WO2023181073A1

WO2023181073A1 - Molecular single-crystal photonic micro-resonators and method of fabricating thereof

Info

Publication number: WO2023181073A1
Application number: PCT/IN2023/050278
Authority: WO
Inventors: Vuppu Vinay PRADEEP; Rajadurai CHANDRASEKAR
Original assignee: University Of Hyderabad
Priority date: 2022-03-23
Filing date: 2023-03-23
Publication date: 2023-09-28

Abstract

The present invention provides micro-resonators comprising a molecular single crystal that are useful in a number of photonic applications. The present invention also provides a method of fabricating a molecular single-crystal photonic micro-resonators. The method of fabricating a micro-resonator can be used to create photonic devices such as resonators, waveguides, lasers, interferometers, gratings, couplers, modulators, beam splitters, photonic crystals, and photonic integrated circuits.

Description

MOLECULAR SINGLE-CRYSTAL PHOTONIC MICRO¬

RESONATORS AND METHOD OF FABRICATING THEREOF

CROSS -REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Indian Patent Application No. 202241016172, filed March 23, 2022, which is incorporated by reference herein in its entirety.

FIELD OF INVENTION

The present invention relates to a molecular single crystal-based optical resonator and a method of fabricating thereof.

BACKGROUND

Frenkel-type molecular optical crystals possess many functional properties such as high exciton binding energy, exciton-polaritons formation, high refractive index, fluorescence, phosphorescence, optical non-linearity, chirality, hardness/softness, bandwidth tunability, surface smoothness, diverse geometries and lightweight. The last decade has seen tremendous progress in utilizing the naturally grown molecular microcrystals as passive and active optical waveguides, resonators, modulators, directional couplers, circuits, and lasers. However, the main bottleneck that hinders the direct utility of molecular single-crystals for commercial high- tech applications such as micro-electromechanical systems, opto -electronic devices, and photonic integrated circuits (PICs) is the lack of controllability of crystal morphology. In PICs, the shape and size of micro components are two critical factors that determine the photonic device characteristics. PICs are useful for quantum communication and processing, sensing, spectroscopy, neuromorphic machinery. Till now, the controllability of microscale shape and size suitable for PICs has been successfully achieved to industrial perfection in silicon and silicon-derivates (Bogaerts, et. al. Nature, Programmable photonic circuits. 586, 207, 2020) and group III-V semiconductors (Thomson, D. et al. Roadmap on silicon photonics. J. Opt. 18, 073003, 2016) using electron-beam-lithography and associated etching processes. As a result, the above-mentioned inorganic materials are widely employed for the commercial-scale manufacturing of microphotonic components such as waveguides, ring-resonators, diskresonators, gratings, and splitters for the construction of monolithic and hybrid PICs. However, monolithic Si-based devices have several drawbacks like indirect bandgap, narrow range visible-near infrared only absorption, high thermo-optical coefficient, centrosymmetry, passive-only light transport, and hardness. Further, cutting-edge PICs require innovative materials platforms providing non-linear optical properties. Thus, hybrid PICs have been manufactured to blend the advantages of many optical materials on a single chip (Leuthold, et. al. Proc. IEEE 97, 1304-1316, 2009).

The realization of all-molecular-crystal PICs (MCPICs) is the next important development step in nanophotonics. Though molecular crystals possess many functional attributes compared to non-molecular materials, their geometry and dimension cannot be controlled with microscale precision during their natural growth. The lack of geometrical and size precisions of crystals severely restricts their entry into the PICs market because the latter demands dimensionally uniform PIC elements with high reproducibility. As a result, the desire to employ molecular crystals as optical elements to create all-molecular PICs with commercially viable precision manufacturing techniques grows day by day.

Optical resonators are quintessential components in MCPICs. An optical resonator circumnavigates or shuttles broad-band light via multiple total internal reflections at the crystal-air interface and subsequently produces optical whispering-gallery-modes (WGMs) or Fabry Perot (FP), respectively via constructive optical interference. The frequency of the modes and their separation (free spectral range, FSR = k_m - k_m+i) depends upon the size and shape of the resonator as FSR « 1/a, where a is diameter or length. Recently, the present inventor reported nearly square-shaped perylene single-crystal optical resonators emitting WGMs which were grown by ambient pressure sublimation technique Pradeep, et. al. Adv. Opt. Mater. 8, 1901317, 2020). Focused ion beam (FIB) milling has been used for etching polymer films and inorganic materials. FIB milling has also been used to convert perylene single crystals into arbitrary shapes. (Li, et. al. Cry st. Growth Des. 20, 1583- 1589, 2020). The creation of well-defined geometrical shapes such as rings, disks, squares, cubes, hexagons, octahedrons, and spheres from various molecular crystals is important to produce microresonators. These crystal resonators are important photonic elements to process and route the light into clockwise and counterclockwise directions when coupled with other photonic elements. Here, the geometrical precision of the single crystal resonator is crucial to trap light and observe the optical resonances. For example, the rectangular crystal geometry supports FP optical resonances by reflecting the light back and forth via their mirror-like opposite facets. Micromachining (by either FIB or electron beam lithography) of a naturally grown crystal into well-defined crystal geometry for direct resonator applications has not been done earlier. Therefore, the present inventors have proposed the FIB milling technique to fabricate geometrically and dimensionally different molecular single crystal micro-resonators.

