Y.C.K. acknowledges financial support from the European Research Council through the project NextPhase (ERC StG 278616). Authors also acknowledge support from the Centre National d'Etudes Spatiales (CNES, France) through the Project SHYRO (Action R&amp;T R-S10/LN-0001-004/DA: 10076201), from the ANR project ORA (BLAN 031202), and from the Region de Franche-Comte, France.

The protocol consists in 5 main stages: In the first one, the whispering-gallery-mode resonator is made. In order to control the progress of the polishing of the resonator, surface state measurements are carried out. In the third stage, we fabricate the tool that will launch light in the resonator. Once those two main tools are manufactured, we use them to visualize optical high-Q resonances. Finally, using a high-power input laser beam, the resonator behaves in a nonlinear fashion and Kerr combs are produced.
1. Polishing the Resonator
In this stage, an optical window crystalline resonator (MgF2 or CaF2, readily available from optical component retailers) is shaped and polished. This polishing procedure converts them into high-quality WGM resonators. The customized polishing tower is presented in Scheme 1.
<ol>
	<li>Glue the crystalline optical window on a stick that can be held by the air bearing spindle motor.</li>
	<li>Coat a V-shaped metallic guide with the appropriate polishing support tissue, and pour 10 &mu;m abrasive powders (aluminum oxide, diamond, or silicon carbide) mixed with water. Approach this guide to the spinning disk (approximately 5,000 rpm, 20 g pressure) and start grinding it. Depending on the material and the spinning speed, this process can last from 2 hr (for CaF2) to 4 hr (for MgF2). This polishing step will give its bi-convex shape to the resonator. At the end of this step the disk should have the shape presented in Scheme 2.</li>
	<li>The next step is generally called "grinding and polishing" procedure24. It typically consists in repeating the preceding step with abrasive particle of size 10 &mu;m, 3 &mu;m, 1 &mu;m, 250 nm, and finally 100 nm. The supporting tissue should be adapted to each particle size, being less stiff for smaller grains. To avoid scratches and stripes, a horizontal translation of the guide can be performed. At each step of successive grinding and polishing, the state of the surface should be improved.</li>
</ol>
2. Controlling the State of the Surface
<ol>
	<li>A visual control under an optical microscope is the first step for a surface state control: the disk is opaque to light in the first stages; however, after a successful 1 &mu;m particle polishing, the disk becomes transparent and its sides reflect light: the so-called optical polish has been reached and the quality factor of the resonator should be in the 105 - 106 range.</li>
	<li>For smaller abrasive, the eye is not able to evaluate the state of the surface, even using a standard microscope. At this point, an interferometric measurement of the surface state is needed. Use a microscope equipped with a Mirau interferometer objective lens and with a white light source. The image of the resonator interferes with a reference plane, thus revealing by a white light phase shifting processing tool the surface height independently at each point with a resolution of a fraction of the wavelength, namely a few nanometers. This measurement can also be used to evaluate the curvature of the disk25.</li>
	<li>By modifying the length between the sample and the objective, determine the optical phase of the resonator reflection and calculate the height variations of the surface. This can be automated thanks to a dedicated computer, and a map of the surface height is created, allowing for the determination of the roughness of the sample. Monitor the surface roughness as explained in Figure 1, and stop the grinding-polishing procedure when the interference fringes are the smoothest possible.</li>
