1. Introduction

In traditional optics, the wavevector and the polarization is treated separately. However, the treatment is inadequate at the subwavelength scale in nanophotonics and plasmonics. In these areas, spin-orbit interaction of light [1,2] comes into play, where the wavevector couples with the polarization. Nowadays, spin-orbit interaction of light is extensively explored. One of the phenomena of spin-orbit interaction of light is the spin-locked photon transmission [3,4].

Spin-locked photon transmission appears in evanescent waves in many subwavelength structures [5,6]. Evanescent waves are confined waves which exponentially decay normal to the transmission direction. Thus, evanescent waves possess components along and normal to the propagation direction [4]. So the optical field possesses transverse spin momentum (TSM). Then, the transmission direction of evanescent waves is locked by its TSM [1]. This phenomenon can induce unidirectional transmission by the means of the "chiral photon-emitter coupling" [7], that is, a circularly polarized emitter couples to evanescent waves. If an emitter with helical emission excites the corresponding TSM of the guided mode, the transmission direction is also confirmed because of spin-locked photon transmission. So the chiral coupling (short for chiral photon-emitter coupling) can induce a unidirectional transmission. The chiral coupling is non-reciprocal because the waves with different TSM interact differently with the emitter [7,8]. The properties of chiral coupling have also been studied from other several aspects. Its physical origin is attributed to the quantum spin Hall effect of light by some researchers [9]. The dynamical properties of TSM are also studied in the light-matter interaction with nanoparticles [3].

The unique non-reciprocal property of unidirectional transmission has attracted lots of research. Theoretical work displays detailed calculations and interpretations of the TSM near and in the nanophotonic constituents for achieving more effective unidirectional transmission [10,11]. Besides, the emitters in chiral coupling are also varied in more degrees of freedom, such as the higher-order multipole sources [12]. Some experimental work displays chiral coupling in various nanophotonic structures, including nanowires [13,14], plasmonic structures [15–17], whispering-gallery-mode microresonators [18], and photonic crystals [19–22]. For applications, the non-reciprocity in chiral coupling inspires various experimental work in integrated non-reciprocal optical components, both in the classical and quantum regime, e.g., low-loss isolators in the single-photon level [23,24], multi-port circulators [25], all-optical switchings [26], quantum gates [22,27] and quantum chiral spin networks [28,29].

However, to date, studies concerning enhancing the coupling strength in chiral coupling are scarce [19,30], although modifying emission rates is crucial to achieve potential applications in integrated photonics. In particular, Purcell effect is used to advance nanoscale photon sources towards practical use, such as bright single-photon sources and nanolasers on integrated photonic circuits [31]. Furthermore, the combination between Purcell enhancement and chiral coupling could bring more possibilities of effective on-chip non-reciprocal devices, such as isolators and circulators. As recently reported, over $98\%$ directionality combined with only a $\sim$10-fold Purcell enhancement was obtained in the photonic crystal [19], and a 148-fold enhancement guided by a waveguide with $\sim 95\%$ directionality has been proposed in a hybrid photonic crystal-Ag nanoparticle structure [30], in which many radiation modes are prohibited. Besides, 400-fold Purcell enhancement is theoretically predicted in the whispering-gallery-mode microresonators, but the work did not take photon routing into consideration [32]. Therefore, at present, higher Purcell enhancement with unidirectional transmission remains to be explored. Here, we adopt gap plasmon structures [33–37] to achieve a $10^4$-fold Purcell enhancement. Gap plasmons are strongly localized optical modes in the gap between a metallic nanoparticle and a substrate [38]. The confinement of gap plasmons induce ultrahigh field enhancement with lengths at 20 nm or less. Such strong fields enable reversible light-matter interaction with a single molecule [39]. The efficient photon routing and enhancement of strong light-matter coupling are also proposed in the gap plasmon system [40,41].

In this article, we demonstrate a gap-plasmon-emitter system that exhibits large Purcell enhancement with unidirectional photon transmission in the chiral photon-emitter coupling. By virtue of a highly localized field and the high TSM at the nanoscale gap, the total decay rate is more than $10^4\gamma _0$ and the transmitted photons in evanescent waves have a $\sim 700$-fold enhancement with 91% of the transmission propagating in a single direction. Furthermore, if different eigenmodes are excited by the same circularly polarized emitter in the near field region, the transmission direction is also different. Meanwhile the high Purcell enhancement is still maintained. The proposed system offers methods to efficient photon routing used in optical circuits and quantum networks.

