We report on monitoring the mode power in dielectric-loaded surface plasmon polariton waveguides (DLSPPWs) by measuring the resistance of gold electrodes, supporting the DLSPPW mode propagation, with internal (on-chip) Wheatstone bridges. The investigated DLSPPW configuration consisted of 1-μm-thick and 10-μm-wide cycloaliphatic acrylate polymer ridges tapered laterally to a 1-μm-wide ridge placed on a 50-nm-thin and 4-um wide gold stripe, all supported by a ~1.7-µm-thick Cytop layer deposited on a Si wafer. The fabricated DLSPPW power monitors were characterized at telecom wavelengths, showing very high responsivities reaching up to ~6.4 μV/μW (for a bias voltage of 245 mV) and the operation bandwidth exceeding 40 kHz.
©2013 Optical Society of America
The massive growth of telecom and data communication traffic in the last decade can be attributed to using optical fibers as the transmission medium which has already taken over the task of long-distance communications from electrical cables and which refines the connections between different parts of large electronic systems. However, in short-distance communications inside information-processing devices on integrated circuit chips and on circuit boards wires still dominate. It means that the optical signals have to be converted to electrical ones, to be amplified, regenerated, or switched, and then they are reconverted to optical signals. It is well known that the optical-to-electronic-to-optical (OEO) conversion is a significant impediment in transmission. Further, a limited capacity of electrical interconnects is a problem for systems even at short distances between chips and on chips. So, replacing existing electronic network switches with optical ones is strongly desired since the need for OEO conversions is removed. Therefore, optical switches can play an important role in applications, including optical cross connection, protection switching, and switching arrays for optical add-drop multiplexing .
Among many available switching technologies the thermo-optic switches  are very attractive due their small size, large scalability, and potentiality for integration with waveguide dense-wavelength division-multiplexing multiplexers. Their optical performances, in terms of cross talk and insertion losses, are acceptable for many applications. Thus, the speed of waveguide devices based on the thermo-optic effect is quite adequate for applications oriented on controllable routing of optical signals.
The design of thermo-optical switches, as well as of switches in general, aims at achieving low switching powers, fast switching times, and high extinction ratios [3, 4]. Additionally, the footprint should be as small as possible to meet a requirements for compact waveguide components and easy in fabrication. But in reality, considering a thermo-optical switch, a trade-off between the temperature switching and power dissipation per unit length must be taken into account. In the case of photonics waveguides, if a thermo-optical switch is realized using a material with a high thermal conductivity, then a short switching time but a high switching power per unit length is obtained. On the contrary, using a material with a small thermal conductivity, a long switching time, but a low switching power per unit length can be achieved. The reason for it is that metallic stripes acting as heaters are deposited on top or lateral of the waveguides next to the switching region, leading, however, to high polarization-dependent losses since the absorption of TE and TM modes by closely placed electrodes is very different. To avoid this one should either move away the electrodes from the propagating region or introduce a thin dielectric layer between the metallic heater and the waveguide what, however, influences on thermal properties of the device.
Compare to dielectric waveguides, plasmonic devices can guide and manipulate optical signals on subwavelength scale and below the diffraction limit of light. Among various SPP-based waveguide configurations, dielectric-loaded SPP waveguides (DLSPPWs) [5, 6] represent an attractive alternative by virtue of being naturally compatible with different dielectric and industrial fabrication using large-scale UV lithography. DLSPPWs satisfy the important requirements of strong mode confinement, relatively low propagation losses, and straightforward integration with control electrodes enabling a thermo-optic control. The main advantage of the plasmonic technology is that gold stripes can be used both as supports of DLSPPWs and electrodes allowing for heating the plasmonic devices where the DLSPPW mode field reaches its maximum at the metal-dielectric interface [6–8]. On the contrary, the main problems in plasmonic technology are high losses related with metal-induced attenuation. This impact can be minimized by integration of short plasmonic waveguides with longer dielectric waveguides . In this way, the small size and low power switching capabilities of plasmonic can be blended with the low loss of dielectric waveguides and processing capacity of electronics, to provide miniaturized and power efficient photonic interconnect routers [10–12]. Additionally, the inevitable propagation losses in plasmonic waveguides can be turned into a useful functionality by DLSPPW mode power monitoring realized via measuring variations in the resistance of metal stripes supporting DLSPPW ridges caused by heating due to the mode absorption [13, 14].
