Fabrication, characterization, and applications of microlenses

Microlenses (MLs) and microlens arrays (MLAs) are assuming an increasingly important role in optical devices. In response to this rapid evolution in technology, emphasis is being placed on research into new manufacturing methods for these devices as well as the characterization of their performance. This paper provides an overview of the fabrication of MLs and MLAs by electrical, mechanical, chemical, and optical methods. As each processing method has distinct advantages and limitations, the most significant characteristic parameters and the measurement of these parameters are discussed for each method. These parameters are then used as indices to evaluate and improve each of the processing methods. Some examples of practical applications of MLAs, especially for micromechanical optoelectronic devices, are also given. This paper aims to summarize the present development and the state of the art in processing technology of MLs and MLAs.

Femtosecond Laser Fabrication of Cavity Microball Lens (CMBL) inside a PMMA Substrate for Super-Wide Angle Imaging

Since microlenses have to date been fabricated primarily by surface manufacturing, they are highly susceptible to surface damage, and their microscale size makes it cumbersome to handle. Thus, cavity lenses are preferred, as they alleviate these difficulties associated with the surface-manufactured microlenses. Here, it is shown that a high repetition femtosecond laser can effectively fabricate cavity microball lenses (CMBLs) inside a polymethyl methacrylate slice. Optimal CMBL fabrication conditions are determined by examining the pertinent parameters, including the laser processing time, the average irradiation power, and the pulse repetition rates. In addition, a heat diffusion modeling is developed to better understand the formation of the spherical cavity and the slightly compressed affected zone surrounding the cavity. A micro-telescope consisting of a microscope objective and a CMBL demonstrates a super-wide field-of-view imaging capability. Finally, detailed optical characterizations of CMBLs are elaborated to account for the refractive index variations of the affected zone. The results presented in the current study demonstrate that a femtosecond laser-fabricated CMBL can be used for robust and super-wide viewing micro imaging applications.

Assembly of silver nanoparticles on nanowires into ordered nanostructures with femtosecond laser radiation

In this work, we show that well-ordered structures of silver nanoparticles on nanowire substrates can be produced by irradiation with femtosecond (fs) laser pulses at fluences ranging from 10.3 to 15.9  mJ/cm² if the direction of polarization is parallel to the long axis of the nanowire. Experimental results show that a uniformly spaced distribution of nanoparticles is more readily produced on nanowires with lengths L≤2λ, where λ=800  nm is the laser wavelength. The distribution of nanoparticles is found to become less well organized as L≥2λ. Finite element method simulations, combined with experimental observations, indicate that nanoparticles are initially distributed in response to the electric field along the clean Ag nanowire arising from optical excitation. This electric field is responsible for the attraction of nanoparticles to certain locations on the nanowire. We show how a fs-laser-driven assembly of nanoparticles on nanowires can be used in the development of a nanoscale optical logic processor. This method of creating periodic arrays of metallic nanoparticles on nanowire substrates then has many possible applications in electro-optics.

High integrity interconnection of silver submicron/nanoparticles on silicon wafer by femtosecond laser irradiation

Welding of nanomaterials is a promising technique for constructing nanodevices with robust mechanical properties. To date, fabrication of these devices is limited because of difficulties in restricting damage to the nanomaterials during the welding process. In this work, by utilizing very low fluence (∼900 μJ cm(-2)) femtosecond (fs) laser irradiation, we have produced a metallic interconnection between two adjacent silver (Ag) submicron/nanoparticles which were fixed on a silicon (Si) wafer after fs laser deposition. No additional filler material was used, and the connected particles remain almost damage free. Observation of the morphology before and after joining and finite difference time domain simulations indicate that the interconnection can be attributed to plasmonic excitation in the Ag submicron/nanoparticles. Concentration of energy between the particles leads to local ablation followed by re-deposition of the ablated material to form a bridging link that joins the two particles. This welding technique shows potential applications in the fabrication of nanodevices.

The formation of glycine and other complex organic molecules in exploding ice mantles

Complex Organic Molecules (COMs), such as propylene (CH₃CHCH₂) and the isomers of C₂H₄O₂ are detected in cold molecular clouds (such as TMC-1) with high fractional abundances (Marcelino et al., Astrophys. J., 2007, 665, L127). The formation mechanism for these species is the subject of intense speculation, as is the possibility of the formation of simple amino acids such as glycine (NH₂CH₂COOH). At typical dark cloud densities, normal interstellar gas-phase chemistries are inefficient, whilst surface chemistry is at best ill defined and does not easily reproduce the abundance ratios observed in the gas phase. Whatever mechanism(s) is/are operating, it/they must be both efficient at converting a significant fraction of the available carbon budget into COMs, and capable of efficiently returning the COMs to the gas phase. In our previous studies we proposed a complementary, alternative mechanism, in which medium- and large-sized molecules are formed by three-body gas kinetic reactions in the warm high density gas phase. This environment exists, for a very short period of time, after the total sublimation of grain ice mantles in transient co-desorption events. In order to drive the process, rapid and efficient mantle sublimation is required and we have proposed that ice mantle ‘explosions’ can be driven by the catastrophic recombination of trapped hydrogen atoms, and other radicals, in the ice. Repeated cycles of freeze-out and explosion can thus lead to a cumulative molecular enrichment of the interstellar medium. Using existing studies we based our chemical network on simple radical addition, subject to enthalpy and valency restrictions. In this work we have extended the chemistry to include the formation pathways of glycine and other large molecular species that are detected in molecular clouds. We find that the mechanism is capable of explaining the observed molecular abundances and complexity in these sources. We find that the proposed mechanism is easily capable of explaining the large abundances of all three isomers of C₂H₄O₂ that are observationally inferred for star-forming regions. However, the model currently does not provide an obvious explanation for the predominance of methyl formate, suggesting that some refinement to our (very simplistic) chemistry is necessary. The model also predicts the production of glycine at a (lower) abundance level, that is consistent with its marginal detection in astrophysical sources.

