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Abstract
Plasmonic interconnection is one kind of the possible methods to construct next-generation optoelectronic integrated circuits. In this paper, the plasmonic interconnection device based on Ge in infrared band is constructed, through efficient electron-hole pair generation, the device can achieve high photocurrent response (0.25A/W). Because of the low plasmon coupling efficiency of the conventional basic periodic gratings, this paper optimized the design of the coupling structure and improved the coupling efficiency by 4 times through constructing a binary Bragg/periodic grating coupler which can realize unidirectional plasmon coupling with a simulated extinction ratio of 12.5 dB. The devices can be easily fabricated by single-step electron beam lithography and lift-off process. The experimental results verified a 3.5 times improvement in the SPPs current of the designed plasmonic interconnection device, which provides a technical path to realize efficient plasmon transmission and detection for on-chip optoelectronic interconnection.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Modern high-speed communication systems and computer chips have higher requirements for information processing and transmission rates, however, in conventional integrated circuits (ICs), problems such as signal delay and high-power consumption become more and more serious [1–4]. Although the traditional optoelectronic integrated circuits (OEICs) have the advantages of high reliability and ultra-high speed, but they are still limited by diffraction limit in practical applications [5–7]. Surface plasmon polaritons (SPPs) can overcome the diffraction limit and have high speed transmission of information, so the plasmonic interconnection circuits (PICs) based on SPPs is currently one of the important development directions in the field of OEICs [1,6–10]. In the recent years, the key components of PICs such as lasers [11–15], waveguides [16–18], modulators [19–26] and photodetectors [27–30] have been reported, and even various kinds of the PICs prototypes have been proposed [31–36].
Three building blocks are indispensable for a complete on-chip PICs device: First, a coupler to connect SPPs with the excitation source, second, a plasmonic waveguide to transport SPPs and, last, a detector decouples the SPPs into the photonic or electronic signal [26,27,37–39]. Subwavelength periodic gratings are often used as the coupling structure of SPPs in surface plasmon functional devices due to its simple structure, high coupling efficiency, easy preparation, and adjustable resonance wavelength [27,40]. At the same time, the periodic grating can also achieve SPPs decoupling, the SPPs propagating along the waveguide can be re-scattered into the free-space to further realize on-chip SPPs-photonic/electronic conversion [25,27]. Based on the above process, Leuthold et al. designed a series of fiber-to-plasmonic modulators with metal gratings and waveguide, these devices are highly compact and have extremely high data modulation rates which realize the interconnection between plasmonic and photonics [25,26].
In many other application cases such as integration with ICs, PICs should have effective and compatible connection with modern planar microelectronic circuits [27,41]. Fukuda et al. discussed the feasibility of PICs for on-chip interconnection, through building the Schottky junction between the the metal gratings and silicon substrate, the SPPs can be converted to a photocurrent based on hot-electron injection in the 1550 nm wavelength band [36–38,42–44]. Panchenko et al. reported a metal-semiconductor-metal (MSM) interdigital grating SPPs detector based on the electron-hole pair generation in the visible light band which is much more efficient than hot-electron injection [27]. However, in the above the PICs devices, the efficiency of the SPPs coupler and detector still have a lot of room to improve [26,45]. One of the problems that cannot be ignored is that only half of the generated SPPs can reach the detector because the SPPs propagated to both sides of the grating, so, it is important to control SPPs to propagate only in a desired direction [37,45]. Furthermore, constructing efficient SPPs-electronic detector in the infrared communication band is also significant for modern electronic applications [36,37,39].
In this paper, we first built the plasmonic interconnection device with basic periodic metal grating coupler and interdigitated electrodes (IDEs) detector. High SPPs responsivity can be achieved by using Ge substrate in 1550 nm wavelength band based on electron-hole pair generation. We numerically analyzed the coupling efficiency and polarization sensitivity of the periodic metal grating, which agree well with the experimental data. In order to achieve unidirectional coupling of SPPs for further improving the efficiency of SPPs coupling and detection, we propose the binary Bragg/periodic grating coupler. Through reasonable design, the coupling efficiency can be increased by 4 times and the extinction ratio of the unidirectional SPPs can reach 12.5 dB investigated by numerical analysis. The experimentally measured SPPs current can been increased by about 3.5 times compared with the basic device without Bragg grating under the same device footprint. All the devices can be easily fabricated by single-step electron beam lithography (EBL) process. Our work provides a simple and effective solution to increase the transmission efficiency of PICs for further on-chip optoelectronic interconnection.
