Higher-order fractal transverse modes observed in microlasers.

Two classes of higher-order, fractal spatial eigenmodes have been predicted computationally and observed experimentally in microlasers. The equatorial plane of a close-packed array of microspheres, lying on one mirror within a Fabry-Pérot resonator and immersed in the laser gain medium, acts as a refractive slit array in a plane transverse to the optical axis. Edge diffraction from the slit array generates the high spatial frequencies (>104 cm-1) required for the formation of high-order laser fractal modes, and fractal transverse modes are generated, amplified, and evolve within the active medium. With a quasi-rectangular (4-microsphere) aperture, the fundamental mode and several higher-order eigenmodes (m = 2,4,5) are observed in experiments, whereas only the m = 1,2 modes are observed experimentally for the higher-loss resonators defined by triangular (3-microsphere) apertures. The fundamental and 2nd-order modes (m = 1,2) for the 4-sphere aperture are calculated to have qualitatively similar intensity profiles and nearly degenerate resonant frequencies that differ by less than <0.1% of the free-spectral range (375 GHz) but exhibit even and odd parity, respectively. For all of the observed fractal modes, the fractal dimension (D) rises rapidly beyond the intracavity aperture array as a result of the high spatial frequencies introduced into the mode profile. Elsewhere, D varies gradually along the resonator axis and 2.2 < D < 2.5. Generating fractal laser modes in an equivalent optical waveguide is expected to allow the realization of new optical devices and imaging protocols based on the spatial frequencies and variable D values available.

CsAr, CsXe, and RbXe B2Σ1/2+ Interatomic Potentials Determined from Absorption Spectra and Calculations of Franck-Condon Factors for Free-Free Optical Transitions of Atomic Collision Pairs.

Interatomic potentials for the B2Σ1/2+ states of CsAr, CsXe, and RbXe have been determined through comparisons of experimental B ← X absorption spectra for alkali vapor-rare gas mixtures with calculations of the Franck-Condon factors (FCFs) associated with free-free transitions of thermal atomic pairs. Simulations of optical transitions of alkali-rare gas atomic pairs between the thermal and vibrational continua of the X2Σ1/2+ and B2Σ1/2+ states of the molecule, responsible for the blue satellites of the Cs and Rb D2 resonance lines in a rare gas background, require the incorporation of ground-state J values above ∼400 into the FCF calculations and proper normalization of the free-particle wave functions. Absorption spectra computed on the basis of several X and B state interatomic potentials available in the literature were found to be sensitive to the height of the B2Σ1/2+ state barrier, as well as the X2Σ1/2+ state repulsive wall contour and the location of the van der Waals minimum. Other spectral simulations entailed iterative modifications to a selected B2Σ1/2+ interatomic potential, again coupled with comparison to experimental B ← X spectra. Comparisons of calculated spectra with experiment yield a CsXe B2Σ1/2+ potential, for example, exhibiting a barrier height of 76 cm-1 at 5.2 Å and yet is nearly flat at smaller values of internuclear separation (R). The latter contrasts with previous theoretical calculations of VB(R) in the vicinity of the barrier maximum. For the CsAr molecule, the B2Σ1/2+ barrier height was found to be 221 cm-1, which is within 3% of the value determined from pseudopotential calculations incorporating the spin-orbit effect. Reproducing Cs-rare gas experimental absorption spectra also requires the existence of a broad, shallow potential well lying beyond the B2Σ1/2+ barrier that, for CsAr, has a dissociation energy (De ∼ 24 cm-1) a factor of 3 larger than values predicted by theory. Similar results are obtained for the RbXe and CsXe complexes.

Flavin mononucleotide biomolecular laser: longitudinal mode structure, polarization, and temporal characteristics as probes of local chemical environment.

