1. Introduction

Periodic metallic slow-wave structures have widely been investigated for terahertz (THz) waveguiding. The surface-confined electromagnetic wave supported by these structures resembles the surface plasmon polaritons at a metal-dielectric interface in the optics regime in terms of field confinement and dispersion characteristics [1-4]. Hence, it is commonly referred to as spoof surface plasmon (SSP) wave in the THz literature [‎2-‎12]. The propagation characteristics of SSP structures depend on their geometry [1]. Hence, various types of SSP waveguiding structures either in planar or cylindrical shape have been introduced. Due to strong field confinement and proper low-profile geometry, planar SSP structures are a promising choice for realizing compact waveguides as a basic structure for implementing THz integrated guided-wave devices and circuits [4-6].

Planar SSP waveguides have been proposed in various designs including holes in a metal plane [‎7,‎8], pillars on a metal surface [9,10], and structured thin metallic strips [11,12]. Among the proposed planar SSP waveguides, the domino and the conformal surface plasmon (CSP) structures provide higher field confinement and have attracted more interest as a result. The former is a one-dimensional (1D) array of metallic boxes on a metal plane while the latter is a 1D array of corrugations on the edge of a thin metal strip. In all of these SSP waveguides, the tradeoff between conductor losses and field confinement is generally a challenge. However, their propagation length is typically long enough for sub-THz applications. The reported propagation length for both the domino and CSP waveguides in the sub-THz band is in the order of tens of wavelengths [4,12]. Spoof localized surface plasmons (LSPs) based on planar plasmonic resonators have also been proposed and used in conjunction with SSP waveguides to realize new THz devices [13-15].

For the realization of THz circuitry, planar dielectric waveguides and defective photonic crystal structures are the other candidates [16-19]. In this regard, dielectric ribbon or ridge waveguides made up of high-resistivity silicon (HR-Si) have been proposed at sub-THz frequencies for very low-loss guided-wave propagation [16-18]. Thanks to the availability of microfabrication techniques for silicon based structures, HR-Si waveguides are promising for sub-THz monolithic circuit integration [17-19]. The mechanism of waveguiding in the mentioned dielectric structures is index-guiding; but, surface guiding in high-contrast gratings (HCGs) has also been reported [20]. The photonic band-gap (PBG) phenomenon in 2D photonic crystals with a line defect is another mechanism that can be exploited for THz waveguiding [21].

While the dielectric and PBG based waveguides outperform the metallic SSP waveguides in terms of the propagation loss, their cross-sectional dimension is diffraction-limited and their single-mode bandwidth is limited [11]. Therefore, at sub-THz frequencies, where loss of metals is tolerable and subwavelength field confinement by miniaturized waveguides is highly demanded, the metallic SSP structures are preferred. Hence, these structures are our main focus in this paper.

In this paper, we report on the design and analysis of a hybrid metal-dielectric SSP waveguide that can support subwavelength field confinement in a broad bandwidth of nearly one octave. The proposed structure is composed of a 1D array of metallic teeth over a metal plate that is surrounded by a customized rectangular channel of HR-Si and covered by a metal plate overlay. The dimensions of the proposed waveguide can be as small as one-tenth of free-space wavelength at the maximum frequency of operation and it can support a sharp bending radius of a quarter of wavelength with a negligible radiation loss.

The outline of this paper is as follows. First, we introduce the structure and explain where its idea is stemmed from. Then, we analyze a 2D version of the proposed structure having an infinite lateral width to gain more insight into its operation. Afterwards, we expand our studies to 3D structures of finite lateral width and characterize their waveguiding properties by full-wave simulation. In this regard, we also investigate different schemes of dielectric coating and different shapes of metallic teeth. And finally, we assess the performance of a sharp waveguide bend based on the presented structures and give some concluding remarks.

