The collective response of a system is profoundly shaped by the interaction dynamics between its constituent elements. In physics, tailoring these interactions can enable the observation of unusual phenomena that are otherwise inaccessible in standard settings, ranging from the possibility of a Kramer’s degeneracy even in the absence of spin to the breakdown of the bulk-boundary correspondence. Here, we show how tailored asymmetric coupling terms can be realized in photonic integrated platforms by exploiting non-Hermitian concepts. In this regard, we introduce a generalized photonic molecule composed of a pair of microring resonators with internal S-bends connected via two directional couplers and a link waveguide. By judiciously designing the parameters of this system, namely, the length of the links and the power division ratio of the directional couplers, we experimentally show the emergence of Hermitian and non-Hermitian-type exchange interactions. The ramifications of such coupling dynamics are then studied in 1D chain and ring-type active lattices. Our findings establish the proposed structure as a promising building block for the realization of a variety of phenomena, especially those associated with phase locking in laser arrays and non-Hermitian topological lattices.

Synthetic gauge fields, enabling photons to flow in an elaborately designed photonic lattice in a similar fashion as electrons in a magnetic flux, are the cornerstone of photonic topological insulators. In optics, such fields have been primarily realized through geometrical design and time/space modulation.1–9 Most early photonic topological lattices have been built on passive platforms, where the topological response can be examined by probing the system with an input and observing the resulting output.4–6 However, it has been recently realized that such topological attributes may be more easily witnessed and consequently utilized in active settings under pumping, especially when the system reaches lasing. In such cases, instead of externally exciting the system, one can deduce the signatures of the underlying topology from the emission spectrum and intensity profile. Along these lines, topological lasers have been demonstrated in which an array of gain elements phase lock to yield a topological edge supermode.10–18 Such lasers have been shown to possess superior properties when compared to their trivial counterparts, in terms of spectral and spatial purity of their emission, higher quantum efficiency, and robustness to defects and disorders.12,16

The use of gain in topological photonics, however, is of more significance than just simplifying measurements or even enabling a new class of lasers. Using optical amplification, one can fundamentally change the nature of interactions between the resonant units in an array.19 In other words, unlike passive systems, in which reciprocity requires the exchange dynamics between elements to be symmetrical, in active/non-Hermitian systems, one can engineer such couplings to be, in general, asymmetric. This is because Lorentz reciprocity is defined in the context of source-free arrangements that are excited by an external source, while in active systems, the presence of gain saturation and spontaneous emission eludes a clear discussion of reciprocity in the abstract definition. In fact, it can be shown that spontaneous emission in judiciously designed resonant structures can lead to non-symmetric coupling interactions.19 

In this article, we propose and experimentally demonstrate a versatile approach for engineering the coupling dynamics in active cavities. In order to do so, we use microring resonators with internal S-bend constructs to enforce unidirectional lasing.12,20,21 The coupling is then established using a link structure. By changing the length of the link and the power division factor of the directional couplers (DCs), various types of Hermitian and non-Hermitian coupling behaviors can be realized. The freedom to design at will the interaction dynamics in a lattice can bring many possibilities to optics and in particular to topological photonics.22–30 Along these lines, some of the applications of such active lattices will be discussed.

This paper is organized as follows. In Sec. II, an active two-resonator system with an adjustable coupling link is introduced. We show how the coupling can be altered from Hermitian- to non-Hermitian-type exchange by varying the lengths and the power division ratio of the directional couplers. In Sec. III, the response of this class of coupled cavities will be verified experimentally. Section IV explores the ramifications of non-Hermitian coupling in 1D chain and ring arrays. Finally, in Sec. V, we will conclude the paper.

Photonic molecules, representing two coupled resonant structures, have been studied extensively in the past few decades.31,32 In standard optical systems that respect time reversal symmetry, the energy exchange between the constituent elements of a photonic molecule tends to be symmetric, resulting in a Hermitian-type dynamical response. Here, we introduce a new type of active photonic molecule that can offer a variety of exchange interactions from Hermitian- to non-Hermitian-type through complex coupling terms. Figure 1(a) shows the schematic of such a molecule. This structure is composed of two active (displaying gain) ring resonators with internal S-bends, connected to each other through a combination of cascaded directional couplers (DCs) that are separated by a link waveguide. Here, the S-bends are designed in such a way to enforce unidirectional lasing in the rings20 [clockwise (CW) for the case depicted in Fig. 1]. The left (right) ports of the first (second) directional coupler are weakly coupled to the opposing sides of ring ① (②).

