Metasurface enables supersensitive, superfast thermal-based photodetector

I’ve always been interested in sensors and their related electronics. These devices are the interface between the real, physical world and the telemechanical systems that make use of their outputs. It’s also fascinating how many basic sensor approaches have been devised and enhanced for basic parameters such as temperature, pressure, distance, light intensity, and more.

Now we are entering a new phase where advances in materials—especially metamaterials, often aided by lasers—are creating breakthrough in sensors that could not be envisioned or implemented just a few years ago.

In short, a metamaterial is an engineered, 2D structure composed of subwavelength-scale elements that precisely control electromagnetic waves, such as light or microwaves, at an interface. The metasurface is an ultra-thin resonant element with special physical properties.

It’s typically composed of sub-wavelength structures (meta-elements) arranged in a 2D plane, enabling control over the propagation and scattering of electromagnetic waves at sub-wavelength scale by adjusting the phase, amplitude, or polarization of the incident waves

A good example of such an innovation is seen in the thermally based photon-detector project at Duke University, where researchers have demonstrated the fastest pyroelectric photodetector to date. It works by absorbing heat generated by incoming light and can capture light from wavelengths across the electromagnetic spectrum. The ultrathin device requires no external power, operates at room temperature, and can be readily integrated into on-chip applications.

Conventional semiconductor photodetectors work by initiating electron flow when struck by visible light. In contrast, the pyroelectric detector approach (also called a thermal detector) generates electric signals when it’s heated up after absorbing light.

Pyroelectric detectors have been in use for decades due to their wideband characteristic, unlike semiconductor sensors that tend to be narrowband devices (which is not necessarily a bad thing, of course). However, these pyroelectric devices are not as responsive as solid-state devices, since they are relatively bulky and have larger thermal mass.

Although using a thermal scheme is normally slow compared to using photons to stimulate electrical current, it does not have to be that way. In the Duke approach, the metasurface-enabled pyroelectric photodetectors are fabricated by layering a well-established nanogap cavity metasurface structure on top of a pyroelectric thin film (Figure 1).

Figure 1 Schematic representation of metasurface-enabled photodetectors illustrating key dimensions (a) with SEM image of the metasurface absorber (b). The red area represents the metasurface array. Finite element simulations of a single plasmonic nanostructure showing a cross-section of the pyroelectric layer 30 ps after resonant excitation of the metasurface (c).

The metallic metasurface consists of an array of nanoscale silver square prisms (90 nm × 90 nm × 35 nm) separated from a gold film by a thin (10 nm) dielectric layer of Al₂O₃ (aluminum oxide or alumina).

When light strikes the surface of a nanocube, it excites the silver’s electrons, trapping the light’s energy through a phenomenon known as plasmonics (the interaction between electromagnetic radiation such as light and conduction electrons at metallic-dielectric interfaces), but only at a specific frequency controlled by the nanocubes’ sizes and spacings.

In the latest iteration, the light-absorbing metasurface is circular rather than rectangular to maximize its exposure while minimizing the distance the signal must travel. This phenomenon is so efficient at trapping light and absorbing its energy that it only requires an extremely thin layer of pyroelectric material beneath it to create an electric signal.

Measuring the performance is another challenge. So, they devised an innovative arrangement with two distributed-feedback lasers that “brightened” when their frequencies became close to the same as the device’s operating speed.

The nearly perfect, spectrally selective absorption of the metasurface, which initiates the photodetector response, is shown by white light reflectivity spectra (Figure 2).

Figure 2 White light reflectance spectrum of a detector is shown with a 1.3 × 10⁻³ mm² active area of 40 μm diameter (a). Photocurrent responsivity spectra of the detector shown in (a) measured upon pulsed 100 nW light excitation as compared to that of a detector in which a gold film rather than a metasurface layer acts as an absorber (b). Photocurrent measured for the device presented in a) and b) upon pulsed 783 nm excitation at the indicated power with the beam size maintained to consistently have a diameter 5 μm smaller than that of the device (c).

The gold mirror alone efficiently reflects near-infrared light, while the metasurface exhibits a relative decrease (>95%) in reflectivity centered at 790 nm. The resonance wavelength is determined by the size of the Ag nanostructures and the thickness of the Al₂O₃ dielectric layer, as it allows the possibility of photodetectors that are spectrally selective across the visible and infrared portions of the spectrum.

The team found that their new thermal photodetector operates at record-breaking 3-dB bandwidth of 2.8 GHz, which corresponds to a rise time of just 125 picoseconds. Also important, these ultrafast speeds were achieved while maintaining competitive responsivities and noise equivalent power (NEP) as low as 96 pW/√Hz.

This is just one of the many innovative applications in the RF and optical worlds which leverage metamaterials and metasurfaces. Among many other uses, these materials enable new ways to manage and channel electromagnetic energy at these wavelengths, often to create sensors of extraordinary accuracy and precision.

The full details of this work by the Duke University team are in their paper “Metasurface-Enhanced Thermal Photodetector Operating at Gigahertz Frequencies” published in Advanced Functional Materials. While that posted paper is behind a paywall, the Duke team has thoughtfully posted an open-source version at their departmental website here.

Have you seen or used any sensors based on metamaterials or metasurfaces? What sensing challenges would you tackle if you had the needed meta resources?

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