SUMMARY

The present invention relates to a micro-resonator comprising a molecular single crystal, wherein the molecular single crystal comprises organic molecules or inorganic molecules. In an aspect, the molecular single crystals of predetermined shape and geometry suitable to act as micro-resonators are fabricated by focused ion beam (FIB) milling method.

In another aspect, the present invention provides micro-resonators comprising different molecular single crystals that are useful in a number of photonic applications.

In yet another aspect, the present invention provides a method of fabricating a molecular single-crystal photonic micro-resonators. In an aspect, the method comprises the steps of: preparing molecular single crystal; coating a conductive layer on the molecular single crystal; geometrical shaping the coated molecular single crystal using FIB milling; and removing the conductive layer

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention will become fully apparent from the following description taken in conjunction with the accompanying figures. With the understanding that the figures depict only several embodiments in accordance with the disclosure and are not to be considered limiting of its scope, the disclosure will be described further through use of the accompanying figures.

Figure 1 illustrates the molecular structures of a) perylene, b) coumarin 153 and c) 3- [4-(dimethylamino)phenyl]-l-(2-hydroxyphenyl)prop-2-en-l-one (HDMAC), respectively.

Figure 1 illustrates the microscopy images of d) perylene (a-form), e) coumarin 153 and f) HDMAC microcrystals prepared using bottom-up self-assembly in tetrahydrofuran (4 mM), ambient pressure vapor deposition technique, and self-assembly in chloroform (2 mM), respectively. Figure 1 g) to i) illustrate the FESEM images displaying milled crystals of various geometries. Scale bar 20 m.

Figure 2 illustrates a) a schematic of FIB milling of naturally grown square-shaped perylene single crystal (I) into a circular disk shape. The corresponding FESEM images of crystal before and after milling, b) and d) the optical microscopy images of a square-shaped perylene single crystal (before milling and gold coating) and a circular disk-shaped crystal (after milling and removal of gold coating), c) and e) the fluorescence microscopy images and f) the fluorescence background- subtracted spectra displaying shape and size-dependent optical resonances.

Figure 3 illustrates a) the schematic of FIB milling of naturally grown rectangularshaped perylene single crystal (II) into a smaller rectangular shape and the FESEM images of crystal before and after milling, b) and c) the optical microscopy images of a rectangularshaped perylene single crystal (before milling and gold coating) and a smaller rectangularshaped crystal (after milling and removal of gold coating), d) and e) the corresponding fluorescence microscopy images and f) the fluorescence background- subtracted spectra displaying size-dependent optical resonances.

Figure 4 illustrates a) to c) the FL lifetime images of a perylene crystal before and after milling, d) to f) the corresponding FL lifetime decay plot with estimated average lifetime values.

Figure 5 illustrates a) to f) washing/ etching/ removing gold layer from the molecular crystals and the substrate using Lugol’s Iodine solution, g) to m) washing/ etching/ removing gold layer from the molecular crystals and the substrate using HCN vapors.

Figure 6 illustrates a) and c) are the SEM images of a perylene single crystal (III) before and after milling, b) is a schematic of FIB milling into a ring shape, d) the photographs displaying various stages of the crystal during FIB milling (taken in iPhone 13), e) is the optical microscopy and FL images of a square-shaped perylene single crystal (before milling) and f) the optical microscopy and FL images of a ring-shaped perylene single crystal (after milling and removal of gold coating) and g) is the FL background-subtracted spectra displaying shape and size-dependent optical resonances from square-shaped (top) and ring-shaped (bottom) perylene single crystals. Figure 7 illustrates a) and c) the SEM images of coumarin 153 single crystal (IV) before and after milling, b) a schematic of FIB milling of the single crystal of coumarin 153 into a smaller disc shape, d) the optical microscopy and FL images of a coumarin 153 single crystal (before milling), e) the optical microscopy and FL images of the disc-shaped crystal (after milling), f) the FL background-subtracted spectra displaying shape and size-dependent optical resonances from coumarin 153 single crystal before (top) and after (bottom) milling into a discshaped crystal.

Figure 8 illustrates a) and c) the SEM images of coumarin 153 single crystal (V) before and after milling, b) a schematic of FIB milling of the single crystal into a disc shape, d) are the photographs displaying various stages of the crystal during FIB milling (taken in iPhone 13), e) is the optical microscopy and FL images of coumarin 153 single crystal (before milling), f) the optical microscopy and FL images of the disc-shaped crystal (after milling) and g) the FL background- subtracted spectra displaying shape and size-dependent optical resonances from coumarin 153 single crystal before (top) and after (bottom) milling into the disc-shaped crystal.

Figure 9 illustrates a) and c) the SEM images of coumarin 153 single crystal (VI) before and after milling, b) a schematic of FIB milling of a single crystal into a smaller rectangular shape, d) the photographs displaying various stages of the crystal during FIB milling, e) the optical microscopy and FL images of coumarin 153 single crystal (before milling), f) the optical microscopy and FL images of the smaller rectangular- shaped crystal (after milling) and g) the FL background- subtracted spectra displaying shape and size-dependent optical resonances from coumarin 153 single crystal before (top) and after (bottom) milling into the rectangularshaped crystal.

Figure 10 illustrates a) and c) the SEM images of HDMAC single crystal (VII) before and after milling, b) a schematic of FIB milling of a portion of a single crystal into a disc shape, d) the photographs displaying various stages of the crystal during FIB milling, e) the optical microscopy and FL images of milled HDMAC single crystal (without removal of gold coating) and f) the FL data displaying optical resonances. DETAILED DESCRIPTION

At the very outset of the detailed description, it may be understood that the ensuing description only illustrates a particular form of this invention. However, such a particular form is only an exemplary embodiment, and without intending to imply any limitation on the scope of this invention. Accordingly, the description is to be understood as an exemplary embodiment and teaching of invention and not intended to be taken restrictively. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the methods will be limited only by the appended claims.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, preferred methods and materials are described. For the purposes of the present invention, the following terms are defined below.

