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Abstract
We present a hybrid device based on graphene-coupled silicon (Si) photonic crystal (PhC) cavities, featuring triple light detection, modulation, and switching. Through depositing single-layer graphene onto the PhC cavity, the light-graphene interaction can be enhanced greatly, which enables significant detection and modulation of the resonant wavelength. The device is designed to generate a photocurrent directly by the photovoltaic effect and has an external responsivity of ∼14 mA/W at 1530.8 nm (on resonance), which is about 10 times higher than that off-resonance. Based on the thermo-optical effect of silicon and graphene, the device is also demonstrated in electro-optical and all-optical modulation. Also, due to the high-quality (Q) factor of the resonate cavity, the device can implement low threshold optical bistable switching, and it promises a fast response speed, with a rise (fall) time of ∼0.4 μs (∼0.5 μs) in the all-optical switch and a rise (fall) time of ∼0.5 μs (∼0.5 μs) in the electro-optical hybrid switch. The multifunctional photodetector, modulator, and optical bistable switch are achieved in a single device, which greatly reduces the photonic overhead and provides potential applications for future integrated optoelectronics.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optoelectronic devices have been implemented for many applications, such as sensing [1,2], communication [3–5], high-performance computing [6,7] and so on due to the development of optoelectronic manufacturing technology. With more and more applications, it is necessary to integrate different discrete optoelectronic devices onto a single substrate, especially in optical information processing. However, various functional devices often require different material systems and device structures, which is an obstacle to the multifunctional integration of optoelectronic devices. In recent years, there has been growing interest in the realization of the single optoelectronic device with multiple functions. For example, Arindam Ghosh’s group presented a graphene-MoS2 hybrid realizing sensitive gate-tunable photodetectors and optical switches, and it also has the potential of optoelectronic memory [8]; Mo Li’s group utilized graphene’s tunable optoelectronic properties to demonstrate the first optoelectronic device that acts as both a modulator and a photodetector [9]; Yalin Lu’s group demonstrated a hybrid meta-device, which provides a multifunctional platform for electrical-controlled terahertz switching and photonic memory devices, as well as ultrafast terahertz modulators [10]; Guijun Li’s group demonstrated a hybrid heterojunction formed between CsPbBr3 halide perovskite and chalcogenide quantum dots, which can serve as a voltage controllable multicolor LED, an efficient solar cell, and a sensitive photodetector [11].
Among these multifunctional devices, two-dimensional (2D) materials are the most commonly used materials. 2D materials, such as graphene, black phosphorus, and transition metal dichalcogenides can directly combine with 3D optoelectronic structures by van der Waals forces due to the unique structure of atomic layer thickness, without considering the lattice mismatch in traditional materials [12]. Besides, the 2D materials have many excellent optical and electrical properties, such as tunable band structure, ultra-fast carrier mobility, and ultra-high nonlinear coefficient. Thus, the integration of 2D materials and optoelectronic structure has been implemented in a large number of high-performance optoelectronic devices, including photodetectors [13–15], modulators [16–18], optical switch [19,20], etc., which lays a foundation for the realization of multifunctional devices based on 2D materials.
Besides, to realize multifunctional optoelectronic devices, structures with strong optical field confinement are required. The microcavity is an excellent structure for realizing the localization of the optical field. 2D material-coupled microcavities have been used in lots of high-sensitivity devices [21–23]. PhC provides the opportunity to shape and mold the flow of light for photonic information technology [24]. By designing structural parameters, we can fabricate photonic crystal microcavities operating in different wavelengths. On-chip PhC microcavities have the advantages of a high ratio of Q factor to mode volume [25], ultra-compact size [26–29], good integration compatibility [30,31], and mature fabrication. Due to the significantly enhanced interaction between the light field and matter in the cavity, PhC microcavities can achieve effective light detection [32], modulation [33–35] as well as low threshold optical bistability [36–38]. Hence, the PhC microcavity is one of the promising candidates to achieve multifunctional optoelectronic devices.
