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 of discrete molecules or nanoparticles. A less sophisticated example of a three-dimensional WGM structure for such applications is a silica microsphere, which can be fabricated with a near atomically smooth surface by simply ablating a drawn optical fiber using a CO₂ laser. As is known, high Q-factors on the order of 10⁹ can be attained¹.

The resonant frequency of a microcavity is conventionally monitored by scanning the optical frequency of a tunable laser source while simultaneously photo-detecting the optical transmission that is captured on an oscilloscope. An inherent drawback of this technique is the uncertainty associated with the location of drops in the transmission that arises from fluctuating laser wavelength or laser jitter. To overcome this complication, an interferometer can be used alongside a microcavity to produce a reference signal to cancel the laser jitter and increase the observed sensitivity². Light input is split into two optical paths: the reference beam that passes through the interferometer (with a free spectral range or FSR large enough to prevent the laser from jittering past one FSR frequency spacing during measurement) and the detection beam that interacts with the WGM microresonator. This feature streamlines experiments in comparison to more advanced configurations, such as that of WGM sensing entailing the combination of a distributed feedback laser (DFB) and periodically poled lithium niobate (PPLN) doubler¹². In this publication, an interferometer technique for ultra-high quality factor microcavity based monitoring of nanoscale matter is described³. The setup and data acquisition procedures that are required to accomplish this are outlined, illustrating how cavity quality factor can be determined through reference interferometry.

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 which in turn leads to an input port of a separate 3 dB directional coupler.
   4. On the 2^nd stage: Form a loop with roughly 16 feet of the optical fiber originating from the other output port of the first 3 dB directional coupler. Direct this fiber to the remaining input port of the second 3 dB directional coupler on the 3^rd stage.
   5. Fill the acrylic box with 50% shaved ice mixed with 50% liquid water, as to fashion an ice bath and consequently maintain the temperature of the optical components near 0 °C.
2. FSR Measurement
   1. Set up the probe laser at the desired wavelength. Employ a function generator such that its output is connected to a 3 dB power splitter. One of the outputs from the 3 dB splitter must be connected to the oscilloscope for monitoring purposes and the other output is to be used to directly tune the frequency of the laser.
   2. Feed the laser output as the input to the 1^st 3 dB directional coupler.
   3. The two outputs of the 2^nd 3 dB directional coupler are to carry photomixed signals to the balanced photodetector (BPD). Finally, connect the output cable of the BPD to a channel input of the oscilloscope.
   4. Linearly scan the laser frequency by supplying the laser module with a ramp signal generated from the waveform generator (with a peak-to-peak voltage of 1 V and scan frequency of 100 Hz). The output signal from the BPD will become sinusoidal on the oscilloscope.
   5. Tune the polarization controller as to maximize the peak-to-peak voltage of the sinusoidal waveform.
   6. To measure the FSR, configure the laser for continuous wave output by setting the waveform generator to DC mode. Tune the waveform generator voltage such that the transmitted signal from the BPD fluctuates around 0 V (i.e. the quadrature point). Inspect the output signal using an electrical spectrum analyzer. The monitored signal should appear as a sinc-square function, where the location of the first zero nearest the global maximum (at zero frequency) corresponds to the FSR. To minimize the measurement noise, set the electrical spectrum analyzer to averaging mode.

2. Fiber Pulling¹³

Preamble: The goal of this procedure is to approximately match the phase of photons travelling in the taper to those of the microcavity so that efficient coupling can occur. As the fiber is pulled, the central section that lies between the two clamps will transition from supporting a single mode within a regular fiber, to multiple modes within a waveguide formed by the original silica cladding becoming the core and air becoming the cladding, and then to a single mode. The silica core of the fiber will virtually vanish in the central section, wherein temporarily satisfied multimode propagation conditions will be counteracted by the continual shrinking of the fiber diameter.

