Cantilever-based photoacoustic detection of electromagnetic radiation Sucheta Sharma 1 , Toni Laurila 1 , Jussi Rossi 2 , Markku Vainio 2,3 and Erkki Ikonen 1,4 1 Metrology Research Institute, Aalto University, Espoo, Finland, 2 Photonics Laboratory, Physics Unit, Tampere University, Tampere, Finland, 3 Department of Chemistry, University of Helsinki, Helsinki, Finland, 4 VTT MIKES, Espoo, Finland Corresponding e-mail address: sucheta.sharma@aalto.fi We report here on a highly sensitive silicon cantilever-based photoacoustic electromagnetic radiation detector of broad spectral bandwidth. The developed detector can be used for measuring the power of radiation in the 100 nW to 10 mW range. Experiments have been carried out using 633 nm and 1523 nm lasers. The results show very good linearity and dynamic range. INTRODUCTION For monitoring environmental pollution or detecting toxic, inflammable and explosive gases [1-2], which pose major risk on health, safety and security, photoacoustic (PA) spectroscopy is one of the applied detection methods because of its high sensitivity and selectivity [3-4]. In the gas sensing applications, the non-radiative relaxation processes of electromagnetic-radiation-induced excited molecules generate heat and consequently produce pressure waves, which give rise to the PA signal. One of the common methods for detecting the PA signal employs a capacitive microphone. However, a capacitive microphone has several limitations including the nonlinear response of the membrane in sensing the external pressure. Apart from that, for improving the sensitivity, the gap between the membrane and the backplane of the capacitive condenser cannot be reduced below a certain limit, as the gas cannot anymore flow freely through such narrow region due to viscous effect [5]. An alternative approach has been developed for PA signal detection where optical beam deflection and micro-mechanical silicon (Si)- cantilevers are used. The cantilever-based approach does not have such drawbacks and it has been successfully used in many applications requiring very sensitive PA signal detection [6]. The cantilever- based PA signal detection method employs a compact interferometer for measuring the deflection of the cantilever tip [7]. In this study, we report on a Si- cantilever-based PA detection system for electromagnetic power measurements. Benefits of the new power measurement method include good linearity and broad spectral bandwidth. In contrast, most available power detectors are highly dependent on the operating wavelengths. Si-photodiode detectors, for example, work mainly in 200 nm-1100 nm range whereas, for infrared wavelengths, InGaAs detectors are used. However, the PA principle employs absorption of radiation into a black absorber material and this can be made rather independent of the operating wavelength over a large spectral range. EXPERIMENT Figure 1 shows the schematic diagram of the experimental setup. The system comprises three main modules: (1) an electromagnetic radiation source, the power of which falls on the absorbing material (for this experiment, candle soot was used) to generate heat, (2) the acoustic section consisting of a gas cell (in this case, it was filled with air) which contains the sample and a cantilever, (3) the optical readout system, to measure the cantilever deflection using a compact Michelson interferometer. The incident radiation first passes through a chopper and gets absorbed by the black absorber material to produce heat. The generated periodic heating causes volume expansion of the gaseous medium within the closed PA cell and produces pressure waves, which are finally detected by monitoring the position of the cantilever tip by using the optical interferometer. RESULTS Experiments were performed using two incident wavelengths: 633 nm and 1523 nm. In both cases, the Figure 1. Schematic diagram of the experimental setup.