Using active gain to maximize light absorption

Using an optical nanoresonator, such as a nanoparticle, to concentrate and absorb light at deep subwavelength scale is of both fundamental and technological significance. To quantify absorption, one typically introduces the absorption cross section and the absorption efficiency, i.e., the ratio of the absorption cross section of a local resonator to its geometric cross section. Exploring a general way to enlarge the absorption cross section of a deep subwavelength nanoparticle and increase the absorption efficiency certainly is important and highly desired. In this work, we show that by introducing a gain medium in the structure, the absorption cross section of any deep subwavelength particle could be boosted and even approach a universal maximum value, of the order λ2. Our approach for controlling the absorption using gain is general and may be useful both for the design of light harvesting materials and high-speed active nano-optics devices.