Methods and apparatus to generate terahertz waves through cascaded nonlinear processes

A system to generate waves having a terahertz frequency, comprising:

• a pump having a pump frequency, wherein the pump frequency comprises a visible wavelength, a near-infrared wavelength, an infrared wavelength or a mid-infrared wavelength;
• a multimodal cavity defined by two or more mirrors, wherein the modes are separated by the terahertz frequency of interest or a fraction of it, and wherein at least one of the two or more mirrors comprises a frequency dependent mirror having a frequency dependent transmissivity or reflectivity profile;
• and a nonlinear medium disposed within the cavity, wherein the frequency dependent mirror is disposed within the cavity and is configured such that transmissivity of modes having a frequency less than the pump frequency comprise a first value, and the transmissivity of at least one mode having a frequency greater than the pump frequency comprises a second value, less than the first value.