Design guidelines for perovskite/silicon 2-terminal tandem solar cells: an optical study.

Perovskite/silicon 2-terminal tandem cells have made significant advances towards >25% efficiency. Despite this, there is limited understanding of how the optical properties of the materials affect the optical losses within the tandem cell. Using an accurate optical model, we investigate, identify and propose solutions to the optical loss mechanisms inherent in a typical perovskite/silicon 2-terminal tandem cell. The results highlight, firstly, the requirement for low absorption in all layers above the perovskite film, and secondly, the importance of the proper choice of refractive index and thickness of charge transport layers of the perovskite cell, in order to minimize reflection at the interfaces formed by these layers. We demonstrate that the proper choice of these parameters is based on, and can be guided by, basic optics principles which serve as design guidelines. With careful selection of charge transport materials, optimization of the perovskite absorber thickness and the introduction of light trapping within the silicon cell, a matched current of over 20 mA/cm2 can be realized, enabling efficiencies greater than 30% using currently available cell processing methods and materials.

Altered resting-state functional connectivity in emotion-processing brain regions in adults who were born very preterm.

BACKGROUND: Very preterm birth (VPT; <32 weeks of gestation) has been associated with impairments in emotion regulation, social competence and communicative skills. However, the neuroanatomical mechanisms underlying such impairments have not been systematically studied. Here we investigated the functional integrity of the amygdala connectivity network in relation to the ability to recognize emotions from facial expressions in VPT adults. METHOD: Thirty-six VPT-born adults and 38 age-matched controls were scanned at rest in a 3-T MRI scanner. Resting-state functional connectivity (rs-fc) was assessed with SPM8. A seed-based analysis focusing on three amygdalar subregions (centro-medial/latero-basal/superficial) was performed. Participants' ability to recognize emotions was assessed using dynamic stimuli of human faces expressing six emotions at different intensities with the Emotion Recognition Task (ERT). RESULTS: VPT individuals compared to controls showed reduced rs-fc between the superficial subregion of the left amygdala, and the right posterior cingulate cortex (p = 0.017) and the left precuneus (p = 0.002). The VPT group further showed elevated rs-fc between the left superficial amygdala and the superior temporal sulcus (p = 0.008). Performance on the ERT showed that the VPT group was less able than controls to recognize anger at low levels of intensity. Anger scores were significantly associated with rs-fc between the superficial amygdala and the posterior cingulate cortex in controls but not in VPT individuals. CONCLUSIONS: These findings suggest that alterations in rs-fc between the amygdala, parietal and temporal cortices could represent the mechanism linking VPT birth and deficits in emotion processing.

You looking at me?: Interpreting social cues in schizophrenia.

BACKGROUND: Deficits in the perception of social cues are common in schizophrenia and predict functional outcome. While effective communication depends on deciphering both verbal and non-verbal features, work on non-verbal communication in the disorder is scarce. METHOD: This behavioural study of 29 individuals with schizophrenia and 25 demographically matched controls used silent video-clips to examine gestural identification, its contextual modulation and related metacognitive representations. RESULTS: In accord with our principal hypothesis, we observed that individuals with schizophrenia exhibited a preserved ability to identify archetypal gestures and did not differentially infer communicative intent from incidental movements. However, patients were more likely than controls to perceive gestures as self-referential when confirmatory evidence was ambiguous. Furthermore, the severity of their current hallucinatory experience inversely predicted their confidence ratings associated with these self-referential judgements. CONCLUSIONS: These findings suggest a deficit in the contextual refinement of social-cue processing in schizophrenia that is potentially attributable to impaired monitoring of a mirror mechanism underlying intentional judgements, or to an incomplete semantic representation of gestural actions. Non-verbal communication may be improved in patients through psychotherapeutic interventions that include performance and perception of gestures in group interactions.

Inefficient cerebral recruitment as a vulnerability marker for schizophrenia.

