Method for Tuning One Or More Resonator(S)

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(11) EP 3 067 723 A1

(12) EUROPEAN PATENT APPLICATION

(43) Date of publication: (51) Int Cl.: 14.09.2016 Bulletin 2016/37 G02B 6/12 (2006.01) G02B 6/136 (2006.01) G02B 6/293 (2006.01) (21) Application number: 15290070.0

(22) Date of filing: 13.03.2015

(84) Designated Contracting States: (72) Inventors: AL AT BE BG CH CY CZ DE DK EE ES FI FR GB • Favero, Ivan GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO 75012 Paris (FR) PL PT RO RS SE SI SK SM TR • Baker, Christopher Designated Extension States: 75008 Paris (FR) BA ME • Gil Santos, Eduardo Designated Validation States: 75015 Paris (FR) MA (74) Representative: Regimbeau (71) Applicants: 20, rue de Chazelles • Université Paris Diderot - Paris 7 75847 Paris Cedex 17 (FR) 75013 Paris (FR) • Centre National de la Recherche Scientifique 75016 Paris (FR)

(54) Method for tuning one or more resonator(s)

(57) The invention concerns a method for tuning at length, into the resonator, a targeted resonance wavelength at least one micro so that the injected light resonates within the resonator and/or nanophotonic resonator, and triggers a photo-electrochemical etching process en- the resonator having dimensions defining resonance abled by the surrounding fluid containing ions, said etch- wavelength of said resonator, the resonator being im- ing process being enhanced by the optical resonance mersed in a fluid containing ions so that the resonator is which amplifies light intensity in the photonic resonator, surrounded by said fluid, the etching decreasing dimensions of the photonic res- wherein the method comprises a step of injecting light, onator, hereby lowering and tuning the resonance wave- having a light wavelength equal to the resonance wavelength of the photonic resonator. EP 3 067 723 A1

Printed by Jouve, 75001 PARIS (FR) 1 EP 3 067 723 A1 2

Description [0004] Several techniques have been developed in the past years in order to tune and control the resonance of FIELD OF THE INVENTION the resonators. We present below an exhaustive over- view of the state of art: [0001] The invention relates to the field of micro- and 5 nano-photonic resonators and in particular to the tuning - K. Srinivasan and 0. Painter, "Optical fiber taper cou- of such resonators to a desired wavelength. pling and high resolution wavelength tuning of micro- Optical resonators, which are part of a photonic device, disk resonators at cryogenic temperatures", Applied generally consist in a cavity in which light can enter and physics letters, vol. 90, no. 3, p. 031114, 2007. resonate, hereby amplifying the signal. 10 In this document, nitrogen is deposited on the optical resonator to shift its resonance wavelength. PRIOR ART Such a technique is non-permanent and requires the sample to be in high-vacuum and at cryogenic tem- [0002] Micro- and nanoresonators are small devices, perature. which are difficult to build with a high precision. Industrials 15 have developed clean room fabrication techniques (such - Reversible tuning of photonic crystal cavities using as photolithography and plasma etching) that typically photochromic thin films. D. Sridharan, E. Waks, G. result in a precision no better than a few nanometers. Solomon, and J. T. Fourkas, "Reversible tuning of These reproducibility constraints due to the manufactur- photonic crystal cavities using photochromic thin ing-relatedimprecisions ultimately affect theultimate res- 20 films", Applied Physics Letters, vol. 96, no. 15, p. onance wavelength of the fabricated optical resonators. 153303, 2010. As a result, fabricating several high Q, wavelength-sized In this document, a thin photochromic layer is de- optical resonators sharing exactly the same resonance posited on the resonator. wavelength is not currently achievable, even through This technique has a limited tuning range, the layer state-of-the art fabrication means. 25 can degrade optical and mechanical properties of [0003] Those resonators have several applications, ei- the device, and is non-applicable for a plurality of ther in academic research or in industrial development resonators. for improved technology. Light-matter interaction (lasers, cavity electrodynamics, - M. Shainline, G. Fernandes, Z. Liu, and J. Xu, "Broad optomechanics, non-linear optics, ...) is a major topic of 30 tuning of whispering-gallery modes in silicon micro- current research. Obtaining optical resonators with pre- disks", Optics express, vol. 18, no. 14, pp. cise resonance frequencies, which can couple to given 14345-14352, 2010. atomic transitions or molecular excitations is of interest. In this document, a tension is applied to the optical Moreover, arrays of resonators and their collective be- resonator, thereby changing its refractive index havior are also a subject of interest. 35 (through the electro-optic effect) and thus its reso- Miniaturized optical resonators are currently the focus of nance frequency. Such a technique requires electric important research and development, with applications tuning with metallic electrodes in contact with the in optical delays, optical routing with micro-nanoscale de- resonator, which degrade the optical properties and vices, photonic force and gas sensing, optical gyro- is non-scalable. scopes, Silicon On Insulator optical devices, semicon- 40 ductor lasers, optical computers, optical metamaterials. - K. Piegdon, M. Lexow, G. Grundmeier, H.-S. Kitze- Resonators can be also used as: row, K. Parschke, D. Mergel, D. Reuter, A. Wieck, and C. Meier, "All-optical tenability of microdisk la- - Sensors,as shown indocument US 2014/03211502, sers via photo-adressable polyelectrolyte function- where acquiring the wavelength of the resonator en- 45 alization", Optics express, vol. 20, no. 6, pp. ables to obtain the temperature. In this case, the best 6060-6067, 2012. the resonance wavelength control, the more accu- In this document, the technique is applied to the rate the obtained temperature value, whole sample,therefore showing alack ofselectivity. - Display devices, as show in document EP 1 585 087, The precision is of the order of nanometers and the where light is guided in a plane comprising fluoro- 50 reproducibility has not been achieved. phores which reemit light when excited. As fluoro- phores are highly selective optically, the resonance - W. von Klitzing, R. Long, V. S. Ilchenko, J. Hare, and wavelength must be well known, V. Lefèvre-Seguin, "Frequency tuning of the whis- - Transistors for logical connections in order to pering-gallery modes of silica microspheres for cav- achieve photonic bits process for optical computers. 55 ity quantum electrodynamics and spectroscopy", The development of such computers where informa- Optics letters, vol. 26, no. 3, pp. 166-168, 2001. tion is transmitted through photons instead of elec- In this document, a mechanical action is exerted on trons is accelerating in the silicon industry. the resonator, consisting in compressing and