2. Setup of the gap-plasmon-emitter system

Our proposed gap-plasmon-emitter system [Fig. 1(a)] is composed of a low-loss dielectric nanowire, a silver nanoblock, and a circularly polarized emitter placed at the nanoscale gap. The gap plasmon between the nanowire and the nanoblock exhibits strong field confinement at the nanoscale and the resonances are utilized to improve the Purcell enhancement. Moreover, the high field TSM ensures a spin-locked photon transmission. In the chiral coupling between the circularly polarized emitter and the gap plasmons, this system demonstrates a large Purcell enhancement with unidirectional photon transmission in numerical analysis. Compared with photonic crystals [30], nanowires can avoid mode cut-off, and the low-loss nanowire in the gap-plasmon system supports long range evanescent waves, which can directly be used for efficient nanoscale photon routing in on-chip devices.

 

Fig. 1. (a) Schematic diagram of the chiral gap-plasmon-emitter system, which consists of a GaAs nanowire, a nanoblock and a circularly polarized emitter placed in the gap. (b) Normalized absorption spectrum of gap plasmons as a function of the side length $L$. These eigenmodes are excited by a right-propagating plane wave from the left end of the nanowire. $L=13.9$ nm and 38.8 nm are chosen as mode 1 and 2. (c) The field intensity $|E|$ and the transverse spin momentum (TSM) $\Sigma$ around the nanoblock of the two eigenmodes. The TSMs near the nanoblock of mode 1 and 2 are opposite, though they both correspond with right-propagating waves (ticked out by arrows).

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First, the optical modes of this gap-plasmon structure are introduced. A silver nanoblock is placed upon a nanowire with a gap $h$ [Fig. 1(a)]. This silver nanoblock is joined by four rods with side lengths $L=10-50$ nm and four spheres with diameters $2r=10$ nm. The radius of the nanowire is set at $R_0=62$ nm to adapt a single-mode transmission (see Supplement 1). The permittivity of the environment is $\varepsilon _0=3$, and the low-loss nanowire is set at $\varepsilon =12.25$. The nanoblock, made of silver, has permittivity $\varepsilon _\textrm {Ag}=-18.28+0.481i$ as reported by Johnson and Christy in 1972 [42]. When the side length $L$ extends, this gap plasmon structure supports several eigenmodes from the fundamental mode to higher-order modes [Fig. 1(b)]. The results in Fig. 1 are obtained under the excitation by a right-propagating wave, which is obtained by setting an excitation port at the left end of the nanowire with TE plane waves in numerical simulations. The normalized absorption power in Fig. 1(b) is obtained from the integral over the entire nanoblock in the numerical simulations [43]. We choose the fundamental mode ($L=13.9$ nm) and one higher-order mode ($L=38.8$ nm) as mode 1 and mode 2 because of their distinct TSM profiles (see Supplement 1). The modes above are obtained when $h=4$ nm and $\lambda =632.8$ nm. Such nanometer gap is achievable in DNA origami assembling and positioning technology by adjusting the numbers of DNA bases [44]. And the gap height $h$ can be relaxed to 15 nm for a considerable enhancement with unidirectional transmission. We will discuss this choice in Section 4. In these eigenmodes, the electric field $|E|$ is strongly confined in the gap [Fig. 1(c)], and the local transverse spin momentum $\Sigma =\frac {\mathrm {Im}(E_y E_z^*)}{|E_y|^2+|E_z|^2}$ is very high. In particular, the two eigenmodes display opposite TSMs in the gap. This special property enables eigenmodes to modulate the transmission direction in the chiral coupling, which we will discuss in Section 3. Besides, although the system is excited under $\lambda =632.8$ nm, its application can be extended to other wavebands such as the infrared and communication bands (see Supplement 1).

