Three-dimensional coupled-wave analysis for square-lattice photonic crystal surface emitting lasers with transverse-electric polarization: finite-size effects

Yong Liang; Chao Peng; Kyosuke Sakai; Seita Iwahashi; Susumu Noda

doi:10.1364/OE.20.015945

1. Introduction

Two-dimensional (2D) photonic-crystal surface-emitting lasers (PC-SELs) are becoming increasingly important due to their promising functionality and improved performance compared to conventional semiconductor lasers. A number of successful demonstrations underpinning this promise have already been made [1–13]. By utilizing the band edge of the photonic band structure, single longitudinal and transverse mode oscillation in two dimensions has been achieved with a large lasing area, enabling high-power, single-mode operation [1, 5]. The output beam of such devices is emitted in the direction normal to the 2D PC plane and has a small beam divergence angle of less than 1° due to the large area of coherent oscillation [5, 6]. Furthermore, both the polarization and pattern of the output beam can be controlled by appropriate design of the PC geometry [2,6]. Recent developments of 2D PC lasers have allowed the lasing wavelength to be extended from the near-infrared regime to the mid-infrared, terahertz, and blue-violet regimes [7–12]. In addition, we have recently demonstrated the operation of a PC-SEL with entirely new functionality: on-chip dynamical control of the emitted beam direction, achieved by using a composite PC structure [13].

Despite the experimental advances that have recently been made in the field of 2D PC-SELs, theoretical studies on these types of lasers have thus far been limited. An important but unresolved issue concerns the mechanisms by which the PC structure determines the output characteristics of the device, thereby limiting progress in optimizing the structural design of devices. Computer simulations based on the 2D plane-wave expansion method (PWEM) [3, 14] or the finite-difference time-domain (FDTD) method [15–17], can provide valuable information about the lasing properties of the PC laser cavity. However, there are some inherent limitations to these computational approaches. The 2D PWEM is only applicable to infinite structures, and the FDTD method requires substantial computational resources in order to model finite structures with realistically large areas. A coupled-wave theory (CWT) approach developed by Kogelnik and Shank [18] can circumvent such limitations. However, extending this approach to accurately model PC-SELs is a rather challenging task, since realistic PC-SELs require three-dimensional (3D) analysis. Very recently, we developed a 3D CWT model that affords an exact analytical treatment of the full 3D structure of typical laser devices and achieves a very accurate and efficient analysis of the surface emission properties. This theory incorporates the key issues in modeling surface-emitting-type PC lasers, i.e. the surface emission in the surface normal direction and the in-plane higher-order coupling effects [22], neither of which was appropriately described in the previous works [4, 19–21].

Nevertheless, in our previous study, we restricted our analyses to infinite periodic structures for simplicity [22]. To predict and improve the performance of the practical PC-SEL device, however, it is essential to consider a finite-size structure. For example, following the arguments of our previous study, two antisymmetric band-edge modes of PCs with circular air holes should be excited simultaneously without emitting a laser beam because both of their radiation constants (i.e. parameters quantifying the surface radiation loss) are shown to be zero. This is in contrast to the experiments where a laser with a doughnut-shaped beam pattern operating stably at one of the two antisymmetric modes was demonstrated [6, 23, 24]. The physical mechanism of this lasing behavior cannot be explained without considering the finite-size effects of the laser device, as we will show later on. This is our motivation for the present work, where we develop a 3D CWT model capable of treating the finite-size laser device by extending our previous work [22]. Our objective in this paper is not to exhaustively quantify the dependence on the large number of parameters that determine device behavior. Rather, we focus on clarifying the underlying physical mechanism of the effects caused by finiteness of the device.

The remainder of this paper is organized as follows. Section 2 describes derivations of the coupled-wave equations for finite systems. Section 3 presents analysis results and discusses the finite-size effects. Section 4 concludes with our findings.

2. Coupled-wave equations for finite-size structures

In this section, we will present the derivations of the coupled-wave equations for finite systems. We shall not give a complete description of the formulation, but highlight only the major differences from our previous study. For a more detailed derivation and discussion, please refer to our previous paper [22].

The schematic structure of PC-SELs is shown in Fig. 1(a), in which the electric field can be expressed as E(r, t) = E(r)e^iωt. By eliminating the magnetic field from Maxwell’s equations, we obtain

\nabla \times \nabla \times E (r) = k_{0}^{2} {\tilde{n}}^{2} (r) E (r),

where k₀(= ω/c) is the free-space wavenumber, ω is the angular frequency, c is the velocity of light in free space and ñ is the refractive index (a complex number) satisfying

k_{0}^{2} {\tilde{n}}^{2} (r) ≃ k_{0}^{2} n^{2} (r) + 2 i k_{0} n_{0} (z) \tilde{α} (z)

[22,25], where n²(r) is a periodic function n₀(z) represents the average refractive index of the material at position z, and α̃(z) represents the gain (α̃ > 0) or loss (α̃ < 0) in each region. Here, we still focus our analysis on the transverse electric (TE) polarization because the lasing mode has been identified as being a TE mode [23]. For TE polarization, E(r) = (E_x(r), E_y(r), 0) can be expanded according to Bloch’s theorem

E_{j} (r) = \sum_{m, n} E_{j, m, n} (x, y, z) e^{- im β_{0} x - in β_{0} y}, j = x, y,

and the periodic function n²(r) can be expanded as

n^{2} (r) = n_{0}^{2} (z) + \sum_{m \neq 0, n \neq 0} ξ_{m, n} (z) e^{- im β_{0} x - in β_{0} y} .

Here, β₀ = 2π/a, a is the lattice constant, m, n are arbitrary integers, and ξ_m,n(z) is the high-order Fourier coefficient term. We note that ξ_m,n(z) is zero outside the PC region. For simplicity, we assume that air holes within the PC region have perfectly vertical sidewalls (tilted case is discussed in Ref. [26]) such that

n_{0}^{2} (z)

represents the average dielectric constant of PC and that ξ_m,n is independent of z within the PC region. By substituting Eqs. (2) and (3) into Eq. (1) and collecting all terms that are multiplied by the factor e^{−imβ₀x−inβ₀y}, we obtain

\begin{matrix} [\frac{\partial^{2}}{\partial z^{2}} + k_{0}^{2} n_{0}^{2} (z) + 2 i k_{0} n_{0} (z) \tilde{α} (z) - n^{2} β_{0}^{2}] E_{x, m, n} & - & 2 in β_{0} \frac{\partial E_{x, m, n}}{\partial y} + \frac{\partial^{2} E_{x, m, n}}{\partial y^{2}} + m n β_{0}^{2} E_{y, m, n} \\ - \frac{\partial^{2} E_{y, m, n}}{\partial x \partial y} + i β_{0} (m \frac{\partial E_{y, m, n}}{\partial y} + n \frac{\partial E_{y, m, n}}{\partial x}) & = & - k_{0}^{2} \sum_{m^{'} \neq m, n^{'} \neq n} ξ_{m - m^{'}, n - n^{'}} E_{x, m^{'}, n^{'}}, \end{matrix}

