Summary of Science materials doctoral thesis: Investigation of lasing emission effect and optical amplification in the cavity conjuncted with 1D, 2D photonic crystal structures

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The objectives of the thesis: Research, fabricate Er3+-doped silica glass microspheres with different sizes by arc discharge method; building an experimental system to investigate WGM mode laser emission spectrum in the optical communication wavelength region ~ 1550 nm of some fabricated microspheres.

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Nội dung Text: Summary of Science materials doctoral thesis: Investigation of lasing emission effect and optical amplification in the cavity conjuncted with 1D, 2D photonic crystal structures

MINISTRY OF EDUCATION VIETNAM ACADEMY AND TRAINING OF SCIENCE AND TECHNOLOGY GRADUATE UNIVERSITY SCIENCE AND TECHNOLOGY ----------------------------------- Nguyen Van An INVESTIGATION OF LASING EMISSION EFFECT AND OPTICAL AMPLIFICATION IN THE CAVITY CONJUNCTED WITH 1D, 2D PHOTONIC CRYSTAL STRUCTURES Major: Materials for Optics Optoelectronics and Photonics Code: 9 44 01 27 SUMMARY OF SCIENCE MATERIALS DOCTORAL THESIS Hanoi – 2020
The thesis was completed at: Graduate University of Science and Technology - Vietnam Academy of Science and Technology. Supervisor 1: Assoc. Prof. Dr. Ngo Quang Minh Supervisor 2: Assoc. Prof. Dr. Pham Van Hoi Reviewer 1: … Reviewer 2: … Reviewer 3: …. The thesis will be defended at Graduate University of Science and Technology - Vietnam Academy of Science and Technology at ……, …..........., 202…. The thesis could be found at: - Library of Graduate University of Science and Technology - National Library of Vietnam
1 INTRODUCTION 1. The urgency of the thesis The transmission of electromagnetic waves inside 1D-PhC was first studied by Lord Rayleigh in 1887. In 1987, the research works about 3D-PhC related to effect of the random emission prohibition in PhC due to existence of PBG was published and proposed by E. Yablonovitch and S. John [1,2]. From that, PhC has attracted the special attention of the researchers in the field of using the new structures based on PhC to conduct, transmit and control electromagnetic waves as well as use electromagnetic waves to information processing. The 2D-PhC controlled waveguide properties, convergence on 2D surface and 1D-PhC fibers for single mode lasers with high output power weve proposed by Birks in 1997 [3]. Until 1999, after O. Painter’s team succeeded in fabricating 2D- PhC lasers with directional feedback distribution effects in the 2D- PhC lattice structure [4], lasers based on photonic materials have been promoted in research in photonic laboratories around the world. The PhC’s cavities with different quasiperiodic lattices have been studied by many groups by theoretical calculation, simulation and experiment to fabricate them [5,6]. The 3D-PhC structure can control the waveguide in 3D space, therefore it has great applicability in micro-optical integrated circuits and ultra-low emission threshold laser, the first 3D-PhC was fabricated by E. Yablonovitch in 1991 based on diamond nanoparticles distributed in organic glass lattice [7]. The micro-resonant cavities with Whispering-Gallery-Mode (WGM) give a (Q) high quality factor and small mode volume so they have been used to decrease the laser emission threshold and other nonlinear optical effects [8-10]. In addition, the micro-resonant
2 cavities with WGM have also been widely used for many fields such as quantum optic, quantum electrodynamics resonance and narrow spectrum laser [11-13]. The microspherical laser with the size from a few microns to several tens of microns is one of the photonic device subjects that is most interested in research due to pump light and emission laser have been strongly held in microsphere thanks to the total reflection on inside surface of device, so ultra-low laser emission threshold and ultra-narrow laser emission spectrum. The optical micro-resonant lasers with features such as ultra-low laser emission threshold, ultra-narrow spectral width, controlled the number of emission modes depending on the structure of the micro- resonance and the techniques to collect the emission signals from micro-resonant cavity have become development research subjects of photonic sensors for biochemical and environment with ultra-high sensitivity [14-18]. Now, the research on optical micro-resonant lasers in general and microspherical lasers, in particular, has given a lot of new information on photonic physics and they are still very active research subjects in the world [19-21]. In general, the research and fabrication of nano and micro- photonic materials and devices in recent years in Vietnam has been achieved many important results. The laboratory for fiber optic applications and materials belong to Institute of Materials Science has been successfully fabricated 1D-PhC structure and has international publications as well as a number of PhDs successfully defended their thesis about this research content including experimental fabrication and related applications [22,23]; the photonic sensors based on wavelength selection in 1D-PhC structure fabricated from porous silicon multi-layer membranes were initially
