Diamond Photoconductive Antenna for Terahertz Generation Equipped with Buried Graphite Electrodes

At a Glance

Metadata	Details
Publication Date	2023-01-09
Journal	Photonics
Authors	T. V. Kononenko, K. K. Ashikkalieva, V. V. Kononenko, E.V. Zavedeev, Margarita A. Dezhkina
Institutions	Prokhorov General Physics Institute
Citations	4
Analysis	Full AI Review Included

Technical Documentation & Analysis: Diamond Photoconductive Antennas for THz Generation

Executive Summary

This research demonstrates a significant advancement in Terahertz (THz) generation using nitrogen-doped diamond Photoconductive Antennas (PCAs) by implementing laser-fabricated buried graphite electrodes.

Core Innovation: Deep-buried graphite pillars, connected by surface stripes, were fabricated inside HPHT diamond to create a highly homogeneous electric field across the entire photo-excited volume.
Material Requirement: The system relies on substitutional nitrogen in the diamond lattice (~20 ppm) to enable effective photoexcitation using the second harmonic of a Ti:sapphire laser ($\lambda$ = 400 nm).
Performance Enhancement: Buried electrodes allowed the reduction of the interelectrode distance from 3.5 mm down to 1.4 mm without any reduction in optical-to-THz conversion efficiency.
Fabrication Insight: Nanosecond laser pulses produced graphite pillars with significantly lower electrical resistance (9 kΩ) compared to femtosecond pulses (800 kΩ), crucial for high-field applications.
Future Potential: Researchers project that fully burying all electrode components could increase the DC breakdown voltage by a factor of 3, enabling field strengths up to 90 kV/cm and a maximum THz fluence of 0.36 µJ/cm².
6CCVD Value Proposition: 6CCVD specializes in the custom growth of high-purity and nitrogen-doped Single Crystal Diamond (SCD) required for replicating and scaling this high-performance THz emitter technology.

Technical Specifications

Parameter	Value	Unit	Context
Diamond Material Type	HPHT Ib	N/A	Single Crystal Diamond (SCD)
Crystal Dimensions Used	3.4 x 3.5 x 0.66	mm³	Basis for PCA fabrication
Substitutional Nitrogen Concentration	~20	ppm	Required for 400 nm excitation
Excitation Wavelength ($\lambda$)	400	nm	Ti:sapphire second harmonic
Absorption Depth (400 nm)	~0.24	mm	At maximum laser fluence
Saturation Fluence Range ($F_{sat}$)	1200-1600	µJ/cm²	Observed across all PCA designs
Minimum Electrode Interspace Tested	0.5	mm	Revealed performance decrease, requires further study
Maximum Measured THz Fluence	0.04	µJ/cm²	At 0.5 mm interspace, 30 kV/cm DC bias
Projected Maximum THz Fluence	0.36	µJ/cm²	Projected for 90 kV/cm field strength
Nanosecond Pillar Resistance	9	kΩ	660 µm long pillars (8 ns pulse)
Femtosecond Pillar Resistance	800	kΩ	660 µm long pillars (330 fs pulse)
Maximum Applied DC Bias Voltage	2.2	kV	Limited by electric breakdown

Key Methodologies

The fabrication and testing of the diamond PCAs involved precise laser microstructuring and high-field electrical testing:

Material Preparation: An HPHT Ib-type SCD crystal (N-doped, ~20 ppm) was selected.
Surface Electrode Formation: Surface graphite stripes (50 µm wide, 2.6 mm long) were formed using a KrF excimer laser ($\lambda$ = 248 nm, $\tau$ = 20 ns) at a fluence of 30 J/cm². Typical resistance was 900 Ω.
Buried Pillar Fabrication (Femtosecond): Most pillars were formed using a femtosecond fiber laser (330 fs, 1035 nm, 1.6 µJ pulse energy). Pillars were grown from the surface stripe, stopping ~50 µm before the front face.
Buried Pillar Fabrication (Nanosecond): Two electrodes were formed using a nanosecond laser (8 ns, 1064 nm, 10 µJ pulse energy). This method resulted in lower resistance graphite (9 kΩ vs. 800 kΩ for fs-pulses) but caused pronounced microcracks in the surrounding diamond matrix.
PCA Assembly: Seven PCAs were configured by combining different pairs of electrodes (surface-to-surface, buried-to-buried, front/rear faces, etc.) with interspaces ranging from 0.5 mm to 3.5 mm.
Electrical Contact: Thin copper wires were attached to the surface electrodes using conductive paste.
THz Measurement: PCAs were pumped by 400 nm radiation (second harmonic of a Ti:sapphire laser). Bias voltage was applied either as DC (up to 2.2 kV) or high-voltage pulses (up to 4 kV, ~10 ns FWHM).