A detailed characterization of the flavin mononucleotide (FMN) biomolecular laser, optically pumped in a stable resonator, is reported here. Photoexcitation of the molecule at 355 nm results in lasing over the ~566.5-573.5 nm spectral region, and the threshold pump energy density is measured to be 110 ± 10 µJ/mm2 for a 10 mM FMN/water solution. Over twenty longitudinal modes are observed when the cavity length L and the energy pump fluence Ep are 375 µm and 300 µJ/mm2, respectively. Partial substitution of glycerol for water as the solvent results in a factor of four reduction in the threshold pump energy fluence (to < 30 µJ/mm2) and a quadrupling of the slope efficiency. This effect is attributed to the O2 - mediated photoconversion of FMN molecules in the triplet state to the singlet species. For pump intensities a factor of 2.5 above threshold, the laser pulse width is ~2 ns FWHM, and the output intensity decays exponentially with a photon lifetime of 1.7 ns. The addition of glycerol to a FMN/water solution also suppresses s-polarized emission (yielding P = 0.78 ± 0.08), presumably as a result of the inhibition of FMN rotational diffusion. The sensitivity of the spectral and optical properties of this and other biomolecular lasers to the chemical environment underscores the value of coherent emission as a biochemical or biomedical diagnostic tool, particularly insofar as molecule-molecule interactions are concerned.

Ultrasound harmonic generation and atomic layer deposition of multilayer, deep-UV mirrors and filters with microcavity plasma arrays.

Abstract: In honor of Professor Kurt Becker's pioneering contributions to microplasma physics and applications, we report the capabilities of arrays of microcavity plasmas in two emerging and disparate applications. The first of these is the generation of ultrasound radiation in the 20-240 kHz spectral range with microplasmas in either a static or jet configuration. When a 10×10 array of microplasma jets is driven by a 20-kHz sinusoidal voltage, for example, harmonics as high as m = 12 are detected and fractional harmonics are produced by controlling the spatial symmetry of the emitter array. The preferential emission of ultrasound in an inverted cone having an angle of ±45∘ with respect to the surface normal of the jet array's exit face is attributed to interference between spatially periodic, outward-propagating waves generated by the arrays. The spatial distribution of ultrasound generated by the arrays is analogous to the radiation patterns produced by Yagi-Uda phased array antennas at RF frequencies for which radiation is emitted broadside to arrays of parallel electric dipoles. Also, the nonperturbative envelope of the ultrasound harmonic spectrum resembles that for high-order harmonic generation at optical frequencies in rare gas plasmas and attests to the strong nonlinearity provided by the pulsed microplasmas in the sub-250-kHz region. Specifically, the relative intensities of the second and third harmonics exceed that for the fundamental, and a "plateau" region is observed extending from the 5th through the 8th harmonics. A strong plasma nonlinearity appears to be responsible for both the appearance of fractional harmonics and the nonperturbative nature of the acoustic harmonic spectrum. Multilayer metal-oxide optical filters designed to have peak transmission near 222 nm in the deep-UV region of the spectrum have been fabricated by microplasma-assisted atomic layer deposition. Alternating layers of ZrO2 and Al2O3, each having a thickness in the 20-50 nm range, were grown on quartz and silicon substrates by successively exposing the substrate to the Zr or Al precursor (tetrakis(dimethylamino) zirconium or trimethylaluminum, respectively) and the products of an oxygen microplasma while maintaining the substrate temperature at 300 K. Bandpass filters comprising 9 cycles of 30-nm-thick ZrO2/50-nm-thick Al2O3 film pairs transmit 80% at 235 nm but < 35% in the 250-280 nm interval. Such multilayer reflectors appear to be of significant value in several applications, including bandpass filters suppressing long wavelength (240-270 nm) radiation emitted by KrCl (222) lamps.

Micro mercury trapped ion clock prototypes with 10[Formula: see text] frequency stability in 1-liter packages.

Modern communication and navigation systems are increasingly relying on atomic clocks. As timing precision requirements increase, demands for lower SWaP (size, weight, and power) clocks rise. However, it has been challenging to break through the general trade-off trend between the clock stability performance and SWaP. Here we demonstrate micro mercury trapped ion clock (M2TIC) prototypes integrated with novel micro-fabricated technologies to simultaneously achieve high performance and low SWaP. The M2TIC prototypes could reach the [Formula: see text]-stability level in 1 day with a SWaP of 1.1 L, 1.2 kg, and under 6 W of power. This stability level is comparable to the widely used rack-mount Microchip 5071A cesium frequency standard. These standalone prototypes survived regular commercial shipping across the North American continent to a government laboratory, where their performance was independently tested. The M2TIC sets a new reference point for SWaP and performance and opens opportunities for high-performance clocks in terrestrial and space applications.

Tunable ring laser with internal injection seeding and an optically-driven photonic crystal reflector.