From theory, the properties of Purcell enhancement and unidirectional transmission of the gap plasmon system can be obtained by extracting different contributions from the total decay rate. The total decay rate $\gamma _\textrm {tot}$ transmits into three main channels: absorption $\gamma _\textrm {ab}$ by the silver nanoblock, transmission $\gamma _\textrm {WG}$ along the nanowire, and far-field radiation $\gamma _\textrm {rad}$. Therefore, the total decay rate is the sum of three contributions $\gamma _\textrm {tot}=\gamma _\textrm {ab}+\gamma _\textrm {WG}+\gamma _\textrm {rad}$, assuming other decay channels are negligible. The normalized decay rate (or Purcell factor) of every fraction is calculated from the ratios of these decay rates to the vacuum decay rate ($\gamma _\textrm {tot}/\gamma _0$, $\gamma _\textrm {ab}/\gamma _0$, $\gamma _\textrm {WG}/\gamma _0$, and $\gamma _\textrm {rad}/\gamma _0$) [41,43]. For measuring the unidirectional transmission, the guided fraction $\gamma _\textrm {WG}$ by the nanowire is separated into two portions $\gamma _R$ and $\gamma _L$, denoting the transmission in two directions by the nanowire. Furthermore, the directionality of transmission is evaluated by $D_{R/L}\equiv \frac {\gamma _{R/L}}{\gamma _R+\gamma _L}$, and the collecting efficiency is evaluated by $\beta =\gamma _\textrm {WG}/\gamma _\textrm {tot}$.

The electromagnetic simulations are performed using finite element methods implemented within the COMSOL software. The model configuration is a 2-$\mu$m-radius, 6.2-$\mu$m-long ($\sim 10\lambda$) cylinder surrounded by a perfect matched layer, which guarantees an infinite environment. The robustness of the model is confirmed because the energy flow calculated from the emitter remains steady when the model length is varied. The axis of the nanowire is placed along the $z$-axis, the nanoblock stretches in the $yz$ plane, and the gap stretches along the $y$-axis. The emitter and the nanoblock are separated by a fixed 2-nm space. The circularly polarized emitter in the gap is set as an in-plane oscillating point dipole with $\sigma _{\pm }=\mu (\hat {y} + e^{\mp i\pi /2}\hat {z})/\sqrt {2}$, with the magnitude of the dipole moment set at $\mu =$ 0.1 pA$\cdot$m.

Next, we describe details of the calculation for the decay rates in the gap-plasmon-emitter system. The total decay rates from the emitter are given by the relation $\gamma _\textrm {tot}/\gamma _0=W_\textrm {tot}/W_0$ [45], where $\gamma _0$ denotes the spontaneous emission rate in vacuum, $W_0$ is the emitted power in vacuum, and $W_\textrm {tot}$ denotes the total emitted power in this system. Similarly, $\gamma _\textrm {rad}$ and $\gamma _\textrm {WG}$ are calculated using the corresponding fraction of power. $\gamma _0$ is extracted from the theory of dipole moment emission, and the electromagnetic powers are calculated using the integral of the Poynting vectors $\iint _{A}\vec {S}\cdot d\vec {A}$ over a given surface [40] in numerical simulations. We choose a 1.8-nm-radius sphere, containing the emitter, as the surface of integration for the total power $W_\textrm {tot}$. The absorbed fraction of power $W_\textrm {ab}$ is extracted from the integral over the entire nanoblock. The transmission fraction of the power $W_\textrm {WG}$ is calculated using an integration at the both ends of the nanowire. After subtracting all the above fractions from $W_\textrm {tot}$, the remaining power is then the far-field radiation $W_\textrm {rad}$.

3. Purcell enhancement with unidirectional photon transmission

We next focus on two main properties in this system–Purcell enhancement and unidirectional transmission. Our numerical analysis show a considerable Purcell enhancement with high unidirectional transmission: strong Purcell enhancement $\gamma _\textrm {tot}$ becomes $10^4$-fold higher than the decay in vacuum $\gamma _0$, and directionality $D_{R/L}$ exceeds 90%. Superior to the photonic crystal system reported previously [30], the guided fraction of the enhancement $\gamma _\textrm {WG}/\gamma _0$ increases $\sim$10-fold due to a smaller mode volume and more emission channels, which is useful for brighter photon sources. Furthermore, the transmission direction can be modulated by two eigenmodes in chiral coupling. The mechanism may extend methods for on-chip photon routing. These features are established by gap plasmons with a strong field confinement and high field TSM, which are detailed in the following.