2. The proposed hybrid SPP waveguide

The proposed structure is illustrated schematically together with several related waveguides in Fig. 1 in order to highlight the background of its idea. Figure 1(a) shows the all-metallic structure of a conventional domino waveguide composed of a 1D array of pillars on a metal plane [4,5]. The confinement of the domino waveguide can be enhanced by integrating the pillars into a customized dielectric channel as shown in part (b) of this figure. In a recent work, it has been shown that the confinement and the bending loss of the waveguide can be improved over a wide bandwidth by coating the pillars by a HR-Si channel of certain dimension while keeping the space between the pillars empty [22]. Figure 1(c) shows the parallel-plate ladder waveguide (PPLWG) [23] which is composed of a similar pillar array sandwiched between parallel metal plates. The guided SSP mode of this structure is closely related to the TE₁ mode of the parallel-plate waveguide (PPWG). Hence, its low frequency cutoff is dictated by the plate spacing [23]. Figure 1(d) shows the ridge gap waveguide [24] which is composed of a central metallic ridge on a metal plane surrounded by a 2D array of metallic posts. Another metal plate is placed at a specific distance above the mentioned metallic structure. The field is confined in the gap between the metal ridge and the overlay and the wave propagation is limited to the ridge axis as a result of the high surface impedance of the metallic post array. In this structure, the guided-wave is closely related to the TEM mode of the PPWG [24]

 

Fig. 1 Schematic drawing of the proposed hybrid SSP waveguide (e) compared with other related planar structures including domino (a), dielectric coated domino (b), parallel-plate ladder (c), and ridge gap waveguides (d). The transparent box is the dielectric channel. The arrow shows the direction of wave propagation.

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Combining the features of the mentioned waveguides, we propose the metallo-dielectric SSP waveguide shown in Fig. 1(e). The proposed structure is composed of a 1D array of dielectric coated metallic teeth on a metal plane above which a metal cladding is overlaid. Both the metal cladding and the dielectric coating can be designed in such a way that the interaction of the SSP wave and the grating is increased over a wide bandwidth. As a result, the proposed structure can provide a higher field confinement in a wider bandwidth as compared to its other counterparts. From a practical standpoint, the proposed structure also features a rigid and shielded structure thanks to the presence of parallel metal plates. The HR-Si is a perfect choice for the dielectric coating due to its low loss, low dispersion, and relatively high refractive index in THz regime. Moreover, the proposed structure can be fabricated by common silicon based techniques and its integration with other active and passive devices is possible. The proposed waveguide can be realized by a variety of available fabrication processes; such as deep reactive ion etching (DRIE), electroplating, micromachining, and other common planar fabrication processes, particularly those related to the silicon technology [19,17 25-27].

Detailed structure of the proposed waveguide in two cross-sections is demonstrated in Fig. 2. The axis of the waveguide is along the x-axis. Firstly, rectangular-shaped pillars of width w and height h are assumed. Then, we show that a similar performance can be obtained by cylindrical posts, too. The period of the pillars along the x-axis is d and the width of the grooves in between is a as shown in Fig. 2(b). The grooves are filled by a dielectric of relative permittivity ε₃. As illustrated by the hashed region in Fig. 2(a), the mentioned metallo-dielectric grating is enclosed by a dielectric channel of width w_d, height h + h₁ and relative permittivity ε₂. The upper metal cladding is placed above the dielectric channel at the distance of h₂ and the remaining region between the parallel plates is filled by a dielectric of relative permittivity ε₁. The waveguiding characteristics of this structure are determined by the introduced geometrical parameters and dielectric materials. We start our analysis by a two-dimensional structure (2D) with no variation along the y-axis; then, we extend our study to three-dimensional (3D) structures.

 

Fig. 2 Details of the proposed structure in the yz (a) and xz (b) planes. The direction of propagation is along the x-axis.

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3. Study of the 2D structure

At first, we analyze a 2D structure by assuming an infinite lateral width for the pillars and the dielectric channel, i.e., w = w_d→∞ in Fig. 2(a). Hence, the structure reduces to a corrugated metal plane with a metal cladding and inhomogeneous dielectric filling as shown in Fig. 2(b). In the resultant 2D structure, we can expand the EM fields of the SSP mode into a series of transverse-magnetic (TM) Bloch waves along the direction of propagation which is the x-axis. The transverse component of the magnetic field in region I (-h₃<z<-h₁) can be written as

By applying the continuity conditions of tangential electric and magnetic fields at z = 0 and z = -h₁ along with some algebraic simplifications, the dispersion relation for the 2D structure can be obtained as follows

The dispersion relation of the SSP wave can be extracted by solving Eq. (4) and the effect of various parameters on the propagation of the SSP wave can be investigated. In particular, the asymptotic or Bragg frequency of the SSP wave and the deviation of its dispersion curve from the light line are of prime importance to us. The extent of separation between the dispersion curve of SSP wave and the light line is an indicator of the degree of interaction between the SSP wave and the hybrid grating which can also be related to the confinement of the SSP wave to the grating surface.

