line resonators

Head_secondary02


	microstrip line resonators

		Half-wavelength microstrip line resonators have been used for EPR spectroscopy since 1974. Large filling factors make these moderate quality factor resonators (Q~100) to compete in sensitivity with high quality EPR geometrical cavities. In addition, their high efficiency in converting the microwave power into ac magnetic field at the sample position, provide an excellent tool for real-time studies of dynamics of low-dimensional magnetic systems.

		Half-wavelength microstrip line resonators have been used for EPR spectroscopy since 1974. The figures below show a sketch of a typical reflection microstrip line resonator and its reflection parameter S11. The fundamental mode (L ~ /2) may be between 5-30 GHz. In our devices, the geometry (i.e., width of the line, w, and thickness of the dielectric substrate, h, separating the line from the metallic bottom plate) has been calculated to match the impedance of the microwave coaxial lines (50 ), to avoid reflections at the device connections. The coupling gap is engineer to critically couple the resonator line to the feeding transmission line in order to optimize the quality factor and minimize power reflection.






	On-chip integration of Hall-effect magnetometry and EPR spectrometry The integrated sensor enables measurement of the magnetization response of micron scale samples upon application of microwave fields. In particular, the combined measurement of the magnetization change and the microwave power under cw microwave irradiation of single-crystal of molecular magnets is used to determine of the energy relaxation time of the molecular spin states. Submitted to: Rev. Sci. Instrum. (2008) Arxiv:0805.0565


	Reflection resonators are not convenient to work with at low temperatures because long coaxial lines, broken at different places for thermal anchoring, are required to transfer the microwave radiation to the sample. Reflections and standing waves generate oscillations of the reflected power (S₁₁) masking the response of the resonator. To solve this problem, we have developed transmission resonators by including a second feed line on the other side of the resonator, separated by a transmission gap, g_t, as shown in the sketch below. The second feed line may perturb the response of the resonator, increasing the losses and decreasing the quality factor substantially. In order to minimize this distortion of the resonator response, we have increased the transmission gap. The figure below shows the Sij-parameters of several transmission resonators with different transmission gap sizes. Both the transmission and the reflection at resonance decrease (Q increases) upon increasing the transmission gap. Beyond a certain gap size the reflected signal is indistinguishable from that of a reflection resonator.


	On-chip integrated HEM-EPR micro-sensors


	We have developed a novel sensor that integrates high sensitivity micro-Hall effect magnetometry and high-frequency electron paramagnetic resonance spectroscopy capabilities on a single semiconductor chip. The Hall-effect magnetometer (HEM) was fabricated from a two-dimensional electron gas GaAs/AlGaAs heterostructure in the form of a cross, with a 50x50 m² sensing area. A high-frequency microstrip resonator is coupled with two small gaps to a transmission line with a 50 Ohms impedance.

	The sketch below shows the design of the HEM-EPR integrated sensor. The sensing area of the HEM lies directly underneath the center of the resonator, where the ac magnetic field is largest at the fundamental resonant mode.



		The figure below shows the simultaneous measurement of the magnetization changes upon aplication of microwave radiation and the power absorved by a microscale single-crystal of Ni₄ single-molecule magnets.

foot_page02