</ol>
3. Drawing the Taper
To couple light in the resonator, a very small optical fiber is needed: its diameter should be around 3 &mu;m (about 20 times smaller than a human hair).
<ol>
	<li>Strip a standard single-mode silica fiber (SMF) off its plastic and polymer coating on an approximate 5 cm length. For monitoring purposes, the fiber should be connected to a laser source at its input, and a photodiode at its output.</li>
	<li>Fix each size of the uncoated section of the fiber to two computer-controlled high-resolution motors. Using the computer interface of the motors, configure them to move with a constantly accelerated motion, so that each side of the fiber will be pulled apart.</li>
	<li>Heat the uncoated fiber between the two fixation points with a blowtorch lamp for about 1 min before starting the stretching. The flame should be gentle in order not to blow the taper away once it is very small.</li>
	<li>Start the motors movement, and thus, the stretching of the fiber. Once the drawing is started, one can monitor the transmission of the taper using a laser source and a photodiode: interference patterns will appear during the process, their frequency will increase, and, finally, they will disappear for a waist diameter near 1 &mu;m. At this stage, the motor and the flame should be stopped simultaneously.</li>
</ol>
4. Coupling Light in the WGM Resonator
In this stage, the taper is used to couple light in the resonator and to observe the high-Q eigenmodes of the cavity, which are represented in Figure 2.
<ol>
	<li>Fix the resonator on a 3-axis Piezo-controlled translation stage. Approach it to the fiber taper at a distance of less than 1 &mu;m. The relative position of the fiber taper and the resonator is monitored, thanks to a microscope, and a mirror is used to control the vertical positioning and tilt angle.</li>
	<li>Connect the fiber taper to a visible laser diode: the resonator should be illuminated when the coupling is efficient, as shown in Figure 3.</li>
	<li>Connect the fiber taper to a mode-hop free laser with a narrow linewidth (narrower than the linewidth of the resonance) to one end, and a photodiode connected to an oscilloscope on the other end. The transmission response of the resonator can be obtained by scanning the wavelength at the input. Evaluate the quality factor of the resonator using the obtained transmission spectrum, by calculating the ratio between the resonance frequency of the modes and their linewidths (full width at half maximum).</li>
	<li>A more accurate measurement is performed with the "cavity-ring-down" experiment26, where the wavelength sweeping is fast enough to obtain interferences between the resonating light decaying in the resonator and the detuned light at a subsequent time. One can fine tune the positioning of the taper and the resonator to increase the coupling Q-factor and obtain the typical pattern shown in Figure 4. The associated fit curve gives the quality factor of the resonator.</li>
</ol>
5. Generating the Comb
In this last stage, a high power pump laser excites nonlinear effects in the resonator.
<ol>
	<li>Insert an optical amplifier between the tunable laser and the resonator.</li>
	<li>Thanks to the photodiode and oscilloscope, fine-tune the laser source so that the input wavelength is next to a resonance.</li>
	<li>Connect the output fiber to a high-resolution optical spectrum analyzer and increase the input power while slightly detuning the pump wavelength. New frequencies will appear on each side of the pump peak: this is a Kerr optical frequency comb.</li>
	<li>Switching back to the photodiode, we can observe the beatings between the different spectral modes created. Using a microwave band-pass filter, one can isolate a pure frequency in this electric signal with very low noise.</li>
</ol>

