Perpendicular Illumination Research Articles

Best practices in measuring absorption at the macro- and microscale

The fraction of light absorbed in a material is a key parameter for a wide range of optoelectronic and energy devices, including solar cells, light emitting diodes, and photo(electro)chemical devices. It can reveal detailed performance information and establish a material’s theoretical efficiency limits. However, measuring absorption accurately is challenging, especially due to scattering effects at the macroscale and achieving perpendicular illumination over a small area at the microscale. In this tutorial, we present concepts and best practices in measuring absorption at both the macro- and micro-scale. We also give examples of using absorption to reveal critical optoelectronic information in energy devices. This work aims at standardizing the recording of absorption measurements across a number of fields, allowing for improved microscale understanding of a wide range of samples.

Plasmonic metaresonances: harnessing nonlocal effects for prospective biomedical applications

Metal nanoparticles (MNPs) possess optical concentration capabilities that can amplify and localize electromagnetic fields into nanometer length scales. The near-fields of MNPs can be used to tailor optical response of luminescent semiconductor quantum dots (QDs), resulting in fascinating optical phenomena. Plasmonic metaresonances (PMRs) form a class of such optical events gaining increasing popularity due to their promising prospects in sensing and switching applications. Unlike the basic excitonic and plasmonic resonances in MNP-QD nanohybrids, PMRs occur in the space/time domain. A nanohybrid experiences PMR when system parameters such as QD dipole moment, MNP-QD centre separation or submerging medium permittivity reach critical values, resulting in the plasmonically induced time delay of the effective Rabi frequency experienced by the QD asymptotically tending to infinity. Theoretical analyses of PMRs available in the literature utilize the local response approximation (LRA) which does not account for the nonlocal effects of the MNP, and neglect the MNP dependence of the QD decay and dephasing rates which hinder their applicability to QDs in the close vicinity of small MNPs. Here, we address these limitations using an approach based on the generalized nonlocal optical response (GNOR) theory which has proven to yield successful theoretical explanations of experimentally observed plasmonic phenomena. Our results indicate that, omission of the MNP nonlocal response and the associated decay/dephasing rate modifications of the QD tend to raise implications such as significant over-estimation of the QD dipole moment required to achieve PMR, under-estimation of the critical centre separation and prediction of significantly lower near-PMR QD absorption rates, in comparison to the improved GNOR based predictions. In light of our observations, we finally suggest two prospective applications of PMR based nanoswitches, namely, aptamer based in vitro cancer screening and thermoresponsive polymer based temperature sensing. To demonstrate the latter application, we develop and utilize a proof of concept (two dimensional) skin tumor model homogeneously populated by MNP-QD nanohybrids. Our simulations reveal a novel near-PMR physical phenomenon observable under perpendicular illumination, which we like to call the margin pattern reversal, where the spatial absorption pattern reverses when the near-PMR QDs switch from the bright to dark state.

Scanning diffracted-light microscopy.

Scanning the direction of the light that is diffracted by a sample permits the achievement of image diversity, which is necessary for implementing the Fourier ptychographic microscopy technique (FPM) using only perpendicular illumination. We also demonstrated that the same method allows for implementation of the illumination-direction-multiplexing FPM technique when the sample is illuminated using a ring-shaped condenser.

Nano-optical single-photon response mapping of waveguide integrated molybdenum silicide (MoSi) superconducting nanowires.

We present low temperature nano-optical characterization of a silicon-on-insulator (SOI) waveguide integrated SNSPD. The SNSPD is fabricated from an amorphous Mo83Si17 thin film chosen to give excellent substrate conformity. At 350 mK, the SNSPD exhibits a uniform photoresponse under perpendicular illumination, corresponding to a maximum system detection efficiency of approximately 5% at 1550 nm wavelength. Under these conditions 10 Hz dark count rate and 51 ps full width at half maximum (FWHM) timing jitter is observed.

Resonant absorption in semiconductor nanowires and nanowire arrays: Relating leaky waveguide modes to Bloch photonic crystal modes

We present a unified framework for resonant absorption in periodic arrays of high index semiconductor nanowires that combines a leaky waveguide theory perspective and that of photonic crystals supporting Bloch modes, as array density transitions from sparse to dense. Full dispersion relations are calculated for each mode at varying illumination angles using the eigenvalue equation for leaky waveguide modes of an infinite dielectric cylinder. The dispersion relations along with symmetry arguments explain the selectivity of mode excitation and spectral red-shifting of absorption for illumination parallel to the nanowire axis in comparison to perpendicular illumination. Analysis of photonic crystal band dispersion for varying array density illustrates that the modes responsible for resonant nanowire absorption emerge from the leaky waveguide modes.

