuning mechanism is demonstrated. The performance is predicted with rigorous full-vector electromagnetic simulations (finite-difference time domain). A prototype has been built and characterized in the W band (70–110 GHz) using a vector network analyzer configured for free-space measurement of S parameters. The measured filter’s passband has a quality factor of 11, a tuning range of 3.5 GHz, and insertion loss of only 1.1–1.7 dB. Device fabrication is straightforward, yielding an inexpensive, robust and compact tunable filter.

Characterization of T-ray binary lenses

Multilevel phase-shift Fresnel diffractive zone plates fabricated on silicon wafers have been used as T-ray imaging lenses. The imaging results, including spatial and temporal distribution of T-rays measured at the focal planes in the frequency range from 0.5 to 1.5 THz, indicate that the performance of the diffractive terahertz (THz) lens is comparable with or better than that of conventional refractive THz lenses. The unique properties of the T-ray binary lens make it possible to fabricate excellent optics for narrow-band THz applications.

Nanolithography in the Evanescent Near Field

New applications in microscopy and nanofabrication have emerged from recent advances in the optical near field. This paper reviews the implications of using evanescently decaying components in the near field of a photomask as a lithography tool for the fabrication of nanoscale structures. Patterning at resolution below the diffraction limit for projection optical lithography has been demonstrated using such evanescent near-field optical lithography (ENFOL) with broadband illumination (365–600 nm). Line widths of 50 nm and gratings with 140 nm period have been achieved. Ultrathin photoresist layers in conjunction with conformable photomasks are employed and both additive and subtractive pattern transfer processes have been developed. Full electromagnetic field simulations of the exposure process show that a high-contrast image is present within the resist layer, and that the exposure is dominated by one polarization for the grating structures studied. These reveal the potential of ENFOL in achieving feature sizes smaller than λ/20.