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The Surface Plasmon Polariton (SPP) planar waveguide with amorphous silicon (α-Si) cladding is studied, for empowering the device modulation response. The device is fabricated with multiple quantum wells (MQWs) as the gain media electrically pumped for compensating SPP propagation loss on Au film waveguide. The SPP propagation greatly benefits from the modal gain for the long-range hybrid mode, which is optimized by adopting an α-Si cladding layer accompanied with minimal degradation of mode confinement. The proposed structure presented more sensitive response to electrical manipulation than the one without cladding in experiment.
© 2014 Optical Society of America
1. Introduction
The understanding of Surface Plasmon Polaritons (SPPs) has been endeavored for decades, for a range of applications on nanoscale electronic and photonic technologies [1, 2]. However, being subject to the significant ohmic loss in metal, SPP-based components for communication and signal processing encounter the bottleneck of SPP transmission efficiency [3]. Besides optimization of low-loss passive waveguides, development of active SPP devices has drawn equally considerable research interest in term of propagation loss compensation, amplification and even lasing of SPPs [4–6]. Based on light-matter interaction between SPPs and active media such as organic molecules, erbium-doped dielectrics and semiconductors, active SPP devices have been implemented with diverse designs [4, 7–9]. Compared to other types of gain media, the low-dimensional semiconductor materials are granted more advantages for active SPP manipulation due to their potent gain efficiency and material stability [10–12]. In our previous work, elongated SPPs propagation was observed experimentally on the Au thin film waveguides made on top of electrically-pumped multiple quantum wells (MQWs) [4, 13].
In active SPP devices, the interface-bounded SPP mode requires a gain medium in the field vicinity in order to maximize the modal gain. Nevertheless, in an electrically-pumped semiconductor device, a spacer with relatively larger energy band gap and low doping concentration is always needed between the contact and the gain region to serve for carrier confinement. This has to compromise with the mode confinement factor of the gain region in optimized design due to the short decay length of the SPP field in transverse direction. An innovated SPP laser structure with a metal-insulator-metal (MIM) SPP waveguide formed by coating Ag film on mesa sidewalls may solve the dilemma as the electrical and optical optimizations can be satisfied separately in two different dimensions [11]. However, perfect process control is required in order to reduce the scalloped fluctuation or roughness of the mesa sidewalls that leads to high scattering loss. In this letter, we investigated an easy-to-fabricate SPP structure with good performance of gain modulation and mode confinement. The parallel coupling is enhanced between the SPP mode and the MQWs’ index-guided mode by introducing a layer of amorphous silicon (α-Si) as the cladding on 20nm-thick Au metal waveguide. The threshold material gain of the MQWs required for infinite SPPs propagation is estimated based on analytical simulation. It is inspiring that the ideal threshold material gain is within the capability of modern low-dimensional III-V semiconductor structures even though attenuation-free propagation of SPPs has not yet achieved in our experiment. The device with optimized cladding layer is realized on InGaAsP MQW substrate and the experimental result shows distinct improvement of SPP output response to electrical modulation, which is in good agreement with the simulation.
2. Device fabrication and measurement scheme
There are three periods of tensile-strained InGaAsP MQWs in the material structure, which were optimized for Transverse Magnetic (TM) gain near the telecom wavelength 1.55µm. The schematic cross section of the fabricated devices is shown in Fig. 1(a).The Au film is separated from the MQWs by 90nm, and in between are a Separate Confinement Heterostructure (SCH) layer and a 50nm-thick p-doped InGaAsP contact layer. Mesas were etched to make the structure for electrical and optical isolation, and then samples were dipped into diluted HCl/HNO3 mixture solution to remove plasma etching damages on side walls. The mesas are 20µm-wide and 40µm to 140µm-long with 20µm increment. A SiO2 passivation layer was deposited on the surface by plasma-enhanced chemical vapor deposition and then selectively removed on top of mesas by chemical wet etching to open the areas for metallization. 2nm-thick Ti and 20nm-thick Au films were deposited as the electric contact and the metal guide. On both ends of a metal guide, coupling gratings were defined by electron beam lithography. An amorphous silicon (α-Si) film was lastly deposited on the top using electron beam evaporator at slow and steady rate in order to guarantee good control on final thickness and film quality. The α-Si was used as the cladding material because it is transparent in the near infrared spectral window and its refractive index matches that of InP. Samples with α-Si film thicknesses of 100nm, 160nm, 180nm and 200nm were prepared for the successive test and analysis. The top view of the fabricated device is shown in Fig. 1(b).