BACKGROUND: Patients with schizophrenia and their first-degree relatives exhibit both abnormally diminished and increased neural activation during cognitive tasks. In particular, excessive task-related activity is often observed when tasks are easy, suggesting that inefficient cerebral recruitment may be a marker of vulnerability for schizophrenia. This hypothesis might best be tested using a very easy task, thus avoiding confounding by individual differences in task difficulty. METHOD: Eighteen people with schizophrenia, 18 unaffected full siblings of patients with schizophrenia and 26 healthy controls performed an easy auditory target-detection task in a 3-T magnetic resonance imaging (MRI) scanner. Groups were matched for accuracy on the task. Blood oxygen level-dependent (BOLD) responses to non-target stimuli in participants with vulnerability for schizophrenia (siblings and patients) were compared with those of healthy controls, and those of patients with those of unaffected siblings. BOLD responses to targets were compared with baseline, across groups. RESULTS: Subjects with vulnerability for schizophrenia showed significant hyperactivation to non-targets in brain areas activated by targets in all groups, in addition to reduced deactivation to non-targets in areas suppressed by targets in all groups. Siblings showed greater activation than patients to non-targets in the medial frontal cortex. Patients exhibited significantly longer reaction times (RTs) than unaffected siblings and healthy controls. CONCLUSIONS: Inefficient cerebral recruitment is a vulnerability marker for schizophrenia, marked by reduced suppression of brain areas normally deactivated in response to task stimuli, and increased activation of areas normally activated in response to task stimuli. Moreover, siblings show additional activation in the medial frontal cortex that may be protective.

Resonant enhancement of dielectric and metal nanoparticle arrays for light trapping in solar cells.

We numerically investigate the light trapping properties of two-dimensional diffraction gratings formed from silver disks or titanium dioxide pillars, placed on the rear of Si thin-film solar cells. In contrast to previous studies of front-surface gratings, we find that metal particles out-perform dielelectric ones when placed on the rear of the cell. By optimizing the grating geometry and the position of a planar reflector, we predict short circuit current enhancements of 45% and 67% respectively for the TiO2 and silver nanoparticles. Furthermore, we show that interference effects between the grating and reflector can significantly enhance, or suppress, the light trapping performance. This demonstrates the critical importance of optimizing the reflector as an integral part of the light trapping structure.

The concept of salience network dysfunction in schizophrenia: from neuroimaging observations to therapeutic opportunities.

A large body of neuroimaging literature suggests that distributed regions in the brain form coordinated largescale networks that show reliable patterns of connectivity when observed using either functional or structural magnetic resonance imaging (MRI) methods. Functional activation within these networks provides a robust and reliable representation of dynamic brain states observed during information processing. One such network comprised of anterior frontoinsular cortex (aFI) and anterior cingulate cortex (ACC) is called the Salience Network (SN). SN has been identified as a system that enables the switch between various dynamic brain states. SN dysfunction has been proposed as a mechanistic model for several core symptoms of schizophrenia. In this review, we explore how various risk factors of schizophrenia could operate through the dysfunctional SN to generate symptoms of psychosis. We also consider the putative neurochemical basis for the SN dysfunction in schizophrenia, and suggest that the SN dysfunction is a viable therapeutic target for a combined pharmacological and cognitive training treatment approach. This combination approach, termed as Brain Network Modulation, could exploit neuronal plasticity to reverse a key pathophysiological deficit in schizophrenia.

fMRI and MEG analysis of visceral pain in healthy volunteers.

BACKGROUND: Although many studies of painful rectal stimulation have found activation in the insula, cingulate, somatosensory, prefrontal cortices and thalamus, there is considerable variability when comparing functional magnetic resonance imaging (fMRI) results. Multiple factors may be responsible, including the model used in fMRI data analysis. Here, we assess the temporal response of activity to rectal barostat distension using novel fMRI and magnetoencephalography (MEG) analysis. METHODS: Liminal and painful rectal barostat balloon inflation thresholds were assessed in 14 female healthy volunteers. Subliminal, liminal and painful 40s periods of distension were applied in a pseudo-randomized paradigm during fMRI and MEG neuroimaging. Functional MRI data analysis was performed comparing standard box-car models of the full 40s of stimulus (Block) with models of the inflation (Ramp-On) and deflation (Ramp-Off) of the barostat. Similar models were used in MEG analysis of oscillatory activity. KEY RESULTS: Modeling the data using a standard Block analysis failed to detect areas of interest found to be active using Ramp-On and Ramp-Off models. Ramp-On generated activity in anterior insula and cingulate regions and other pain-matrix associated areas. Ramp-Off demonstrated activity of a network of posterior insula, SII and posterior cingulate. Active areas were consistent with those identified from MEG data. CONCLUSIONS & INFERENCES: In studies of visceral pain, fMRI model design strongly influences the detected activity and must be accounted for to effectively explore the fMRI data in healthy subjects and within patient groups. In particular a strong cortical response is detected to inflation and deflation of the barostat, rather than to its absolute volume.