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stretching a microsphere. - Tune a single optical resonator to a targeted wave- Such a technique is non-scalable and non-compat- length, ible with micro- and nano-photonic devices, is non- - Tune multiple resonators to a common targeted permanent and is technically impractical. wavelength. 5 - N. Niu, T.-L. Liu, I. Aharonovich, K. J. Russell, A. SUMMARY OF THE INVENTION Woolf, T. C. Sadler, H. A. El-Ella, M. J. Kappers, R. A. Oliver, and E. L. Hu, "A full free spectral range [0006] It is an object of the invention to propose an tuning of pin doped gallium nitride microdisk cavity", improved method for tuning a resonator at a targeted Applied Physics Letters, vol. 101, no. 16, p. 161105, 10 resonance wavelength at least one micro and/or nano- 2012. photonic resonator, the resonator having dimensions de- In this document, the whole sample containing res- fining resonance wavelength of said resonator, the resonators is tuned with photoelectrical etching by UV onator being immersed in a fluid containing ions so that light. The UV light is absorbed by the whole sample. the resonator is surrounded by said fluid, wherein the Such a technique lacks spectral precision, is not site- 15 method comprises a step of injecting light, having a light specific, is non-scalable to a large number of reso- wavelength equal to the resonance wavelength, into the nators, and does not allow real time in-situ monitor- resonator, so that the injected light resonates within the ing of the resonance wavelength. resonator and triggers a photo-electrochemical etching process enabled by the surrounding fluid containing ions, - Y. Shen, I. B. Divliansky, D. N. Basov, and 20 S. said etching process being enhanced by the optical res- Mookherjea, "Electric-field-driven nano-oxidation onance which amplifies light intensity in the photonic res- trimming of silicon microrings and interferometers", onator, the etching decreasing dimensions of the phot- Optics letters, vol. 36, no. 14, pp. 2668-2670, 2011. onic resonator, hereby lowering and tuning the reso- In this document, oxidation through peak effect is nance wavelength of the photonic resonator. applied, which changes the resonance wavelength 25 [0007] The method of the invention may comprise the of the resonator through refractive index changes in following features in combination or not. The photonic the resonator material. resonator has a bandgap wavelength set by its constitu- Such a technique is slow, non-scalable to arrays of tive material and the wavelength of the injected light is resonators, andhas not beenproven to be applicable above the resonator bandgap wavelength. to nano-resonators with the same accuracy. 30 The injected light is monochromatic. - M. Zhang, G. S. Wiederhecker, S. Manipatruni, A. The method comprises the steps of: Barnard, P. McEuen and M. Lipson, "Synchroniza- tion of micromechanical oscillators using light", - Starting the step of injecting the light, Physical review letters, vol. 109, no. 23, p. 233906, 35 - Adjusting the light wavelength of the injected 2012. light so as to attain the resonance wavelength In this document, a laser light is used to heat a res- of the resonator, onator to change its resonance wavelength. - Lowering the light wavelength so as to follow the Such a technique is non-permanent, non-scalable resonance wavelength during the etching proc- to large numbers of resonators, and complex, involv- 40 ess until a targeted resonance wavelength is ing several feedback loops. reached, - Stopping the light injection at the targeted res- - P. Dong, W. Qian, H. Liang, R. Shafiiha, D. Feng, onance wavelength. G. Li, J. E. Cunningham, A. V. Krishnamoorthy, and M. Asghari, "Thermally tunable silicon racetrack res- 45 [0008] The tuning procedure consists in lowering the onators with ultralow tuning power," Optics express, resonance wavelength of the photonic resonator by con- vol. 18, no. 19, pp. 20298-20304, 2010. secutive sweeps, each sweep consisting in sweeping the In this document, the resonator is thermo-electrically light wavelength of the injected light between values tuned. which comprise the resonance wavelength of the phot- Such a technique is non-permanent and has only 50 onic resonator. been demonstrated on micro-resonators. [0009] The step of injecting light is realized by far-field or near-field optical coupling technique, and where light [0005] No one of the cited papers disclose an efficient is injected into one or several resonators. technique which would be permanent, scalable, address- [0010] The step of injecting light is implemented by a able, site-specific, and accurate at different orders of55 waveguide which is on a substrate onto which the phot- magnitude. onic resonator is positioned, the waveguide being in the But all the above-mentioned applications require a cost- vicinity of the photonic resonator in order to allow near- effective, precise and simple technique to: field optical coupling between the two, the light wave-