Fig. 1 (a) Device cross section schematic. (b) The top view of fabricated device with waveguide length of 40µm.
In order to enhance the sensitivity of signal detection, a measurement system was customized based on the setup described in [13]. The system configuration and corresponding working mechanism are shown in Fig. 2.SPP modes were excited by 1.55µm polarized laser on the Au waveguide via a coupling grating. The SPP signals were decoupled from an output grating and collected by a monochromator via a multimode fiber probe. A free-space chopper at 1000Hz frequency was inserted between the laser source and the input coupler for input signal modulation. In parallel, the device under test was biased by a high precision pulse current source at 1200Hz frequency and 30% duty cycle. The bias was added vertically on the diode with the Au waveguide as the anode. The monochromator provided a wavelength window centered at 1.55µm with 0.1nm passband to suppress the influence of spontaneous emission on readings, and the integration time of the lock-in amplifier was set as 2s for noise suppression. The measurements were repeated and the averaged data were derived for alleviating the random noise. By taking the differential frequency between those of the optical chopper and the pulse current source for reference, the lock-in amplifier only extracted the signal sent from the input and modulated by the electrical current source, while rejected the out-of-phase components given by spontaneous emission and direct transmission [14, 15]. Attributed to this measurement customization, the signal can thus be regarded as the SPP response to the amplification by the gain material. The SPPs propagation lengths could be estimated by analyzing the output from waveguides of different lengths, with the assumption that the input and output gratings’ coupling efficiencies were consistent for all samples in the measurement.
Fig. 2 Experiment setup. A high precision pulse current source is used for electrical injection. A square wave signal generator provides the driving signal for current source f2, optical chopper f1 and the phase-locked pulse with calculated frequency f2-f1. The differential pulse is the input of lock-in amplifier as reference signal.
3. Simulation and theoretical analysis
Prior to device fabrication, the multilayer semiconductor structure was evaluated with analytical simulation. The refractive indices of semiconductor layers used in the simulation are referred to [16]. Apart from the active layer, the rest semiconductor layers in the system are considered transparent at working wavelength, i.e., with zero imaginary part of refractive indices. Refractive indices of other layers are retrieved from [17].
Dispersion curves were plotted as functions of cladding layer thickness in Fig. 3(a) using the Transfer Matrix Method (TMM). Unlike the waveguide in [13] where both SRSPP and LRSPP were supported, it can be seen that only one TM mode, i.e. short-range SPP (SRSPP), is sustained in the structure with the cladding layer thickness less than 140nm on the 20nm-thick metal layer. A hybrid mode with much longer propagation lengths emerges when the α-Si cladding layer goes above this critical thickness. The two bound TM modes, i.e. SRSPP and LRSPP, are supported simultaneously when the cladding thickness is less than 690nm, above which higher order LRSPP mode will be activated. As the SRSPP mode attenuates rapidly in a few microns due to high intrinsic loss, the signal observed thereafter would be dominated by the LRSPP mode.
Fig. 3 (a) Plots of mode indices versus cladding thickness labeled with the critical thickness for each mode. (b) TM field distributions for modes with cladding thickness 0nm, 100nm, 160nm, 180nm and 200nm. (c) Mode size and threshold material gain versus cladding thickness for modes with cladding thickness varying from 150nm to 360nm. (d) Plots of Im(neff) versus Im(nMQW) showing the relationship between the modal loss and the material gain. The threshold material gains for attenuation-free propagation of SPPs are labeled on the plots.
