An XPS study of pulsed plasma polymerised allyl alcohol film growth on polyurethane

The growth of highly functionalised poly allyl alcohol films by pulsed plasma polymerisation of CH2 double bond; length as m-dashCHCH2OH on biomedical grade polyurethane has been followed by X-ray photoelectron spectroscopy (XPS) and contact angle measurements. Film thickness is observed to increase approximately linearly with plasma modification time, suggesting a layer-by-layer growth mode of poly allyl alcohol. Water contact angle measurements reveal the change in the surface free energy of wetting decreases linearly with plasma modification up to the monolayer point after which a constant limiting value of −24 mJ m−2 was attained. Films prepared at 20 W plasma power with a duty cycle of 10 μs:500 μs exhibit a high degree of hydroxyl (single bondOH) retention with minimal fragmentation of the monomer observed. Increasing the plasma power up to 125 W is found to improve single bondOH retention at the expense of ether formation generating films close to the monomer stoichiometry. Duty cycle plays an important role in controlling both film composition and thickness, with longer off times increasing single bondOH retention, while longer on times enhance allyl alcohol film growth.

Colour management of InGaN/GaN based monolithic two-wavelength LEDs

The recent advancement in the growth technology of InGaN/GaN has decently positioned InGaN based white LEDs to leap into the area of general or daily lighting. Monolithic white LEDs with multiple QWs were previously demonstrated by Damilano et al. [1] in 2001. However, there are several challenges yet to be overcome for InGaN based monolithic white LEDs to establish themselves as an alternative to other day-to-day lighting sources [2,3]. Alongside the key characteristics of luminous efficacy and EQE, colour rendering index (CRI) and correlated colour temperature (CCT) are important characteristics for these structures [2,4]. Investigated monolithic white structures were similar to that described in [5] and contained blue and green InGaN multiple QWs without short-period superlattice between them and emitting at 440 nm and 530 nm, respectively. The electroluminescence (EL) measurements were done in the CW and pulse current modes. An integration sphere (Labsphere “CDS 600” spectrometer) and a pulse generator (Agilent 8114A) were used to perform the measurements. The CCT and Green/Blue radiant flux ratio were investigated at extended operation currents from 100mA to 2A using current pulses from 100ns to 100μs with a duty cycle varying from 1% to 95%. The strong dependence of the CCT on the duty cycle value, with the CCT value decreasing by more than three times at high duty cycle values (shown at the 300 mA pulse operation current) was demonstrated (Fig. 1). The pulse width variation seems to have a negligible effect on the CCT (Fig. 1). To account for the joule heating, a duty cycle more than 1% was considered as an overheated mode. For the 1% duty cycle it was demonstrated that the CCT was tuneable in three times by modulating input current and pulse width (Fig. 2). It has also been demonstrated that there is a possibility of keeping luminous flux independent of pulse width variation for a constant value of current pulse (Fig. 3).

Bit-error rate performance of 20 Gbit/s WDM RZ-DPSK non-slope matched submarine transmission systems

Applying direct error counting, we assess the performance of 20 Gbit/s wavelength-division multiplexing return-to-zero differential phase-shift keying (RZ-DPSK) transmission at 0.4 bit/(s Hz) spectral efficiency for application on installed non-zero dispersion-shifted fibre based transoceanic submarine systems. The impact of the pulse duty cycle on the system performance is investigated and the reliability of the existing theoretical approaches to the BER estimation for the RZ-DPSK format is discussed.