Millimeter Waves Precisely Detect Nanoscale Materials

Scientists have developed a novel method for sensitively probing nanoscale materials using millimeter-wave photonic crystals, offering a crucial advancement for material characterisation. Kevin K. S. Multani from the Department of Physics, Stanford University, alongside Zhurun Ji, Wentao Jiang from the Department of Applied Physics & E.L. Ginzton Laboratory, Akasha G. Hayden from the Department of Electrical Engineering & E.L. Ginzton Laboratory, Gitanjali Multani, Sharon Ruth S. Platt, Emilio A. Nanni from SLAC National Laboratory, Zhi-Xun Shen and Amir H. Safavi-Naeini and colleagues demonstrate a dielectric platform compatible with strong magnetic fields, overcoming limitations of existing superconducting technologies and enabling studies in previously inaccessible conditions. Their research, conducted in collaboration between the Department of Physics, Department of Applied Physics & E.L. Ginzton Laboratory, Department of Electrical Engineering & E.L. Ginzton Laboratory, and SLAC National Laboratory at Stanford University, showcases a silicon photonic crystal cavity achieving a quality factor exceeding at 96GHz and successfully detects the perturbative response of a hexagonal boron nitride-multilayer graphene heterostructure, paving the way for sensitive, on-chip spectroscopy at millimeter-wave frequencies.

Detecting the properties of materials at the nanoscale has long demanded complex and bulky equipment. Now, a new photonic crystal design enables precise measurements using millimetre waves, opening doors to studying materials in previously inaccessible conditions. This compact technology promises to revolutionise how we analyse tiny structures and their behaviour.

Scientists have developed a new platform for probing nanoscale materials using millimeter-wave light, offering a potential route to study quantum materials in conditions inaccessible to existing techniques. Current methods, such as terahertz time-domain spectroscopy, can be challenging to implement in extreme environments like strong magnetic fields or at very low temperatures.

This approach utilizes silicon photonic crystal cavities, tiny structures engineered to trap and manipulate millimeter-wave radiation, to overcome these limitations and enable sensitive, on-chip spectroscopy. At millimeter-wave frequencies, the small wavelength of light and the properties of silicon allow for the creation of compact devices using standard silicon micromachining techniques. Scientists are reporting the cryogenic performance of these silicon photonic crystal cavities for the first time. The core principle relies on detecting shifts in the cavity’s resonant frequency and internal linewidth when a material introduces a change to them.

By positioning a hexagonal boron nitride-multilayer graphene (hBN-MLG) heterostructure, a layered material combining boron nitride and graphene, at a point of maximum electric field within the cavity, researchers observed a measurable change in the cavity’s resonance at room temperature. From this change, they extracted a total sample conductivity of approximately 5.1×106 S/m, validating the platform’s sensing capabilities.

The design process begins with engineering a photonic bandgap through periodic index modulation, then introducing a defect to confine light. A genetic algorithm maximised the radiation-limited quality factor of the fundamental defect mode, important for detecting minute changes in the material’s properties. Here, this perturbative sensing technique allows for frequency-domain detection, providing a more detailed analysis of material characteristics. By carefully analysing the shifts in the cavity’s resonance, scientists can determine the complex permittivity of the material under investigation. Beyond fundamental materials science, this technology holds promise for applications in areas such as advanced telecommunications and the development of new sensing systems.

Cryogenic characterisation of high-quality millimeter-wave silicon photonic crystal cavities for nanoscale material detection

Millimeter-wave silicon photonic crystal cavities serve as the central component in this effort, providing a platform for detecting nanoscale materials through perturbative sensing. These cavities, fabricated within a silicon substrate, were chosen because their dielectric nature allows operation in strong magnetic fields, a capability unavailable to superconducting counterparts.

To characterise performance, researchers cryogenically cooled a silicon photonic crystal cavity to 4.3 K. Fabrication involved electron-beam lithography and inductively coupled plasma reactive-ion etching to define the crystal structure within the silicon. Then, they positioned a hexagonal boron nitride-multilayer graphene (hBN-MLG) heterostructure at an electric-field antinode of the cavity to demonstrate sensing potential.

