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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 1 — Jan. 2, 2012
  • pp: 474–488
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Steady-state ab initio laser theory for N-level lasers

Alexander Cerjan, Yidong Chong, Li Ge, and A. Douglas Stone  »View Author Affiliations


Optics Express, Vol. 20, Issue 1, pp. 474-488 (2012)
http://dx.doi.org/10.1364/OE.20.000474


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Abstract

We show that Steady-state Ab initio Laser Theory (SALT) can be applied to find the stationary multimode lasing properties of an N-level laser. This is achieved by mapping the N-level rate equations to an effective two-level model of the type solved by the SALT algorithm. This mapping yields excellent agreement with more computationally demanding N-level time domain solutions for the steady state.

© 2011 OSA

1. Introduction

Semiclassical laser theory, which neglects the quantum fluctuations of the electromagnetic field, is widely used to describe and simulate lasers [1

1. H. Haken, Light: Laser Dynamics (North-Holland Phys. Publishing, 1985), Vol. 2.

,2

2. W. E. Lamb, “Theory of an optical maser,” Phys. Rev. 134, A1429 (1964). [CrossRef]

]. In principle, it correctly describes the laser thresholds and frequencies, the spatial pattern of the lasing modes, and the laser output power, including all classical non-linear effects, such as spatial hole-burning, gain saturation, and mode and phase locking. Essentially the theory describes Maxwell’s equations in an open cavity, coupled to the non-linear polarization of the gain medium. The gain polarization can be described using either a classical non-linear oscillator model [3

3. A. E. Siegman, Lasers (University Science Books, 1986).

], or a quantum-mechanical model of N atomic levels in which the polarization and level populations obey the equations of motion of the quantum density matrix. The simplest version of the theory, used widely in textbooks, is the two-level Maxwell-Bloch (MB) model [1

1. H. Haken, Light: Laser Dynamics (North-Holland Phys. Publishing, 1985), Vol. 2.

]; however, most design and characterization simulations of lasers use models with N = 3 or more levels. In addition, most theoretical solutions for the semiclassical laser equations employ a large number of simplifying assumptions in order to make them analytically tractable, most notably neglecting the openness of the cavity and/or treating only simple one-dimensional (1D) or ring cavities, as well as approximating the nonlinear interactions to cubic order. The results are typically not useful for quantitative modeling. Until recently, the only useful way to obtain quantitative results for non-trivial laser structures was to integrate the semiclassical laser equations in space and time. For novel and interesting modern laser structures with non-trivial 2D and 3D cavity geometries, such simulations are at the limits of computational feasibility, making it difficult to study a large parameter space or ensemble of designs.

In the past five years a new approach to finding the stationary solutions of the semiclassical laser equations has been developed, known as Steady state Ab initio Laser Theory (SALT) [6

6. H. E. Türeci, A. D. Stone, and B. Collier, “Self-consistent multimode lasing theory for complex or random lasing media,” Phys. Rev. A 74, 043822 (2006). [CrossRef]

9

9. L. Ge, Y. D. Chong, and A. D. Stone, “Steady-state ab initio laser theory: generalizations and analytic results,” Phys. Rev. A 82, 063824 (2010). [CrossRef]

]. SALT treats the openness of the cavity exactly, and the multimode non-linear interactions to infinite order (with two approximations, to be discussed below). It is applicable to cavities of arbitrary complexity in 2D and 3D, although we will discuss here only the scalar wave equation coupled to a gain medium. Importantly, this approach eliminates the need to perform a time integration to steady-state, dramatically reducing the computational effort and allowing one to study cavities with high spatial complexity, such as 2D random lasers [8

8. H. E. Türeci, L. Ge, S. Rotter, and A. D. Stone, “Strong interactions in multimode random lasers,” Science 320, 643–646 (2008). [CrossRef] [PubMed]

, 10

10. H. Cao, “Review on the latest developments in random lasers with coherent feedback,” J. Phys. A 38, 10497–10535 (2005). [CrossRef]

], photonic crystal lasers [11

11. O. Painter, R. K. Lee, A. Scherer, A. Yariv, J. D. O’Brien, P. D. Dapkus, and I. Kim, “Two-dimensional photonic band-gap defect mode laser,” Science 284, 1819–1821 (1999). [CrossRef] [PubMed]

, 12

12. S. Chua, Y. D. Chong, A. D. Stone, M Soljac̆ić, and J. Bravo-Abad, “Low-threshold lasing action in photonic crystal slabs enabled by Fano resonances,” Opt. Express 19, 1539 (2011). [CrossRef] [PubMed]

] and chaotic disk microlasers [13

13. C. Gmachl, F. Capasso, E. E. Narimanov, J. U. Nöckel, A. D. Stone, J. Faist, D. L. Sivco, and A. Y. Cho, “High-power directional emission from microlasers with chaotic resonators,” Science 280, 1556–1564 (1998). [CrossRef] [PubMed]

, 14

14. Ge Li, Yale PhD thesis, 2010.

].