2. Experimental arrangement
The substrate used was standard Si wafer that was covered with a spin-coated ~1.7μm-thick Cytop realized using a procedure similar to those reported in ref [11, 12]. Cytop grade CTL-809M and the corresponding solvent CT-SOLV180 used for dilution were obtained from AGC Chemicals Europe Ltd. The other processing resists used: AZnLOF 2070 and LOR-A were from Microchemicals Gmbh and MicroChem Corp., respectively. The investigated DLSPPW-based Mach-Zehnder interferometers (Fig. 1 ) were fabricated by a UV lithography process using a Süss Microtech MJB4 mask aligner in the vacuum contact mode and using consecutively two (commercial) masks. In the first step, a bilayer i-line (365nm) lithography process using AZnLOF resist (the imaging layer) on top of a LOR-A underlayer resist (facilitating the lift-off) followed by gold evaporation and lift-off in N-methyl-2-pyrrolidone was used to pattern the 50-nm-thick gold electrodes deposited on Cytop. In order to get a good adhesion of LOR on Cytop, we introduced a short oxygen plasma ashing step prior to LOR coating. In the second step, a spin-coated (~0.6 μm-thick) layer of polymer resist - a cycloaliphatic acrylate polymer (CAP) was exposed through the second mask at a wavelength of 250 nm and developed, thus defining the dielectric (polymer) ridges of the DLSPPW circuitry. The polymer ridges were ~1 μm-wide in principal regions of the components and were on both sides connected via 25 μm-long funnel structures with access (10 μm-wide) polymer waveguides extending outside gold covered regions of DLSPPWs all the way up to the substrate edges [Fig. 1(a)], facilitating thereby the end-fire coupling of photonics waveguide with single-mode tapered optical fibers.
To reduce the influence of environmental temperature fluctuations, the internal Wheatstone bridge configuration was implemented with all conductors being stripes similar to that used to guide the DLSPP mode [Fig. 1(b)], so that, in the absence of DLSPP radiation, the bridge was almost perfectly balanced. The internal Wheatstone bridge was connected with the external a bias voltage source and lock-in amplifier by the aluminum wires connected to the bonding pads on the sample by ultrasonic wire bonding. The 4-μm-wide and 46-μm-long DLSPPW gold stripe was electrically isolated from the rest of the DLSPPW structure with 2-μm-wide gaps which introduce additional scattering loss.
3. Operation principle and results
The optical power absorbed by the metal stripe is dissipated into both the top polymer ridge and a substrate and an amount of heat dissipated to ridge depends on thermal resistance and capacity of the surroundings materials. The dissipated power by stripe depends on the SPP attenuation coefficient (propagation length) and the length of the active region. The dynamic increase of the metal stripe temperature ΔT(t) due to the absorption of the SPP mode power at time t can be expressed as
The temperature rise causes an increase in the metal resistivity and, consequently, in the stripe resistance that can be evaluated as follow
To monitor changes in the DLSPPW gold stripe resistance the Wheatstone bridge configuration was used. For the balanced bridge being constructed to ensure the maximum response of the Wheatstone bridge, i.e., when R1 = R2 = R3 = Rx(Pin = 0) with the latter resistance being that of the DLSPPW metal stripe in the absence of the SPP radiation, the signal voltage can be expresses as followsFig. 1(b)] and the scattering contribution was neglected (i.e., it was assumed that all propagation losses goes into the absorption losses, which is a reasonable assumption for DLSPPWs). In the Wheatstone bridge configuration for the connections presented in Fig. 1 and in the absence of DLSPP radiation the signal voltage is given by
The responsivity of the investigated power monitors was evaluated by first measuring the signal voltage Vs of the Wheatstone bridge in the absence of the DLSPPW excitation and then measuring the signal voltage for different laser powers [Fig. 2(a) ]. The changes in the signal voltage were found linear with respect to the input laser power, showing the possibility of monitoring the (DLSPPW mode) power-induced changes in the stripe resistance. The slope of the response, which defines the responsivity of the power monitor, was found to be 6.4 µV/µW with Vb = 255 mV what gives responsivity per applied a bias voltage ~25.1 µV/µW∙V. It is over 14-25 times higher compared to the previously reported  (1.0-1.8 µV/µW∙V) for structure with MgF2 substrate and with the 1 µm-thick a PMMA ridge. A higher responsivity can be attributed to several factors from which higher absorption losses of the DLSPP mode consequent on the lower ridge height and a higher refractive index of the ridge is the most important factor. Furthermore, the ratio of thermal conductivity coefficients of the ridge to the substrate is much higher for analyzed structure mostly due to much higher thermal conductivity coefficient of Cytop compared to MgF2 (MgF2 – κ = 11.6 (W/mK), Cytop - κ = 0.12 (W/mK)) what influence on the direction of heat dissipation and the amount of heat transferred to the ridge. The ridge temperature increase is proportional to the DLSPP mode power coupled to the plasmonic waveguide and influences the signal voltage drop, which was also calculated theoretically showing very good agreement with measurements [Fig. 2(a)].