2. Experimental
2.1 Device fabrication and characterization
All the proposed devices were fabricated on an intrinsic Ge wafer with a bulk resistivity of 50 Ω·cm. As a first step, the Ge wafer was cleaned by ultrasonication in acetone, isopropyl alcohol, and deionized water for 15 minutes, respectively. The designs of the grating coupler, waveguide and detector were subsequently patterned via a JBX5500ZA EBL system after being covered with a 250-nm-thick layer of PMMA-A4 resist. Once the sample was developed, a 100 nm Au layer for the device with basic periodic grating coupler and 200 nm Au layer for the device with binary Bragg/periodic grating coupler was respectively deposited using a thermal evaporator, and the sample was placed in warm acetone maintained at 50°C for several hours. After lift-off, the sample was cleaned in isopropyl alcohol and dried with nitrogen gas. Finally, the contact pads were led out by an ultrasonic aluminium wire welder for subsequent characterization. The morphology of the device was characterized by high-resolution scanning electron microscopy (SEM, FEI-Quanta 200).
2.2 Optical characterization
The characterization of the device was carried out using homemade 1550 nm laser sources. A combination of an infrared polarizer (Thorlabs, LPNIRC100-MP2) and an infrared half-wave plate (Thorlabs, HWP20-1550B) was used for the generation of a linearly polarized light beam and the polarization rotation. In addition, an infrared non-polarizing 50:50 beam splitter (Thorlabs, MBS1455-C) was utilized to align the beam, which was subsequently focused on the sample through an infrared microscope objective (Thorlabs, MY50X-825, ×50, numerical aperture of 0.42). The measurements of the current-voltage (I-V) and photocurrent were performed with a Keithley 2400 source meter.
2.3 Numerical analysis
Numerical simulations were carried out via the two-dimensional (2D) finite-difference time-domain (FDTD) method. The mesh size of 5 nm and the perfectly matched layer (PML) boundary condition are used in both the x direction and the z direction. The optical properties of Au and Ge used in the model were taken from the experimental data by Johnson et al. [46].
3. Results and discussion
3.1 Plasmonic interconnection device with basic periodic grating coupler
In order to deeply understand the basic working characteristics of the designed plasmonic interconnection device in the infrared band, we first focus on the device with basic periodic grating coupler. The schematic diagram of the basic device in x-y plane is shown in Fig. 1(a), which consists of four components, including an Au SPPs grating coupler, an Au waveguide which is designed to be tapered for matching the size difference between the coupler and detector, a MSM detector with IDEs, and a contact pad connected to external circuit. Different periods (coupler: P, detector: p) and slit widths (coupler: W, detector: w) are combined to constitute the coupling and decoupling area of the SPPs. The grating couplers (P = 1520 nm and W = 800 nm) were designed for 1550 nm normal incident illumination, an Au-tapered waveguide and IDEs detector (p = 1500 nm and w = 750 nm) were utilized for the propagation and decoupling of the SPPs (Fig. 1(b)). In addition, for the SPPs detector with basic periodic grating coupler, the thickness (h) of the grating couplers, waveguide, and IDEs detector is 100 nm to ensure that the SPPs at the Au/air and Au/Ge interfaces are decoupled and to suppress the direct leakage of the SPPs into the Ge substrate [27].