3 applied in equipments for measuring biochemical environments [23,24]; the fiber optic devices with wavelength selection structure based on FBG have been researched and developed for optical communication networks and optical sensors [25,26],… With the 3D-PhC structure, the researchers have also successfully fabricated spherical micro-resonant cavity lasers based on Er3+-doped silica- alumina glass emitting 1550 nm region WGM modes for optical communication and the visible region applied to sensors with rather strong intensity, ultra-narrow spectrum width and controlled the number of emission modes from micro-resonant cavity [27,28]. Besides, there are some research works on photonic devices based on 2D-PhC by simulation have achieved very positive results [29-32] and opened up new research directions on photonic devices, including optical amplification by experimental method combined with simulation calculation. On the basis of the PhC research results, coupling the micro- resonant cavity with the PhC structure for laser emission is necessary direction to demonstrate high orientation in integrated photonic devices fabricated technology. To continue developing the research direction of nano and micro-meter photonic structure, towards the application in optical communication and sensors, we choose the thesis topic with title: “Investigation of lasing emission effect and optical amplification in the cavity conjuncted with 1D, 2D photonic crystal structures”. 2. The objectives of the thesis - Research, fabricate Er3+-doped silica glass microspheres with different sizes by arc discharge method; building an experimental system to investigate WGM mode laser emission spectrum in the
4 optical communication wavelength region ~ 1550 nm of some fabricated microspheres. - Design and simulation of integrated structure of silica glass microsphere with 2D-PhC waveguide based on SOI material to study on WGM mode laser emission effect in the optical communication wavelength region ~ 1550 nm. - Design and construct a sensor system for refractive index measurement of some liquids using the FBGs that are integrated into the fiber ring laser configuration without using spectrometer. Research method The thesis uses both simulated and experimental calculation methods. The simulated calculation method was used to determine PBG, waveguided mode, guided-mode resonance, WGM mode and field distribution in the structure of PhC. The experimental method was used to fabricate microspheres, FBG, design and construct laser emission spectrometry system of Er3+-doped silica microspheres coupled with pump source and receiver by the single-mode optical tapered fibers, design and construct liquid sensor system based on FBG integrated in fiber ring laser configuration. 3. The main contents of the thesis: - Overview of photonic crystal and its application in research on laser fabrication. - The research methods. - Calculate and simulate some optical devices using two- dimensional photonic crystal structure. - Study on laser emission effect of microspheres based on Er3+- doped silica glass, photonic devices and its application. Thesis layout
5 The thesis includes the introduction, four chapters of content and the general conclusion. The main contents of the thesis is presented in four chapters. The first chapter presents the concepts, research situation of PhC structure and its application. The second chapter introduces the research methods, including theoretical models of the waveguide - resonator coupling, simulated calculation and experiment. The third chapter presents the results of calculation and simulation of some optical devices using 2D-PhC structure. The fourth chapter presents the results of the emission of microspherical lasers on Er3+-doped silica glass, simulation of the integrated structure of Er3+-doped silica microsphere with 2D-PhC waveguide and some test results of liquid refractive index measurement using PBG elements integrated in fiber ring laser configuration. CHAPTER 1. OVERVIEW OF PHOTONIC CRYSTAL AND ITS APPLICATION IN RESEARCH ON LASER FABRICATION - Introduction to the photonic crystal structure. - Presenting features of 2D-PhC such as photonic band gap, waveguided and wave confinement, guided-mode resonance. - Presentation of the optical processes in spherical micro- resonance cavities: WGM mode of dielectric microspheres, equations of state for modes and microsphere - waveguide coupling methods. - Apply the 1D-PhC structure in fiber optic (FBG) to developing optical sensor. CHAPTER 2. THE RESEARCH METHODS 2.1. The theoretical models of the resonator-waveguide coupling 2.1.1. The theory of resonator - waveguide coupling Mode coupling method has been applied to many physical