FIG. 1.

(a) Schematic of a two-element system with unidirectional microring resonators and a link structure. The cascaded directional couplers provide a π phase difference between the two coupling directions. The coupling phase βL can be changed by altering the length L. An asymmetric coupling coefficient can be introduced by selecting the length of the directional couplers. (b) Three types of couplings under different settings of βL and rt can be achieved. (c) Two-element system with an outcoupling structure for device characterization.

FIG. 1.

(a) Schematic of a two-element system with unidirectional microring resonators and a link structure. The cascaded directional couplers provide a π phase difference between the two coupling directions. The coupling phase βL can be changed by altering the length L. An asymmetric coupling coefficient can be introduced by selecting the length of the directional couplers. (b) Three types of couplings under different settings of βL and rt can be achieved. (c) Two-element system with an outcoupling structure for device characterization.

Close modal

In order to analyze this system, we can write the spatial coupled mode equations relating the fields in the two rings—without a priori assumption regarding the unidirectional flow of light in the cavities (see  Appendix A for details). The result, however, appears to fully agree with the temporal coupled mode analysis performed with the assumption that the two rings are unidirectional. Without loss of generality, here we use the latter approach. In addition, in order to keep the number of variables limited, we study the case where all coupling strengths between resonators and waveguides are equal and the two directional couplers are identical. In general, each ring, when uncoupled, can support a number of spectral lines. We also limit our analysis to a single longitudinal mode since these modes can be treated independently. We also assume that the waveguiding section is designed so as to primarily support the TE0 transverse mode. In this respect, the interplay between the electric modal fields in the two identical rings can be effectively described through a set of time-dependent coupled equations,

iȧ1+ω0a1+κ21a2=0iȧ2+ω0a2+κ12a1=0
(1)

where a1,2 represent the modal amplitudes in the two cavities. The angular frequency ω0 is determined by the resonance conditions for each resonator in the absence of coupling. The coupling from resonator ① → ② is given by κ1→2 = 2r2eiβL, while that from ② → ① is expressed by κ2→1 = −2r2eiβL (see  Appendix B for the derivation). Here, r and t are the through and cross terms for the directional coupler. In a lossless device, these two coefficients are related through r2 + t2 = 1. The coupling strengths between the rings and bus waveguides are all set to be κ. Finally, the overall length of the link part is given by L = L0 + 2Lc, where L0 and Lc are the lengths of the waveguide sections between the two directional couplers and from each directional coupler to the adjacent ring, respectively.

Depending on the values of βL and r (or equivalently t), the Hamiltonian describing this system can become Hermitian, non-Hermitian with two coupling coefficients of equal magnitudes but opposing signs, or non-Hermitian with interactions displaying dissimilar magnitude and/or phase. For example, in the case of r = t (3 dB directional couplers), when βL = , this system exhibits a fully Hermitian behavior, while for βL, the coupling between the elements and therefore the Hamiltonian becomes non-Hermitian, even though the magnitude of the exchange interactions remains the same. On the other hand, for rt, the Hamiltonian is inevitably non-Hermitian since the magnitude of the coupling terms will be different. If non-Hermitian interactions that are purely imaginary are desired, one can change the length Lc of the upper arms vs those on the lower arms. The table provided in Fig. 1(b) summarizes the conditions, leading to the aforementioned various scenarios. It should be noted that even though the non-Hermitian coupling here is enabled by gain, the type of non-Hermiticity we observe in these systems is very different from those observed in PT-symmetric arrangements that are realized by the presence of a gain–loss contrast.33–36 

In order to verify the response of the generalized photonic molecule introduced in Sec. II, here we fabricate the proposed resonant systems on a III–V semiconductor wafer (six quantum wells of InGaAsP with an overall thickness of 200 nm). The ring resonators and all waveguides have a width of 500 nm and a height of 200 nm. The high-contrast core (ncore = 3.4) is embedded in silicon dioxide (nSIO2=1.45) and is exposed to air on top. This structure is designed to support the TE0 mode with an effective index of neff = 2.273 and a group index of ng ≈ 4 at a wavelength of 1580 nm. The separations between the waveguides in the directional couplers, the bus WGs and resonators, and the S-bends and resonators are all nominally set to be 100 nm.