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The authors declare that they have no competing financial interests.

This protocol allows producing high-Q optical resonators, to couple light into them and trigger nonlinear phenomena for various microwave photonics applications.
The first step of rough grinding should give its shape to resonator. After an hour of grinding with the 10 &mu;m abrasive powder, one side of the rim of the resonator should be conveniently shaped (see Scheme 2). The following step will smooth the surface of the resonator and when reaching the stage of the 1 ...

Whispering gallery mode resonators are disks or spheres of micro- or millimetric radius1,2,3,4. Provided that the resonator is almost perfectly shaped (nanometer-size surface roughness), laser light can be trapped by total internal reflection within its eigenmodes, which are usually referred to as whispering-gallery modes (WGMs). Their free-spectral range (or intermodal frequency) can vary from GHz to THz depending on the resonator's radius, while their quality factor Q can be exceptionally high5, ranging from 107 - 1011. Due to their unique property of stockpiling and slowing down light, WGM optical resonators have been used to perform many optical signal processing tasks3: filtering, amplification, time-delaying, etc. With the continuous improvement of fabrication technologies, their unprecedented quality factors make them suitable for even more demanding application in metrology or quantum-based applications6-13.
In these ultra-high Q resonators, the small volume of confinement, high photon density, and long photon lifetime (proportional to Q) induce a very strong light-matter interaction, which may excite the various WGMs through various nonlinear effects, like Kerr, Raman, or Brillouin for example14-19. Using nonlinear phenomena in whispering gallery mode resonators was proposed as a promising paradigm shift for ultra-pure microwave and lightwave generation. The fact that this topic intersects so many areas of fundamental science and technology is a clear indicator of its very strong potential impact on a wide range of disciplines. In particular, aerospace and communication engineering technologies are currently in need of versatile microwave and lightwave signal with exceptional coherence. The WGM technology has several advantages over existing or other prospective methods: conceptual simplicity, higher robustness, smaller power consumption, longer lifetime, immunity to interferences, very compact volume, frequency versatility, easy chip integration, as well as a strong potential for integrating the mainstream of standard photonic components for both microwave and lightwave technologies.
In aerospace engineering, quartz oscillators are overwhelmingly dominant as key microwave sources for both navigation systems (planes, satellites, spacecrafts, etc.) and detection systems (radars, sensors, etc.). However, it is unanimously recognized today that frequency stability performance of quartz oscillators is reaching its floor, and will not improve significantly anymore. Along the same line, their frequency versatility is limited and will hardly allow for ultra-stable microwave generation beyond 40 GHz. Microwave photonic oscillators are expected to overcome these limitations. On the other hand, in communication engineering, microwave photonic oscillators are also expected to be key components in optical communication networks where they would perform the lightwave/microwave conversion with unprecedented efficiency. They are also compatible with the ongoing trend of compact full-optical components in lightwave technology, which enable ultra-fast processing [up/down conversion, (de)modulation, amplification, multiplexing, mixing, etc.] without the need to manipulate massive (and then, slow) electrons. This concept of compact photonic circuits where photons control photons via nonlinear media aims to circumvent the bottleneck originating from virtually unlimited optical bandwidth versus limited optoelectronic processing speed. Optical communications systems are also very demanding for ultra-low phase noise microwaves in order to satisfy both clocking (low phase noise is equivalent to low time-jitter) and bandwidth (bit-rates increase proportionally to the clock frequency) requirements. In fact, in high-speed communication networks, such ultra-stable oscillators are fundamental references for several purposes (local oscillator for up/down frequency conversion, network synchronization, carrier synthesis, etc.).
Nonlinear phenomena in WGM resonators also open new horizons of research for other applications, such as Raman and Brillouin lasers. More generally, these phenomena can be merged within the broader perspective of nonlinear phenomena in optical cavities and waveguides, and it is a fruitful paradigm for crystalline or silicon photonics. The strong confinement and very long lifetime of photons into the torus-like WGMs also offer an excellent test-bench to investigate fundamental issues in condensed matter and quantum physics. The race to ever increased accuracy in electromagnetic signals also contributes to answer quintessential questions in physics, related to relativity (tests for Lorentz invariance), or the measurement of fundamental physical constants and their possible variation with time.
In this article, the different steps required to obtain crystalline optical whispering-gallery-mode (WGM) resonators are described and their characterization is explained. Also presented is the protocol to obtain the high quality tapered fiber needed to couple laser light into these resonators. Finally, a flagship application of these resonators in the field of microwave photonics, namely ultra-stable microwave generation using Kerr combs, is presented and discussed.
In the first section, we detail the protocol followed to obtain ultra-high Q WGM resonators. Our method relies on a grind and polish approach, which is reminiscent to the standard techniques used to polish optical components such as lenses or telescope mirrors. The second section is devoted to the characterization of surface roughness. We use a non-contact white light interferometric profilometer to measure the surface roughness which leads to surface scattering-induced losses and thereby lower the Q factor performance. This step is an important experimental test to evaluate the quality of the polishing. The third section is concerned with the fabrication a tapered silica fiber with diameter in the micrometer range in order to launch light in the resonator. To reach such small diameters, we adopt the "flame-brushing" technique, using simultaneously computer-controlled motors to pull the fiber apart, and a blowtorch to heat the fiber area to be tapered20. In the fourth section, the resonator and the tapered fiber are approached to one another to visualize the resonance signal of the whispering gallery modes using a wavelength-scanning laser. We show in the fifth section how, by increasing the optical power in the resonator, we manage to trigger nonlinear phenomena until we observe the formation of Kerr optical frequency combs, with a spectrum made of equidistant spectral lines. As emphasized above, these Kerr comb spectra have exceptional characteristics that are suitable for several applications in both science and technology21-23. We will consider one of the most noteworthy applications of WGM resonators by demonstrating an optical multi-wavelength signal whose intermodal frequency is an ultra-stable microwave.