SALT was originally formulated to solve the steady-state two-level Maxwell-Bloch equations [6

6. H. E. Türeci, A. D. Stone, and B. Collier, “Self-consistent multimode lasing theory for complex or random lasing media,” Phys. Rev. A 74, 043822 (2006). [CrossRef]

, 7

7. H. E. Türeci, A. D. Stone, and L. Ge, “Theory of the spatial structure of nonlinear lasing modes,” Phys. Rev. A 76, 013813 (2007). [CrossRef]

] in the standard slowly-varying envelope approximation (SVEA), and an iterative algorithm was developed [7

7. H. E. Türeci, A. D. Stone, and L. Ge, “Theory of the spatial structure of nonlinear lasing modes,” Phys. Rev. A 76, 013813 (2007). [CrossRef]

] to solve the resulting SALT equations. Subsequently it was realized that the SVEA afforded no advantage in numerical solutions of the SALT equations, so this approximation was dropped, leading to slightly different SALT equations [14

14. Ge Li, Yale PhD thesis, 2010.

, 15

15. L. Ge, R. J. Tandy, A. D. Stone, and H. E. Türeci, “Quantitative verification of ab initio self-consistent laser theory,” Opt. Express 16, 16895 (2008). [CrossRef] [PubMed]

], which were used in all subsequent work [8

8. H. E. Türeci, L. Ge, S. Rotter, and A. D. Stone, “Strong interactions in multimode random lasers,” Science 320, 643–646 (2008). [CrossRef] [PubMed]

, 9

9. L. Ge, Y. D. Chong, and A. D. Stone, “Steady-state ab initio laser theory: generalizations and analytic results,” Phys. Rev. A 82, 063824 (2010). [CrossRef]

, 12

12. S. Chua, Y. D. Chong, A. D. Stone, M Soljac̆ić, and J. Bravo-Abad, “Low-threshold lasing action in photonic crystal slabs enabled by Fano resonances,” Opt. Express 19, 1539 (2011). [CrossRef] [PubMed]

]. Recently an important generalization of the SALT solution algorithm was developed, improving its performance for laser cavities with complex spatial index variation and inhomogeneous pumping [9

9. L. Ge, Y. D. Chong, and A. D. Stone, “Steady-state ab initio laser theory: generalizations and analytic results,” Phys. Rev. A 82, 063824 (2010). [CrossRef]

]. In its present form, SALT only employs two approximations, the stationary inversion approximation (SIA) and the rotating wave approximation (RWA), which are both well satisfied for most lasers of interest. In Ref. [15

15. L. Ge, R. J. Tandy, A. D. Stone, and H. E. Türeci, “Quantitative verification of ab initio self-consistent laser theory,” Opt. Express 16, 16895 (2008). [CrossRef] [PubMed]

], the results of SALT were compared to the full time-dependent solution of the MB equations for a simple 1D cavity in the multimode regime, well above threshold. Excellent agreement was found in the parameter regime for which the SIA holds. This was, to our knowledge, the first demonstration of a frequency-domain method which agrees with exact time-domain methods for above-threshold multimode lasing.

2. Effective two-level systems

2.1. Four level system analysis

We illustrate the approach using the semi-classical laser equations [1

1. H. Haken, Light: Laser Dynamics (North-Holland Phys. Publishing, 1985), Vol. 2.

] for the four-level atomic gain medium shown in Fig. 1:
4πP¨+=c22E+εc(r)E¨+
(1)
P˙+=(iωa+γ)P++g2ih¯E+(ρ22ρ11)
(2)
ρ˙33=𝒫(ρ00ρ33)γ23ρ33
(3)
ρ˙22=γ23ρ33γ12ρ221ih¯E+((P+)*P+)
(4)
ρ˙11=γ12ρ22γ01ρ11+1ih¯E+((P+)*P+)
(5)
ρ˙00=γ01ρ11𝒫(ρ00ρ33).
(6)
The RWA has already been made, and we have assumed a lasing structure with one or two directions of translational symmetry, so that TM and TE polarizations are conserved and Maxwell’s equations reduce to a scalar Helmholtz equation [16

16. The equations are written for the TM case, the modifications for TE are straightforward.

]. E+ and P+ are the positive frequency components of the scalar electric and polarization fields respectively. ρii is the population density of level |i〉, ωa is the frequency of the gain center, γ is the gain width (polarization dephasing rate), g is the dipole matrix element, 𝒫 is the pump rate, and γij is the decay rate from level |j〉 to level |i〉. The four levels are labelled from 0 – 3 in order of increasing energy (Fig. 1). The polarization equation, Eq. (2), is obtained from the four-level density matrix equation of motion, assuming that only the level 2 → 1 transition will be inverted and lase. Often in FDTD calculations a real classical oscillating dipole equation is used to describe the polarization [12

12. S. Chua, Y. D. Chong, A. D. Stone, M Soljac̆ić, and J. Bravo-Abad, “Low-threshold lasing action in photonic crystal slabs enabled by Fano resonances,” Opt. Express 19, 1539 (2011). [CrossRef] [PubMed]

]; in Appendix A, we show that this yields essentially the same results, with an appropriate identification of parameters. In the rate equations, Eqs. (3)(6), the pump is coherently acting between levels |0〉 and |3〉.

Fig. 1 Schematic of a four-level gain medium.