In this work, we demonstrate a multifunctional device operating at a communication wavelength band by coupling graphene with a silicon-based PhC cavity, which can simultaneously realize photodetection, electro-optical/all-optical modulation, and electro-optical/all-optical bistable switching. Figure 1(a) illustrates the configuration of the device. The Si PhC structure is doped by ion implantation. The concentration in both p-type (boron doped) and n-type (phosphorus doped) regions are 5×1018 cm-3. The i-region is lightly p-doped (1016 cm-3). Therefore, a p-i-n junction is formed in the Si PhC structure. An air-suspended PhC cavity was fabricated on a doped silicon-on-insulator (SOI) wafer via deep ultraviolet lithography, reactive ion etching, and wet etching. Single-layer graphene prepared by chemical vapor deposition was wet-transferred onto the PhC cavity, coupling with the cavity evanescent field. The Fermi-level in graphene is shifted due to the electrostatic doping from the Si homojunction. The graphene on p-Si is positive, and the graphene on n-Si is negative. A lateral p-i-n junction is formed along the graphene plane, and a Schottky barrier is formed at the graphene-Si interface, as shown in Fig. 1(b). Figure 1(c) shows the optical micrography of the device, and Fig. 1(d) is the scanning electron micrograph (SEM) image of the device covered by monolayer graphene. Resonance tuning of the PhC cavity structure has been discussed in detail in Refs. [39]. Here, we mainly focus on cavity-enhanced photoresponsivity, efficient modulation, low-power optical bistability, and switching.
Fig. 1. Structure and band diagram of the device. (a) A schematic diagram of the graphene-coupled PhC cavity structure. (b) Energy band diagrams of the graphene lateral homojunction and graphene-Si Schottky junction. (c) Optical microscope image of the device. (d) SEM image of the device covered by monolayer graphene.
2. Experiment and discussions
The transmission of our graphene-coupled PhC cavity was characterized utilizing an amplified spontaneous emission (ASE) light source and an optical spectrum analyzer (OSA, Yokogawa AQ6370C). The source was launched into the PhC cavity through a polarizer, a polarization controller (PC) and an input tapered fiber probe. These devices were utilized to adjust the polarization states of the laser and couple laser into the waveguide. Then the laser in the cavity was output to the OSA with a resolution of 0.02 nm through an output tapered fiber probe. Figure 2 shows the transmission spectrum of the device under TE polarization. The insert in Fig. 2 shows obvious sidebands of the PhC structure. Figure 2 is the close-up of the resonance peaks of the graphene-coupled PhC cavity, which include cavity modes and Fabry-Perot modes [39]. The third mode has a full width at half maximum (FWHM) bandwidth of ∼0.03 nm, and the Q factor is calculated to be ∼50000.
2.1 Graphene-integrated PhC cavity photodetectors
For detailed studying of the photodetection function, photocurrent responses of the device with both p-i-n and Schottky junctions were measured. When the light source at the communication band is injected into the waveguide, the photons are mainly absorbed by graphene as the photon energy (∼0.8 eV at 1530 nm) is smaller than the bandgap of Si. Since the absorption of monolayer graphene is only 2.3%, the responsivity of graphene-silicon waveguide Schottky junction photodetector is relatively low. Relying on the localized optical field in the PhC cavity, the interaction between graphene and the PhC cavity’s evanescent field is significantly enhanced, which enables more photo-generated carriers in the graphene-Si interface for photocurrent generation. We experimentally characterized the photodetection characteristics of the device. A narrow-linewidth tunable laser (Santec TSL-710) was employed as the light source. When the incident light was launched into the device through a PC, electron-hole pairs were generated at the interface between graphene and Si, and their directional migration under the action of electric field result in photocurrent. The current-voltage (I-V) characteristic was measured with a sensitive source meter (Keithley 2400), which can apply DC bias voltage to the aluminum electrodes and measure the photocurrent simultaneously. Then, setting the wavelength of the tunable laser at 1530.8 nm (on resonance) and 1529.0 nm (off-resonance) respectively, we measured the I-V characteristic curves under different laser powers, as shown in Fig. 3(a) and (b), which show the typical characteristics