Optical resonators (or cavities) are structures that keep light confined in space, typically bouncing in between a set of mirrors. They are a critical component in optical devices such as lasers and optical filters, making a thorough understanding of their properties technologically important. While resonators at the macro-scale are useful, they really shine at the micro-scale. Optical microcavities confine light into spaces so tight that only very specific wavelengths (known as cavity modes) of light can fit. Furthermore, they can actually modify how the confined light interacts with matter, allowing for new and improved optical devices.

The most widespread method for testing microcavities is by means of photoluminescence spectroscopy. In this technique, the microcavity needs to have light emitters within it (nowadays, the most commonly used emitters are semiconductor quantum dots, but also atoms and dyes can be used). When these emitters are excited with a laser beam, they emit light into the cavity modes. By measuring the spectrum of the photoluminescence it is possible to see the signature of the microcavity as peaks at the wavelengths corresponding to the cavity modes. This is a very convenient technique, since it is quite straightforward and it does not require any extra equipment beyond that regularly used for photoluminescence spectroscopy of materials. Although this method does not have a very high resolution, it gives a very good overview of the microcavity's mode structure. The main factors that limit the resolution are the capabilities of the equipment, and the spectral broadening of the emitters (in real life, the emitters do not emit at a single wavelength but rather within a range which becomes broader with higher temperatures).

Another method for investigating microcavities is evanescent field spectroscopy. This is a completely different technique, which requires holding the microcavity very close to a specially tapered optical fiber. As the wavelength of a laser coupled into the optical fiber is scanned, dips in the transmission (when light from the fiber "hops" onto the microcavity) show the presence of modes. Since the lasers can be scanned very finely, the resolution of this measurement technique can be very high. This high resolution allows for a very detailed study of the mode structure of a microcavity; on the other hand, this is a much more complicated experiment requiring specialized equipment so it is not as popular as photoluminescence spectroscopy.

Given the complementary features of photoluminescence and evanescent field spectroscopy when applied to the characterization of microcavities, they fulfill different research needs. However, there has not been to this date a clear comparison of the results of both techniques when used on the same microcavity. In this article, Zhi et al. perform this comparison. What they find is that even for very complicated mode structures (as measured using evanescent fields) photoluminescence spectra tend to be simple, showing only a smoothed picture of the modes and hiding the fine structure. This smoothing goes beyond what is expected from the instrumental resolution alone. However, to complicate the picture, the smoothing is much less pronounced than what would be expected if it were dominated by the spectral broadening of the emitters. Based on their modelling the authors propose that another mechanism, spectral diffusion, is responsible for the reduction in the smoothing; the term spectral diffusion designates the random changes of the emission wavelength of a luminescent system due to a fluctuating environment. According to their idea, these random changes increase the probability for the emission to couple into a microcavity mode with respect to when the emission wavelength is constant in time. As a result the peaks in the photoluminescence spectrum associated to microcavity modes should become sharper as the temperature (and the associated spectral diffusion) increases; this conclusion is supported by their experimental data that show narrower luminescence peaks at higher temperatures.

Since photoluminescence spectroscopy is so often used to characterize microcavities, this article is very useful, as it improves our understanding of its limitations when applied to these kind of studies. It also shows how nature can sometime surprise us, as the idea that spectral diffusion results in narrowing of the mode peaks is quite counterintuitive for people versed in the field.

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