components of electro-magnetic waves. The simplest wave plates have a homogeneous uniaxial birefringence (Δn) with a thickness (d) and induce a phase shift between the two orthogonal waves according toin which λ is the wavelength and k the induced phase shift divided by 2π. [1,8,9] Liquid crystals (LCs) are frequently used as wave plate materials. By programming the Δn and d of a uniaxial aligned LC quarter-and half-wave plates can be easily fabricated. Also achromatic polarization converters can be obtained by stacking multiple wave plates using the principle of retardation compensation or using configurations involving twisted nematic LCs. [1,[8][9][10][11][12][13][14][15][16][17][18] Wave plates based on nonpolymerized LCs in cells can also be switched using electric fields, which among others resulted in the billion dollar LC display industry. [1,19,20] To widen its application range it would be appealing to make wave plate films that are responsive to other stimuli such as temperature. However, stimuli-responsive polymer wave plates have never been reported so far.We recently reported on a reflective LC coating based on a semi-interpenetrating polymer network (semi-IPN) composed of a LC elastomer (LCE) and a helical LC network (LCN). The coating showed a fast and reversible decrease in reflection band intensity with increasing temperature, which could be tuned by the polymer network density. [21] In this work, we developed temperature-responsive polymer wave plates, which reversibly change from a full-wave to a half-wave plate upon heating. The wave plate consists of a uniaxial aligned nematic semi-IPN in which a non-crosslinked LCE interpenetrates through an LCN. Upon heating above the nematic-to-isotropic transition temperature (T N-I ) of the semi-IPN, the LCE loses order and the effective Δn of the material halves, while d remains constant, thereby changing the polymer film from a full-wave to a half-wave plate. We demonstrate its function by sandwiching the temperatureresponsive wave plate between two identical cholesteric LC (CLC) polarizers reflecting right-handed circular polarized (CP) light around the λ for which the wave plate is operative (Figure 1). Below the T N-I , the left-handed CP light transmitted by the first polarizer is effectively not affected by the full-wave plate and thus also transmitted by the second polarizer, resulting in an overall reflectivity of 50%. Upon heating above T N-I , the wave plate turns into a half-wave plate, which converts the A temperature-responsive polarization converter, which reversibly changes from a full-wave to a half-wave plate upon heating, is developed. The polymer wave plate has a controlled thickness and is based on a uniaxial aligned nematic semi-interpenetrating network coating containing a specific concentration of a non-crosslinked liquid crystal elastomer. Upon heating, the effective birefringence of the wave plate halves without changing the thickness. The function of the wave plate is demonstrated by sandwiching the tunable polarization conver...