## 2 Chemo-Mechanical Sensing with Optical Detection
### Optical Detection System  
The deflection of a cantilever sensor can be detected by various techniques [16]. The most frequently used method is beam deflection readout by the \"beam bounce method\" [17]. The method is relatively simple and is used to obtain cantilever deflection with very high accuracy (a schematic representation of our system is given in Figure 4a). A collimated light from a 5 mW laser diode with wavelength \(\lambda=635\) nm passes through a small aperture (diameter around 1 mm) and is focused on the backside of a cantilever using \(f=50\) mm spherical lens. The reflected beam is directed onto a position sensitive four-quadrant photodiode (PSD) using an adjustable mirror. The PSD consists of four closely spaced segments, and the photocurrent from each segment is proportional to the amount of light impinging on that segment from the cantilever. Thus, the changes in position of the reflected light can be precisely determined by measuring the difference in photocurrents from each photodiode segment using a differential amplifier. Cantilever displacements of the order of 0.1 nm are easily detected in this way. Another optical method for precisely detecting cantilever bending is the optical interferometric method [16]. It is much more sensitive, but requires an elaborate optical set-up. The electrical methods of detection of cantilever bending include the Scanning Tunnelling Microscope principle of operation. A sharp conductive tip is positioned in close proximity to the back side of the AFM conducting cantilever and the tunnelling current between the tip and the cantilever is measured. This method was used in the first AFM experiment by [PERSON] and [PERSON] [11]. In comparison to the beam deflection system it is much more sensitive, but is also quite complicated to set up and relatively bulky and thus rarely used. Other electrical methods of detection of cantilever bending include piezoresistive and capacitive detection [18,19]. While the piezo resistive method is quite handy, the force sensitivity is nearly two orders of magnitude lower compared to the optical detection [20,21] and thus impractical for high sensitivity sensor devices. The complete optical detection system is shown in Figure 4b. The outer part of the system is made out of stainless steel and serves as a carrier for mirrors, lenses, PSDs, and cantilever holders. This part has to be as rigid as possible to minimize any vibration between components of the measuring system. It is mounted on a vibration-isolated optical table. The inner part is made out of polished and gold plated copper and serves as a temperature stabilization cage. Its temperature is stabilized by 12 Peltier elements to a temperature within 1 mK. The copper cage is mounted onto a stainless steel outer part by only four Teflon holders in order to minimize the heat transfer between the two parts. All light beams enter and exit the inner part through thin glass windows. The testing gas enters the system from one side and exits the system on the other. The system consists of two mirror layouts for beam deflection readout and uses two different cantilevers for the operation (red and blue paths in Figure 4b). Simultaneous and independent measurement of the deflection of both cantilevers is possible. The top mirrors are mounted using tilt mirror mounts and used to direct the beams from the laser diodes onto the cantilever ends. The laser light enters the system through a small aperture and is focused using a spherical lens with a focal length of \(f\) = 50 mm. The reflected light is directed out of the inner copper part by mirrors, mounted on rotatable holders. The exact position of the reflected beam relative to the PSD is adjusted by translation mounts which hold the PSD. Each of the two cantilevers is mounted on a holder with a flat spring, and the distance between them in the system is about 1 mm.