Implementation of a Reference Interferometer for Nanodetection

Summary

A reference interferometer technique, which is designed to remove undesirable laser jitter noise for nanodetection, is utilized for probing an ultra-high quality factor microcavity. Instructions for assembly, setup, and data acquisition are provided, alongside the measurement process for specifying the cavity quality factor.

Abstract

A thermally and mechanically stabilized fiber interferometer suited for examining ultra-high quality factor microcavities is fashioned. After assessing its free spectral range (FSR), the module is put in parallel with a fiber taper-microcavity system and then calibrated through isolating and eliminating random shifts in the laser frequency (i.e. laser jitter noise). To realize the taper-microcavity junction and to maximize the optical power that is transferred to the resonator, a single-mode optical fiber waveguide is pulled. Solutions containing polystyrene nanobeads are then prepared and flown to the microcavity in order to demonstrate the system’s ability to sense binding to the surface of the microcavity. Data is post-processed via adaptive curve fitting, which allows for high-resolution measurements of the quality factor as well as the plotting of time-dependent parameters, such as resonant wavelength and split frequency shifts. By carefully inspecting steps in the time-domain response and shifting in the frequency-domain response, this instrument can quantify discrete binding events.

Introduction

Research interest has risen significantly on the use of whispering-gallery mode (WGM) microcavities for the purpose of nanodetection and biosensing^1-8. This involves ultra-high quality factor (Q) optical cavities that are proficient in identifying miniscule biological particles, down to the single-protein level². That is, monitoring shifts in resonance and split frequency for transmission with extraordinary sensitivity^9-11 can be enabled by the cavity’s confinement of light energy within a small mode volume. Variations in the optical properties of a resonator are the cause of these shifts, which in turn originate from the binding....

Protocol

1. Reference Interferometer Construction and FSR Measurement

1. Construction
   1. Create an open-top acrylic box. This structure should be large enough to fit snugly into a 16 in x 16 in x 16 in Styrofoam box.
   2. Fabricate a 3-stage shelving unit to house optical components, which will sit in the open-top acrylic box and will be completely enclosed by the Styrofoam box for thermal isolation. Two elevated holes on the Styrofoam box must be present to allow for fibers to enter and exit the entire enclosure.
   3. On the 3^rd stage: One output fiber from the 3 dB directional coupler should be clamped to a polarization controller whi....

Results

After following the protocol, the traces can be compiled and fitted. Figure 3a shows the typical resonance structure of the microsphere as presented in the video, for which frequency splitting is observed in a DPBS medium. A least-square fit to the double-Lorentzian function indicates that the quality factor of the left and right resonance dips are respectively 2.1 x 10⁸ and 3.8 x 10⁸ in an aqueous environment. The optical frequencies of the FWHM are obtained by comparing the cavity.......

Discussion

This current setup is capable of probing a variety of WGM microcavities, such as microdisks, microspheres, and microtoroids, without requiring any feedback control for the probe laser source. A considerable signal-to-noise ratio (SNR) for detection can be obtained due to the step shift enhancements provided by path length and particle-induced backscattering effects. Given the simplicity and low cost of the reference interferometer itself, this method is an efficient technique for studying or exploiting the properties of .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Polystyrene  Microspheres	PolyScience
Dulbecco’s Phosphate Buffered Saline (DPBS)	Life Technologies	14190
Piezoelectric Nanopositioner System	Physik Instrumente	P-611.3S
Balanced Photodetector	Thorlabs	PDB120A
Photodetector	Newport	1801-FC
3 dB Fiber Optical Directional Coupler	Thorlabs	FC632-50B
10 dB Fiber Optical Directional Coupler	Thorlabs	FC632-90B
Drop-In Polarization Controller	General Photonics	PLC-003-S-25
Function Generator	Hewlett-Packard	33120A
Fusion Splicer	Ericsson	FSU-925
High-Speed Oscilloscope	Agilent	DS09404A
Motorized Translation Stage with Controller	Thorlabs	MTS25-Z8E
Single Mode Optical Fiber, 600-800 nm, Ø125 μm Cladding	Thorlabs	SM600
Real-Time Electrical Spectrum Analyzer	Tektronix	RSA3408B
Optical Spectrum Analyzer	Agilent	70951A
632.5 – 637 nm Tunable Laser	New Focus	TLB-6304
Filtration Pump	KNF
Ultrasonic Cleaner	Crest Ultrasonics	Powersonic 1100D
Mini Vortexer	VWR	VM-3000
Centrifuge	Beckman Coulter	Microfuge 22R