of a Schottky junction. Limited by the sensitivity of the source meter, the dark current was measured to be about 0.1 nA. Since the photocurrent is opposite to the forward voltage, the reverse current increases with the input optical power. Figure 3(c) shows the photocurrent (Ip) and responsivity in the device as a function of incident power under the bias voltage of -4 V. At a fixed bias voltage, the photocurrent increases with the optical power, while the responsivity decreases with the increase of the optical power due to the saturation of optical absorption in graphene. At lower incident power, the absolute device responsivity RI ($\textrm{ = }{{{I_\textrm{p}}} / P}$) for wavelength in cavity exceeds 14 mA/W, which is significantly larger than that out of the cavity (<1 mA/W). The responsivity increases more than tenfold due to the localization of the light field in the cavity. In our experiment, the photocurrent generated in the device without graphene was also detected, which was compared with the photocurrent generated in the device covered by graphene in the inset in Fig. 3(a). We observed that the photocurrent was very low, and the responsivity was calculated to be only ∼0.15 mA/W. The weak photocurrent generated in the Si p-i-n structure is probably result from the surface state absorption of Si, as nanostructured Si has optically accessible trap states below its bandgap [40]. According to the I-V curve under illumination (the green dotted lines in the inset of Fig. 3(a)), we observed that there was a significant deviation of the I-V curve from the conventional photodiode-like response when the device is covered by graphene, which is a result of the unique electronic structure of graphene near its Fermi level [41].
Fig. 3. I-V characteristic curves under different laser powers for (a) 1530.8 nm (on resonance) and (b) 1529 nm (off-resonance). Inset in (a): the I-V curves of the device with and without graphene. (c) Photocurrent and responsivity under different optical power. Circles indicate wavelength in the cavity, and squares indicate wavelength out of the cavity.
Next, with the bias voltage fixed at -3 V and the input power kept at 0.016 mW, we measured the photocurrent while sweeping the incident wavelength from 1530 nm to 1532 nm to obtain the photocurrent spectrum, as displayed in Fig. 4(a) (red curve). The photocurrent shows multiple spectral dips, which are consistent with the resonance dips in the transmission spectrum of the graphene-coupled PhC cavity (black curve in Fig. 4(a)). As the cavity effect enhances the light-matter interaction, the light absorption at the dip wavelength is significantly amplified, which results in a higher photo-responsivity.
Fig. 4. (a) The optical transmission spectrum, and photocurrent as a function of wavelength. (b) The input light waveform and the current in response to the input light.
The temporal dynamical response of the device as a photodetector was measured by an oscilloscope (OSC, ROHDE&SCHWARZ, RTO 1022). The continue-wave laser was modulated to 10 kHz. The device generated a photocurrent in response to this optical signal. This photocurrent was then converted to a voltage signal and amplified using an I-V conversion amplifier module. The amplified voltage signal was transmitted to the oscilloscope to test the generated waveform. Figure 4(b) shows the typical rise and fall times in response to the square wave optical signal. We obtained a rise (fall) time ∼12 μs (11 μs). (It should be noted that the oxidation of electrodes and silicon-based structures occurs during the cleaning process of the device, which greatly increases the resistance and capacitance time constant of the relevant RC circuit and slows down the response speed of the device. In the further optimization design of the device, good package protection will effectively increase the response speed of the device. Besides, the test result is also limited by the speed of the I-V conversion amplifier module.)
The FWHM of the cavity is only ∼0.03 nm, which affords the device better wavelength selectivity, thus, a photodetector based on this structure can be applied in wavelength division multiplexing (WDM) system. And it is more convenient for on-chip integration with other optoelectronic structures, compared with a vertical-cavity enhanced photodetector.
2.2 Graphene-integrated PhC cavity modulators
The PhC cavity enhances the light-graphene interaction, which contributes to the effective modulation of resonant wavelength. We realized electro-optical and all-optical modulation based on this device.
