Internal field distribution measurement in 1-D strongly anisotropic sub-wavelength periodic structures of finite length.

We report measurements of the internal field intensity distribution in finite length one dimensional strongly anisotropic sub-wavelength periodic structures in the vicinity of the photonic band gap (PBG) edge. The strong in-plane anisotropy of more than 10% index contrast is obtained via form birefringent sub-wavelength gratings. The structures have a period of less than half the wavelength. Depending on the excitation frequency, both standing wave and evanescent Bloch modes can be identified and observed experimentally. The field enhancement near the PBG edge is confirmed also but at a significantly reduced strength attributed to the small but finite material loss.

Tunable negative group index in metamaterial structures with large form birefringence.

We experimentally verify the anomalous phase behavior in metamaterial structures with birefringent materials predicted by Mandatori, et. al. using form birefringent structures. Large birefringence as much as Deltan/n = 0.7 has been achieved by surface-treated form birefringent discs, making compact single layer Mandatori structures viable. With a reduced model of a single layer birefringent structure, the relationship between design parameters (thickness and orientation angle) and device operation and performance parameters (such as the center operation frequency, bandwidth, effective negative index, negative group index of refraction, and the transmission throughput) are derived and verified experimentally. Tunable group index of refraction from strong slow light of ng = 29.6 to fast light of ng = -1.1 are measured experimentally.

Ultrasound induced improvement in optical coherence tomography (OCT) resolution.

Optical coherence tomography (OCT) is a rapidly emerging technology for high-resolution biomedical imaging. The axial resolution of this technology is determined by the bandwidth of the source. Commercial sources generally provide resolutions of 10-20 microm whereas laboratory-based solid state lasers have resolutions of approximately 4 microm. The resolution in tissue depends almost exclusively on detecting single scattered events. However, the phenomenon known as multiple scattering results in a deterioration of resolution as a function of depth. In this study, OCT was combined with ultrasound in an attempt to reduce the effect of multiple scattering. The theory is that, with parallel ultrasound and OCT beams, multiply scattered light with a momentum component significantly perpendicular to the OCT beam will be reduced because the light is Doppler shifted outside the bandpass filter of the OCT detection electronics. A 7.5-MHz ultrasound transducer was used to introduce the photon/phonon interaction. A reflecting metal plate was placed within biological tissue, and the point spread function (PSF) was assessed off the reflector. The PSF was determined in the presence of no ultrasound, pulsed ultrasound, and continuous-wave (CW) ultrasound. CW ultrasound resulted in a 17% improvement (P < 0.001) in resolution and pulsed ultrasound resulted in 8% (P < 0.01). Image noise reduction could also be noted. Combining OCT with a parallel ultrasound beam results in an improvement in resolution through a reduced effect of multiple scattering due to photon/phonon interaction. With higher frequencies, better control of the acoustical beam, and tests in media with higher rates of multiple scattering, improved results are anticipated.