Nonlinear Optics and Spectroscopy

Nonlinear optics describes the regime in which the response of a material to an applied electromagnetic field depends nonlinearly on the field amplitude. Whereas in linear optics the induced polarization P is proportional to the field E, in nonlinear optics one must expand P as a power series in E. The higher‐order terms give rise to a rich array of phenomena—frequency conversion, optical rectification, intensity‐dependent refractive index, multi‐wave mixing and more—that underpin ultrafast spectroscopy, all‐optical signal processing and novel light sources. From second‐harmonic generation (SHG) in noncentrosymmetric crystals to four‐wave mixing and carrier–envelope phase‐sensitive processes, nonlinear spectroscopy provides both fundamental tests of light–matter interaction and practical tools for material characterisation on femtosecond to picosecond time scales.

Research from Nature Portfolio

Advances in ultrafast scanning tunnelling microscopy have pushed nonlinear spectroscopy into real‐space atomic resolution. An externally triggered optical pump–probe STM achieves tens of picoseconds temporal resolution under stable laser illumination, electrically controlling pulse timing and beam stability to resolve carrier decay at specific GaAs surface features. In a complementary development, lightwave‐driven terahertz STM and spectroscopy of atomically precise graphene nanoribbons have attained simultaneous sub‐ångström spatial and sub‐picosecond temporal resolution. Single‐cycle THz transients coupled to the STM tip enable coherent control of local wavefunctions and yield energy‐resolved spectra inaccessible to conventional STM, opening routes for ultrafast probing of electron dynamics in low‐dimensional materials. Earlier work on nanoparticle suspensions has demonstrated stimulated scattering in gold‐nanorod–water samples, revealing transitions between stimulated Raman, Brillouin and thermal scattering regimes under ns‐pulsed 532 nm illumination and establishing thresholds for nonlinear gain in colloidal media.

Research from all publishers

In bulk nonlinear media, high‐energy sub‐phonon‐lifetime pulse compression by stimulated Brillouin scattering (SBS) in liquids has compressed nanosecond, multijoule pulses down to sub‐200 ps durations with joule‐level energies, by matching interaction length and phonon lifetime, achieving stable, high‐energy output for material processing applications. A comprehensive review of picosecond laser developments based on SBS‐pulse compression synthesises decades of schemes—seed‐initiated, non‐focusing and adaptive designs—optimising compression ratio, efficiency and system complexity for various pump regimes. More recently, nonlinear gain modulation has been employed to stabilise high‐repetition‐rate SBS phase‐conjugate mirrors: by synchronously adapting radial and axial focusing one mitigates thermal loading and holds energy reflectivity at ~60 % with <2 % pulse‐energy fluctuations under MHz‐class pumping. These works illustrate the continued innovation in harnessing material nonlinearities for spectral broadening, pulse shortening and adaptive wavefront control.

Nonlinear Optics and Spectroscopy publication trend

The graph below shows the total number of articles in nonlinear optics and spectroscopy across all publications each year (not limited to Nature Index journals).

Technical terms

Nonlinear optics: The study of light–matter interactions in which the induced polarization P responds as a nonlinear function of the electric field E, P=ε₀(χ^(1)E+χ^(2)EE+χ^(3)EEE+…).

Second-harmonic generation (SHG): A χ^(2) process in noncentrosymmetric media whereby two photons at frequency ω combine to produce one photon at 2ω.

Stimulated Brillouin scattering (SBS): A third‐order nonlinear process in which light scatters off acoustic phonons, enabling pulse compression and phase conjugation near the phonon lifetime limit.

Pump–probe spectroscopy: A time‐resolved technique using one pulse to excite (pump) a system and a delayed pulse to monitor (probe) its transient response.

Phase matching: The condition k_pump(ω₁)+k_pump(ω₂)=k_output(ω₁+ω₂) required for efficient frequency conversion, often achieved by birefringence or temperature tuning.

Carrier-envelope phase (CEP): The offset between the peak of the pulse envelope and the phase of the carrier oscillation, crucial for waveform‐sensitive nonlinear processes.

Four-wave mixing (FWM): A χ^(3) interaction in which photons at three frequencies combine to generate a fourth, underpinning wavelength conversion and optical parametric amplification.

References

1. Externally-triggerable optical pump-probe scanning tunneling microscopy with a time resolution of tens-picosecond. Scientific Reports (2023).
2. Lightwave-driven scanning tunnelling spectroscopy of atomically precise graphene nanoribbons. Nature Communications (2021).
3. Efficient and Continuous Carrier-Envelope Phase Control for Terahertz Lightwave-Driven Scanning Probe Microscopy. ACS Photonics (2023).
4. Stimulated scattering effects in gold-nanorod-water samples pumped by 532 nm laser pulses. Scientific Reports (2015).
5. High-energy sub-phonon lifetime pulse compression by stimulated Brillouin scattering in liquids.. Optics Express (2017).
6. Developments of Picosecond Lasers Based on Stimulated Brillouin Scattering Pulse Compression. Frontiers in Physics (2021).
7. Stability enhancement with nonlinear gain modulation in high-power SBS-PCM. APL Photonics (2024).