But in order to tune to different frequencies, either the capacitance or inductance needs to change. So, as a first attempt, Chen adds silicon to the metal squares to change the capacitance of the structure. Silicon is responsive to light, so when Chen shines a near-infrared laser onto the strips, their capacitance changes, which selects a specific frequency from the incoming terahertz light. By adjusting the power from the infrared laser, Chen was able to tune in to specific frequencies from 850 gigahertz to 1.06 terahertz.
While Chen’s recent work is important for tuning across a range of terahertz frequencies, bringing it closer to a practical application, his approach could also be used for other frequencies. “What’s interesting here is, the way they’ve chosen to achieve [tunability] is versatile,” he says. The same approach of adding silicon strips could be used for devices at different wavelengths, including microwave and optical frequencies. While there are already devices that can tune across these, a tunable metamaterial is of interest to researchers for several reasons. Metamaterials are able to perform bizarre feats: lenses made of metamaterials can focus light tighter than conventional lenses can, and metamaterials have even been used to make a rudimentary cloaking device. A tunable metamaterial for many different wavelengths could expand the potential of these early applications.
Chen says that his next step is to tune the filter using electrical current instead of optical laser pulses. While it was more straightforward to change silicon’s properties by shining light on it, it’s ultimately more practical to use an electric current in an imaging system or wireless device.