\begin{matrix} [\frac{\partial^{2}}{\partial z^{2}} + k_{0}^{2} n_{0}^{2} (z) + 2 i k_{0} n_{0} (z) \tilde{α} (z) - m^{2} β_{0}^{2}] E_{y, m, n} & - & 2 im β_{0} \frac{\partial E_{y, m, n}}{\partial x} + \frac{\partial^{2} E_{y, m, n}}{\partial x^{2}} + m n β_{0}^{2} E_{x, m, n} \\ - \frac{\partial^{2} E_{x, m, n}}{\partial x \partial y} + i β_{0} (m \frac{\partial E_{x, m, n}}{\partial y} + n \frac{\partial E_{x, m, n}}{\partial x}) & = & - k_{0}^{2} \sum_{m^{'} \neq m, n^{'} \neq n} ξ_{m - m^{'}, n - n^{'}} E_{y, m^{'}, n^{'}}, \end{matrix}

\frac{\partial}{\partial z} [(\frac{\partial E_{x, m, n}}{\partial x} + \frac{\partial E_{y, m, n}}{\partial y}) - i β_{0} (m E_{x, m n} + n E_{y, m n})] = 0.

Here, as we are considering a finite system, we have retained the spatial derivative terms of E_x and E_y with respect to x and y. As defined previously [22], the wavevectors can be classified into three groups according to their in-plane wavenumber,

\sqrt{m^{2} + n^{2}} β_{0}

: basic waves (

\sqrt{m^{2} + n^{2}} = 1

), high-order waves (

\sqrt{m^{2} + n^{2}} > 1

), and radiative waves (m = n = 0). By assuming a separable form of solutions for the fields, we can solve Eqs. (4)–(6) analytically. In the resonant case at the second-order Γ point shown in Fig. 1(b), the four basic waves can be expressed as [22]

E_{x, 1, 0} = 0, E_{y, 1, 0} = R_{x} (x, y) Θ_{0} (z),

E_{x, - 1, 0} = 0, E_{y, - 1, 0} = S_{x} (x, y) Θ_{0} (z),

E_{x, 0, 1} = R_{y} (x, y) Θ_{0} (z), E_{y, 0, 1} = 0.

E_{x, 0, - 1} = S_{y} (x, y) Θ_{0} (z), E_{y, 0, - 1} = 0 .

Here, R_x(x, y) and S_x(x, y) represent the amplitudes of basic waves propagating in the +x and −x directions, respectively, and likewise, R_y(x, y) and S_y(x, y) represent the amplitudes of waves propagating in the +y and −y directions, respectively. These four basic waves are assumed to have identical field profiles in the z-direction, denoted by Θ₀(z), which is the same as the field profile of the fundamental guided mode for a multilayer structure without PCs [25]. We express the wave equation for the fundamental guided mode in terms of Θ₀(z) as

\frac{\partial^{2} Θ_{0}}{\partial z^{2}} + [k_{0}^{2} n_{0}^{2} (z) - β^{2}] Θ_{0} = 0,

where β is the propagation constant, which satisfies β ≃ β₀ in the vicinity of the second-order Γ point. In this work, we calculate β and Θ₀(z) in Eq. (11) by employing the transfer matrix method [27] and normalize Θ₀(z) as

\int_{- \infty}^{\infty} {| Θ_{0} (z) |}^{2} d z = 1

.

Fig. 1 (a) Schematic structure of square-lattice PC-SEL device with circular (CC) and equilateral triangular (ET) air holes (inset: scanning electron microscope images). (b) A typical photonic-band structure calculated by 2D-PWEM. There exist four band-edge modes in the vicinity of the second-order Γ point, which we refer to as modes A, B, C and D, in order of increasing frequency.

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In order to obtain the coupled-wave equations, Eqs. (7)–(10) are substituted into Eqs. (4)–(6) for (m, n) = {(1, 0), (−1, 0), (0, 1), (0, −1)}. Here, without loss of generality, we focus on the case for which (m, n) = (1, 0). We then only need to consider Eqs. (7) and (5). Substitution of Eq. (7) into Eq. (5) gives

\begin{array}{r} [\frac{\partial^{2} Θ_{0}}{\partial z^{2}} + (k_{0}^{2} n_{0}^{2} (z) + 2 i k_{0} n_{0} (z) \tilde{α} (z) - β_{0}^{2}) Θ_{0}] R_{x} - 2 i β_{0} \frac{\partial R_{x}}{\partial x} Θ_{0} \\ = - k_{0}^{2} \sum_{m^{'} \neq 1, n^{'} \neq 0} ξ_{1 - m^{'}, - n^{'}} E_{y, m^{'}, n^{'}} . \end{array}

Here, the basic wave R_x is assumed to vary slowly compared to exp(−iβ₀x) so that its second spatial derivative terms in Eq. (5) can be neglected. Then Eq. (11) is substituted into Eq. (12) to yield

(β^{2} - β_{0}^{2}) R_{x} Θ_{0} + 2 i k_{0} n_{0} (z) \tilde{α} (z) R_{x} Θ_{0} - 2 i β_{0} \frac{\partial R_{x}}{\partial x} Θ_{0} = - k_{0}^{2} \sum_{m^{'} \neq 1, n^{'} \neq 0} ξ_{1 - m^{'}, - n^{'}} E_{y, m^{'}, n^{'}} .

The term E_y,m′,n′ on the right-hand side of Eq. (13) represents all the waves that may couple to R_x, including basic, high-order, and radiative waves as described above. Specifically, we express the radiative waves [for which (m′, n′) = (0, 0)] as

E_{x, 0, 0} = Δ E_{x} (z), E_{y, 0, 0} = Δ E_{y} (z) .

