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	European Organization for Nuclear Research, Clinica Universidad de Navarra
Citations	4
Analysis	Full AI Review Included

Tunable Diamond Raman Lasers for Resonance Photo-Ionization: 6CCVD Technical Analysis

This document analyzes the research demonstrating the superior performance of diamond Raman lasers (DRLs) in resonance photo-ionization (RPI) applications, highlighting 6CCVD’s capabilities in supplying the necessary high-purity, single-crystal diamond (SCD) materials for replicating and advancing this technology.

Executive Summary

The research validates the use of diamond Raman lasers (DRLs) as highly efficient, tunable, all-solid-state sources for resonance photo-ionization (RPI) and ion beam production, offering a compelling alternative to conventional Ti:Sapphire (Ti:Sa) systems.

Efficiency Demonstrated: The DRL achieved ion current production comparable to, and in some cases superior to, the standard Ti:Sa laser when ionizing ¹⁵²Sm⁺ isotopes.
Spectral Advantage: The DRL’s broader, “noisy” spectral modes resulted in a significantly better spectral overlap (8.3 GHz FWHM convolution) with the Doppler-broadened atomic transition line (1.81 GHz FWHM) compared to the cleaner Ti:Sa spectrum (5.9 GHz FWHM convolution).
Material Requirement: The core component is a high-quality, 6 mm thick diamond crystal acting as the Raman medium in a hemi-spherical cavity.
Solid-State Solution: DRLs provide wide, continuous tunability while preserving the pump linewidth, offering a robust, low-maintenance, all-solid-state replacement for complex dye laser systems in nuclear and quantum applications (e.g., CERN/ISOLDE).
Saturation Performance: The DRL exhibited a saturation power (P_s) of 13.54 mW, confirming its suitability for high-efficiency ionization saturation regimes.

Technical Specifications

Parameter	Value	Unit	Context
Raman Medium	Diamond Crystal	N/A	Used in hemi-spherical cavity design.
Crystal Length	6	mm	Thickness of the SCD used for Raman conversion.
Pump Wavelength (First Step)	433.9	nm	Resonant excitation step for Sm atoms.
Non-Resonant Step Wavelength	355	nm	Ionization above the IP using a frequency-tripled Nd:YAG laser.
Raman Laser Output Power	400	mW	Maximum output power at 433.9 nm (10 kHz repetition rate).
Ti:Sa Laser Output Power	900	mW	Maximum output power at 433.9 nm (10 kHz repetition rate).
Ionized Element	Samarium (¹⁵²Sm⁺)	N/A	Ion beam produced in a hot metal cavity.
Oven Temperature (Sm)	~2000	°C	Temperature of the hot cavity ion source.
Sm Doppler Broadening (FWHM)	1.81	GHz	Calculated linewidth of the atomic transition.
Raman Linewidth (Convolution)	8.3	GHz	Measured total linewidth (laser + transition).
Ti:Sa Linewidth (Convolution)	5.9	GHz	Measured total linewidth (laser + transition).
Raman Saturation Power (P_s)	13.54	mW	Power required to reach ionization saturation.
Ti:Sa Saturation Power (P_s)	13.97	mW	Power required to reach ionization saturation.
Diamond Raman Shift	1332	cm^-1	Fundamental vibrational frequency of diamond.

Key Methodologies

The experiment utilized a two-step resonance photo-ionization (RPI) scheme to compare the performance of a diamond Raman laser (DRL) against a conventional Ti:Sapphire (Ti:Sa) laser.

