Rtrt The 9thInternational Workshop on - CSDL Khoa học

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Vietnam Academy of Science and Technology National Institute for Materials Science of Japan

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Ninh Binh City, Vietnam, November 7-11

^th

, 2018

xvii

Nguyen Tien Dai, Nguyen Quang Liem, Doan Dinh Phuong, Tran Quoc Tien, Dao Khac An, Kim Jun Oh, Jeon Jiyeon, Hwang Jehwan, Lee Sang Jun

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Fabrication of hydrogen gas sensor based on SnO

2

/Pt thin film on polyimide substrate

Vo Thanh Duoc, Nguyen Xuan Thai, Nguyen Van Duy, Nguyen Van Hieu

NMD-O15 247

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Synthesis and surface functionalization of mesoporous MCM-41 silica nanoparticles by APTES and their drug doxorubicin hydrochloride loading

Dau Tran Anh Nguyet, Tran Thi Thanh Van, Pham Thi Kim Hong

NMD-O18 252

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Application of the Cu-CNTs hybrid nanofluid to enhance heat dissipation for 200W LED streetlight

Hung Thang Bui, Viet Phuong Nguyen, Ngoc Anh Nguyen, Van Trinh Pham, Dinh Quang Le, Ngoc Minh Phan

NMD-O20 261

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Whispering-gallery-mode lasers in Er

³⁺

-doped silica glass microspheres towards integration in SOI photonic crystal waveguides

Van An Nguyen, Van Dai Pham, Thanh Binh Pham, Hoang Thu Trang, Thi Hong Cam Hoang, Quang Minh Ngo, Van Hoi Pham

NMD-P01 269

40

Structural, optical and impedance spectroscopy studies of zinc doped nickel titanate ceramics

Luong Huu Bac, Pham Van Thang, Nguyen Hoang Thoan, Nguyen Ngoc Trung

NMD-P03 276

41

Synthesis of flower-like gold nanoparticles for SERS application

Tuan Anh Cao, Tran Cao Dao, Truc Quynh Ngan Luong, Ngoc Minh Kieu, Van Chung Pham, Van Vu Le

NMD-P06 284

42

Possible existence of spinodal decomposition in MnBi magnets

Truong Xuan Nguyen, Ky Hong Vu, Vuong Van Nguyen

NMD-P08 289

The 9^th International Workshop on Advanced Materials Science and Nanotechnology (IWAMSN 2018) – November 7^th-11^th, 2018 – Ninh Binh, Vietnam

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CODE: NMD-P01

Whispering-gallery-mode lasers in Er

³⁺

-doped silica glass microspheres towards integration in SOI photonic crystal waveguides

Van An Nguyen^1,2,*, Van Dai Pham³, Thanh Binh Pham³, Hoang Thu Trang³, Thi Hong Cam Hoang¹, Quang Minh Ngo^1,3 and Van Hoi Pham^1,3

1Graduate University of Science and Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Road, Cau Giay District, Hanoi, Vietnam

2Hue College of Sciences, Hue University, 77 Nguyen Hue Street, Hue City, Vietnam

3Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Road, Cau Giay District, Hanoi, Vietnam

*Email: ngvanan2009@gmail.com

Abstract. Er³⁺-doped silica glass microspheres (S) with diameters of ~30-40 m were fabricated by using an electrical discharge method. The single-mode optical tapered fibers were used to guide the pumped laser into the S surface and collect the resulting lasing emission.

The observation of whispering-gallery-mode (WGM) at telecom regime is quantitatively analysed. With the presented scheme, the selective single- or multi-emitted modes of the S laser can be obtained by adjusting the coupling gap between the collection tapered fiber and the

S surface. To overcome the collection tapered fiber‟s vibration at close range of the coupling gap, the guiding and coupling losses and to be compact to fit with CMOS device, the integration of a S with silicon-on-insulator (SOI) slotted photonic crystal waveguide is proposed and modelled with the help of finite-difference time-domain (FDTD) simulations.