The articles "a" and "an" are used herein to refer to one or to more than one (i.e. to at least one) of the grammatical object of the article. By way of example, "an element" means one element or more than one element.

As used herein, the term "comprises", "comprising", or “comprising of’ is generally used in the sense of include, that is to say permitting the presence of one or more features or components.

The term “aromatic hydrocarbons” refers to molecules containing one or more aromatic rings. Examples of aromatic hydrocarbons include, but are not limited to, benzene, toluene, naphthalene, anthracene, tetracene, perylene, pentacene, rubrene, and the like.

"Aromatic" is a term that describes unsaturated compounds containing at least one closed ring consisting of at least 6 atoms. All of the ring atoms in such a compound are either co-planar or nearly co-planar and are covalently bonded to one another. Furthermore, all the ring atoms in such a compound are part of a mesomeric system. The term "carbazole" refers to a class of compounds that contain the carbazole group or a compound that is derived from the carbazole group.

The term "heterocycle" encompasses aliphatic and aromatic heterocycles. It denotes a ring system that is either saturated or unsaturated, comprising carbon atoms and heteroatoms that are independently chosen from N, O, or S. This ring system may be substituted or unsubstituted. The term "saturated heterocyclic ring," unless otherwise specified, refers to a heterocyclic ring that comprises carbon atoms and heteroatoms that are independently selected from N, O or S. This ring system may be substituted or unsubstituted.

Some examples of heterocycles include, but are not limited to, pyrrolyl, thienyl (thiophenyl), thiazolyl, imidazolyl, pyrazinyl, piperidinyl, piperazinyl, pyrimidinyl, pyrrolidinyl, morpholinyl, furyl, tetrahydrofuryl, tetrahydropyranyl, oxiranyl, pyranyl, pyridyl or tetrahydropyridinyl, coumarin 153, and the like.

As used herein, the term “invention” or “present invention” as used herein is a nonlimiting term and is not intended to refer to any single embodiment of the particular invention but encompasses all possible embodiments as described in the specification and the claims.

As used herein, the term "molecular single crystal micro-resonator" refers to a part fabricated from a molecular single crystal.

The expression "molecular single crystal” refers to a solid composed of molecules.

The term "Schiff base" denotes an imine functional group, which is formed through the condensation of an amine group with the carbonyl group of either an aldehyde or ketone In certain embodiments, "Schiff base" refers to a (RI)(R2)C=NR3 moiety, wherein Ri, R2 and R3 are independently hydrogen, alkyl or aryl groups.

As used herein the term “substrate” refers to a container or crucible used for growing crystals. Various types of containers can be used for growing crystals depending on the type and size of crystal and the method of growth, such as beakers, test tubes, Petri dishes, and crystallization dishes, glass coverslips and any coated glass.

As used herein, “vinylene” refers to -CH=CH-. Each embodiment is provided by way of explanation of the invention and not by way of limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made to the compounds, compositions and/or methods described herein without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be applied to another embodiment to yield a still further embodiment. Thus, it is intended that the present invention includes such modifications and variations and their equivalents. Other objects, features and aspects of the present invention are disclosed in or are obvious from, the following detailed description. It is to be understood by one of ordinary skill in the art that the present discussion is a description of exemplary embodiments only and is not to be construed as limiting the broader aspects of the present invention.

The present invention provides a micro-resonator comprising a molecular single crystal, wherein the molecular single crystal comprises organic molecules or inorganic molecules. In an embodiment, molecular single crystals of predetermined shape and geometry are fabricated by focused ion beam (FIB) milling method. In certain embodiments, the molecular single crystals of predetermined shape and geometry are fabricated by FIB milling method with an acceleration voltage of about 30 kV and current of at least about 0.1 nA. In further embodiments, the molecular single crystal is fabricated by FIB milling method with an acceleration voltage of about 500 V-30 kV and current of about 1.0 pA-1.0 nA. In some instances, the molecular single crystal is fabricated by FIB method with an acceleration voltage of 30 kV and current of about 1.0 pA. In certain embodiments, the focused ion beam is gallium ion beam.

In certain embodiments, the molecular single crystal is an organic molecule. In some embodiments, the molecular single crystal is an inorganic molecule.

The molecular single crystal can be prepared by “top-down” (such as FIB milling, electron beam lithography, and photolithography) method, " or “bottom-up” (such as selfassembly, crystallization, sublimation, and ambient pressure vapor deposition) method, or a combination of thereof. In certain embodiments, the molecular single crystal is prepared by top-down method. In some embodiments, the molecular single crystal is prepared by bottom- up method. In certain embodiment, “bottom-up” method is used to naturally grow molecular single crystals. In certain embodiments, the molecular single crystal is in a shape of ring, disc, ellipse, square, rectangle, cube, pentagon, hexagon, octagon, octahedron, or sphere.