<table><tbody><tr><td>Step motors 50 mm course</td><td>Thorlabs</td><td></td><td></td></tr><tr><td>3 axis nanostage</td><td>Physik Instrumente</td><td></td><td></td></tr><tr><td>TUNICS tunable laser source</td><td>Yenista</td><td></td><td></td></tr><tr><td>Optical spectrum analyzer APEX</td><td>APEX Technologies</td><td></td><td></td></tr></tbody></table>

microwave photonics systems based on whispering-gallery-mode resonators

Microwave photonics systems rely fundamentally on the interaction between microwave and optical signals. These systems are extremely promising for various areas of technology and applied science, such as aerospace and communication engineering, sensing, metrology, nonlinear photonics, and quantum optics. In this article, we present the principal techniques used in our lab to build microwave photonics systems based on ultra-high Q whispering gallery mode resonators. First detailed in this article is the protocol for resonator polishing, which is based on a grind-and-polish technique close to the ones used to polish optical components such as lenses or telescope mirrors. Then, a white light interferometric profilometer measures surface roughness, which is a key parameter to characterize the quality of the polishing. In order to launch light in the resonator, a tapered silica fiber with diameter in the micrometer range is used. To reach such small diameters, we adopt the "flame-brushing" technique, using simultaneously computer-controlled motors to pull the fiber apart, and a blowtorch to heat the fiber area to be tapered. The resonator and the tapered fiber are later approached to one another to visualize the resonance signal of the whispering gallery modes using a wavelength-scanning laser. By increasing the optical power in the resonator, nonlinear phenomena are triggered until the formation of a Kerr optical frequency comb is observed with a spectrum made of equidistant spectral lines. These Kerr comb spectra have exceptional characteristics that are suitable for several applications in science and technology. We consider the application related to ultra-stable microwave frequency synthesis and demonstrate the generation of a Kerr comb with GHz intermodal frequency.

This five-step protocol enables to obtain WGM resonators with very high quality factors for microwave photonic applications.
The first step aims to give to the resonator the desired shape, as represented on Scheme 2. The main difficulty here is to manufacture a disk whose rim is sharp enough so that can it strongly confine the trapped photons, without leading to structural fragility from a mechanical standpoint. This polishing tower also possesses remarkable versatility as it ...

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Microwave Photonics Systems Based on Whispering-gallery-mode Resonators

The customized techniques developed in our lab to build microwave photonics systems based on ultra-high Q whispering gallery mode resonators are presented. The protocols to obtain and characterize these resonators are detailed, and an explanation of some of their applications in microwave photonics is given.

Microwave photonics systems rely fundamentally on the interaction between microwave and optical signals. These systems are extremely promising for various ...

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16.9K Views.  FEMTO-ST Institute. The customized techniques developed in our lab to build microwave photonics systems based on ultra-high Q whispering gallery mode resonators are presented. The protocols to obtain and characterize these resonators are detailed, and an explanation of some of their applications in microwave photonics is given.