6CCVD Solutions & Capabilities

6CCVD provides the foundational diamond materials and advanced processing services necessary to replicate, scale, and optimize this high-performance THz generation technology.

Applicable Materials

To replicate the high-efficiency THz generation demonstrated in this paper, researchers require diamond with precise nitrogen incorporation to ensure strong absorption at 400 nm.

Material Requirement	6CCVD Solution	Technical Advantage
Nitrogen-Doped Substrate	Optical Grade SCD (N-Doped Grade)	MPCVD growth allows precise control over substitutional nitrogen concentration (N_s) to optimize 400 nm absorption depth and carrier generation efficiency.
High Dielectric Strength	High Purity SCD	Diamond’s intrinsic 2000 kV/cm dielectric strength is critical for achieving the projected 90 kV/cm bias fields, minimizing breakdown risk.
Large Area Scaling	PCD Plates up to 125 mm	While SCD was used here, 6CCVD offers large-area PCD plates (up to 125 mm diameter) for scaling up Large-Aperture PCAs (LAPCAs) for higher power output.

Customization Potential for THz Emitters

The fabrication of buried graphite electrodes requires highly specialized laser processing and subsequent electrical contacting. 6CCVD offers comprehensive services to support the full device architecture:

Custom Dimensions and Thickness: The paper used a 0.66 mm thick crystal. 6CCVD supplies SCD and PCD plates with custom thicknesses from 0.1 µm up to 500 µm, and substrates up to 10 mm, allowing optimization of the active volume relative to the 400 nm absorption depth (0.24 mm).
Precision Polishing: Achieving high optical quality for laser excitation is paramount. 6CCVD guarantees Ra < 1 nm polishing for SCD, ensuring minimal scattering losses for the 400 nm pump beam.
Advanced Metalization Services: The paper relied on conductive paste for contacts. For reliable, high-field operation, 6CCVD offers in-house, high-precision metalization (e.g., Ti/Pt/Au, W, Cu) patterned directly onto the diamond surface, providing superior ohmic contact and stability compared to paste.
Laser Processing Support: While the paper details laser graphitization, 6CCVD offers custom laser cutting and patterning services to define precise electrode geometries and facilitate subsequent metalization steps.

Engineering Support

6CCVD’s in-house PhD team possesses deep expertise in diamond material science and its application in high-field electronics and optics. We can assist researchers in optimizing material selection (e.g., N-doping level, crystal orientation) for similar High-Power THz Generation projects, ensuring the diamond substrate meets the stringent requirements for high breakdown voltage and efficient carrier generation.

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

View Original Abstract

It has been shown recently that a photoconductive antenna (PCA) based on a nitrogen-doped diamond can be effectively excited by the second harmonic of a Ti:sapphire laser (λ = 400 nm). The THz emission performance of the PCA can be significantly increased if a much stronger electric field is created between the close-located electrodes. To produce a homogeneous electric field over the entire excited diamond volume, the laser fabrication of deep-buried graphite electrodes inside the diamond crystal was proposed. Several electrodes consisting of the arrays of buried pillars connected by the surface graphite stripes were produced inside an HPHT diamond crystal using femtosecond and nanosecond laser pulses. Combining different pairs of the electrodes, a series of PCAs with various electrode interspaces was formed. The THz emission of the PCAs equipped with the buried electrodes was measured at different values of excitation fluence and bias voltage (DC and pulsed) and compared with the emission of the same diamond crystal when the bias voltage was applied to the surface electrodes on the opposite faces. All examined PCAs have demonstrated the square-law dependencies of the THz fluence on the field strength, while the saturation fluence fluctuated in the range of 1200-1600 µJ/cm2. The THz emission performance was found to be approximately the same for the PCAs with the surface electrodes and with the buried electrodes spaced at a distance of 1.4-3.5 mm. However, it noticeably decreased when the distance between the buried electrodes was reduced to 0.5 mm.

Tech Support

Original Source

DOI: https://doi.org/10.3390/photonics10010075

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