This review article explains the basic theory and working of the Whispering Gallery Mode (WGM) principle and types of WGM resonators. The various fabrication methods of torodial shape microdisk resonator with comparative pros and cons and area of application of the same are also described. Finally, fabrication with focused ion beam is recommended the best method with valid reasons.
Optical resonators can imprison and storage light energy. They include travelling wave cavities based on total internal reflection (TIR), Febry Perot Interferometer formed by two parallel mirrors, and Bragg cavities based on distributed Bragg reflection (DBR). Optical Micro resonators based on TIR has sensitivity to surrounding environment, can realize small mode volumes and internal field intensity which leads to strong coupling between atoms and optical field, so they have basic applications in optoelectronics technologies such as cavity quantum electrodynamics (QED) experiments, temperature and pressure sensing, bio-chemical sensing, add/drop multiplexers, spatial switching and quantum information processing.
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Dielectric microdisk resonators offers a great advantage in obtaining simulated emission in small volume since the works on Whispering gallery modes (WGM), which are very confined resonances with maximum intensity near the disk edge. The performance of WGM resonator is mainly depends on the surface roughness of walls of disk. Microdisk with low surface roughness walls will produce less scattering losses. The surface finish of the wall depends on the fabrication method of it. In this article various method of fabrication are described and ultimately found that focused ion beam method followed by post stress relieving process is the most suitable.
Theory of WGM:-
This principle is highly used in the field of optics. Whispering-gallery mode resonances in optics correspond to trapping of light in circular orbits like sphere or disk. Â When light wave is supplied to circular path at particular interface some of it will be reflected and some will be transmitted. However, in the case of glass sphere the transmission of light is very less which could easily negligible, which causes total internal reflection in circular orbit. The light wave travels along zigzag paths around the orbit as shown in below figure. Since the light will make many millions of circulations of the sphere before being transferred to other end. This means that only certain wavelengths of light can 'fit' around the edge of the sphere. The discrete frequency wavelengths depend on the index of refraction and radius of sphere.
Figure:- Working principle of Whispering Gallery Mode. 
The best practical example of whispering gallery mode is the dome of St Paul's Cathedral, London, it has the curious property that if two people stand at opposite sides of the gallery, at a distance of 42 meters, and one whispers into the wall of the dome, then the other person can hear what is being said. The reason for this strange effect is that the sound bounces along the wall of the gallery with very little loss, and so can be heard at a great distance. And it does not work if you speak at increased amplitude of noise because it circulates in gallery multiple times and mixed up and not heard properly. On base of this gallery in St Paul's Cathedral this phenomena is known as a "Whispering Gallery Mode".
The circular orbit for wave reflection could be obtain by different geometries in WGM resonators, e.g. Sphere, Cylinder, Ring on disk, Toroid
Figure:- WGM resonator geometries.
Among all these shapes the high Q factor can be observed in the sphere shape because of the exceptional smoothness (low scattering loss) of the dielectric boundary that forms the whispering gallery. However, such micro-resonators are not easily fabricated in large numbers, nor can they be integrated with other optical elements or electronics. These problems can be eliminated by a recently developed wafer-based toroidal shape resonator capable of ultra-high-Q performance.
The working of WGM resonator is described by a light wave that circumnavigates near the surface of microdisk and returns back upon itself in phase. It is done by evanescent coupling between microdisk and light guide like optical fiber or wave guide, which is driven in turn by a laser at wavelength Î». Resonance is achieved by tuning the laser to a specific wavelength Î»r for which the microsphere circumference is an integer multiple of orbital wavelengths. This closure relation forces the resonance wavelength to change for the slightest change in microsphere radius, R, or orbital refractive index of the sphere, n, according to âˆ†Î»r /Î»r = âˆ†R/R + âˆ†n/n. This relationship provides the sensing principle of temperature, pressure or bio sensing application. The tapered fiber not only serves to drive the resonance but also leads to a means for detecting the wavelength shift. As the laser wavelength is tuned through Î»r, power is extracted from the fiber, causing a dip in the light transmitted to the detector as shown in below figure.
Figure:- Phase shift in WGM for sensing application 
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In above figure solid line shows dip in intensity of light at normal Î»r wavelength, and dotted line shows change in dip wavelength, which is shown by âˆ†Î»r in figure. By calculating the value of âˆ†Î»r the sensing could be measured.
The performance of the WGM resonators depends on the method of fabrication. To obtain a maximum efficiency of the resonator it should has good circularity, smooth boundary, vertical side walls and low roughness of the walls of the disk to reduce optical scattering losses and low surface recombination velocity to increase internal quantum efficiency.The method of fabrication varies according to the shape of the resonators. This review concentrates specifically on Toroid Shape microdisk resonator. Following are the various methods fabrication of it.