The designed structures may find application for the promising photonic devices such as the ultrahigh sensitivity sensors, the lasing sources and also towards the advanced quantum communication technologies and larger adoption of quantum-secured communications.

Keywords: microspheres, whispering-gallery-mode, finite-difference time-domain method, two- dimensional photonic crystal waveguide.

1. Introduction

Optical micro-resonators have been the subject of much attention due to their special characteristics and many promising applications for quantum electrodynamics, nonlinear optics, data storages, chemical or bio-sensing, lasers and filters [1-4]. In dielectric microspheres(S) lights can be confined in the form of the whispering-gallery-modes (WGMs), which have small modal volumes and ultrahigh quality (Q)factors. If S is composed of an active material, the lasing emission is modulated by the WGM spectrum, therefore the spontaneous emission rate is significant enhanced [3,5]. Numerous materials including polymers, crystalline, or glassy compounds doped with a high enough concentration of rare earth ions have been used to fabricate the Ss and they create active cavities, which act as both the gain media and the resonators for the emitted lasers. In the measurement setup, the tapered single-mode optical fibers were used to guide the pump laser beam coupling into theS surface and collect the resulting lasing emission. The achieved S lasers were emitted by the WGMs

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with narrowed linewidth multi-mode lines in the telecom wavelength range of 1550 nm. In addition, by adjusting the coupling gap smaller than the critical length, it is possible to obtain the single-mode of high power with the selected wavelength. As our results shown, there have been wide applications, such as telecom-like spectroscopy, optical switches, high density wavelength division multiplexing (DWDM) communications and lab-on-a-chip sensors [6]. However, there was a drawback of using a narrow-linewidth emission for spectroscopy and sensors, owing to the low power per emission line and the difficulty of resolving features that are smaller than the emitted mode spacing [7]. In addition, the vibration of the coupling gap, which makes the loss and the unstability of the laser and signal coupled into and out theS, respectively [8] hinders the practical application of these optical devices.

As the works presented on the progress and infrastructure of integrated photonics for communication and lab-on-chip sensors in terms of complexity, robustness and scalability, silicon photonics can offer almost complete suite with further benefits of miniaturization, cost-effective device manufacture and compatibility with CMOS technology [9,10]. From the general point of view, silicon-on-insulator (SOI) slotted waveguide geometry has been considered for integrated optics due to the high degree of spatial light confinement. It means that light is confined in a narrow slotted region with low refractive index etched into a high refractive index medium. With the two dimensional (2D) slotted photonic crystal (PhC) waveguide, which provides the control over the dispersion of light in air, both in the temporal and the spatial domains [11]. It inherits the peculiarity from its geometry that combines two confining mechanisms, such as in slot and in 2D-PhC lattices. Thus, low loss propagation in slotted PhC components is possible and that it is comparable to losses in non-slotted PhC waveguides or in slotted ridge waveguides. Therefore, propagation losses do not restrict the further exploration of high light-matter interaction in slotted waveguides, e.g. for applications of sensing or nonlinear optics [12].

In this paper, based on the Er³⁺-doped silica glass Ss with diameters of ~30-40 m were fabricated, two tapered optical fibers were used to guide the pumped laser (~1470 nm), restricted to TE polarization (H-field normal to the WGM propagation plane), coupling into the S surface and collect the resulting signals, respectively. The observed WGM lasing emissions at telecom regime of

~1550 nm are analyzed quantitatively. Depending on the coupling gap between the collection tapered fiber and theS surface, the WGM single-mode line at given wavelength or multi-mode lines of the

S lasers can be obtained. In order to visualize the WGMs formed by the S lasers, the finite- difference time-domain (FDTD) simulations were carried out on the silica glass spherical shapes that closely identical to the real samples fabricated in this work. The electric dipole emitters producing the sinusoidal pulses centered at the lasing emission wavelength were placed in the zone internally near the S surface. These results in a greater number of observable emission modes, which has good agreement with the analytic model results. Because of using two tapered optical fibers for pumping laser into theS surface and collecting the lasing emission, the vibration of the fiber due to the close coupling gap induces the coupling losses and unstability of the emitted signals. To overcome these limitations and also create a compatibility with CMOS devices, a large-scale integrated optics configuration between aS and SOI slotted PhC waveguides is designed and modeled with the help of FDTD simulations [13]. The integrated structure in this work may find applications in the ultrahigh sensitivity sensors, the lasing sources and the integrated quantum communications.