Response of an SPP mode to the MQW gain material is also evaluated. The TM field distributions shown in Fig. 3(b) for different cladding thicknesses reveal that the SRSPP mode’s field magnitude decay exponentially on both sides of the metal/semiconductor interface, with trivial coupling to the active layer. The field intensity peak lies on the Au/Semiconductor interface when the cladding thickness is less than the critical thickness, but moves to the Au/α-Si interface for greater cladding thickness when the LRSPP is activated. With the cladding thickness being further increased from 160nm to 200nm, the SRSPP field distribution shows little change and slight suppression in the active layer. On the contrary, the modulation on the LRSPP would become more efficient as it couples to the active layer more strongly. The cladding layer influences the LRSPP mode field distribution by improving the refractive index symmetry of the SPP waveguide. As shown in Fig. 3(b), the improved confinement factor, which originates from a second field peak of the hybrid LRSPP mode around the MQWs, implies the greatly enhanced efficiency of electrical manipulation on SPPs. This feature, more ideally, can be tailored by varying the cladding thickness, given the diverse field distribution corresponding to 160nm, 180nm and 200nm cladding thickness.
Being a flexible and effective factor for customizing the hybrid LRSPP mode, the effect of cladding thickness is worthy of further investigation. The required material gain for attenuation-free propagation of SPPs, i.e. Im(ksp) = 0, versus the cladding layer thickness is shown in Fig. 3(c). With an LRSPP mode being activated, increasing cladding thickness quickly pulls down the threshold material gain, which illustrates the enhancement of SPP amplification by stimulated emission. The threshold material gain keeps decreasing till the cladding thickness increases to ~260nm, where the threshold has been as low as ~1 × 103cm−1.
In addition, the effective mode size versus the cladding layer thickness is also shown in Fig. 3(c). The calculated effective mode size as the merit of mode confinement ability is defined as the lateral distance over which the TM field magnitude decays to 1/e of its global field maximum [18]. The abrupt rise of the mode size occurs for the cladding thickness changing from 170nm to 180nm because the field around the MQW region exceeds 1/e of the global field maximum. The maximum around the cladding thickness of 260nm can be explained by the most symmetrically distributed SPP field at both sides of the Au film. The opposite trends of the two curves shown in Fig. 3(c) shows that the proposed structure does not break the trade-off between tight mode confinement and low SPP propagation attenuation [19]. As implied by mode size curves shown in Fig. 3(c), the growth of cladding thickness decreases the confinement of the hybrid mode, although the SPP mode benefits more from the MQWs’ material gain. Therefore, these two characteristic parameters need to be synergized in device design. The experimental investigation on cladding thickness around 160nm, 180nm, and 200nm were advised, in consideration of achieving both small mode size and reduced threshold material gain. Figure 3(d) shows the imaginary part of the mode effective index as the function of the MQWs’ material gain, with cladding thickness being 200nm, 180nm, and 160nm. The threshold material gains for attenuation-free propagation of SPPs are denoted as 1583cm−1, 2745cm−1, and 5431cm−1, respectively. The material gain of 1583cm−1 is well achievable for low-dimensional semiconductor materials, like quantum wells, quantum dots and nanowires/nanorods. A threshold material gain of ~1583cm−1 for attenuation-free propagation requires carrier concentration ~4 × 1018cm−3, according to the k·p calculation [4].
4. Measurement result analysis and propagation length fitting
Waveguides with lengths ranging from 40µm to 140µm with 20µm increment were tested for obtaining the SPP propagation length by curve-fitting the output intensities for samples with different cladding thicknesses. A structure with cladding thickness 187nm was set as an example to illustrate the procedure of measurement data analysis and derivation of propagation length. The output intensities from the abovementioned measurement scheme are plotted as the functions of the current density in Fig. 4(a).Because the device was biased vertically, the current density could be easily derived from current amplitude divided by the metal strip area. The data were recorded with biasing current starting from 400A/cm2 instead of zero for the sake of the lock-in measurement scheme described before. The experiment showed that noticeable signal response took place at about 1200A/cm2. The intensity values at high injection level before saturation were as great as 5~20 times of those at low injection level. For the devices with longer waveguides, especially for that with 140µm-long waveguide, the detected intensity at low injection level almost kept constant without perceptible change, because the signal was below the detection limit of the experimental setup due to severe intrinsic loss.