Photolithography and Dry etching
This is a traditional method of fabrication on WGM resonators. It is most commonly used method worldwide because of easiness of fabrication. The fabrication is done in two stages; first silicon wafers coated with certain thickness of SiO2 is painted with photoresistant (PR) after that mask pattern is transferred to PR over silica with standard photolithography; and in second step PR pattern is transferred to silica layer with Reactive ion beam etching (RIE); silicon under the silicadisk get etched with isotropic silicon etching to form mushroom-like toroid shape micro resonators.
The processes of fabrication experiments were as follows:
AZ1400 Photoresist is painted on structure.
Photolithograph including exposal and developing.
Etching of SiO2 layer with reactive ion beam etching (RIE) with Ar + CHF3 gas mixture to transfer exposed PR figure to SiO2 layer. The ratio of Ar/ CHF3 mixture is 1:2 and etching is done at 3 x 10-2 Pa pressure with 40 mA ion beam , 500 eV ion energy, 280- V acceleration range and 48- mA neutralization current with etch rate of 25 nm/ min.
Etching of Si under SiO2 microdisk. This is done by forming silicon micropillars. This will form a disk like structure. Optical microresonators fabricated with this etching method could be easily controlled with high precision and little pollution.
Removal of residual PR.
Figure:- SEM photograph of a 60-Î¼m-diameter circle microdisk resonator. 
WGM resonator fabricated by this method has a achieve geometry control and high Q factor. But it has major drawback that the side walls of resonator has very low surface finish. The reason for poor surface quality is dry etching by Reactive ion etching (RIE).
Photolithography and Wet etching
The steps of fabrication process for this method is almost same as first method except the difference in method of etching used to selectively etch substrate layer. In this method two step of chemical wet etching is used instead of dry etching. In the first etching is done isotropic way to form an undercut beneath the photoresist mask. And in the second method the undercut is selectively etched forming a pedestal underneath the microdisk. Following are the steps of fabrication;
First the sample is grown on GaAs substrate. The layer structure consist of 1000 nm Al0.68Ga0.32As and 265 nm GaAs.
Photo resist is applied and circular shape is defined by lithography.
Then GaAs layer is etched by isotropic etching to form an undercut beneath the PR mask by using a mixture of HBr:H2O2:H2O with the ratio 4:1:25.
Finally, Al0.68Ga0.32As layer is selectively etched by using dilute hydrofluoric acid (HF) forming a toroid shape underneath the GaAs disk.
Figure:- a)Top-view and b) tilt-view scanning electron microscope images of a GaAs disk on top of an AlGaAs pedestal. The diskdiameter is 627 nm and the disk thickness is 265 nm. 
The advantages of this fabrication method compare to first method are;
less fabrication cost because machinery required for this method is less costly
This method facilitates mass production
It creates less surface damage compare to dry etching method
Along with these advantages it has few major drawbacks which make them doubtable for wide application like; etching time is very long, isotropic etching may over etch AlGaAs layer, and controlling wet etching is very difficult.
Photolithography, Chemical etching and Selective oxidation
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In this method of fabrication the microdisk is obtained by lift- off technique followed by chemical etching and selective oxidation of aluminum- rich pedestal. The steps of fabrication are as follows;
First sample is grown by molecular beam epitaxy on a GaAs substrate with the buffer layer of Al0.95Ga0.05As. This active material consists of three layers which are 240 nm thick layer of GaAs, 20 nm thick Al 0.2 Ga 0.8 As and 20 nm thick GaAs.
After that 300 nm thick layer of Si3N4 is deposited
The fabrication is performed by electron beam lithography, a lift-off of a 10 nm Ni mask followed by a reactive ion etching. Which form circular disk with size varying from 3 to 4.8 Âµm by step of 0.2 Âµm. For lift of technique PMMA is used as a sacrificial layer.
Then the sample is etched by isotropic chemical etching with dichromate based solution. After vertical etching of 1.8 Âµm the diameter of disk is reduced by 1 Âµm.
After that the sample is immediately rinsed with de-ionized water before being introduced into a humid oxidation furnace.
The aluminum-rich pedestal is selectively oxidized at 400 Â°C for 90 min under water vapor carried out by the N2 gas at a flow rate of 1 l /h.
Finally, the Ni/Si3N4 mask is removed with a diluted HNO3 solution followed by a well-diluted Hydrofluoric solution.
Below figure shows various steps of fabrication and a side view of 4Âµm diameter microdisk obtained by scanning electron microscope (SEM).
Figure:- Process description and SEM image. 
Advantages of this process compare to chemically etched microdisks are as follows;
The size of AlOx pedestal is as large as microdisk diameter so it provide a robustness to structure, while in AlGaAs disk, the pedestal has very small lateral size
Thermal heating of resonator is very low because AlOxis better heat conductor than air.
