The implausible combination of centrifugal disc microfluidics and un-covered channels provides a simple way in which Raman spectroscopy can be implemented in industrially-relevant lab-on-a disc technology. Here we demonstrate these advantages by detecting very low concentrations of melamine in liquid milk, without pre-processing, without surface enhancement of the Raman signal and with no evidence of spectral contamination from the polymeric chip itself. A limit of detection (LOD) of 203 ppm for melamine in milk was achieved from Raman spectra of milk after drying. The centrifugal disc rotation and microchannel geometry results in rapid and reliable filling of the channels and in meniscus shape control, enabling reproducible Raman detection with quantitative precision.

A series of multi-branched two-photon photoinitiators (PIs) based around the well-known triphenylamine donor core were synthesised for use in two-photon polymerisation (TPP) and are designated as compounds 6, 7 and 8. The use of a phenylene-vinylene π-system was used with an ethyl ester acceptor moiety which gives dipolar (6), quadrupolar (7) and octupolar (8) branching. Two-photon absorption cross-sections (δ2PA) of 126 GM, 358 GM and 590 GM were measured at 780 nm for 6, 7 and 8, respectively. The fluorescence quantum yields (ϕF) were determined in both MeOH and the acrylate system employed for TPP, and demonstrate the impact of viscosity upon photophysical properties of multi-branched molecules. Excellent polymerisation thresholds were demonstrated in the μW region: namely 45 μW (6), 61 μW (7) and 27 μW (8) at a writing speed of 50 μm s−1. Finally, an explanation for the disparity of polymerisation thresholds is proposed for these PIs and provides insight into the future development of low threshold PIs for TPP.

Photoreduction of graphene oxide (GO) to reduced graphene oxide (rGO) under ambient conditions was evaluated for three laser processing approaches: 800 nm fs pulses, 248 nm ns pulses, and 788 nm continuous wave (CW) illumination. Resultant features were compared using Raman, XPS, optical profilometry and SEM. The most effective approach for photoreduction and graphenization with minimal defects was nanosecond processing (0.12 J cm−2; 40 pulses overlapped). The Raman 2D band intensity confirmed a graphene-like structure. The ablation threshold for GO (248 nm, 5 ns) is ∼10 mJ cm−2. Laser photoreduction with CW 788 nm yielded similar oxygen removal, but conversion to a graphene-like structure was poorer, and more defects were introduced. Femtosecond pulse illumination resulted in oxygen removal from the surface but the transformation to an sp2 graphene-like structure was not observed. This result suggests that the photochemical reduction of GO is not thermally mediated, but the structural reorganization from sp3 to sp2 requires heat deposition into the material. This is the first such comparison on a consistent GO substrate under comparable ambient conditions, and the results provide insight into the fundamental mechanism and the utility of the method for direct laser writing electronically active materials.

•The fs laser ablation threshold of alumina was measured using the D-scan method.•The focal length and depth, power, speed, and number of passes were optimised.•The optimal conditions allowed complete cutting at an overall processing speed of 143 µm/s.•This overall processing speed is more than 4 times faster than previously achieved using fs laser ablation.Fast, accurate cutting of technical ceramics is a significant technological challenge because of these materials' typical high mechanical strength and thermal resistance. Femtosecond pulsed lasers offer significant promise for meeting this challenge. Femtosecond pulses can machine nearly any material with small kerf and little to no collateral damage to the surrounding material. The main drawback to femtosecond laser machining of ceramics is slow processing speed. In this work we report on the improvement of femtosecond laser cutting of sintered alumina substrates through optimisation of laser processing parameters. The femtosecond laser ablation thresholds for sintered alumina were measured using the diagonal scan method. Incubation effects were found to fit a defect accumulation model, with Fth,1=6.0 J/cm2 (±0.3) and Fth,∞=2.5 J/cm2 (±0.2). The focal length and depth, laser power, number of passes, and material translation speed were optimised for ablation speed and high quality. Optimal conditions of 500 mW power, 100 mm focal length, 2000 µm/s material translation speed, with 14 passes, produced complete cutting of the alumina substrate at an overall processing speed of 143 µm/s – more than 4 times faster than the maximum reported overall processing speed previously achieved by Wang et al. [1]. This process significantly increases processing speeds of alumina substrates, thereby reducing costs, making femtosecond laser machining a more viable option for industrial users.

High energy density per pulse (−15 dBm nm−1) supercontinuum (SC) source has been developed as a probe for transient absorption (TrA) spectroscopy of systems with lifetimes from nanoseconds to a few milliseconds. We have generated a 600–1600 nm, broadband SC by pumping a 15 m photonic crystal fiber (PCF) with relatively high power, 7 ns, 1064 nm pulses. The SC generated at peak pump power of 7.1 kW was randomly polarized and maintained a stable output (6.5% rms average power; 9.1% rms shot-to-shot power). Co-pumping with both 1064 and 532 nm light extended the wavelength range of the SC by about 20%, to 500–1700 nm. Power conversion efficiency and spectral flatness were improved as well. In the visible range, the single-pump SC shows a flatness of 5 dB while the dual-pump SC exhibits 3 dB. In the NIR (1100–1600 nm), the flatness in single- and dual-pump configurations were 3 and 2 dB, respectively. Optically induced fiber breakdown was characterized.

The details of the photophysics of a diphosphene DmpPPDmp (Dmp: 2,6-Mes2C6H3) have been examined experimentally and computationally. Femtosecond transient absorption spectroscopy has been used to probe the dynamics of the S1 and S2 excited states of DmpPPDmp, through excitation at 480 and 400 nm, respectively. The molecule returns to S0 on sub-nanosecond timescales; no irreversible photochemistry is observed. The S2 state is observed in the transient spectra with an absorption feature at the red end of the visible spectrum. Its lifetime was measured to be 275 fs. The S1 state does not absorb appreciably in the probe wavelength range. Excitation into either of these states leads to transient absorption signals in the 400–600 nm region that exhibit a rise time longer than the measured instrument response function, indicating that they do not arise from the initially excited state. These bands decay biexponentially, with lifetimes of ∼20 ps and of a few hundred ps. Calculations at the CASSCF(8,6)/6-31G** and CASPT2(8,6)/6-31G**//CASSCF(8,6)/6-31G** levels support these assignments, and underpin an initial working model that involves participation of phenyl torsional twisting motions and the possibility of rapid intersystem crossing to the low-lying triplet manifold.

The details of the photophysics of a diphosphene DmpPPDmp (Dmp: 2,6-Mes2C6H3) have been examined experimentally and computationally. Femtosecond transient absorption spectroscopy has been used to probe the dynamics of the S1 and S2 excited states of DmpPPDmp, through excitation at 480 and 400 nm, respectively. The molecule returns to S0 on sub-nanosecond timescales; no irreversible photochemistry is observed. The S2 state is observed in the transient spectra with an absorption feature at the red end of the visible spectrum. Its lifetime was measured to be 275 fs. The S1 state does not absorb appreciably in the probe wavelength range. Excitation into either of these states leads to transient absorption signals in the 400–600 nm region that exhibit a rise time longer than the measured instrument response function, indicating that they do not arise from the initially excited state. These bands decay biexponentially, with lifetimes of ∼20 ps and of a few hundred ps. Calculations at the CASSCF(8,6)/6-31G** and CASPT2(8,6)/6-31G**//CASSCF(8,6)/6-31G** levels support these assignments, and underpin an initial working model that involves participation of phenyl torsional twisting motions and the possibility of rapid intersystem crossing to the low-lying triplet manifold.