3 5 EP 3 067 723 A1 6 length of the injected light being larger than a material mon targeted resonance wavelength value. bandgap wavelength. [0011] The light wavelength of the injected light is com- [0020] This method is thus scalable to a large number prised amongst visible and infrared spectrum. of photonic devices and still requires a single tuning step [0012] The coupling between the photonic resonator 5 anda shortamount of time (order of the minute), therefore and the waveguide is achieved by evanescent waves. overwhelming the drawbacks of the prior art. [0013] The fluid is a liquid containing ions. [0021] The fact that the tuning step automatically etch- [0014] The method presented here allows for three or- es the photonic devices of largest size, and then auto- ders of magnitude higher precision, both in controlling matically etches all the cavities of identical size at iden- the effective optical device size and in its spectral reso- 10 tical speed, provides a fast tuning mechanism and does nance, reaching picometer precision for these two as- not require the one-by-one identification of the numerous pects. resonators involved in a collective architecture. [0015] The method is applicable to an industrial setting: [0022] Indeed, the spectral misalignment that arises in it is permanent, scalable and fast, requires little equip- arrays of nominally identical photonic resonator due to ment. 15 fabrication variability is solved by the method. [0016] The permanence of the tuning is a major ad- [0023] According to a third aspect, the invention con- vantage for an industrial application, and the method al- cerns a photonic device comprising at least one micro lows for a broad range of wavelength tuning, from picom- and/or nanophotonic resonator (10) on the substrate eters to hundreds of nanometers. (30), obtainable by a method according to the first aspect [0017] Over the prior art, moreover the method does 20 or the second aspect of the invention. not require any additional component on the photonic [0024] The photonic device may be tuned with a phys- device (polymer coating, electrical connects, thermal ical precision of dimensions of the photonic resonator controls, as explained before), making it simple and non- less than 10pm. degrading. [0025] The photonic device may be tuned at the same [0018] The surrounding fluid can easily be removed 25 value with a physical precision of dimensions of the pho- after the tuning step without damaging the characteristics tonic resonators less than 10pm. of the resonator. [0026] The photonic device may comprise a [0019] According to a second aspect, the method of waveguide which enables transmission of the light to the invention allows tuning several resonators, each one each photonic resonator, by being on the substrate in the having a different resonance wavelength. In that case, 30 vicinity of the at least one resonator. this method comprises the following steps: [0027] Each photonic resonator is a disk, a photonic crystal resonator, a ring or racetrack resonator, a Bragg - Setting a light wavelength of the injected light to a resonator, a distributed feedback grating resonator, a value larger than the resonance wavelength of either Fabry-Perot resonator or a plasmonic resonator. resonator, 35 [0028] The photonic resonator, the substrate and the - Lowering light wavelength of the injected light so as waveguide are made of GaAs, Silicon, and/or any other to attain a first resonance wavelength of a first pho- semi-conductor and dielectric material where photoelec- tonic resonator and start the etching process, while trochemical etching operates. a second photonic resonator having a second resonance wavelength inferior to the first resonance40 DRAWINGS wavelength remains intact, - Lowering light wavelength of the injected light so as [0029] Other features, embodiments, aim and advan- to follow the first resonance wavelength, which de- tages of the invention will be pointed out in the following creases until the first resonance wavelength of the specification, which is illustrative and non-limitative, that first photonic resonator that is being etched overlaps 45 shall be read in view of the drawing, on which: with theresonance of thesecond photonic resonator, the photonic resonators being hereby tuned. - Figure 1 shows a disk resonator to which the method The method according to the second aspect may of the invention can be applied, comprise: - Figure 2 shows a finite element simulation showing 50 the distribution of the electric field inside the disk, - the step of lowering light wavelength so as to - Figure 3 shows a spectrum of the resonance doublet etch both photonic resonators at the same time of the disk resonator of figure 1, and the same rate, such that both photonic res- - Figure 4 shows a set-up adapted for a method ac- onators can be tuned at a common targeted res- cording to the invention, onance wavelength value and have the same 55 - Figure 5 shows a plurality of resonators fed by a plu- dimensions and optical resonance wavelength; rality of waveguides, - the repetition of the step of lowering light wave- - Figure 6 shows a droplet covering the plurality of length, so as to tune all the resonators at a com- resonators and waveguides. In this figure, each