By providing adequate pumping to this system, the eigenmodes with the larger imaginary part of the eigenvalues (gain) are expected to lase, allowing the emission to display the signatures of the underlying coupling. In particular, one can measure the spectrum as well as the phase difference between the outputs from the two resonators. For measuring the phase, we collect the light from the two resonators with bus waveguides and allow them to interfere using an additional 3 dB coupler, as depicted in Fig. 1(c). The output arms of the directional coupler are equipped with surface emitting second-order gratings.

Figure 2 depicts three different scenarios of interest, where the coupling turns from non-Hermitian to Hermitian and back to non-Hermitian again, simply by changing the length of the link L0. When βL = 2mπ + π/2, the coupling terms κ12andκ21 are both real but with opposing signs. The non-Hermitian coupling allows the system to support two modes with eigenvectors 1±iT and corresponding eigenvalues iκ12. This presents a situation where the structure supports two modes with equal field intensity in resonators ① and ②. The electric field components from the mode with the higher quality factor (subject to gain) show a −π/2 phase difference, while this quantity for a mode with a lower quality factor (experiencing loss) is +π/2. Figure 2(a) shows the lasing properties of such a photonic molecule. The emission intensity clearly corroborates the expected interference behavior, as the light exclusively exits the bottom grating port. In addition, the spectrum shows a single peak emission, an indication that one of the modes is largely suppressed and the laser is now single-moded.

FIG. 2.

Experimental results of coupling coefficients for three values of βL: (a) βL = 2mπ + π/2, (b) βL = , and (c) βL = 2mππ/2. The insets show the spectra collected at the output gratings. The spectrum in (a) and (c) indicate that there is only one lasing mode when a non-Hermitian coupling is implemented. The spectrum in (b) shows there are two lasing modes when the two resonators interact through a Hermitian coupling.

FIG. 2.

Experimental results of coupling coefficients for three values of βL: (a) βL = 2mπ + π/2, (b) βL = , and (c) βL = 2mππ/2. The insets show the spectra collected at the output gratings. The spectrum in (a) and (c) indicate that there is only one lasing mode when a non-Hermitian coupling is implemented. The spectrum in (b) shows there are two lasing modes when the two resonators interact through a Hermitian coupling.

Close modal

The above photonic molecule, however, exhibits a very different kind of response when the length of link structure satisfies βL = . In this case, the two coupling terms become imaginary and complex conjugate of each other. The resulting Hermitian Hamiltonian gives rise to two eigenmodes of 1±iT with eigenvalues of iκ12. In the absence of mode discrimination, this system is expected to support two lasing modes that are separated by a frequency difference of 2κ12. This situation can be clearly seen in Fig. 2(b) where not only both grating ports have a balanced output intensity but also the spectrum shows two lasing lines. Finally, Fig. 2(c) shows a similar Hermitian scenario as in Fig. 2(a), albeit with βL = 2mππ/2. In this case, the system is again single-moded, but the two components of the electric field associated with the lasing mode are displaying a +π/2 phase difference, resulting in a higher output power emerging from the upper grating.

The nature and strength of the interconnectivity between the elements of a lattice can profoundly affect its band structure. In this section, we study the ramifications of generalized coupling dynamics on the band shape, energy states, modal content, and particularly the emergence of edge modes in lattices. In this regard, we consider several 1D chains in which the elements are coupled through imaginary Hermitian and non-Hermitian interactions. Subsequently, we extend our analysis to the ring-type lattice configurations.

Generally, the eigenvalues of a 1D chain of length N with nearest neighbor right-to-left κrl and left-to-right κlr couplings, as depicted in Fig. 3(a), are given by

λh=ω0κlrκrlcoshπN+1

Here, h is the integer identifying the mode number h=1N. The corresponding eigenvectors are then given by

Vh=vh1vh2vhNT

where vhk=κlrκrlk2sinhkπN+1k=1N

FIG. 3.

(a) 1D optical chain with asymmetric couplings. (b) Photonic realization of a chain where the of length of bus waveguides Lcij, L0, and the coupling ratios of the directional couplers rt can be modified to realize various types of interactions. Eigenvalues associated with different modes as well as the amplitude of the components of the fundamental mode for chains with (c) imaginary Hermitian coupling κlr=κrl=iκ22, (d) balanced imaginary non-Hermitian coupling κlr=κrl=iκ22, and (e) unbalanced imaginary non-Hermitian coupling 2κlr=κrl=iκ2, resulting in a non-Hermitian topological lattice.