Modulation response of the power monitor was studied by chopping the input laser light at various frequencies with the signal voltage being measured by a lock-in amplifier for two different bias voltages [Fig. 2(b)]. It was observed that the signal voltage level is weakly frequency dependent in the analyzed frequency range from 1 Hz to 800 Hz (the upper modulation frequency was limited by use of a mechanical chopper). However, the signal voltage level varies considerably with change in the bias voltage. For a bias voltage Vb = 200 mV, the recorded signal voltage is negative compared to positive a signal voltage observed for increased a bias voltage to Vb = 250 mV. For a larger bias voltage and thereby a larger current in bridge electrodes, the polymer ridge decreases its refractive index increasing the DLSPP mode propagation loss and consequently increasing the stripe resistance through increasing the stripe temperature. Additionally, it was observed that for frequencies below 2 Hz the signal voltage decreases gradually suggesting that, for longer light switching, the heat originated from the light absorption dissipates into the entire system, decreasing in this way a temperature difference between the stripe supporting the DLSPP mode and rest of the system.
It should be noted that the signal voltage and, therefore, the responsivity depends also upon the wavelength used in accord with the dependence of the DLSPP propagation loss caused by metal absorption, which is wavelength dependent. Furthermore, the presence of gaps on both sides of the DLSPP waveguide makes it similar to a Fabry-Perot cavity so the transmission depends strongly on the wavelength exhibiting a periodic dependence with respect to the phase accumulated by the cavity mode per circulation. Smaller transmission implies that the DLSPP mode power is larger in the cavity, increasing the absorption losses and, as a consequence, increasing the signal voltage and responsivity (Fig. 3 ). It should be mentioned that the observed wavelength dependence can be significantly attenuated with the oscillations being strongly suppressed by removing the gaps between the Wheatstone bridge electrodes and the rest of waveguide circuitry. These gaps are really needed only in configurations containing other active plasmonic components, i.e., with other paths for bias and signal currents. The measurements of the resistance between pads 1-4, 2-3, 1-3, and 2-4 [Fig. 1(a)] allow one to evaluate the metal stripe resistance supporting the DLSPP mode to be R≈5.8 Ω what fits very good with calculated one R≈5.7 Ω, using the relation R = ρ(L/w·t) with L = 46 µm, t = 50 nm and w = 4 µm.
To investigate further the monitoring of the power coupled to the plasmonic waveguide we decided to modulate the bias voltage while keeping the in-coupled radiation power constant. The modulated bias voltage was changed from 0 to 50 mV or 100 mV at the desired frequency and then the signal voltage was measured using a lock-in amplifier. In the absence of light coupled to the DLSPPW, the signal voltage was measured to be −131.05 μV and −263.2 μV for a bias voltage 50 mV and 100 mV, respectively, with the latter being modulated at the frequency of 1 kHz. It should be noted that, even for a perfectly balanced Wheatstone bridge, one should expect nonzero signal voltages in the absence of light, because the bias current would heat different bridge arms differently due to the presence of a polymer ridge on the top of waveguide stripe electrode. Heating of the polymer-loaded electrode would, in this case, occur similar to its heating by the DLSPPW mode, since the light absorption takes place also inside this stripe. One should therefore expect similar frequency responses in both cases and benefit from a relative ease, with which current modulation can be implemented as compared to the optical power modulation. Also, similar frequency responses when modulating the bias voltage are expected with and without the (non-modulated) radiation being coupled into the DLSPPW mode.
The frequency response was observed to be almost flat in the frequency range from 10 Hz to 40 kHz with local maxima and minima not exceeding however ~5% of the average value [Fig. 4(a) ]. The situation changes for lower and higher frequencies, at which the signal voltage drops to zero. In the case of low frequencies, the signal voltage decreases to zero as a result of equalization of temperatures between the heated part of the Wheatstone bridge and the rest of the structure as mentioned above. On the contrary, for high frequencies, the ridge experiences a problem to follow rapid changes in the temperature, decreasing thereby the difference in current induced heating of the polymer-loaded and other bridge electrodes, and the signal voltage drops down. The measured frequency cutoff is in the range of 75-100 kHz, which corresponds to the response time of 10-13μs. Additionally, the wavelength dependence of the signal voltage (with the light coupled into the DLSPPW) and transmission show similar behavior to the observed previously with a DC bias voltage and modulated light power, with a maximum and minimum transmission and voltage for wavelength 1550 nm and 1600 nm, respectively [Fig. 4(b)].
