Tunable diamond raman lasers for resonance photo-ionization and ion beam production

At a Glance

Metadata	Details
Publication Date	2022-07-22
Journal	Frontiers in Physics
Authors	Daniel T. Echarri, K. Chrysalidis, V. N. Fedosseev, Reinhard Heinke, B. A. Marsh
Institutions	Clinica Universidad de Navarra, European Organization for Nuclear Research
Citations	4
Analysis	Full AI Review Included

Executive Summary

Performance Equivalence: Tunable Diamond Raman Lasers (DRLs) demonstrated comparable, and in some cases superior, overall ion current production efficiency compared to conventional Ti:Sapphire (Ti:Sa) lasers for Resonance Photo-Ionization (RPI) of Samarium (Sm) isotopes.
Spectral Advantage: A spectrum-dependent excitation model showed that DRLs, characterized by broader and noisier spectral modes, achieve a significantly better spectral overlap (up to 2x higher integral in saturation) with the Doppler-broadened atomic transition line (FWHM 1.81 GHz at 2000°C).
Solid-State Solution: The DRL provides an agile, all-solid-state alternative to operationally challenging dye lasers, offering wide and continuous tunability while preserving the narrow linewidth of the pump source.
Ionization Saturation: Experimental saturation power (P_s) measurements were nearly identical: 13.54 mW for the DRL and 13.97 mW for the Ti:Sa laser, validating the DRL’s suitability for high-efficiency RPI applications.
Design Optimization: A multi-Stokes Raman scattering simulator was developed to optimize DRL cavity design, predicting lasing thresholds and maximizing cascading efficiency based on pump polarization (optimal angle: ±54.7° relative to the [001] crystallographic axis).
Wavelength Coverage: The technology effectively bridges the difficult-to-access spectral gap between 450 nm and 650 nm using existing solid-state pump sources.

Technical Specifications

Parameter	Value	Unit	Context
Raman Medium	Diamond	N/A	6 mm crystal length
Cavity Type	Hemi-spherical	N/A	Uncoated output coupler, 50 mm ROC concave mirror
Optimal Pump Polarization	±54.7°	Degrees	Relative to the [001] crystallographic axis
Pump Spot Size	~50	µm	Diameter (1/e²) focused into the diamond
Pump Wavelength (Resonant Step)	433.9	nm	First resonant excitation step for Sm
Non-Resonant Step Wavelength	355	nm	Ionization into the continuum (Nd:YAG)
Maximum DRL Output Power	400	mW	At 433.9 nm, 10 kHz repetition rate
Repetition Rate	10	kHz	Used for both DRL and Ti:Sa comparison
Hot Cavity Temperature	~2000	°C	Environment for Sm atom ionization
Sm Doppler Broadening (FWHM)	1.81	GHz	Calculated at 2000°C
Simulated DRL Envelope FWHM	6.4	GHz	Gaussian envelope
Simulated DRL Mode FSR	2	GHz	Free Spectral Range
Simulated Ti:Sa Envelope FWHM	3.1	GHz	Gaussian envelope
Measured Linewidth (Raman Convolution)	8.3	GHz	Convolution of DRL spectrum and Sm transition
Measured Linewidth (Ti:Sa Convolution)	5.9	GHz	Convolution of Ti:Sa spectrum and Sm transition
Saturation Power (P_s) - DRL	13.54	mW	Fitted value for ¹⁵²Sm⁺ ionization
Saturation Power (P_s) - Ti:Sa	13.97	mW	Fitted value for ¹⁵²Sm⁺ ionization
Extraction Voltage	30	kV	Ion beam extraction potential

Key Methodologies

Raman Laser Modeling: Developed a multi-Stokes Raman scattering simulator based on coupled differential equations to predict temporal dynamics and output power. This model incorporates wavelength-dependent Raman gain and optimizes cascading based on pump polarization (maximizing gain when Stokes polarization is parallel to the <111> axis).
Spectral Excitation Modeling: Created a spectrum-dependent excitation model to quantify the efficiency of RPI by calculating the spectral overlap integral (Eq. 11) between the laser spectrum (simulated Lorentzian or Gaussian modes) and the Doppler-broadened Sm atomic transition (Gaussian lineshape).
Ion Source Operation: Samarium (Sm) atoms were produced in a hot metal cavity operating at approximately 2000°C, resulting in a Doppler-broadened transition FWHM of 1.81 GHz.
Two-Step Ionization Scheme: Ionization of ¹⁵²Sm⁺ utilized a resonant first step (4f⁶6s² to 4f⁵(⁶F°)5d6s²) at 433.9 nm, provided alternately by the DRL and the Ti:Sa laser, followed by a non-resonant second step (into the continuum) at 355 nm (provided by a synchronized Nd:YAG laser).
Ion Current Measurement: The produced ion beam was extracted, mass-separated via a dipole magnet to select ¹⁵²Sm⁺, and measured using a Faraday-cup (FC). Saturation curves were generated by varying the output power of the first-step laser (0.15-100 mW range).
Data Analysis: Measured ion current curves were normalized and fitted using the standard RPI saturation function (Eq. 12) to determine the saturation power (P_s) and compare the relative ionization efficiency of the DRL and Ti:Sa systems.

Commercial Applications

Nuclear Physics and Isotope Production: Essential for Isotope Separation On-Line (ISOL) facilities (like CERN ISOLDE) requiring selective and efficient ionization of radioactive isotopes for subsequent study. DRLs provide a robust, all-solid-state replacement for high-maintenance dye lasers.
Quantum Computing and Sensing: The ability to perform high-fidelity, selective atomic excitation is crucial for manipulating atomic hyperfine qubits and producing atomic quantum states with high precision.
High-Power Laser Systems: Diamond’s exceptional thermal conductivity and high Raman gain enable the construction of highly efficient, high-power, broadly tunable solid-state lasers for industrial and scientific applications, particularly in the visible and UV ranges.
Advanced Spectroscopy: Provides a widely tunable, narrow-linewidth light source suitable for high-resolution laser spectroscopy of complex atomic and molecular structures.
Photonics and Integrated Devices: The underlying diamond material science supports the development of integrated photonic devices and resonators, offering wide tunability and spectral synthesis capabilities.

View Original Abstract

Lasers with wide tunability and tailored linewidth are key assets for spectroscopy research and applications. We show that diamond, when configured as a Raman laser, provides agile access to a broad range of wavelengths while being capable of efficient and selective photo-excitation of atomic species and suitable spectroscopic applications thanks to its narrow linewidth. We demonstrate the use of a compact diamond Raman laser capable of efficient ion beam production by resonance ionization of Sm isotopes in a hot metal cavity. The ionization efficiency was compared with a conventional Ti:sapphire laser operating at the same wavelength. Our results show that the overall ion current produced by the diamond Raman laser was comparable -or even superior in some cases-to that of the commonly used Ti:sapphire lasers. This demonstrates the photo-ionization capability of Raman lasers in the Doppler broadening-dominated regime, even with the considerable differences in their spectral properties. In order to theoretically corroborate the obtained data and with an eye on studying the most convenient spectral properties for photo-ionization experiments, we propose a simple excitation model that analyzes and compares the spectral overlap of the Raman and Ti:Sapphire lasers with the Doppler-broadened atomic spectral line. We demonstrate that Raman lasers are a suitable source for resonance photo-ionization applications in this regime.

Tech Support

Original Source

DOI: https://doi.org/10.3389/fphy.2022.937976

References

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