In some embodiments, the molecular single crystal is in a shape of ring, disc, square, rectangle or hexagon. In certain embodiments, the creation of well-defined geometrical shapes such as rings, disks, squares, and spheres are important to produce whispering-gallery-mode (WGM) micro-resonators. In certain embodiments, these geometries are important photonic elements to process and route the light into clockwise and counterclockwise directions when coupled with other photonic elements or themselves. Thus, the geometrical precision of the single crystal resonator is crucial to trap light and observe the optical resonances. For example, the rectangular crystal geometry supports Fabry Perot (FP) optical resonances by reflecting the light back and forth via their mirror-like opposite facets.

In certain embodiments, the molecular single crystal is in a shape of ring or disc or rectangle having a dimension of about 0.1 to about 100 pm. In certain embodiments, the dimension is about 0.1 to about 95 pm, about 0.1 to about 90 pm, about 0.1 to about 85 pm, about 0.1 to about 80 pm, about 0.1 to about 75 pm, about 0.1 to about 70 pm, about 0.1 to about 65 pm, about 0.1 to about 60 pm, about 0.1 to about 55 pm, about 0.1 to about 50 pm, about 0.1 to about 45 pm, about 0.1 to about 40 pm, about 0.1 to about 35 pm, about 0.1 to about 30 pm, about 0.1 to about 25 pm, about 0.1 to about 20 pm, about 0.1 to about 15 pm, about 0.1 to about 10 pm, or about 0.1 to about 5 pm. In some instances, the molecular single crystal is having a thickness of about 0.1 to about 10 pm.

In certain embodiments, the organic molecule is selected from a group comprising organic dyes, vinylenes, carbazoles, aromatic hydrocarbons, heterocycles and Schiff bases. In some embodiments, the organic molecule is selected from a group comprising perylene, anthracene, tetracene, pentacene, rubrenes, phenylene, thiophene, coumarin 153 and 3-[4- (dimethylamino)phenyl]-l-(2-hydroxyphenyl)prop-2-en-l-one. In one embodiment, the organic molecule is selected from perylene, coumarin 153 and 3-[4-(dimethylamino)phenyl]- l-(2-hydroxyphenyl)prop-2-en-l-one (HDMAC). In some instances, the organic molecule is a-form of perylene microcrystal.

The present disclosure further provides a method of fabricating a micro-resonator. In certain embodiments, the method of fabricating a micro-resonator comprises the steps of: • preparing a molecular single crystal;

• coating a conductive layer on the molecular single crystal;

• geometrical shaping the coated molecular single crystal using FIB milling; and

• removing the conductive layer.

In certain embodiments of the method, the molecular single crystal comprises organic molecules or inorganic molecules. In some embodiments, the organic molecule is selected from a group comprising organic dyes, vinylenes, carbazoles, aromatic hydrocarbons, heterocycles, and Schiff bases. In some instances, the organic molecule is selected from a group comprising perylene, anthracene, tetracene, pentacene, rubrenes, phenylene, thiophene, coumarin 153 and 3 - [4-(dimethylamino)phenyl] - 1 -(2-hydroxyphenyl)prop-2-en- 1 -one (HDM AC) .

In certain embodiments of the method, the molecular single crystal is prepared by top- down method or bottom-up method. In some embodiments, the molecular single crystal is prepared by bottom-up method. In some instances, “bottom-up” method is used to naturally grow molecular single crystals. In certain embodiments, the molecular single crystal is in the shape of ring, disc, ellipse, square, rectangle, cube, pentagon, hexagon, octagon, octahedron, or sphere.

In certain embodiments of the method, the molecular single crystal is in a shape of ring or disc or rectangle having a dimension of about 0.1 to about 100 pm.

In certain embodiments of the method, the molecular single crystal is having a thickness of about 0.1 to about 10 pm.

In certain embodiments of the method, the conductive layer comprises a conductive agent. In one embodiment, the conductive agent is selected from a group comprising copper, gold, platinum, tungsten, and silver.

In certain embodiments, the present invention provides a micro-resonator comprising a molecular single crystal that is useful in several photonic applications, particularly nanophotonic applications. The molecular single crystal can comprise any molecular material. In certain embodiments, the molecular single crystal comprises the organic or inorganic material or molecule having suitable optical and/or photonic properties. In certain embodiments, the present invention provides a method of fabricating a molecular single-crystal micro-resonator for direct resonator applications. In one embodiment, the method of fabrication comprises micromachining by focused ion beam (FIB) milling of a naturally grown molecular single crystal into predetermined and well-defined crystal geometry. In certain embodiments, the focused ion beam is gallium ion beam.

In certain embodiments of the method, the conductive layer can be removed by any method known in the art. In one embodiment, the conductive layer from the molecular crystal can be removed with an etchant solution. In certain embodiments, the etchant solution comprises Lugol’s Iodine solution (Kl/iodinc solution). In some embodiments, the conductive layer from the molecular crystal is removed by washing it multiple times with Kl/Iodinc solution. In certain embodiments, the conductive layer from the substrate is removed by HCN such as HCN vapors.

In certain embodiments, the method of fabrication can be used to fabricate molecular single-crystal micro-resonators of different geometry and size such as disk- and rectangularshaped photonic resonators, circular, ring, rod-shaped, hexagonal, octagonal and any possible geometries to create photonic modules such as resonators, waveguides, lasers, interferometers, gratings, couplers, modulators, and photonic crystals suitable for the fabrication of PICs. As the geometry and dimension of the molecular crystals can be precisely controlled down to microscale during the milling process, the present method can also be applied to the industrialscale production of photonic modules for PICs. In some embodiments, the method of fabricating a micro-resonator can be used to create photonic devices such as resonators, waveguides, lasers, interferometers, gratings, couplers, modulators, and photonic crystals.