Fig. 4 (a) Plots of output intensity versus current density for devices with various waveguide lengths. The logarithmic scale shows increments at small intensities clearly. (b) Measured intensity versus waveguide length at various current densities and the fitting plots using the exponential form. Experimental samples have α-Si cladding thickness 187nm and waveguide width 20μm.
The plots of output intensity versus waveguide length for various injection current densities shown in Fig. 4(b) can be regarded as intensity distributions along the propagation path, although experimental error did exist due to repetition of aligning the objective lens and the fiber probe above the coupling gratings among samples. As the SPP intensity attenuates exponentially along the propagation path, an exponential function can be adopted for curve-fitting the data. The distance where the magnitude decays to 1/e of that at the origin is defined as the mode propagation length, and the plot of derived propagation length versus injection current density is shown in Fig. 5 (sample D). The propagation length is elongated from 20µm to nearly 50µm in a rapid growth with increasing current density before reaching saturation at a high injection level. It clearly demonstrates the gain contribution to the SPP propagation, although the stimulated emission of SPPs has not overcome the intrinsic propagation loss in order to achieve the net output amplification.
Fig. 5 The propagation length fitted from the measurement data under different α-Si film thicknesses. The device without cladding is plotted as the reference. The dash lines are visual aid for indicating the growth trend of propagation length with increasing current density.
5. The influence of cladding thickness to propagation length
Structures without cladding (denoted as sample A) and with cladding of various thicknesses (practically 105nm, 156nm, 187nm and 203nm, denoted as samples B, C, D, E, respectively) were prepared for experimentally studying the influence of cladding thickness on the mode performance. Following the curve-fitting approach adopted in Fig. 4(b), the derived propagation lengths versus current density are plotted in Fig. 5 for the abovementioned samples. According to TMM simulation for the case without gain and dielectric absorption, their propagation lengths are 3, 1, 16, 30, and 42μm, respectively. For samples A and B, the error of derived propagation lengths is large due to weak signal and thus poor signal-to-noise ratio. Detection limit restricted by noise in the measurement is manifested by the curve of the 140μm-long waveguide at intensity level of ~10−8 for current density less than 2000A/cm2. For samples C~E, the theoretical propagation lengths could be reached in presence of gain compensation, i.e., under current injection at 1700~2400A/cm2, and higher current density is needed for greater cladding thickness. This suggests that the dielectric absorption greatly affected the transmission of SPPs. The plots show a trend of increasing propagation length with increasing current density till 3200A/cm2, beyond which saturation occurred due to device overheating and thus quantum efficiency degradation. The experiment did not achieve attenuation-free propagation of SPPs predicted by the theoretical analysis for several reasons. Our theoretical calculation underestimates the threshold material gain and the corresponding carrier concentration, e.g., ~1583cm−1 and ~4 × 1018cm−3, respectively, for cladding thickness 200nm, as the dielectric absorption were not taken into consideration. In real case, dielectric absorption exists in both α-Si and the InGaAsP compound material. Furthermore, the nonideal carrier injection limits the growth of carrier concentration in practical case due to (1) appreciable spontaneous emission in MQWs under low optical input and high current injection level and (2) excess carrier leakage caused by device overheating under the present uncooled test condition.
Nevertheless, it is clearly seen that plots of samples C~E whose cladding thicknesses are greater than 140nm are distinguished from those of samples A and B in Fig. 5, due to their different propagation modes. Apparently, samples C~E have much longer propagation lengths than samples A and B, as the former employed the LRSPP mode while the latter employed the SRSPP mode. These experimental results are in agreement with the numerically calculated ones in Fig. 3(a). Moreover, it can be found from Fig. 3(b) that the coupling strength of the SRSPP mode to the MQWs decreases gradually with increasing thickness of the cladding layer. Thus, the introduction of the cladding layer can play a role in suppressing the modal gain of the SRSPP mode. This means that the SRSPP was undetectable because of its fast attenuation such that there was merely noise in readings under low current injection for samples A and B. For samples C~E, even under high current injection, the SRSPP is insignificant in the output compared to the LRSPP due to this cladding effect.