Numerical analysis of deep sub-wavelength integrated plasmonic devices based on Semiconductor-Insulator-Metal strip waveguides

We report the first study of nanoscale integrated photonic devices constructed with semiconductor-insulator-metal strip (SIMS) waveguides for use at telecom wavelengths. These waveguides support hybrid plasmonic modes transmitting through a 5-nm thick insulating region with a normalized intensity of 200-300 μm(-2). Their fundamental mode, unique transmission and dispersion properties are consistent with photonic devices for guiding and routing of signals in communication applications. It has been demonstrated using Finite Element Methods (FEM) that the high performance SIMS waveguide can be used to fabricate deep sub-wavelength integrated plasmonic devices such as directional couplers with the ultra short coupling lengths, sharply bent waveguides, and ring resonators having a functional size of ≈1 µm and with low insertion losses and nearly zero radiation losses.

Spectroscopic characterization of carbon chains in nanostructured tetrahedral carbon films synthesized by femtosecond pulsed laser deposition

A comparative study of carbon bonding states and Raman spectra is reported for amorphous diamondlike carbon films deposited using 120 fs and 30 ns pulsed laser ablation of graphite. The presence of sp(1) chains in femtosecond carbon films is confirmed by the appearance of a broad excitation band at 2000-2200 cm(-1) in UV-Raman spectra. Analysis of Raman spectra indicates that the concentrations of sp(1)-, sp(2)-, and sp(3)-bonded carbon are approximately 6%, approximately 43%, and approximately 51%, respectively, in carbon films prepared by femtosecond laser ablation. Using surface enhanced Raman spectroscopy, specific vibrational frequencies associated with polycumulene, polyyne, and trans-polyacetylene chains have been identified. The present study provides further insight into the composition and structure of tetrahedral carbon films containing both sp(2) clusters and sp(1) chains.

Polycyclic aromatic hydrocarbons, carbon nanoparticles and the diffuse interstellar bands

Observational data on the appearance and properties of the diffuse interstellar bands (DIBs) are reviewed in the context of a model in which the proposed carriers of these bands are large carbon molecules and carbon nanoparticles containing between 30 and several hundred carbon atoms. The abundance of these carriers, as estimated from the observed strengths of the DIBs, place strong constraints on their rates of formation and destruction, and suggest that the strongest bands, including that at 4428 A, could be produced via the decomposition of larger carbon particles, possibly those particles that have been postulated to be the source of the 2175 A extinction feature. Such particles are of mixed sp2 and sp3 carbon composition, with sizes between that of large molecules and small macroscopic solids. Any description of their characteristics must combine aspects of molecular and condensed matter physics, and this is incorporated in the present discussion. I discuss recent experimental and theoretical data related to these matters.

Treatment of metal surfaces with excimer laser radiation for radiative applications

In this study the effects of intense excimer laser irradiation (10(8)-10(9) W cm(-2)) on some radiative properties of copper, aluminum, magnesium, tantalum, and molybdenum are investigated. The short pulse duration of the excimer laser (30 ns FWHM) and the wavelength of the radiation initiate a processing regime that is dominated by the effects of a laser-supported plasma above the workpiece surface. A laser-supported detonation wave is observed that acts on a thin molten surface layer. The resultant structure that is frozen on the surface is dependent on the thermophysical properties of the metal and the number and manner in which multiple pulses are applied to the surface. This surface roughness has features on both the macro- and microscopic scale that affect the radiative properties of the surface. The surface plasma also initiates very fast reaction rates that can create a thick oxidized layer on the metal surface. The surface oxide layer may either be lightly (CuO particles on a copper substrate) or tightly adhered (Al(2)O(3) on aluminum) to the surface. The effect of excimer laser irradiation is characterized with scanning electron microscopy and Fourier transform infrared spectroscopy. Data on solar absorptivity α(s), spectral reflectivity ρ'(λ), bidirectional reflectivity ρ″(λ), and total near-normal total emissivity ε'(T) are also presented that show that in some cases heavily irradiated samples simulate the radiative behavior of a flat absorber.

Interaction of TEA CO(2) laser radiation with aerosol particles

Some effects arising from the interaction of TEA CO(2) laser pulses with individual aerosol particles are described. The time dependence of thermal radiation emitted from aerosol particles heated with pulses from a TEA laser is shown to be related to the size of individual particles and to the distribution of sizes within an aerosol. Charge and mass changes have been determined for single particles on absorption of 10.6-microm CO(2) laser radiation. The predominant charging effect at low intensities ( approximately 10(5) W/cm(2)) involves a loss of positive charge. Splitting of particle aggregates has also been observed.

Some Limitations on the Interpretation of Specular Reflection Spectra with an Application to Solid CO

A simple theoretical framework is used to calculate specular reflection spectra in the vicinity of an absorption band in a solid. The energy of the reflectivity peak is shown to differ from the energy of the absorption band even at normal incidence with thick samples and with thin film samples supported on a substrate. By considering the absorption band system of solid CO, synthetic reflection spectra of the Fourth Positive System of solid CO at 45 degrees incidence are obtained that successfully match the experimental results, including striking time-dependent changes.