The Flow Cell Device (FCD)

The central part of the coupling and measurement setup is the flow cell device (FCD), where the coupling to the resonator takes place. The FCD is machined from

2.1 Coupling setup 25

Figure 2.1.: The flow cell device. (a) Drawing of the FCD. The fiber crosses the coupling chamber at an angle of 25◦ through slits that are sealed with latex adhesive. The angle is a compromise between a narrow flow channel and a sufficient length of the tapered fiber. The chip that carries the resonators is glued to a mount with UV adhesive. (b) Photograph of the FCD and a sample. The sample mount is attached to a 3-axis nano-positioning system via magnets. The fluid arrives from the left through a PTFE tube and is withdrawn on the right. The fiber entry slits in the coupling chamber are sealed with latex adhesive (black material in the center). (c) A view through the bottom window shows the sample inside the coupling chamber together with the tapered fiber. Here we send ∼5 mW of power though the tapered fiber for better visibility. In a measurement around 10µW are used. The exact coupling point on the tapered fiber (and thus the thickness) can be adjusted within the limits of lateral displacement of the chip. (d) The fluid channel and the coupling chamber are highlighted using a colorant.

acrylic glass and the sample is inserted through the open ceiling of the coupling chamber, while the coupling is monitored through a window in the bottom part of the FCD (cf. Figure 2.1 (a) ). To couple to the resonator, we use a tapered optical fiber [103, 104, 105], which is firmly mounted in the FCD at an angle of 25◦ with respect to the coupling chamber, as shown in Figure 2.1 (c). Such design allows us to maintain a minimum length of 20 mm for the tapered region of the fiber, while at the same time minimizing the width of the coupling chamber.1 _{The former avoids}

transmission loss at the points of entry and exit and the latter is important for efficient flushing of the coupling chamber. The fiber enters the FCD through 1 mm slits, which are sealed using a fast curing and easily removable adhesive (Microset 101RF; cf. Figure2.1 (b) ).

1_{The angle of 25}◦_{is set by the diameter and distance of the resonators on the chip, such that the}

26 2. The experimental setup

To establish a constant water or buffer flow in the FCD, we use two syringe pumps equipped with 50ml BD Plastipak syringes for fluid insertion and extraction (both Chemyx Fusion). Two supplementary pumps (New Era NE-1000) on the insertion side are used to add the analyte (cf. Figure2.5 for a schematic layout). All pumps are computer interfaced and can be used to run synchronized insertion/flushing protocols. For valves, connectors and tubing we mostly rely on sterile medical supplies (e.g. B. Braun Discofix-3 and extensions) that are regularly exchanged. It is important to note that the flow needs to be controlled precisely to maintain a constant fluid level in the coupling chamber. Therefore, all measurements were conducted with the syringe plunger position between 15−45 ml, where the plunger movement is smooth and flow is constant. Additionally, a camera was installed to monitor the fluid level in the coupling chamber. In the experiment a constant flow is important, because distortions lead to a drift of the resonance frequency, probably due to local heating.

On the mechanical side, we use a motorized xyz-translation stage (Newport Gothic- Arch 65-mm Platform) for coarse positioning of the sample. Fine alignment is achieved with an attached, inverted piezo nano-positioning system (PI, nanocube). For fast characterization of a sample, the motorized actuators are synchronized to achieve a linear movement of the chip in the coupling chamber. A translation script allows us to hop from one resonator to the next and to store absolute positions of resonators. This maximizes throughput and helps to minimized the air exposure time when the sample is mounted, as it can directly be moved to a predefined position. The whole coupling setup is installed on an optical table in a climate controlled laboratory and which is surrounded by an enclosure with sliding windows. A flow box establishes a constant air flow that prevents heat convection and ensures a constant temperature above the optical table. The syringe pumps are placed on a rack next to the optical table to grant accessibility and to decouple the vibrations from the syringe pump motors. However there is typically a temperature difference between the inside and the outside of the table enclosure, such that the fluid needs to thermalize with the environment of the coupling setup before entering the FCD. To this end we installed a thin PTFE tube (inner diameter of 0.8 mm) of ∼ 2 m length, which leads to the FCD (cf. Figure 2.1 (b) ) and which is taped to the surface of the optical table that serves in this case as a heat sink.

In general the inverted setup holds several advantages compared to an upright ar- rangement where the samples on the chip point upwards and the fiber taper is approached from the top. i) The tapered fiber does not need to be removed to exchange the sample. In practice it takes several tries to produce a fiber taper that meets the requirements for coupling in aqueous environment. Moreover an average number of 20−40 resonators needs to be tested and characterized to find a high-Q optical resonance that lies within the thermal tuning range of the Nd:YAG laser. The inverted design thus allows for high sample throughput that results eventually in higher sample quality. ii) The fixed mounting of the tapered fiber leads to an increase of coupling stability. iii) The complete coupling setup is designed in a way that it fits on commercial inverted microscopes (e.g. Leica DMI3000, Nikon Eclipse MA100, Olympus GX51, Zeiss Axio Vert). (iv) Gas bubbles that could rupture

2.1 Coupling setup 27

the fiber can easily escape. (v) Lastly, the sample mount itself facilitates sample handling, which will be reported on in section2.1.3.