Fig. 1. (a) Schematic diagram of the plasmonic interconnection device with basic periodic grating coupler in x-y plane (Due to figure size limitation, only part of the coupling and decoupling gratings is drawn). (b) Illustration of the working processes of the device in x-z plane, SPPs can be excited at the area of periodic coupling gratings (red dotted border) under the incident light (red straight arrow) and propagate along the waveguide (red curved arrow), finally outcoupling to air (green arrow), localized surface plasmon (orange arrow) and Ge substrate (blue arrow) on the IDEs detector. (c) Band diagram of a biased Au/Ge heterojunction explaining the two generation mechanisms of the photocurrent: direct generation of electron-hole pairs and hot electrons based on internal photoemission, qΦB and EF represent the height of the barrier and Fermi energy levels, respectively.
The working processes of the device can be divided into the following four steps as shown in Fig. 1(b). In step 1, as a SPPs excitation structure, the coupling gratings are used to compensate the wave vector mismatch between the SPPs and the incident light [47]. Subsequently, the excited SPPs are transmitted along the tapered waveguide at the Au-air interface (step 2). Because of the wavevector mismatch caused by the IDEs, the SPPs that propagate to the end of the waveguide is directly decoupled into radiation, allowing photons to radiate upward into the air (green arrow) or downward into the Ge substrate (blue arrow). Another part of the SPPs can be coupled back into the localized surface plasmon mode (orange arrow) at the IDEs (step 3). In step 4, as shown in Fig. 1(c), when a positive and negative bias is applied to the IDEs, the electron-hole pairs generated by the SPPs scattered into the Ge substrate can be quickly separated and collected by the Au IDEs and leads to photocurrent production. At the same time, the hot-electron generation and injection through the Au-Ge Schottky barrier into the substrate will lead to an additional photocurrent attributed to internal photoemission (IPE) [27,39,40]. So, the device can detect the SPPs both above and below the band gap of Ge in the infrared band.
Figure 2(a) illustrates the scanning electron microscopy (SEM) image of the fabricated plasmonic interconnection device with basic periodic grating coupler. Except that the width of the coupling gratings and IDEs is slightly widened, the error of other structures is controlled within 15 nm compared with design. Though the size of the entire coupled grating is about 55 × 55 µm2 which is bigger than the similar SPPs grating coupler reported in other literature to match the incident spot size in our experiment platform, but this structure presented in our work is flexible and permits varying the parameters to match the different types of incident light source and spot size [27,40]. A short (5 µm) and tapered waveguide is utilized in this case for ensuring low propagation loss and reducing the effect of the localized surface plasmon and incident light for the further photoelectric test. The dark I-V characteristic of the IDEs detector is shown in Fig. 2(b), the detector consists of two identical Schottky diodes connected back-to-back, which shows similar behaviour under both positive and negative bias. The detector shows a large dark current because of the large metal-semiconductor contact area and small band gap of Ge, the dark current could be reduced by introducing barrier layer or guard ring [27]. However, on the other hand, the higher carrier mobility of Ge can improve the responsivity of the SPPs detector.
Fig. 2. (a) SEM image of the fabricated plasmonic interconnection device with basic periodic grating coupler, the blue and black dotted circles represent the different light incident areas. (b) The measured I–V curve of the IDEs detector. (c) Photocurrent as a function of polarization angle with the light irradiating the different positions. (d) Generated photocurrent as a function of different incident power (direct illumination, 10 mV bias, 1550 nm wavelength). The average responsivity of the SPPs detector is 0.25 A/W.
The photocurrent (bias voltage of 10 mV for the small dark current) under the same illumination (86 µW, the incident spot diameter is 40 µm which is about 3/4 of the grating coupler to meet the same illumination with the binary Bragg/periodic grating device, 1550 nm wavelength) at different positions (position 1 and position 2 as shown by the blue and black dotted circle in Fig. 2(a), respectively) and polarizations was measured as shown in Fig. 2(c). When the polarization direction of the incident light is perpendicular to the lines of the coupling grating (TM-polarization, 0°/180°), the photocurrent reaches the highest value of Iperp ≈ 22 µA. While the photocurrent is minimum (Ipar ≈ 11 µA) when the polarization direction is parallel to the lines of the coupling grating (TE-polarization, 90°/270°). It can be found from the blue fitting curve that the photocurrent has a cos2(θ) relationship with the polarization angle of the incident light which consistent with the SPPs excitation conditions of periodic grating coupler [27,40].