Continuous tuning over a 1.6 THz region in the near-infrared (842.5-848.6 nm) has been achieved with a hybrid ring/external cavity laser having a single, optically-driven grating reflector and gain provided by an injection-seeded semiconductor amplifier. Driven at 532 nm and incorporating a photonic crystal with an azobenzene overlayer, the reflector has a peak reflectivity of ~80% and tunes at the rate of 0.024 nm per mW of incident green power. In a departure from conventional ring or external cavity lasers, the frequency selectivity for this system is provided by the passband of the tunable photonic crystal reflector and line narrowing in a high gain amplifier. Sub - 0.1 nm linewidths and amplifier extraction efficiencies above 97% are observed with the reflector tuned to 842.5 nm.

Inactivation and sensitization of Pseudomonas aeruginosa by microplasma jet array for treating otitis media.

Otitis media (OM), known as a middle ear infection, is the leading cause of antibiotic prescriptions for children. With wide-spread use of antibiotics in OM, resistance to antibiotics continues to decrease the efficacy of the treatment. Furthermore, as the presence of a middle ear biofilm has contributed to this reduced susceptibility to antimicrobials, effective interventions are necessary. A miniaturized 3D-printed microplasma jet array has been developed to inactivate Pseudomonas aeruginosa, a common bacterial strain associated with OM. The experiments demonstrate the disruption of planktonic and biofilm P. aeruginosa by long-lived molecular species generated by microplasma, as well as the synergy of combining microplasma treatment with antibiotic therapy. In addition, a middle ear phantom model was developed with an excised rat eardrum to investigate the antimicrobial effects of microplasma on bacteria located behind the eardrum, as in a patient-relevant setup. These results suggest the potential for microplasma as a new treatment paradigm for OM.

Microplasmas for Advanced Materials and Devices.

Microplasmas are low-temperature plasmas that feature microscale dimensions and a unique high-energy-density and a nonequilibrium reactive environment, which makes them promising for the fabrication of advanced nanomaterials and devices for diverse applications. Here, recent microplasma applications are examined, spanning from high-throughput, printing-technology-compatible synthesis of nanocrystalline particles of common materials types, to water purification and optoelectronic devices. Microplasmas combined with gaseous and/or liquid media at low temperatures and atmospheric pressure open new ways to form advanced functional materials and devices. Specific examples include gas-phase, substrate-free, plasma-liquid, and surface-supported synthesis of metallic, semiconducting, metal oxide, and carbon-based nanomaterials. Representative applications of microplasmas of particular importance to materials science and technology include light sources for multipurpose, efficient VUV/UV light sources for photochemical materials processing and spectroscopic materials analysis, surface disinfection, water purification, active electromagnetic devices based on artificial microplasma optical materials, and other devices and systems including the plasma transistor. The current limitations and future opportunities for microplasma applications in materials related fields are highlighted.

Photolithography in the vacuum ultraviolet (172 nm) with sub-400 nm resolution: photoablative patterning of nanostructures and optical components in bulk polymers and thin films on semiconductors.

Precision photoablation of bulk polymers or films with incoherent vacuum ultraviolet (VUV) radiation from flat, microplasma array-powered lamps has led to the realization of a photolithographic process in which an acrylic, polycarbonate, or other polymer serves as a dry photoresist. Patterning of the surface of commercial-grade, bulk polymers (or films spun onto Si substrates) such as poly-methyl methacrylate (PMMA) and acrylonitrile butadiene styrene (ABS) with 172 nm lamp intensities as low as ∼10 mW cm-2 and a fused silica contact mask yields trenches, as well as arbitrarily-complex 3D structures, with depths reproducible to ∼10 nm. For 172 nm intensities of 10 mW cm-2 at the substrate, linearized PMMA photoablation rates of ∼4 nm s-1 are measured for exposure times t≤ 70 s but a gradual decline is observed thereafter. Beyond t∼ 300 s, the polymer removal rate gradually saturates at ∼0.2 nm s-1. Intricate patterns are readily produced in bulk acrylics or 40-200 nm thick acrylic films on Si with two or more exposures and overall process times of typically 10-300 s. The photoablation process is sufficiently precise that the smallest lateral feature size fabricated reproducibly to date, ∼350 nm, appears to be limited primarily by the photomask itself. Examples of the versatility and precision of this photolithographic process include the fabrication of arrays of aluminum nanomirrors, each atop a 350 nm or 1 µm-diameter Si post, as well as optical components such as transmission gratings or Fresnel lenses photoablated into PMMA.