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waveguide is coupled to three distinct resonators, toa type ofwave that can travelaround a concave surface - Figure 7 shows the step of a method according to (see figure 2). As shown in figure 3, the resonance as- the invention, sociated to WGMs appear as dips in an optical spectrum. - Figure 8 shows the step of another method according Such disk resonator 10 typically presents an optical res- to the invention, 5 onance doublet. For clarity reasons in the description, - Figure 9a, 9b, 10a, 10b, 11a, 11b illustrate different this doublet will sometimes be considered as a single steps of a method according to the invention. The resonance value λr. curves represent the optical transmission of the sys- One major criterion for a resonator is its quality factor Q, tem formed by a waveguide coupled to two resona- which can be defined as: tors, as a function of the wavelength, 10 - Figure 12 shows the spectrum for three resonators tuned with a method according to the invention, - Figure 13 shows the evolution of the resonance wavelength of the three resonators of figure 12, at different steps of the method. 15 [0035] Where λr is a resonance wavelength of the resonator and Δλ is the mode resonance full width at half DETAILED DESCRIPTION maximum (FWHM). The higher the Q, the narrower the dip in the spectrum. [0030] In order to give a detailed view of the method, the specification will be based on a special type of reso- 20 Installation set-up nator. This shall not be limitative, in so far as the method can be applied to any optical resonator. By resonator, it [0036] Light at a wavelength λ is injected into the res- is meant any kind, or any shape, or any material that onator through the waveguide 20 by a laser source 40. would enable a photonic resonance and the confinement Wavelength λ is variable and the laser source 40 delivers of light in a given volume. 25 preferably monochromatic light. [0037] The optical spectroscopy of the resonator 10 The photonic device and waveguide 20 is performed in the present case in the 1260-1350 nanometers wavelength range (infrared) [0031] The nano- or micro-photonic resonant cavities, using the laser source 40 which is in the present case a or photonic resonators 10, hereby referred as the reso- 30 continuous wave external cavity diode laser. The beam nator 10, employed here for proof of principle experi- of laser light is focused onto the coupling waveguide input ments consist of disks 11 (see figure 1), made from a facet 22 using a micro-lensed fiber 41. Output light at the semi-conductor such as Gallium Arsenide (GaAs). A disk waveguide output facet 23 is collected by a microscope is typically micrometer-sized in lateral dimensions and objective 42 and sent on a photodetector 43 (see path few hundreds of nanometers thick. The resonator 10 is 35 on figure 4). coupled to an integrated optical waveguide 20, whose For example, as shown in figure 3, when scanning the central region 21 is tapered in the vicinity of the resonator laser wavelength, the several dips appearing in the trans- 10 to allow optical coupling. Typically, and not only in the mission spectrum correspond to the WGMs of the disk disk case, said coupling is achieved by evanescent 11 of the resonator 10. Linewidths are about 20 to 50 pm. waves. The waveguide 20 enables transmission of the 40 The optical quality factor Q is about 70 000 to 25000. light from an external light source to the resonator 10. [0038] On figure 5 is represented a plurality of reso- For instance, the distance between the waveguide 20 nators 10 and waveguides 20 on the sample substrate and the disk 11 is between 100 to 800 nm (hundred to 30. There is only one resonator 10 per waveguide 20. eight hundred nanometers). On figure 6 is represented a collective architecture of [0032] An Aluminium Gallium Arsenide pedestal 12 45 resonators 10 and waveguides 20 on the sample sub- isolates the disk 11 from a sample substrate 30 onto strate. As it can be observed, three resonators 10a, 10b, which the waveguide 20 is positioned. 10c are located in the tapered central region 21 of each Resonators 10, waveguides 20 and the substrate 30 will waveguide 20, said three resonators 10 being thus opti- be referred as the photonic device. cally fed by the same waveguide 20. [0033] The photonic resonator 10 has an energy band- 50 [0039] Fora typical optical tuning application, each res- gap set by its constitutive material. A bandgap wave- onator 10 has its resonance wavelengths λr, which are length lambda_B is associated to said bandgap set. Light to be tuned at a targeted resonance wavelength λt. Each whose energy is below the energy bandgap is generally resonance wavelength λr depends on the dimension of not absorbed. In the present case of GaAs, the bandgap the resonator 10. In the present case, we remind that wavelength lambda_B is around 870 nm, and GaAs be- 55 each resonator 10 has in fact two resonance wavelength comes transparent to wavelengths above that. λr corresponding to the doublet, but as the two values [0034] The GaAs disk 11 supports high quality optical are bound, we can refer to a single one. Whispering Gallery Modes (WGMs), which correspond The method for tuning resonator(s) relies upon photo-

5 9 EP 3 067 723 A1 10 electrochemical (PEC) etching. PEC is a light activated - Absorption of light generates electron and holes, chemical etching. - GaAs + 6h+ → Ga3+ + As3+, To enable such an etching, the resonator 10 has to be - The generated ions are dissolved in the surrounding immersed in a fluid 60 containing ions (see figure 6). fluid 60, terminating the etching. 5 Method for tuning an optical resonator (figure 7) [0045] In standard PEC technique, UV light, that is to say light with a wavelength below 400nm, is often used [0040] First of all, the photonic device, in particular the because it is absorbed by the semi-conductor thanks to resonators 10a, 10b are immersed in the fluid 60 con- its energy above the energy bandgap of the resonator. taining ions. A preliminary step E0 is thus to deposit a 10 In some cases, visible light with energy above the band- droplet of said fluid 60 onto the resonator 10, or more gap can be used as well. The optical absorption, coupled generally to immerse resonators in the said fluid. The to the presence of ions in the fluid 60, leads to the etching resonators 10 are thus surrounded by said fluid 60. of the semiconductor material. [0041] The method comprises a step of injecting light [0046] In the preferred embodiment described here, into the resonator 10 at the resonance, that is to say the 15 light whose wavelength λ is above the wavelength band- wavelength λ of the injected light is equal or sensibly gap lambda B (i.e. light whose energy is below the energy equal to the resonator wavelength λr, such that the in- bandgap) is used. Infrared light is preferably used (light jected light λ resonates within the resonator 10 and trig- wavelength above 800 nanometers), but visible light gers a photoelectrochemical etching (PEC) process. (light wavelength between 400 and 800 nanometers) can As it will be explained below, the resonance amplifies 20 be used as well, depending on the characteristics of the light intensity in the resonator 10 and in particular in the materialof the resonator(s) to be etched and tuned. Semi- disk 11, hereby enhancing the etching process. conductor resonator 10, in the present case GaAs disk The presence of surrounding fluid 60 containing ions en- 11, absorbs a residual fraction of the confined infrared ables such process. light even at energy below the energy bandgap due to Thus, as the disk is etched, its dimensions decrease, 25 various mechanisms. hereby lowering the resonance wavelength λr of the res- Among those mechanisms are mid-gap levels present at onator 10. the semi-conductor surface, as well as two-photon ab- When no more light is injected, the etching process stops. sorption at higher light intensity. [0042] The previous step is part of the following ones: Therefore, by using light wavelength above the bandgap 30 wavelength, light can be injected and highly intensity- - Starting the step E1 of injecting the light, amplified within the resonator 10 thanks to the reso- - Adjusting E2 the light wavelength λ of the injected nance. The circulating power in the disk is enhanced over light so as to attain the resonance wavelength λr of the power circulating in the waveguide by a factor pro- the resonator 10, portional to the resonator finesse, which in these devices - Lowering E3 the light wavelength λ so as to follow 35 is of the order of 1000. The resonance increases the in- the resonance wavelength λr during the etching tensity of the injected light at the level of the resonator process until a targeted resonance λt wavelength is 10 and enables the etching process described above, reached, eventhough the injected lightwavelength λhas itsenergy - Stopping E4 the light injection at the targeted reso- below the energy bandgap of the material. Indeed, the nance wavelength λt. 40 etching process is enabled thanks to the resonance of the resonator 10. Preferred embodiment of the method This directly leads to a high selectivity in etching only the resonator 10, where the light intensity is enhanced at [0043] In a preferred embodiment, the wavelengthλ most, over the surroundings (waveguide 20, substrate of the injected light is above the resonator bandgap wave- 45 30). Besides, the technique provides both an extremely length lambda_B. This means that the light energy is be- high control in the etch speed and the possibility to mon- low the bandgap energy of the material. itor in real time with high spatial and spectral resolution. [0044] Indeed, standard PEC etching technique consists in illuminating a semiconductor immersed in an elec- Method for tuning several resonators at a targeted value trically conductive liquid, such as the fluid 60 containing 50 (figure 8) ions, with light whose energy is above the bandgap. This generates electrons and holes within the material and [0047] A second aspect of the method is to tune several these photogenerated free carriers lead to the formation resonators 10a, 10b, 10c at a common targeted wave- of ionic species at the surface of the semiconductor that length λt. The resonators 10a, 10b, 10c are typically cou- are dissolved in the presence of ions provided by the fluid 55 pled to a waveguide 20 (see figure 6 for instance) 60. Figures 9a to 11b show the principle of the method ap- In GaAs this process is described by the following chem- plied to a plurality of resonators. After micro-nanofabri- ical reaction: cation, the photonic resonators 10a, 10b have different