When the incident light illuminates Ge substrate at position 2, the photocurrent IGe does not change with the polarization angle as shown in Fig. 2(c). It is worth noting that IGe is greater than Ipar and less than Iperp. This is because that when the incident light whose polarization angle is parallel to the lines of the coupling grating, the SPPs mode cannot be excited, the Ipar is generated by the Ge substrate directly absorbing the incident light on the grating coupler. Since the incident light cannot completely pass through the coupling gratings, Ipar is smaller than IGe generated by the incident light directly irradiating on the Ge substrate. When the incident light whose polarization angle is perpendicular to the lines of the coupling grating, the SPPs mode on the Au-air interface is excited, Iperp is greater than IGe. The photocurrent detected at the IDEs is the combination of SPPs current (ISPPs = Iperp-Ipar) caused by SPPs and ISub caused by the absorption by the Ge substrate at the bottom of the coupling gratings. This result shows that the detection of SPP signals has been successfully realized in the fabricated SPPs interconnection device with basic periodic grating coupler.
In order to characterize the linearity of the SPPs detector, we linearly fit the photocurrent value under different incident light power, as shown in Fig. 2(d), the light responsivity of the detector is about 0.25 A/W which is much larger than the previously reported silicon-based SPPs detection structure [27,36,37,42,43].
Figure 3(a) shows the normalized electric field intensity distribution of |Ez| of the basic grating coupler side in the x-z plane. It is clearly observed that the SPPs traveling to the Au waveguide are coupled by the basic grating on the Au-air interface (z = 0.1 µm) under illumination by 1550 nm total-field-scattered-field (TFSF) source. In the simulation process, a power monitor at 2 µm length away from the leftmost end of the waveguide as shown by the white dotted line in Fig. 3(a) is observed to reduce the effect of the localized surface plasmon and the incidence field, the SPPs power is obtained by integrating the Poynting’s vector on the vertical power monitor line. The coupling efficiency can be defined as the ratio of the power of SPPs traveling to the waveguide and the incident light. The calculated coupling efficiency of the basic periodic grating coupler is about 6% as shown in Fig. 3(a). It is worth noting that because the fabricated device has lots of gratings which requires considerable computing space, we simplified the grating coupler in the FDTD simulation. Figure 3(b) shows the coupling efficiency as a function of different number of coupling gratings in the 1000-1800 nm wavelength range, we can find that when the number of gratings exceeds 6, there is no obvious change in the simulated curve of the coupling efficiency versus wavelength, except that the full width at half maxima (FWHM) gradually decreases. Therefore, we believe that 7 coupling gratings used in the simulation model are sufficient to meet the simulation accuracy, since the fabricated device contains far more coupling gratings, the wavelength selection characteristic will be better. The coupling efficiency as a function of polarization angle in the 1400 to 1700 nm wavelength range is shown in Fig. 3(c), which indicates the same relationship with polarization angle as the photocurrent as shown in Fig. 2(c). The blue dotted line in Fig. 3(c) is the coupling efficiency at the center wavelength of 1550 nm which shows the maximum of about 6% under TM-polarization (0°/180°) illumination.
Fig. 3. (a) Normalized electric field distribution of the device with basic periodic grating coupler in the grating coupler side, the coupling efficiency is about 6% towards the waveguide under illumination by 1550 nm total-field–scattered-field source. (b) Coupling efficiency as a function of different number of coupling gratings in the 1000-1800 nm wavelength range. (c) Coupling efficiency as a function of polarization angle in the 1400 to 1700 nm wavelength range, the blue dotted line shows the the coupling efficiency at the center wavelength of 1550 nm.
