A Diamond Terahertz Large Aperture Photoconductive Antenna Biased by a Longitudinal Field

At a Glance

Metadata	Details
Publication Date	2023-10-20
Journal	Photonics
Authors	V. V. Kononenko, V. V. Bukin, М. С. Комленок, E.V. Zavedeev, T. V. Kononenko
Institutions	Moscow Institute of Physics and Technology, Prokhorov General Physics Institute
Citations	5
Analysis	Full AI Review Included

Technical Documentation & Analysis: Diamond THz LAPCA Emitter

This document analyzes the research paper, “A Diamond Terahertz Large Aperture Photoconductive Antenna Biased by a Longitudinal Field,” to provide technical specifications and align the findings with 6CCVD’s advanced MPCVD diamond capabilities, driving material sales for high-power THz applications.

Executive Summary

The research successfully demonstrates a novel Large Aperture Photoconductive Antenna (LAPCA) design utilizing nitrogen-doped synthetic monocrystalline diamond (SCD) with a longitudinally oriented bias field. This configuration leverages diamond’s extreme material properties for high-intensity THz generation.

Material Validation: Nitrogen-doped SCD is confirmed as the optimal material for high-power THz emitters, boasting an exceptional electrical breakdown threshold (10 MV/cm) and high thermal diffusivity (>10 cm²/s).
Design Innovation: The longitudinal bias field configuration allows for a significantly reduced interelectrode gap (substrate thickness, 0.5 mm), resulting in a higher effective bias field for the same applied voltage compared to conventional transverse designs.
Performance Metrics: The longitudinal LAPCA achieved a maximum THz yield of 0.62 nJ and an optical-to-THz conversion efficiency of 0.0008% when pumped by 400 nm femtosecond pulses.
Electrode Robustness: Laser-graphitized grid electrodes provided stable ohmic contact and proved robust against repetitive high-voltage pulsing, overcoming the rapid degradation observed with Indium Tin Oxide (ITO) films.
Future Roadmap: The study paves the way for next-generation, high-power diamond LAPCAs by suggesting the encapsulation of graphite electrodes within the diamond bulk to eliminate surface discharge currents.

Technical Specifications

The following hard data points were extracted from the experimental results and material properties cited in the research.

Parameter	Value	Unit	Context
Substrate Material	Nitrogen-doped SCD	N/A	Synthetic monocrystalline diamond
Nitrogen Doping Level	~10	ppm	Used to enable 400 nm absorption (2.2 eV transitions)
Substrate Dimensions (Tested)	5.2 x 2.6 x 0.5	mm	HPHT grown crystal
Electrical Breakdown Threshold	10	MV/cm	Intrinsic diamond strength
Thermal Diffusivity (Diamond)	>10	cm²/s	Enables efficient heat dissipation
Electron Mobility (Room Temp)	~4500	cm² V⁻¹ s⁻¹	High carrier transport
Pump Wavelength	400	nm	Second Harmonic Generation (SHG)
Pump Pulse Duration	~120	fs	Femtosecond Ti:sapphire laser system
Maximum Bias Field (Tested)	~10	kV/cm	Limited by surface electrical breakdown
Saturation Fluence (F_sat)	255 ± 10	µJ/cm²	Independent of electrode design
Max THz Yield (Longitudinal E-Field)	0.62	nJ	Grid graphitized surface
Optical-to-THz Conversion (Longitudinal)	0.0008	%	Highest reported efficiency
Graphite Electrode Resistance	~1	kΩm	Ohmic contact achieved
Electric Field Modulation Depth	~40	µm	Subsurface layer modulation due to grid electrodes
Absorption Depth (1/e level, 400 nm)	≈220	µm	Selected crystal property for effective bulk pumping

Key Methodologies

The experiment focused on comparing conventional transverse biasing with the novel longitudinal biasing using high-quality diamond substrates and advanced electrode fabrication techniques.