6 11 EP 3 067 723 A1 12 size due to fabrication variability (seefigure 9a), and largest resonator first, last or in the middle) is not impor- therefore express different resonance wavelengthλ ra tant, thanks to the wavelength selective nature of the and λrb (see figure 9b), one being higher than the other tuning. one (for instance in this example resonator 10b is initially [0056] Figures 12 shows the spectrum of resonance slightly larger than resonator 10a, hence λrb> λra). 5 of resonators 10a, 10b, 10c being tuned. Each resonator [0048] This method is based on the first method pre- 10 is associated to a resonant doublet. For clarity reason, viously described, that is to say a photoelectrical etching we will refer to a single value of resonance λra, λrb, λrc processtriggered byoptical resonance and aprogressive for each resonator 10. decrease of the injected light wavelength λ so as to shift As it can be observed in curve 1 of figure 12, the three the resonance wavelength λr to the targeted wavelength 10 resonators 10 have distinct resonance wavelengths λra λt. < λrb < λrc. Thanks to the etching, those wavelengths [0049] The photonic device, in particular the resona- have been grouped (see the evolution in curves 2 to 9) tors 10a and 10b, are immersed in the fluid 60 containing λra = λrb = λrc. ions (see figure 10a). The same preliminary step E0 (de- [0057] Figure 13 shows the convergence of said res- posit a droplet of said fluid onto the resonator 10, or more 15 onant doublet wavelength towards a unique value, which generally immersing in the ionic fluid) is achieved. is the targeted wavelength λt (again, the two close curves [0050] The next step consists in setting (E1’) a light represent the resonant doublet). wavelength λ of the injected light to a value larger than the resonance wavelengths λra, λrb of the resonators 10. Different embodiments for varving the laser wavelength In a similar way as before, the laser source 40 is coupled 20 in order to tune optical resonators to the waveguide 20. [0051] Then, the light wavelength λ is lowered (E2’) so [0058] Different techniques have been employed to as to reach the first resonance wavelengthλ rb of the vary the laser source wavelength λ in order to control the photonic resonator 10b. The etching process starts on tuning method. A first one is using discrete sweeps, and this latter resonator. Due to the site-specific character of 25 a second one is using a continuous shift. the method, the other resonator 10a, whose resonance [0059] Thefirst technique fortuning at a targeted wave- wavelength λra is lower, is not etched and remains intact. length λt with successive sweeps of laser λ can be the [0052] Indeed, the injected light naturally enters first following one: the resonance of resonator 10b and shifts its resonance wavelength λrb towards lower values (see arrow in figure 30 - Lowering the resonance wavelength λr of the phot- 10b). onic resonator 10 by consecutive sweeps, each [0053] The next step consists in lowering (E31’) the sweep consisting in sweeping the injected laser light wavelength λ of the injected light, so as to follow reso- wavelength λ over a range that contains the resonance wavelength λrb of the resonator 10b, which is de- nance wavelength λr of the photonic resonator 10, creasing because of the etching. The wavelengthλ is 35 thereby at each sweep injecting short bursts of light lowered until the resonance wavelength λrb of the reso- (with a fixed amount of energy depending on the nator 10b reaches the resonance wavelength λra of the sweep speed) into the resonator 10. other resonator 10a (see figures 11a, 11b). Once it is achieved, both resonators 10a, 10b share a [0060] Instead of using discrete successive sweeps, common resonance wavelength and the same dimen- 40 the second technique for tuning at a targeted wavelength sions, meaning that they are tuned. λt can use a continuous shift of the laser source wave- [0054] The process can be completed by lowering length λ.In this respect, a lasersource 40 must be chosen (E32’) again the wavelength λ of the injected light, which whose wavelength is continuously adjustable. triggers the etching of both photonic resonators 10a, 10b The process consists in a unique and slow sweep. at the same time and the same speed. It is therefore45 A control unit 50 can be connected to the laser source possible to reach the targeted resonance wavelength λt 40 so as to control the wavelength of the laser source 40 for both resonators 10a, 10b. in order to precisely follow the resonance wavelength λr. Indeed, when the two resonances overlap, any further [0061] The adjustment of the light wavelength typically decrease of the injected light wavelengthλ simultane- consists in lowering the wavelengthλ , as long as the ously etches both resonators 10a, 10b and maintains 50 starting wavelength has been chosen above the highest them with exactly the same effective dimensions and op- resonance wavelength λr of the resonators 10. tical resonance values. [0062] Those two techniques apply for tuning one res- [0055] This method can be applied for tuning any onator 10 as well as for tuning several resonators 10a, number of resonators 10 coupled to a waveguide 20, as 10b, 10c, or more. long as the value of the wavelength λ of the injected light 55 is above the highest resonance value λr of said resonators. Note that the order in which the resonators 10 are positioned along the waveguide (for example with the