In such a plasmonic interconnection device, transmission loss is a serious problem because SPPs signals experience large ohmic and scattering losses on the Au-air interface [37]. Though the intensity of the SPPs decays strongly as they propagate along the waveguide, the theoretical transmission distance can meet the requirements for constructing plasmonic circuits is secured in ICs under the limit of bit error rate (BER) and energy loss [36,37]. To investigate the decay of SPPs, five plasmonic interconnection devices with different waveguide lengths were fabricated and characterized using the same setup. The incident light with the same power (86 µW) and spot size (40 µm) was irradiated on the grating coupler to excite SPPs. For normalization, we use the SPPs current (ISPPs = Iperp-Ipar) as the indicator to evaluate the attenuation of the device with different waveguide lengths. Figure 4(a-e) show the relation between the registered ISPPs and the polarization angle of the incident light for 5, 7, 10, 15 and 20 µm long waveguides, respectively, the amplitude of the ISPPs decreases with increasing length. The average value of the SPPs current under TM-polarization illumination as a function of the waveguide length is shown in Fig. 4(f) together with the simulated power flow normalized by the value at the 5-µm waveguide length. The simulation result approximates well the corresponding experimental result which shows the similar exponential decay as the waveguide length increases. The measured exponential decay characteristic of the SPPs current with waveguide length proves that the device can work as a SPPs detector [27,43].
Fig. 4. (a-e) SPPs current of the the device with basic periodic grating coupler as a function of polarization angle for different waveguide lengths. (f) SPPs current at each waveguide length. An exponential fit of the experiment data (red line) and the normalized power flow simulated by FDTD at different distances (blue dotted line) are plotted on the same graph.
3.2 Plasmonic interconnection device with binary Bragg/periodic grating coupler
Due to the symmetry of the SPPs excitation under direct illumination, the SPPs propagating to the left and right waveguide is equal in the basic plasmonic interconnection which leads to the reduction of the energy coupled from the SPPs traveling to the right waveguide. Therefore, the coupling efficiency of SPPs needs to be further improved to realize the efficient excitation and detection of SPPs to meet the requirements of high efficiency, low power consumption and high signal-to-noise ratio in the future PICs. Adding Bragg reflection gratings on one side of the coupling gratings has been proven to be a simple and effective method to achieve unidirectional SPPs transmission [48–50], so we build the binary Bragg/periodic grating coupler based on the above basic plasmonic interconnection device as shown in Fig. 5.
Fig. 5. Schematic diagram of the plasmonic interconnection device with binary Bragg/periodic grating coupler in x-y plane, the blue and red dotted border represents the area of Bragg and basic periodic gratings, respectively.
When the SPPs propagate through the Bragg gratings in the left side of the basic periodic coupling gratings as shown in Fig. 5, they can be diffracted to different angles. By adjusting the period (PBragg) of the Bragg gratings, the SPPs traveling in the Bragg gratings can be mainly reflected and constructively interfere with each other [50]. The relation for constructive interference can be written as [50]:
where kSPPs and λSPPs is the SPPs wave vector and wavelength, respectively and m is an integer. When m is 1, then the PBragg is half of the SPPs wavelength, which is about 728 nm. Due to the structural particularity of the Bragg gratings, the width of a single Bragg grating (WBragg = 364 nm) is designed to be half of PBragg, that is, one quarter of the SPPs wavelength. The reflected SPPs from the left Bragg gratings will interfere with the SPPs wave on the right basic periodic coupling gratings. By optimizing the distance D between the center of the rightmost bar of the Bragg gratings and the center of the basic coupling gratings, the reflected SPPs from the left Bragg gratings can have constructive interference with the original SPPs on the right basic periodic coupling gratings. The phase difference Δφ between the two above SPPs waves can be expressed as [50]:In the previous design, m = 1 has been taken. In order to make the two SPPs waves meet the condition of constructive interference, the phase difference should be Δφ=2nπ, where n is also an integer, so the distance D needs to satisfy the following expression:
Due to the limit of the grating coupler size in the FDTD simulation model, the distance D must be greater than 4800 nm, so take n = 8, D is set to 5460 nm in the simulation model. It must be noted that the size of the actual device is much larger than the simulation model, we adjusted the corresponding value of D to fit the fabricated device. Because the Bragg gratings are only used to reflect the SPPs traveling to the left and make the reflected SPPs have constructive interference with the original SPPs on the right instead of coupling the incident light, the TFSF source only covers the area of the basic periodic gratings. In addition, the thickness (h) of the Bragg gratings will also affect its reflection effect on the propagating SPPs, we find that a thickness of about 200 nm Au layer has the best result through FDTD simulation. Figure 6(a) shows the normalized electric field intensity distribution of |Ez| of the binary Bragg/periodic grating coupler in the x-z plane. It is obvious that after using the designed binary grating coupler, the excited SPPs can almost all propagate to the right waveguide. The calculated coupling efficiency in the basic periodic grating side (ηright) is about 23% compared with the Bragg grating side (ηleft) which is about only 1.2%. Figure 6(b) is the calculated ηright and ηleft in the in the 1500 to 1600 nm wavelength range, we can find that in the entire 100 nm wavelength range, the ηright is greater than 15%, while ηleft is lower than 2%. Therefore, the simulated extinction ratios ER = 10log(ηright /ηleft) of the SPPs wave traveling to the different side is more than 10.5 dB in the 1500 to 1600 nm wavelength range and the maximum of ER is about 12.5 dB at 1550 nm as shown in Fig. 6(c).