2. Research methods

Numerous studies have realized the WGMs of the Ss using several analytic models [14]. In here, a simplified 2D model for understanding the physical and optical properties of the 3D optically coupled S systems was considered. As it is well known that the optical WGM is formed by the repeated total internal reflection of the light on the S surface (Figure 1(a)). When the phase matching condition is satisfied, standing waves will appear along the corresponding perimeter of the S (Figure 1(b)). Three mode numbers describe it: radial (n), angular (l) and azimuthal (m) [15]. The radial number, n, indicates the number of intensity maxima along the radial direction; the angular number, l, represents the number of modal wavelengths that fit into the circumference of the equatorial plane of the S. It means that 2l is the number of maxima in the angular variation of the resonant field around the S equator. The azimuthal mode number, m, describes the field variation in the polar direction,

The 9^th International Workshop on Advanced Materials Science and Nanotechnology (IWAMSN 2018) – November 7^th-11^th, 2018 – Ninh Binh, Vietnam

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with the number of intensity maxima along this direction being equal to l-|m|+1. In the case of light beams transmitting in a large circle that is inclined at an angle  to the equatorial plane (Figure 1(c)), the azimuthal mode number is m = lcos. In practical, we are interested in exciting low-order WGMs with small n and with m ≈l for maximizing the resonant advantages of the S. These modes have the optical field distributions near the S surface approximated to the equator and the E-fields are compressed into the smallest modal volume.

Figure 1. Resonant microcavity formed when the phase matching condition is satisfied (a), standing waves appear on the perimeter of the S (b) and the orbit of the photon along the perimeter of the S (c).

In simulation, we used the plane-wave expansion (PWE) method embedded in MIT open-source, called MPB and the commercial Lumerical FDTD software [13,16]. To obtain the spectra, resonant patterns of the S laser and of the integration between a S and SOI slotted PhC waveguides, the perfectly matched absorbing boundary conditions were used at the surroundings of these structures [13]. The excitation characteristics: the TE polarized point-source electric dipoles in the vicinity of the

S surface have been taken for the WGM-S lasers and whereas, the Bloch-mode source located at the input SOI slotted PhC waveguide after the perfectly matched absorbing layer was used for the S integrated with SOI slotted PhC waveguide structure.

3. Results and discussion

Figure 2. Experimental setup for laser emission spectrum in a Er³⁺-doped silica glass S.

The scheme of Lab-made setup for measuring the WGMs of S is depicted in Figure 2. The laser diode at  = 1470 nm with the minimum output power ~180 mW was polarized at TE mode for excitation of the Er³⁺ ions. The pump laser beam and the resulting lasing emission from the S are guided by different tapered fibers. This technique has an advantage of flexibility in controlling the coupling gap between the collection fiber and S surface, while the position of excited fiber was fixed.

The spectral characteristics of the lasing emissions were analyzed by the optical spectrum analyzer (OSA)-Advantest Q8384 with the resolution of 0.01 nm. The coupling gap was adjusted by a 3D micro-precision stage with the accuracy of 0.10 m. Two types of mode corresponding to the two possible directions of propagation (clockwise-CW and counterclockwise-CCW) have been measured.

In order to couple light into the S efficiently, the evanescent wave mode of the tapered fiber must match with the WGM of the S. When the Er³⁺-doped silica glass S is adequately coupled with the

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tapered fiber, single-mode emission can be achieved. Otherwise, a multi-mode emission can be observed [17,18]. In the experiment, single- and multi-mode emissions have been measured. As shown in Figures 3(a,b), the S with the diameter of ~38.5 μm and coupling gap of 1.80.1 m produced the multi-mode emissions. As it is shown, these emissions have the CW sense of circulation. It means that there was strong coupling in the CW geometry, which can be known as the in-phase matched coupling [19].