Reality distortion is related to the structure of the salience network in schizophrenia.

BACKGROUND: An intrinsic cerebral network comprising the anterior cingulate and anterior insula (the salience network) is considered to play an important role in salience detection in healthy volunteers. Aberrant salience has been proposed as an important mechanism in the production of psychotic symptoms such as delusions and hallucinations (reality distortion). We investigated whether structural deficits in the salience network are associated with the reality distortion seen in schizophrenia. METHOD: A sample of 57 patients in a clinically stable state of schizophrenia and 41 controls were studied with high-resolution magnetic resonance imaging. RESULTS: Bilateral volume reduction was seen in the anterior cingulate and anterior insula in patients with schizophrenia. Reduced volume in the two left-sided regions of the salience network was significantly correlated with the severity of reality distortion. CONCLUSIONS: These findings suggest that a deficit of grey matter in the salience network leads to an impaired attribution of salience to stimuli that is associated with delusions and hallucinations in schizophrenia.

Four-wave mixing in slow light engineered silicon photonic crystal waveguides.

We experimentally investigate four-wave mixing (FWM) in short (80 µm) dispersion-engineered slow light silicon photonic crystal waveguides. The pump, probe and idler signals all lie in a 14 nm wide low dispersion region with a near-constant group velocity of c/30. We measure an instantaneous conversion efficiency of up to -9dB between the idler and the continuous-wave probe, with 1W peak pump power and 6 nm pump-probe detuning. This conversion efficiency is found to be considerably higher (>10 × ) than that of a Si nanowire with a group velocity ten times larger. In addition, we estimate the FWM bandwidth to be at least that of the flat band slow light window. These results, supported by numerical simulations, emphasize the importance of engineering the dispersion of PhC waveguides to exploit the slow light enhancement of FWM efficiency, even for short device lengths.

Optical signal processing on a silicon chip at 640Gb/s using slow-light.

We demonstrate optical performance monitoring of in-band optical signal to noise ratio (OSNR) and residual dispersion, at bit rates of 40Gb/s, 160Gb/s and 640Gb/s, using slow-light enhanced optical third harmonic generation (THG) in a compact (80microm) dispersion engineered 2D silicon photonic crystal waveguide. We show that there is no intrinsic degradation in the enhancement of the signal processing at 640Gb/s relative to that at 40Gb/s, and that this device should operate well above 1Tb/s. This work represents a record 16-fold increase in processing speed for a silicon device, and opens the door for slow light to play a key role in ultra-high bandwidth telecommunications systems.

Loss engineered slow light waveguides.

Slow light devices such as photonic crystal waveguides (PhCW) and coupled resonator optical waveguides (CROW) have much promise for optical signal processing applications and a number of successful demonstrations underpinning this promise have already been made. Most of these applications are limited by propagation losses, especially for higher group indices. These losses are caused by technological imperfections ("extrinsic loss") that cause scattering of light from the waveguide mode. The relationship between this loss and the group velocity is complex and until now has not been fully understood. Here, we present a comprehensive explanation of the extrinsic loss mechanisms in PhC waveguides and address some misconceptions surrounding loss and slow light that have arisen in recent years. We develop a theoretical model that accurately describes the loss spectra of PhC waveguides. One of the key insights of the model is that the entire hole contributes coherently to the scattering process, in contrast to previous models that added up the scattering from short sections incoherently. As a result, we have already realised waveguides with significantly lower losses than comparable photonic crystal waveguides as well as achieving propagation losses, in units of loss per unit time (dB/ns) that are even lower than those of state-of-the-art coupled resonator optical waveguides based on silicon photonic wires. The model will enable more advanced designs with further loss reduction within existing technological constraints.

Optical filter with very large stopband (approximately 300 nm) based on a photonic-crystal vertical-directional coupler.

We have designed, fabricated, and demonstrated a vertical directional coupler based on the coupling between a polymer waveguide and a W1 photonic crystal waveguide. The filters have a bandwidth of approximately 2 nm within a stopband of Delta lambda approximately 300 nm and an on-chip insertion loss of 1 dB. This is the first (to our knowledge) demonstration of a filter with such a large stopband that overcomes the bandwidth limitation of existing filters.

Efficient coupling into slow light photonic crystal waveguide without transition region: role of evanescent modes.