FIG. 3.

(a) 1D optical chain with asymmetric couplings. (b) Photonic realization of a chain where the of length of bus waveguides Lcij, L0, and the coupling ratios of the directional couplers rt can be modified to realize various types of interactions. Eigenvalues associated with different modes as well as the amplitude of the components of the fundamental mode for chains with (c) imaginary Hermitian coupling κlr=κrl=iκ22, (d) balanced imaginary non-Hermitian coupling κlr=κrl=iκ22, and (e) unbalanced imaginary non-Hermitian coupling 2κlr=κrl=iκ2, resulting in a non-Hermitian topological lattice.

Close modal

Figure 3(b) shows an implementation of such a 1D lattice using our generalized photonic molecules as the constituent elements. When the coupling is designed to be purely imaginary, but the Hamiltonian is Hermitian (all Lcijs are equal, and βL is a multiple integer of 2π resulting in lr = −rl); all eigenvalues are expected to be real, thus representing modes that are separated in the spectral domain while having the same quality factors. Figure 3(c) shows the eigen-spectrum as well as the fundamental mode of this system. On the other hand, when the coupling terms are purely imaginary but equal in magnitude (Lci1=Lci2=Lci+11=2qπβ, Lci+12=Lci+21=2q1πβ, and βL0 = 2mπ), resulting in a Hamiltonian that is non-Hermitian iκlr=iκrl, all the eigenvalues become entirely imaginary with the in-phase mode experiencing a net gain (positive imaginary term), while the out-of-phase mode undergoes the same amount of loss. The eigenvalue distribution and the amplitude of the fundamental mode of this arrangement are depicted in Fig. 3(d). This situation may be of interest in laser phase locking where the mode discrimination offered by the difference between the eigenvalues forces the array into lasing in the in-phase supermode. This scheme may be of practical use in fiber laser systems where one can control the locking dynamics against the random variations of the individual cavity lengths by adjusting the coupling strength. It should be noted that the intensity distribution across the various elements of the lattice can be further evened out by slightly changing the coupling strengths between those elements of the chain that are closer to the ends.

Finally, when the coupling between the array elements become imaginary and uneven (all Lcij’s are equal, βL = 2mπ, and rt ≠ 1), a new type of topological edge mode appears in these lattices. This situation known as the Hatano–Nelson model was first proposed to describe the localization transition in systems with non-Hermitian couplings κlrκrl26,27 Despite a number of proposals, the implementation of this lattice has yet remained elusive in optics. Using our generalized photonic molecules, the Hatano–Nelson chain can be realized by modifying the directional couplers in the lattice shown in Fig. 3(b) to have unequal through and cross coupling terms rt. When a chain structure is formed based on these molecules, all eigenvalues split in the imaginary part of the frequency domain, while the eigenvectors/field distributions become unbalanced, tilting toward one end of lattice [Fig. 3(e)]. Unlike the standard Su–Schrieffer–Heeger (SSH) arrays,37 in this case, all modes of the system are of edge-type, leading to a situation that is coined by bulk-boundary correspondence.29,38 The appearance of edge-type modes in this system should be of no surprise since the left-right coupling unbalance pushes the energy toward one end of the array. In such arrangements, the edge mode that represents the closest to a uniform distribution of field intensity at various elements experiences the highest level of gain.

Ring-type lattices offering periodic boundary conditions in addition to their inter-element spacing are used to study a broad range of physical phenomena from spin-squeezing to various topological and synthetic gauge structures. Figure 4(a) shows a schematic of a four-element ring-shaped lattice where the left-to-right coupling differs from right-to-left exchange. Unlike the 1D chains, the circulation of power in such toroidal lattices leads to the formation of modes with equal intensities across the array elements. In such systems, a Hermitian imaginary coupling lr = rl [that is realized by having all Lcij’s to be equal and βL to be a multiple integer of 2π in Fig. 4(b)] does not provide any mode selectivity, but it gives rise to a fundamental mode with equal intensity in all sites [see Fig. 4(c)]. On the other hand, when the coupling between elements is of non-Hermitian and imaginary type, while the magnitudes of the coupling terms are equal κ12=κ21 (for example, by choosing Lci1=Lci2=Lci11=2qπβ, Lci+12=Lci+21=2q1πβ, and βL0 = 2mπ), the eigenfrequencies exhibit a splitting in the complex domain, resulting in a mode discrimination [Fig. 4(d)], where the in-phase super-mode exhibits the highest quality factor. Finally, when the coupling is non-Hermitian and imaginary and κ12κ21 (having all Lcij’s to be equal, βL = 2mπ, and rt ≠ 1), mode discrimination can be observed both in real and imaginary parts of the eigenfrequency [Fig. 4(e)]. As a result, various modes not only differ in their quality factors but also appear at different parts of the spectrum. Here, no apparent edge mode can be identified, even though the modes show natural robustness against the variation in couplings between the elements.