The graphene-coupled cavity is working close to the critical coupling condition at resonance wavelength. Considerable Joule heating is generated in the graphene layer and Si membrane by applying bias voltage or pumping light, which changes their refractive index. A slight change of the index in the Si PhC cavity and graphene would influence the resonance wavelength significantly, which promises electro-optical and all-optical modulation of this device. We experimentally demonstrate this function. An ASE source (10 mW) was launched into the device, and the output light was investigated utilizing an OSA and a power meter (Thorlabs PM100D). The total insertion loss of the device is ∼14 dB, including the absorption of graphene, which can be further optimized either by electrical gating or by chemical doping. A voltage source (Keithley 2231A-30-3) and a radio frequency (RF) probe were used to forward bias the integrated p-n junction of the PhC cavity. Figure 5(a) shows the normalized transmission spectrum under increasing bias voltage. We observe that the resonant peaks are redshifted when the voltage increases. A large amount of heat induced by the strong electrothermal effect changes the refractive index of both graphene and PhC-cavity so that the resonant conditions of the signal light are changed accordingly. When modulation voltage reaches 3 V, the cavity mode resonance peak shift over 0.12 nm, which is well in excess of the FWHM of the resonance peak (∼0.03 nm). The modulation depth is ∼3 dB. The normalized transmission spectrum of the device without the graphene layer under increasing bias voltage is shown in the inset of Fig. 5(a). At the same bias voltage (4 V), the cavity resonance peak shifts only about 0.05 nm, which is much less than that of the device covered by the graphene layer (∼0.27 nm).
Fig. 5. (a) Red-shifts of the resonant peaks with the increasing bias voltages of the device covered by graphene sheet. Inset: the red-shifts of the device without graphene. (b) Temporal response of the electro-optical modulator.
To investigate the modulation speed, a tunable continue-wave laser was used to take the place of ASE. A Square-wave electrical signal generated by the signal generator was applied on the aluminum electrodes to modulate the device continually. The electrical signal has a frequency of 20 kHz and a duty ratio of 50%, as shown in the red line in Fig. 5(b). The output optical signal was measured utilizing a photodetector (Conquer KG-PR-200M-A-FC) and oscilloscope. The black line in Fig. 5(b) shows the output signal waveform. The initial resonant wavelength was on the left side of the incident wavelength, so the output power was high. When the positive voltage was applied, the electro-induced thermo-optic effect made the resonant wavelength red shift and the detuning reduce, thus the output power decreased. That is, heating corresponded to the falling edge of the output waveform. The output signal has the same frequency as the input signal, indicating that the optical signal has been successfully modulated. From the output waveform, we observed that the rise time (cooling time) of the device was ∼0.3 µs, and the fall time (heating time) was ∼1.3 µs.
Then, the signal light generated by ASE and the pump light generated by TSL are coupled into the device through an optical fiber coupler. All-optical modulation characteristic of the device was investigated by tuning the power of the pump light with the signal light fixed. OSA recorded the normalized transmittance spectra under different pump light intensities, as shown in Fig. 6(a). We can observe that due to the photothermal effect, the heat generated in the device causes the refractive index of both the PhC cavity and graphene to change with the increase of the pump light power, which results in the red-shift of the resonant peaks. We think there is no saturable absorption in this all-optical modulation. The saturable absorption threshold of graphene is quite high (>1 MW/cm2) [42]. While the power used in our all-optical modulation was only ∼2 mW, and our device had an effective cross-sectional mode area (Aeff) of about 0.55 μm2. The pumping light was not at the resonance wavelength, so the pumping light was mainly in the PhC waveguide. We can calculate the power density in the graphene covered on the waveguide is ∼0.18 MW/cm2, which is far below the saturable absorption threshold.
Fig. 6. (a) Red-shifts of the resonant peaks with the increasing input powers. (b) Temporal response of the all-optical modulator.
