A microwave heater (100, 200, 300) comprising: a cylindrical housing having an inner surface defining an internal cavity (102, 202, 302); a microwave generator (106, 206, 306) secured to the housing adjacent a first end thereof; and a sample holder received inside the internal cavity, the sample holder comprising: an annular base member (130, 230, 330) positioned adjacent the first end, the base member (130, 230, 330) having a central axis coinciding with a longitudinal axis (114, 214, 314) of the housing and a first opening (128, 228, 328) formed along the central axis, an outlet (120, 220, 320) of the microwave generator extending through the first opening (128, 228, 328) along the central axis for emitting into the internal cavity (102, 202, 302) electromagnetic waves centered at a given microwave wavelength, and a circular support plate (112, 212, 312) substantially parallel to the base member and concentric therewith, characterised by the support plate being separated from the base member (130, 230, 330) by a distance less than the given microwave wavelength for establishing a Transverse Magnetic 01 mode as a predominant resonance mode inside a cavity portion defined between the support plate and the base member, thereby suppressing a multimode effect from the cavity portion. a cylindrical housing having an inner surface defining an internal cavity (102, 202, 302); a microwave generator (106, 206, 306) secured to the housing adjacent a first end thereof; and a sample holder received inside the internal cavity, the sample holder comprising: an annular base member (130, 230, 330) positioned adjacent the first end, the base member (130, 230, 330) having a central axis coinciding with a longitudinal axis (114, 214, 314) of the housing and a first opening (128, 228, 328) formed along the central axis, an outlet (120, 220, 320) of the microwave generator extending through the first opening (128, 228, 328) along the central axis for emitting into the internal cavity (102, 202, 302) electromagnetic waves centered at a given microwave wavelength, and a circular support plate (112, 212, 312) substantially parallel to the base member and concentric therewith, characterised by the support plate being separated from the base member (130, 230, 330) by a distance less than the given microwave wavelength for establishing a Transverse Magnetic 01 mode as a predominant resonance mode inside a cavity portion defined between the support plate and the base member, thereby suppressing a multimode effect from the cavity portion. an annular base member (130, 230, 330) positioned adjacent the first end, the base member (130, 230, 330) having a central axis coinciding with a longitudinal axis (114, 214, 314) of the housing and a first opening (128, 228, 328) formed along the central axis, an outlet (120, 220, 320) of the microwave generator extending through the first opening (128, 228, 328) along the central axis for emitting into the internal cavity (102, 202, 302) electromagnetic waves centered at a given microwave wavelength, and a circular support plate (112, 212, 312) substantially parallel to the base member and concentric therewith, characterised by the support plate being separated from the base member (130, 230, 330) by a distance less than the given microwave wavelength for establishing a Transverse Magnetic 01 mode as a predominant resonance mode inside a cavity portion defined between the support plate and the base member, thereby suppressing a multimode effect from the cavity portion.