Energy conversion in a physical system requires time-translation invariance breaking according to Noether's theorem. Closely associated with this symmetry-conservation relation, the frequencies of electromagnetic waves are found to be converted as the waves propagate through a temporally varying medium. Thus, effective temporal control of the medium, be it artificial or natural, through which the waves are propagating, lies at the heart of linear frequency conversion. Here, we explain the basic principle of linear frequency conversion in a rapidly time-variant metadevice and show various interesting properties and future prospects of rapidly time-variant metadevices.
The plethora of nonlinear optical phenomena can provide an innovative route for developing subwavelength-scale functional optical devices. One of the examples may be the nonlinear mixing of low energy photons (of which the wavelength is a few hundred micrometers) in atomically-thin materials. Here,the experimental proof on the optically-induced nonlinear mixing of terahertz resonances in graphene-integrated metadevices will be presented. Upon ultrafast optical excitation, the conductivity of graphene is reduced for a few picoseconds due to the increase in the Dirac-fermion scattering rate. This fast temporal change of graphene conductivity provides time-varying perturbation to the graphene-integrated metadevices and generates a difference frequency component by the mixing of meta-atoms’ two electric dipole resonances. Ultrafast terahertz spectroscopy corroborates that the characteristic difference-frequency resonance indeed originates from the coupled interaction between graphene and meta-atoms. Further elaborating this concept, it will be shown that the sudden merging of distinct meta-atoms’ resonances by ultrafast optical excitation can also result in frequency conversion.
It is well known from transformation optics that a light pathway can be designed with artificial materials. When a coordinate transform technique is applied to optically resonating dielectric structures, interesting phenomena can be observed as well. Generally, a long-lived whispering gallery mode (WGM) has no preferential direction of radiation because of its rotationally symmetric structure. However, if the space inside the resonator is transformed so that the discontinuity of coordinates exists, it becomes possible to reconcile directional emission with WGMs. Here, we transform only the inner space of a deformed optical cavity, e.g., the Limaçon cavity into a virtual perfect disk via a conformal mapping and show these two seemingly incompatible behaviors can be observed simultaneously. The refractive index profiles that realize the transformed space can be obtained from the conformal space transformation. The resonant mode calculated with a transformed boundary element method shows that the WGMs can restore for the deformed cavity. The Husimi function calculated for this transformed cavity shows a weighted band-like profile, which implies that the optical rays inside the cavity is maintaining its reflecting angle as is for the original cavity. However, the far-field pattern shows anisotropic emission of radiation because it is determined by tunneling through the rotationally asymmetric boundary. For example, the conformal WGMs in Limaçon and center-shifted triangular cavities exhibit bidirectional and uni-directional emission patterns in the far-field, respectively. These conformal WGM cavities with both the ultra-high quality factor and the directional light emission may be used in the realization of efficient directional light sources.
Increasing demands on the high capacity wavelength division multiplexed (WDM) transmission system now require newly developed transmission windows beyond the gain bandwidth supported by erbium-doped fiber amplifiers (EDFA). With the intensive development efforts on new rare-earth dopants and fiber nonlinearity (Raman process) for fast few years, wideband optical amplifiers now can support easily over 4-5 fold wider gain bandwidth than it was formerly possible with the conventional EDFAs. Of various breeds for this application, there exist three distinct approaches near 150nm band, accessible in the commercial market. These include: Thulium-doped fluoride fiber amplifiers (TDFA) for S+band (1450-1480 nm) and S band (1480-1530 nm), EDFAs for C band (1530-1560nm) and L band (1570-1610nm) and L band (1570-1610nm), Raman amplifiers with 100 nm's of gain bandwidth (with flexible location from S+ to L Band), and hybrid amplifiers with serial/parallel combinations of above techniques. Even though there have been much increased experimental reports for all of these amplifiers, the complexity of the amplification dynamics from the number of involving energy levels and difficulty in measuring experimental parameters make it harder than ever to predict the performance of wideband amplifiers in general. This lack of serious study on the analytic or numerical analysis on wideband amplifiers could cause the future impairments for the prediction and estimation of the amplifier performances for different applications, restricting the successful deployment of wideband amplifier based transmission systems. In this paper, we present the numerical model and analysis techniques for wideband amplifiers (C/L band EDFA, Raman amplifier, and TDFA),along with their application examples.