ABBREVIATIONS

FIB- Focused Ion Beam; PIC- Photonic Integrated Circuit; FL- Fluorescence; FSR- Free Spectral Range; WGM- Whispering-Gallery-Mode; FP- Fabry Perot; FESEM-Field Emission Scanning Electron Microscopy; mM- Millimolar; pm- Micrometer; MCPIC- all- Molecular Crystal Photonic Integrated Circuit; AFM- Atomic Force Microscope, HDMAC- 3- [4-(dimethylamino)pheny 1] ■■ 1 -(2 -hydro xyphenyl)prop-2-en- 1 -one. The present invention is further described with reference to the following examples, which are only illustrative in nature and should not be construed to limit the scope of the present disclosure in any manner.

EXAMPLES

PREPARATION OF MICROCRYSTALS:

Example 1: Preparation of perylene microcrystals a-form of perylene microcrystals were used in this. Bottom-up method was used to naturally grow a-form perylene microcrystals on a clean coverslip (Figure la, d). Tetrahydrofuran (HPLC grade) solution of perylene (1 mg/1 mL, 4 mM) was used to fabricate microcrystals. The solution was sonicated for 30 s and kept for 5 min without any disturbance. Later, 2-3 drops (~20 pl) of perylene solution was drop-casted on the clean glass coverslip and allowed to evaporate. As the solvent evaporates, the microcrystals started growing. Finally, the solvent got evaporated completely and resulted in microcrystals of different sizes. The growth of numerous square- and rectangular- shaped microcrystal of various sizes were observed and confirmed by an optical microscopy study.

Optical microscopy studies reveal the growth of numerous square- and rectangular- shaped microcrystal of various sizes. As test cases, we identified three crystals labeled I, II and III for the FIB milling (Figure Id). As per our plan, the shape of the selected perylene single-crystals I, II and III were precisely converted to disk-, rectangular- and ring-shaped crystals, respectively with size reduction (Figure 1g). Remarkably, single-particle micro-spectroscopy studies reveal that micromachined disk-, rectangular- and ring-shaped crystals act as microresonators by emitting resonance modes.

To generalize these experiments, FIB milling along with micro-spectroscopic studies were carried out on two other chemically different compounds, namely, Coumarin 153 and 3-[4- (dimethylamino)phenyl]-l-(2-hydroxyphenyl)prop-2-en-l-one (HDMAC), and the details were discussed in example 2 and 3, respectively (Figure Ib-c). Example 2: Preparation of Coumarin 153 microcrystals

Coumarin 153 is a green, fluorescent compound, which on subjected to ambient pressure vapor deposition resulted in microcrystals. For this <1 mg of the compound was placed on a clean coverslip. Later, another coverslip was placed on it using two supporting stubs of height 1 mm. The whole setup was placed on a hot plate and heated slowly up to 110 °C. It was allowed to sublime at 110 °C for about a minute. Then, the temperature slowly cooled down to room temperature. The optical microscopy studies of the coverslip revealed the growth of numerous microcrystals on the top coverslip (Figure le).

As test cases, we identified three crystals labeled IV, V and VI for FIB milling (figure le). As per our plan, the shape of selected coumarin 153 single crystals IV, V and VI were precisely converted to two different disc-shaped crystals (IV, V) and one rectangular- shaped (VI) crystal, respectively (Figure Ih). As expected, single-particle micro-spectroscopy studies revealed that micromachined discs- and rectangular- shaped crystals act as micro-resonators by emitting resonance modes.

Example 3: Preparation of 3-r4-(dimethylamino)phenyl1-l-(2-hvdroxyphenyl)prop-2-en- 1-one (HDMAC) microcrystals

HDMAC is a red emissive compound synthesized using 2-hydroxy aminophenol, 4-dimethyl amino benzaldehyde, and KOH in methanol. The reactants were stirred for 10 hrs, at room temperature. Filtration of precipitate, followed by flash column (eluent: CHCh) resulted in a pure HDMAC compound. Microcrystals of this were obtained using self-assembly (lmg/1 mL CHCL3 solution of HDMAC) as well as using ambient pressure vapor deposition technique (amount: <1 mg, sublimation temperature: 165 °C, separation between the coverslips: 1 mm, and sublimation time: 10 min). The growth of numerous square- and rectangular- shaped microcrystal of various sizes were observed and confirmed by an optical microscopy study.

As a test case, we identified a crystal labeled VII for FIB milling (figure If). As per our plan, one comer of the HDMAC single crystal VII was precisely converted to a disc-shaped crystal (Figure li). Single-particle micro-spectroscopy studies exhibited that the micromachined discshaped crystal act as a micro-resonator by emitting resonance modes. FIB Milling:

