The Transient Current Technique (TCT) is a well‑established method for the characterization of silicon particle detectors, both irradiated and non‑irradiated. In conventional laser‑TCT implementations based on single‑photon absorption (SPA), spatial resolution is limited to two dimensions, as charge carriers are generated along the full propagation path of the laser beam inside the detector. As a result, sensitivity to the position of the device along the beam axis is fundamentally limited.
Two‑Photon Absorption Transient Current Technique (TPA‑TCT) overcomes this limitation by exploiting nonlinear absorption in silicon. When using sub‑bandgap photon energies, linear absorption is strongly suppressed and charge carriers are generated exclusively at the focal point of a strongly focused laser beam. This enables true three‑dimensional spatial resolution of carrier generation inside the silicon detector bulk, significantly enhancing resolution both perpendicular and parallel to the beam axis.
For TPA‑TCT measurements in silicon, laser wavelengths around 1550 nm are particularly suitable, as a single photon does not carry sufficient energy to create electron‑hole pairs. Only at sufficiently high peak intensities are carriers generated via two‑photon absorption, confining charge generation to a micrometric excitation volume around the focal spot. Achieving stable and reproducible two‑photon absorption therefore requires femtosecond laser pulses, precise control of pulse energy, and robust optical stability.
FYLA has developed Pulsar, a compact all‑fiber femtosecond laser platform specifically designed to meet the demanding requirements of TPA‑TCT excitation. Operating around 1550 nm, Pulsar provides ultrashort femtosecond pulses together with independent control of pulse energy and repetition rate, enabling the optimization of nonlinear absorption conditions while minimizing parasitic single‑photon absorption. Its fiber‑based architecture ensures mechanical stability, repeatable pulse delivery, and seamless integration into table‑top detector characterization systems.
This laser architecture allows stable pulse generation, wide‑range pulse energy tuning, and repetition rate selection, which are essential for high‑resolution mapping of electric field distributions, charge transport properties, and radiation‑induced effects in advanced silicon detectors. The compact form factor and all‑fiber design make the system suitable for deployment in detector R&D laboratories without the complexity of large‑scale solid‑state laser infrastructures.
CERN–FYLA collaboration and experimental validation
The development and validation of TPA‑TCT as a silicon detector characterization technique has been strongly driven by the RD50 collaboration at CERN, in close cooperation with academic and industrial partners. In this context, FYLA has developed Pulsar as the femtosecond excitation laser used in the current table‑top TPA‑TCT systems operating at CERN. Based on an all‑fiber femtosecond laser architecture emitting ultrashort pulses around 1550 nm, Pulsar enables sub‑bandgap excitation and localized charge carrier generation through two‑photon absorption in silicon particle detectors.
The experimental implementation and performance of this approach—including true three‑dimensional spatial resolution, applicability to both irradiated and non‑irradiated detectors, and detailed analysis of the TPA‑TCT methodology—are described in the peer‑reviewed publication “Development of a Tabletop Setup for the Transient Current Technique Using Two‑Photon Absorption in Silicon Particle Detectors”, published in IEEE Transactions on Nuclear Science by a CERN‑led research team.
TPA‑TCT measurements enabled by 1550 nm femtosecond fiber lasers offer several advantages over conventional SPA‑based TCT methods, including true 3D resolution, improved transverse spatial resolution, reduced sensitivity to unwanted optical reflections, and the ability to separate linear and nonlinear absorption mechanisms in irradiated detectors. This makes the technique particularly valuable for the characterization of thin sensors, CMOS detectors, LGADs, and devices with complex electrode geometries.
TPA‑TCT has proven to be a powerful extension of established silicon detector characterization techniques and is increasingly adopted in high‑energy physics detector R&D. Compact ultrafast laser systems such as FYLA Pulsar, developed and validated in collaboration with CERN, enable laboratory‑scale implementation of this method, providing researchers with a stable, reproducible, and high‑precision tool for advanced silicon particle detector development.