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Parameters and experimental data Sweeping rate

Fluid [0070] In an experiment, the injected light has been injected discontinuously, by sweeping the laser source [0063] The fluid 60 is typically deposited as a microliter 5 40 several times back and forth across one optical res- droplet of water on the sample substrate with a micro- onance of the resonator 10. This way, at each sweep, a pipette. This droplet covers several adjacent resonator small and reproducible amount of energy was injected and waveguides (see figure 6, 10a). Besides, such a inside the resonator 10 (nano Joule range). fluid 60 leaves the overall optical transmission through In figure 14 the WGM resonance wavelength λr has been waveguides unaltered in average power. 10 represented as a function of the sweep count. The power [0064] The fluid 60 can be, as non-limitative example, of the laser source 40 was low, at 0,2 mW at the display a liquid containing a few ions, such as deionized water of the laser source controller. or ammoniac, or a gas containing ions, such as steam. It has been estimated that a rate of 7 pm per round-trip In order to be efficient, the fluid 60 must have a non-zero sweep, i.e. 3.5 pm per single sweep has been achieved. conductivity, that is to say an ability to conduct an electric 15 This corresponds to the removal of the order of 10 -2 mon- current. As explained below, the value of said conduc- olayerof materialper sweep(for instance, one monolayer tivity is not of a major importance. of GaAs is approximatively 300 pm). The error bar rep- [0065] Several experiments (not reported here) have resents the 20 pm relative wavelength accuracy of the been run with higher conductivity liquids to non-conduc- employed laser source 40 over repeated wavelength tive ones. By reducing by orders of magnitude the con- 20 sweeps. ductivity ofthe liquid 60, the etching processis completely suppressed due to the absence of ions. Conversely the Etching and laser source power etching speed is not highly modified if the concentration of ions is increased. Those experiments suggest that the [0071] In figure 15, the wavelength shift per sweep etching rate is limited by the electrons and holes gener- 25 has been represented as a function of the laser power. ation in the disk 11, and not by the dissolution of the ionic A linear dependence can be observed; therefore the compounds in the liquid 60. etching speed can be precisely controlled by controlling [0066] The fluid 60, especially in the case of a liquid, the power of the laser source 40. can be easily removed after the tuning procedure by Several orders of magnitude in the etching speed are blowing air or nitrogen, without any damage to the pho- 30 possible. tonic device. Characteristics of the tuned optical resonator Material and resonator geometry [0072] When tuning a resonator, it is crucial to check [0067] The resonant PEC tuning of optical resonators 35 that no degradation occurs, especially when a significant is not restricted to GaAs resonators shown above as ex- amount of material is removed. ample. Indeed the PEC process operates on all semi- The spectrums (measured in air) of a resonator 10 in conductor materials in the standard configuration of light figures 16 (before tuning) and 17 (after tuning) show an wavelength λ below the material bandgap wavelength optical resonance doublet corresponding to the fine lambda B. Hence the technique presented here of PEC- 40 structure of each resonator 10. This is partly due to the assisted optical tuning applies to optical resonators fab- fact that the disk is not perfectly round-shaped. Between ricated out of any semiconductor material, with the spe- spectrum of figure 16 and the one of figure 17, a shift cificity of now having the laser light wavelength λ larger in the resonance wavelengths λr is observed (around 4 than the bandgap wavelength lambda B. nm), corresponding to the result of the etching process. [0068] The technique extends as well to dielectric op- 45 But the quality factors of the doublet of resonator 10 be- tical resonators, even if PEC processes are less trivial to fore the tuning are around 42 000 / 42 000 ( λr ≈ 1334 nm use in such materials, because of constraints due to the and Δλ ≈ 32 pm for both dips of the doublet) and the high bandgap energy of these materials. quality factors afterwards are around 53 000 / 70 000 ( λr [0069] The technique also operates on different ge- ≈ 1330 nm and Δλ ≈ 25 pm and 19 pm for the dips of the ometries of optical resonators, and not only on disk res- 50 doublet). As a result, it turns out that the method does onators sustaining WGMs like discussed above. Of tech- not damage the properties of the resonators 10, and nological relevance are photonic crystal micro and nano- would even improve their optical quality factor Q. resonators, race-track and ring optical resonators, plas- [0073] It is important to note that after the tuning pro- monic resonators, Fabry-Perot resonators, Bragg mirror cedure, the resonators are permanently tuned, even resonators and distributed feedback grating resonators. 55 when the tuning laser is switched off and the fluid re- Anymicro or nano-optical resonator providingoptical res- moved. onance, with the associated light confinement and am- The photonic devices, especially the resonators 10, are plification, is suited for the technique. thus permanently tuned with a physical precision on the

