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The substrate has a cell attached to its surface to contain the particles. This also facilitates the easy change of the fluid samples above the waveguides without the need to remove the substrate and cell, clean the substrate, reapply the cell and realign the

substrate. Instead the cell can be cleaned by flowing deionised water through it for a minute at high speed.

There are two design limitations for the cell. Firstly the part of the cell in contact with the waveguide must be of lower refractive index than the effective index of the modes supported by the waveguides. If this is not the case the waveguide would cease to contain the light at that point and the light would leak into the superstrate making the waveguide very lossy. Secondly the superstrate should have low loss so as to not increase the loss of the waveguide as a whole. However more importantly, if the superstrate is not transparent to the wavelength used, then at the high powers used, damage to the superstrate is likely to occur due to overheating. Typically the damage would in itself lead to change of form of the material e.g. by burning and thus make it much more lossy. These two conditions rule out most photoresists, many coatings, many glues and many tapes. The four options considered were: evaporated Teflon AF (with a refractive index of∼1.31), poly-dimethyl siloxane (PDMS) (∼1.45), HS-2 UV-curing glue (∼1.45) and sputtered silica (∼1.45).

Three approaches were regularly used, each for slightly different reasons. The easiest design for rapid trials was to use a cell made of PDMS. This sealed well to the waveguide surface and would typically cause approximately 1dB of extra loss. This was fabricated by thoroughly mixing a 10:1 ratio of PDMS to hardener, pouring over a mould and allowing to settle for at least one hour (see figure 6.2). The mould design typically used was a coverslip of approximate dimensions 30×10×0.2 mm glued on top of a microscope slide. In order to remove the air bubbles the sample was then exposed to a weak vacuum (∼0.1bar) for some hours. Finally it was removed and baked for approximately one hour at 800C until set. The cell was then cut with a scalpel and could be peeled off the mould. Two holes could be punched in either end of the cell in order to allow liquid in and air out. Teflon tubes (Ismatec) with an inner diameter of 1mm and an outer diameter of 1.8mm may be inserted into these holes. If inlet and outlet tubes were both used then it was required that they be positioned at least 35mm apart in order that microscope objectives could approach close enough without touching these tubes. A peristaltic pump (Pharmacia, LKB Pump, P-1) could be used to pump the required solutions over the waveguides.

The advantages of this cell were that it was easy to clean and apply. In addition, the processing of PDMS is a well known technology that can be defined much more accurately photolithographically [141]. This would require much more processing time, however it would also allow the use of much smaller samples (as may be required in a process involving the analysis of a biological sample where the sample volume is often very small). The main disadvantage with PDMS was that in order for it to be strong enough not to tear and so that the cell did not cave in, a thickness of a least two millimetres was required. This meant that the imaging was not optimal as microscope

objectives are typically adjusted for imaging through a coverslip, which has a standard thickness of approximately 180µm. This was more of a problem in imaging the gold particles than the latex particles, due to their smaller size.

Petri dish coverslip microscope slide PDMS reusable cell flow/ return pipes substrate a) b) c) d)

Figure 6.2: Fabrication and use of PDMS cell. a) Mould is made by gluing a coverslip, microscope slide, and petri dish together, b) PDMS is poured over mould, degassed, baked and cut, c) the cell can be pulled out of the mould and d) the cell is applied to the substrate. Tubes can then be pushed into the cell to allow the pumping of fluid

over the waveguides.

A second design consisted of a cell made by glueing thin segments of a coverslip to the substrate with UV-curing, optically-clear glue (see figure 6.3). The segments of coverslip are made by marking with a diamond scribe using a ruler and then breaking. A microscope slide is cut to size and two holes of diameter 1.8mm are drilled with an ultrasonic drill. The required shape of a waxy film (Parafilm) is cut out with a scalpel and is used to affix the slide to the substrate to form the cell. The substrate, Parafilm cut out and the drilled microscope slide are heated to 700_{C, pressed firmly together}

and allowed to cool (with the pressure still applied). This produced a watertight cell of approximate thickness 230µm, equal to the thickness of the coverslip (180µm) plus the thickness of the Parafilm cut-out (50µm). Tubes could then be attached to the holes with superglue and a solution pumped through, as before.

There were three principal problems with this method. Firstly it was impossible to wipe away any excess glue when gluing the coverslips as this would lead to streaks of glue being left on the substrate. This meant the beam encountered multiple changes in the superstrate refractive index and thus increased the loss. Wiping with acetone that acted as a solvent improved this, however it also left an opaque film when it dried that drastically increased the loss. Secondly the damage threshold of the glue was not high and led to observations of burnt glue above waveguides which had suddenly become very

lossy at the high powers required for trapping. Lastly, it was very hard to remove the coverslips once they had been glued down.

coverslip flow/ return pipes a) _b) c) _d) substrate ParafilmTM drilled slide glue

Figure 6.3: Fabrication and use of cell using UV-curing glue. a) Segments of coverslip are glued to the substrate, b) a cut-out sheet of Parafilm is laid over coverslips, c) the microscope slide fixed to top by heating the substrate and pressing together and d) the

tubes glued in with superglue.

The most satisfactory method for the liquid cell was with the use of a silica (SiO₂) isolation layer defined with photoresist on top of the waveguides (see figure 6.4). A film of thickness 1.5µm SiO2 was sputtered over the whole surface. The substrate was

then washed in acetone that lifted off the photoresist leaving a gap in the silica film. This allowed the particles access to the superstrate of the waveguide. A Parafilm film cut-out and the same cut and drilled microscope slide as above then formed the cell. The thickness of this cell was therefore approximately 50µm. The sputtered silica was found to give no measurable extra loss.

Unless otherwise stated results presented in this thesis were collected using one of the latter two cells. The PDMS cell was only used for screening samples and quick tests. The method using the sputtered silica was the preferred method, however acceptable sputtering conditions for silica were not developed until part way through the project.