Infrared detection and materials characterization using microwave resonators




Dev, Sukrith Umesh

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The radio frequency (RF) and microwave regions of the electromagnetic (EM) spectrum have seen incredible advances over the last few decades. RF components have become more compact, inexpensive, and accessible while still maintaining high performance. Applications such as communications or sensing that were originally intended for the military have now become commercial and used regularly by the majority of citizens. On the other hand, the mid-infrared (MIR) region of the EM spectrum, although gaining significant attention in recent years, does not have the same widespread material and device infrastructure associated with the RF. This dissertation seeks to exploit the maturity of the RF spectral range for infrared applications. Specifically, we utilize resonant microwave circuits, microwave split ring resonators (SRRs), as an effective means of mid-infrared detection and materials characterization. The presented work is primarily divided into three research thrusts. Ultimately, because the goal of this dissertation is to employ microwave resonators to interact with semiconductor materials, to determine the position of greatest field strength in our microwave circuits, the first technique introduced is the microwave mapping by optically induced conductance (MMOIC). In the MMOIC, a resonant microwave circuit is driven by a continuous wave (CW) RF source, while a laser simultaneously optically excites the semiconductor on which the circuit is fabricated. It is shown that the optically modulated signal is proportional to the square root of the RF power, suggesting that the response provides a measurement of relative electric field strength. It is demonstrated that when the circuit is driven on resonance, the spatial position of greatest field is located within the capacitive split-gap of the SRR. In the second presented technique, the micro-scale time-resolved microwave resonator response (µ-TRMRR), the SRR circuit is used to characterize the time-response of a micro-scale infrared pixel capacitively loaded in the split-gap. While driving the circuit on resonance, a pulsed laser excites electron-hole pairs (EHPs) in the pixel, modulating the amplitude of the transmitted carrier wave. By reading out the modulated carrier amplitude as a function of time via a Schottky diode RF detector, the minority carrier lifetime of the micro-scale material is effectively characterized. Results are compared with time-resolved photoluminescence (TRPL), and it is demonstrated that for this material system, the µ-TRMRR has a > 10⁵ improvement in sensitivity relative to TRPL. Next, the coupled pixel-SRR architecture, dubbed as the resonant microwave photoconductor (RMPC), is evaluated as a candidate for room temperature MIR detection. A noise analysis is performed on the RMPC, and it is found that the resonator can shape and suppress Johnson noise generated from the reactively coupled pixel. When compared to a standard DC photoconductor (DCPC) utilizing similar infrared absorber material, the RMPC architecture demonstrates a factor of three improvement in the Johnson-limited specific detectivity (D*). Finally, the dissertation concludes by summarizing the results and suggesting future work in this branch of RF-photonics.


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