# Electromagnetic Power Harvester Using Wide-Angle and Polarization-Insensitive Metasurfaces
## 1 Introduction  
With the rise of the internet of things (IoT) technology, the micro wireless sensor networks (WSN) are being extensive development. In recent years, with the development of the ultra-low power consumption chip technology, the power consumption of wireless sensor node has already entered a microwatt magnitude range. At the same time, an increasing number of wireless network and mobile communication base stations and other radio transmitting equipment that is full of rich electromagnetic energy in the surrounding environment. It is also a research hotspot to reduce the demand of the battery by absorbing the ambient energy to power a sensor node automatically and adaptively in the electromagnetic environment.  
Current researches show that there are about 12 million wireless communication base stations in the global in 2012, about 68 billion equipments using a radio frequency identification card (RFID) in 2013, and the world has about 40 billion WiFi consumer electronic equipments in 2014 [1]. Figure 1 shows that the ambient power spectrum distribution of the electromagnetic field measured in the campus environment of Xidian University, China. We can find that the ambient energy peaks appear in the existing wireless communication frequency bands. These micro electromagnetic energies that distribute in the environment are expected to be used again after harvesting efficiently.  
Metamaterials are the artificial composite structures or composite materials with superior physical properties that some natural materials do not have. Metasurfaces (MS) are two-dimensional metamaterial structures, which have been attracting more attention, due to a wide range of potential applications for wireless power transfer and harvesting [2]. A perfect metamaterial absorber has been firstly proposed in 2008 by [PERSON] et al. [3]. Different from the electromagnetic metamaterial absorber [4; 5; 6], the small split-ring resonators (SRR) were first used to collect energy in2012 [7]. After that, some other SRRs [8; 9; 10], electric-inductive-capacitive (ELC) resonators [11], and ground-backed complementary split-ring (G-CSRR) resonator have also been proposed for electromagnetic energy harvesting [12; 13]. The recently reported wideband ground-backed complementary split-ring (WG-CSRR) resonator structure can improve the bandwidth [14]. Each unit in the WG-CSRR structure requires four ports in order to collect energy at the same time.  
This paper uses a new and simple electromagnetic MS as the ambient energy harvester in the wireless communication systems. The proposed MS is a two-dimensional energy harvesting array that is composed of subwavelength electrical small square-ring resonator unit. Because the incident angles and polarizations of environmental electromagnetic waves are random and unknown, the conventional energy harvesting antenna or polarization-related metamaterial harvester cannot maintain the high collection efficiency for arbitrary direction and polarization. No matter whether the incident wave is transverse electric (TE) or transverse magnetic (TM) polarization mode, the proposed MS unit or array can keep a high RF power harvesting efficiency. Due to the frequency distribution of electromagnetic energy in the environment has the narrow-band characteristics, this work uses only one harvesting port designed at the appropriate location for spatial filtering, which can simplify the back-end matching and rectifier circuit complexity, and realize the high efficiency of energy harvesting. The proposed MS is sub-wavelength, wide-angle, and polarization-insensitive, which is suitable for ambient power harvesting.