The polarization equation, Eq. (2), incorporates the inversion D(x,t) ≡ ρ22(x,t) – ρ11(x,t), similar to the polarization equation for a two-level laser. By assuming that the non-lasing populations are stationary, ρ̇00 = ρ̇33 = 0, we can show that D(x,t) obeys
D˙=γ||(DD0)2ih¯E+((P+)*P+),
(7)
which is precisely the form of the inversion equation for the two-level medium [1

1. H. Haken, Light: Laser Dynamics (North-Holland Phys. Publishing, 1985), Vol. 2.

, 18

18. H. Fu and H. Haken, “Multifrequency operations in a short-cavity standing-wave laser,” Phys. Rev. A 43, 2446–2454 (1991). [CrossRef] [PubMed]

]. The parameters D0 and γ|| serve as an effective equilibrium inversion and inversion relaxation rate respectively, and are given by [14

14. Ge Li, Yale PhD thesis, 2010.

]:
γ||=2γ12(1+S2+γ01𝒫+2γ01γ23)
(8)
D0=S𝒫nγ01+(S+2+2γ01γ23)𝒫,
(9)
where S = (γ01γ12)/γ12 and n = ∑iρii is the total density of gain atoms. Equation (9) for the inversion in the absence of laser emission (which here acts as the effective pump parameter) has been discussed by Siegman [3

3. A. E. Siegman, Lasers (University Science Books, 1986).

], while Eq. (8) for the effective relaxation rate has been derived for a special case by Khanin [19

19. Y. I. Khanin, Principles of Laser Dynamics (Elsevier, 1995).

]. These expressions have not been used previously to solve the four-level lasing equations in terms of the two-level solutions, as we do here. If we use an incoherent pump, the 𝒫(ρ00ρ33) terms in Eqs. (3) and (6) would be replaced by 𝒫ρ00; the effect of this would be the removal of the factor of 2 preceding the ratio γ01/γ23 in the denominators of Eqs. (8) and (9). For typical laser systems, each denominator is dominated by the γ01/𝒫 term, so the difference between coherently and incoherently pumping the system is negligible [17

17. The observation that coherent and incoherent pumping are nearly equivalent for most systems, is invalid when the coherent pumping is supplied at a similar frequency to the atomic lasing transition and thus interactions between the lasing field and pumping field must be taken into account.

].

Given the parameters {γ01,γ12,γ23, 𝒫} describing the four-level medium of Fig. 2, we can calculate the effective pump and relaxation rates, and feed those (together with the cavity dielectric function, lasing transition frequency ωa, and gain linewidth γ) into the SALT algorithm. That will yield the steady-state lasing properties of this four-level laser.

Fig. 2 Modal intensities as functions of the normalized equilibrium inversion D0/Dc (effective pump) in a 1D microcavity edge emitting laser (schematic inset). The cavity is bounded on one side by a perfect mirror and on the other side by air, and has uniform refractive index n = 1.5. Solid lines are results obtained by the time-independent SALT method; open circles are results of FDTD simulations with a coherently pumped four-level medium (Fig. 1); solid triangles are results of FDTD simulations with a coherently pumped six-level medium with a lasing transition between |3〉 and |1〉. Simulation parameters are given in Appendix D. Both the four-level and six-level media are chosen to satisfy SIA. The dephas-ing rate is γ = 4.0. The four-level system is in the linear regime described by Eqs. (16) and (17). The six-level system is calculated using the formula in the appendix B, but is in the non-linear regime described by Eqs. (18) and (19). The spectra at D0/Dc = 0.488, and the gain curve, are shown in the upper left inset.

2.2. Arbitrary number of levels

A general N-level laser can be treated via an analogous procedure. Suppose we have a gain medium with an arbitrary number of levels, N. We assume that there is only a single lasing transition, between two levels which we denote by |u〉 and |l〉. The population of each of the N – 2 non-lasing levels obeys
ρ˙ii=jγijρjjjγjiρii+γiuρuu+γilρll(γui+γli)ρii,
(10)
where the sums are taken over all of the non-lasing states and γij is the rate at which atoms transition from state |j〉 to |i〉 which is either a decay or pump rate depending on the relative energies of the states. In this way, we can incorporate decay and pump processes between any levels. We again assume that ρ̇ii = 0 for all non-lasing levels. Then
j[(si+γui+γli)δijγij]ρij=γiuρuu+γilρll,
(11)
where si = ∑jγji and δij is the Kronecker delta. The term in brackets on the left hand side corresponds to an (N – 2)×(N – 2) matrix, which we denote as R. Upon inverting Eq. (11) and substituting it into the equations of motion for the lasing levels, we obtain
γ||=BlTuBuTlTu+Tl,D0=Bu+BlTu+Tlnγ||,
(12)
where
Tu/l=1+ij[R1]ijγj,u/l
(13)
Bu=su+ij(γuiγli)[R1]ijγju,
(14)
Bl=sl+ij(γuiγli)[R1]ijγjl.
(15)
The details of this calculation are given in Appendix B.

3. Physical limits of interest

Returning to the typical four-level case, we take note of two important physical regimes. The first, the linear regime, is γ23γ01γ12 ≫ 𝒫, for which one recovers the expected behavior that the equilibrium inversion increases linearly with the pump and that γ|| is a constant:
γ||2γ12,
(16)
D0𝒫γ12n.
(17)
In this case, varying the equilibrium inversion and the pump strength are essentially equivalent.

