We describe the so-called $\lambda$ -Zeeman method to investigate individual hyperfine transitions between Zeeman sublevels of atoms in an external magnetic field of $0.1\mathrm{mT}–0.25\mathrm{T}$ . Atoms are confined in a nanocell with thickness $L=\lambda$ , where $\lambda$ is the resonant wavelength (794 or $780\mathrm{nm}$ for $D_1$ or $D_2$ line, respectively, of Rb). Narrow resonances in the transmission spectrum of the nanocell are split into several components in a magnetic field; their frequency positions and transition probabilities depend on the $B$ field. Possible applications are described, such as magnetometers with nanometric spatial resolution and tunable atomic frequency references.

Among the different types of photonic sensor devices, optical whispering gallery mode resonators (WGMRs) have attracted interest due to their high level of sensitivity, small size, and ability to perform real-time temperature measurements. Here we demonstrate the applicability of temperature measurements using WGMR in both air and liquid environments. We also show that WGMR allowed measurements of the refractive index variations in an evaporating glucose–water solution droplet. The thermal tuning of WGMR can be reduced by coating WGMRs with a thin layer of polymethyl methacrylate (PMMA). Dip-coating the silica microsphere multiple times significantly reduced the resonance shift, partially compensating for the positive thermo-optical coefficient of silica. The shift direction changed the sign eventually.

Whispering gallery mode optical resonators have attracted attention due to their simplicity and applicability for sensing. In this paper, analytical formulas are provided that describe resonance conditions in optical resonators. Basic terms (resonance wavelengths and frequencies, free spectral range,Q-factor, summation principle ofQ-factors of various processes, finesse, etc.) are introduced. A description of interference of an infinite number of waves of progressively smaller amplitudes and equal phase differences is given. A description of a Fabry–Perot resonator with nonequal reflection coefficients is also given as well as analysis of all-pass and add-drop optical filters. The presented description of resonators will help to analyze the effects of optical resonators, interpret the results of experiments, and guide the development of novel applications of microresonators.

We demonstrate a highly sensitive whispering gallery mode (WGM) relative humidity (RH) sensor based on a glycerol microdroplet. WGMs were excited using a 760 nm tunable semiconductor laser. We used free space coupling, which is effective when using a liquid resonator. A detailed analysis of different parameters influencing the sensor’s characteristics (sensitivity, hysteresis, resolution, stability, and temperature) is presented. The sensitivity of the sensor is one of the highest reported (2.85 nm/% RH in the range 50–70% RH with the resolution 1 × 10⁻⁴% RH). This type of humidity sensor has several advantages, such as high sensitivity, extended lifetime, good repeatability, and low cost, as well as the use of a non-toxic and environmentally friendly liquid.