Scientists at Rice University have demonstrated a method to pair atomic vibrations with light waves in perovskite thin films, potentially transforming how engineers design next-generation optoelectronic devices.
This achievement, which occurs at room temperature without extreme conditions, could lead to more efficient solar cells and LEDs by providing unprecedented control over energy flow at the quantum level.
The research team, led by doctoral alumnus Dasom Kim and Smalley-Curl Institute director Junichiro Kono, successfully created hybrid states of matter called phonon-polaritons. They achieved this by engineering nanoscale gold slots that trap and tune light to match the natural vibrations of perovskite crystals. This approach represents the first room-temperature demonstration of ultrastrong coupling between two phonons and a single engineered terahertz resonance in perovskite thin films.
The breakthrough centres on manipulating phonons-quantised vibrations in crystal lattices-to interact strongly with electromagnetic waves. In lead halide perovskites, materials already known for their exceptional optoelectronic properties, the researchers achieved what physicists call “ultrastrong coupling”, where the interaction between light and matter becomes so intense that they form entirely new hybrid quantum states.
To accomplish this, the team fabricated arrays of gold slots measuring 100 nanometers in width-about one thousand times thinner than plastic wrap. These metallic structures function as resonant cavities that confine electromagnetic radiation at terahertz frequencies. By varying the slot lengths across seven different sizes, engineers could precisely tune the trapped light frequency to match specific vibrational modes in the perovskite material above.
“Designing the slot geometry allowed us to shape the interaction between light and the perovskite phonons without using high-power laser pulses or bulky crystals,” explains Kim. The longer slots captured lower-frequency light waves, while shorter ones trapped higher frequencies, enabling systematic exploration of the coupling phenomenon across different energy scales.
Understanding Phonon-Polariton Engineering
At the heart of this breakthrough lies the fundamental physics of coupled oscillators-a concept engineering students encounter in classical mechanics and quantum mechanics courses. When two oscillating systems interact strongly enough, they cease to behave independently and instead form new normal modes with properties distinct from either original system.
In this case, the oscillating systems are phonons (lattice vibrations) and photons (electromagnetic field oscillations) confined in the engineered slots. When these two systems couple, they create phonon-polaritons-quasiparticles that exhibit characteristics of both light and mechanical vibration. The coupling strength, measured as a ratio to the phonon frequency, reached approximately 30% at room temperature-well into the ultrastrong regime where quantum effects dominate the system behaviour.
The engineering challenge involved matching the electromagnetic resonance of the gold slots to the intrinsic phonon frequencies of the perovskite, which occur in the terahertz range (10^12 Hz). Engineers achieved this through careful electromagnetic modelling and nanofabrication techniques. The slot dimensions determine the cavity resonance frequency according to the relationship f = c/(2nL), where c is the speed of light, n is the refractive index, and L is the effective cavity length.
The perovskite thin films, deposited on top of the gold slot arrays, needed to be uniform and defect-free to maintain coherent phonon modes. Lead halide perovskites were chosen because they exhibit strong optical phonon modes at specific frequencies and demonstrate excellent optoelectronic properties. The material’s crystal structure allows for efficient coupling between electronic transitions and lattice vibrations, making it ideal for this application.
From an engineering design perspective, this approach offers several advantages over previous methods. Traditional techniques for achieving strong light-matter coupling required either cryogenic temperatures, high-power pulsed lasers, or bulk crystals with limited device integration potential. By contrast, the thin-film architecture with engineered metallic structures operates at room temperature and integrates readily with existing semiconductor fabrication processes.
The formation of three distinct hybrid quantum states, as observed in the experiments, corresponds to the theoretical prediction for a system where two phonon modes simultaneously couple to a single electromagnetic mode. This can be understood through the quantum mechanical Hamiltonian that describes the coupled system, where the interaction terms lead to energy level splitting and the emergence of new eigenstates that are superpositions of the original phonon and photon states.
The experimental validation involved detailed optical spectroscopy measurements that revealed the characteristic anticrossing behaviour of strongly coupled systems. When the slot resonance frequency approached the phonon frequencies, instead of simply crossing through each other on an energy diagram, the modes repelled, forming the telltale signature of strong coupling. Numerical simulations using finite-element methods confirmed that the observed spectral features matched theoretical predictions for ultrastrong coupling.
This advancement could fundamentally alter how engineers approach energy management in optoelectronic devices. In conventional solar cells, for instance, energy absorbed from sunlight often dissipates as waste heat through phonon scattering before it can be converted to electricity. If engineers can control phonon-polariton states, they might direct this energy more efficiently toward charge collection, potentially increasing device efficiency beyond current theoretical limits.
Similarly, in LED technology, phonon-polaritons could provide new pathways for light emission. Current LEDs lose significant energy to phonon interactions that don’t contribute to light output. By engineering the phonon-photon coupling, future devices might channel more energy directly into useful light emission while minimising thermal losses.
The room-temperature operation makes this approach particularly attractive for practical applications. Previous demonstrations of similar effects required cryogenic cooling or specialised laboratory conditions incompatible with real-world devices. The thin-film geometry and compatibility with standard nanofabrication techniques mean this technology could integrate into existing semiconductor manufacturing processes.
If this technology proves successful in device applications, engineers might extend the concept to other material systems and frequency ranges. The principles demonstrated with perovskites and terahertz radiation could apply to different semiconductors and optical frequencies, potentially enabling a new class of quantum-engineered materials where light-matter interactions are designed at the nanoscale.
- Rice University researchers successfully coupled atomic vibrations with light waves in perovskite thin films at room temperature
- Created hybrid quantum states called phonon-polaritons using engineered gold nanoslots just 100 nanometers wide
- Achieved \”ultrastrong coupling\” where light and matter form entirely new quantum states with 30% coupling strength
- First room-temperature demonstration of dual phonon coupling to a single terahertz electromagnetic resonance
- Could enable more efficient solar cells and LEDs by providing unprecedented control over energy flow
- Technology integrates with existing semiconductor manufacturing processes, unlike previous methods requiring extreme conditions
- Opens pathway for quantum-engineered materials where light-matter interactions are designed at the nanoscale
