Infrared Regime Research Articles

Fiber delivery of mid-IR lasers

Fiber optics for the visible to near infrared (NIR) wavelength regimes (i.e. = 0.42 {mu}m) have proven to be extremely useful for a myriad of applications such as telecommunications, illumination, and sensors because they enable convenient, compact, and remote delivery of laser beams. Similarly, there is a need for fiber optics operating at longer wavelengths. For example, systems operating in the mid-IR regime (i.e., = 314 {mu}m) are being developed to detect trace molecular species with far-reaching applications, such as detecting explosives on surfaces, pollutants in the environment, and biomarkers in the breath of a patient. Furthermore, with the increasing availability of quantum cascade lasers (QCLs) which are semiconductor lasers that operate in the mid-IR regime additional uses are rapidly being developed. Here, we describe the development of hollow-core fibers for delivery of high-quality mid-IR laser beams across a broad spectral range.

Continuum random combs and scale-dependent spectral dimension

Numerical computations have suggested that in causal dynamical triangulation models of quantum gravity (CDT) the effective dimension of spacetime in the ultraviolet (UV) is lower than in the infrared (IR). In this paper we develop a simple model based on the previous work on random combs, which share some of the properties of CDT, in which this effect can be shown to occur analytically. We construct a definition for short- and long-distance spectral dimensions and show that the random comb models exhibit scale-dependent spectral dimension defined in this way. We also observe that a hierarchy of apparent spectral dimensions may be obtained in the cross-over region between UV and IR regimes for suitable choices of the continuum variables. Our main result is valid for a wide class of tooth length distributions thereby extending previous work on random combs by Durhuus et al.

TH-E-214-04: MR-Guided Thermal Therapy Using Gold Nanoshells

MRI is increasingly being used to provide imaging-guidance of high- temperature thermal therapies primarily due to its inherent sensitivity to temperature changes which make it attractive for monitoring and control, as well as multiple contrast mechanisms useful for planning, targeting and verifying therapy delivery in a single setting. These advantages have opened the door for a new generation of FDA cleared laser ablation systems which can use MRI guidance. Using compact, solid state lasers operating in the near infrared (NIR) regime along with actively cooled catheters which facilitate use of higher powers and larger lesions, interstitial laser ablation of deep seated lesions in areas such as brain, prostate, liver, kidney and bone are being investigated clinically. Gold nanoparticles (AuNP) can be designed to strongly absorb in the NIR portion of the spectrum. Even in the presence of relatively low applied power, which does not raise tissue temperatures to ablative levels, biologically relevant concentrations of AuNP aptly absorb the NIR energy generating ablative temperatures making them potent mediators of targeted laser induced thermal therapy. Because of their small size and ability to have their surface chemistry modulated for functionality, AuNPˈs may be designed to preferentially accumulate in a variety of diseased tissue. Additionally, unlike previous investigations using molecular dyes, these particles tend to maintain their effectiveness even at higher temperatures or laser power. Here we provide an overview of our experience using MR-guidance and temperature imaging to investigate the potential for using AuNP as mediators for delivering highly conformal laser induced thermal therapy of soft-tissue tumors using phantoms as well as in vivo small and large animal models. MR temperature imaging provides a means to demonstrate feasibility of these therapies in a variety of scenarios as well as monitor and control therapy delivery. In addition, temperature feedback can be used to validate computational models of AuNP mediated therapy as well. In this manner, MRI is used to both verify and shape the approach to these therapies. Educational Objectives: 1. Gain perspective on the potential role of gold nanoparticles in laser induced thermal therapies. 2. Understand the growing role of MR temperature imaging in clinical laser induced thermal therapy. 3. Understand the role of MR temperature imaging in the development and validation of gold nanoparticle mediated laser induced thermal therapies.

Narrowband plasmonic excitation on gold hole-array nanostructures observed using spectroscopic ellipsometer

Surface plasmon polaritons (SPPs) modes on gold hole-array nanostructures were studied using spectroscopy ellipsometer in reflection mode. Using background free techniques in the optical ellipsometer, clear SPP bands on gold nanostructure/air interface were measured in UV-Visible-Near infrared (NIR) regime (300 nm-1800 nm). Plasmonic excitation with bandwidth of 13 nm (FWHM) was observed in reflection measurement, and it is much narrower than that observed in transmission measurement mode. In addition, the plasmonic excitation bands were characterized in both amplitude and phase domain. Theoretical analysis using reciprocal lattice vector method and rigorous coupled wave analysis (RCWA) agreed well with the experimental results. By measuring the TM and TE waves simultaneously, the spectroscopic ellipsometer provides an important method to analyze both amplitude and phase information in plasmonic nanostructures and metamaterials.

