Space-time duality in paraxial optical wave propagation implies the existence of intriguing effects when light interacts with a material exhibiting two refractive indexes separated by a boundary in time. The direct consequence of such time-refraction effect is a change in the frequency of light while leaving the wavevector unchanged. Here, we experimentally show that the effect of time refraction is significantly enhanced in an epsilon-near-zero (ENZ) medium as a consequence of the optically induced unity-order refractive index change in a sub-picosecond time scale. Specifically, we demonstrate broadband and controllable shift (up to 14.9 THz) in the frequency of a light beam using a time-varying subwavelength-thick indium tin oxide (ITO) film in its ENZ spectral range. Our findings hint at the possibility of designing (3 + 1)D metamaterials by incorporating time-varying bulk ENZ materials, and they present a unique playground to investigate various novel effects in the time domain.

It is now possible to exercise a high degree of control over the velocity at which light pulses pass through material media. This velocity, known as the group velocity, can be made to be very different from the speed of light in a vacuum c. Specifically, the group velocity of light can be made much smaller than c, greater than c, or even negative. We present a survey of methods for establishing extreme values of the group velocity, concentrating especially on methods that work in room-temperature solids. We also describe some applications of slow light.

We analyze the influence of atmospheric turbulence on the propagation of an optical vortex beam having the form $V(r,\theta )=A_0{e}^{im\theta }$. The probability that a detected photon after propagating through the atmosphere has the same value of the orbital angular momentum as the launched photon is found to be given by $⟨s_0⟩={[1+{(1.845D∕r_0)}^2]}^{-1∕2}$, where D is the aperture diameter and $r_0$ is the Fried coherence diameter. These vortex beams behave very similarly to Laguerre–Gauss beams under the influence of atmospheric turbulence. These results have important implications for atmospheric laser communication systems that employ quantum encryption.

We describe a photonic device based on a high-finesse, whispering-gallery-mode disk resonator that can be used for the detection of biological pathogens. This device operates by means of monitoring the change in transfer characteristics of the disk resonator when biological materials fall onto its active area. High sensitivity is achieved because the light wave interacts many times with each pathogen as a consequence of the resonant recirculation of light within the disk structure. Specificity of the detected substance can be achieved when a layer of antibodies or other binding material is deposited onto the active area of the resonator. Formulas are presented that allow the sensitivity of the device to be quantified and that show that, under optimum conditions, as few as 100 molecules can be detected.

Resonant systems with high bandwidth: The performance of an active system, whether it is optical, electrical, or mechanical, is often described by its quality ( Q ) factor. Typically, one learns the rule that the higher the Q factor, the sharper the resonance—that is, the bandwidth of the device is reduced. Tsakmakidis et al. show that this is indeed the case, but only for symmetric systems. However, for the case of asymmetric (or nonreciprocal) systems, the rule need not be obeyed. They show theoretically that the more asymmetric a system with high Q is, the wider the bandwidth can be. The effect raises the prospect of designing high- Q devices operating over large bandwidths. Science , this issue p. 1260