Directional emission from asymmetric microlaser resonators of ∏-conjugated polymers

Journal ArticleA ∏-conjugated polymer film was fabricated into an asymmetric microlaser resonator having a quadrapole deformation with irregular boundaries and a Q factor of 600. At high excitation intensities above the threshold for lasing, we observed multimode laser emission spectra and directional emission at four different angles. Chaotic ray dynamics explains the observed emission pattern

Spectral analysis of polymer microring lasers

Journal ArticleThe emission spectra of microring lasers made of p-conjugated polymer films that coat glass optical fibers are analyzed with the help of a Fourier transform. This method allows for the assignment of photopumped emission lines to integral Bessel functions and a more precise determination of the laser threshold excitation intensity based on harmonic analysis

Spatially mapping random lasing cavities

ManuscriptA mapping technique is developed to spatially resolve random laser emission spectra from disorder solid media with optical gain above the threshold excitation intensity for lasing; the technique is applied to rr-conjugated polymer lms. By mapping the spatial extent of emission peaks in the random laser spectrum, bright areas that correspond to naturally-formed lasing microcavities are unraveled. The size of the obtained microcavities matches the size extracted from the Fourier transform analysis of the laser emission spectrum. Mapping at increased excitation intensities show multiple resonant microcavities that lase at increasing threshold intensities

Multiple resonances in microdisk lasers of ∏-conjugated polymers

Journal ArticleWe have fabricated microdisk lasers from p-conjugated polymers that show multiline emission spectrum upon optical excitation. Using Fourier transform analysis, each emission line is assigned integer to an interger Bessel function that helps to estimate the field distribution inside the photoexcited polymer microdisk. We found that the microdisk can sustain two different laser modes with different, though complementary, field distributions and that the index of refraction decreases with increasing excitation intensity

Spectral analysis of near threshold random lasers

ManuscriptEmission spectra from random lasing systems typically have numerous narrow resonant lines. When excited very near to the laser threshold there are fewer resonant lines which clarify the emission spectrum analysis. We studied three different random lasing systems including ?-conjugated polymer films, zinc oxide and TiO2 scatterers in dye solution. Fourier transform analysis of the laser emission spectra near threshold of each system shows that all the sharp lines are highly correlated, indicating that they originate from a single high symmetry resonant structure. The naturally formed microresonators have a circular geometry in the two-dimensional films, and transient spherical geometry in the scatterers/dye suspension

Organic random lasers in the weak-scattering regime

Journal ArticleWe used the ensemble-averaged power Fourier transform (PFT) of random laser emission spectra over the illuminated area to study random lasers with coherent feedback in four different disordered organic gain media in the weak scattering regime, where the light mean free path, ℓ* is much larger than the emission wavelength. The disordered gain media include a p-conjugated polymer film, an opal photonic crystal infiltrated with a laser dye (rhodamine 6G; R6G) having optical gain in the visible spectral range, a suspension of titania balls in R6G solution, and biological tissues such as chicken breast infiltrated with R6G. We show the existence of universality among the random resonators in each gain medium that we tested, in which at the same excitation intensity a dominant random cavity is excited in different parts of the sample. We show a second universality when scaling the average PFT of the four different media by ℓ*; we found that the dominant cavity in each disordered gain medium scales with ℓ*. The excellent agreement obtained with computer simulations using a distribution of random microdisks, each contributing a number of longitudinal whispering gallery modes within the gain spectrum, unambiguously shows that random lasers in the weak scattering regime cannot be described by gain amplification of localized photon states

Universal properties of random lasers

Journal ArticleThe design and fabrication of laser resonators is often difficult. However, random lasers occur in gain media with numerous scatterers and produce coherent laser emission without any predesigned cavity. The generation of coherent emission from multiple scattering is quite general and its basic principles are shown here using two model systems, namely rr-conjugated polymer films and rhodamine-TiO2 suspensions. Above a threshold excitation intensity, both systems show narrow emission lines (<0.5 nm), coherence that is determined by photon statistics, and a fundamental cavity length in the disordered material that is revealed by averaging multiple power Fourier transform spectra

The Rich Get Richer: Enabling Conditions for Knowledge Use in Organizational Work Teams

Individuals on the periphery of organizational knowledge sharing networks, due to inexperience, location, or lack of social capital, may struggle to access useful knowledge at work. An electronic knowledge repository (KR) has the potential to help peripheral individuals gain access to valuable knowledge because a KR is universally and constantly available and can be used without social interaction. However, for it to serve this equalizing function, those on the periphery of the organization must actually use it, possibly overcoming barriers to doing so. In this paper, we develop a multi-level model of knowledge use in teams and show that individuals whose experience and position already provide them access to vital knowledge use a KR more frequently than individuals on the organizational periphery. We argue that this occurs because the KR – despite its appearance of equivalent accessibility to all – is actually more accessible to central than peripheral players due to their greater experience and access to colleagues. Thus, KR use is not driven primarily by the need to overcome limited access to other knowledge sources. Rather KR use is enabled when actors know how to reap value from the KR, which ironically improves with increasing access to other sources of knowledge. Implications for both team effectiveness and knowledge management research are offered. We conclude that KRs are unlikely to serve as a knowledge equalizer without intervention

SYSTEMS-2: a randomised phase II study of radiotherapy dose escalation for pain control in malignant pleural mesothelioma

SYSTEMS-2 is a randomised study of radiotherapy dose escalation for pain control in 112 patients with malignant pleural mesothelioma (MPM). Standard palliative (20Gy/5#) or dose escalated treatment (36Gy/6#) will be delivered using advanced radiotherapy techniques and pain responses will be compared at week 5. Data will guide optimal palliative radiotherapy in MPM