Cosmic Strings in a Model of Non-relativistic Gravity

Hořava proposed a non-relativistic renormalizable theory of gravitation, which is reduced to general relativity (GR) in large distances (infra-red regime (IR)). It is believed that this theory is an ultra-violet (UV) completion for the classical theory of gravitation. In this paper, after a brief review of some fundamental features of this theory, we investigate it for a static cylindrical symmetric solution which describes Cosmic string as a special case. We have also investigated some possible solutions, and have seen that how the classical GR field equations are modified for generic potential V(g). In one case there is an algebraic constraint on the values of three coupling constants. Finally as a pioneering work we deduce the most general cosmic string in this theory. We explicitly show that how the coupling constants distort the mass parameter of cosmic string. We deduce an explicit function for mass per unit length of the space-time as a function of the coupling constants. We compare this function with another which Aryal et al. (Phys. Rev. D 34:2263, 1986) have found in GR. Also we calculate the self-force on a massive particle near Hořava-Lifshitz straight string and we give a typical order for the coupling constant g 9. This order of magnitude proposes a cosmological test for validity of this theory.

Method for depth-resolved quantitation of optical properties in layered media using spatially modulated quantitative spectroscopy

We have demonstrated that spatially modulated quantitative spectroscopy (SMoQS) is capable of extracting absolute optical properties from homogeneous tissue simulating phantoms that span both the visible and near-infrared wavelength regimes. However, biological tissue, such as skin, is highly structured, presenting challenges to quantitative spectroscopic techniques based on homogeneous models. In order to more accurately address the challenges associated with skin, we present a method for depth-resolved optical property quantitation based on a two layer model. Layered Monte Carlo simulations and layered tissue simulating phantoms are used to determine the efficacy and accuracy of SMoQS to quantify layer specific optical properties of layered media. Initial results from both the simulation and experiment show that this empirical method is capable of determining top layer thickness within tens of microns across a physiological range for skin. Layer specific chromophore concentration can be determined to <±10% the actual values, on average, whereas bulk quantitation in either visible or near infrared spectroscopic regimes significantly underestimates the layer specific chromophore concentration and can be confounded by top layer thickness.

Long-wavelength infrared quantum-dot based interband photodetectors

We report on the design and fabrication of (Al)GaAs(Sb)/InAs tensile strained quantum-dot (QD) based detector material for thermal infrared imaging applications in the long-wavelength infrared (LWIR) regime. The detection is based on transitions between confined dot states and continuum states in a type-II band lineup, and we therefore refer to it as a dot-to-bulk (D2B) infrared photodetector with expected benefits including long carrier lifetime due to the type-II band alignment, suppressed Shockley–Read–Hall generation–recombination due to the relatively large-bandgap matrix material, inhibited Auger recombination processes due to the tensile strain and epitaxial simplicity. Metal–organic vapor-phase epitaxy was used to grow multiple (Al)GaAs(Sb) QD layers on InAs substrates at different QD nominal thicknesses, compositions, doping conditions and multilayer periods, and the material was characterized using atomic force and transmission electron microscopy, and Fourier-transform infrared absorption spectroscopy. Dot densities up to 1 × 10 11 cm −2, 1 × 10 12 cm −2 and 3 × 10 10 cm −2 were measured for GaAs, AlGaAs and GaAsSb QDs, respectively. Strong absorption in GaAs, AlGaAs and GaAsSb multilayer QD samples was observed in the wavelength range 6–12 μm. From the wavelength shift in the spectral absorption for samples with varying QD thickness and composition it is believed that the absorption is due to an intra- valance band transition. From this it is possible to estimate the type-II interband transition wavelength, thereby suggesting that (Al)GaAs(Sb) QD/InAs heterostructures are suitable candidates for LWIR detection and imaging.

Emergent relativistic-like kinematics and dynamical mass generation for a Lifshitz-type Yukawa model

We study the Infra Red (IR) limit of dispersion relations for scalar and fermion fields in a Lifshitz-type Yukawa model, after dressing by quantum fluctuations. Relativistic-like dispersion relations emerge dynamically in the IR regime of the model, after quantum corrections are taken into account. In this regime, dynamical mass generation also takes place, but in such a way that the particle excitations remain massive, even if the bare masses vanish. The group velocities of the corresponding massive particles of course are smaller than the speed of light, in a way consistent with the IR regime where the analysis is performed. We also comment on possible extensions of the model where the fermions are coupled to an Abelian gauge field.