We show that efficient coupling between fast and slow photonic crystal waveguide modes is possible, provided that there exist strong evanescent modes to match the waveguide fields across the interface. Evanescent modes are required when the propagating modes have substantially different modal fields, which occurs, for example, when coupling an index-guided mode and a gap-guided mode.

Reduced event-related low frequency EEG activity in schizophrenia during an auditory oddball task.

This study examines EEG low frequency characteristics which have been linked to specific cognitive functions such as stimulus encoding and attention during an auditory oddball task in schizophrenia patients and healthy controls. EEG data was recorded from 17 young schizophrenia patients in a stable phase of their illness and 17 healthy controls performing an auditory oddball task. Evoked and induced delta and theta activity, N100, P300 amplitude were computed. Between 200-500 ms after a stimulus was presented, patients displayed significantly reduced P300, less evoked and induced delta and theta activity than controls. We conclude that the well known finding of P300 reduction in schizophrenia can be linked to reductions in delta and theta activity, which are a manifestation of impaired stimulus evaluation, memory retrieval, and a lack of sustained attention.

Efficient slow-light coupling in a photonic crystal waveguide without transition region.

We consider the coupling into a slow mode that appears near an inflection point in the band structure of a photonic crystal waveguide. Remarkably, the coupling into this slow mode, which has a group index ng>1000, can be essentially perfect without any transition region. We show that this efficient coupling occurs thanks to an evanescent mode in the slow medium, which has appreciable amplitude and helps satisfy the boundary conditions but does not transport any energy.

Silica-embedded silicon photonic crystal waveguides.

We report on the fabrication and characterization of silicon photonic crystal waveguides completely embedded in silica. These waveguides offer a robust alternative to air-membranes and are fully compatible with monolithic integration. Despite the reduced refractive index contrast compared to the air-membranes, these waveguides offer a considerable operating range of approximately 10 nm in the 1550 nm window. While the reduced index contrast weakens the perturbations due to surface roughness, we measure losses of 35 +/- 3dB/cm compared to 12 +/- 3 dB/cm for nominally identical air-membranes. Numerical analysis reveals that the difference in loss results from the different mode distribution and group index of the respective waveguide modes. Radius disorder is used as a fitting parameter in the numerical simulations with the best fits found for disorder levels of 1.4 - 1.7 nm RMS, which attest to the high quality of our structures.

Photonic crystal laser with mode selective mirrors.

The mini-stopband (MSB) of a W3 line-defect photonic crystal waveguide is used as a mirror for a GaAs based quantum-dot laser. Single mode, continuous-wave lasing is demonstrated for broad area lasers up to a current of 125 mA (2.7 x laser threshold), which demonstrates the high degree of mode selectivity of the MSB mirror. FDTD calculations indicate that optimisation of the mirror interface could lead to a further fourfold increase in reflectivity resulting in significantly reduced thresholds.

Coupling into slow-mode photonic crystal waveguides.

We study efficient injectors for coupling light from z-invariant ridge waveguides into slow Bloch modes of single-row defect photonic crystal waveguides. Two-dimensional vectorial computations performed with a Bloch mode theory approach predict that very high efficiencies (>90%) can be achieved for injector lengths of only a few wavelengths in length, even for small group velocities in the range of c/100-c/400. This result suggests that photonic crystal devices operating with slow waves can be interfaced with classical waveguides without sacrificing compactness.

Wide-angle coupling into rod-type photonic crystals with ultralow reflectance.

We describe the surprising phenomenon of near-perfect coupling from free space into uniform two-dimensional rod-type photonic crystals over a wide range of incident angles. This behavior is shown to be a generic feature of many rod-type photonic crystal structures that is related to strong forward scattering resonances of the individual cylinders. We explain these results using both semianalytic analysis and two-dimensional numerical calculations and identify the conditions under which efficient, wide-angle coupling can occur. The results may lead to more efficient designs for in-band photonic crystal devices such as superprisms and self-collimation based photonic circuits.

Bloch mode scattering matrix methods for modeling extended photonic crystal structures. I. Theory.

We present a rigorous Bloch mode scattering matrix method for modeling two-dimensional photonic crystal structures and discuss the formal properties of the formulation. Reciprocity and energy conservation considerations lead to modal orthogonality relations and normalization, both of which are required for mode calculations in inhomogeneous media. Relations are derived for studying the propagation of Bloch modes through photonic crystal structures, and for the reflection and transmission of these modes at interfaces with other photonic crystal structures.