Expanding Near-Field Terahertz Optics: Nonlinear Optics with Blue Light  

The integration of terahertz optical techniques with scattering-type scanning near-field optical microscopy has enabled groundbreaking advances in nanoscale spectroscopy and imaging. By overcoming the diffraction limit, these techniques have allowed researchers to probe material properties with unprecedented spatial resolution. Until now, however, nearly all examples of s-SNOM have relied on long-wavelength sources, typically at energies below 2.5 eV. The coupling of short-wavelengths, in particular blue light, to nanoscale tips has remained a significant challenge, preventing the application of nonlinear terahertz near-field optics to a wide range of technologically important materials. 

Featuring the Inspire fs OPO from Radiantis, this work presents the first experimental demonstration of near-field terahertz nonlinear optics using blue light. By employing femtosecond pulses at 410 nm, the group of Prof. Daniel Mittleman from Brown University, US, has generated terahertz emission directly from bulk silicon and spatially resolved at the nanoscale.¹ This achievement represents a major advancement in ultrafast spectroscopy and nonlinear optical microscopy, as it enables the study of wide-bandgap materials such as silicon and gallium nitride, which require photoexcitation at higher energies. The ability to perform terahertz near-field measurements with blue light expands the capabilities of s-SNOM and establishes a new approach to investigating ultrafast carrier dynamics and nonlinear optical processes at the nanoscale. 

Figure 1 | Experimental setup for the s-SNOM experiment. NIR, blue light, and THz beams are generated separately, with the THz pulses generated using a conventional photoconductive antenna (PCA). All three beams then overlap and are coupled into the AFM. Scattered or emitted THz pulses are coherently detected on the other side via free-space electro-optic sampling (EOS). Adapted from [1]. 

The obstacles that have historically prevented the realization of blue-light-driven near-field terahertz experiments are primarily related to the difficulty of efficiently coupling a short-wavelength beam to nanoscale tips. Traditional s-SNOM techniques rely on a sharp metallic tip to confine the incident wave, creating a strong localized field at the tip apex. While this approach works well at longer wavelengths, shorter wavelengths introduce significant challenges. The increased scattering losses and reduced field enhancement at visible wavelengths make it extremely difficult to achieve efficient near-field coupling. Furthermore, the alignment of tightly focused short-wavelength beams with the nano-tip is inherently complex, requiring high precision and stability that have been difficult to achieve in practical experiments.  

This work overcomes these limitations by utilizing an alternative approach. Instead of relying on direct coupling of blue light to the nano-tip, the terahertz emission originates from a macroscopic photo-generated dipole in the sample and is out-coupled by the tip itself. This novel configuration bypasses the need for precise near-field confinement of short-wavelength light while still achieving nanoscale resolution. The experimental setup is shown in Figure 1. 

The theoretical framework developed for this process builds upon the finite dipole model, adapted to describe the interaction between the AFM tip and a subsurface terahertz-emitting dipole. The strength of the emitted terahertz signal is directly related to the dielectric properties of the sample, allowing for quantitative extraction of material parameters such as carrier concentration and nonlinear optical response. 

Experimental validation of this approach has been demonstrated using both indium arsenide (InAs) and silicon (Si) materials. In InAs, terahertz pulses induced by blue-light pumping exhibit spectral narrowing compared to those generated by near-infrared pulses. This effect is attributed to energy-dependent carrier dynamics, where higher-energy excitation leads to increased scattering and reduced high-frequency terahertz emission. In silicon, blue-light excitation produces strong, spatially localized terahertz pulses, revealing clear contrasts between regions of differing doping concentrations. Unlike conventional terahertz s-SNOM, which relies on elastic scattering, this technique enables direct imaging of charge carrier distributions based on their nonlinear optical response. The observed decrease in terahertz emission with increasing doping density confirms that free carriers screen the nonlinear interaction, providing a highly sensitive probe of carrier dynamics in semiconductors.  

Figure 2 | a) Time-domain THz emission waveforms from InAs by using a NIR pump (red) and a blue light pump (blue), measured at 2nd harmonic demodulation. b) Near-field THz emission waveforms from a blue light pump, emitted by the Si substrate (light blue) and implanted region (dark blue). Inset: top-down AFM camera view of the implant-substrate boundary region. Adapted from [1]

Beyond its fundamental implications, this technique offers new opportunities for ultrafast pump-probe spectroscopy and nonlinear optical microscopy. Traditional terahertz time-domain spectroscopy is fundamentally limited by diffraction, restricting spatial resolution to the micrometer scale. The combination of femtosecond blue-light pulses with s-SNOM overcomes this limitation, enabling terahertz pump-probe experiments with nanoscale resolution. This capability is particularly valuable for studying ultrafast carrier dynamics in quantum materials, two-dimensional heterostructures, and complex oxides, where nanoscale heterogeneities play a critical role in determining material behavior. 

Furthermore, the ability to generate and detect nanoscale terahertz signals using blue-light excitation opens new possibilities for exploring nonlinear optical effects such as sum-frequency generation and difference-frequency generation at nanometer-length scales. These effects, which are highly sensitive to local symmetry and electronic structure, can now be investigated in semiconductors, plasmonic systems, and metamaterials with unprecedented spatial resolution. The demonstrated capability of blue-light-driven s-SNOM to resolve nanoscale terahertz emission establishes a new paradigm in near-field optics, bridging the gap between ultrafast spectroscopy and high-resolution imaging. 

These results suggest that the continued integration of tunable femtosecond laser sources with near-field terahertz techniques will further enhance the versatility and impact of this emerging field. 

The Inspire product is a trademark from Newport Corporation (Spectra-Physics). 

1. Pizzuto, A., Ma, P. & Mittleman, D.M. Near-field terahertz nonlinear optics with blue light. Light Sci Appl 12, 96 (2023). https://doi.org/10.1038/s41377-023-01137-y