For the milling of microcrystals, Thermo Scientific SCIOS 2 Dual Beam instrument was used. Initially, the microcrystals were imaged in SEM mode with a 0.0° tilt angle, 0.4 nA beam current and an accelerating beam voltage of 5.00 kV. Later, the sample was tilted 52° to align orthogonal to FIB (Gallium ion source). Using pre-defined shapes in the software (in this case rectangular and circular shapes), the milling portion was selected. Then, 30 kV accelerating beam voltage and 0.1-0.4 nA probe current were applied to mill the microcrystals to desired shapes. a) To fabricate a circular disk-shaped crystal of perylene of diameter 4.63 m (Figure 1g and 2a, d), the crystal (I) was milled after gold coating using gallium ion beam. The gold coating on the sample was removed by repeated washing with Lugol’s Iodine solution (method 1, Figure 5). The energy dispersive X-ray analysis (ED AX) indicated the presence of only a tiny fraction of Ga ions in the sample. b) To fabricate a rectangular- shaped crystal of perylene of dimension « 1.84x1.44 pm² (Figure 1g and 3a, c), the crystal (II) was milled after gold coating using gallium ion beam. The gold coating on the sample was removed by repeated washing with Lugol’s Iodine solution (method 1, Figure 5). c) To fabricate a ring-shaped crystal of perylene of diameters (outcr~6.3 pm and inncr~2.35 pm) (Figure 1g and 6c, f), the crystal (III) was milled after gold coating using gallium ion beam. The photographs taken on iPhone 13 during crystal milling are presented in figure 6d. The gold coating on the sample was removed by HCN vapors (method 2, Figure 5). d) To fabricate two disc-shaped crystals of Coumarin 153 of diameters 2.5 pm and 7.47 pm (Figure Ih), crystals IV and V were selected. Once the micro spectroscopy experiments were done, contact mode atomic force microscopy (AFM) was performed to get the thickness of the crystal. While performing this, the crystal edges were damaged due to high scan speed (Figures 7a and 8a). Finally, the centre of the crystals was milled after gold coating using gallium ion beam (Figures 7c and 8c). Figure 8d shows photographs displaying various stages of crystal V during FIB milling. e) To fabricate a rectangular- shaped crystal of coumarin 153 of dimension ^6.3x5.5 pm² (Figure Ih), crystal VI was selected. Once the micro -spectroscopic experiments were done, contact mode atomic force microscopy was performed to get the thickness of the crystal. While performing this, the edges were damaged due to the high scan speed (Figure 9a). Finally, the crystal was milled after gold coating using gallium ion beam (Figure 9c). Figure 9d shows the intermediate photographs during FIB milling. f) To fabricate a circular disk-shaped crystal of HDMAC of diameter 7.3 m (Figure li), the crystal VII was milled after gold coating using gallium ion beam (Figure 10c). The intermediate photographs are shown in figure lOd.

Gold Coating:

The coverslip containing microcrystals was placed in QUORM Tech, sputter coater. By applying 15 pA current for about 80 sec, a thin layer of gold was coated on the sample.

Washing/etching/removing gold from the molecular crystal and substrate:

Method 1: Lugol’s Iodine solution was used to etch the gold from the substrate.

Preparation of Lugol’s Iodine solution: 10g of potassium iodide was dissolved in 30 ml of distilled water. 5 g of iodine was added to it and heated gently with constant stirring until it was dissolved. Later, the solution was diluted to 100 ml with distilled water.

Etching: The FIB milled microcrystals containing coverslip was placed in a petridish. The freshly prepared Lugol’s iodine solution was added gently via the walls of the petridish. The coverslip was allowed to soak for about 1-2 minutes, then the solution was pipetted out using a dropper. Later, it was washed with distilled water. This procedure was repeated with a soaking time of 2 minutes (Figure 5).

Method 2: HCN vapors were used to etch the gold from the substrate (Figure 5)

Materials required: NaCN, tissue paper, cardboard, glass container having a lid. The complete procedure was carried out in a suitable fume hood.

Preparation: 1 g of NaCN was placed on the tissue paper and made a packet of it. Later, the packet was placed in the glass container, and a cardboard having 2 holes was placed over it. Now, the gold-coated coverslip (FIB milled microcrystals containing coverslip) was gently placed on the cardboard. Finally, 3-4 drops of water were added through the holes of the cardboard and the glass container was closed using the lid and tightly sealed using parafilm tape.

Etching: The added water droplets react with NaCN, which in turn results in the generation of HCN vapors. These vapors slowly react with the gold layer on the coverslip. Within a day, the gold layer was completely reacted with HCN and the formed AuCN was settled on the coverslip. Finally, the gold-layer free coverslip was obtained after washing with distilled water.

Polarised Light Microscopy:

The images of fabricated and milled microcrystals / micro-resonators were captured using a NIKON eclipse LV100N POL polarising microscope. It was equipped with an epi-illuminator (NIKON 12V 50W), DS-Fi3 camera having a 5.9 megapixel CMOS sensor, which enables superior color reproduction and NIKON TU plan fluor EPI P series objectives (4x, lOx, 20x and 50x) for pin-sharp aberration-free images regardless of magnification.

Confocal Micro-Spectroscopy:

Fluorescence spectra of the microcrystals / micro-resonators were recorded on a WI-Tec confocal spectrometer equipped with a Peltier-cooled CCD detector. Using 300 grooves/mm grating BLZ = 750 nm. All measurements were performed in transmission mode geometry. A solid-state 405 nm laser was used as an excitation source. To collect the output signals from the specific area of microcrystals / micro-resonators, 150x objective (N. A.: 0.95) was used. For acquiring a single spectrum before FIB milling, the laser power, integration time and accumulations were optimized to 0.05 mW, 2 s and 5, respectively. Whereas, after FIB milling, the power is varied from compound to compound and are represented in the microscopy studies section individually. The images were processed by using WI-Tec 5.2 software.

Atomic Force Spectroscopy (AFM):

All the AFM experiments were carried out on an Oxford Asylum Research MFP-3D Origin. The image processing was carried out by using AR 16.25.226 software provided by the manufacturer. The images were recorded in a contact mode topography using a silicon cantilever (NSG 10_DLC) with a diamond-like carbon tip (NT-MDT). The dimension of the tip is as follows: cantilever length = 100 (±5) pm, cantilever width = 35 (±3) pm, and cantilever thickness = 1.7-2.3 pm, resonance frequency = 190-325 kHz, force constant = 5.5-22.5 N/m, tip height = 10-20 nm.