To study the impact of metal cladding on the dispersion relation of the SSP wave, we consider a sample structure with d = 50μm, a = 40μm, and h = 55μm which is homogeneously filled with HR-Si, i.e., ε₁ = ε₂ = ε₃ = 11.9 [17]. If the distance of metal cladding and the grating (h₃) approaches infinity, the asymptotic frequency of this structure is nearly 300GHz. By reducing the mentioned distance to a certain limit, the deviation of the dispersion curve from the light line is increased without a noticeable change in the asymptotic frequency. Further reduction of the distance results in a drastic decrease of the asymptotic frequency and the bandwidth. In this regard, Fig. 3(a) shows the dispersion curves of the mentioned structure for different values of h₃. It is clear that for distances greater than or equal to 25μm, the asymptotic frequency is negligibly changed despite the deviation of the dispersion curve from the light line. Therefore, by placing the metal cladding at a proper distance from the grating, the SSP wave becomes more confined to the grating surface without reducing the bandwidth. In other words, the wideband enhancement of field confinement is possible by introducing the metal cladding. From this viewpoint, the distance of metal cladding has an optimum value which is nearly h₃ = 25μm in this example.

 

Figure 3 Dispersion diagram of a sample 2D structure with d = 50μm and a = 40μm; (a) for homogeneous filling with HR-Si, h = 55μm and different distances of the metal cladding (h₃), (b) for h₃ = 25μm and different schemes of dielectric filling. In each scheme, height of corrugations (h) is chosen in order to fix the Bragg frequency at 300GHz.

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In a recent work, we have shown that the confinement can also be improved by filling the grooves by a dielectric with lower permittivity than that of the exterior region, i.e., ε₃<ε₂ [22]. To assess this fact in the presence of the metal cladding, we revise the previous study to incorporate an inhomogeneous dielectric filling. The distance of metal cladding is kept fixed at the optimum value of h₃ = 25μm while the period and the groove width are unchanged. In this case, we study the dispersion characteristics of the structure for different scenarios of dielectric filling. Each region of the structure, as shown in Fig. 2(b), is considered to be filled either with air or with HR-Si. The Bragg frequency depends on the effective height of the corrugations. Thus, when the structure is filled with a different dielectric, the Bragg frequency is changed. Therefore, to make the comparison of dispersion curves straightforward, we design the height of corrugations (h) accordingly in order to keep the Bragg frequency fixed at 300GHz. For example, when the grooves are filled with air while the remaining region is HR-Si (ε₁ = ε₂ = 11.9, ε₃ = 1) the height of corrugations should be increased to h = 98μm as compared to the case of homogeneous filling with HR-Si (ε₁ = ε₂ = ε₃ = 11.9) in which the height is h = 54μm. Following this routine, the dispersion diagram of the structure for different dielectric filling schemes is calculated and illustrated in Fig. 3(b). By comparing the obtained curves it becomes evident that the optimum dielectric filling scheme is to fill the grooves with air while the rest of space is filled with HR-Si which is ε₁ = ε₂ = 11.9, ε₃ = 1. In this case, the dispersion diagram shows the maximum deviation from the light line. To reveal the important impact of metal cladding, the dispersion diagram in the absence of metal cladding (h₃→∞) is also illustrated in this figure.

4. Modal analysis of 3D structures

The analysis of the 2D structure revealed the basic features of the proposed structure including the impact of metal cladding, the optimum distance of metal cladding, and the optimum scheme of dielectric filling. Relying on the ideas inferred from the simple 2D structures, we extend our studies to the realistic 3D structures in which the corrugations are of a finite lateral dimension. In order to extract the modal characteristics of the 3D hybrid SSP waveguide, we have used the CST Microwave Studio commercial software. For extracting the dispersion diagram of 3D waveguides under investigation, we performed an Eigen mode analysis on only one period of the structure. The periodic boundary condition with a pre-determined phase shift is applied to the faces of the unit cell normal to the direction of the propagation. By sweeping the phase shift from 0 to π, the dispersion diagram can be obtained.