FIG. 4.

(a) Schematic of a four-element loop-type lattice with dissimilar κlr and κrl coupling terms. (b) Photonic realization of a four-element loop. To implement different types of couplings, one can engineer the length of the bus waveguides Lcij, L0, and the coupling ratios of the directional couplers rt, resulting in (c) imaginary Hermitian coupling κlr=κrl=iκ22, (d) balanced non-Hermitian coupling κlr=κrl=iκ22, and (e) unbalanced imaginary non-Hermitian coupling 2κlr=κrl=iκ2, respectively. Eigenenergy corresponding to the various modes and the amplitude of the components of the fundamental mode for loops are plotted. With imaginary coupling coefficients, the field profiles are found to be uniform. The fundamental modes in (d) and (e) have the highest quality factors.

FIG. 4.

(a) Schematic of a four-element loop-type lattice with dissimilar κlr and κrl coupling terms. (b) Photonic realization of a four-element loop. To implement different types of couplings, one can engineer the length of the bus waveguides Lcij, L0, and the coupling ratios of the directional couplers rt, resulting in (c) imaginary Hermitian coupling κlr=κrl=iκ22, (d) balanced non-Hermitian coupling κlr=κrl=iκ22, and (e) unbalanced imaginary non-Hermitian coupling 2κlr=κrl=iκ2, respectively. Eigenenergy corresponding to the various modes and the amplitude of the components of the fundamental mode for loops are plotted. With imaginary coupling coefficients, the field profiles are found to be uniform. The fundamental modes in (d) and (e) have the highest quality factors.

Close modal

We introduced a new active photonic molecule capable of exhibiting a wide range of interaction dynamics by adjusting its parameters. What enables this system to work is the unidirectional flow of energy in the rings due to the presence of the S-bends. The emergence of various types of coupling coefficients in this molecule has been experimentally verified, resulting in Hermitian and non-Hermitian-type Hamiltonians. The exchange terms can be, in general, complex, even though in most of our analysis we focused on purely imaginary couplings. We have also explored the role these interaction dynamics play in determining the response of 1D chains and ring-shaped lattices. This building block may be used to study a variety of effects, especially those arising in non-Hermitian topological lattices. In addition, engineering the interaction dynamics in laser arrays comprising of such molecules may lead to new regimes of phase locking as well as near-field and far-field steerable emission.

The authors would like to thank the support from DARPA (Grant No. D18AP00058), the Office of Naval Research (Grant Nos. N00014-20-1-2522, N00014-20-1-2789, N00014-16-1-2640, N00014-18-1-2347, and N00014-19-1-2052), the Army Research Office (Grant No. W911NF-17-1-0481), the Air Force Office of Scientific Research (Grant Nos. FA9550-14-1-0037 and FA9550-20-1-0322), the National Science Foundation (Grant Nos. CBET 1805200, ECCS 2000538, and ECCS 2011171), the US-Israel Binational Science Foundation (BSF; Grant No. 2016381), and the Polish Ministry of Science and Higher Education (Mobility Plus, Grant No. 1654/MOB/V/2017).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

To derive the aforementioned coupling coefficients with spatial coupled mode analysis, we introduce the transfer matrices associated with the various elements involved in the links between the two resonators, as shown in Fig. 5. In particular,

M1=1ϵ12iϵ1iϵ11ϵ12M2=1ϵ22iϵ2iϵ21ϵ22
(A1)
M3=rititr
(A2)

where 1ϵj2 and j represent the portion of the field that propagates through and cross the coupling regions, respectively. The M3 matrices correspond to the two directional couplers, while the M1 and M2 matrices account for weak coupling to the S-bends and waveguide buses, respectively. Consider the fields propagating along the direction of the S-bend, i.e., counterclockwise in Fig. 5. The coupling between the S-bends and the resonators can be written as

a2sa2=M1a1sa1a6sa6=M1a5sa5b2sb2=M1b1sb1b6sb6=M1b5sb5
(A3)

and the coupling between the resonator and the links can be expressed via

a4c2=M2a3c1a8d8=M2a7d7b4d2=M2b3d1b8c8=M2b7c7
(A4)