6 systems to process the resonant modes or propagating modes. We can choose a simple LC circuit to illustrate the significance of the related physical parameters [94]. - If the loss is small, then: da/dt = joa – (1/o)a (2.10) where a, 1/o are the mode amplitude and the decay rate due to the loss, respectively. - When the resonator is coupled to an external waveguide, due to escaping into the waveguide, equation (2.10) must be modified: da/dt = joa – (1/o + 1/e)a (2.15) where 1/e expresses the additional rate of decay due to escaping power. - In case, the waveguide carries a wave traveling toward the resonator of amplitude s+ due to a source, there will be a coupling of waveguide and resonator so (2.15) must be written: da/dt = joa – (1/o + 1/e)a + ks+ (2.19) where k is a coefficient expressing the degree of coupling between the resonator and the wave s+. We normalize s+ so that 2 s = power carried by incident wave; here s+ to designate a wave incident upon the resonator; the reflected wave will be denoted by s-, respectively. - If the source is at frequency , then the response is at the same frequency, from (2.19), that: ja = joa – (1/o + 1/e)a + ks+ from then a = ks+ /[j(-o) + (1/o + 1/e)] (2.20) The relationship between k và e are given by: k  2 /e (2.28) From (2.19), we have:
7 da/dt = joa – (1/o + 1/e)a + 2 /  e s+ (2.29) (2.29) is the equation describing excitation of the resonator mode by an incident wave. 2.1.2. The coupling of a micro-resonator - two waveguides The simple model is shown in Figure 2.4. U, o are the amplitude and frequency of the resonator mode excited in the resonator, respectively. The resonator mode couples to two waveguides () and () and obeys the equation [96]: dU/dt=joU–(1/e +1/e +1/o)U + 2 /  e a1 + 2 /  e a4 (2.30) where a1 and a4 are the incident waves in the two waveguides, 2 2 normalized so that a1 and a4 are equal to the incident power in the two waveguides; 1/  e and 1/  e are the coupling ratios between the micro-resonator with the two waveguides () and (), respectively; 1/0 is the decay rate due to the loss (radiation and other losses combined). The resonant mode U couples back into the outgoing waves in the waveguides in the clockwises direction: b2  a1  2 /  e U (2.31) b3  a4  2 /  e U (2.32) Figure 2.4. Coupled-mode model of resonator with two waveguides 2.1.3. The coupling of the micro-resonator - waveguide when considering backscattering The coupling between a waveguide and a micro-resonant cavity
8 when considering backscattering can be illustrated in Figure 2.5. Figure 2.5. Schematic of the coupling of a micro-resonator with a waveguide when considering backscattering The equations of motion for counter-propagating modes (CCW and CW) that are coupled to one another as well as to a waveguide mode can be described by the couple-mode equations similar to those presented in [96,97]: dacw/dt = j.acw - (1/2)(1/e + 1/o)acw + (j/2)accw + k.s (2.33) daccw/dt = j.accw - (1/2)(1/e + 1/o)accw + (j/2)acw (2.34) here acw and accw are the amplitude of the clockwise and counterclockwise modes of the resonator, respectively; s denotes the input wave, which is selected to excite the CW mode; the scattering rate 1/ describers the mutual coupling of the CW and CCW mode. 2.2. The simulation calculation method 2.2.1. The finite-difference time-domain method (FDTD) FDTD is a method of directly solving the system of Maxwell equations in the time-domain [117,118]. The relationship between the time steps of the FDTD method is as follows: at any point in  space, the next value of the electric field E over time depends on the  value of the previous electric field E and the numerical rota of the  local distribution of the magnetic field H in space [117]. Similarly,  for the time-step progression of the magnetic H . K. Yee proposed the “leap-frog” leap-frogging scheme for the