Using the notation introduced in Fig. 2, we assume a metallo-dielectric SSP waveguide with d = 50μm, a = 0.8d, and w = 50μm. As explained before, depending on the scheme of dielectric filling, we can change the asymptotic frequency by designing the height of pillars (h). As before, we set the asymptotic frequency around 300GHz by proper choice of h. It should be stressed that the asymptotic frequency is increased by decreasing the height of pillars, and vice versa. Thus, in each scheme of dielectric filling the change of asymptotic frequency can be compensated by the varying the height of metallic pillars. At first, we study the effect of the metal cladding distance on the asymptotic frequency of the structure at hand. We assume homogeneous filling with HR-Si (ε₁ = ε₂ = ε₃ = 11.9) and set the height of pillars at h = 57μm. The resulting Bragg frequency as a function of metal cladding distance is illustrated in Fig. 4(a). Based on this figure, the minimum distance at which the Bragg frequency undergoes a negligible change is around h₃ = 25μm. Thus, we set the distance of cladding at this optimum value in all of the next simulations. Similar to what mentioned in the previous section, this choice leads to an enhanced confinement without sacrificing the operational bandwidth of the waveguide.

 

Fig. 4 (a) Asymptotic frequency of 3D structure versus the distance of metal cladding (h₃) for d = 50μm, a = 0.8d, w = 50μm, h = 57μm and homogeneous filling with HR-Si. (b) The dispersion diagram in the presence and absence of the metal cladding for rectangular and cylindrical-shaped pillars. The dimensions are given in the text.

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In Fig. 4(b), the calculated dispersion diagram of the mentioned HR-Si filled structure for d = 50μm, a = 0.8d, h = 57μm, and w = 50μm in the presence (h₃ = 25μm) and in the absence (h₃→∞) of the metal cladding is compared. It is evident that the presence of metal cladding results in more deviation of dispersion curve from the light line which is an indication of improved field confinement. The shape of pillars in the proposed waveguide is not limited to rectangular. In Fig. 4(b), the dispersion diagram for a structure with cylindrical posts is also presented. This structure is composed of a 1D array of cylindrical pillars with diameter 38μm, height h = 58μm, period d = 50μm, and a metal cladding distance h₃ = 20μm. It is clear that nearly the same performance is achievable by cylindrical posts, too.

As mentioned in the previous section, the confinement can further be increased by a customized dielectric filling. To investigate this issue, we revise the previously-studied structure by enclosing the pillar array by a HR-Si ridge with finite lateral width of w_d = 2w. The grooves are filled with air and the metal cladding is placed on the HR-Si ridge without any gap (ε₁ = ε₂ = 11.9, ε₃ = 1). Other dimensions of this structure are d = 50μm, a = 0.8d, w = 50μm, h = 82μm, and h₃ = 25μm. We compare the performance of this structure with that of the conventional domino waveguide, the dielectric coated domino waveguide, and the parallel-plate ladder waveguide (PPLWG) mentioned in Fig. 2(a), 2(b), and 2(c), respectively. The mentioned structures are all designed to have a Bragg frequency of 300GHz. The domino waveguide is composed of rectangular pillars of width w = 50μm, height h = 57μm, period d = 50μm, and groove width a = 0.8d in a homogeneous HR-Si medium. The dielectric coated domino waveguide is similar to the DRAF waveguide proposed in [22]. It consists of rectangular metallic pillars of height h = 82μm, width w = 50μm, period d = 50μm, and groove width a = 0.8d in a HR-Si ridge of height 182μm and width w_d = 200μm. The grooves and the exterior space are filled with air in DRAF structure [22]. The PPLWG is composed of 1D array of rectangular metallic posts of width 120μm, thickness 15μm, and period 50μm which are sandwiched by parallel metal plates of spacing 360μm and immersed in homogeneous HR-Si medium.

In part (a) of Fig. 5, the calculated dispersion diagram of the proposed SSP waveguide is compared to that of the PPLWG, DRAF and domino waveguide. The PPLWG has a low-frequency cutoff of nearly 120GHz which is similar to the cutoff frequency of the TE₁ mode of the PPWG. As can be seen in this figure, the phase constant of the proposed waveguide is noticeably larger than that of the other structures nearly at all frequencies. Hence, the proposed structure outperforms the mentioned counterparts in terms of the field confinement. In part (b) of this figure, the group velocity of the mentioned waveguides normalized by the speed of light in vacuum is illustrated. It is evident that the group velocity of the proposed waveguide is lower than that of the domino and DRAF waveguides at lower frequencies which indicates the higher field confinement of the proposed waveguide.