Concave Plasmonic Particles: Broad-Band Geometrical Tunability in the Near-Infrared

Optical resonances spanning the near-infrared spectrum (1-2 microm) were exhibited experimentally by arrays of plasmonic nanoparticles with concave cross-section. The concavity of the particle was shown to be the key component for enabling the broad band tunability of the resonance frequency, even for particles with dimensional aspect ratios of order unity. The atypical flexibility of setting the resonance wavelength is shown to stem from a unique interplay of local geometry with surface charge distributions.

Photocatalytic properties of porous silicon nanowires

Porous silicon nanowires are synthesized through metal assisted wet-chemical etch of highly-doped silicon wafer. The resulted porous silicon nanowires exhibit a large surface area of 337 m(2)·g(-1) and a wide spectrum absorption across the entire ultraviolet, visible and near infrared regime. We further demonstrate that platinum nanoparticles can be loaded onto the surface of the porous silicon nanowires with controlled density. These combined advancements make the porous silicon nanowires an interesting material for photocatalytic applications. We show that the porous silicon nanowires and platinum nanoparticle loaded porous silicon nanowires can be used as effective photocatalysts for photocatalytic degradation of organic dyes and toxic pollutants under visible irradiation, and thus are of significant interest for organic waste treatment and environmental remediation.

Potentiality of Metal–Oxide–Semiconductor Silicon Optical Modulator Based on Free Carrier Absorption

Light propagation in a metal–oxide–semiconductor optical modulator based on free carrier absorption is analyzed theoretically. The analysis is based on Marcatili's approximation taking account of absorption by free carriers in the inversion layer. The gate voltage and wavelength dependences of propagation loss and extinction ratio are also evaluated. The free carrier absorption in the proposed modulator is appropriately confirmed by comparing theoretical results with the experimental results of fabricated optical modulators on silicon-on-insulator wafers; however, practical use of the device is limited since the extinction ratio remains small owing to a weak interaction between light and inversion carriers at 1.55-µm-wavelength light. By theoretical analyses, a large extinction ratio is obtained for possible applications in the deeper infrared regime because free carrier absorption increases with wavelength. A much larger extinction ratio is expected when the interaction between guided waves and surface plasmons occurs.

High-efficiency photonic crystal solar cell architecture

Thin silicon solar cells suffer from low light absorption compared to their thick counterparts, especially in the near infra-red regime. In order to obtain high energy conversion efficiency in thin solar cells, an efficient light trapping scheme is required. In this paper, we theoretically demonstrate significant enhancement in efficiency of thin crystalline silicon solar cells by using photonic crystals as the light absorbing layer. In particular, a relative increase of 11.15% and 3.87% in the energy conversion efficiency compared to the optimized conventional design is achieved for 2 microm and 10 microm thicknesses, respectively.

Clickable fluorophores for biological labeling—with or without copper

The synthesis of a set of new clickable fluorophores that virtually cover the whole visible spectrum reaching the near infra-red regime is presented herein. Besides dyes that are capable of participating in classical copper catalyzed 1,3-dipolar cycloaddition reactions with the counterparting function we have also prepared dyes containing a cyclooctyne moiety, an alkyne derivative that enables copper free clicking to azides. The suitability of these dyes for fluorescent labeling of biomolecules is presented by examples on model frameworks representing major biopolymer building blocks. The versatility of these dyes is presented in cell labeling experiments as well as by labeling the azide modified surface glycans of CHO-cells either by copper catalyzed or copper-free click reaction. These dyes are expected to have a large variety of applications in (bio)orthogonal labeling schemes both in vivo and in vitro.

Two-photon absorption and optical power limiting properties in femtosecond regime of novel multi-branched chromophores based on tri-substituted olefinic scaffolds

Two new multi-branched model fluorophores based on tri-substituted olefinic skeletons had been synthesized and their degenerate two-photon absorption properties were characterized by ultra-short laser pulses in the femtosecond regime. The experimental results show that the studied chromophore structures exhibit relatively strong and wide-dispersed two-photon activities in the near infra-red (NIR) regime. It is demonstrated that molecular structures with expanded π-frameworks derived from multi-substituted olefin moieties would possess large molecular nonlinear absorptivities within the studied spectral region. Two-photon absorption based optical power limiting behaviors in the femtosecond time domain of these model compounds were also explored and the results indicate that such a structural motif could be a useful molecular design approach towards strong two-photon absorbing material systems for quick-responsive and broadband optical-suppressing related applications, especially when against ultra-short laser pulses.