Purcell enhancement is strong in the nanoscale gap because the emitter overlaps the optical field with a high density of states. The calculation shows that $10^4$-fold enhancement is accessible in the gap plasmon system. To obtain a higher Purcell enhancement, the emitter should be placed at the field hotspots in the nanoscale gap. By changing the emitter’s position $L_\textrm {em}$ [the inset of Fig. 2(d)] (the origin of $L_\textrm {em}$ is set at the center of the gap), the fractions of enhancement $\gamma _\textrm {tot}/\gamma _0, \gamma _\textrm {WG}/\gamma _0, \gamma _\textrm {rad}/\gamma _0$ have peaks and valleys when the emitters are in the hotspots and darker areas [Fig. 1(c)]. In numerical calculations, the highest values of the total Purcell factor $\gamma _\textrm {tot}/\gamma _0$ are 19063 in mode 1 with $L=13.9$ nm, and 11055 in mode 2 with $L=38.8$ nm. The difference between two eigenmodes is mainly because of the size of their mode volumes [Fig. 1(c)]. At the same time, the transmission fraction $\gamma _\textrm {WG}/\gamma _0$ reaches its maximum of 1037 for mode 1 and 125 for mode 2.

 

Fig. 2. Purcell enhancement with unidirectional transmission by chiral photon-emitter coupling in the gap-plasmon-emitter system. The electric field excited by a $\sigma _-$ emitter under $\lambda =$632.8 nm in (a) mode 1 and (b) mode 2. The TSM around the nanoblock is highlighted in the right insets, and the circularly polarized emitters are pointed out by red dots. Normalized decay rates $\gamma _\textrm {tot}/\gamma _0$, $\gamma _\textrm {rad}/\gamma _0$ and $\gamma _\textrm {WG}/\gamma _0$ as functions of $L_\textrm {em}$ in (c) mode 1 and (d) mode 2. The dashed lines in (c, d) stress the optimal value of $L_\textrm {em}$ in mode 1 and 2, which are -3.5 nm and -13 nm respectively. (e–h) Directionality $D_{R/L}\equiv \frac {\gamma _{R/L}}{\gamma _R+\gamma _L}$ and the TSM of the field $\Sigma$ of mode 1 and 2 as functions of $L_\textrm {em}$. Under the chiral photon-emitter coupling in modes 1 and 2, the unidirectional transmission is in the opposite direction, which is attributed to different corresponding relations between the TSM profile and the transmission direction. The vertical dashed lines in (e–h) emphasize this correspondence of maximums between directionality $D_{R/L}$ and TSM $\Sigma$.

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Aside from the large Purcell enhancement, the emission from a circularly polarized emitter has unidirectional transmission. The asymmetrical transmission is characteristic of chiral photon-emitter coupling. If the system is excited by linearly polarized light, the chiral coupling will not occur, and the transmission is bidirectional. However, in the chiral coupling, the wavevectors are locked to the field TSM excited by the helical emission from a circularly polarized emitter, so the transmission becomes unidirectional. The numerical calculations reveal that the directionality $D_{R/L}$ reaches 90% in both eigenmodes. To optimize the results, we change the emitter’s position in the gap $L_\textrm {em}$, and the transmittance [Fig. 2(e, f)] along the nanowire is varied with respect to the changes. The highest directionalities $D_{R/L}$ are $D_L=93.0\%$ in mode 1 and $D_R=95.6\%$ in mode 2. The high directionality is a consequence of placing the emitter at high field TSM. The correlation between the directionality $D_{R/L}$ and the field TSM $\Sigma$ are pointed out by vertical dashed lines in Fig. 2(e, g) and (f, h). The peaks and valleys of $D_{R/L}$ and $\Sigma$ correspond well (pointed out by vertical dashed lines in Fig. 2) both in mode 1 and 2. Hence, by selecting the position of the emitter at which the gap plasmons have both field hotspots and high TSM, a balance between Purcell enhancement and unidirectional transmission is achieved. In mode 1, when $L_\textrm {em}=-3.5$ nm, the total decay rate $\gamma _\textrm {tot}/\gamma _0$ reaches 11585, and the guided fraction $\gamma _\textrm {WG}/\gamma _0$ approaches 704 and directionality approaches 91.5$\%$. In mode 2, when $L_\textrm {em}=-13$ nm, the total decay rate $\gamma _\textrm {tot}/\gamma _0$ reaches 5671 and the guided fraction $\gamma _\textrm {WG}/\gamma _0$ approaches 80, directionality approaches 95.6%. The optimal values of both eigenmodes are stressed by dashed lines in Fig. 2(c, d). Because of the Ohmic loss and scattering of the silver nanoblock, the coupling efficiency $\beta =\gamma _\textrm {WG}/\gamma _\textrm {tot}$ is limited below 7%. The low efficiency is attributed to a mismatch between gap plasmon modes and guided modes. It can be improved under further design of the diameters of the nanoblock and the nanowire to reduce the mismatch [41,43]. Besides, a planar surface morphology also increases the coupling efficiency, which is discussed in Section 4.

