Laser-Inscribed Diamond Waveguide Resonantly Coupled to Diamond Microsphere

At a Glance

Metadata	Details
Publication Date	2020-06-10
Journal	Molecules
Authors	Nurperi Yavuz, Mustafa Mert Bayer, Hüseyin Ozan Ҫirkinoğlu, Ali Serpengüzel, Thien Le Phu
Institutions	University of California, Irvine, Center for Biomolecular Nanotechnologies
Citations	4
Analysis	Full AI Review Included

Executive Summary

This analysis summarizes the implementation and performance of the first reported all-diamond photonic circuit, integrating a femtosecond (fs)-laser-written waveguide (WG) with a diamond microsphere resonator.

All-Diamond Integration: An integrated photonic circuit was successfully demonstrated using a shallow, stress-induced (Type II) diamond waveguide coupled evanescently to a 1 mm diameter CVD diamond microsphere.
High-Q Resonance Achieved: The system exhibited high-quality factor (Q-factor) Whispering-Gallery Modes (WGMs) in the near-infrared region (1427 nm), reaching a maximum Q-factor of 1.6 x 10⁵ for TM-polarized light.
Fabrication Method: The waveguide was fabricated using fs-laser photo-inscription (515 nm, 230 fs pulses) at a shallow depth (20 µm) to maximize evanescent coupling.
Spectral Performance: Measured WGM spacing was 0.33 nm, closely matching the theoretical calculation (0.34 nm). Fabry-Pérot (FP) resonances within the WG were also observed, yielding a Free Spectral Range (FSR) of 87 pm and Q-factors up to 10⁴.
Future Potential: The platform is highly promising for integrated quantum photonics, leveraging existing nitrogen-vacancy (NV) centers in the diamond components for filtering, high-resolution sensing, and nonlinear optical applications.
Limiting Factors: Current Q-factor limits (10⁵) are primarily determined by material absorption losses (Q_mat) and external coupling losses (Q_ext), both estimated to be in the order of 10⁵. Optimization is projected to achieve Q-factors of 10⁶.

Technical Specifications

Parameter	Value	Unit	Context
WGM Q-Factor (Max)	1.6 x 10⁵	Dimensionless	TM-polarized light, 1427.96 nm
WGM Spacing (Measured)	0.33	nm	Diamond microsphere resonance
FP Resonance Q-Factor (Max)	10⁴	Dimensionless	Diamond Waveguide (WG)
FP FSR (Measured)	87	pm	Diamond WG (5 mm length)
WG Insertion Loss	12.4	dB	At 1550 nm wavelength
WG Depth (Center)	20	µm	Measured from surface
WG MFD (Mode Field Diameter)	16 x 20	µm	Elliptical mode shape
Microsphere Diameter	1.0	mm	Type-Ib CVD diamond
Microsphere Form Accuracy	<250	nm	Roundness achieved by lapping
Diamond Refractive Index (n)	2.4	Dimensionless	Near-IR region
Surface Roughness (WG/Sphere)	<2	nm	After cleaning/polishing
Estimated Coupling Efficiency	~3.3	%	Between SMF and WG mode
Target Q-Factor (Optimized)	10⁶	Dimensionless	Projected maximum

Key Methodologies

The all-diamond photonic circuit was realized through precise femtosecond laser writing and careful component integration and alignment.

Waveguide (WG) Fabrication:
- Laser System: Yb:KGW femtosecond pulsed laser (Pharos).
- Parameters: 515 nm central wavelength, 500 kHz repetition rate, 230 fs pulse duration.
- Focusing: 100x oil immersion objective (NA 1.25).
- Material: Optical grade diamond (5 mm x 5 mm x 0.5 mm) with ~100 ppb Nitrogen impurities.
- Writing Process: Type II fabrication method (stress-induced waveguiding) using laser powers between 30 mW and 40 mW.
- Geometry: Tracks spaced 19 µm apart, written at a scan speed of 0.5 mm/s, resulting in a WG center depth of 20 µm.
Microsphere Preparation:
- Material: 1 mm diameter Type-Ib diamond microsphere (CVD grown, >5 ppm Nitrogen).
- Cleaning: Ultrasonic bath using isopropanol and acetone mixture to achieve surface roughness <2 nm.
Experimental Setup and Coupling:
- Light Source: Distributed Feedback (DFB) semiconductor laser (1427.7 nm CW mode), tunable with 1 pm spectral resolution.
- WG Injection: Light coupled into the diamond WG using a bare Single Mode Fiber (SMF) via butt-coupling. Polarization adjusted via a fiber polarization controller.
- Microsphere Placement: The diamond microsphere was positioned onto the diamond WG using a needle tip connected to a vacuum pump, ensuring contact for evanescent coupling (estimated impact parameter b = 520 µm).
- Detection: 90° elastically scattered light (WGMs) collected via a 10x objective and detected by PD1. Transmitted light (FP resonances) collected via an output SMF and detected by PD2. A Glan polarizer differentiated TE- and TM-polarized scattered light.

Commercial Applications

The integration of high-Q diamond resonators with robust diamond waveguides creates a platform suitable for extreme environments and quantum technologies.

Quantum Information Processing (QIP):
- Exploiting existing Nitrogen-Vacancy (NV) centers within the diamond components for integrated quantum emitters.
- Realization of integrated quantum memories and scalable photonic circuits.
High-Resolution Sensing:
- Biosensing: Utilizing the high Q-factors of WGMs for label-free detection down to single molecules, suitable for harsh chemical environments due to diamond’s inertness.
- Temperature/Strain Sensing: Leveraging diamond’s extreme mechanical and thermal stability.
Integrated Photonics and Telecommunications:
- Optical Filtering: Realizing ultra-compact, high-resolution optical filters and channel dropping devices based on the sharp WGM resonances (0.33 nm spacing).
- Nonlinear Optics: Utilizing diamond’s high Raman gain and wide transparency window for integrated frequency comb generation and high-power Raman lasers.
Extreme Environment Applications:
- Diamond’s superior hardness and high thermal conductivity make this platform ideal for use in high-radiation, high-temperature, or chemically aggressive environments where silicon or silica components fail.

View Original Abstract

An all-diamond photonic circuit was implemented by integrating a diamond microsphere with a femtosecond-laser-written bulk diamond waveguide. The near surface waveguide was fabricated by exploiting the Type II fabrication method to achieve stress-induced waveguiding. Transverse electrically and transverse magnetically polarized light from a tunable laser operating in the near-infrared region was injected into the diamond waveguide, which when coupled to the diamond microsphere showed whispering-gallery modes with a spacing of 0.33 nm and high-quality factors of 105. By carefully engineering these high-quality factor resonances, and further exploiting the properties of existing nitrogen-vacancy centers in diamond microspheres and diamond waveguides in such configurations, it should be possible to realize filtering, sensing and nonlinear optical applications in integrated diamond photonics.

Tech Support

Original Source

DOI: https://doi.org/10.3390/molecules25112698

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