Armand Rosenberg

Guided resonances in asymmetrical GaN photonic crystal slabs observed in the visible spectrum

We demonstrate that guided resonant modes can be readily observed in asymmetrical photonic crystal slabs on high-index substrates. In spite of the high radiative loss associated with all optical modes in these cases, the guided resonant modes are found to give rise to strong high-Q features in the transmission spectra. Since these photonic crystal structures are far more robust and easier to fabricate than the free-standing photonic crystal membranes used in previous studies of guided resonant modes, detailed studies of relevant optical phenomena and the implementation of proposed applications are greatly simplified.

Optical properties of a bio-inspired gradient refractive index polymer lens

The design, fabrication, and properties of one of a new class of gradient-index lenses are reported. The lens is an f/2.25 GRIN singlet based on a nanolayered polymer composite material, designed to correct for spherical aberration. The light gathering and focusing properties of the polymer lens are compared to a homogeneous BK7 glass singlet with a similar f-number. The modulation transfer function of the polymer GRIN lens exceeded that of the homogeneous glass lens at all spatial frequencies and was as much as 3 times better at 5 cyc/mm. The weight of the polymer lens was approximately an order of magnitude less than the homogeneous glass lens.

Refractive index measurements of poly(methyl methacrylate) (PMMA) from 0.4-1.6 μm.

Using a transmission-spectrum-based method, the refractive index of a 50 μm thick sample of poly(methyl methacrylate) (PMMA) was measured as a function of wavelength. To mitigate the effects of nonplane-parallel surfaces, the sample was measured at 16 different locations. The technique resulted in the measurement of index at several thousand independent wavelengths from 0.42 to 1.62 μm, with a relative RMS accuracy <0.5×10(-4) and absolute accuracy <2×10(-4).

Minimizing Heat Generation in Solid-State Lasers

Novel high-power ytterbium YAG lasers are described. These lasers incorporate the principle of anti-Stokes fluorescence cooling to reduce or eliminate detrimental heating. Lasers with net heating and net cooling are demonstrated. By balancing the spontaneous and stimulated emission, we have reduced the net thermal loading to below 0.01% of the lasers average output power. Design, testing, and analysis are reported for lasers up to 500 W average power and pulsed operation up to 30 s. Issues and limitations of this approach are discussed.

Fabrication of GaN suspended photonic crystal membranes and resonant nanocavities on Si(111)

The authors demonstrate the fabrication of suspended photonic crystal membranes in GaN films deposited on Si(111). The photonic crystal patterns fabricated in these membranes consisted of triangular arrays of holes having diameters in the range of 70–175nm, with a lattice spacing of 200–500nm. The patterns included optical cavity structures (groups of missing holes), which are predicted to exhibit resonances in the visible to near-IR spectral region. Such suspended photonic crystal membranes may serve as the basis for efficient wavelength-scale GaN-based light emitters monolithically integrated with Si.

Using Ni masks in inductively coupled plasma etching of high density hole patterns in GaN

High density patterns of holes in metalorganic chemical vapor deposition grown GaN films on sapphire have been fabricated by inductively coupled plasma etching using a nickel mask. Pattern transfer from the e-beam lithographically patterned resist to the nickel etch mask was accomplished by ion beam milling. A Cl2–BCl3 inductively coupled plasma was used to anisotropically etch patterns of 200–250-nm-diam holes with 300nm center-to-center spacing through the entire thickness of a 265-nm- and a 300-nm-thick GaN film. This work demonstrates a pattern transfer technique to Ni by ion beam milling and Ni as a durable etch mask under a chlorine environment for high density patterning of GaN films.

Near IR nonlinear absorption of an organic supermolecule [Invited]

The photophysics of bis(terpyridyl)osmium-(porphinato)zinc-bis(terpyridyl)osmium (OsPZnOs), a D-π-A-π-D symmetric supermolecule, were investigated in the femtosecond and nanosecond regimes. The supermolecule exhibits a two-photon absorption (δpeak ~900 GM) in the near IR (900-1300 nm) and optical pumping by two-photon absorption leads to a broad excited state absorption (σpeak ~1.1 × 10−16 cm2) in the same near IR region. Since the excited state has a long lifetime, OsPZnOs exhibits a strong nanosecond nonlinear absorption in this region. That nonlinear absorption is substantially enhanced when OsPZnOs is incorporated into a multimode waveguide. When two-photon pumping is the dominant mechanism, an additional enhancement of up to ~100 × in the nonlinear absorption is observed in a microchannel waveguide. OsPZnOs is a promising material for photonic applications such as optical noise suppression and optical limiting in the near IR.

Diode-pumped dysprosium laser materials

We are investigating materials for direct blue solid-state lasers assuming UV excitation with GaN based laser diodes. Room temperature spectroscopy is reported relevant to a proposed quasi-three level laser from the 4F9/2 level in trivalent dysprosium. Modeling based on these measurements suggests this is a promising new laser transition.

GaN-based photonic crystals and integrated optics

We have developed growth techniques for GaN-based multilayer films suitable for low-loss optical waveguides, as well as precision patterning for integrated optics and photonic crystal structures with sub-100 nm critical features.

Raman spectroscopy of Bi2Sr2CaCu2O8+δ with varying oxygen stoichiometry

Abstract Measurements of Raman scattering from the electronic continuum are reported for crystals of the cuprate superconductor Bi 2 Sr 2 CaCu 2 O 8+δ (Bi 2212) post-annealed at both high and low oxygen partial pressures to adjust T c . For temperatures below T c , a peak forms in the continuum at an energy shift which depends upon the polarizations of the incident and collected light as well as oxygen concentration. The results are discussed in terms of an anisotropic superconducting energy gap.