A design method of broadband metalens using time-domain topology optimization

Flat metalenses have attracted attention due to an increasing demand for compact electromagnetic devices. For such applications, broadband are highly desirable; however, conventional show relatively narrow band operation. Here, we propose a design method of free-form using topology optimization operate with enhanced bandwidths. In contrast preceding reports methods metalenses, developed based on the time domain formulation deal frequencies simultaneously. this purpose, group delay optical pulses in domain, which is equivalent phase matching condition frequency employed objective function. A level set applied obtain clear optimal configuration. To demonstrate effectiveness proposed method, provide examples metalens unit cells at millimeter frequency. We confirm that optimized superior performance compared both transmittance efficiency and error wavelength.