Finally, we can obtain the coupled-wave equation for (m, n) = (1, 0) by multiplying Eq. (13) by $Θ_{0}^{*} (z)$ on both sides and integrating over (−∞, ∞) along the z direction. Three more coupled-wave equations for (m, n) = {(−1, 0), (0, 1), (0, −1)} can be derived in analogous fashion. We write the four coupled-wave equations in the following form:

\begin{array}{l} - i \frac{\partial R_{x}}{\partial x} + (δ + i α) R_{x} = & κ_{2, 0} S_{x} - \frac{k_{0}^{2}}{2 β_{0}} ξ_{1, 0} \int_{P C} Δ E_{y} (z) Θ_{0}^{*} (z) d z \\ - \frac{k_{0}^{2}}{2 β_{0}} \sum_{\sqrt{m^{2} + n^{2}} > 1} ξ_{1 - m, - n} \int_{P C} E_{y, m, n} (z) Θ_{0}^{*} (z) d z, \end{array}

\begin{array}{l} i \frac{\partial S_{x}}{\partial x} + (δ + i α) S_{x} = & κ_{- 2, 0} R_{x} - \frac{k_{0}^{2}}{2 β_{0}} ξ_{- 1, 0} \int_{P C} Δ E_{y} (z) Θ_{0}^{*} (z) d z \\ - \frac{k_{0}^{2}}{2 β_{0}} \sum_{\sqrt{m^{2} + n^{2}} > 1} ξ_{- 1 - m, - n} \int_{P C} E_{y, m, n} (z) Θ_{0}^{*} (z) d z, \end{array}

\begin{array}{l} - i \frac{\partial R_{y}}{\partial y} + (δ + i α) R_{y} = & κ_{0, 2} S_{y} - \frac{k_{0}^{2}}{2 β_{0}} ξ_{0, 1} \int_{P C} Δ E_{x} (z) Θ_{0}^{*} (z) d z \\ - \frac{k_{0}^{2}}{2 β_{0}} \sum_{\sqrt{m^{2} + n^{2}} > 1} ξ_{- m, 1 - n} \int_{P C} E_{x, m, n} (z) Θ_{0}^{*} (z) d z, \end{array}

\begin{array}{l} i \frac{\partial S_{y}}{\partial y} + (δ + i α) S_{y} = & κ_{0, - 2} R_{y} - \frac{k_{0}^{2}}{2 β_{0}} ξ_{0, - 1} \int_{P C} Δ E_{x} (z) Θ_{0}^{*} (z) d z \\ - \frac{k_{0}^{2}}{2 β_{0}} \sum_{\sqrt{m^{2} + n^{2}} > 1} ξ_{- m, - 1 - n} \int_{P C} E_{x, m, n} (z) Θ_{0}^{*} (z) d z . \end{array}

Here,

δ = (β^{2} - β_{0}^{2}) / 2 β_{0} ≃ β - β_{0} = n_{eff} (ω - ω_{0}) / c

is the deviation from the Bragg condition, ω₀ is the Bragg frequency, n_eff is the effective refractive index for the fundamental guided mode, and is the mode loss given by

α = \frac{k_{0}}{β_{0}} \int_{- \infty}^{\infty} n_{0} (z) \tilde{α} (z) {| Θ_{0} (z) |}^{2} d z

. κ_±2,0 and κ_0,±2 are the backward coupling coefficients defined as

κ_{\pm 2, 0} = - \frac{k_{0}^{2}}{2 β_{0}} ξ_{\pm 2, 0} \int_{P C} {| Θ_{0} (z) |}^{2} d z,

κ_{0, \pm 2} = - \frac{k_{0}^{2}}{2 β_{0}} ξ_{0, \pm 2} \int_{P C} {| Θ_{0} (z) |}^{2} d z,

respectively, which are equivalent to the conventional one dimensional (1D) coupling coefficients [18]. The second and third terms on the right-hand sides of Eqs. (15)–(18) represent the out-of-plane and 2D optical couplings, respectively. It should be noted that integrals in Eqs. (15)–(18), as well as those in Eqs. (19)–(20), extend only over the PC region because the Fourier coefficient ξ_mn = 0 outside that range.

As the fields of the radiative waves (ΔE_x(z), ΔE_y(z)) and the high-order waves (E_x,m,n(z), E_y,m,n(z)) are unknown, we cannot yet evaluate the right-hand sides of Eqs. (15)–(18). In order to determine these fields, Eqs. (4)–(6) must be solved for these waves. Details concerning the solutions of ΔE_x(z), ΔE_y(z), E_x,m,n(z), and E_y,m,n(z) are shown in Appendix A. Finally, the coupled-wave Eqs. (15)–(18) can be rewritten in matrix form as

(δ + i α) (\begin{matrix} R_{x} \\ S_{x} \\ R_{y} \\ S_{y} \end{matrix}) = C (\begin{matrix} R_{x} \\ S_{x} \\ R_{y} \\ S_{y} \end{matrix}) + i (\begin{matrix} \partial R_{x} / \partial x \\ - \partial S_{x} / \partial x \\ \partial R_{y} / \partial y \\ - \partial S_{y} / \partial y \end{matrix}) .

Here, C is a 4 × 4 matrix (see Appendix A). By considering a finite laser cavity area with 0 ≤ x, y ≤ L (L: laser cavity length in both x and y directions) and defining appropriate boundary conditions, Eq. (21) can be discretized using the staggered-grid finite-difference method [28]. This creates an eigenvalue problem with eigenvectors

{(R_{x}^{j, k}, S_{x}^{j, k}, R_{y}^{j, k}, S_{y}^{j, k}, R_{x}^{j + 1, k}, S_{x}^{j + 1, k}, R_{y}^{j + 1, k}, S_{y}^{j + 1, k}, \dots)}^{t}

and normalized eigenvalues (δ + iα)L.

3. Analysis results and discussions

In this section, we will show numerical results of the coupled-wave equations derived above. By solving the coupled-wave Eq. (21), we can obtain various properties of interest, including mode frequency, threshold gain, field intensity envelope profile within the device, far-field patterns (including the polarization profile) of the band-edge modes. The effects of finite size on these properties will be discussed.

The structural parameters of the laser structure [see Fig. 1(a)] to be studied are shown in Table 1. The PC layer is embedded inside the multilayer structure, which is assumed to support only a single guided mode. The average refractive index of the PC layer is given by $n_{a v} = \sqrt{f n_{a}^{2} + (1 - f) n_{b}^{2}}$ , where n_a is the refractive index of air, n_b is the refractive index of the background dielectric material (GaAs), and f is the air-hole filling factor (i.e., the fraction of the area of a unit cell occupied by air holes). In this work, we focus our analyses on two typical air-hole shape designs [see the inset of Fig. 1(a)]: circular (CC) and equilateral triangular (ET), both of which have been studied experimentally in our previous works [6, 24]. We consider a boundary condition that is defined as

R_{x} (0, y) = S_{x} (L, y) = R_{y} (x, 0) = S_{y} (x, L) = 0,

which includes fixing the field amplitude to zero at only one of two sides, generalizing a similar concept for 1D distributed feedback (DFB) laser structrue originally proposed by Kogelnik and C. V. Shank [18]. Unless otherwise noted in the following examples, the lattice constant a = 295 nm and the air-hole filling factor f = 0.16, the device length L=70 μm (all these values were inferred from experimental data). Here, it is important to note that L=70 μm approximately corresponds to a 240a × 240a lasing area, which is almost impossible to use 3D FDTD method to compute.