On the other hand, consider the fields propagating against the direction of the S-bend, i.e., clockwise in Fig. 5. The coupling between the S-bends and the resonators can be written as

x2sx2=M1x1sx1x6sx6=M1x5sx5y2sy2=M1y1sy1y6sy6=M1y5sy5
(A5)

and the coupling between the resonator and the links can be expressed via

x4f2=M2x3f1x8e8=M2x7e7y4e2=M2y3e1y8f8=M2y7f7
(A6)

In addition, the field amplitudes at the directional couplers are

c4f4=M3c3f3c6f6=M3c5f5d4e4=M3d3e3d6e6=M3d5e5
(A7)

Notice that the ends of the S-bends and the open arms of the directional couplers are tapered to minimize the reflection. sx1, sx5, sy1, sy5, d5, and f5 are assumed to be 0 in the analysis.

FIG. 5.

Two microring resonators coupled through a link involving two directional couplers. Consider fields with opposite directions.

FIG. 5.

Two microring resonators coupled through a link involving two directional couplers. Consider fields with opposite directions.

Close modal

Assuming α to be the amplifying/damping rate per unit length and Lr to be the perimeter of each resonator. The field in resonator ① propagates according to

a1=eiβLr4αLr4a8a3=eiβLr4αLr4a2a5=eiβLr4αLr4a4a7=eiβLr4αLr4a6
(A8)

Similarly, the field components inside resonator ② obey

b1=eiβLr4αLr4b8b3=eiβLr4αLr4b2b5=eiβLr4αLr4b4b7=eiβLr4αLr4b6
(A9)

In addition, the length from the resonator to the directional coupler is Lc, the length between two directional couplers is Lm, and the length of the S-bend is Ls, as shown in Fig. 5. By neglecting any amplification/loss in the link, the fields propagate according to

c3=eiβLcc2c5=eiβLmc4c7=eiβLcc6
(A10)
d3=eiβLcd2d7=eiβLcd6
(A11)
e3=eiβLce2e5=eiβLme4e7=eiβLce6
(A12)
f3=eiβLcf2f7=eiβLcf6
(A13)
sa2=eiβLssx6sa5=eiβLssx2
(A14)
sb2=eiβLssy6sb5=eiβLssy2
(A15)

The transfer matrix of this system can be written as

KaaKbaKxaKyaKabKbbKxbKybKaxKbxKxxKyxKayKbyKxyKyya1b1x1y1=K̂a1b1x1y1=Λa1b1x1y1
(A16)

where Λ is the system eigenvalue with a1b1x1y1T representing the system eigenvectors. The elements Kmn in the matrix K̂ are

Kaa=eLrα+iLrβ1+ϵ121+ϵ22
(A17)
Kba=12e12Lrαiβ+i2βLc+βLm1ϵ12ϵ22
(A18)
Kxae3Lrα2+iLrβ2+iβLsϵ121ϵ22eLrα+eiLrβ1+ϵ121+ϵ22
(A19)
Kya=12e3Lrα2+iLrβ2+2iβLs+iβLc1ϵ12ϵ22×ieLrα2+e12iLrβ+2βLsϵ121ϵ22
(A20)
Kab=12e12Lrαiβ+i2βLc+βLm1ϵ12ϵ22
(A21)
Kbb=eLrα+iLrβ1+ϵ121+ϵ22
(A22)
Kxb=12eLrα+iLrβ2+2iβLc+iβLm1ϵ12ϵ22×ieLrα2+e12iLrβ+2βLsϵ121ϵ22
(A23)
Kyb=e3Lrα2+iLrβ2+iβLsϵ121ϵ22×eLrα+eiLrβ1+ϵ121+ϵ22
(A24)
Kax=0
(A25)
Kbx=12ie12Lrαiβ+i2βLc+βLm1ϵ12ϵ22
(A26)
Kxx=eLrα+iLrβ1+ϵ121+ϵ22
(A27)
Kyx=12eLrα+iLrβ2+2iβLc+iβLc1ϵ12ϵ22×ieLrα2+e12iLrβ+2βLsϵ121ϵ22
(A28)
Kay=12e12Lrαiβ+i2βLc+βLm1ϵ12ϵ22
(A29)
Kby=0
(A30)
Kxy=12eLrα+iLrβ2+2iβLc+iβLm1ϵ12ϵ22×ieLrα2+e12iLrβ+2βLsϵ121ϵ22
(A31)
Kyy=eLrα+iLrβ1+ϵ121+ϵ22
(A32)

