A New Way to “Squeeze” Infrared Wavelengths Down to Size

SCIENTIFIC ACHIEVEMENT

Using the Advanced Light Source (ALS), researchers demonstrated a new way to confine, or “squeeze,” infrared light by coupling photons with phonons (lattice vibrations) within a certain type of thin film.

SIGNIFICANCE AND IMPACT

The work heralds a new class of optical materials for controlling infrared light, with potential applications in photonics, sensors, and microelectronic heat management.

Title at top says "Synchrotron infrared nanoscopy." A ball-and-stick depiction of the SrTiO3 membrane (with green, blue, and red balls) floats in space. A needle on a cantilever arm is poised above the membrane. Red arrows pointing toward and away from the needle tip indicate direction of infrared beams. A packet of squiggly lines above the membrane surface is labeled "Phonon polaritons." — In this experiment, an atomic force microscope tip focuses broadband synchrotron infrared light onto the surface of a strontium titanate (SrTiO₃) membrane, just 100 nm thick. The infrared light excites phonon polaritons—quasiparticles that arise when light strongly interacts with dipole oscillations in the material’s lattice. Spectroscopic analysis of the scattered light enabled researchers to determine the properties of phonon polaritons on the material surface.

A light squeeze

Researchers have demonstrated that thin films of strontium titanate (SrTiO₃, or STO) can confine, or “squeeze,” infrared light 10 times more than its bulk form can—a finding that holds promise for next-generation microelectronic and photonic devices. While this unusual behavior had been theoretically predicted for STO membranes, it had not yet been experimentally observed.

The researchers took advantage of advances in the synthesis of freestanding, large-scale crystalline oxide membranes, then used a combination of infrared micro- and nanospectroscopy to observe how infrared light couples to lattice vibrations in the membranes. They found that the coupling produced hybrid vibrational and electromagnetic waves (phonon polaritons) in the material, with different modes characterized by highly compressed wavelengths or greatly enhanced fields inside the sample.

Transferable membranes

Theoretical studies have suggested that ultrathin STO and other perovskite membranes can host highly confined surface phonon polaritons (SPhPs) with good propagation quality. Other compounds may have higher figures of merit, but because they are typically manually exfoliated, their lateral size is constrained to the micrometer range, which limits their potential for large-scale device fabrication.

Yearbook-style photos of a woman and a man. — North Carolina State University co-authors Ruijuan Xu and Yin Liu.

For this work, the researchers prepared single-crystal membranes of STO, grown on a substrate with a water-soluble buffer layer. When the buffer layer is dissolved, a millimeter-scale STO film just 100 nm thick is released and can be transferred onto arbitrary materials. This approach avoids the problem of lattice mismatches, which can lower membrane quality in heterostructures that are epitaxially grown. Here, the STO membrane was transferred onto a SiO₂/Si substrate partially coated with gold, which allowed direct comparison of SPhP properties on metals versus insulators.

Symmetric-antisymmetric splitting

Using lab-based Fourier-transform infrared (FTIR) spectroscopy, the researchers were able to associate absorption peaks with what’s known as a Berreman mode, in which the normal component of the electric field inside the sample is strongly enhanced.

For further insights, the researchers used synchrotron infrared nanospectroscopy (SINS) at ALS Beamline 2.4 to probe the SPhPs with nanoscale resolution across a wide frequency range. The data revealed a splitting of the SPhP modes into symmetric and antisymmetric branches. These branches result from interactions between SPhPs at the top and bottom surfaces as the membrane thickness decreases. In the symmetric branch, the normal component of the electric field is greatly enhanced. In the antisymmetric branch, propagating SPhPs are highly confined (by a factor of 10 compared to the corresponding bulk mode). Notably, the antisymmetric mode is fully suppressed on the metallic substrate—an important observation for theory and applications. Theoretical modeling fully corroborates the experimental results.

Two panels, labeled "a" on the left and "b" on the right. Each panel is further divided into a white half (left) and a blue half (right). The white halves show line spectra (experimental and calculated), rotated 90 degrees clockwise from the usuall orientation to line up with the calculated dispersions on the right halves (blue is low intensity and red is high intensity). For both a and b, the data on the left aligns well with the dispersions on the right. — (a) Left panel: SINS spectra (purple symbols) for STO membrane on SiO₂/Si, compared to a model calculation (solid black line). Right panel: Corresponding (calculated) dispersion map showing symmetric and antisymmetric SPhP modes corresponding to the SINS peaks. Green crosses represent experimental data extracted from SINS imaging. (b) Same analyses as in (a), but for the STO membrane on gold.

A broader palette of properties

Finally, SINS scans across a sample edge allowed the researchers to extract from the data important SPhP parameters such as momentum, confinement factor, propagation length, and quality factor.

In general, the results open up a broad new class of complex oxide materials that, as transferable membranes, can be easily integrated with other photonic materials to generate SPhPs for light manipulation. The work expands scientists’ options for controlling infrared-wavelength light and establishes the enormous potential of transition-metal oxide membranes as building blocks for future long-wavelength nanophotonics.

Three line graphs, as described in caption. In the first graph, momentum increases vs frequency for STO membranes but is flat and near zero for the bulk. In the second graph, the confinement factor vs frequency also shows an increasing trend for STO membranes and a flat line near zero for STO bulk. In the third graph, propagation length and quality factor Q are plotted together, mirroring each other with generally downward trends as frequency increases. — Left: Momentum of SPhPs in the membrane and bulk. The observed momenta in the membrane (open circles) are in excellent agreement with theory (solid line) and are much higher than the momenta in the bulk (dashed red line). Center: Confinement factor of SPhPs in the membrane, which is larger than in the bulk by up to a factor of 10. Right: Propagation length (purple circles) and quality factor (blue squares) of SPhPs in STO.

Contact: Yin Liu

Researchers: R. Xu and Y. Liu (North Carolina State University); I. Crassee, A. Bercher, L. Korosec, C.W. Rischau, J. Teyssier, and A.B. Kuzmenko (University of Geneva, Switzerland); H.A. Bechtel and S.N. Gilbert Corder (ALS); Y. Zhou (University of Geneva, Switzerland, and Capital Normal University, China); K.J. Crust, Y. Lee, J. Li, and H.Y. Hwang (Stanford University and SLAC National Accelerator Laboratory); and J.A. Dionne (Stanford University).

Funding: National Science Foundation, North Carolina State University, State of North Carolina, Swiss National Science Foundation, and US Department of Energy (DOE), Office of Science, National Quantum Information Science Research Centers. Operation of the ALS and SLAC is supported by DOE, Office of Science, Basic Energy Sciences program.

Publication: R. Xu, I. Crassee, H.A. Bechtel, Y. Zhou, A. Bercher, L. Korosec, C.W. Rischau, J. Teyssier, K.J. Crust, Y. Lee, S.N. Gilbert Corder, J. Li, J.A. Dionne, H.Y. Hwang, A.B. Kuzmenko, and Y. Liu, “Highly confined epsilon-near-zero and surface phonon polaritons in SrTiO₃ membranes,” Nat. Commun. 15, 4743 (2024), doi:10.1038/s41467-024-47917-x.

ALS SCIENCE HIGHLIGHT #510