This method can produce very small size microdisk(< 0.5 Âµm diameter ) Which is not possible in case of normal selective etching because of the unrealistic pedestal size (< 50 nm) is required
Focused ion beam machining and Chemical etching
In this method fabrication is done by focused-ion beam (FIB) followed by wet-chemical etching. The main advantage of this method is that it fabricates microdisks with walls of excellent quality and any size. The fabrication procedure steps are as follows;
Firstly epitaxial layers grown on a n-type (001) which consists of layers of Si-doped InP substrate lower cladding layer, undoped 300 nm InGaAsP waveguide layer, followed by a1.9 Âµm thick Zn-doped top p-type cladding layer.
Disk shape is formed by using conventional lift-off technique and electron beam evaporation. And then sample is alloyed in forming gas for 30s at 420 C.
Focused ion beam milling is done by Ga+ ions using FIB to remove the field around the microdisk by obtaining several pillars. The milling is done with an emission current of 20nA and 30 keV for about 5 min.
Polishing milling is done on same machine to obtain pillers with very smooth walls. It is done with an emission current of 1nA and 30 keV for about 6 min.
Finally wet chemical etching is done using H2O and HCL to selectively remove InP material and form disk structure of InGaAs contact layer.
Figure:- Outline of the process. a)Metallization of contacts and liftoff; b) focused ion beam milling of disks; c) smoothing process with FIB resulting in vertical and very smooth walls; d) InP selective etching 
Application of Toroidel Microdisk Resonators:-
The WGM principle has a wide application in fields of photonics, quantum electrodynamics (QED), atom optics and telecommunication. The shape of WGM resonator varies according to the application. The toroidal shape resonator is generally used in temperature and pressure sensing, bio sensing, underwater impurity measurement.
A recent research work done by Vahala. et al:  on toroidal resonator shows that it could be used as Micro mechanical oscillator. This method works on a micro-mechanical oscillation that can occur within these structures and which is driven by radiation pressure. High circulating optical power within the toroidal whispering gallery leads to radiation pressure caused by circulating photons in cavity will induce deformation of the toroidal structure and change the diameter the silica structure. This, in turn, modifies the optical path length for the resonant mode, changing its resonant frequency. This resonant shift either lowers or raises the coupled optical pump power, depending upon the sigh of the detuning of pump frequency relative to the micro cavity resonant frequency. When pump laser is detuned to the high frequency of optical mode, the phase relationship between optical pressure and microdisk deformation results in net power transfer from optical pump to mechanical mode. Interaction of the vibrating resonator with photons inside the cavity results in creation of photons down shifted or up shifted in energy from original photons by the RF frequency of the vibration. This dynamic unfolds result in regenerative oscillation of the toroidal micromechanical structure at radio-frequency rates. These micro-mechanical oscillations, in turn, modulate the optical power of the incident pump wave. This modulation provides a convenient means by which the oscillations can be studied.
All the fabrication method mentioned above has its own advantages and disadvantages. Also microdisk resonator fabricated by each method is used for specific application. For example application like radiation-pressure driven micro-mechanical oscillator where high Q factor is required is fabricated by using either method number  or . And application with low accuracy could be achieved by using method number  or  with average surface finish of the side walls of resonator.
After studying all above described methods with respect to cost of manufacturing, time of fabrication, availability of equipment, easiness of fabrication and final quality of the product, method  fabrication with focused ion beam and wet etching could be the most reliable one. Because of the following advantages:
This technique offers superior flexibility because of its direct write capability, means it allows placement of devices anywhere in the sample.
There is no need for a resist and no dry etching step, and one can easily alter existing devices which is very difficult with resist based methods.
This method could produce structure with less than 50 nm because ion beam is focused in a spot about 10 nm.
The surface finish of side walls obtained by this method is very good which is very important to reduce the scattering losses. Below figure shows a comparison of resonator fabricated with wet chemical etching and with FIB.
Figure:- Left: with wet chemical etching; Right: with FIB polishing 
From the figure it we clearly see that FIB allows a smooth disk with minimized photon scattering.
The only difficulty faced in this method is induced crystal damage by Ga+ ions and as per Barea et al:  it is important to address this issue and further research is required to improve the performance of resonator. To remove this difficulty Schrauwen et al:  has described a various methods like annealing and use of nonvolatile etch layer. In thermal annealing method structure is heated above 800 oC and then cooled to relieve the damage by refining the crystal structure of material. However, such high temperatures are not compatible with low melting point. Therefore an alternative approach of applying nonvolatile etch layer of iodine on silicon. This layer will protect silicon from the impinging ions. After finishing the process it can be easily removed by backing it at 300 C temperature. This yields structures with relatively low optical loss.
In review article various methods of fabrication of dielectric Toroidal Microdisk Resonator are studied. After comparative study of all method it is found that fabrication with Focused Ion Beam (FIB) is best alternative, because of its superior flexibility, extremely good surface finish and it can produce microdisk with any size. So, this technique is proven to be very efficient and reliable for fabrication of dielectric Toroidal Microdisk Resonator.