Table 1. Structural Parameters of the 2D PC-SEL Device

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3.1. Threshold gain behavior and field intensity envelope

In finite systems, mode frequency and threshold gain correspond to the real part and imaginary part of the normalized eigenvalue of Eq. (21), (δ +iα)L, respectively. As an example, we show in Fig. 2(i) a plot of the normalized threshold gain (αL) as a function of deviation from the Bragg condition (δL) for a PC-SEL with ET air holes. Though a large number of modes exist due to the numerical calculation, we identify the four band-edge modes (A, B, C and D) using the techniques described in Ref. [21]. First, we evaluate the field intensity envelope of the individual modes throughout the laser cavity, which modulates the fast-varying Bloch waves in the PC lattice and can be determined by [21]

I (x, y) = {| R_{x} (x, y) |}^{2} + {| S_{x} (x, y) |}^{2} + {| R_{y} (x, y) |}^{2} + {| S_{y} (x, y) |}^{2} .

We extract the solutions of interest that have a singled-lobed profile throughout the laser cavity. Then, we further identify the four band-edge modes by plotting their field distribution patterns inside a unit cell located at the center of the cavity and comparing these patterns with those calculated by 2D-PWEM. Figure 2(ii) shows the field intensity envelope profiles of the individual band-edge modes, indicating that energy can be well confined inside the laser cavity. Figure 2(iii) shows field distributions of each mode inside a unit cell located at the center of the cavity. Modes A and B correspond to the antisymmetric (nonleaky) mode of 1D DFB lasers, whereas modes C and D correspond to the symmetric (leaky) mode of 1D DFB lasers [29].

Fig. 2 (i) Normalized threshold gain (αL) as a function of normalized mode frequency deviation (δL) for a PC-SEL with ET air holes. The four band-edge modes A–D are indicated by arrows. (ii) Field intensity envelopes of the individual band-edge modes. (iii) Field distributions inside a unit cell located at the center of the cavity, in which colors and arrows represent H- and E-fields, respectively. Thick black triangles indicate the air holes. In the calculations, we use the structural parameters shown in Tab. 1, the air-hole filling factor f = 0.16, the lattice constant a=295 nm, and the device length L=70 μm.The laser is divided into 14 sections for which the eigenvalues (αL and δL) converge well.

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The normalized threshold gains of modes A–D are shown in Table 2 in which the values for the CC air-hole case were obtained in the same manner. We note that modes C and D exhibit a much higher threshold gain compared with modes A and B, which is attributed to the symmetric nature of their electric-field distributions with respect to the air holes as shown in Fig. 2(iii). As the lasing action occurs at the mode with the lowest threshold gain (loss), Table 2 indicates that mode A is favored for lasing for both CC and ET air holes when the device length L = 70 μm, with a large threshold-gain discrimination of over 20 cm⁻¹ against mode B. This is consistent with the experimental observations [23, 24] where stable single-mode operations at mode A were demonstrated.

Table 2. Normalized Threshold Gain (αL) of the Four Band-Edge Modes A–D for CC and ET Air-Hole Shapes (f = 0.16, a=295 nm, and L=70 μm)

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It is important to note that the single-mode lasing behavior at mode A cannot be explained based on the infinite solutions [22]. For an infinite periodic system with the vertical structure shown in Table 1, the radiation constants α_inf of modes A and B are α_inf,A = α_inf,B = 0 cm⁻¹ for CC and α_inf,A = 12.35 cm⁻¹, α_inf,B = 0.82 cm⁻¹ for ET air holes, respectively. From these results, one might expect that both modes A and B are favored for lasing for the CC air holes because both of their radiation constants (losses) are equal to zero; and for the ET air holes, mode B lases due to its much smaller radiation constant compared with that of mode A. Apparently, these predictions are in contrast to our finite analysis results and experimental findings. We suggest that the in-plane loss of finite structures might have a great impact on the mode selection of PC-SELs, which will be discussed in detail in the following sections.

3.2. Far-field pattern and polarization

The far-field pattern and polarization of the output laser beam is one of the most unique features of 2D PC-SELs. As demonstrated in Refs. [2, 6], both far-field pattern and polarization can be tailored by appropriately engineering the PC geometry. Theoretically, it is possible to calculate the far-field patterns of 2D PC-SELs based on our 3D CWT model since the the radiation field emanating from the surface of the finite-size laser structure is formulated explicitly (see Appendix A). The time-dependent amplitude of the far field F_j=x,y (subscripts denote the x and y components, respectively), a function of the deflection angle (θ_x, θ_y) from the surface normal, can be found by taking the Fourier transform of the radiation field and multiplying by an obliquity factor (cosθ_x + cosθ_y − 1) [30],

\begin{array}{r} F_{j} (θ_{x}, θ_{y}, t) \propto (cos θ_{x} + cos θ_{y} - 1) \iint_{0}^{L} Δ E_{j} (x, y, z = d_{P C} / 2) e^{i ω t} \\ \cdot e^{- i k_{0} (tan θ_{x} x + tan θ_{y} y)]} d x d y, j = x, y, \end{array}

where ΔE_j(x, y, z = d_PC/2) is the complex radiation field just above the PC surface [for further detail, see Eq. (A4) in Appendix A], d_PC is the depth of the PC layer, and z = 0 is defined at the center of the PC layer. Then, the time-averaged far-field intensity (i.e. beam pattern) is given as

〈 B (θ_{x}, θ_{y}) 〉 = 〈 B_{x} (θ_{x}, θ_{y}) 〉 + 〈 B_{y} (θ_{x}, θ_{y}) 〉,

where

〈 B_{j} (θ_{x}, θ_{y}) 〉 = \frac{1}{T} \int_{0}^{T} {| Re [F_{j} (θ_{x}, θ_{y}, t)] |}^{2} d t, j = x, y, T = 2 π / ω .

By calculating the x and y components of the far field using Eq. (26), we can directly evaluate the effect of the air holes on the polarization of the output beam.