Fluorescence Lifetime Imaging:

FL decays and FL lifetime images were recorded on a time -resolved (Micro-Time 200, Pico Quant) confocal FLIM setup, which was equipped with an inverted microscope (Olympus IX 71). Measurements were performed at room temperature, on microcrystals deposited coverslip. The microcrystals / micro-resonators were excited by a 405 nm ps diode pulse laser (power ~ 5 pw) with a stable repetition rate of 20 MHz (FWHM: 176 ps) through a water immersion objective (Olympus UPlans Apo; 60 x; NA 1.2). The signal from the microcrystal / microresonator was collected by the same objective and passed through the dichroic mirror, filtered by using a 430 nm long-pass filter to cut off any exciting light. The signal was then focused onto a 50 pm diameter pinhole to remove the out-of-focus signal, recollimated, and directed onto a (50/50) beam splitter prior to entering two single -photon avalanche photodiodes. The data acquisition was carried out with a SymPhoTime software-controlled PicoHarp 300 time- correlated single-photon counting module in a time-tagged time-resolved mode. The overall resolution of the setup was 4 ps.

FESEM Analysis:

A thin layer of gold was coated on the substrate using a 15 pA current for 80 sec. The size and morphology of the milled micro-resonators were examined by using a Zeiss field emission scanning electron microscope operating at an accelerating voltage of 5 kV.

Microscopy Studies:

The crystal (I) was excited with a continuous-wave 405 nm laser (Excitation: 0.05 mW; objective: 60x), and it displayed a bright yellow fluorescence (FL) at its four edges. The recorded broad FL (objective 150x, numerical aperture: 0.95) spectrum covering the bandwidth of *525-775 nm region exhibited a series of pairs (transverse magnetic, TM and transverse electric, TE) of sharp peaks, confirming that the crystal is a WGM micro-resonator. The FSR value of the crystal resonator is *8.10 nm. The resonator characteristics of the square-shaped perylene crystal shown in the field emission scanning electron microscope (FESEM) image arise due to multiple circulations of FL by the four-light-reflective edges of the crystal (Figure 2a, c). Photonic experiments performed on the disk-shaped crystal exhibited an intense FL spectrum supporting relatively broad optical modes with an FSR of 15.39 nm (Figure 2f). For acquiring this single spectrum, the laser power, integration time and accumulations were 4 mW, 1 s and 20, respectively. The number of optical modes depends on the geometry and size of the resonators.

The crystal (II) exhibited optical resonances with an FSR value of 12.16 nm during singleparticle micro-spectroscopy experiments. Milling the crystal into a smaller rectangular crystal of dimensions 1.84x1.44 pm² and subsequent optical experiments display relatively broader modes with an FSR of 31.29 nm (Figure 3f) The increase in the full-width-at-half-maximum of optical resonant modes and FSR values is in line with the inverse relationship of FSR with the resonator dimension. For acquiring this single spectrum, the laser power, integration time and accumulations were 4 mW, 1 s and 20, respectively. Further, the crystals were stable up to 20 mW laser pump power.

The crystal (III) also exhibited optical resonances with an FSR of 3.18 nm during singleparticle micro-spectroscopy experiments. AFM studies were performed in contact mode and the thickness was found to be 0.35 pm, respectively (inset of 6a). Milling the crystal into a ring-shaped crystal of diameters (outer and inner) 6.3 pm and 2.35 pm and subsequent optical experiments display relatively broader modes with an FSR of 20.75 nm (Figure 6g). For acquiring this single spectrum, the laser power, integration time and accumulations were 0.5 mW, 1 s and 20, respectively.

All the coumarin 153 crystals (IV, V and VI) were excited with a continuous-wave 405 nm laser (Excitation: 0.05 mW; objective: 60x), and it displayed a bright green FL at two opposite edges. The recorded broad FL (objective 150x, numerical aperture: 0.95) spectrum covering the bandwidth of *485-725 nm region exhibited sharp peaks, confirming that the crystal is a micro-resonator. The FSR values of the crystal resonators IV, V and VI are *4.8, *3.81 and *4.07, nm, respectively. The resonator characteristics of the microcrystal of coumarin 153 shown in the SEM image arise due to multiple reflections of FL by the two opposite light- reflective edges of the crystal (Figure le, 7d, 8e and 9e). AFM studies in contact mode revealed the thickness of the crystals (IV, V and VI), and are 1.33, 1.18 and 0.91 pm, respectively (insets of 7a, 8a and 9a). The edges of these crystals V and VI were ruptured due to high scan speed in contact mode topography. To perform SEM imaging, gold coating was done and the SEM image before milling clearly shows the damaged edges of these crystals (Figures 8a and 9a). FIB milling was done in such a way that, the final structures were defect- free (Figures 7c, 8c and 9c). Interestingly, without the removal of gold coating, photonic experiments on these three crystals exhibited an intense FL spectrum supporting relatively broad optical modes with an FSR of 23.14, 6.25 and 8.15 nm, respectively (Figure 7f, 8g and 9g). For acquiring these single spectrums, the laser power, integration time and accumulations were 0.5 mW, 0.5 s and 10, respectively.