Also, it is seen in Fig. 5 that the plots for samples C~E have larger growth rate with increasing current density than those for samples A and B, denoting better sensitivity to gain modulation. Figure 3(b) implies that the LRSPP mode’s confinement factor in the MQWs was much greater than the SRSPP mode’s. This is different from the results in [13], where there was a highly asymmetric structure and the SRSPP’s mode confinement factor in the MQWs was greater than that of the LRSPP. As shown in Fig. 5, sample E could present a propagation length increment of 32µm (i.e. from 21µm under low current injection level increased to 53µm under high current injection level), and equivalently the modal gain for LRSPP increased by 288cm−1. Here, the modal gain increment is estimated as ∆g = 1/LSPPs(Il) – 1/LSPPs(Ih), where LSPPs is the mode propagation length, and Il and Ih are the low and high current density, respectively. Therefore, by improving the plasmonic waveguide symmetry using the cladding layer, the MQW gain medium become capable of operation by the LRSPP mode. With the increased coupling strength between the SPP mode and the gain medium, loss compensation is more efficient than that demonstrated in [13] whose operation was by the SRSPP mode.
Plots for samples C~E in Fig. 5 show that the propagation length is greater for larger cladding layer thickness. Nevertheless, it is also noticed that the difference between the plots for samples D and E is not as appreciable as that between the plots for samples C and D, which seems to deviate from the numerical simulation forecast in Figs. 3(c) and 3(d). This is because dielectric absorption was not considered in the numerical simulation. Absorption of dielectric layers rises with increasing cladding thickness as the field confinement in the dielectric layers goes up. Numerical calculation suggests that the increase of dielectric absorption becomes quite appreciable for large cladding layer, i.e. ~200 nm, and greatly counteracts reduction of the metal absorption.
To further increase the compensation efficiency, optimization of metallization for ohmic contact for SPP waveguide and top epitaxial semiconductor would be critical. Improving device heating dissipation and reducing the contact resistance will make the device better for practical applications, such as modulators and lasers. Even though the gain threshold was predicted for classic SPP waveguides on MQWs [20, 21], the feasibility of electrically pumped SPP devices was suspected due to the modal loss issue and shortened carrier recombination lifetime [22]. The hybrid SPP mode is one of the solutions for separating the active layers from the high electrical field area at metal interface. Fabricating SPP waveguides on atomic level smooth surface of epitaxial samples not only greatly simplifies the processing, but also provides the flexibility of SPP mode conversion by spatially patterning the cladding layer [23].
6. Conclusion
We have demonstrated improved active manipulation of SPP propagation on electrically-pumped MQWs-based plasmonic device with high refractive-index dielectric cladding. The proposed structure with the thickness of the α-Si cladding layer below the critical value only supports an SRSPP mode. Above the critical thickness, a hybrid LRSPP mode with minimal degradation of mode confinement emerges as a better choice for loss compensation than the SRSPP mode studied in our previous work, due to improved coupling strength between the SPP field and the MQW gain medium. Improving the waveguide symmetry with a dielectric cladding layer changes the situation that only the SRSPP mode can be actively modulated in the asymmetric SPP waveguide, and further suppresses the SRSPP modal gain. A propagation length increment of 32µm, which is equivalent to a modal gain increment of 288cm−1, has been observed experimentally by harnessing the LRSPP mode. This work demonstrated an effective approach to active manipulation of SPPs.
Acknowledgments
The authors acknowledge financial support from the National Natural Science Foundation of China (grant no. 61176085 & 61377055), the Department of Education of Guangdong Province, China (grant no. gjhz1103) and the open-project fund support from the State Key laboratory of Opto-Electronic Material and Technologies (Sun Yatsen University), Guangzhou, China. N. Zhu acknowledges financial support from Natural Science Foundation of China (61308007) and Specialized Research Fund for the Doctoral Program of Higher Education (20134407120012).
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