Experiments on Tree Reflectivity at C, X, K and Ka-bands

Dedicated field portable radars have been used to measure the reflectivity of identified deciduous and coniferous trees at C to Ka frequency bands with perpendicular illumination. Actual test radars, their electrical parameters including antennas and calibration devices are described. First results show summer time reflectivities from −30 to −10 dB (m2/m2) for deciduous trees and from −30 to 0 dB (2/m2) for coniferous samples, depending on frequency band. Late autumn and spring measurements show particularly well in Ka-band the absence of leaves. Variations in histogram detail, from one frequency to another, exceed 10 dB despite the cumulative probabilities for all bands are quite similar. The measured power spectral density of deciduous trees in the spring was typically 20 dB/Hz at X-band and 30 dB/Hz in Ka-band. Midsummer tests gave values close to 15 and 25 dB/Hz.

Dispersion, Wave Propagation and Efficiency Analysis of Nanowire Solar Cells

We analyze the electromagnetic properties of InP/InAs nanowire solar cells for different geometries. We address both eigenvalue calculations to determine the wave propagation as well as source problems to simulate direct perpendicular illumination by three-dimensional finite element calculations. We demonstrate the validity of a 2D waveguide modal analysis as a method of estimating the results of the computationally far more demanding 3D analysis. The resulting data is employed in a detailed balance analysis in order to determine the optimum set of bandgap energies for a single-junction and dual-junction cell as well as the corresponding efficiency limit. The efficiency of the nanowire design can approach the efficiency of conventional thin-film designs despite the low volume fill-factor.

Polarimetric Ka-band backscattering measurements at grazing-angle extremes

Polarimetric millimeter-wave backscattering data has been measured in true operational environments for perpendicular illumination and for grazing angles below one degree. Manmade objects, such as vehicles and light poles, were found to give a 5-10 dB lower response for vertical polarization, and the cross-polarized returns were about 10-20 dB below the reference level obtained with horizontal polarization. Forest and soil backscattering showed similar characteristics of -7 to -9 dB (vertical) and -14 to -17 dB (cross-polar). An anomaly was observed for horizontally transmitted/vertically received data, where gravel soil showed only about 2 dB attenuation compared to the co-polar reference level. The dynamic variations of mobile target signals at different polarizations indicated that millimeter-wave cross-polarized returns call for high transmitter power if true polarimetric target classification is attempted. Close-to-ground radars seem to benefit from multipath and thus from using horizontal polarization if only maximum range is defined. Variations in the cross-polarized echo amplitudes of moving manmade objects were found to be within 3-5 dB of their typical mean values

Photoswitchable Smectic Liquid‐Crystalline Elastomers

AbstractSummary: We succeeded in the synthesis of azo side chain containing polysiloxanes with broad smectic C* and A phases. In these polymers the phase transition temperatures can be shifted reversibly by up to 17 °C by irradiation with UV (cis) or VIS (trans) light. Thin films of these polymers in the smectic phase (both on substrates and as free‐standing films) orient perfectly in a homeotropic manner. As a consequence, the azo chromophores do no longer absorb during a perpendicular illumination with light (dichroism). It is thus possible to crosslink these films photochemically to prepare “photoswitchable smectic LC elastomers”.The trans‐cis isomerization in homeotropically oriented LC elastomers.magnified imageThe trans‐cis isomerization in homeotropically oriented LC elastomers.

Chloroplast movements in the field

ABSTRACTAn ecophysiological understanding of chloroplast movements in leaves requires measurement of these movements under field conditions. A field‐portable instrument was constructed, based on a pulsed measuring beam and lock‐in detection that measures chloroplast movements in attached leaves by sensing the resultant changes in leaf transmittance. In the instrument and generally in nature, leaves are illuminated obliquely, in contrast with the perpendicular illumination used in most laboratory experiments on chloroplast movement. Microscopic analysis of cells illuminated obliquely with bright light verified that chloroplasts move out of the light path, and transmittance changes in response to oblique light were robust. Chloroplast movements in Alocasia brisbanensis under natural sunlight express kinetics and light requirements expected from laboratory observations: chloroplasts were in the periclinal position at dawn and dusk, anticlinal position in full sunlight midday, and in an intermediate position at night. Movement in response to bright light was rapid allowing responses to brief sunflecks. Movements in Helianthus tuberosum, Eustrephus latifolius and Cissus hypoglauca were qualitatively similar with differing kinetics and magnitude. In all four species, chloroplasts were in motion most of the time, rarely achieving the extreme anticlinal or periclinal positions.