Figures 3(a) and 3(b) show the calculated far-field patterns of mode A (i.e. the lowest threshold mode) for the two air-hole shapes. A doughnut-shaped profile is obtained for CC, whereas a single-lobed profile is obtained for ET air holes. The insets show the x and y components of the far field, indicating that CC air holes exhibit an azimuthal polarization, and ET ones exhibit an almost linear polarization in the y direction. These semi-analytical results closely match our experimental observations shown in Figs. 3(c) and 3(d) [6, 24]. The beam divergence angle of all of the far-field patterns is around 1°, reflecting the large area of coherent oscillation. The small side lobes of the calculated far-field patterns are caused by the abrupt termination of the radiation field at the edges of the laser cavity.

Fig. 3 Calculated far-field patterns (FFPs) of mode A (i.e. the lowest threshold mode) for (a) CC and (b) ET air holes and experimentally observed FFPs for (c) CC and (d) ET air holes. The insets in (a) and (b) represent the x and y components of the far field. Parameters used for the calculations are the same as those shown in Fig. 2. The yellow arrows in (c) and (d) indicate the directions of polarization.

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It is interesting to note that the quantity of surface emission for the symmetric CC air-hole shape is zero when an infinite periodic structure is considered [22], whereas it has a finite value for a finite periodic structure. This difference can be understood by considering the effect of finite device length on the electromagnetic field distribution within the device. For an infinite periodic structure, the light from mode A cannot be coupled to the external system because perfect destructive interference occurs everywhere due to the antisymmetric field distribution pattern with respect to the perfectly symmetric air holes. On the other hand, the mode field is spatially restricted for a finite system, leading to a small shift of the electromagnetic field with respect to the air holes. Figure 4(a) shows the electromagnetic field distribution patterns at (i) the center of the device and toward (ii–vi) the edges of the device. Although the mode field is kept antisymmetric with respect to the air holes in the center region, the field in the regions away from the center is no longer antisymmetric with respect to the air holes, thereby resulting in an imperfect destructive interference.

Fig. 4 (a) Illustrative field distribution patterns (plotted in a single unit cell) of mode A for CC air holes at the center of the cavity (i): ( $\frac{L}{2}$ , $\frac{L}{2}$ ) and toward the edges (ii): ( $\frac{6 L}{7}$ , $\frac{L}{2}$ ), (iii): ( $\frac{L}{7}$ , $\frac{L}{2}$ , (iv): ( $\frac{L}{2}$ , $\frac{6 L}{7}$ ), and (vi): ( $\frac{L}{2}$ , $\frac{L}{7}$ ). Colors and arrows represent H- and E-fields, respectively. Thick black circles indicate the locations of air holes with respect to the unit cell. (b) Field intensities of basic waves propagating in the ±x directions, |R_x|² and |S_x|² (red and blue curves), and the radiation field intensity represented by |R_x + S_x|² (black curve) along the axis y = L/2. Note that R_x and S_x are zero at x = 0 and x = L, respectively, which corresponds to the boundary condition described by Eq. (22).

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To examine the resultant interference effect in further detail, we plot in Fig. 4(b) the intensities of basic waves propagating in the ±x directions (i.e. |R_x|² and |S_x|²) and the radiation field intensity along the line y = L/2. The radiation field intensity can be represented by |R_x + S_x|², which reflects the interference effects of the two counter-propagating basic waves R_x and S_x [see Eq. (A4) in Appendix A and Eq. (B11) in Appendix B by noting that Fourier coefficient terms (ξ_m,n) for CC air holes are real numbers]. From Fig. 4(b), we can clearly see that the interference is indeed perfectly destructive at the center but becomes imperfect toward the edges of the device. The situation for ET can be understood in the same manner except that even at the central region the perfect destructive interference is suppressed because of the asymmetric air-hole effects [31].

The divergence angle of the far-field pattern is closely related to the finite size of the device. When the device is infinitely large, the surface emission is strictly in the surface normal direction. However, the realistic device has a finite length L, and therefore allows a wavenumber fluctuation of δk ≃ 2π/L from the Γ point, which was demonstrated by 3D FDTD simulations for finite structures [16, 17]. In fact, this wavenumber fluctuation corresponds to the slightly shifted electromagnetic field distributions discussed above [see Fig. 4(a)]. Due to this fluctuation, the emitted beam deviates from the surface normal direction with a deflection angle δθ. The relation between δθ and the wavenumber fluctuation can be expressed as

k_{0} sin (δ θ) = δ k,

where k₀ = 2π/λ (λ : the resonance wavelength in free space). For example, when λ = 960 nm, L=70 μm, δθ ≃ 0.8°, which is a good approximation of the results shown in Fig. 3.

3.3. Device length dependence of threshold gain

The analysis for an infinite system corresponds to the case when L → ∞. In this case, threshold gain and mode selection properties are determined only by the surface radiation loss (for simplicity, in this work we do not consider absorption loss). However, for a finite structure, influence of the in-plane loss (i.e. the power escaping from the edges of the laser cavity) has to be taken into account. To clarify the effects of the in-plane loss and highlight the difference between finite and infinite analyses, we investigate the threshold dependence on device length.

We plot in Fig. 5 the normalized mode frequency and threshold gain as a function of the device length L for CC and ET air holes. Also included as dashed lines are the infinite solutions (δ and α are first calculated for infinite periodic structures and then normalized by using (δ + β₀)/(n_eff β₀) [22] and αL, respectively). From Figs. 5(a) and 5(b), we note that mode frequency increases with respect to L and converges quickly to the infinite solutions (dashed lines). The threshold gain, in contrast, shows a quite different behavior: as shown in Figs. 5(c) and 5(d), in both cases, the laser threshold gain (solid curves) initially decreases with increasing L and then begins to be asymptotic to the infinite solutions (dashed lines). This behavior can be intuitively understood as follows. For small values of L, the dominant loss mechanism in the laser is the in-plane loss. In particular, at very short lengths, the optical power exits the gain region at the edges of the laser cavity without being diffracted out of the PC surface, leading to a high threshold gain. However, for larger values of L, the in-plane loss steadily decreases and the surface radiation loss becomes dominant. In fact, infinite solutions can correctly predict the lasing mode (i.e. the lowest threshold mode) of the device only when L is large enough, e.g. L > 400 μm (see Fig. 5). For a relatively small L, e.g. L < 100 μm, the influence of the in-plane loss on threshold behavior should not be neglected.

Fig. 5 Normalized mode frequency (a, b) and threshold gain (c, d) as a function of the device length L for CC and ET air holes (f = 0.16 and a=295 nm). The solid curves and dashed lines are calculated for finite and infinite structures, respectively.

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To evaluate the influence of the in-plane loss quantitatively, we derive a formula (Appendix B) to describe the power flow in 2D PC-SELs by extending the energy conservation theorem developed for 1D DFB lasers [32], which can be expressed as

P_{stim} = P_{edge} + P_{rad} .