8 15 EP 3 067 723 A1 16 dimensions which is less than 10 pm, more precisely less wherein the method comprises a step of injecting than 4 pm. This means that, for each resonator, an ab- light, having a light wavelength equal to the reso- solute precision at less than 10pm can be reached. For nance wavelength (λr), into the resonator (10), several resonators, a relative precision of few tens of pm so that the injected light ( λ) resonates within the res- is demonstrated and less than 10 pm can be reached. 5 onator (10) and triggers a photo-electrochemical [0074] The same precision is also found for the value etching process enabled by the surrounding fluid of the optical resonance wavelength. These tuning pre- (60) containing ions, said etching process being en- cisions are given here for the specific GaAs disk resona- hanced by the optical resonance which amplifies tors presented above, but finer spectral precision can be light intensity in the photonic resonator (10), reached by resonators of larger dimensions or by em- 10 the etching decreasing dimensions of the photonic ployinga more stablelaser source for controlling the etch- resonator, hereby lowering and tuning the reso- ing. nance wavelength λ (r) of the photonic resonator [0075] The methods presented here allows for three (10). orders of magnitude higher precision over prior art, both in controlling the effective optical device size and in its 15 2. Method according to claim 1, wherein the photonic spectral optical resonance, reaching picometer precision resonator (10) has a bandgap wavelength (lambda for these two aspects. B) set by its constitutive material and the wavelength [0076] As the etching is triggered when the optical res- (λ) of the injected light is above the resonator band- onance is reached, the methods provide a high spatial gap wavelength (lambda B) selectivity: the resonator 10 is selectively etched while 20 its nearby environment, such as the waveguide 20, the 3. Method according to any one of claims 1 to 2, where- substrate 30, or other resonator of different resonance in the injected light is monochromatic. wavelength remain intact. [0077] By adjusting the etching rate (with laser source 4. Method according to any one of the preceding intensity for example) and spectrally monitoring the etch 25 claims, comprising the steps of: progress, a high quality factor resonator 10 can be tuned to resonance with a high precision. Similarly, several res- - Starting the step (E1) of injecting the light, onatorsof high qualityfactor Qcan be tuned to resonance - Adjusting (E2) the light wavelength (λ) of the as well. Thanks to the scalability of the method, hundreds injected light so as to attain the resonance wave- of resonators can be rapidly tuned using a single sweep 30 length (λr) of the resonator (10), of laser wavelength λ. - Lowering (E3) the light wavelength λ( ) so as Indeed, as described above, there is no need to identify to follow the resonance wavelength (λr) during and to act individually on each resonator; instead, the the etching process until a targeted resonance method automatically brings optical resonances at a (λt) wavelength is reached, common targeted wavelength λt, with a precision less 35 - Stopping the light injection at the targeted res- that 10pm. onance wavelength (λt). [0078] It is therefore now possible to manufacture net- works or arrays of high quality factor optical resonators, 5. Method according to claim 4, wherein the tuning pro- which are resonantly coupled. cedure consists in lowering (E2) the resonance [0079] Amongst the resonators 10 that can be tuned 40 wavelength (λr) of the photonic resonator (10) by with those methods, one can cite photonic crystal reso- consecutive sweeps, each sweep consisting in nators, Bragg resonators or plasmonic resonators. sweeping the light wavelength ( λ) of the injected light The resonator 10, the waveguide 20 and the substrate between values which comprise the resonance 30 can be also made of Silicon, or any other semi-con- wavelength (λr) of the photonic resonator (10). ductor or dielectric material with a bandgap where pho- 45 toelectrochemical (PEC) etching operates. 6. Method according to any of claims 1 to 5, wherein the step of injecting light is realized by far-field or near-field optical coupling technique, and where light Claims is injected into one or several resonators. 50 1. Method for tuning at a targeted resonance wave- 7. Method according to any of claims 1 to 6, wherein length (λt) at least one micro and/or nanophotonic the step of injecting light is implemented by a resonator (10), waveguide (20) which is on a substrate (30) onto the resonator having dimensions defining resonance which the photonic resonator (10) is positioned, the wavelength (λr) of said resonator (10), 55 waveguide (20) being in the vicinity of the photonic the resonator (10) being immersed in a fluid (60) con- resonator (10) in order to allow near-field optical cou- taining ions so that the resonator is surrounded by pling between the two (10, 20), the light wavelength said fluid, (λ) of the injected light being larger than a material