 

Fig. 5 Comparing the dispersion characteristics (a), the normalized group velocity (b), the attenuation constant (c), and the normalized propagation length (d) of the proposed waveguide with those of PPLWG, the conventional domino waveguide, and the dielectric coated domino waveguide (DRAF [‎22]).

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Like all of the metallic SSP waveguides, higher confinement is achieved at the cost of higher conductor losses. Assuming the conductivity of σ_c = 6.3 × 10⁷S/m (silver) for the conductors and σ_si = 5 × 10⁻²S/m for HR-Si, the attenuation constant of the structures under investigation is calculated and depicted in Fig. 5(c). The propagation length of the mentioned structures is also illustrated in Fig. 5(d). In this figure, the propagation length is defined as the distance along which the transmitted power reduces to 1/e of its initial value and is normalized by the wavelength in HR-Si. Figures 5(c) and 5(d) reveal that the proposed waveguide has a higher attenuation constant and a shorter propagation length which is an inevitable result of the higher field confinement. Depending on the application demands, the proposed structure has the flexibility to be designed for high confinement or for low propagation loss. In this paper, we seek high confinement to build very low leakage sharp bends.

As mentioned before, the high field confinement of the proposed waveguide is attributed to both the customized dielectric coating and the metal cladding. The lower refractive index of the grooves enhances the penetration of electric field inside them as a consequence of normal electric field discontinuity. In addition, the finite width of HR-Si channel along with the presence of metal cladding tends to confine a considerable portion of power in the area between the metal cladding and the pillars, i.e., in |y|<w/2 and –h₃<z<0. The fraction of propagated power in this area is calculated for the proposed structure and shown in Fig. 6(a). It is observed that more than one-third to one half of transmitted power is confined in this small area all over the bandwidth. In this figure, the result of a similar calculation for the DRAF waveguide is also provided. In the DRAF waveguide the fraction of transmitted power in the semi-infinite rectangular area above the metallic pillars, i.e. |y|<w/2 and z<0, is calculated. The hashed regions in the insets of Fig. 6(a) show the mentioned areas. It is apparent that the fraction of transmitted power above the metallic pillars of the DRAF waveguide is less than that of the proposed waveguide. This finding proves that the presence of metal cladding in the proposed waveguide increases the field confinement, especially at lower frequencies. It can be attributed to the enhancement of the z-component of the electric field in the gap between the pillars and the metal cladding.

 

Fig. 6 (a) The fraction of transmitted power confined in the gap between the pillars and the metal cladding in the proposed waveguide (solid line) compared with the fraction of transmitted power above the pillars in the dielectric coated domino waveguide (DRAF) (dashed line), (b) the intensity of electric field in the cross-section the proposed waveguide (bottom) and the dielectric coated domino waveguide (DRAF) (top) at 225GHz.

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The normalized intensity of the electric field in the cross section of the proposed waveguide and the DRAF waveguide is also calculated at 225GHz and illustrated in Fig. 6(b). Again, the outstanding confinement of the electric field in the proposed waveguide is evident. It is also evident that the proposed waveguide has smaller cross-sectional dimensions in comparison with the DRAF waveguide. Based on these results, an extremely low radiation loss at sharp bends is expected for the proposed waveguide.

5. Performance of sharp 90° bends

In this section, we investigate the performance of 90° bends based on the proposed hybrid SSP waveguide studied in Figs. 5 and 6. In this regard, a waveguide bend of radius 250μm which is a quarter of free-space wavelength at 300GHz is simulated. A customized rectangular waveguide adapter is used to excite the SSP mode of the mentioned structure and extract the scattering (S) parameters of the waveguide bend in a similar manner as reported in [22,28]. The adapter realizes a high efficiency transition from the rectangular waveguide to the proposed waveguide. It is comprised of a tapered dielectric channel and metallic pillar array which convert the TE₁₀ mode of the input rectangular waveguide to the SSP mode of the proposed waveguide. The adapter is designed to obtain very low reflection and radiation loss and is composed of lossless materials in the simulations. The designed adapter has an overall length of 500μm and its reflection (magnitude of S₁₁ parameter) is less than −10dB all over the bandwidth of the proposed waveguide.