Figure 3. WGM emission spectra extracted from Er³⁺-S with diameter of ~38.5 m and coupling gap of 1.80.1 m: The CW geometry (a); CCW geometry (b) as shown in Figure 2 and CW geometry when the

coupling gap between the tapered fiber and the S surface 0.5 m  0.1 m (c).

To explore the characteristics of the single WGMs, when the coupling gap is smaller than ~0.7 m, we have collected its output power by precise adjustment of the coupling gap. In the presented scheme, the diameter of S and pumped power were kept as same as the one shown in Figure 3(a,b), while Figure 3(c) presents the single-mode spectra extracting from the CW geometry for coupling gap of 0.5  0.1 m. This technique has good reproducibility in practice and we can extract most of the WGMs that can oscillate in the S with a suitable adjustment of the coupling gap.

Figure 4. Refractive index of the silica glass S with diameter of 38.5 m (a) and Hz component distribution confining inside and on the equatorial plane of the S for the WGM at  = 1549.01 nm (b).

For visualizing the modes and fields, the 3D-FDTD method combining with perfectly matched layers (PML) were implemented to simulate the radiation patterns of WGMs from S [13]. As mentioned above, the electric-dipole emitters located near the surface and oriented along the radials of

S, producing the ~20 fs sinusoidal pulses centered at emission wavelengths interact with S. Figures 4(a,b) show the WGM H-field distribution for the S with the diameter of 38.5 m and its refractive index of 1.44 in air at the wavelength 1549.01 nm, respectively. The estimated resonance by analytical

The 9^th International Workshop on Advanced Materials Science and Nanotechnology (IWAMSN 2018) – November 7^th-11^th, 2018 – Ninh Binh, Vietnam

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approach and simulation in our 2D model was found to be a reasonable agreement with that determined by fitting the WGM peaks in the experimental spectra in Figure 3.

In order to excite the WGM of the S, we propose an integration configuration based on the SOI platform. This structure consists of two slotted PhC waveguides coupling to the 38.5 m diameter S, one guides the pumped signal into the S while the other transports the output signal. The schematic view of the proposed configuration is shown in Figure 5(a). For serving the lasing behavior of the Er³⁺-S, the input slotted PhC waveguide has the function to guide the TE-polarized light of ~1470 nm and the output slotted PhC waveguide operates at ~1550 nm.

Figure 5. Schematic diagram of the intergration of the Er³⁺-S and the SOI slotted PhC waveguides (a), Projected band diagram for PhC structure and the E-field distribution in the two waveguides at  = 1470 nm

and  = 1549.01 nm (b).

The two slotted PhC waveguide dispersions were calculated by using 3D PWE method [20]. They were both conducted in the lattice constant a of 400 nm and the thickness of silicon slab of 220 nm. In this PhC platform, the waveguide width and the slot width were modified in order to generate the integrated waveguides with different properties on the same SOI chip. The waveguide width was enlarged from the original W1 (W1a 3). For the case of the input waveguide, the waveguide width was enlarged to 1.18W1 according with the slot width of 165 nm, while for the case of the output waveguide, a 1.25W1 waveguide width and the slot width of 125 nm was considered. Figure 5(b) depicts the dispersion diagram of the two input and output waveguides while the insets are E-field profiles at the edge wavelength of these slotted PhC waveguides. The E-fields are strongly localize in the waveguide area with low refractive index. These slotted PhC waveguides were designed considering air cladding in order to match with the S mentioned above.