The second regime of interest, the non-linear regime, is γ23γ01γ12 ∼ 𝒫, i.e. when the slow decay rate between the lasing levels is on the same order as the pump rate. In this regime, γ|| increases with increasing pump and D0 saturates with increasing pump:
γ||2(γ12+𝒫),
(18)
D011+𝒫γ12(𝒫γ12)n.
(19)
This regime is also interesting from the viewpoint of SALT. As γ|| increases with 𝒫, a laser could satisfy the inequality γ||γ near threshold, leading to stationary inversion and an accurate solution via SALT, but fail to satisfy the inequality as the pump becomes stronger, leading to a decrease in the accuracy of SALT.

For a system with an arbitrary number of levels, the first regime always occurs at sufficiently small pump values; the second regime is obtainable if electrons in the upper lasing level are relatively long-lived compared to electrons in other levels.

4. Brief summary of SALT

5. Numerical comparison

As shown in Fig. 2, we find close agreement between SALT calculations and FDTD simulations. At a representative pump strength D0 = 0.488Dc, the mode intensities produced by SALT differ from those of the four- and six-level FDTD simulations by ∼ 1%, while the frequencies differ by < 0.1%. The difference in mode frequencies between SALT and FDTD also exists at the first lasing threshold, for which an analytical value can be calculated. There, we find that the FDTD simulation has a 0.2% error in the first mode frequency, while SALT has a 0.08% error; this error arises from the spatial discretization of the cavity employed in both approaches [21

21. For any 1D cavity which is uniformly pumped the TCF states for solving SALT can also be found using a transfer matrix method which does not require discretizing space. We use a more general TCF solver in the calculations presented here which does discretize space.

]. It is worth emphasizing that SALT treats the non-linearity to infinite order; in the earlier work on the Maxwell-Bloch model [15

15. L. Ge, R. J. Tandy, A. D. Stone, and H. E. Türeci, “Quantitative verification of ab initio self-consistent laser theory,” Opt. Express 16, 16895 (2008). [CrossRef] [PubMed]

] it was shown that for this same cavity the common cubic approximation for the non-linearity fails both quantitatively and qualitatively.

These results demonstrate that so long as the system satisfies SIA, the mapping between systems with an arbitrary number of levels to an effective two-level system is nearly exact, and SALT is able to very accurately determine the steady state properties of the cavity. If two cavities, each with an arbitrary number of levels, have the same effective parameters D0 and γ||, and otherwise have the same polarization relaxation rate and atomic transition frequency, the cavities are equivalent from the electromagnetic point of view, and will have identical lasing properties.

The mapping between the N-level laser and two-level SALT breaks down at large pump strengths, when the condition γ||γ is violated due to the increase of γ|| with 𝒫. In Ref. [15

15. L. Ge, R. J. Tandy, A. D. Stone, and H. E. Türeci, “Quantitative verification of ab initio self-consistent laser theory,” Opt. Express 16, 16895 (2008). [CrossRef] [PubMed]

], following an argument by Haken [18

18. H. Fu and H. Haken, “Multifrequency operations in a short-cavity standing-wave laser,” Phys. Rev. A 43, 2446–2454 (1991). [CrossRef] [PubMed]

], it was demonstrated that violating this condition causes the SIA for the two-level model to break down. This effect can be seen in the four-level laser data in Fig. 3, where γ|| = 0.1, γ = 4.0 and accuracy is already lost for the third lasing mode. For the six-level data of Fig. 3, which is in the non-linear parameter regime, SIA is satisfied and SALT agrees with the FDTD simulations for small values of the normalized equilibrium inversion; for larger values of D0, the SALT and FDTD results begin to diverge.

Fig. 3 Breakdown of the equivalence between SALT and FDTD when SIA is not valid is shown here in two different ways. Here, modal intensities as a function of the normalized equilibrium inversion D0/Dc (effective pump) are shown for a 1D microcavity edge emitting laser with γ = 4.0 and n = 1.5. Solid lines again represent results obtained from SALT, while open circles represent FDTD simulations of a simple four-level system with γ|| = 0.1. Triangles represent FDTD simulations of a six-level system in the non-linear parameter regime in which γ|| ∼ 0.001 for D0 ≤ 0.1, and thus satisfying SIA, but γ|| ∼ 0.01 for D0 ≥ 0.45, and consequently no longer satisfying SIA.

Fig. 4 SALT and FDTD results for a 1D random laser. Modal intensities are plotted against the normalized equilibrium inversion D0/Dc (effective pump). Solid lines represent SALT results, and circles represent FDTD simulations for a four-level system with γ|| = 0.001. The refractive index distribution of the edge emitting random laser is described in the text. The gain medium has γ = 4.0 and is in the regime described by Eqs. (16) and (17). Left inset: log-log plot of the indicated region where three modes turn on in close proximity. Right inset: schematic of the cavity structure.