9   progression over time of E and H . The computational processes   for E and H are illustrated by the flowchart in Figure 2.7. The   relationship of E and H calculation is as follows:  - Calculate the components of E at a point in space at the time n t .  - Calculate the components of H at that point at the next moment  n  1/ 2  t .   Figure 2.7. Flowchart illustrating the E and H calculation procedures at different times in space With the “leap-frog” algorithm proposed by K. Yee, the E-field value in the space at the specified time is calculated according to the previous electric field value and its four adjacent magnetic field values. The same goes for calculating the value of the magnetic field. 2.2.2. The plane wave expansion methode (PWE) The PWE method has simple manipulation; it is used in the studies of PhC structure [121-123]. The PWE method allows solving the complete wave vector equation of the electromagnetic field, calculates eigenfrequency with standard accuracy and suitable timing, It can be used to calculate the energy band structure of the PhC structure, the transmission spectrum [121,124,125],… 2.2.3. The Boundary conditions and convergence of the algorithm There are many different boundary conditions, but the two basic
10 types mentioned are Bloch periodic boundaries and perfectly matched layers PML. The periodic boundary conditions are useful in periodic systems. For periodic boundaries, in a cell of size L, the field components satisfy f(x + L)= f(x) . To simulate open boundary conditions, we need the boundaries to absorb all the waves towards them without reflection. This is done by the PML. 2.3. Fabrication method for silica glass microspheres and FBG 2.3.1. Fabrication of microspheres by arc discharge method The silica and Er3+-doped silica microspheres have been fabricated by us by the arc discharge method on standard telecom fiber and Er3+-doped fiber according to the process: - Peel off the coating at the start of the fiber-optic to a length of  1.0 cm. - Using HF solution to chemically etch peeled optical fiber head with a length of  0.4 cm. - Arc discharge at the start of the optical fiber has been etched. 2.3.2. Fabrication of FBG using photolithography technique Figure 2.9. Diagram of fabrication principle of FBG by interfering mirror system In this thesis, we present only method to fabricated FBG by interferometer system; this is method that we used to make FBG. The
11 wavelength of the UV beam we used is UV = 248 nm and the optical fiber with the SiO2 core is highly doped with GeO2 photosensitive material (14% to 20%). When illuminating UV beam at a certain location of the optical fiber, the structure of GeO2 there is broken. The region that receives high UV intensity, the refractive index increases; the region that receives low UV intensity, the refractive index remains; on that basis, we obtained the FBG structure. Diagram of fabrication principle of FBG is illustrated in Figure 2.9. 2.4. Some experimental configurations to survey laser emission spectra This section presents experimental configurations to survey laser emission spectra based on the coupling of Er3+-doped silica microsphere with the tapered fibers and fiber laser configuration of the liquid sensor system using e-FPG. 2.5. Scanning Electron Microscope (SEM) This section presents the meaning of the SEM method and the general operating principle of the SEM machines. CHAPTER 3. CALCULATE AND SIMULATE SOME THE OPTICAL DEVICES USING 2D-PhC STRUCTURE 3.1. The photonic band gap of 2D-PhC slab structure The structure of the 2D-PhC waveguide is modelled as shown in Figure 3.1: 2D-PhC triangular lattice structure with lattice constant a of air holes of radius r, depth h =220 nm is designed on the dielectric background Si with thickness d = 220 nm and refractive index n1 = 3.48; this lattice is placed on SiO2 substrate with refractive index n2 = 1.44. The PBG simulation was performed using 3D-PWE method, PML boundary conditions are placed above and below the slab (parallel to the structural surface), Bloch periodic boundary
12 conditions are applied according to the periodic directions of the structure, the resolution to perform the simulation is 10 nm. Figure 3.1. The triangular lattice structure of the 2D-PhC slab with lattice constant a of cylindrical air holes of radius r, depth h is designed on the dielectric background Si with thickness d = h = 220 nm In the case of a = 400 nm, r = 100 nm, the result is shown in Figure 3.2. Figure 3.2. Photonic band structure for the 2D-PhC slab even mode Figure 3.2 shows the existence of a complete PBG with even mode and wavelengths in the range from  1369 nm to  1607 nm corresponding to normalized frequencies 0, 2922( a / 2 c ) and 0, 2489( a / 2 c ) . The selected structure has PBG containing wavelengths 1470nm and 1550nm, that means this structure can be used to fabricate waveguide channels with wavelengths 1470nm and 1550nm.