They accomplished this placement using a micromanipulator system, carefully aligning the heterostructure within the cavity’s electromagnetic field. Room temperature measurements then followed, monitoring the cavity’s response to the introduced material. A vector network analyzer measured the resonance shift induced by the heterostructure, enabling extraction of the sample’s total conductivity.

Careful consideration was given to understanding material losses within the silicon itself. Detailed calculations, based on the Drude model, estimated the frequency-dependent complex permittivity of the silicon substrate. Through relating the resistivity of the silicon wafer to free-carrier concentration and mobility, they determined parameters for the Drude model. Including the plasma frequency and scattering relaxation time.

These calculations, alongside modelling of asymmetric Lorentzian lineshapes, provided a foundation for interpreting the observed resonance shifts and accurately determining the conductivity of the hBN-MLG heterostructure. At the same time, this high-Q resonance forms the basis for sensitive perturbative sensing of nanoscale materials. Indicating a substantial electronic response within the heterostructure at millimeter-wave frequencies. Here, the design process began with engineering a photonic bandgap through periodic index modulation, then introducing a defect for mode confinement. Fabricators created the cavity from intrinsic silicon with resistivity exceeding 20 kΩ-cm. In turn, the cavity’s performance closely matched the simulated characteristics. They integrated the hBN-MLG heterostructure using a dry-transfer technique.

Through measurement of the perturbative effects of this material, researchers directly encoded the complex permittivity, ε = ε′ −jε′′. Within changes to the cavity resonance frequency and internal linewidth. At the same time, at room temperature, the heterostructure’s presence altered the cavity’s resonance, allowing for the extraction of its conductivity. A change in resonance frequency of even a few megahertz can be linked to specific material properties.

The periodic mirror cells create a photonic bandgap by coupling transverse-electric modes. By utilising a genetic algorithm, they maximised the radiation-limited quality factor of the fundamental defect mode. Inside the quasi-1D cavity, the resonance frequency decreases with increasing longitudinal mode number, due to the corresponding increase in effective refractive index. These results establish silicon photonic crystal cavities as a promising tool for on-chip spectroscopy of nanoscale materials at millimeter-wave frequencies.

Millimeter-wave silicon photonics enable nanoscale material sensing under demanding conditions

Scientists have long sought ways to probe the properties of materials at the nanoscale without disturbing them, and this effort presents a significant advance. For years, detecting faint signals from extremely small samples has demanded either exquisitely sensitive superconducting resonators, which struggle in the presence of strong magnetic fields, or cumbersome, low-throughput techniques.

Scientists demonstrate a silicon photonic crystal cavity capable of sensing nanoscale materials using millimeter waves. A frequency range offering unique advantages for studying certain quantum phenomena. The real step forward lies in establishing a platform compatible with both nanoscale samples and challenging experimental conditions. By fabricating these delicate silicon structures required careful etching and release processes, highlighting the practical difficulties of translating designs into functioning devices.

By securing the sample, a layered hexagonal boron nitride and graphene heterostructure, onto the cavity presented another hurdle. Necessitating a modified transfer technique involving heating to improve adhesion. The successful measurement of conductivity from this tiny sample confirms the potential of the approach. Interpreting these signals isn’t straightforward.

At these millimeter-wave frequencies, aligning the transmitting and receiving antennas within a cryostat, the device used to maintain extremely low temperatures, proved problematic, generating background noise that initially obscured the desired signal. By carefully optimising the setup, the team overcame this obstacle, but it underscores the sensitivity of The trial to external factors.

Future work might explore integrating the antenna directly into the chip, reducing signal loss and simplifying the experimental arrangement — the prospect of performing on-chip spectroscopy at these frequencies opens up new avenues for materials science and condensed matter physics. Unlike traditional methods, this platform allows for The effort of materials in extreme environments, and potentially revealing novel behaviours under high magnetic fields or at cryogenic temperatures. The versatility of the system suggests it could be adapted to investigate a wide range of nanoscale materials, while offering a powerful new tool for exploring the fundamental properties of matter.