Here, P_stim, P_edge, and P_rad represent the stimulated emission power, the power escaping from the edges of the laser cavity (i.e. the in-plane loss), and the surface radiation power, respectively. Therefore, the percentage of the in-plane loss power with respect to the total stimulated power is given by

P_{edge} / P_{stim} = 1 - P_{rad} / P_{stim} .

Table 3 shows the calculated P_edge/P_stim of modes A and B for CC and ET air holes with different device lengths. We can clearly see that, for both air-hole shapes, when L = 70 μm, the in-plane loss is indeed noticeably larger than the surface radiation loss, indicating that in-plane loss plays a very important role in the mode selections of the laser device. While when L=400 μm, the in-plane loss drastically decreases and its influence on mode selections might be neglected. Here, it is important to note that, in the case of ET air holes, when the device length L is increased from 70 μm to 400 μm, the lasing mode likely switches from mode A to mode B which has a smaller radiation constant [see Fig. 5(d)].

Table 3. Percentage of the In-Plane Loss with Respect to the Total Stimulated Power, P_edge/P_stim for CC and ET Air Holes with Different Device Lengths (f = 0.16 and a=295 nm)

View Table | View all tables in this article

4. Conclusion

In conclusion, we have developed a CWT model for finite-size square-lattice PC-SELs with TE polarization by extending our previous 3D CWT framework. Using this model, we calculated various properties of 2D PC-SELs, including threshold gain, mode frequency, field intensity envelope within the device, and the profile of the output beam (far-field pattern and polarization). The theoretical predictions of the lowest threshold mode and the output beam profile showed good agreement with our experimental findings, thereby affirming the validity of our analyses.

Our finite-size analysis results demonstrate that finite device length indeed strongly influences surface emission and mode selection properties of the PC-SEL device. Phenomena that cannot be explained by the infinite CWT analysis are clarified by considering the finite-size effects of the device. We showed that the finite device length may lead to a small shift of the electromagnetic field with respect to the air holes. This slightly shifted field suppresses the otherwise perfect destructive interference and thus enhances surface emission from the regions away from the center of the cavity, which accounts for the finite surface radiation loss emitted from the non-radiative (antisymmetric) mode of the CC air holes. By investigating the device length dependence of threshold gain, we find that, at a very large device length L (e.g. L > 400 μm), the finite-size analysis results become asymptotic to the infinite solutions, while at a relatively small L (e.g. L < 100 μm), the predicted lasing mode obtained using the finite-size analysis may be quite different from that based on the infinite structures. This is attributed to the influence of the in-plane loss, which can be noticeably large at short device lengths. The in-plane loss can provide a strong mode selection mechanism with a large threshold-gain discrimination of over 20 cm⁻¹, which allows us to explain the stable single-mode operation observed in our previous experiments.

The insights obtained in this work are essential for understanding the performances and physics of a practical PC-SEL device. Moreover, this theory also forms the basis for more complicated analyses of the properties of the device, including slope efficiency and the nonlinear effects [33] such as gain saturation [32], non-uniform gain spatial distribution, and hole burning. We believe that further theoretical work based on this semi-analytical 3D CWT framework will enable efficient optimization of the structures of 2D PC-SELs for a range of applications as well as facilitating a comprehensive understanding of the physics of PC-SELs.

Appendix A: Solutions of radiative and high-order waves

We assume, following Ref. [25], that: (1) Only basic waves are important in generating radiative and high-order waves; (2) For radiative and high-order waves, their derivatives in Eqs. (4)–(6) are negligible compared to the others; (3) α̃ is small and thus may be neglected. Under these assumptions, Eqs. (4)–(6) are reduced to

[\frac{\partial^{2}}{\partial z^{2}} + k_{0}^{2} n_{0}^{2} (z) - n^{2} β_{0}^{2}] E_{x, m, n} + m n β_{0}^{2} E_{y, m, n} = - k_{0}^{2} \sum_{m^{'} \neq m, n^{'} \neq n} ξ_{m - m^{'}, n - n^{'}} E_{x, m^{'}, n^{'}},

[\frac{\partial^{2}}{\partial z^{2}} + k_{0}^{2} n_{0}^{2} (z) - m^{2} β_{0}^{2}] E_{y, m, n} + m n β_{0}^{2} E_{x, m, n} = - k_{0}^{2} \sum_{m^{'} \neq m, n^{'} \neq n} ξ_{m - m^{'}, n - n^{'}} E_{y, m^{'}, n^{'}},

\frac{\partial}{\partial z} [m E_{x, m n} + n E_{y, m n}] = 0,

which are in the same form as that of those derived for the infinite periodic structures [22]. Then E_x,m,n and E_y,m,n can be solved in a similar manner as that described in Ref. [22], thus we shall only show the final solutions of radiative and high-order waves.

Radiative waves

The radiative waves [for which (m, n) = (0, 0)] can be expressed as

\begin{array}{l} Δ E_{x} (x, y, z) = [ξ_{0, - 1} R_{y} (x, y) + ξ_{0, 1} S_{y} (x, y)] k_{0}^{2} \int_{P C} G (z, z^{'}) Θ_{0} (z^{'}) d z^{'}, \\ Δ E_{y} (x, y, z) = [ξ_{- 1, 0} R_{x} (x, y) + ξ_{1, 0} S_{x} (x, y)] k_{0}^{2} \int_{P C} G (z, z^{'}) Θ_{0} (z^{'}) d z^{'} . \end{array}

Here, Green’s function

G (z, z^{'}) ≃ - \frac{i}{2 β_{z}} e^{- i β_{z} | z - z^{'} |}

and β_z = k₀n₀(z); It should be noted that ΔE_x (or ΔE_y) is a function of x and y. As a consequence, the second terms of the right-hand sides of the coupled-wave Eqs. (15)–(18) can be replaced by terms only associated with the basic waves.