To analyze the behavior of the aforementioned coupled microring laser system and to simplify the calculation, we consider a lossless, resonating, non-Hermitian system with two identical cavities and ignore the phase noise introduced by the S-bend. Substituting the following parameters in Eq. (A16): α = 0, βLr = 2π, βLc = π/2, βLm = π, βLs = 2π, ε1 = 0.4, and ε2 = 0.2, one can obtain the eigenvalues,

Λ̂=0.8850.8050.068i0.805+0.068i0.731=0.8850.808ei0.0840.808ei0.0840.731
(A33)

Here, the magnitudes of the elements represent the roundtrip amplification/damping of the fields and element arguments represent the roundtrip phase change of the fields. In a laser system, the eigenmode with the highest amplification will reach lasing threshold first under pumping. The eigenvector that is related to an eigenvalue of Λ = 0.885 represents the lasing mode in this condition. Its corresponding eigenvector is

abxy=0.6830.683+0.037i0.1750.048i0.178+0.039i
(A34)

In this case, the amplitudes of fields a and b are almost similar and significantly larger than those of x and y because of the S-bend. It is therefore conceivable that the system indeed supports a mode that is to a good approximation (especially after the gain nonlinearity is factored in) that is predicted from the temporal coupled mode with a priori assumption that the two rings are unidirectional.

In this section, we show how a Hermitian antisymmetric imaginary coupling can be achieved by introducing a link with two directional couplers between two unidirectional ring resonators (Fig. 6). When operating as a laser, the perimeter of the ring matters as it may cause lasing in multiple longitudinal modes. In order to avoid such a behavior, in practice, we chose the radius of the rings as well as the other lengths small. This increases the free spectral range to over 1 THz and further allows us to have larger couplings between elements. While in many applications we prefer single longitudinal mode operation for lasers, the physics of interactions remains pretty much unchanged even if the system is multimoded.

FIG. 6.

Two microring resonators coupled through a link involving two directional couplers.

FIG. 6.

Two microring resonators coupled through a link involving two directional couplers.

Close modal

When applying the antisymmetric imaginary coupling between two identical resonators (coupled to bus lines), in the tight-binding picture, one obtains the following time evolution equations:6 

iddta=iγiniγexa+it2bi2γexϵiniddtb=iγiniγexbit2a
(B1)

where a and b are the complex field modal amplitudes in the two identical unidirectional ring resonators. γex is the coupling coefficient between the resonators and the input/output bus waveguides. γin represents the gain/loss in the two resonators, and t2 is associated with the tunneling rate. Let us assume an input of amplitude ɛin at an angular frequency ω0 + δω injected from the bus waveguide to resonator ① on the right, where ω0 is the resonance frequency of the resonators and δω represents a shift from this value. The output fields propagating in the two bus waveguides are given by bout=2γexb and aout=ϵin+2γexa. As a result, the reflection and transmission coefficients of this system as obtained from the temporal coupled mode theory are

RCM=2γexbϵin=2t2γext22+iδωγexγin2
(B2)
TCM=2γexa+ϵinϵin=1+2γexiδωγinγext22+iδωγexγin2
(B3)

To self-consistently relate the above results (obtained from the temporal coupled mode theory) to those expected from a continuous wave analysis, we introduce the transfer matrices20,21 associated with the various elements involved in the links between the two resonators, as shown in Fig. 6. In particular,

M1=1ϵ12iϵ1iϵ11ϵ12M2=1ϵ22iϵ2iϵ21ϵ22
(B4)
M3=121ii1
(B5)

where 1ϵj2 and j represent the portion of the field that propagates through and cross the coupling regions, respectively. The M3 matrices correspond to the two 3 dB couplers, while the M1 and M2 matrices account for weak coupling to the waveguide buses, i.e., ϵj ≪ 1. The coupling between the bus waveguides and the resonators is now written as

x2a2=M1x1a1y2b2=M1y1b1
(B6)

and the coupling between the resonator and the links can be expressed via

a4c2=M2a3c1b4d2=M2b3d1
(B7)
b6c8=M2b5c7a6e4=M2a5e1
(B8)