Cylindrical localized approximation to speed up computations for Gaussian beams in the generalized Lorenz–Mie theory for cylinders, with arbitrary location and orientation of the scatterer

A cylindrical localized approximation to speed up numerical computations in generalized Lorenz-Mie theory for cylinders, in a special case of perpendicular illumination, was recently introduced and rigorously justified. We generalize this approximation to the case when the cylinder is arbitrarily located and arbitrarily oriented in a Gaussian beam.

Applicability conditions of the Kubelka–Munk theory

The description of optical properties of light-scattering materials has made extensive use of radiative transfer models. One of the most successful and simplest models is that of Kubelka and Munk (KM). With this model, optical properties of particulate films under diffuse illumination can be predicted from effective absorption and scattering coefficients of the material. We consider the applicability conditions of this kind of model. An extended KM model for the case of perpendicular collimated illumination is compared with results from a more general four-flux approach, and the differences between them are characterized in terms of a correction factor that depends on particle scattering and absorption, concentration of the scatterers, and film thickness. It is proved formally that the extended KM model under perpendicular illumination is a good approximation for the cases of optically thick films that contain weakly or nonabsorbing particles.

Analysis of Coincidence Losses for a Monitor of Particle Contamination on Surfaces

A widely used instrument (manufactured by Inspex, Incorporated) for detecting, sizing, and counting particles on silicon wafer surfaces determines particle size by the number of pixels on a vidicon‐type screen that are activated by light scattered from particles during oblique observation of perpendicular illumination. The particle counting ability of this surface monitor is affected by the number of particles present, their sizes, the sensitivity of the tool, and the system optical and electrical noise. In this system, the overlapping of the responses of two or more particles causes them to be counted as a single particle of a larger size. We present a method for estimating the fraction of the particles that are not involved in coincidence. The full solution for the fraction of particles in any size category overlapped by those in any other category is presented. Two limits are developed: one for particles all the same size (that is, each giving the same response) and one for particles of very different sizes, the smallest being much more numerous. Practical situations are expected to fall between these two limits. It is shown that for only 5% of the pixels of the surface monitor's screen activated, between 5% and 18% (on average) of the particles would not register as singlets, due to coincidence. For a given total number of pixels activated, coincidence errors are greatest where the particles each give the same size response.

Phototropic Response to Vectorial Light in Leaves of Lavatera cretica L

The mechanism by which the mature leaf of certain plants reorients its lamina to face the sun throughout the day was studied in Lavatera cretica L. The photoreceptor for this response differs fundamentally from the one involved in the phototropic growth response, by sensing light as a vector, rather than as a difference in luminous flux. The photoreceptor is located in the veins, which radiate in the plane of the lamina from the pulvinus situated at the junction between the lamina and petiole. The integrated response to the messages from the different veins takes place by differential turgor changes in a motor tissue surrounding the central vascular cylinder of the pulvinus, in which the veins coalesce. The differential turgor in the different segments of the motor tissue determines the orientation of the lamina. The photoreceptor reacts only to a parallel light beam striking the vein obliquely (from above). When half of the lamina is shaded, the leaf does not reorient in response to perpendicular illumination and its reorientation in response to an oblique beam is slower and partial, to a greater extend when the half-leaf is centrifugally illuminated than when it is centripetally illuminated. Application of 2,3,5 tri-iodobenzoic acid to the base of the veins in the shaded half-leaf eliminated all restrictions from the response to centrifugal illumination and totally inhibited the response to centripetal illumination. The results are consistent with a hypothesis that centrifugally illuminated veins generate turgor in their associated motor tissue in the pulvinus by activating K(+) uptake, while centripetally illuminated veins cause loss of turgor in their associated motor tissue by deactivating K(+) uptake, which leads to passive leakage of K(+). When the entire lamina is exposed to oblique illumination, the centrifugally illuminated half and the centripetally illuminated half cooperate in the full response. Shaded parts of the lamina apparently interfere with the response by supplying their associated motor tissue with auxin, which presumably causes in it an active export of protons and concomitant uptake of K(+), thereby establishing a static "dark turgor" in it.