High-order waves

The integrals of high-order waves (for which $\sqrt{m^{2} + n^{2}} > 1$ ) included in the third terms of the right-hand sides of the coupled-wave Eqs. (15)–(18) can be expressed as

\begin{array}{l} (\begin{matrix} \int_{P C} E_{x, m, n} (z) Θ_{0}^{*} (z) d z \\ \int_{P C} E_{y, m, n} (z) Θ_{0}^{*} (z) d z \end{matrix}) \\ = & \frac{1}{m^{2} + n^{2}} (\begin{matrix} n & m \\ - m & n \end{matrix}) (\begin{matrix} - m μ_{m, n}^{(1, 0)} & - m μ_{m, n}^{(- 1, 0)} & n μ_{m, n}^{(0, 1)} & n μ_{m, n}^{(0, - 1)} \\ n ν_{m, n}^{(1, 0)} & n ν_{m, n}^{(- 1, 0)} & m ν_{m, n}^{(0, 1)} & m ν_{m, n}^{(0, - 1)} \end{matrix}) \cdot V, \\ \equiv & (\begin{matrix} ς_{x, m, n}^{(1, 0)} & ς_{x, m, n}^{(- 1, 0)} & ς_{x, m, n}^{(0, 1)} & ς_{x, m, n}^{(0, - 1)} \\ ς_{y, m, n}^{(1, 0)} & ς_{y, m, n}^{(- 1, 0)} & ς_{y, m, n}^{(0, 1)} & ς_{y, m, n}^{(0, - 1)} \end{matrix}) \cdot V, \end{array}

where

V = {(\begin{matrix} R_{x}, & S_{x}, & R_{y}, & S_{y} \end{matrix})}^{t},

μ_{m, n}^{(r, s)} = k_{0}^{2} \iint_{P C} ξ_{m - r, n - s} (z) G_{m, n} (z, z^{'}) Θ_{0} (z^{'}) Θ_{0}^{*} (z) d z^{'} d z,

ν_{m, n}^{(r, s)} = - \int_{P C} \frac{1}{n_{0}^{2} (z)} ξ_{m - r, n - s} (z) {| Θ_{0} (z) |}^{2} d z,

G_{m, n} (z, z^{'}) = \frac{1}{2 β_{z}, m, n} e^{- β_{z, m, n} | z - z^{'} |}, β_{z,m,n} = \sqrt{(m^{2} + n^{2}) β_{0}^{2} - k_{0}^{2} n_{0}^{2} (z)} .

The elements of matrix C

Finally, the matrix C in Eq. (21) can be written as

C = C_{1 D} + C_{rad} + C_{2 D},

and

C_{1 D} = (\begin{matrix} 0 & κ_{2, 0} & 0 & 0 \\ κ_{- 2, 0} & 0 & 0 & 0 \\ 0 & 0 & 0 & κ_{0, 2} \\ 0 & 0 & κ_{0, - 2} & 0 \end{matrix}),

C_{rad} = (\begin{matrix} ζ_{1, 0}^{(1, 0)} & ζ_{1, 0}^{(- 1, 0)} & 0 & 0 \\ ζ_{- 1, 0}^{(1, 0)} & ζ_{- 1, 0}^{(- 1, 0)} & 0 & 0 \\ 0 & 0 & ζ_{0, 1}^{(0, 1)} & ζ_{0, 1}^{(0, - 1)} \\ 0 & 0 & ζ_{0, - 1}^{(0, 1)} & ζ_{0, - 1}^{(0, - 1)} \end{matrix}),

C_{2 D} = (\begin{matrix} χ_{y, 1, 0}^{(1, 0)} & χ_{y, 1, 0}^{(- 1, 0)} & χ_{y, 1, 0}^{(0, 1)} & χ_{y, 1, 0}^{(0, - 1)} \\ χ_{y, - 1, 0}^{(1, 0)} & χ_{y, - 1, 0}^{(- 1, 0)} & χ_{y, - 1, 0}^{(0, 1)} & χ_{y, - 1, 0}^{(0, - 1)} \\ χ_{x, 0, 1}^{(1, 0)} & χ_{x, 0, 1}^{(- 1, 0)} & χ_{x, 0, 1}^{(0, 1)} & χ_{x, 0, 1}^{(0, - 1)} \\ χ_{x, 0, - 1}^{(1, 0)} & χ_{x, 0, - 1}^{(- 1, 0)} & χ_{x, 0, - 1}^{(0, 1)} & χ_{x, 0, - 1}^{(0, - 1)} \end{matrix}),

and

ζ_{p, q}^{(r, s)} = - \frac{k_{0}^{4}}{2 β_{0}} \iint_{P C} ξ_{p, q} ξ_{- r, - s} G (z, z^{'}) Θ_{0} (z^{'}) Θ_{0}^{*} (z) d z^{'} d z,

χ_{j, p, q}^{(r, s)} = - \frac{k_{0}^{2}}{2 β_{0}} \sum_{\sqrt{m^{2} + n^{2}} > 1} ξ_{p - m, q - n} ς_{j, m, n}^{(r, s)}, j = x, y .

Note that the three square matrices: C_1D, C_rad and C_2D correspond to the conventional 1D feedback coupling, out-of-plane coupling via radiative waves, and 2D optical coupling via high-order waves, respectively. It should also be noted that C_1D and C_2D are Hermitian matrices, whereas C_rad is not an Hermitian one.

Appendix B: Derivation of energy conservation theorem for 2D PC-SELs

The coupled-wave Eq. (21) can be rewritten as

- i (δ + i α) R_{x} = \partial R_{x} / \partial x - i (C_{11} R_{x} + C_{12} S_{x} + C_{13} R_{y} + C_{14} S_{y}),

- i (δ + i α) S_{x} = - \partial S_{x} / \partial x - i (C_{21} R_{x} + C_{22} S_{x} + C_{23} R_{y} + C_{24} S_{y}),

- i (δ + i α) R_{y} = \partial R_{y} / \partial y - i (C_{31} R_{x} + C_{32} S_{x} + C_{33} R_{y} + C_{34} S_{y}),

- i (δ + i α) S_{y} = - \partial S_{y} / \partial y - i (C_{41} R_{x} + C_{42} S_{x} + C_{43} R_{y} + C_{44} S_{y}),

where C_ij(1 ≤ i, j ≤ 4) = C_1D,ij +C_rad,ij +C_2D,ij [see Eqs. (A10)–(A13)]. Multiplying Eq. (B1) by

R_{x}^{*}

, Eq. (B2) by

S_{x}^{*}

, Eq. (B3) by

R_{y}^{*}

, and Eq. (B4) by

S_{y}^{*}

, and adding the four equations and their conjugates, we obtain

\begin{array}{l} 2 α ({| R_{x} |}^{2} + {| S_{x} |}^{2} + {| R_{y} |}^{2} + {| S_{y} |}^{2}) = & \frac{\partial}{\partial x} ({| R_{x} |}^{2} - {| S_{x} |}^{2}) + \frac{\partial}{\partial y} ({| R_{y} |}^{2} - {| S_{y} |}^{2}) \\ + 2 κ_{v, i} ({| ξ_{- 1, 0} R_{x} + ξ_{1, 0} S_{x} |}^{2} + {| ξ_{0, - 1} R_{y} + ξ_{0, 1} S_{y} |}^{2}), \end{array}

where the C_1D,ij and C_2D,ij terms have vanished due to their Hermitian property, and only the term

