Sound and noisy light: Optical control of phonons in photoswitchable structures

We present a means of controlling phonons via optical tuning. Taking as a model an array of photoresponsive materials (photoswitches) embedded in a matrix, we numerically analyze the vibrational response of an array of bistable harmonic oscillators with stochastic spring constants. Changing the intensity of light incident on the lattice directly controls the composition of the lattice and therefore the speed of sound. Furthermore, modulation of the phonon band structure at high frequencies results in a strong confinement of phonons. The applications of this regime for phonon waveguides, vibrational energy storage, and phononic transistors is examined.

Figure 1

Schematic model of system. Photoswitches (green) sandwiched between damped regions (brown) which are clamped at the far ends. Light (orange curves) is applied to the photoswitches, which are also driven mechanically (blue arrow) to produce phonons. Photoswitches are modeled as a series of 1D SHOs composed of photosensitive materials (e.g., anthracene, see bottom) embedded in a matrix, creating a bistable system with two spring constants.

Figure 2

Phonon amplitude under photoswitching. The $x$ axis denotes the distance along the 1D chain, and the $y$ axis denotes time. Contours (colored) are isoval curves of phonon amplitude $u_i\left(t\right)$ (see the side bar). Gray segments are the ground state (no illumination), white segments are excited (illuminated), and gradients are transitions. (a) Low frequency ($\omega =0.5/t_{\gamma }$). The bottom inset (red, vertical slice) shows $u(N/2,t)$ (red, solid curve) and $k(N/2,t),k(N/2+1,t)$ (black, dashed curves) as a function of time. The side inset (blue, horizontal slice) shows a snapshot $u(x,t_0)$ (blue, solid curve) and $k(x,t_0)$ (black dots) as a function of position. (b) Midfrequency ($\omega =1.5/t_{\gamma }$). (c) High frequency ($\omega =2.5/t_{\gamma }$).

Figure 3

Dispersion $\omega \left(q\right)$ for a 1D chain of photoswitches. The color denotes the strength of illumination from blue (no illumination, all in the ground state) through red (full illumination, all in the excited state). Dotted lines and dots (with error bars) indicate numerical results. Solid lines denote fits to the sinusoid dispersion. Inset: Speed of sound as a function of fraction in the excited state. The colored, solid line is the numerical fit (the color corresponds to dispersion) and the black, dashed line is the kinetic model.

Figure 4

Transmitted signal (amplitude vs time) of photoswitches in a switch/transistor regime. The blue curve is the response, the black curve is the rms average over dark period, and the red curve is the rms average over the illuminated interval. The separation of the black and red curves is the switch's efficacy.