Method and apparatus for conjugate quadrature interferometric detection of an immunoassay

A method for detecting the presence of an antigen on a photonic structure, the method comprising:

• exposing the photonic structure to a sample that may contain the antigen, the photonic structure having a reflectance-band and side bands;
• illuminating at a wavelength in at least one of an edge of the reflectance-band and the side bands of the photonic structure with an illumination source to create a return beam;
• detecting the return beam with a detector system having an intensity-sensitive detection function and a phase-sensitive detection function;
• detecting an in-line quadrature signal from the return beam using the intensity-sensitive detection function of the detector system;
• detecting a differential phase contrast quadrature signal from the return beam using the phase-sensitive detection function of the detector system;
• receiving the in-line quadrature signal and the differential phase contrast quadrature signal;
• combining the in-line quadrature signal and the differential phase contrast quadrature signal to generate a joint signal, and
• determining one of the presence or the absence of the target material on the photonic structure using the joint signal.

A method of using a resonant response modulus to provide a signal robust against operating system drift, the method comprising:

• providing a substrate including a plurality of target molecules distributed about the substrate;
• illuminating the substrate with a illumination source to produce a reflected beam and cause a resonant response;
• detecting the modulus of the resonant response with a photodetector by separately detecting an in-phase intensity resonant response and a conjugate phase resonant response;
• and
• combining the in-phase intensity resonant response and the conjugate phase resonant response to provide a signal robust against operating system drift, the signal being representative of the detected modulus of the combined responses.