κ_{v, i} = - Im {\frac{k_{0}^{4}}{2 β_{0}} \int_{P C} G (z, z^{'}) Θ_{0} (z^{'}) Θ_{0}^{*} (z) d z^{'} d z}

which is closely associated with the out-of-plane coupling remains [see Eq. (A14)]. In order to model the power flow in the 2D PC-SELs, we integrate Eq. (B5) over the whole laser cavity area, i.e., 0 ≤ x, y ≤ L. Then we have

\begin{array}{l} 2 α \iint_{0}^{L} ({| R_{x} |}^{2} + {| S_{x} |}^{2} & , & {| R_{y} |}^{2} + {| S_{y} |}^{2}) d x d y = \\ \iint_{0}^{L} [\frac{\partial}{\partial x} ({| R_{x} |}^{2} - {| S_{x} |}^{2}) + \frac{\partial}{\partial y} ({| R_{y} |}^{2} - {| S_{y} |}^{2})] d x d y \\ + & 2 κ_{v, i} \iint ({| ξ_{- 1, 0} R_{x} + ξ_{1, 0} S_{x} |}^{2} + {| ξ_{0, - 1} R_{y} + ξ_{0, 1} S_{y} |}^{2}) d x d y . \end{array}

For a laser structure with boundary condition given by Eq. (22), the field amplitudes of basic waves at the four edges can be expressed as

\begin{array}{l} R_{x} (0, y) = 0, S_{x} (0, y) = S_{ex} (y), \\ R_{x} (L, y) = R_{ex} (y), S_{x} (L, y) = 0, \\ R_{y} (x, 0) = 0, S_{y} (x, 0) = S_{ey} (x), \\ R_{y} (x, L) = R_{ey} (x), S_{y} (x, L) = 0. \end{array}

Then Eq. (B6) can be written as

P_{stim} = P_{edge} + P_{rad},

if we define

P_{stim} = 2 α \iint_{0}^{L} ({| R_{x} |}^{2} + {| S_{x} |}^{2} + {| R_{y} |}^{2} + {| S_{y} |}^{2}) d x d y,

P_{edge} = \int_{0}^{L} ({| R_{ex} |}^{2} + {| S_{ex} |}^{2}) d y + \int_{0}^{L} ({| R_{e y} |}^{2} + {| S_{ey} |}^{2}) d x,

P_{rad} = 2 κ_{v, i} \iint_{0}^{L} ({| ξ_{- 1, 0} R_{x} + ξ_{1, 0} S_{x} |}^{2} + {| ξ_{0, - 1} R_{y} + ξ_{0, 1} S_{y} |}^{2}) d x d y .

Here, P_stim describes the total stimulated power inside the laser structure, P_edge represents the power escaping from the edges of the laser cavity (i.e. the in-plane loss), and P_rad represents the radiation power emitted from the device surface [note that this quantity is proportional to the intensity of the radiative waves described by Eq. (A4)], respectively. Equation (B8) states the fact that the power generated inside the laser structure is equal to the sum of the power escaping from the edges of the structure and the surface radiation power.

Acknowledgments

The authors are grateful to Dr. K. Ishizaki, Dr. M. Yamaguchi, Dr. A. Oskooi, and Mr. T. Okino for helpful discussions and valuable suggestions. This work was partly supported by the Core Research for Evolution Science and Technology program of the Japan Science and Technology Agency (CREST-JST) and by the Global COE Program. Three of the authors (Y. Liang, C. Peng, and S. Iwahashi) are supported by research fellowships of Japan Society for the Promotion of Science (JSPS).

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31. Fundamentally, radiation fields emitted from the center of the laser cavity have similar properties to those emitted from an infinite periodic structure described in Ref. [22]. Unlike CC air holes, Fourier coefficients (ξ_m,n) of the dielectric function ε(r) for ET air holes are complex numbers. Therefore, the radiation field intensity is proportional to |ξ_−1,0R_x + ξ_1,0S_x|² [see Eq. (A4) in Appendix A and Eq. (B11) in Appendix B]. These complex Fourier coefficient terms multiplied to basic waves may change the phase difference of the waves diffracted vertically, resulting in a suppression of the destructive interference.

32. H. A. Haus, “Gain saturation in distributed feedback lasers,” Appl. Opt. 14, 2650–2652 (1975). [CrossRef] [PubMed]

33. S. H. Macomber, “Nonlinear analysis of surface-emitting distributed feedback lasers,” IEEE J. Quantum Electron. 26, 2065–2074 (1990). [CrossRef]

Layer	Thickness (nm)	Refractive index
n-clad (AlGaAs)	1500	3.307
Spacer (GaAs)	30	3.524
Active (3QW, InGaAs/GaAs)	64	3.553
Spacer (GaAs)	40	3.524
Cladding (AlGaAs)	40	3.279
GaAs	22	3.524
PC	118	n_av
GaAs	20	3.524
p-clad (AlGaAs)	1500	3.307

Shape	α_AL	α_BL	α_CL	α_DL
CC	0.23	0.52	2.18	2.18
ET	0.43	0.57	2.08	2.01

Layer	Thickness (nm)	Refractive index
n-clad (AlGaAs)	1500	3.307
Spacer (GaAs)	30	3.524
Active (3QW, InGaAs/GaAs)	64	3.553
Spacer (GaAs)	40	3.524
Cladding (AlGaAs)	40	3.279
GaAs	22	3.524
PC	118	n_av
GaAs	20	3.524
p-clad (AlGaAs)	1500	3.307

Shape	α_AL	α_BL	α_CL	α_DL
CC	0.23	0.52	2.18	2.18
ET	0.43	0.57	2.08	2.01

Three-dimensional coupled-wave analysis for square-lattice photonic crystal surface emitting lasers with transverse-electric polarization: finite-size effects

Abstract

1. Introduction

2. Coupled-wave equations for finite-size structures

3. Analysis results and discussions

3.1. Threshold gain behavior and field intensity envelope

3.2. Far-field pattern and polarization

3.3. Device length dependence of threshold gain

4. Conclusion

Appendix A: Solutions of radiative and high-order waves

Radiative waves

High-order waves

The elements of matrix C

Appendix B: Derivation of energy conservation theorem for 2D PC-SELs

Acknowledgments

References and links

Cited By

Figures (5)

Tables (3)

Equations (55)

Optics Express

Shape/mode		L=70 μm	L=400 μm
CC	mode A	67%	26%
CC	mode B	69%	26%
ET	mode A	58%	2%
ET	mode B	71%	19%

Shape/mode		L=70 μm	L=400 μm
CC	mode A	67%	26%
CC	mode B	69%	26%
ET	mode A	58%	2%
ET	mode B	71%	19%