Based on the signal voltage increase with the applied voltage the responsivity was calculated taking into account the 6.5 μW laser power coupled to the plasmonic waveguide. Obtained responsivity of 0.12 μV/μW for Vb = 100 mV was much lower compared to the previously reported with a DC bias voltage. However, it should be noted that in the case of measurements with a DC bias voltage the bias voltage level was higher compared to the measurements with the AC bias voltage. The power dissipated by stripe and, in consequence, the temperature of the ridge while heated increases quadratically with the applied voltage what influences on the DLSPP mode absorption. The higher a ridge temperature is the lower ridge refractive index and the higher the DLSPP mode absorption will be. So, increasing the ridge temperature via increasing a bias voltage causes a higher DLSPP mode absorption for the same optical power coupled to the plasmonic waveguide what influences on the further ridge temperature increases and higher signal voltage drop measured by lock-in amplifier.
To confirm the high cut off frequency of the power monitor the temporal response was recorded for a modulation bias voltage 284 mV and 428 mV connected to the pads 1 and 2 (Fig. 5 ) with the total resistance measured on 26.83 Ω.
The total power dissipated by the stripe together with connecting electrodes and wires was evaluated on ~3 mW and ~6.8 mW for a bias voltage 284 mV and 428 mV with a power dissipated by the metal stripe supporting the DLSPP mode evaluated on 355 μW and 792 μW respectively. However, it should be emphasized that only 20-30% of the dissipated power contribute to the temperature increase of the ridge as a results of ridge contact area with the metal electrode and a thermal conductivity coefficient difference between materials which are in contact with electrode. At the same time a temporal response of the light modulated by voltage of 428 mV showed a switching on/off time being in the range of 15 μs what corresponds to the cut off frequency of 70 kHz [Fig. 5(a)] and fits very good with the cut off frequency of the power monitor [Fig. 4(a)]. The power dissipated by metal stripe supporting the DLSPP mode depends quadratically on the bias voltage and for the low bias voltage of 50 mV is 20 μW. However, only part of dissipated power is transferred to the ridge. The transfer amount depends on the ridge contact’s area with the metal electrode, thermal conductivity coefficient’s difference between ridge and substrate below metal electrodes, and the amount of dissipated power. The smaller is the dissipated power, the larger amount of heat is transferred to the ridge. It was calculated that, for a bias voltage of 50 mV, almost 75% of power is transferred to the ridge, while for the 100 mV it is only 30%.
Compact fiber-coupled DLSPPW-based power monitoring with an internal Wheatstone bridge configuration was demonstrated showing weaker influence of the environment on the overall performance of the power monitor, which in the absence of DLSPP radiation was almost perfectly balanced. Compare to our previous results, the Cytop was used as a supporting material below gold electrodes and the PMMA was replaced by the Cyclomer as a ridge material. In addition, the height of the Cyclomer ridge was reduced from 1 um to 0.6 um what influenced on the higher absorption losses of the DLSPP mode showing significant improvement in the performance of the DLSPP-based power monitor. Using single-mode polarization-maintaining fiber for in- and out-coupling of radiation, the DLSPPW mode power monitoring at telecom wavelength has been realized with very high responsivities reaching up to 6.4 µV/µW, for a bias voltage of 245 mV, and showing pronounced wavelength dependence. It should be mentioned that the latter feature can be significantly attenuated by removing the gaps between the Wheatstone bridge electrodes and the rest of waveguide circuitry. These gaps are really needed only in configurations containing other active plasmonic components, i.e., with other paths for bias and signal currents. The investigated power monitor has exhibited a flat frequency response in the range of 1-800 Hz with a DC bias voltage, and almost flat frequency response in the range of 0.001-40 kHz with the AC bias voltage. It has to be emphasized that the measurements with the DC bias voltage were performed to the maximum frequency of 800 Hz and it was limited only by the limit of external mechanical chopper used in the measurements. Our measurements of the power monitor response to the modulated bias signal indicate that its time response is at the level of 10-13 µs, which is similar to that achieved in the best DLSPPW-based components demonstrated insofar .
This work was supported by the European Union (EU) project FP7-249135 (PLATON) and by the Danish Council for Independent Research (contract no. 09-072949).
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