Video: Microwave Photonics Systems Based on Whispering-gallery-mode Resonators

Fabrication of Silica Ultra High Quality Factor Microresonators

Whispering gallery resonant cavities confine light in circular orbits at their periphery.1-2 The photon storage lifetime in the cavity, quantified by the quality factor (Q) of the cavity, can be in excess of 500ns for cavities with Q factors above 100 million. As a result of their low material losses, silica microcavities have demonstrated some of the longest photon lifetimes to date1-2. Since a portion of the circulating light extends outside the resonator, these devices can also be used to probe the surroundings. This interaction has enabled numerous experiments in biology, such as single molecule biodetection and antibody-antigen kinetics, as well as discoveries in other fields, such as development of ultra-low-threshold microlasers, characterization of thin films, and cavity quantum electrodynamics studies.3-7
The two primary silica resonant cavity geometries are the microsphere and the microtoroid. Both devices rely on a carbon dioxide laser reflow step to achieve their ultra-high-Q factors (Q&gt;100 million).1-2,8-9 However, there are several notable differences between the two structures. Silica microspheres are free-standing, supported by a single optical fiber, whereas silica microtoroids can be fabricated on a silicon wafer in large arrays using a combination of lithography and etching steps. These differences influence which device is optimal for a given experiment.
Here, we present detailed fabrication protocols for both types of resonant cavities. While the fabrication of microsphere resonant cavities is fairly straightforward, the fabrication of microtoroid resonant cavities requires additional specialized equipment and facilities (cleanroom). Therefore, this additional requirement may also influence which device is selected for a given experiment.
Introduction
An optical resonator efficiently confines light at specific wavelengths, known as the resonant wavelengths of the device. 1-2 The common figure of merit for these optical resonators is the quality factor or Q. This term describes the photon lifetime (&tau;o) within the resonator, which is directly related to the resonator's optical losses. Therefore, an optical resonator with a high Q factor has low optical losses, long photon lifetimes, and very low photon decay rates (1/&tau;o). As a result of the long photon lifetimes, it is possible to build-up extremely large circulating optical field intensities in these devices. This very unique property has allowed these devices to be used as laser sources and integrated biosensors.10
A unique sub-class of resonators is the whispering gallery mode optical microcavity. In these devices, the light is confined in circular orbits at the periphery. Therefore, the field is not completely confined within the device, but evanesces into the environment. Whispering gallery mode optical cavities have demonstrated some of the highest quality factors of any optical resonant cavity to date.9,11 Therefore, these devices are used throughout science and engineering, including in fundamental physics studies and in telecommunications as well as in biodetection experiments. 3-7,12
Optical microcavities can be fabricated from a wide range of materials and in a wide variety of geometries. A few examples include silica and silicon microtoroids, silicon, silicon nitride, and silica microdisks, micropillars, and silica and polymer microrings.13-17 The range in quality factor (Q) varies as dramatically as the geometry. Although both geometry and high Q are important considerations in any field, in many applications, there is far greater leverage in boosting device performance through Q enhancement. Among the numerous options detailed previously, the silica microsphere and the silica microtoroid resonator have achieved some of the highest Q factors to date.1,9 Additionally, as a result of the extremely low optical loss of silica from the visible through the near-IR, both microspheres and microtoroids are able to maintain their Q factors over a wide range of testing wavelengths.18 Finally, because silica is inherently biocompatible, it is routinely used in biodetection experiments.
In addition to high material absorption, there are several other potential loss mechanisms, including surface roughness, radiation loss, and contamination loss.2 Through an optimization of the device size, it is possible to eliminate radiation losses, which arise from poor optical field confinement within the device. Similarly, by storing a device in an appropriately clean environment, contamination of the surface can be minimized. Therefore, in addition to material loss, surface scattering is the primary loss mechanism of concern.2,8
In silica devices, surface scattering is minimized by using a laser reflow technique, which melts the silica through surface tension induced reflow. While spherical optical resonators have been studied for many years, it is only with recent advances in fabrication technologies that researchers been able to fabricate high quality silica optical toroidal microresonators (Q&gt;100 million) on a silicon substrate, thus paving the way for integration with microfluidics.1
The present series of protocols details how to fabricate both silica microsphere and microtoroid resonant cavities. While silica microsphere resonant cavities are well-established, microtoroid resonant cavities were only recently invented.1 As many of the fundamental methods used to fabricate the microsphere are also used in the more complex microtoroid fabrication procedure, by including both in a single protocol it will enable researchers to more easily trouble-shoot their experiments.

We describe the use of a carbon dioxide laser reflow technique to fabricate silica resonant cavities, including free-standing microspheres and on-chip microtoroids. The reflow method removes surface imperfections, allowing long photon lifetimes within both devices. The resulting devices have ultra high quality factors, enabling applications ranging from telecommunications to biodetection.

Whispering gallery resonant cavities confine light in circular orbits at their periphery.1-2 The photon storage lifetime in the cavity, quantified by the ...

Development of Whispering Gallery Mode Polymeric Micro-optical Electric Field Sensors