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bandgap wavelength (lambda B). 15. Photonic device with several photonic resonators (10) according to any of claims 13 to 14, tuned at the 8. Method according to any one of the preceding same value with a physical precision of dimensions claims, wherein the light wavelength ( λ) of the inject- of the photonic resonators less than 10pm. ed light is comprised amongst visible and infrared 5 spectrum. 16. Photonic device according to any of claims 13 to 15, comprising a waveguide (20) which enables trans- 9. Method according to any one of the preceding claims mission of the light to each photonic resonator (10), 7, 8, wherein the coupling between the photonic res- by being on the substrate (30) in the vicinity of the onator (10) and the waveguide (20) is achieved by 10 at least one resonator (10). evanescent waves. 17. Photonic device according to any of claims 13 to 16, 10. Method according to any one of the preceding wherein each photonic resonator (10) is a disk (11), claims, wherein the fluid (60) is a liquid containing a photonic crystal resonator, a ring or racetrack res- ions. 15 onator, a Bragg resonator, a distributed feedback grating resonator, a Fabry-Perot resonator or a plas- 11. Method according to any one of claims 1 to 10, for monic resonator. tuning two photonic resonators (10a, 10b), each one having a different resonance wavelength (λra, λrb), 18. Photonic device according to any of claims 13 to 17, the method comprising the following steps: 20 wherein the photonic resonator (10), the substrate (30) and the waveguide (20) are made of GaAs, Sil- - Setting (E1’) a light wavelength λ( ) of the in- icon, and/or any other semi-conductor and dielectric jected light to a value larger than the resonance material where photoelectrochemical etching oper- wavelength (λra, λrb) of either resonator (10a, ates. 10b), 25 - Lowering (E2’) light wavelength of the injected light so as to attain a first resonance wavelength (λrb) of a first photonic resonator (10b) and start the etching process, while a second photonic resonator (10a) having a second resonance30 wavelength (λra) inferior to the first resonance wavelength (λrb) remains intact, - Lowering (E31’) light wavelength (λ) of the injected light so as to follow the first resonance wavelength (λrb), which decreases until the first 35 resonance wavelength ( λrb) of the first photonic resonator (10b) that is being etched overlaps with the resonance ( λra) of the second photonic resonator (10a), the photonic resonators (10a, 10b) being hereby tuned. 40

12. Method according to claim 11, comprising the following step of lowering (E32’) light wavelength ( λ) so as to etch both photonic resonators (10a, 10b) at the same time and the same rate, such that both phot- 45 onic resonators (10a ,10b) can be tuned at a common targeted resonance wavelength value (λt) and have the same dimensions and optical resonance wavelength. 50 13. Photonicdevice comprising atleast one micro and/or nanophotonic resonator (10) on the substrate (30), obtainable by a method according to claim 1 to 12.

14. Photonic device comprising a micro and/or nanopho- 55 tonic resonator (10) according to claim 13, tuned with a physical precision of dimensions of the photonic resonator less than 10pm.

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REFERENCES CITED IN THE DESCRIPTION

This list of references cited by the applicant is for the reader’s convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.

Patent documents cited in the description

• US 201403211502 A [0003] • EP 1585087 A [0003]

Non-patent literature cited in the description

• K. SRINIVASAN ; 0. PAINTER. Optical fiber taper •N.NIU; T.-L. LIU ; I. AHARONOVICH ; K. J. coupling and high resolution wavelength tuning of RUSSELL ; A. WOOLF; T. C. SADLER ;H. A. microdisk resonators at cryogenic temperatures. Ap- EL-ELLA ; M. J. KAPPERS ; R. A. OLIVER ; E. L. plied physics letters, 2007, vol. 90 (3), 031114 [0004] HU. A full free spectral range tuning of pin doped • D. SRIDHARAN ; E. WAKS ; G. SOLOMON ; J. T. gallium nitride microdisk cavity. Applied Physics Let- FOURKAS. Reversible tuning of photonic crystal ters, 2012, vol. 101 (16), 161105 [0004] cavities using photochromic thin films. Applied Phys- • Y. SHEN ; I. B. DIVLIANSKY ;D. N. BASOV; S. ics Letters, 2010, vol. 96 (15), 153303 [0004] MOOKHERJEA. Electric-field-driven nano-oxidation • M. SHAINLINE ; G. FERNANDES ; Z. LIU ; J. XU. trimming of silicon microrings and interferometers. Broad tuning of whispering-gallery modes in silicon Optics letters, 2011, vol. 36 (14), 2668-2670 [0004] microdisks. Optics express, 2010, vol. 18 (14), • M. ZHANG ; G. S. WIEDERHECKER S. ; 14345-14352 [0004] MANIPATRUNI ; A. BARNARD ; P. MCEUEN ; M. • K. PIEGDON ; M. LEXOW ; G. GRUNDMEIER ; LIPSON. Synchronization of micromechanical oscil- H.-S. KITZEROW ; K. PARSCHKE ; D. MERGEL ; lators using light. Physical review letters, 2012, vol. D. REUTER ; A. WIECK ; C. MEIER. All-optical ten- 109 (23), 233906 [0004] ability of microdisk lasers via photo-adressable pol- • P. DONG ; W. QIAN ; H. LIANG ; R. SHAFIIHA ; D. yelectrolyte functionalization. Optics express, 2012, FENG ; G. LI ; J. E. CUNNINGHAM ;A. V. vol. 20 (6), 6060-6067 [0004] KRISHNAMOORTHY ; M. ASGHARI. Thermally • W. VON KLITZING ; R. LONG ; V. S. ILCHENKO ; tunable silicon racetrack resonators with ultralow tun- J. HARE ; V. LEFÈVRE-SEGUIN. Frequency tuning ing power. Optics express, 2010, vol. 18 (19), of the whispering-gallery modes of silica micro- 20298-20304 [0004] spheres for cavity quantum electrodynamics and spectroscopy. Optics letters, 2001, vol. 26 (3), 166-168 [0004]