Thursday, November 26, 2009

University of Oregon Metamaterial Photonic Funnels Form Functional Nanoscale Waveguides

Photonic funnels promote efficient transfer of electromagnetic energy to and from subwavelength scales and can be used to build high-performance near-field microscope tips, subwavelength probes, single molecule detectors, and achieve miniaturization of terahertz sources and increase nonlinearities, according to University of Oregon Professor  Viktor A. Podolskiy and Penn State University Professor Alexander A. Govyadinov
in U.S. Patent 7,623,745

Left- and right-handed funnel designs allow effective phase manipulation at subwavelength scale. Since the mode structure of the funnels is identical to that of conventional waveguides (e.g., optical fibers), the design easily integrates with modern technologies. Applications of the wave guide include second- and higher-harmonic generation on the nanoscale, low-loss direct coupling to ultra-compact all-optical circuits, near-field optical microscopes with positive or negative refraction with efficiency several orders of magnitude better than current state of the art, sensors with subwavelength resolution (e.g., near-field probes, single molecule detectors, miniaturization of energy sources), low-loss all-optical couplers between optical transistors/processors/circuits and long-range optical fibers, signal amplification, phase advance in positive-index materials, phase retardation in negative-index materials, ultra-slow and superluminal pulse propagation.

Compressing the light beyond the diffraction limit is one of the most fundamental problems of modern photonics and plasmonics. Efficient coupling between diffraction-limited and sub-diffraction scales will strongly benefit the areas of near-field sensing, nm-scale optical control, single-molecule spectroscopy, high-energy focusing and compact optoelectronics. While light emission by atoms, molecules, quantum wells, quantum dots and other quantum objects occurs from nm-sized regions, light propagation takes place on .mu.m-wide (wavelength) scales. Such a huge scale difference introduces fundamental limitations on (i) the size of waveguiding structures and (ii) efficiency of coupling between nano- and micro-domains.

These limitations, in turn, restrict the resolution and sensitivity of near-field microscopes, prevent fabrication of ultra-compact all-optical processing circuits, integrated optoelectronic devices and other photonic systems. However, despite the ever-increasing number of opportunities offered by modern technology, straightforward reduction of size of conventional dielectric waveguides is not possible since the onset of diffraction will lead to either cut-off of waveguide modes or to their leakage into the dielectric surrounding.

Podolskiy and Govyadinov's waveguide device and method support volume propagation modes of electromagnetic waves even when the waveguide radius is significantly smaller than the free-space wavelength. The waveguide can be tapered to provide efficient coupling to and from the nanoscale with 10% or more efficiency. For example, a photonic funnel is a waveguide with cross-sectional dimensions progressively decreasing along the direction of mode propagation, concentrating light energy to the nanoscale.

The waveguides are characterized by a nonmagnetic, nonresonant core material having a highly anisotropic dielectric constant with either positive or negative refractive index. Such a core material may be realized as a nano-layered metamaterial, e.g., alternating layers of dielectric and metallic layers. The dielectric layers can be gain regions to provide amplification or loss suppression and/or to introduce a versatile control over group velocity of optical pulses in the waveguide. Specific exemplary material systems are provided for five different operational frequencies (near IR: Si/Ag, mid IR: Si/SiC, far IR: doped InGaAs/AlInAs).

The waveguide designs may use a mode structure identical to that in conventional optical waveguides, allowing for efficient energy transfer therebetween. As an example, the metamaterial waveguide core can be realized as a 1D photonic crystal (PC) medium, e.g., periodic array of thin dielectric and metallic layers. Both positive and negative refractive index structures are proposed, as well as the combination of different refractive indices to provide phase manipulation.

FIG. 3 illustrates a comparison between the light propagation in a homogenous core photonic funnel with a PC-based photonic funnel in accordance with Podolskiy and Govyadinov's invention, with FIG. 3(a) showing a homogeneous Si core structure and FIG. 3(b) showing the field concentration in a Si--SiC PC photonic funnel.

Photonic funnels and anisotropic waveguides for subdiffraction light compression and pulse management at the nanoscale earned U.S. Patent 7,623,745 for The State of Oregon Acting By and through the State Board at Higher Education (Corvallis, OR).

No comments:

Post a Comment