The bend (radiation) loss can be calculated from the S-parameters in the absence of dielectric and conductor losses of the structure. The radiation loss is calculated by comparing the transmission coefficient (magnitude of S₂₁ parameter) of the bend with that of a straight waveguide section of exactly the same overall length. In Fig. 7(a), the bend loss of the mentioned structure for two different widths of the HR-Si channel (w_d = 100, 200μm) and that of the DRAF waveguide [22] is illustrated. In part (b) of this figure, the magnitude of the z-component of electric field at 225GHz in the plane of bend for the case of w_d = 200μm is illustrated. It is clear that the radiation loss is extremely low. The maximum bend loss for the structure with w_d = 100μm is below 0.5 dB all over the frequency range of 160GHz~300GHz which is much lower that of the DRAF waveguide and the conventional domino waveguide (reported in [22]) at lower frequencies. This finding is consistent with the results of Fig. 6(a) and is attributed to the presence of the proposed metal cladding.

 

Fig. 7 (a) Radiation loss of a bend of radius 250μm based on the proposed hybrid SPP waveguide for HR-Si channel width of 100 and 200μm and the same bend based on the DRAF waveguide [22], (b) the z-component of electric field in the plane of the bend at 225GHz for the proposed waveguide with w_d = 200μm.

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To assess total insertion loss of the mentioned waveguide bend, the overall transmission coefficient of a bend of radius 250μm and total length of 900μm is calculated for both the proposed waveguide and DRAF waveguide and presented in Fig. 8. The overall transmission of a straight waveguide section of length 900μm is also shown in this figure. In these calculations, material losses are taken into account (σ_c = 6.3 × 10⁷S/m and σ_si = 5 × 10⁻²S/m). Figure 8 reveals that the proposed waveguide outperforms the DRAF waveguide at lower frequencies in terms of the insertion loss. The metal cladding increases the modal confinement at low frequencies and reduces the radiation loss of the bends. As shown in Fig. 8, both the straight and the bent DRAF waveguides suffer from a high insertion loss at low frequencies as a result of radiation loss of both the adapters and the bend which is a consequence of low field confinement.

 

Fig. 8 Overall transmission coefficient along bent and straight sections of the proposed waveguide (with w_d = 200μm) and the dielectric coated domino waveguide (DRAF). The straight sections and bends have the same total length of 900μm and material losses are included in the simulations.

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The achieved bend performance can further be highlighted by comparing the proposed structure with a HR-Si ridge waveguide. In a recent work, a silicon–BCB–quartz (SBQ) THz dielectric waveguide is proposed and its bend insertion loss is assessed [18]. The SBQ structure is composed of a HR-Si ridge bonded to a quartz substrate by a thin adhesive layer. The cross-sectional dimension HR-Si ridge is nearly one-third of free-space wavelength (λ₀/3) at its operational frequencies and can reach a bend loss of less than 0.5dB for bending radius of almost one wavelength (λ₀). In comparison, the proposed metallo-dielectric SSP waveguide reaches almost the same bend performance with a cross-sectional dimension less than one-tenth of free-space wavelength (λ₀/10) and the bend radius of a quarter of wavelength (λ₀/4) over a 1.8:1 bandwidth. Thus, realization of low loss subwavelength bends is made possible by the proposed subwavelength hybrid SSP waveguide.

6. Conclusions

In this paper, a metallo-dielectric SSP waveguide is proposed and analyzed. The proposed structure is composed of a conventional metallic grating on a metal plane coated by a rectangular channel of HR-Si and overlaid by a metal cladding. Both modifications improve the field confinement of the SSP wave in a broad bandwidth. This claim is proved by the analysis of 2D and 3D structures and by simulating sharp 90° bands of the proposed waveguide. The results of full-wave simulation of the mentioned structure in 160-300 GHz band show a considerable improvement in the field confinement with a waveguide cross-section of one-tenth to one-fifth of free-space wavelength at the asymptotic frequency. It is also shown that the proposed structure can be bent at a sharp radius of one-quarter of a wavelength with a negligible radiation loss all over the bandwidth. The achieved results reveal that the proposed hybrid SSP waveguide outperforms the other related structures such as metallic SSP and dielectric ridge waveguides in terms of field confinement and radiation loss. Since the proposed structure is easy to fabricate by conventional microfabrication processes especially those based on silicon technology, and by considering its outstanding performance in terms of field confinement and bend loss, it is a promising choice for realizing compact sub-THz guided-wave devices and circuits. Being integrated between parallel metal plates, the proposed hybrid SSP waveguide needs no additional packaging and possess a shielded and ruggedized structure suitable for real-world applications. This feature can be considered as an important advantage of the proposed structure compared to other mentioned SSP and dielectric waveguides from a practical point of view.