Besides, the response time of the saturable absorption effect and thermo-optical effect is very different, due to the different mechanism. The response time of thermo-optical effect is in the order of millisecond to sub-microsecond, and the response time of saturable absorption is in the order of sub-picosecond. We utilize a tunable continue-wave laser (∼1531 nm, on resonance) as the signal light and a modulated tunable laser (1530 nm) as pump pulse. At the output end of the device, a reflective fiber Bragg grating (RFBG) is employed to make sure that only signal light can pass through. When the device was heated by pump light, the output power increased. Figure 6(b) shows the waveform of all-optical modulated output optical signal. We observed that the rise time (heating time) of the device was ∼0.7 µs, and the fall time (cooling time) was ∼0.5 µs. The response time also confirmed the modulation mechanism was thermo-optical effect rather than saturable absorption effect.
Compared Fig. 5(b) with Fig. 6(b), the heating rate of all-optical modulation was faster than electro-optical modulation, and the cooling rate of all-optical modulation was slower than electro-optical modulation. This was because only the waveguide and microcavity were heated in all-optical modulation, but the whole device covered by graphene sheet was heated in electro-optical modulation. Thus, the heating rate was faster in all-optical modulation. (Actually, the all-optical method is a more efficient way to achieve the thermo-optical modulation as we only need to heat a small cavity area to tune the resonance peak.) In the cooling process, the thermal energy was distributed across a larger area in electro-optical modulator, enabling faster conduction and dissipation of heat, and result in a faster cooling rate in electro-optical modulation.
2.3 Optical bistability switching in graphene-integrated PhC cavities
Although the speed of the thermo-optic switch is slow, it still plays an important role in the spatial switch. Graphene has strong optical absorption, especially in the coupling with high-Q cavities, and Si has a remarkably high thermo-optic coefficient (∼1.85×10−4 K-1). Therefore, graphene-integrated Si PhC cavity can produce a significant thermo-optic effect and realize a low threshold thermo-optical bistable switch. Although bistable switching in graphene-clad nanocavities have researched in previous works [38], the PhC side-coupled cavity used in this experiment has a higher Q-factor (∼50000) than previous PhC L3 cavity (∼7500), which will result in lower bistability threshold power. Meanwhile, as a flexible conductor, graphene can be heated by electric heating to achieve an electro-induced thermo-optic switch.
Sweeping the wavelength of the tunable laser source from 1530 nm to 1532.5 nm at a speed of 0.5 nm/s, we measured the optical transmission spectra at different input powers by a photodetector (PDA10CS2) and a data acquisition (DAQ) card. Figure 7(a) shows the TSL scanned transmission spectra of the device. The resonant peaks shift to the long-wavelength direction and become asymmetric with the input power increases gradually. Even triangular curves are deformed with sub-milliwatt incident power. This phenomenon is due to the thermal hysteresis effects caused by thermo-optic nonlinearity in the cavity.
Fig. 7. (a) TSL scanned transmission spectra of the device. (b) Hysteresis curves of the optical bistability at wavelengths of 1531.10, 1531.11, 1531.12, and 1531.13 nm, respectively.
By simultaneously taking advantage of both the strong thermo-optic effect and resonant enhancement, optical bistability is expected with a relatively low optical power. We verify the optical bistability phenomenon in the graphene-PhC cavity by monitoring the power-dependent output at the wavelength around the resonance with the input power increase and decrease circularly. Figure 7(b) shows the hysteresis curves of the optical bistability at 1531.10, 1531.11, 1531.12, and 1531.13 nm with threshold powers as lower as 0.033, 0.044, 0.060, and 0.079 mW, respectively. With cavity-input laser detuning increasing, the threshold power increase, and the interval of the bistability hysteresis become broader.
To operate the bistability switching, we should choose the values for the laser operating power. According to the input-output power hysteresis curves for both power increasing and decreasing (Fig. 8(a)), we choose the power of 0.27 mW in the middle of the hysteresis loop as a “standby” power. At this power, the output power can take one of the two states: higher output power as the state “1”, or lower output power as the state “0”. We choose a power near the left (right) of the hysteresis loop as “on” (“off”) power.