6. Computational efficiency of SALT for N-level systems

In this section we present a set of benchmarks comparing the computational efficiency of SALT to FDTD. SALT calculations enjoy three main advantages over FDTD simulations of the semi-classical laser equations. First and foremost, SALT directly finds the steady-state solutions, so no time integration is involved, which substantially decreases computational effort. Second, SALT unambiguously determines how many modes are lasing at a given pump, whereas it can be difficult to determine, especially for multimode lasing, when an FDTD simulation has reached the steady-state with all modes that will lase “on”. Third, within SALT, with minimal additional computational effort, it is possible to monitor modes which are below threshold via a modified threshold matrix [8

8. H. E. Türeci, L. Ge, S. Rotter, and A. D. Stone, “Strong interactions in multimode random lasers,” Science 320, 643–646 (2008). [CrossRef] [PubMed]

], and hence to ascertain if more modes are likely to turn on in some interval of pump.

Fig. 5 Comparison of SALT and FDTD run-times. Modal intensities are shown as a function of the run-time for SALT (squares) and four-level FDTD simulations (circles), using the parameters of Fig. 2. FDTD simulations that have not begun to lase are marked as crosses. Plot (a) shows data for D0/Dc = 0.071, just above the first lasing threshold. SALT determined the steady-state single modal intensity in under three minutes, while the FDTD required ∼ 5000 minutes to reach steady state. Plot (b) shows data for D0/Dc = 0.486, well above the third lasing threshold. SALT calculated all data up to and including this pump value in under 90 minutes, whereas FDTD required > 500 minutes for the first two modes to reach steady-state, with the third mode intensity (green circles) still fluctuating after 5000 minutes (not shown).

As mentioned before, the bulk of the computational effort required for the SALT algorithm, especially in higher dimensions, is in solving for the TCF states. The TCF generalized eigenvalue problem is essentially identical in computational complexity to solving for a portion of the linear resonance spectrum of a cavity, something which is challenging in higher dimensional complex structures but feasible, particularly if one can use the scalar wave equation as in 2D. In practice the number of TCF states needed is often ∼ 10 – 20 and should not exceed ∼ 100 in cases of interest. Advanced finite element codes can be adapted to solve the TCF problem and promise to increase its efficiency in the future. It should be noted that the TCF basis library needs to be generated at a few dozen k values, which does increase the full computation compared to a single resonance calculation. Once one has a TCF basis library, using the SALT algorithm to iterate above threshold does not directly scale with the dimensionality of the system.

Finally, the current implementations of SALT assumes the electric field or magnetic field is perpendicular to the direction of wave propagation, leading to a scalar wave equation; the vectorial generalization of SALT is under investigation and will complicate the TCF calculation to the same extent that they do for more traditional linear Maxwell solvers.

7. Conclusion

Appendix A: Classical polarization in the laser-rate equations

Throughout this paper, we have used the density matrix equations of motion for the polarization field. However, much of the literature uses the classical oscillating dipole equation, in which the gain atoms are assumed to be dipoles undergoing harmonic oscillations and the quantum density matrix is neglected. This appendix briefly demonstrates that the two models are equivalent, and derives the relevant parameter redefinitions. A more thorough discussion has been given by Boyd [24

24. R. W. Boyd, Nonlinear Optics (Academic Press, 2008).

].

The polarization equation used here,
P˙+=(iωa+γ)P++g2ih¯E+(ρuuρll)
(A1)
is derived from the density matrix equation
ρ˙ul=(iωa+γ)ρulih¯gE(ρuuρll)
(A2)
where |u〉 and |l〉 are the upper and lower lasing states, ρij is the density matrix element ij and gul = glu = g is the coupling constant of the lasing states to the electric field, which allows for the definition P+ = ul. Alternatively, following the notation of Boyd [24

24. R. W. Boyd, Nonlinear Optics (Academic Press, 2008).

], we could consider the equation of motion for M = ul + c.c. = P+ + P, which is the expectation value of the dipole moment induced by the applied field, i.e. the classical oscillating dipole field. Thus
M˙=gρ˙ul+c.c.=(iωa+γ)ρulih¯gED+c.c.,
(A3)
using Eq. (A2), and
M¨=(ωa2+2iωaγ+γ2)gρulωah¯g2ED+c.c..
(A4)
This can be rewritten as
M¨+2γM˙+ωa2M=γ2M2ωag2h¯ED,
(A5)
where D = ρuuρll is the inversion density of the lasing states. For ωa2γ2, we can discard the term γ2M, resulting in the traditional form of the classical oscillating dipole polarization field equation. This gives the classical coupling constant σ = 2ωag2/ [3

3. A. E. Siegman, Lasers (University Science Books, 1986).

].

A similar analysis can used to show that the inversion equation can be rewritten as
D˙=γ||(DD0)+2Eh¯ωaM˙,
(A6)
which demonstrates how the change in the inversion is dependent upon the classical polarization field. Therefore, so long as ωa2γ2, a condition that is usually satisfied, these two formulations of the polarization dynamics are equivalent.

