## 1 Introduction  
Diffractive Optical Elements (DOEs) are photonics structures that focus and redirect light, using diffraction rather than refraction. 'Volume' or 'thick' Diffractive Optical Elements utilize Bragg diffraction and can very efficiently transfer energy from the incident beam into a diffracted beam [1]. DOEs have the potential to replace optical components in a range of applications and have the advantages of being low cost, lightweight and in most cases, much thinner than their refractive counterparts. The DOE element can be thought of as a set of localized gratings, each potentially having a different slant, orientation and spatial frequency (set by the patterning process) so that the direction and divergence of the diffracted output beam can be chosen as desired [2]. As a result, DOEs can replace standard optical components (e.g., lenses and mirrors) in some applications. Bragg diffraction, however, introduces wavelength and angular selectivity, which can be particularly restrictive if the element/grating is thick [1]. Photopolymer materials are typically prepared as thin, photosensitive films that polymerize when they absorb light, and can record patterns with very high spatial resolution. They can, therefore, be used to form volume DOEs in a low-intensity, single-exposure step, which is readily customized. Since the patterning step involves holographic recording, they are also often referred to as Holographic Optical Elements or HOEs. Currently, Diffractive Optical Elements are not routinely used with broadband sources. Some practical applications utilizing surface gratings in the management of light are reported in the literature; for example, LED light is re-directed through a 30\({}^{\circ}\) angle by a binary diffractive lens [3] and broadband multicolor imaging is demonstrated by using multi-level surface holograms [4]. Fabrication of surface gratings is challenging, however, since it requires multiple manufacturing steps--lithography and etching--and the surface of the photonic structures is difficult to protect. Volume photopolymer HOEs [5] offer a significant improvement since they can be patterned (with low intensity exposure) [6] through the volume of the materials, can be laminated [7; 8] into a stack, and importantly, they can achieve near 100% re-direction of the incident light into a single output beam [9; 10]. This optical patterning can potentially replace the etching and stamping process and facilitate customization and small batches. The main features of surface and volume HOEs are compared, at a glance, in Table 1.  
Research into beam patterning and shaping with HOEs has continued to advance [11; 12; 13], but solutions for broad wavelength sources are needed to increase the application range. Many sources, such as white light LEDs, output a set of wavelength ranges rather than a continuous broad band of wavelengths (as for solar radiation, for example). This paper explores a method for producing a compound element useful for situations where elements that are designed for two separate target wavelengths can be advantageously combined to achieve a highly efficient HOE. The authors previously reported stacked gratings for operation in broader wavelength range by calculating the correct angle of diffraction for each target wavelength and recording at a single wavelength in green in two separate layers that are then stacked after recording [14]. This approach is somewhat limited by the complex modeling and recording arrangements needed for elements such as lenses, as they contain a wide range of spatial frequencies and slants that need to be matched to the different wavelength, as well as the difficulty of assembling the stack after recording without introducing damage and/or shifts in the slant angle. This paper presents a method of stacking that utilizes selectively pre-sensitized layers stacked together so that dual wavelength recording can take place independently in the two stacked layers and explores the process as a means to combine lens elements.