1. Fix the fiber holder to the motorized translational stage.
2. Connectorize two sections of optical fiber with FC/APC connectors on one end of each section. Remove the buffer coating from the unconnected ends with a fiber stripper, clean them with acetone first and then isopropanol, cleave the end facets, and fusion splice them together.
3. To monitor the loss in the taper, connect a probe laser in constant power mode to one end of the fiber while the other end of the fiber is connected to a photodetector (PD). The output of the PD must be connected to an oscilloscope. Adjust the oscilloscope settings as to measure the PD output voltage, which is proportional to the transmitted laser power.
4. Record the initial value of the PD output voltage and continue to monitor it until step 2.9.
5. Clamp the fiber to the fiber holder and image the fiber with an optical microscope.
6. Release hydrogen such that it begins to flow near the taper, waiting for air to exit the tube and for the pressure of the channel to stabilize. Once the flow rate for the hydrogen gas reaches 110 ml/min, ignite it near the outlet with a lighter to heat the fiber.
7. Using a custom LabVIEW program, linearly pull the fiber. Note that during the pulling process, the fiber core gradually vanishes whilst multiple cladding modes become dominant in guiding the light through the tapered fiber section. The transmitted intensity through the optical fiber should oscillate due to multimode interference.
8. Continue pulling the fiber to reduce the fiber taper width until it only supports a single cladding mode. Once the transmitted intensity ceases to vary, stop pulling the fiber.
9. Release the fiber holder from the translation stage and secure it near the piezoelectric stage.

3. Preparation and Delivery of Solutions

1. Prepare 10 pM, 1 pM, and 100 fM solutions composed of 50 nm radius monodisperse polystyrene microspheres in Dulbecco’s phosphate buffered saline (DPBS). Additionally, create a pure DPBS solution.
2. Place the solutions in a centrifuge, stagger their positions within it for balance purposes, and initiate a 30 min spinning cycle.
3. Upon completion, securely place the solutions in a desiccator, evacuate it, and bombard the solutions with ultrasound waves for 30 min.
4. Remove the solutions and set them aside near the experiment setup.
5. Build a stand for a small fluid delivery system.
   1. Upon cleaning two ferrules, insert syringe tips onto both ends of a microtubule segment and screw on the ferrules to the syringe tips. Individually connect one of the ferrules to a third syringe tip and the other to the Luer lock fitting of a barrel-plunger assembly.
   2. Fasten the exposed syringe tip to the stand and prop it behind the sample. The fluids should be able to flow onto the sample without significant spillage.
6. In terms of Section 5 of the Protocol, load the barrel with an appropriate solution and manually inject it through the microfluidic system during experiment.

4. System Configuration and Interconnections

1. Connect the probe laser to a 10 dB directional coupler. The coupled port is connected to the input port of the reference interferometer while the transmitted port is connected to a polarization controller followed by the tapered fiber.
2. Refocus the microscope objectives to acquire two sharp images of the fiber taper.
3. Connect the output of the tapered fiber to a PD. The output of this PD should be attached to a different channel input of the oscilloscope.
4. Mount the sample on the nanopositioner and make coarse adjustments to displace it so that it is proximate to the center of the fiber taper.
5. Inject DPBS to the sample. Make coarse adjustments such that the fiber taper comes into view of the two CCD cameras. Adjust the nanopositioner to establish coupling from the fiber taper to the microcavity.
6. Scan the laser wavelength as to obtain an appropriate resonance dip on the oscilloscope.

5. Nanoparticle Detection

To acquire data: Configure the oscilloscope’s trigger settings and, using homemade software, collect oscilloscope traces for further processing.

1. Record the data for the buffer solution as a reference.
2. Record the data for the nanoparticle solutions from lowest to highest concentration.
3. Observe the frequency shifts that take place due to nanoparticle binding on the microcavity.

6. Post-Processing of Data

The collected data can be further processed by a self-written MATLAB program. The program should:

1. Read the reference interferometer traces and conduct a least-square fit to the sinusoidal curves. The phases of the fitted sinusoidal are used to estimate the laser jitter on the fly.
2. Read the cavity transmission traces and conduct a least-square fit to the double-Lorentzian function. Optical frequencies corresponding to the resonance dips (ν₁, ν₂) and their full widths at half maximum (FWHM’s, represented by δν₁,δν₂) are determined by comparing the transmission signal to the interferometer signal.
3. Obtain the quality factor of each individual dip from Q_i = ν_i / δν_i, where i can either be 1 (left resonance) or 2 (right resonance).
4. Calculate, as is conventional, the optical frequencies of the resonance dips via the laser scan-voltage, where the laser jitter yields larger measurement noise.
5. Collect the average resonance frequency ν_avg = (ν₁ + ν₂) / 2 and split frequency Δν = ν₂ - ν₁ for each measurement and plot them as a function of time. When a nanoparticle binds onto the surface of the microcavity, sudden shifts of both average resonance frequency and split frequency should be observed.

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