Optical modes of dielectric micro-cavities have received significant attention in recent years for their potential in a broad range of applications. The optical modes are frequently referred to as "whispering gallery modes" (WGM) or "morphology dependent resonances" (MDR) and exhibit high optical quality factors. Some proposed applications of micro-cavity optical resonators are in spectroscopy1, micro-cavity laser technology2, optical communications3-6 as well as sensor technology. The WGM-based sensor applications include those in biology7, trace gas detection8, and impurity detection in liquids9. Mechanical sensors based on microsphere resonators have also been proposed, including those for force10,11, pressure12, acceleration13 and wall shear stress14. In the present, we demonstrate a WGM-based electric field sensor, which builds on our previous studies15,16. A candidate application of this sensor is in the detection of neuronal action potential.
The electric field sensor is based on polymeric multi-layered dielectric microspheres. The external electric field induces surface and body forces on the spheres (electrostriction effect) leading to elastic deformation. This change in the morphology of the spheres, leads to shifts in the WGM. The electric field-induced WGM shifts are interrogated by exciting the optical modes of the spheres by laser light. Light from a distributed feedback (DFB) laser (nominal wavelength of ~ 1.3 &mu;m) is side-coupled into the microspheres using a tapered section of a single mode optical fiber. The base material of the spheres is polydimethylsiloxane (PDMS). Three microsphere geometries are used: (1) PDMS sphere with a 60:1 volumetric ratio of base-to-curing agent mixture, (2) multi layer sphere with 60:1 PDMS core, in order to increase the dielectric constant of the sphere, a middle layer of 60:1 PDMS that is mixed with varying amounts (2% to 10% by volume) of barium titanate and an outer layer of 60:1 PDMS and (3) solid silica sphere coated with a thin layer of uncured PDMS base. In each type of sensor, laser light from the tapered fiber is coupled into the outermost layer that provides high optical quality factor WGM (Q ~ 106). The microspheres are poled for several hours at electric fields of ~ 1 MV/m to increase their sensitivity to electric field.

A high-sensitivity photonic micro sensor was developed for electric field detection. The sensor exploits the optical modes of a dielectric sphere. Changes in the external electric field perturb the sphere morphology leading to shifts in its optical modes. The electric field strength is measured by monitoring these optical shifts.

Optical modes of dielectric micro-cavities have received significant attention in recent years for their potential in a broad range of applications. The ...

Fabrication And Characterization Of Photonic Crystal Slow Light Waveguides And Cavities

Slow light has been one of the hot topics in the photonics community in the past decade, generating great interest both from a fundamental point of view and for its considerable potential for practical applications. Slow light photonic crystal waveguides, in particular, have played a major part and have been successfully employed for delaying optical signals1-4 and the enhancement of both linear5-7 and nonlinear devices.8-11
Photonic crystal cavities achieve similar effects to that of slow light waveguides, but over a reduced band-width. These cavities offer high Q-factor/volume ratio, for the realization of optically12 and electrically13 pumped ultra-low threshold lasers and the enhancement of nonlinear effects.14-16 Furthermore, passive filters17 and modulators18-19 have been demonstrated, exhibiting ultra-narrow line-width, high free-spectral range and record values of low energy consumption.
To attain these exciting results, a robust repeatable fabrication protocol must be developed. In this paper we take an in-depth look at our fabrication protocol which employs electron-beam lithography for the definition of photonic crystal patterns and uses wet and dry etching techniques. Our optimised fabrication recipe results in photonic crystals that do not suffer from vertical asymmetry and exhibit very good edge-wall roughness. We discuss the results of varying the etching parameters and the detrimental effects that they can have on a device, leading to a diagnostic route that can be taken to identify and eliminate similar issues.
The key to evaluating slow light waveguides is the passive characterization of transmission and group index spectra. Various methods have been reported, most notably resolving the Fabry-Perot fringes of the transmission spectrum20-21 and interferometric techniques.22-25 Here, we describe a direct, broadband measurement technique combining spectral interferometry with Fourier transform analysis.26 Our method stands out for its simplicity and power, as we can characterise a bare photonic crystal with access waveguides, without need for on-chip interference components, and the setup only consists of a Mach-Zehnder interferometer, with no need for moving parts and delay scans.
When characterising photonic crystal cavities, techniques involving internal sources21 or external waveguides directly coupled to the cavity27 impact on the performance of the cavity itself, thereby distorting the measurement. Here, we describe a novel and non-intrusive technique that makes use of a cross-polarised probe beam and is known as resonant scattering (RS), where the probe is coupled out-of plane into the cavity through an objective. The technique was first demonstrated by McCutcheon et al.28 and further developed by Galli et al.29

Use of photonic crystal slow light waveguides and cavities has been widely adopted by the photonics community in many differing applications. Therefore fabrication and characterization of these devices are of great interest. This paper outlines our fabrication technique and two optical characterization methods, namely: interferometric (waveguides) and resonant scattering (cavities).

Slow light has been one of the hot topics in the photonics community in the past decade, generating great interest both from a fundamental point of view ...

Implementation of a Reference Interferometer for Nanodetection

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&#8217;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.

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.

