Vladimir Liberman

A Nanoparticle Convective Directed Assembly Process for the Fabrication of Periodic Surface Enhanced Raman Spectroscopy Substrates

Surface enhanced Raman scattering was discovered over 30 years ago, when it was noted that the usually weak molecular Raman scattering cross section was increased by orders of magnitude in the vicinity of metal surfaces. [ 1 , 2 ] Despite the early reports of single-molecule detection, [ 3 ] the promise of the technique as the basis for portable chemical sensors has not been fully realized. The reason for this gap between the science and the engineering of SERS lies in the formidable nanofabrication challenges it poses, which is the need to prepare large numbers of very small yet highly controlled “hot spots” as the sensing device. The need for “hot spots” arises because the SERS enhancement is composed of an electromagnetic effect and a chemical (or resonance) enhancement, with the electromagnetic effect being responsible for the majority of the enhancement. The “hot spots” are the manifestations of this fi eld enhancement, occurring only for select plasmonic materials. [ 4 ] Modeling studies of the electromagnetic effect indicate that dimerized plasmonic metal structures offer a signifi cantly higher electromagnetic fi eld enhancement than isolated structures, with the maximum fi elds occurring in the gap between the structures. [ 5 , 6 ] The fi eld enhancement dependence on the gap size is highly nonlinear: it becomes signifi cant when the gap is less than ∼ 10 nm, and then rises steeply with decreasing gap size. The importance of the small gap size on signal strength has been confi rmed experimentally. [ 7 , 8 ] While the SERS effect may be very high in a localized volume of the order of a few nm 3 , the probability that target analyte molecules adsorb there is very small. [ 9 ] Consequently, practical SERS-based sensors require the engineering of a very large number of such “hot spots” over areas that are at least a few mm 2 . Small interparticle spacing with a large number of metal particles has been realized for SERS experiments performed in nanoparticle solutions, which have been “activated” by electrolyte-induced aggregation. [ 10 ] However, solution-based

Periodically poled BaMgF 4 for ultraviolet frequency generation

Ferroelectric domain inversion has been demonstrated in BaMgF(4) . Transparency has been measured to <140nm, and no change in transmission was measured under 157-nm irradiation for >1.1x10(9) shots at 2mJ/cm(2) per pulse. First-order quasi-phase-matched generation of 157 nm is predicted by use of grating periods as long as 1.5mum. This material should permit shorter-wavelength chi((2)) frequency-mixing processes than with any other crystalline material.

Materials issues for optical components and photomasks in 157 nm lithography

Photolithography using 157 nm pulsed fluorine lasers has emerged as the leading candidate technology for the post-193-nm generation. Preliminary data have indicated that at 157 nm there are optical materials transparent enough to enable the fabrication of refractive elements, both in the projection and illumination part of the optical train. However, a number of critical issues still remain. Optical materials must show no appreciable degradation with laser irradiation. The availability of transparent photomask substrates must be ascertained. Optical coatings must be developed and qualified. At this short wavelength, interface effects, subsurface damage, and adsorbate effects become increasingly prominent. We present recent experimental results on the durability tests of calcium fluoride, modified fused silica, and optical coatings for 157 nm applications. Our initial assessment of several grades of modified fused silica demonstrates that at least one grade already meets transparency and durability require...

Super‐resolution microscopy by movable thin‐films with embedded microspheres: Resolution analysis

Microsphere-assisted imaging emerged as a surprisingly simple way of achieving optical super-resolution imaging. In this work, we use movable PDMS thin films with embedded high-index barium titanate glass microspheres a sample scanning capability was developed, thus removing the main limitation of this technology based on its small field-of-view.

Movable thin films with embedded high-index microspheres for super-resolution microscopy

Microsphere-assisted imaging emerged as a surprisingly simple way of achieving optical super-resolution imaging. In this work, we use movable PDMS thin films with embedded high-index barium titanate glass microspheres a sample scanning capability was developed, thus removing the main limitation of this technology based on its small field-of-view.

Liquid immersion lithography: Why, how, and when?