The two slotted PhC waveguides were used to excite the WGMs inside the Er³⁺-S and to guide the resulting lasing signal out in the planar SOI PhC platform. The input slotted PhC wavegudie is able to excite the Er³⁺ ions at wavelength ~1470 nm while the output slotted PhC waveguide guides the lasing mode ~1549.01 nm as depicted in Figure 6, the E-field intensity at wavelength ~1470 nm and

~1549.01 nm at the access regions of input and output slotted PhC waveguides are shown, respectively. Although there is the mismatch between two different kinds of mode: Bloch-mode of the slotted PhC waveguide and the WGM of the S, this integrated system shows the possibility of coupling the Bloch-mode to the WGM. This configuration offers a compatible and mechanically robust integrated device in the silicon photonic domain and it can also enrich the diversity of on-chip silicon devices.

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Figure 6. E- field profile of the WGM at 1549.01 nm and insets are the access of input waveguide and output waveguide and the S at 1470 nm and 1549.01 nm, respectively.

4. Conclusion

In conclusion, base on the Er³⁺-S with diameters of ~30-40 m were fabricated, their WGM lasing emissions at telecom regime were quantitatively analyzed. In measurement, the coupling gap between the collection tapered fiber and the S surface results in the single- or the multi-emitted modes. The integration of a S with SOI slotted PhC waveguides was proposed and modelled with the help of 3D- FDTD simulations. It was shown that this compact structure works and a further research has to be processed for applications of ultrahigh sensitivity sensors, lasing sources and also towards the advanced quantum communications.

Acknowledgments

This research is funded by Vietnam Academy of Science and Technology (VAST) under grant numbers “VAST03.05/18-19” and “CSTX08.18”.

References

[1] Vahala K J 2003 Nature 424 839.

[2] Berman P R 1994 Cavity Quantum Electrodynamics (Academic Press, New York)

[3] Fürst J U, Strekalov D V, Elser D, Aiello A, Andersen U L, Marquardt C, Leuchs G 2010 Phys.

Rev. Lett. 105 263904.

[4] Asano T, Ochi Y, Takahashi Y, Kishimoto K, Noda S 2017 Opt. Exp. 25 18165.

[5] Toncelli A, Capuj N E, Garrido B, Sledzinska M, Sotomayor-Torres C M, Tredicucci A, Navarro- Urrios D 2017 J. Appl. Phys. 122 053101.

[6] Righini G C, Soria S 2016 Sensors 16 905.

[7] Del'Haye P, Arcizet O, Gorodetsky M L, Holzwarth R, Kippenberg T J 2009 Nat. Photon. 3 529.

[8] Gorodetsky M L, Ilchenko V S 1999 J. Opt. Soc. Am. B 16 147.

[9] Nozaki K, Kuramochi E, Shinya A, Notomi M 2014 Opt. Express 22 14263.

[10] Sibson P, Kennard J E, Stanisic S, Erven C, O‟Brien J L, Thompson M G 2017Optica 4 172.

[11] Xu K 2016 IEEE Sens. J. 16 6184.

[12] Makino S, Ishizaka Y, Saitoh K, Koshiba M 2013 IEEE Photon. J. 5 2700309.

[13] Taflove A 1995 Computational Electrodynamics (Artech House, Boston).

[14] Little B E, Laine J -P, Haus H A 1999 J. Lightwave Technol. 17 704.

[15] Spillane S M, Kippenberg T J, Painter O J, Vahala K J 2003 Phys. Rev. Lett. 91 043902.

[16] Joannopoulos J D, Johnson S G, Meade R D, Winn J N 2008 Photonic Crystals: Molding the Flow of Light (Second edition, Princeton Univ. Press).

[17] Pham V H, Bui H, Pham T S, Nguyen T A, Nguyen T V, Le H T, Bui T N, Nguyen V P, Coisson

The 9^th International Workshop on Advanced Materials Science and Nanotechnology (IWAMSN 2018) – November 7^th-11^th, 2018 – Ninh Binh, Vietnam

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R 2013 J. Opt. Soc. Am. B 30 1586.

[18] Lissillour F, Messager D, Stéphan G, Féron P 2001 Opt. Lett. 26 1051.

[19] Dong C, Xiao Y, Yang Y, Han Z, Guo G, Yang L 2008 Opt. Lett. 6 300.

[20] Johnson S G, Joannopoulos J D 2001 Opt. Exp. 8 173.