## 1 Introduction  
The use of metasurfaces operating in the terahertz (THz) band for sensing applications has gained increased attention in the last few years. This is because of the recent and unprecedented advances achieved in the development of technology in this spectral region [0.1-10 THz], such as more efficient sources and detectors added to commercial instrumentation like THz time-domain spectrometers (THz-TDS) [1]. The particular characteristics of the THz band such as the sensitivity to weak molecular interactions, water absorption, and penetrability through non-polar materials, have attracted the attention of the scientific community in a wide variety of fields such as security,medicine, communications [2], as well as sensing applications [3; 4; 5; 6; 7; 8; 9]. Metasurfaces are usually designed by assembling arrays of subwavelength resonators (meta-atoms), whose electromagnetic response can be arbitrarily manipulated by selecting the best suitable materials, geometries, and spatial arrangement. Of particular interest in sensing applications is the fact that meta-atoms produce a high electric field intensity at their resonance frequency, leading to an enhancement of the interaction of any substance or analyte under analysis placed on, or at the vicinity of, the metasurface. As a result of such light-matter interaction enhancement, observable changes in its frequency response arise. This is advantageous compared to traditional detection methods, whose main weakness lies in the difference of wavelength and analyte size: the THz wavelength size is of the order of tens to thousands of microns, whereas typical analyte sample sizes are often below one micrometer [4; 5; 9; 10; 11; 12]. For this reason, there is usually not enough interaction between the radiation and the sample, and thus, the sensing becomes very difficult or even impossible. Thin-film sensing is of high interest in practical applications, especially when measuring substances in a thin homogeneous sample becomes essential or when the sample amount is too small. Additionally, it can be used for situations in which it is easier to process a thin-film form of the sample due to the nature of the substance, such as in biological and chemical sensing [13]. There are different examples in the literature of metasurfaces designed to perform this type of sensing such as designs based on split-ring resonators (SRRs) [10; 14; 15], crosses [8; 16], metastructures that exploit very sharp resonances such as Fano devices or hole arrays [10; 17], or more complex geometries or materials such as graphene-based metasurfaces [18]. There are recent reviews that summarize all this information and the different types of device proposed in the literature [5]. Whichever the type of metasensor employed, one of the biggest challenges of THz thin-film sensing is how to achieve a high-quality device in terms of sensitivity (a parameter that gives a measure of the frequency shift compared to the response of the empty structure) and figure of merit (FOM, that relates the sensitivity and resonance linewidth). In practice, it is found that the higher the electric field confinement, the higher the sensitivity and FOM [4]. The main advantage of metastructures based on meta-atoms lies in the ease of their design and manufacture since their unit-cell geometries are not very complex. Meta-atoms based on crosses or SRRs have been extensively studied in other works [8; 9; 10; 12; 14; 15; 16]. Motivated by the promising capabilities of metasurfaces for sensing and their importance for thin-film detection at THz frequencies, in this work we propose metasensors using meta-atoms with a geometry consisting of three blades separated from each other by 120 degrees, called \"tripod metasurfaces\".