13 3.2. Waveguide in plane using 2D-PhC slab structure Using 2D-PhC slab structure in Figure 3.1 with the selected parameters: a = 400 nm, h = d = 220 nm, r = 100 nm. The PML boundary condition was used and placed around the structure, the resolution to perform the simulation is 10 nm. The source is placed at the input waveguide and behind PML layer, the receiver is placed around the structure and in the PML layer. 3.2.1. W1 waveguide and field distribution in waveguide W1 waveguides are created by filling a row of air holes of the structure as shown in Figure 3.1. To extend the band [132], we reduce the radius of two adjacent rows of air holes with waveguide W1 from r = 100 nm to r1 = 95 nm. Figure 3.7. Dispersion diagram and electric field distribution of the Ey component in the waveguide at wavelength  = 1550 nm By using 3D-PWE method, we get the dispersion diagram, the electric field distribution of the Ey component in the structure at wavelength  = 1550 nm as shown in Figure 3.7. The simulation results show that the selected 2D-PhC structure that conduct wave is good with wavelength  = 1550 nm. 3.2.2. Slotted waveguide and field distribution in waveguide The model of the slotted 2D-PhC waveguide structure is shown in Figure 3.8. The dispersion diagram, the electric field distribution in the
14 structure corresponding to wavelengths  = 1470 nm,  = 1550 nm and the refractive index distribution of the structure was simulated by the 3D-PWE method. Simulation results for the case w = 165 nm, W = 1.18 W1 and w = 125 nm, W = 1.25 W1 are shown in Figure 3.9 and Figure 3.10, respectively. The results showed that the 2D-PhC with the selected parameters has good wave conductivity with  = 1470 nm and 1550 nm. Figure 3.8. Slotted 2D-PhC waveguide structure of triangular lattice Figure 3.9. Dispersion diagram, E-field distribution of the Ey component in the waveguide at  = 1470 nm and refractive index distribution of the structure Figure 3.10. Dispersion diagram, E-field distribution of the Ey component in the waveguide at  = 1550 nm and refractive index distribution of the structure
15 3.3. The optical wave filter based on GMR effect To test and evaluate the wavelength selection of the 2D-PhC slab structure, we perform the investigation, calculation and simulation of the optical wave filters using 2D-PhC slab based on the GMR effect. The characteristics parameters for filter such as resonance wavelength 0, quality factor Q… are determined indirectly through using characteristics expression of the Fano spectrum. The simulation results for GMR spectra, field distributions is given by a single lattice structure and two types of dual lattice structures are presented in detail in this section. CHAPTER 4. LASER EMISSION OF MICROSPHERE BASED ON Er3+-DOPED SILICA, PHOTONIC DEVICE AND ITS APPLICATION 4.1. Fabrication results of Er3+-doped silica microsphere Figure 4.3. SEM images of Er3+-doped silica glass microsphere Figure 4.3 shows SEM images of some Er3+-doped silica glass microspheres that we fabricated by arc discharged method. 4.2. Emission spectrum of Er3+-doped silica microsphere laser The emission spectrum of Er3+-doped silica microsphere laser with diameter ∼ 29.7 μm obtained from experiment with a number of different coupling gaps according to two configurations CW and CCW are shown in Figure 4.12-4.14.
16 Figure 4.12. WGM mode emission spectra extracted from Er3+-doped silica microsphere: coupling gap  1.5  0.1 m according to CW configuration Figure 4.13. WGM mode emission spectra extracted from Er3+-doped silica microsphere: coupling gap  1.5  0.1 m according to CCW configuration Figure 4.14. WGM mode emission spectra depends on the coupling gap according to the CW configuration 4.3. Simulate the WGM mode of silica microspheres 4.3.1. WGM mode of microsphere with diameter of 38.5 m This section presents some simulation results of WGM mode of
17 silica microsphere with diameter of 38.5 m on the equatorial plane of the microsphere. 4.3.2. WGM mode of microsphere with diameter of 29.7 m Figure 4.16 shows some simulation results of WGM mode of silica microsphere with diameter of 29.7 m on the equatorial plane of the microsphere. Figure 4.16. Reflection spectrum on the surface of the microsphere (a), E-field magnitude distribution of WGM mode at  = 1551.53 nm with TM mode (b), field distribution of the EZ component at  = 1551.53 nm with TM mode (c) and HZ component at  = 1550.82 nm with TE mode (d) 4.3.3. Calculate quantum mode numbers using numerical method Table 4.1 presents the results about sets of values (l, n) that characterize WGM modes distributed on the equatorial plane of two silica microspheres is calculated by numerical method using approximate expressions from (1.40) to (1.44) [87] and 3D-FDTD simulation method.
18 The Table 4.1 shows that there is a good suitability between one of quantum value sets (l, n) when calculating numerically with set of values (l, n) is determined by 3D-FDFD simulation method. Table 4.1. Sets of values (l, n) are calculated using two different methods Diameter Resonance Numerical 3D-FDTD of S Polarization wavelength method simulation (m) (nm) (104, 1), (98, 2), (92, 3) 38.5 TM 1550.74 (80, 6) (88, 4), (84, 5), (80, 6) (105, 1), (99, 2), (93, 3) 38.5 TE 1549.01 (85, 5) (88, 4), (84, 5), (80, 6) (79, 1), (73, 2), 29.7 TM 1551.53 (68, 3) (64, 4) (64, 4), (61, 5) (80, 1), (74, 2), 29.7 TE 1550.82 (69, 3) (66, 4) (65, 4), (61,5) 4.4. Integrated photonic device based on the coupling of the microsphere with the SOI slotted 2D-PhC waveguides 4.4.1. Design proposal Figure 4.17. Schematic diagram of the intergration of the Er3+-doped silica microsphere and the two SOI slotted PhC waveguides The integrated structure consists of two SOI slotted PhC