Monolithic mtesla-level magnetic induction by self-rolled-up membrane technology.

Monolithic strong magnetic induction at the mtesla to tesla level provides essential functionalities to physical, chemical, and medical systems. Current design options are constrained by existing capabilities in three-dimensional (3D) structure construction, current handling, and magnetic material integration. We report here geometric transformation of large-area and relatively thick (~100 to 250 nm) 2D nanomembranes into multiturn 3D air-core microtubes by a vapor-phase self-rolled-up membrane (S-RuM) nanotechnology, combined with postrolling integration of ferrofluid magnetic materials by capillary force. Hundreds of S-RuM power inductors on sapphire are designed and tested, with maximum operating frequency exceeding 500 MHz. An inductance of 1.24 µH at 10 kHz has been achieved for a single microtube inductor, with corresponding areal and volumetric inductance densities of 3 µH/mm2 and 23 µH/mm3, respectively. The simulated intensity of the magnetic induction reaches tens of mtesla in fabricated devices at 10 MHz.

Fractal modes and multi-beam generation from hybrid microlaser resonators.

Fractals are ubiquitous in nature, and prominent examples include snowflakes and neurons. Although it has long been known that intricate optical fractal patterns can be realized with components such as gratings and reflecting spheres, generating fractal transverse modes from a laser has proven to be elusive. By introducing a 2D network of microspheres into a Fabry-Pérot cavity bounding a gain medium, we demonstrate a hybrid optical resonator in which the spheres enable the simultaneous generation of arrays of conventional (Gaussian) and fractal laser modes. Within the interstices of the microsphere crystal, several distinct fractal modes are observed, two of which resemble the Sierpinski Triangle. Coupling between adjacent fractal modes is evident, and fractal modes may be synthesized through design of the microsphere network. Owing to a unique synergy between the gain medium and the resonator, this optical platform is able to emit hundreds of microlaser beams and probe live motile cells.

Disintegration of simulated drinking water biofilms with arrays of microchannel plasma jets.

Biofilms exist and thrive within drinking water distribution networks, and can present human health concerns. Exposure of simulated drinking water biofilms, grown from groundwater, to a 9 × 9 array of microchannel plasma jets has the effect of severely eroding the biofilm and deactivating the organisms they harbor. In-situ measurements of biofilm structure and thickness with an optical coherence tomography (OCT) system show the biofilm thickness to fall from 122 ± 17 µm to 55 ± 13 µm after 15 min. of exposure of the biofilm to the microplasma column array, when the plasmas are dissipating a power density of 58 W/cm2. All biofilms investigated vanish with 20 min. of exposure. Confocal laser scanning microscopy (CLSM) demonstrates that the number of living cells in the biofilms declines by more than 93% with 15 min. of biofilm exposure to the plasma arrays. Concentrations of several oxygen-bearing species, generated by the plasma array, were found to be 0.4-21 nM/s for the hydroxyl radical (OH), 85-396 nM/s for the 1O2 excited molecule, 98-280 µM for H2O2, and 24-42 µM for O3 when the power density delivered to the array was varied between 3.6 W/cm2 and 79 W/cm2. The data presented here demonstrate the potential of microplasma arrays as a tool for controlling, through non-thermal disruption and removal, mixed-species biofilms prevalent in commercial and residential water systems.

Microplasma Jet Arrays as a Therapeutic Choice for Fungal Keratitis.

The clinical impact of microplasma jets on rabbit eyes infected by Candida albicans has been investigated. Arrays of such jets produce low-temperature plasma micro-columns suitable for ophthalmic therapeutics and fungal infections, in particular, and the technology is capable of being scaled to surface areas of at least 10 cm2. Keratitis was induced in the right central corneas of rabbits, whereas the left eyes served as a normal group. The rabbits were divided into the plasma non-treated group (control) and plasma treatment group. Histologic analyses of both groups showed marked reductions in the thickness, angiogenesis, and opacity of all rabbit corneas following plasma treatment. Indeed, for treatment times beyond 14 days, infected eyes exhibited no significant differences from the normal group. Healing of rabbit eyes infected by Candida albicans apparently proceeds by disrupting corneal epithelial proliferation, and by reducing fibrotic changes in the stroma. This study demonstrates that low-temperature plasma jets are remarkably effective in healing Candida albicans-infected corneas, thereby providing a promising medical treatment option for keratitis.

