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Crystal Report 8.5 Crack
The relationship between the structure and function of haemoglobin has mainly been studied by comparing its X-ray crystal structures with its function in solutions. To make a direct comparison we have studied the functional properties of haemoglobin in single crystals, an approach that has been an important part of the investigation of several enzyme mechanisms. Here we report on the oxygen binding by single crystals of human haemoglobin grown in solutions of polyethylene glycol. Unlike haemoglobin crystals formed in concentrated salt solution, which crack and become disordered on oxygenation, crystals grown in polyethylene glycol remain intact. X-ray studies have shown that the T (deoxy) quaternary structure of haemoglobin in this crystal at pH 7.0 is maintained at atmospheric oxygen pressure, and that the salt-bridges are not broken. We find striking differences between oxygen binding by haemoglobin in this crystal and by haemoglobin in solution. Not only is oxygenation of the crystal noncooperative, but the oxygen affinity is independent of pH in the range 6.0-8.5, and is much lower than that of the T state in solution. The lack of cooperativity without a change in quaternary structure is predicted by the two-state allosteric model of Monod, Wyman and Changeux. The absence of a Bohr effect without breakage of salt-bridges is predicted by Perutz's stereochemical mechanism. In contrast to the X-ray result that oxygen binds only to the alpha haems, our measurements show that the alpha haems have only a slightly higher affinity than the beta haems.
We introduce the fabrication and use of microcracks embedded in glass as an optical element for manipulating light propagation, in particular for enhancing waveguide performance in silica integrated optics. By using a femtosecond laser to induce a strong asymmetric stress pattern in silica, uniform cracks with set dimensions can be created within the substrate and propagated along a fixed path. The smoothness of the resulting cleave interface and large index contrast can be exploited to enhance waveguide modal confinement. As a demonstration, we tackle the longstanding high bend-loss issue in femtosecond laser written silica waveguides by using this technique to cleave the outer edge of laser written waveguide bends, to suppress radiative bend loss. The microcrack cross section is estimated to be 15 μm in height and 30 nm in width, for the 10 \(\times\) 10 μm waveguides. At 1550 nm wavelength, losses down to 1 dB/cm at 10 mm bend radius were achieved, without introducing additional scattering. Both the cleave stress pattern and waveguide are fabricated with the same multiscan writing procedure, without requiring additional steps, and re-characterisation of the waveguides after 1 year confirm excellent long term performance stability.
Strong research interest in ultrafast laser material processing has been motivated by the ability to modify refractive index1, machine or ablate2,3, and nanostructure4,5,6,7 with excellent resolution all using a single system, with a host of applications in the fabrication of low-cost optical chips such as quantum circuits8,9,10,11 and integrated biophotonics12,13. In particular, direct laser writing of such optical chips by focusing femtosecond pulses into transparent media, wherein nonlinear effects including multiphoton absorption and avalanche lead to a localised and permanent index change \(\Delta n\) at the focal volume14, allows the fabrication of embedded waveguide devices1,15,16,17 with submicron feature size and 10 nm resolution. However, one fundamental limitation is the low induced \(\Delta n\), typically \(10^-3\) in fused silica, which restricts optical functionality of the resulting devices. While it is possible create a high index-contrast by inducing voids via microexplosions18 and also by dicing the glass to create an interface with air19, the former is highly scattering and the latter cannot be locally integrated into devices. To overcome this shortcoming, we introduce and demonstrate an additional processing technique to locally cleave the glass: by focusing femtosecond laser pulses into a glass substrate to induce an appropriate stress pattern, we can create and accurately guide the localised formation of microcracks, i.e. cracks with a typical cross section height of the order of 10 μm or more and a submicron width. To date, cracks are often considered as damage during laser inscription and parameters are chosen specifically to avoid their formation, whether for optical20 or mechanical structures21. However, contrary to this common notion, we demonstrate that it is possible to create mechanically stable cracks with optically smooth non-scattering interfaces (indeed, even for macroscopically stress-diced glass the surface roughness is on the order of only 10 nm22), cleaved along pre-defined 3D paths embedded in the glass and thus confined with precise dimensions and position.
Experimental set up and fabrication procedure. (a) Femtosecond writing system for inscription of waveguides. The focusing objective is mounted on a vertical stage to adjust height, and the silica substrate is placed on a two-axis translation stage. (b) Procedure for fabricating microcrack along outer edge of curved waveguides.