Material Preparation: A single HPHT-grown monocrystalline diamond plate (5.2 x 2.6 x 0.5 mm) was laser-cut and mechanically polished to optical quality (Ra < 1 nm equivalent).
Electrode Fabrication (Transverse): Conventional LAPCA used graphite electrodes written by laser on the diamond edge, glued to a PCB.
Electrode Fabrication (Longitudinal - ITO): Transparent Indium Tin Oxide (ITO) films were pressed onto both sides of the diamond substrate in a sandwich configuration. (This configuration failed due to rapid degradation under HV pulses).
Electrode Fabrication (Longitudinal - Graphite Grid): Robust electrodes were fabricated by direct laser writing (KrF excimer, 248 nm) on both diamond surfaces, creating 10 µm wide graphite wires spaced 100 µm apart.
Biasing: The longitudinal LAPCA was driven by synchronized pulse voltage (up to 3 kV, ~10 ns duration) at a 1 kHz repetition rate. The maximum applied E-field was limited to ~10 kV/cm by surface breakdown.
Optical Pumping: The LAPCA was pumped by 400 nm femtosecond pulses (SHG from 800 nm Ti:sapphire laser). The pump beam was incident obliquely (45° to 50°) to the surface normal to generate the necessary orthogonal E-field component.
THz Measurement: THz power was measured using a Golay cell, and the THz pulse waveform was characterized via electro-optical sampling using a ZnTe crystal.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the high-quality SCD material and advanced processing required to replicate, scale, and advance the diamond LAPCA technology demonstrated in this research. Our capabilities directly address the material specifications, custom dimensions, and future engineering challenges outlined by the authors.

Paper Requirement/Challenge	6CCVD Solution & Capability	Technical Advantage
Material Specification: Nitrogen-Doped SCD (HPHT/CVD equivalent)	Optical Grade SCD Wafers: We supply high-purity Single Crystal Diamond (SCD) grown via MPCVD, with precise control over nitrogen doping levels (e.g., N-doped SCD) to optimize 400 nm absorption.	Guarantees the required 2.2 eV defect transitions for efficient 400 nm pumping, coupled with diamond’s superior thermal and electrical properties.
Custom Dimensions & Thickness: Plates used were 5.2 x 2.6 x 0.5 mm. Future scaling requires larger apertures.	Custom Dimensions & Thickness: We offer SCD plates from 0.1 µm to 500 µm thick, and substrates up to 10 mm. Our custom laser cutting and shaping services ensure precise geometry (e.g., 5.2 x 2.6 mm).	Supports scaling of LAPCA aperture size up to 125 mm (PCD) or large-area SCD, crucial for increasing THz yield (as yield scales with aperture area).
Surface Quality: Mechanically polished to optical quality.	Precision Polishing: Our standard SCD polishing achieves surface roughness Ra < 1 nm. Inch-size PCD can achieve Ra < 5 nm.	Essential for minimizing scattering losses during oblique optical incidence and ensuring high-quality interfaces for electrode deposition.
Electrode Integration: Need for robust, ohmic contacts (Graphite Grid alternative).	Custom Metalization Services: Internal capability for depositing high-adhesion metal stacks (Au, Pt, Pd, Ti, W, Cu). We can engineer Ti/Pt/Au stacks to provide stable, low-resistance ohmic contacts superior to the tested ITO films.	Ensures stable performance under high bias fields and repetitive pulsing, eliminating electrode degradation issues.
Future Goal: Encapsulating electrodes in the diamond bulk to eliminate surface discharge.	Engineering Support & Thick Substrates: Our in-house PhD team can assist with material selection and processing strategies for burying electrodes, utilizing our thick SCD substrates (up to 10 mm) and advanced laser processing techniques.	Enables the realization of next-generation LAPCAs capable of operating at the maximum intrinsic breakdown field (10 MV/cm) of diamond, maximizing THz output.

6CCVD provides global shipping (DDU default, DDP available) for all custom diamond solutions.

For custom specifications or material consultation regarding high-power THz emitters, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

The novel design of a terahertz large aperture photoconductive antenna (LAPCA) is reported. It features a longitudinal orientation of the bias electric field within the photoconductive substrate, and has the advantage of a small interelectrode gap, resulting in a higher field for the same applied voltage. The proposed LAPCA configuration has been tested with a nitrogen-doped (∼10 ppm) synthetic monocrystalline diamond, which is a promising material for high-intensity and high-power terahertz sources. Two antennas with different high-voltage electrode realizations were assembled, pumped by a 400 nm femtosecond laser, and tested for THz emitter function. The experimental data are found to be in good correlation with the numerical simulation results. The performance of antennas with the conventional transverse E-field configuration and the novel longitudinal configuration is compared and discussed.

Tech Support

Original Source

DOI: https://doi.org/10.3390/photonics10101169

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