This disclosure describes various microfluidic devices that may be used in thermal cyclic fluid samples. Some of these devices may include a plurality of microwells that may be coupled by interconnected fluidic channels. These microwells may not be physically separated and yet may include features allowing for effective isolation of target molecules within each microwell. Other devices may include a plurality of microwells that may not be interconnected. The devices may also include mechanisms for causing a fluid to flow across the device. The devices may also include light-absorbing films for converting light energy to heat so as to allow for thermal cycling of samples within the microwells.