Simulated effect of crack on modal confinement in curved waveguides. (a, b) Fundamental mode electric field profile and cutline across \(z=0\) for the case \(r_b=7\) mm without crack for x polarization, (c, d) with crack for y polarization and (e, f) with crack for x polarization. Waveguide is 10 \(\times\) 10 μm; crack is 30 nm wide and 15 μm high located on outer bend edge of waveguide indicated by dotted red line; \(\lambda =1.55\) μm. Shaded grey region in cutline plots indicates extent of core.
To illustrate the mechanism by which the microcrack reduces bend loss, we simulated the \(\lambda =1.55\) μm fundamental mode field profiles of a curved waveguide as shown in Fig. 3, for a \(10\times 10\) μm core silica waveguide of bend radius \(r_b=7\) mm, with and without a crack on the outside bend edge of the core. The core-cladding index difference was taken as \(4.1\times 10^-3\), based on previous experimental measurements, and crack width was estimated as \(w_c=30\) nm from comparisons of the simulated results for different widths. In Fig. 3a, b, the waveguide without the crack is poorly guiding with a high radiative loss, as can be seen by the significant portion of the field to the right of the waveguide. The cutline also shows the mode peak skewed toward the outer bend direction, rather than centered on the waveguide. Since the core-cladding index here is weak, the x and y polarization mode profiles are almost identical, with a calculated bend loss of 6.5 dB/cm.
Bright field microscopy of fabricated waveguides and integrated cracks. (a) Top-down images of waveguides before polishing and (b) after polishing, with cleaved microcrack visible on lower edge. (c) Waveguide bend arrays. (d) Examples of cracks fabricated along sinusoidal and (e) stepped 45\(^\circ\) angle paths. (f) Cross sections of waveguides inscribed at writing powers \(P_w\) from 8.5 to 10.5 mW (\(\pm 0.05\) mW), and (g) using different multiscan line orders, for \(P_w\) = 9.5 mW. All waveguides written with width 10 μm, pulse density \(D=3\times 10^5\) pulses/mm, scanline spacing \(s=200\) nm, \(\lambda _w = 515\) nm, 0.4 NA objective lens, and circular polarization.
Microscope images in Fig. 4a, b confirm successful cleaving of the outer bend edge of the waveguides after lapping/polishing, clearly visible as a dark edge due to the microcrack \(\Delta n\) of 0.45 being 100 times larger than that of the core-cladding. All cracks appear visually identical and propagated successfully along the length of each waveguide as shown in Fig. 4c, while end facet cross sections verify the cleave covers the entire edge of the waveguide. It also partially wraps above and below the core, as might be expected intuitively from the non-zero but weaker stress field in those regions, giving an approximate total height of 15\(\pm 1\) μm.
To confirm that more complex geometries can be produced using this process, Fig. 4d, e show cracks following sinusoidal and stepped 45\(^\circ\) angle paths. These are intended to demonstrate the localised cleaving as a more general micromachining technique, rather than only for enhancing waveguides. Note in Fig. 4e the crack is seen to cross over from one side of the multiscan to the other; this occurs if the induced stress is too high on both sides of the waveguide, although here the higher stress at the corners due to segmented multiscan is also a contributing factor. For waveguides, such a crossing across the core would be lossy, but can be inhibited by adjusting the writing beam power according to the geometry or optimising the multiscan path to minimise high stress points.
Multiscan line order was also investigated with straight waveguides in Fig. 4g. Inverting the order forces the crack to form on the other edge of the waveguide, as expected. However, if the multiscan begins from the center and scanlines alternate consecutively left and right, the symmetric stress forms the crack with equal chance on either side. Correct scanning order is thus straightforward but crucial for accurate placement of the crack.
Experimentally measured waveguide mode profiles and bend loss. Mode intensity profiles for straight waveguides (a) without microcrack for y polarization (b) with microcrack for x polarization and (c) y polarization. Crack is on right side of waveguide and peak intensity normalised to unity. The 1/\(e^2\) mode field diameters in the x and y dimensions are labelled. (d) Measured bend loss \(\alpha _b\) against bend radius \(r_b\), for both polarizations at \(\lambda =1550\) nm and 1310 nm. Shaded region denotes polarization-dependent loss. Vertical lines divide the bend radius into 3 ranges A, B and C, where dominant losses are excessive confinement loss, bend loss, and scattering losses, respectively. 2ff7e9595c
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