In addition, the field amplitudes at the 3 dB directional couplers are

d4d6=M3d3d5d8e2=M3d7e1
(B9)
c4=12c3c6=12c5
(B10)

We now assume that the effective propagation constant within the resonators is k = 2πneffλ0, where neff is the effective refractive index of the lasing mode, λ is the corresponding vacuum wavelength, and λ0 is the amplifying/damping rate per unit length. Given that α represents the perimeter of each resonator, the field in resonator ① propagates according to

a1=eikLr4αLr4a6a3=eikLr4αLr4a2a5=eikLr2αLr2a4
(B11)

Similarly, the field components inside resonator ② obey

b1=eikLr4αLr4b6b3=eikLr4αLr4b2b5=eikLr2αLr2b4
(B12)

In addition, the length from the resonator to the directional coupler is Lc and the length between two directional couplers is Lm, as shown in Fig. 6. By neglecting any amplification/loss in the link, the fields propagate according to

c3=eikLcc2c5=eikLmc4c7=eikLcc6
(B13)
d3=eikLcd2e3=eikLce2
(B14)

For lasing modes that operate near the resonant frequency in the unidirectional resonators, we assume that eikLr1 since kLr − 2mπ = δkLr ≪ 1, where δk=δωcneff. Moreover, the loss in the resonators is negligible, i.e., αLr ≪ 1, which results in eαLr1. By expanding the exponential functions and square roots in Eqs. (B13) and (B14) and keeping in mind that ɛj, δkLr, αLr ≪ 1, we obtain the reflection coefficient of the system in the continuous wave picture,

RTM=2ϵ12ϵ22P
(B15)

where P=isin2kLc+kLm2αLr2iδkLr+ϵ12+2ϵ222ϵ24cos2kLc+kLm2αLr2iδkLr+ϵ12+2ϵ222+ϵ24. To compare this reflection coefficient to that obtained from the temporal coupled mode theory in Eq. (B2), we introduce the substitutions

ϵ122γexFSRϵ222t2FSRαLr=γinFSRδkLr=δωFSRFSR=cngLr
(B16)

where ng is the group index. In this case, the reflection coefficient can be written as

RTM=2t2γexQ
(B17)

where Q=isin2kLc+kLmiδωγexγin2t22t22cos2kLc+kLmiδωγexγin2t22+t22. Notice that when ϕ = 2kLc + kLm = 2 (N = integer), the above reflection coefficient reduces to

RTM=2t2rexiδωγexγin2t22+t22
(B18)

which has exactly the same form as the reflection coefficient obtained from the temporal coupled mode theory [Eq. (B2)], once the coupling loss −2t2 introduced by the link is also considered. In the case when ϕ=2kLc+kLm=2N1π (N = integer), the directionality of the antisymmetric hopping is reversed, giving t2=t2

The two eigenfrequencies of this system are given by

δω12=±t2iγexiγin2it2
(B19)

Similarly, one can calculate the transmission coefficient of the system from the CW approach,

TTM=12ϵ122αLr2iδLr+ϵ12+2ϵ22e2ik2Lc+Lmϵ24+2αLr2iδkLr+ϵ12+2ϵ222
(B20)

The transmission coefficient can also be written as

TTM=1+2γexiδωγexγin2t2e2i2kLc+kLmt22+iδωγexγin2t22
(B21)

Again, when ϕ = 2kLc + kLm = , where N is an integer, the above transmission coefficient becomes

TTM=1+2γexiδωγexγin2t2t22+iδωγexγin2t22
(B22)

which has the same form as the transmission coefficient obtained from the temporal coupled mode theory [Eq. (B3)], apart from an additional term of −2t2 introduced by the loss of the link.

As it is provided above, the reflection coefficient and transmission coefficient of the structure shown in Fig. 6 are given by Eqs. (B18) and (B22). This matches the forms of reflection and transmission coefficients of the system given by Eqs. (B2) and (B3). The structure in Fig. 6 hence behaves the same as such a system, and Eq. (B16) is used to relate these parameters to the parameters used in the Hermitian Hamiltonian.

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