# Holographic Lens Array for Solar Collector with Large Angle and Expanded Spectral Width
## 1 Introduction  
A solar collector is a device that modulates sunlight to achieve solar energy concentration by various optical elements. The capture and accumulation of solar energy avoids the high cost and low portability of large-area solar cells, offering competitive applications in industries requiring energy harvesting, such as construction [1, 2, 3], agriculture [4], and transportation [5]. Compared with traditional geometric optical elements [6, 7] and diffractive optical elements [8, 9, 10], holographic solar collectors [11, 12, 13, 14] have the advantages of lightweight, small size, high flexibility, and high multiplexing capability, demonstrating irreplaceable application as compact and efficient solar collectors. Nevertheless, the limited reconstruction angle and wavelength of holographic optical elements are important challenges to overcome in order to achieve a tracking-free solar energy collector that can work all day. On the one hand, due to the constant movement of the sun throughout the day, a small collection angle will result in a shorter effective working time of the system. Adding solar tracking equipment [15] can extend the effective working time, but this additional device will also increase the burn, cost, and complexity of the system. On the other hand, a narrow collection spectrum means that only a small portion of the wide-spectrum of sunlight is efficiently collected. Moreover, in some solar collectors that require perspective observation [16], a narrow collection spectrum will also cause strong color deviations between observed and real environments. There have been some early efforts to address the angular and spectral limitations of holographic solar collectors (HSC). In order to improve the acceptance angle of HSC, a low spatial frequency recording method [17, 18, 19], angle multiplexing method [20, 21, 22, 23], and multi-layer method [11, 24] have been proposed, which led to several discoveries. Firstly, reducing the spatial frequency of the recorded microstructure can expand the reconstruction angle, but the expanded angle is unable to exceed \(5^{\circ}\), which is far from the requirement of tens of degrees. Secondly, multiplexing several holographic lenses with different angles can expand the collection angle to approximately \(60^{\circ}\), but this expansion often comes at the cost of decreased diffraction efficiency. Finally, regarding the angle multiplexing and multi-layer methods, the efficiency within the entire large angle range is not always high, but there are several central angles with obvious advantages in efficiency. In order to broaden the collection spectral width of HSC, researchers have proposed a spectral separation method [25, 26, 27, 28] and wavelength multiplexing method [29]. However, the former method can only expand the collection spectrum into a few discrete narrow bands, while the latter method can only widen the spectrum at a fixed incident angle. Ultimately, various methods have been proposed that improve the collection angle and spectrum of HSC. However, expanding both the collection angle and spectrum simultaneously is still a problem that needs to be solved. This paper proposes a holographic lens array solar collector (HLASC) composed of an off-axis holographic lens array, which can expand the collection angle and spectrum at the same time. We first analyze the relationship between the parameters of the holographic lens array and the collection angle of the HLASC. In addition, in order to expand the collection spectral width, a spectrum spatial allocation method based on single wavelength recording for multi-wavelength reconstruction is proposed. Subsequently, a universal design method for flexible HLASCs with different parameters is established. Finally, the feasibility and rationality of this method for an HLASC with a large angle and wide spectrum are verified through optical experiments.