A thermally and mechanically stabilized fiber interferometer suited for examining ultra-high quality factor microcavities is fashioned. After assessing ...

Fabrication and Testing of Microfluidic Optomechanical Oscillators

Cavity optomechanics experiments that parametrically couple the phonon modes and photon modes have been investigated in various optical systems including microresonators. However, because of the increased acoustic radiative losses during direct liquid immersion of optomechanical devices, almost all published optomechanical experiments have been performed in solid phase. This paper discusses a recently introduced hollow microfluidic optomechanical resonator. Detailed methodology is provided to fabricate these ultra-high-Q microfluidic resonators, perform optomechanical testing, and measure radiation pressure-driven breathing mode and SBS-driven whispering gallery mode parametric vibrations. By confining liquids inside the capillary resonator, high mechanical- and optical- quality factors are simultaneously maintained.

Parametric optomechanical excitations have recently been experimentally demonstrated in microfluidic optomechanical resonators by means of optical radiation pressure and stimulated Brillouin scattering. This paper describes the fabrication of these microfluidic resonators along with methodologies for generating and verifying optomechanical oscillations.

Cavity optomechanics experiments that parametrically couple the phonon modes and photon modes have been investigated in various optical systems including ...

Using Microwave and Macroscopic Samples of Dielectric Solids to Study the Photonic Properties of Disordered Photonic Bandgap Materials

Recently, disordered photonic materials have been suggested as an alternative to periodic crystals for the formation of a complete photonic bandgap (PBG). In this article we will describe the methods for constructing and characterizing macroscopic disordered photonic structures using microwaves. The microwave regime offers the most convenient experimental sample size to build and test PBG media. Easily manipulated dielectric lattice components extend flexibility in building various 2D structures on top of pre-printed plastic templates. Once built, the structures could be quickly modified with point and line defects to make freeform waveguides and filters. Testing is done using a widely available Vector Network Analyzer and pairs of microwave horn antennas. Due to the scale invariance property of electromagnetic fields, the results we obtained in the microwave region can be directly applied to infrared and optical regions. Our approach is simple but delivers exciting new insight into the nature of light and disordered matter interaction.

Our representative results include the first experimental demonstration of the existence of a complete and isotropic PBG in a two-dimensional (2D) hyperuniform disordered dielectric structure. Additionally we demonstrate experimentally the ability of this novel photonic structure to guide electromagnetic waves (EM) through freeform waveguides of arbitrary shape.

Disordered structures offer new mechanisms for forming photonic bandgaps and unprecedented freedom in functional-defect designs. To circumvent the computational challenges of disordered systems, we construct modular macroscopic samples of the new class of PBG materials and use microwaves to characterize their scale-invariant photonic properties, in an easy and inexpensive manner.

Recently, disordered photonic materials have been suggested as an alternative to periodic crystals for the formation of a complete photonic bandgap (PBG). ...

Fabrication and Characterization of Superconducting Resonators

Superconducting microwave resonators are of interest for a wide range of applications, including for their use as microwave kinetic inductance detectors (MKIDs) for the detection of faint astrophysical signatures, as well as for quantum computing applications and materials characterization. In this paper, procedures are presented for the fabrication and characterization of thin-film superconducting microwave resonators. The fabrication methodology allows for the realization of superconducting transmission-line resonators with features on both sides of an atomically smooth single-crystal silicon dielectric. This work describes the procedure for the installation of resonator devices into a cryogenic microwave testbed and for cool-down below the superconducting transition temperature. The set-up of the cryogenic microwave testbed allows one to do careful measurements of the complex microwave transmission of these resonator devices, enabling the extraction of the properties of the superconducting lines and dielectric substrate (e.g., internal quality factors, loss and kinetic inductance fractions), which are important for device design and performance.

Superconducting microwave resonators are of interest for detection of light, quantum computing applications and materials characterization. This work presents a detailed procedure for fabrication and characterization of superconducting microwave resonator scattering parameters.

Superconducting microwave resonators are of interest for a wide range of applications, including for their use as microwave kinetic inductance detectors ...

