The increasing number of datacenter applications with heavy one-to-many communications has raised the need for an efficient group data delivery solution. This paper presents an unconventional clean-slate architecture called λ group that uses optical networking technologies to enable ultra-fast, energy-efficient, low cost, and highly reliable group data delivery in the datacenter. By replicating data purely optically, λ group enables reliable one-to-many communication independent of modulated data rate, while simultaneously being energy-efficient. λ group makes use of software defined network switches to facilitate direct interactions between applications and the network, eliminating the need for conventional multicast routing and group management protocols without sacrificing link-stress or latency performance. From an end-to-end perspective, λ group provides predictable latency and near-zero packet loss between application end points, allowing even simple end-to-end flow control and packet loss recovery mechanisms to work well. We implement a λ group prototype to measure its physical layer characteristics and end-to-end performance benefits to applications. Extensive simulations using synthetic traffic show λ group provides an order of magnitude performance improvement relative to a number of existing alternative group data delivery approaches.