The HDMAC crystal (VII) was subjected to FIB milling at one of its corners. It was milled into a disc of diameter 7.3 pm and subsequent optical experiments display resonance modes with an FSR of 3.58 nm (Figure 7f). For acquiring this single spectrum, the laser power, integration time and accumulations were 4 mW, 1 s and 20, respectively. From the FESEM images, the thickness of the crystal was found to be «2.53 pm.

The FL lifetime of perylene crystals before and after gallium ion milling was investigated using an FL lifetime microscope (pico-second 405 nm pulse laser) with a time-correlated single photon counter (Figure 4a-c). The images show the distribution of FL lifetime values within the crystal. The disk-shaped resonator showed a well-resolved image with a high FL signal from the rim of the circular cavity due to circumnavigating light at the crystal-air interface. On the other hand, for the smaller rectangular cavity, a nearly equal spread of FL was observed. Unlike ordinary crystals, the lifetime values of crystal resonators are different as the quality factor of the resonator determines the photon lifetime (trapped light) of the FL within the crystal by the relation, Q « Tp. The average lifetime decay values of milled crystals are slightly lowered compared to crystals before milling (Figure 4d-f).

Claims

We Claim:

1. A micro-resonator comprising a molecular single crystal; wherein the molecular single crystal comprises organic molecules or inorganic molecules.

2. The micro-resonator as claimed in claim 1, wherein the molecular single crystal microresonator is fabricated by focused ion beam (FIB) milling method.

3. The micro-resonator as claimed in claim 1, wherein the molecular single crystal is an assembly of organic molecules or inorganic molecules.

4. The micro-resonator as claimed in any one of claims 1 to 3, wherein the molecular single crystal is prepared by top-down method or bottom-up method.

5. The micro-resonator as claimed in any one of claims 1 to 4, wherein the molecular single crystal is prepared by bottom-up method.

6. The micro-resonator as claimed in any one of claims 1 to 5, wherein the molecular single crystal is in a shape of ring, disc, ellipse, square, rectangle, cube, pentagon, hexagon, octagon, octahedron, or sphere.

7. The micro-resonator as claimed in any one of claims 1 to 6, wherein the molecular single crystal is in a shape of ring or disc or rectangle having a dimension of about 0.1 to about 100 pm.

8. The micro-resonator as claimed in any one of claims 1 to 7, wherein the molecular single crystal is having a thickness of about 0.1 to about 10 pm.

9. The micro-resonator as claimed in claim 1, wherein the organic molecule is selected from a group comprising organic dyes, vinylenes, carbazoles, aromatic hydrocarbons, heterocycles, and Schiff bases.

10. The micro-resonator as claimed in claim 1, wherein the organic molecule is selected from a group comprising perylene, anthracene, tetracene, pentacene, rubrenes, phenylene, thiophene, coumarin 153, and 3-[4-(dimethylamino)phenyl]-l-(2- hydroxyphenyl)prop-2-en- 1-one (HDMAC).

11. The micro-resonator as claimed in any one of claims 1 to 10, wherein the molecular single crystal resonator is fabricated by FIB milling method with acceleration voltage of about 30 kV and current of at least about 0.1 nA.

12. The micro-resonator as claimed in any one of claims 10, wherein the accelerating voltage is about 500 V-30 kV, and the current is about 1.0 pA-1.0 nA. A method of fabricating a micro-resonator comprises the steps of:

• preparing molecular single crystal;

• coating a conductive layer on the molecular single crystal;

• removing the conductive layer. The method as claimed in claim 13, wherein the molecular single crystal comprises organic molecules or inorganic molecules. The method as claimed in claim 13, wherein the molecular single crystal is prepared by top-down method or bottom-up method. The method as claimed in any one of claims 13 to 15, wherein the molecular single crystal is in a shape of ring, disc, ellipse, square, rectangle, cube, pentagon, hexagon, octagon, octahedron, or sphere. The method as claimed in any one of claims 13 to 16, wherein the molecular single crystal is in a shape of ring or disc or rectangle having dimension of 0.1 to 100 pm. The method as claimed in any one of claims 13 to 17, wherein the molecular single crystal is having a thickness of about 0.1 to about 10 pm. The method as claimed in any one of claims 13 to 18, wherein the organic molecule is selected from a group comprising organic dyes, vinylenes, carbazoles, aromatic hydrocarbons, heterocycles and Schiff bases, perylene, anthracene, tetracene, pentacene, rubrenes, phenylene, thiophene, coumarin 153, and 3-[4- (dimethylamino)phenyl] - 1 -(2-hydroxyphenyl)prop-2-en- 1 -one (HDMAC). The method as claimed in claim 13, wherein the conductive layer comprises a conductive agent selected from a group comprising copper, gold, platinum, tungsten, and silver. The method as claimed in claim 13, wherein the conductive layer from the molecular crystal is removed with an etchant solution. The method as claimed in claim 13, wherein the conductive layer from substrate is removed with HCN. The method as claimed in claim 21, wherein the etchant solution is Kl/iodinc solution. The method as claimed in any one of claims 13 to 23, wherein about 30 kV accelerating voltage and at least about 0.1 nA current are applied to mill the crystals into desired or predetermined shapes. The method as claimed in claim 24, wherein the accelerating voltage is about 500 V- 30 kV, and the current is about 1.0 pA-1.0 nA. The method as claimed in any one of claims 1 to 25, wherein the method of fabricating a micro-resonator is used to create photonic devices such as resonators, waveguides, lasers, interferometers, gratings, couplers, modulators, beam splitters, photonic crystals, and photonic integrated circuits.