Appendix B: Effective two-level parameters from N-level rate equations

This appendix derives the effective equilibrium inversion and relaxation rate for a gain medium with an arbitrary number of levels. We allow decays between any two levels, even if they are not adjacent. We assume there is a single lasing transition, between levels |u〉 and |l〉, which need not be adjacent. The rate equation for an arbitrary non-lasing level in the system is
ρ˙ii=jγijρjjjγjiρii+γiuρuu+γilρll(γui+γli)ρii,
(B1)
where the summations are taken over all non-lasing levels. Here we do not distinguish between decay rates and pumping rates; γij is simply interpreted as the rate at which level |j〉 transitions into level |i〉, regardless of the energies of those states. If the populations of all the non-lasing transitions are stationary, i.e. ρ̇ii = 0, then we can rewrite Eq. (B1) as
jRijρjj=γiuρuu+γilρll,
(B2)
Rij(si+γui+γli)δijγij.
(B3)
Here, si ≡ ∑jγji and δij is the Kronecker delta. Inverting Eq. (B2) gives
ρii=j[R1]ij(γjuρuu+γjlρll).
(B4)
Hence, we can express the total number density of gain atoms as
n=iρii+ρuu+ρll
(B5)
=Tuρuu+Tlρll
(B6)
where
Tu=1+ij[R1]ijγju,
(B7)
Tl=1+ij[R1]ijγjl.
(B8)
Noting that D = ρuuρll, we can write the populations of the lasing states as
ρuu=n+TlDTl+Tu,
(B9)
ρll=nTuDTl+Tu.
(B10)

From the equations of motion for the lasing levels, we have the inversion equation
D˙=ρ˙uuρ˙ll=i(γuiγli)ρiisuρuu+slρll2ih¯E+((P+)*P+),
(B11)
where su = ∑jγju + γlu and sl is defined similarly. Inserting Eq. (B4) into this equation gives
D˙=Buρu,u+Blρl,l2ih¯E+((P+)*P+),
(B12)
where
Busu+ij(γuiγli)[R1]ijγju,
(B13)
Blsl+ij(γuiγli)[R1]ijγjl.
(B14)
Plugging in Eqs. (B9) and (B10) now yields the inversion equation in the desired form Eq. (7), with
γ||=BlTuBuTlTu+Tl,
(B15)
D0=Bu+BlBlTuBuTln.
(B16)

Appendix C: Inversion as a function of the pump

Fig. 6 (a) Unscaled modal intensity of the six-level simulations from Fig. 2 as a function of the pump. A cross sectional area of 1m2 is assumed to calculate the power. (b) Inversion as a function of the pump at the cavity boundary. Dashed lines in plots a and b correspond to the pump values shown in plot c. (c) Inversion as a function of position within the cavity for three different pump values, cyan corresponds with 𝒫 = 3.75 × 108s−1, magenta with 𝒫 = 1.65 × 109s−1, and orange with 𝒫 = 4.85 × 109s−1 to show the evolution of the inversion within the cavity as a function of the pump strength.

To understand this formally, we begin by rewriting Eq. (22) and removing the scaling factors Ec and Dc gives
2g2h¯2ν=1NΓν|Ψν(r)|2=γ||(D0D(r)1).
(C1)
Substituting in Eqs. (18) and (19), which are valid for this simulation, gives
g2h¯2ν=1NΓν|Ψν(r)|2=𝒫(ND(r)1)γ12.
(C2)
The inversion D is a function of both position and the pump. However, for r⃗ = L corresponding to the cavity edge, D should be mostly independent of the pump, as at this location every mode is at its maximum intensity and the effect of spatial hole-burning is most pronounced. The FDTD simulation results, shown in Fig. 6, demonstrate that D indeed varies very weakly with 𝒫 at the cavity edge. Since the left hand side of Eq. (C2) evaluated at r = L is proportional to the total output power, and this must hold for any number, N, of lasing modes, pump-independence of D(r) implies a linear behavior of the unscaled output power of each mode with pump, as observed in Fig. 6(a).

Appendix D: Simulation constants

In this appendix we list the parameters used in each of the FDTD simulations described above. These constants are matrices, with γij denoting the decay rate from |j〉 to |i〉. These values are given in their dimensionless form, i.e. γmeas = γrealL/c. Unlisted entries are zero. We also note that throughout this paper |0〉 denotes the ground state, so these matrices are 0 indexed.

For the four-level simulations in Fig. 2,
γ4lv,fig2=(·0.85×1040.8).
(D1)
The dipole matrix element is g = 2.3 · 10−12m3/2, and the number of gain atoms is n = 5 · 1023m−3. The pump 𝒫 was varied between 3 · 10−6 and 3 · 10−5.

Thus, for an optical wavelength of λ = 628nm, the requirement in Fig. 2 that n0kL = 60 means that L = 4μm. Using this length, the decay rates can be converted to their unit-full values as γ = 3 · 1014s−1, γ23 = γ01 = 6 · 1013s−1, γ12 = 3.75 · 1010s−1, and the pump at threshold is 𝒫 = 3 · 108s−1. Similarly, the dipole matrix element also acquires units of inverse time, and can be expressed as g2/ = 3.98 · 10−9m3/s, which corresponds to a coupling constant in the classical oscillating dipole picture of σ = 10−4C2/kg. These constants can be seen to be similar to those used in other studies of optical microcavities [4

4. A. S. Nagra and R. A. York, “FDTD analysis of wave propagation in nonlinear absorbing and gain media,” IEEE Trans. Antennas Propag. 46, 334–340 (1998). [CrossRef]

, 12

12. S. Chua, Y. D. Chong, A. D. Stone, M Soljac̆ić, and J. Bravo-Abad, “Low-threshold lasing action in photonic crystal slabs enabled by Fano resonances,” Opt. Express 19, 1539 (2011). [CrossRef] [PubMed]

].