Lesions of ultrasound-induced lung hemorrhage are not consistent with thermal injury.

Thermal injury, a potential mechanism of ultrasound-induced lung hemorrhage, was studied by comparing lesions induced by an infrared laser (a tissue-heating source) with those induced by pulsed ultrasound. A 600-mW continuous-wave CO2 laser (wavelength approximately 10.6 microm) was focused (680-microm beamwidth) on the surface of the lungs of rats for a duration between 10 to 40 s; ultrasound beamwidths were between 310 and 930 microm. After exposure, lungs were examined grossly and then processed for microscopic evaluation. Grossly, lesions induced by laser were somewhat similar to those induced by ultrasound; however, microscopically, they were dissimilar. Grossly, lesions were oval, red to dark red and extended into subjacent tissue to form a cone. The surface was elevated, but the center of the laser-induced lesions was often depressed. Microscopically, the laser-induced injury consisted of coagulation of tissue, cells and fluids, whereas injury induced by ultrasound consisted solely of alveolar hemorrhage. These results suggest that ultrasound-induced lung injury is most likely not caused by a thermal mechanism.

An experimental burn wound-healing study of non-thermal atmospheric pressure microplasma jet arrays.

In contrast with a thermal plasma surgical instrument based on coagulative and ablative properties, low-temperature (non-thermal) non-equilibrium plasmas are known for novel medicinal effects on exposed tissue while minimizing undesirable tissue damage. In this study we demonstrated that arrays of non-thermal microplasma jet devices fabricated from a transparent polymer can efficiently inactivate fungi (Candida albicans) as well as bacteria (Escherichia coli), both in vitro and in vivo, and that this leads to a significant wound-healing effect. Microplasma jet arrays offer several advantages over conventional single-jet devices, including superior packing density, inherent scalability for larger treatment areas, unprecedented material flexibility in a plasma jet device, and the selective generation of medically relevant reactive species at higher plasma densities. The therapeutic effects of our multi-jet device were verified on second-degree burns in animal rat models. Reduction of the wound area and the histology of the wound after treatment have been investigated, and expression of interleukin (IL)-1α, -6 and -10 was verified to evaluate the healing effects. The consistent effectiveness of non-thermal plasma treatment has been observed especially in decreasing wound size and promoting re-epithelialization through collagen arrangement and the regulation of expression of inflammatory genes.

Injection-seeded optoplasmonic amplifier in the visible.

A hybrid optoplasmonic amplifier, injection-seeded by an internally-generated Raman signal and operating in the visible (563-675 nm), is proposed and evidence for amplification is presented. Comprising a gain medium tethered to a whispering gallery mode (WGM) resonator with a protein, and a plasmonic surface, the optical system described here selectively amplifies a single (or a few) Raman line(s) produced within the WGM resonator and is well-suited for routing narrowband optical power on-a-chip. Over the past five decades, optical oscillators and amplifiers have typically been based on the buildup of the field from the spontaneous emission background. Doing so limits the temporal coherence of the output, lengthens the time required for the optical field intensity to reach saturation, and often is responsible for complex, multiline spectra. In addition to the spectral control afforded by injection-locking, the effective Q of the amplifier can be specified by the bandwidth of the injected Raman signal. This characteristic contrasts with previous WGM-based lasers and amplifiers for which the Q is determined solely by the WGM resonator.

External cavity laser biosensor.

Utilizing a tunable photonic crystal resonant reflector as a mirror of an external cavity laser cavity, we demonstrate a new type of label-free optical biosensor that achieves a high quality factor through the process of stimulated emission, while at the same time providing high sensitivity and large dynamic range. The photonic crystal is fabricated inexpensively from plastic materials, and its resonant wavelength is tuned by adsorption of biomolecules on its surface. Gain for the lasing process is provided by a semiconductor optical amplifier, resulting in a simple detection instrument that operates by normally incident noncontact illumination of the photonic crystal and direct back-reflection into the amplifier. We demonstrate single-mode, biomolecule-induced tuning of the continuous-wave laser wavelength. Because the approach incorporates external optical gain that is separate from the transducer, the device represents a significant advance over previous passive optical resonator biosensors and laser-based biosensors.