Liquid immersion lithography, especially at 193nm, is a serious candidate for extending projection optical lithography to the 65nm node and beyond. This article reviews the status of this technology, the potential pitfalls that it may still encounter, and also the potential to extend it to 157nm and to higher-index liquids. At 193nm, no fundamental obstacles have been found yet, although defect control and materials compatibility must still be worked out. At 157nm, significant progress has been made in developing suitable liquids. The next hurdle is to increase their refractive index, in order to make the transition in wavelengths cost-effective.

157 nm: Deepest deep-ultraviolet yet

Lithography at 157 nm is rapidly emerging as the industry-preferred technology for the post-193 nm era. Its target application is for the 100 to 70 nm generations, and it is therefore widely viewed as a “bridge” technology before the next-generation lithographies are ready for insertion into manufacturing. Its attractiveness stems from the overlap in many areas with current practice and shared infrastructure developed for longer wavelengths. This article will review the present status of 157 nm lithography, emphasizing the technological challenges in the various subsystems: lasers, optical materials and coatings, photomask materials, photoresists, and projection tool development. Viewed as a whole, recent developments in these diverse areas are cause for cautious optimism that indeed 157 nm lithography will be ready in time, without encountering unforeseen obstacles.

Excimer-laser-induced degradation of fused silica and calcium fluoride for 193-nm lithographic applications.

We report the initial results of a large-scale evaluation of production-grade fused silica and calcium fluoride to be used in 193-nm lithographic applications. The samples have been provided by many different suppliers of materials. A marathon irradiation chamber permits simultaneous exposure of as many as 36 samples at 800 Hz, at fluences from 0.2 to > or =4 (mJ/cm(2))/pulse and pulse counts in excess of 10(9) . The initial absorption and the laser-induced absorption are found to vary over a wide range. The compaction of each fused-silica sample follows a power law, but its parameters can differ widely from sample to sample.

Rational design and optimization of plasmonic nanoarrays for surface enhanced infrared spectroscopy

We present an approach for rational design and optimization of plasmonic arrays for ultrasensitive surface enhanced infrared absorption (SEIRA) spectroscopy of specific protein analytes. Motivated by our previous work that demonstrated sub-attomole detection of surface-bound silk fibroin [Proc. Natl. Acad. Sci. U.S.A. 106, 19227 (2009)], we introduce here a general framework that allows for the numerical optimization of metamaterial sensor designs in order to maximize the absorbance signal. A critical feature of our method is the explicit compensation for the perturbative effects of the analytes refractive index which alters the resonance frequency and line-shape of the metamaterial response, thereby leading to spectral distortion in SEIRA signatures. As an example, we leverage our method to optimize the geometry of periodic arrays of plasmonic nanoparticles on both Si and CaF2 substrates. The optimal geometries result in a three-order of magnitude absorbance enhancement compared to an unstructured Au layer, with the CaF2 substrate offering an additional factor of three enhancement in absorbance over a traditional Si substrate. The latter improvement arises from increase of near-field intensity over the Au nanobar surface for the lower index substrate. Finally, we perform sensitivity analysis for our optimized arrays to predict the effects of fabrication imperfections. We find that <20% deviation from the optimized absorbance response is readily achievable over large areas with modern nanofabrication techniques.

Aluminum plasmonics: optimization of plasmonic properties using liquid-prism-coupled ellipsometry

We have established a method to quantify and optimize the plasmonic behavior of aluminum thin films by coupling spectroscopic ellipsometry into surface plasmon polaritons using a liquid prism cell in a modified Otto configuration. This procedure was applied to Al thin films deposited by four different methods, as well as to single crystal Al substrates, to determine the broadband optical constants and calculate plasmonic figures of merit. The best performance was achieved with Al films that have been sputter-deposited at high temperatures of 350°C, followed by chemical mechanical polishing. This combination of temperature and post-processing produced aluminum films with both large grain size and low surface roughness. Comparing these figures of merit with literature values of gold, silver, and copper shows that at blue and ultraviolet wavelengths, optimized aluminum has the highest figure of merit of all materials studied. We further employ the Ashcroft and Sturm theory of optical conductivity to extract the electron scattering times for the Drude and effective interband transitions, interband transition energies, and the optical mass of electrons.