For the six-level simulations in Fig. 2,
γ6lv,fig2=(0.81051051051050.81051051045×1051051050.81050.8).
(D2)
Furthermore, γ15 = 10−4, and the lasing transition is between levels |3〉 and |1〉 (where the ground state is again |0〉 and the states are numbered in order of increasing energy).

For the four-level simulations in Fig. 3,
γ4lv,fig3=(0.85×1020.8).
(D3)

For the six-level simulations in Fig. 3,
γ6lv,fig3=(0.81051051051050.81051051045×1051051050.81050.8).
(D4)

The four-level simulations of the random cavity in Fig. 4 used the same parameters as the four-level simulations in Fig. 2.

Acknowledgments

We thank Robert J. Tandy, Hui Cao, and Peter Bermel for helpful discussions. This work was partially supported by NSF grant No. DMR-0908437.

References and links

1.

H. Haken, Light: Laser Dynamics (North-Holland Phys. Publishing, 1985), Vol. 2.

2.

W. E. Lamb, “Theory of an optical maser,” Phys. Rev. 134, A1429 (1964). [CrossRef]

3.

A. E. Siegman, Lasers (University Science Books, 1986).

4.

A. S. Nagra and R. A. York, “FDTD analysis of wave propagation in nonlinear absorbing and gain media,” IEEE Trans. Antennas Propag. 46, 334–340 (1998). [CrossRef]

5.

K. S. Yee, “Numerical solution of the initial boundary value problems involving Maxwell’s equations in isotropic media,” IEEE Trans Antennas Propag . 14, 302–307 (1966). [CrossRef]

6.

H. E. Türeci, A. D. Stone, and B. Collier, “Self-consistent multimode lasing theory for complex or random lasing media,” Phys. Rev. A 74, 043822 (2006). [CrossRef]

7.

H. E. Türeci, A. D. Stone, and L. Ge, “Theory of the spatial structure of nonlinear lasing modes,” Phys. Rev. A 76, 013813 (2007). [CrossRef]

8.

H. E. Türeci, L. Ge, S. Rotter, and A. D. Stone, “Strong interactions in multimode random lasers,” Science 320, 643–646 (2008). [CrossRef] [PubMed]

9.

L. Ge, Y. D. Chong, and A. D. Stone, “Steady-state ab initio laser theory: generalizations and analytic results,” Phys. Rev. A 82, 063824 (2010). [CrossRef]

10.

H. Cao, “Review on the latest developments in random lasers with coherent feedback,” J. Phys. A 38, 10497–10535 (2005). [CrossRef]

11.

O. Painter, R. K. Lee, A. Scherer, A. Yariv, J. D. O’Brien, P. D. Dapkus, and I. Kim, “Two-dimensional photonic band-gap defect mode laser,” Science 284, 1819–1821 (1999). [CrossRef] [PubMed]

12.

S. Chua, Y. D. Chong, A. D. Stone, M Soljac̆ić, and J. Bravo-Abad, “Low-threshold lasing action in photonic crystal slabs enabled by Fano resonances,” Opt. Express 19, 1539 (2011). [CrossRef] [PubMed]

13.

C. Gmachl, F. Capasso, E. E. Narimanov, J. U. Nöckel, A. D. Stone, J. Faist, D. L. Sivco, and A. Y. Cho, “High-power directional emission from microlasers with chaotic resonators,” Science 280, 1556–1564 (1998). [CrossRef] [PubMed]

14.

Ge Li, Yale PhD thesis, 2010.

15.

L. Ge, R. J. Tandy, A. D. Stone, and H. E. Türeci, “Quantitative verification of ab initio self-consistent laser theory,” Opt. Express 16, 16895 (2008). [CrossRef] [PubMed]

16.

The equations are written for the TM case, the modifications for TE are straightforward.

17.

The observation that coherent and incoherent pumping are nearly equivalent for most systems, is invalid when the coherent pumping is supplied at a similar frequency to the atomic lasing transition and thus interactions between the lasing field and pumping field must be taken into account.

18.

H. Fu and H. Haken, “Multifrequency operations in a short-cavity standing-wave laser,” Phys. Rev. A 43, 2446–2454 (1991). [CrossRef] [PubMed]

19.

Y. I. Khanin, Principles of Laser Dynamics (Elsevier, 1995).

20.

B. Bidégaray, “Time discretizations for Maxwell-Bloch equations,” Numer. Meth. Partial Differential Equations 19, 284–300 (2003). [CrossRef]

21.

For any 1D cavity which is uniformly pumped the TCF states for solving SALT can also be found using a transfer matrix method which does not require discretizing space. We use a more general TCF solver in the calculations presented here which does discretize space.

22.

X. Jiang and C. M. Soukoulis, “Time dependent theory for random lasers,” Phys. Rev. Lett. 85, 70 (2000). [CrossRef] [PubMed]

23.