Fig. 6. (a) Normalized electric field distribution of the device with binary Bragg/periodic grating coupler in the grating coupler side, the coupling efficiency is about 23% towards the right waveguide and 1.2% towards the left waveguide. (b) The coupling efficiency at the different side of the binary Bragg/periodic grating coupler in the 1500 to 1600 nm wavelength range, the red area indicates that the coupling efficiency is greater than 20%. (c) Extinction ratios of the device with respect to the wavelength, the blue area indicates that the ER is more than 12 dB.
Figure 7(a) shows an SEM image of the proposed plasmonic interconnection device with binary Bragg/periodic grating coupler, we replaced about 1/4 of the basic periodic coupling gratings of the basic device as shown in Fig. 2(a) with the designed Bragg gratings and keep the same device footprint. Except that the thickness of the Au layer of the device with binary grating coupler is 100 nm thicker than the basic device, other parameters such as waveguide length, IDEs size and etc. remain the same. Corresponding to the simulation model in Fig. 6, the laser only irradiates the area of the basic periodic gratings in the actual photoelectric test as shown by the red dotted circles in Fig. 7(a). The thicker Bragg gratings are good for reflecting SPPs propagating to the left but may weaken the scattering of SPPs into the Ge substrate. However, the experimental SPPs current of the SPPs detector with binary grating coupler (red line) is still about 3.5 times (at TM-polarization) larger than the basic device (black line) under the same illumination (86 µW, incident spot diameter is 40 µm) as shown in Fig. 7(b), which confirms the improvement of the Bragg gratings on the detection efficiency of the plasmonic interconnection device due to the effective increase in coupling efficiency as shown in Fig. 6.
Fig. 7. (a) SEM image of the plasmonic interconnection device with binary Bragg/periodic grating coupler, the red dotted circles represent the light incident areas. (b) The SPPs current of the device with basic and binary Bragg/periodic grating coupler as a function of polarization angle.
4. Conclusion
In summary, we have proposed and experimentally demonstrated a form of the plasmonic interconnection device consisting periodic grating coupler, waveguide and IDEs detector to realize on-chip excitation, transmission and electric detection of SPPs, respectively. We have designed the basic plasmonic interconnection device with high photocurrent response in the infrared band and introduced binary Bragg/periodic grating coupler to further improve the SPPs coupling efficiency. We numerically demonstrated that under the normal illumination condition, the designed binary Bragg/periodic grating coupler can obtain about 4 times increase in the unidirectional coupling efficiency compared with the basic periodic grating coupler, as a result, the SPPs detection efficiency of the whole device has been increased by about 3.5 times. The coupling efficiency of the grating SPPs coupler can be further improved by using secondary etching process and refractive index matching, etc. The device fabrication in our work is simple and can be compatible with the standard CMOS process. Our technique of applying unidirectional SPPs transmission and communication band optoelectronic material to the plasmonic interconnection points out a promising direction for future on-chip efficient optoelectronic interconnection and communication.
Funding
National Natural Science Foundation of China (61805037).
Disclosures
The authors declare no conflicts of interest.
Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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