The transmission direction can be modulated by two eigenmodes mentioned above. In mode 1, chiral coupling yields unidirectional transmission to the left, whereas, in mode 2, the transmission direction reverses [Fig. 2(a, b)]. Such property is relevant to the locking between the profiles of TSM and the transmission direction in these two eigenmodes. In mode 1, the right-propagating waves are locked to the negative TSM near the nanoblock, which also reveals that the left-propagating waves correspond with the positive TSM. However, in mode 2 the relation is reversed, and the right-propagating waves are locked to the positive TSM. The lower insets of Fig. 1(c) illustrate the different corresponding relations. In the chiral coupling, the transmission direction is locked to the TSM excited by a circularly polarized emitter. For example, in mode 1, the TSM excited by a $\sigma _-$ emitter [inset of Fig. 2(a)] is positive near the nanoblock [inset of Fig. 2(e)]. So the transmission direction is the left due to the locking relations mentioned above. The mechanism also takes effect in mode 2, i.e., the $\sigma _-$ emitter excites the positive TSM, so chiral coupling yields a right-propagating wave [insets of Fig. 2(b, f)]. Therefore, the different corresponding relations between the TSM profile and the transmission direction in the two eigenmodes induces the direction modulation in chiral coupling. Note that this modulation of transmission direction need no substitution of a $\sigma _{-}$ emitter for a $\sigma _{+}$ one, which is different from common methods in previous work [7,14]. Additionally, it is worth mentioning that the geometry of the nanoblock highly influences the modulation properties. The opposite TSM profiles in the eigenmodes are necessary in our system, and it cannot be replaced by similar structures, such as a U-shaped nanoblock (see Supplement 1). We numerically obtain that the opposite corresponding relations between the wavevector and TSM do not exist in gap plasmons supported by a U-shaped nanoblock, so the structure cannot modulate transmission direction. Moreover, the modulation can also be realized in a fixed nanoblock size and a dynamical wavelength. The calculation reveals that the transmission direction is opposite at 300 nm and 632.8 nm (see Supplement 1). The eigenmode here is different from that at 632.8 nm because the real part of the permittivity of silver is positive at 300 nm, so it acts like a dielectric with Ohmic losses. The unidirectional transmission established by gap plasmons provides a new way to control unidirectional transmission and gives inspiration for photon routing of different frequencies.

4. Discussions

We focus now on the dependence of Purcell enhancement and unidirectional transmission on structural and material parameters. The gap width $h$, the shape of the nanowire and nanoblock, and the material parameters of the nanowire are chosen to explore their influences on the chiral coupling. First, the gap height $h$ determines the confinement of gap plasmon modes and influences the Purcell factor. The calculation shows that the guided fraction of the enhancement $\gamma _\textrm {WG}/\gamma _0$ reaches 464 when $h=5$ nm, and then decreases to $183\gamma _0$ when $h=15$ nm [Fig. 3(a)]. Besides, with $h$ increasing, the directionality $D_{L}$ raises from 84% to 88%. The transform is a result of the mode decoupling between the nanoblock and the nanowire. To obtain a considerable enhancement and directionality, the optimal choice of the gap width $h$ is less than 15 nm.

 

Fig. 3. Normalized decay rates $\gamma _\textrm {tot}/\gamma _0, \gamma _\textrm {WG}/\gamma _0$ and the corresponding directionality $D_L$ of four structures in mode 1 by chiral coupling. (a) $\gamma _\textrm {tot}/\gamma _0, \gamma _\textrm {WG}/\gamma _0$ and $D_L$ as functions of the gap width $h$. The optimal choice of $h$ is less than 15 nm. (b–d) $\gamma _\textrm {WG}/\gamma _0$ and $D_L$ as functions of the emitter’s position $L_\textrm {em}$ in structures: (b) a pentagonal dielectric nanowire and a silver nanoblock with curved surfaces, (c) a cylindrical dielectric nanowire and a silver nanoblock with planar surfaces, and (d) a cylindrical silver nanowire and a silver nanoblock with curved surfaces. Directionality in all cases are over $80\%$ with large Purcell enhancement. The radius $R_0$ of nanowires is 62 nm.