X. Jiang, S. Feng, C. M. Soukoulis, J. Zi, J. D. Joannopoulos, and H. Cao, “Coupling, competition, and stability of modes in random lasers,” Phys. Rev. B 69, 104202 (2004). [CrossRef]

24.

R. W. Boyd, Nonlinear Optics (Academic Press, 2008).

OCIS Codes
(140.3430) Lasers and laser optics : Laser theory
(140.3945) Lasers and laser optics : Microcavities

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: September 27, 2011
Revised Manuscript: November 21, 2011
Manuscript Accepted: November 22, 2011
Published: December 21, 2011

Citation
Alexander Cerjan, Yidong Chong, Li Ge, and A. Douglas Stone, "Steady-state ab initio laser theory for N-level lasers," Opt. Express 20, 474-488 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-1-474


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References

  1. H. Haken, Light: Laser Dynamics (North-Holland Phys. Publishing, 1985), Vol. 2.
  2. W. E. Lamb, “Theory of an optical maser,” Phys. Rev.134, A1429 (1964). [CrossRef]
  3. A. E. Siegman, Lasers (University Science Books, 1986).
  4. A. S. Nagra and R. A. York, “FDTD analysis of wave propagation in nonlinear absorbing and gain media,” IEEE Trans. Antennas Propag.46, 334–340 (1998). [CrossRef]
  5. K. S. Yee, “Numerical solution of the initial boundary value problems involving Maxwell’s equations in isotropic media,” IEEE Trans Antennas Propag. 14, 302–307 (1966). [CrossRef]
  6. H. E. Türeci, A. D. Stone, and B. Collier, “Self-consistent multimode lasing theory for complex or random lasing media,” Phys. Rev. A74, 043822 (2006). [CrossRef]
  7. H. E. Türeci, A. D. Stone, and L. Ge, “Theory of the spatial structure of nonlinear lasing modes,” Phys. Rev. A76, 013813 (2007). [CrossRef]
  8. H. E. Türeci, L. Ge, S. Rotter, and A. D. Stone, “Strong interactions in multimode random lasers,” Science320, 643–646 (2008). [CrossRef] [PubMed]
  9. L. Ge, Y. D. Chong, and A. D. Stone, “Steady-state ab initio laser theory: generalizations and analytic results,” Phys. Rev. A82, 063824 (2010). [CrossRef]
  10. H. Cao, “Review on the latest developments in random lasers with coherent feedback,” J. Phys. A38, 10497–10535 (2005). [CrossRef]
  11. O. Painter, R. K. Lee, A. Scherer, A. Yariv, J. D. O’Brien, P. D. Dapkus, and I. Kim, “Two-dimensional photonic band-gap defect mode laser,” Science284, 1819–1821 (1999). [CrossRef] [PubMed]
  12. S. Chua, Y. D. Chong, A. D. Stone, M Soljac̆ić, and J. Bravo-Abad, “Low-threshold lasing action in photonic crystal slabs enabled by Fano resonances,” Opt. Express19, 1539 (2011). [CrossRef] [PubMed]
  13. C. Gmachl, F. Capasso, E. E. Narimanov, J. U. Nöckel, A. D. Stone, J. Faist, D. L. Sivco, and A. Y. Cho, “High-power directional emission from microlasers with chaotic resonators,” Science280, 1556–1564 (1998). [CrossRef] [PubMed]
  14. Ge Li, Yale PhD thesis, 2010.
  15. L. Ge, R. J. Tandy, A. D. Stone, and H. E. Türeci, “Quantitative verification of ab initio self-consistent laser theory,” Opt. Express16, 16895 (2008). [CrossRef] [PubMed]
  16. The equations are written for the TM case, the modifications for TE are straightforward.
  17. The observation that coherent and incoherent pumping are nearly equivalent for most systems, is invalid when the coherent pumping is supplied at a similar frequency to the atomic lasing transition and thus interactions between the lasing field and pumping field must be taken into account.
  18. H. Fu and H. Haken, “Multifrequency operations in a short-cavity standing-wave laser,” Phys. Rev. A43, 2446–2454 (1991). [CrossRef] [PubMed]
  19. Y. I. Khanin, Principles of Laser Dynamics (Elsevier, 1995).
  20. B. Bidégaray, “Time discretizations for Maxwell-Bloch equations,” Numer. Meth. Partial Differential Equations19, 284–300 (2003). [CrossRef]
  21. For any 1D cavity which is uniformly pumped the TCF states for solving SALT can also be found using a transfer matrix method which does not require discretizing space. We use a more general TCF solver in the calculations presented here which does discretize space.
  22. X. Jiang and C. M. Soukoulis, “Time dependent theory for random lasers,” Phys. Rev. Lett.85, 70 (2000). [CrossRef] [PubMed]
  23. X. Jiang, S. Feng, C. M. Soukoulis, J. Zi, J. D. Joannopoulos, and H. Cao, “Coupling, competition, and stability of modes in random lasers,” Phys. Rev. B69, 104202 (2004). [CrossRef]
  24. R. W. Boyd, Nonlinear Optics (Academic Press, 2008).

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