Broadband Dipole Absorption in Dispersive Photonic Time Crystals

Abstract

Photonic media modulated periodically in time, termed photonic time crystals (PTCs), have attracted considerable attention for their ability to open momentum bandgaps hosting amplifying modes. These momentum gaps, however, generally appear only at the system's parametric resonance condition which constrain many features derived from amplification to a narrow frequency band. Moreover, they are accompanied by exceptional points (EPs) and may drive the system into an instability, which render their analysis more intricate. Here, we show that a careful consideration of dispersion and absorption can overcome these issues. By investigating the dissipated power of a point-dipole embedded in a dispersive and absorptive PTC, we unveil that temporal modulation enables the conversion of dipole emission into dipole absorption within a broadband frequency window free of EPs. We demonstrate that this effect is general, emerging in both the stable and unstable regimes, and occurs from weak modulation strength to low modulations frequencies that could be achieved for various material platforms.

Figures (4)

• Figure 1: (a) Sketch of the system under consideration. A three-dimensional Drude-Lorentz medium, with either its plasma or resonance frequency modulated sinusoidally in time, contains an oscillating dipole. (b)-(c) Formation of dispersive momentum gaps. (b) Complex bandstructure of the static medium ($\alpha=0$). Dashed lines represent Floquet replicas of the original bands, shifted by a modulation frequency $\Omega=\omega_{\mathrm{p}}$. (c) Complex bandstructure of a medium with a modulated plasma frequency ($\alpha=0.05$) in the first FBZ. The red area highlights the modulation-induced gain region. In both panels, $\omega_0=0.6\omega_{\mathrm{p}}$ and, as in the remaining of this paper, $\gamma=0.02\omega_{\mathrm{p}}$.
• Figure 2: Broadband dipole absorption through weak modulation of the plasma frequency. (a) Momentum-resolved LDOS. The gray dashed lines show the real Floquet bandstructure Re$[\omega_\perp]$. (b) Imaginary Floquet bandstructure. (c) Positive $\bar{P}_\omega^{\perp,\mathrm{loss}}$ and negative $\bar{P}_\omega^{\perp,\mathrm{gain}}$ parts of the power dissipated by a point dipole with frequency $\omega$, in units of the nonmodulated value $\bar{P}_\omega^{\perp,0}$. (d) Total dissipated power $\bar{P}_\omega^{\perp,\mathrm{total}} = \bar{P}_\omega^{\perp,\mathrm{loss}} - \bar{P}_\omega^{\perp,\mathrm{gain}}$.
• Figure 3: Broadband absorption and inhibition of dissipated power. Same quantities as in Fig. \ref{['fig:weak modulation']}, but considering a stronger yet lower modulation of the plasma frequency.
• Figure 4: Drastic enhancement of emission and absorption in a stable PTC. Same quantities as in Figs. \ref{['fig:weak modulation']} and \ref{['fig:strong modulation']}, but considering a modulation of the resonance frequency around the value $\omega_0=0.3\omega_\mathrm{p}$.