Fig. 8. (a) Steady-state output power in response to input powers increasing and decreasing. (b) The waveform of input laser power and the output waveform in response to the input waveform.
Figure 8(b) shows the measured input power and the corresponding output power as a function of time, respectively. The laser pulses were programmed with a duration of 1 ms. In the on-stage, the output is in the state “1”. At the end of the “off” pulse, the output power falls to the state “0”. The “off” pulse thus changes the switch state from on to off. Then we continue to input the “off” pulse, the output state keeps the state “0”. Applying an “on” pulse, output state changes from “0” back to “1”, and subsequent “on” pulses do not influence the output state. This result demonstrates the switch function of the device.
The speed of our optical switching based on the graphene-PhC cavity primarily depends on the photothermal response time of the device. To verify the bistable switching dynamics, we input a square-wave optical signal to the graphene-PhC cavity. Figure 9 presents the output transient response of the device. We observe that the switching time for on and off are about 0.5 μs and 0.6 μs, respectively.
The all-optical bistable switch is primarily determined by the thermo-optic nonlinearity. With the tuning of input power, the resonant wavelength is shift, thus changing the output power of the device. Meanwhile, the shift of the resonant wavelength is strongly dependent on the circulating optical power in the cavity, which in turn depends on the detuning between the laser wavelength and the resonant wavelength. The two mechanisms interact to lead to bistability.
The shift of the resonant wavelength can also be varied by the electro-induced thermo-optic effect caused by applying a bias voltage. Meanwhile, the photocurrent depends on the detuning between the laser wavelength and the resonator wavelength, which in turn determines the shift of resonant wavelength. Thus, electro-optical hybrid bistability can be realized in this device. Figure 10 presents the electro-optical hybrid bistable phenomenon under voltage tuning. With the input power of the tunable laser fixed, varying the forward bias voltage applied to the drain-source electrode, we observed that the output current and optical power presented obvious bistable hysteresis phenomenon, due to the electro-induced thermo-optic nonlinearity, as shown in Fig. 10(a). Analogously, we tested the dynamic electro-optical switching characteristic by applying a square-wave voltage signal to the drain-source electrode. Figure 10(b) shows the input voltage waveform and output optical power waveform. The switch on and off time are ∼0.5 μs.
Fig. 10. Observation of the electro-optical hybrid bistable phenomenon. (a) Current and output power as a function of voltage. (b) Input voltage waveform and output optical power waveform.
3. Conclusions
In conclusion, we have presented a scheme to realize light detection, modulation, and switching in a single device by integrating graphene with a Si-based p-i-n PhC cavity. Due to the localization of the light field in the cavity, the interaction between graphene and the evanescent field is significantly enhanced, which provides efficient optical absorption in graphene. In the fabricated device, the photocurrent responsivity is up to ∼14 mA/W at a wavelength on resonance, which is one order of magnitude larger than that at a wavelength off-resonance. Electro-optical and all-optical modulation are demonstrated with the rise time of ∼0.3 µs (∼0.7 µs) and the fall time of ∼1.3 µs (∼0.5 µs) following the 10%-90% rule, respectively. Due to the high-Q factor of the PhC cavity, optical bistability is achieved with a threshold low to 0.033 mW. And it promises all-optical bistability switching and electro-optical hybrid bistability with a rise time of ∼0.4 µs (∼0.5 µs), and a fall time of ∼0.5 µs (∼0.5 µs), respectively. While the integration of graphene increases the absorption, it also reduces the Q factor due to optical loss. A trade-off between the two aspects will be achieved by further controlling the contact area of the graphene layer and PhC cavity, which will allow the device to show better performance. The graphene-coupled Si PhC cavity hybrid platform realized multifunctional photodetector, modulator, and optical bistable switching in a single device. It is important for the application of 2D materials-coupled Si-based structure. It also paves the way for the multifunctional integration of future optoelectronics.
Funding
National Natural Science Foundation of China (61705148).
Disclosures
The authors declare no conflicts of interest.
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