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In experimental preparations, nanowires generally have different cross sections due to the constraints from the crystal structures, which also change properties of chiral coupling. We substitute a round cross section for a pentagonal one [Fig. 3(b)]. The pentagonal nanowire has a radius (defined as the distance from the center to one vortex) of 62 nm, for which the effective wavevectors are very close to those of a cylindrical nanowire. In this structure, numerical simulations demonstrate the highest total decay rate $\gamma _\textrm {tot}=19527\gamma _0$ because the emitter feels the strongest field near edges with abrupt changes in surface curvatures. The guided fraction $\gamma _\textrm {WG}$ shows a decrease possibly stemming from its lower symmetry. If $L_\textrm {em}=-4.5$ nm, the total Purcell factor $\gamma _\textrm {tot}$ reaches 14680$\gamma _0$, the guided fraction $\gamma _\textrm {WG}$ reaches 489$\gamma _0$, and the directionality approaches 86%.

Next, surface morphology exerts an influence on chiral coupling. Here, the nanoblock is configured with planar surfaces rather than curved surfaces [Fig. 3(c)]. Planar surfaces retain fewer electrons than curved surfaces. Hence, a weaker field confinement leads to a lower Purcell enhancement. Besides, a planar surface suppresses absorption from the nanoblock, so the coupling efficiency $\beta =\gamma _\textrm {WG}/\gamma _\textrm {tot}$ proceeds from 6% to 11%. The total and guided fractions of the Purcell enhancement $\gamma _\textrm {tot}/\gamma _0$, $\gamma _\textrm {WG}/\gamma _0$ reach highest values of 11539 and 1066, respectively. By optimizing parameter values, at $L_\textrm {em}=-2.5$ nm, the total Purcell factor $\gamma _\textrm {tot}$ approaches 6683$\gamma _0$, and simultaneously the guided fraction $\gamma _\textrm {WG}$ reaches 724$\gamma _0$. Meanwhile the directionality approaches $84\%$.

We also substitute the dielectric nanowire for a silver nanowire. This all-metal gap plasmon structures enable a stronger Purcell enhancement than structures described above. However, non-negligible Ohmic loss hinders its collecting efficiency. The gap plasmon in the all-metal structure yields the total Purcell factor of $\gamma _\textrm {tot}/\gamma _0=31856$ [Fig. 3(d)]. However, its transmission fraction is only up to $\gamma _\textrm {WG}/\gamma _0=637$ due to substantial Ohmic loss, and the directionality reaches only 77%. For the optimal value of $L_\textrm {em}$, when $L_\textrm {em}=-3$ nm, the total Purcell factor $\gamma _\textrm {tot}/\gamma _0$, transmission fraction $\gamma _\textrm {WG}/\gamma _0$ and directionality $D_L$ reach 14726, 400, 75% respectively. Therefore, the all-metal structure is more suitable for enhancement of spontaneous emission rather than photon transmission.

Finally, we discuss the fabrication possibilities of our system. Dielectric or metallic nanowires are commonly applied in the fabrication of nanophotonic structures, and has mature preparing process [6,37]. And the entire system could be assembled by scanning tunneling microscopy to position a nanoblock and an emitter [46]. The position of the emitter has a tolerance of 5–10 nm along the gap for a considerable Purcell enhancement and unidirectional transmission. In our system, the preparation of a nanoblock is the most challenging task. In recent work, a 60-nm-long plasmonic chiral nanostructure is precisely assembled by nanoparticle-dressed DNA origami [44]. It is possible for more complicated nanostructures to be fabricated with further progress in assembling technology. Besides, the emission frequency and polarization of the emitter can be tuned by external magnetic fields or applying a static electric field [47].

5. Conclusion

We have demonstrated a chiral gap-plasmon-emitter system for achieving enhanced chiral photon-emitter coupling with both large Purcell enhancement and nanoscale non-reciprocal photon transmission in numerical analysis. The total decay rate $\gamma _\textrm {tot}$ and the guided fraction $\gamma _\textrm {WG}$ rise above $10^4\gamma _0$ and 700$\gamma _0$, respectively. Simultaneously, the directionality $D_{R/L}$ is above $90\%$. We also found that if the same circularly polarized emitter excites different eigenmodes with opposite TSM, the emission will transmit in the opposite direction. The proposed system will benefit on-chip non-reciprocal devices as optical isolators and circulators. Additionally, gap-plasmon-emitter systems can be further explored in the quantum regime for qubit routing in quantum information processing.