Ion Source Setup: Sm atoms were heated in a hot refractory metal cavity (~2000 °C) to create a cloud of Doppler-broadened atoms, which were subsequently ionized and extracted at 30 kV.
Raman Laser Configuration:
- A 6 mm diamond crystal was used as the Raman medium in a minimalist hemi-spherical cavity.
- The output coupler was the uncoated side of the crystal (approximate reflectivity of 17%).
- The high reflector was a 50 mm ROC concave mirror (~99% reflectivity), broadband coated for 450-850 nm.
- The pump source was a tunable Ti:Sa laser (frequency-doubled by a BiBO crystal) producing 1.1 W maximum power.
Ionization Scheme:
- First Step (Resonant): Both the DRL and the Ti:Sa laser were tuned to 433.9 nm to excite the 4f⁶6s² → 4f⁵(⁶F°)5d6s² transition.
- Second Step (Non-Resonant): A high-power, frequency-tripled Q-switched Nd:YAG laser (10 W @ 355 nm) was used to surpass the ionization potential (IP).
Performance Measurement: Ion current of ¹⁵²Sm⁺ was measured using a Faraday-cup (FC) across a range of laser output powers (0.15-100 mW) to determine the saturation curves and saturation power (P_s).
Spectral Analysis: Frequency sweep scans were performed to measure the convolution linewidths of the laser spectra with the Sm transition, confirming the DRL’s broader spectral profile (8.3 GHz) provided superior spectral overlap and excitation efficiency.

6CCVD Solutions & Capabilities

The successful implementation of the diamond Raman laser relies critically on the quality, purity, and precise geometry of the CVD diamond material. 6CCVD is uniquely positioned to supply the required optical-grade diamond components for both research replication and commercial scaling of this technology.

Applicable Materials

To replicate or extend this research, high-purity, low-loss Single Crystal Diamond (SCD) is essential for maximizing Raman gain and minimizing cavity losses.

6CCVD Material	Specification	Application Relevance
Optical Grade SCD	High purity (low nitrogen/defects), low birefringence.	Required for high-power Raman conversion and narrow linewidth preservation.
Custom SCD Substrates	Thicknesses up to 500 µm (wafers) or up to 10 mm (substrates).	Directly addresses the need for the 6 mm thick diamond crystal used in the resonator.
Polycrystalline Diamond (PCD)	Plates up to 125 mm diameter.	Suitable for high-power, large-area Raman applications where single-crystal size is a limitation, or for thermal management components.

Customization Potential

The paper describes a specific hemi-spherical cavity design requiring precise material dimensions and optical coatings. 6CCVD offers comprehensive customization services to meet these exact engineering requirements:

Custom Dimensions and Thickness: We provide SCD plates and substrates tailored to the exact 6 mm length (or custom lengths for optimization) required for specific resonator designs (e.g., Z-fold or hemi-spherical cavities).
Advanced Polishing: Achieving low-loss operation requires exceptional surface quality. 6CCVD guarantees optical-grade polishing:
- SCD: Surface roughness (Ra) < 1 nm.
- PCD: Surface roughness (Ra) < 5 nm (for inch-size plates).
Integrated Metalization and Coatings: The research utilized uncoated and AR-coated surfaces. 6CCVD offers internal metalization capabilities (Au, Pt, Pd, Ti, W, Cu) and can assist in integrating custom dielectric coatings (AR/HR) directly onto the diamond surface, simplifying resonator assembly and reducing alignment complexity.
Laser Cutting and Shaping: We provide precise laser cutting services to achieve specific crystal geometries and orientations (e.g., <110> crystallographic axis propagation) necessary for maximizing Raman gain and optimizing polarization conversion.

Engineering Support

The finding that “noisy” laser sources with broadened spectral modes are more suitable for Doppler-broadening-dominated RPI applications is counter-intuitive and requires specialized material knowledge for optimization.

Raman Optimization: 6CCVD’s in-house PhD team specializes in CVD diamond growth and optical properties. We can assist researchers in selecting the optimal SCD grade, thickness, and crystal orientation to tune the spectral properties (e.g., FSR, mode linewidth) of the DRL to maximize spectral overlap for similar Resonance Photo-Ionization (RPI) or Quantum Technology projects.
Thermal Management: Diamond’s exceptional thermal conductivity is crucial for high-power operation. We ensure the material supplied maintains the thermal stability required for continuous, high-repetition-rate operation (10 kHz in this study).

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

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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2004 - The effect of dye photodegradation on the performance of dye lasers [Crossref]
2019 - Continuously tunable diamond Raman laser for resonance laser ionization [Crossref]
2020 - Broadly tunable linewidth-invariant Raman Stokes comb for selective resonance photoionization [Crossref]
2021 - Cascaded Stokes polarization conversion in cubic Raman crystals [Crossref]