Observation of Quantum Beating from Two Coupled Parametric Six-Wave Mixing Signals in Rb

Two processes of coupled difference-frequency axially phase-matched parametric six-wave mixing are carried out in Rb vapour by two-photon excitation using fs laser pulses, and parametric six-wave mixing signals in the infrared and near infrared regime are detected. The infrared parametric six-wave mixing signals are up-converted into the visible spectral range by sum-frequency mixing with the pump laser in a LHO3 crystal. Moreover, quantum beating at 608 cm-1, corresponding to the 7 s – 5d energy difference in Rb, is observed from the sum-frequency signal at 495 nm. As a result, we obtain modulated light signals in the visible, near infrared and infrared spectral ranges, and study the interference between 7s and 5d states of Rb.

Temperature measurements of expansion products from shock compressed materials using high-speed spectroscopy

Results from spectral radiance measurements using optical multi-channel analyzer over the visible and near infrared regime provide estimates of temperature from expansion products resulting from shocked materials. Specifically, we have made spectral radiance measurements over the wavelength regime of 300–1500 nm. Experiments conducted on aluminum, cerium, and Composition-B high explosive span a wide regime of E/E_v, where E is the internal energy increase of the shocked material, and E_v, is the specific energy required to vaporize the material. For the materials investigated, the ratio is ~ 1, 3 and 5 for aluminum, cerium, and Composition-B, respectively. The basic assumption made to deduce these temperatures is that the debris cloud is radiating as a blackbody with emissivity of one and independent of the wavelength. We are also assuming that the probe is monitoring the debris, which is at a single temperature and that there is no spatial temperature gradient. Temperatures at or above the boiling point are confirmed for aluminum and cerium, while the results for Composition-B provide the time-dependent temperature expansion history for shocked Composition-B over the stress regime of 28–130 GPa. These are the first measurements of temperature obtained from the expansion products from materials that have been shocked to very high pressures.

Fabrication of near infrared metallodielectric photonic crystal using metal-coated dielectric spheres

A metallodielectric photonic crystal with photonic band gaps in near infrared regime has been constructed using layer-by-layer stacking of two-dimensional micro-size metal-coated dielectric spheres array. In transmission spectra two photonic band gaps are observed at 1.38 μm and 2.46 μm, which are in agreement with theoretical calculations. Experimental results show that the photonic band gaps can be realized with about ten layers. The structure with metallic microspheres provides us a novel way for fabrication of near infrared metallic photonic crystals.

Semiclassical spinning strings and confining gauge theories

We study multi-charged rotating string states on type IIB regular backgrounds dual to confining SU( N) gauge theories with (softly broken) N=1 supersymmetry, in the infra red regime. After exhibiting the classical energy/charge relations for the folded and circular two-charge strings, we compute in the latter case the one loop sigma-model quantum correction. The classical relation has an expansion in positive powers of the analogous of the BMN effective coupling, while the quantum corrections are non-perturbative in nature and are not subleading in the limit of infinite charge. We comment about the dual field theory multi-charged hadrons and the implications of our computation for the AdS/ N=4 duality.

Softening the supersymmetric flavor problem in orbifold grand unified theories

The infra-red attractive force of the bulk gauge interactions is applied to soften the supersymmetric flavor problem in the orbifold SU(5) GUT of Kawamura. Then this force aligns in the infra-red regime the soft supersymmetry breaking terms out of their anarchical disorder at a fundamental scale, in such a way that flavor-changing neutral currents as well as dangerous CP-violating phases are suppressed at low energies. It is found that this dynamical alignment is sufficiently good compared with the current experimental bounds, as long as the diagonalization matrices of the Yukawa couplings are CKM-like.

Reversibly strain-tunable elastomeric photonic crystals

Reversibly strain-tunable polymeric photonic crystals made of thermoplastic polyester elastomer (TPEE) were fabricated by using the self-assembled silica opals as templates. The stop band of the polymeric photonic crystal locates at the near infrared (IR) regime in its transmission spectrum, and exhibits a blue shift with the increase of the incident angle. Because of the elasticity of the TPEE, the stop band of the TPEE photonic crystal can also be reversibly tuned at ambient temperature through to and fro uniaxially or biaxially stretching and recovering by changing the lattice spacing and the symmetry of the crystal along (1 1 1) plane.