Stimulated Stokes and Antistokes Raman Scattering in Microspherical Whispering Gallery Mode Resonators

Dielectric microspheres can confine light and sound for a length of time through high quality factor whispering gallery modes (WGM). Glass microspheres can be thought as a store of energy with a huge variety of applications: compact laser sources, highly sensitive biochemical sensors and nonlinear phenomena. A protocol for the fabrication of both the microspheres and coupling system is given. The couplers described here are tapered fibers. Efficient generation of nonlinear phenomena related to third order optical non-linear susceptibility &#935;(3) interactions in triply resonant silica microspheres is presented in this paper. The interactions here reported are: Stimulated Raman Scattering (SRS), and four wave mixing processes comprising Stimulated Anti-stokes Raman Scattering (SARS). A proof of the cavity-enhanced phenomenon is given by the lack of correlation among the pump, signal and idler: a resonant mode has to exist in order to obtain the pair of signal and idler. In the case of hyperparametric oscillations (four wave mixing and stimulated anti-stokes Raman scattering), the modes must fulfill the energy and momentum conservation and, last but not least, have a good spatial overlap.

Efficient generation of nonlinear phenomena related to third order optical non-linear susceptibility &#935;(3) interactions in triply resonant silica microspheres is presented in this paper. The interactions here reported are: Stimulated Raman Scattering (SRS), and four wave mixing processes comprising Stimulated Anti-stokes Raman Scattering (SARS).

Dielectric microspheres can confine light and sound for a length of time through high quality factor whispering gallery modes (WGM). Glass microspheres ...

Recombination Dynamics in Thin-film Photovoltaic Materials via Time-resolved Microwave Conductivity

A method for investigating recombination dynamics of photo-induced charge carriers in thin film semiconductors, specifically in photovoltaic materials such as organo-lead halide perovskites is presented. The perovskite film thickness and absorption coefficient are initially characterized by profilometry and UV-VIS absorption spectroscopy. Calibration of both laser power and cavity sensitivity is described in detail. A protocol for performing Flash-photolysis Time Resolved Microwave Conductivity (TRMC) experiments, a non-contact method of determining the conductivity of a material, is presented. A process for identifying the real and imaginary components of the complex conductivity by performing TRMC as a function of microwave frequency is given. Charge carrier dynamics are determined under different excitation regimes (including both power and wavelength). Techniques for distinguishing between direct and trap-mediated decay processes are presented and discussed. Results are modelled and interpreted with reference to a general kinetic model of photoinduced charge carriers in a semiconductor. The techniques described are applicable to a wide range of optoelectronic materials, including organic and inorganic photovoltaic materials, nanoparticles, and conducting/semiconducting thin films.

A Time Resolved Microwave Conductivity technique for investigating direct and trap-mediated recombination dynamics and determining carrier mobilities of thin film semiconductors is presented here.

A method for investigating recombination dynamics of photo-induced charge carriers in thin film semiconductors, specifically in photovoltaic materials ...

Fabrication of Polymer Microspheres for Optical Resonator and Laser Applications

This paper describes three methods of preparing fluorescent microspheres comprising &#960;-conjugated or non-conjugated polymers: vapor diffusion, interface precipitation, and mini-emulsion. In all methods, well-defined, micrometer-sized spheres are obtained from a self-assembling process in solution. The vapor diffusion method can result in spheres with the highest sphericity and surface smoothness, yet the types of the polymers able to form these spheres are limited. On the other hand, in the mini-emulsion method, microspheres can be made from various types of polymers, even from highly crystalline polymers with coplanar, &#960;-conjugated backbones. The photoluminescent (PL) properties from single isolated microspheres are unusual: the PL is confined inside the spheres, propagates at the circumference of the spheres via the total internal reflection at the polymer/air interface, and self-interferes to show sharp and periodic resonant PL lines. These resonating modes are so-called "whispering gallery modes" (WGMs). This work demonstrates how to measure WGM PL from single isolated spheres using the micro-photoluminescence (&#181;-PL) technique. In this technique, a focused laser beam irradiates a single microsphere, and the luminescence is detected by a spectrometer. A micromanipulation technique is then used to connect the microspheres one by one and to demonstrate the intersphere PL propagation and color conversion from coupled microspheres upon excitation at the perimeter of one sphere and detection of PL from the other microsphere. These techniques, &#181;-PL and micromanipulation, are useful for experiments on micro-optic application using polymer materials.

Protocols for the synthesis of microspheres from polymers, the manipulation of microspheres, and micro-photoluminescence measurements are presented.