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Abstract
Two novel top mirror designs of high contrast gratings (HCG) are used as the top mirrors of the resonant-cavity enhanced photodetector (RCE PD) operating at 940 nm. The bottom mirror is composed of 36-pair AlAs/GaAs, while the top mirror is a thin-layer grating providing reflectivity higher than 99%. With grating periods varying from 450 to 490 nm, different designs with FWHM of about 0.2∼3 nm are attained. A broadband HCG as top reflector can result in significantly improved manufacturing cost, as well as near unity quantum efficiency (QE). A resonator HCG can result in a new splitting responsivity spectrum with on-off ratio of 14 dB, which has the potential to serve as the basic elements of ternary system, polarization dichroism or diattenuation, and optical switch.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Resonant-Cavity-Enhanced photodetectors (RCE PD) [1] are promising for many applications such as optical communications, optical interconnections, 3D imaging and sensing [2–5]. For many applications, the receivers not only need to have high speed but also low noise. Typically, a narrow bandpass optical filter is included in front of a high-speed photodetector to eliminate noise due to other light sources. A RCE PD is a good candidate as it has a built-in narrow spectral bandwidth. By combining two mirrors with the absorption region to form a cavity, the quantum efficiency (QE) is selectively enhanced at resonant wavelength to near unity, forming an effective bandpass filter [6].
Previous work on RCE photodetector comprises of metal or Distributed Bragg Reflector (DBR) as the mirrors to support the light circulation inside the cavity [7–10]. To improve the QE, the top and bottom reflectors need to be designed to match the absorption region [11–13]. By etching the top DBR designed for 830nm, two cases with FWHM of 6.5 nm and 36% QE and FWHM of 1.7 nm and 73% QE were obtained in the experiment [14]. An optimization of FWHM was proposed with value of 30nm [15]. There is significant trade-off between QE, bandwidth and speed, resulting in challenging experimental realization [16–19].
A conventional RCE using Distributed Bragg Reflectors (DBRs) as top mirror has a large thickness, which makes it difficult for the device to extract photogenerated carriers [13]. And there are large thermal effect and a longer transit time [20]. Compared to the DBR devices, a High Contrast Grating (HCG) only has a layer with thickness less than 300nm. The HCG has been demonstrated to provide reflectivity higher than 99% as well as the polarization selectivity. An HCG has been demonstrated as the top mirror in a RCE detector with potential advantages such as ease of epitaxy and reflectivity design flexibility [21].
In this paper, the design is for wavelength of 940 nm and the HCG reflectivity is calculated by the RCWA method. For a series of HCG dimensions with airgap ranging from 260 to 300 nm, the resonant wavelength of the cavity slightly varies due to the reflection phase change. The responsivity spectrum is simulated by transfer-matrix method (TMM) and finite-difference time-domain (FDTD) method [22]. By theoretically analyzing the optimum condition, the requirement for top mirror reflectivity is discussed for high quantum efficiency. In addition, we propose a novel grating into the RCE photodetector with the HCG designed as a resonator [5,23,24] to provide a high design flexibility for broadband detector. We introduce a novel HCG design with a Fano-resonance reflection spectrum, referred to as HCG resonator (HCG-R). Using this unique HCG-R as the top mirror, a novel surface-normal optical switch with high extinction can be achieved.
2. Device design
The designed structure has an HCG and DBR as the top and bottom reflector, respectively. A wet thermal oxide layer beneath grating bars serving as low index material results in a large index contrast, providing high reflectivity. A Fabry–Perot cavity is formed by the top and bottom reflectors. Compared with the conventional RCE PD, top mirror thickness is significantly reduced. The P contact is deposited on the GaAs and the cavity between the two mirrors is designed to have resonance at around 940nm. And there is an oxide aperture placed on top of the active region to provide lateral confinement in the cavity. The photon generated current by surface normal incident light is extracted through the reversely biased photodiode junction.
An RCE PD structure with HCG serving as top mirror is shown in Fig. 1(a). The active region is composed of multiple quantum wells, providing absorption coefficient as high as 1e4 cm−1 at the wavelength around 940 nm. The bottom mirror with high reflectivity is 36 pairs of DBR. This device configuration is similar to the one reported in [5] for the HCG oxide-spacer vertical cavity surface emitting laser. A simplified model is shown in Fig. 1(b), where the main components top mirror, active region and bottom mirror determine the QE.
3. Broadband HCG RCE photodetector
The HCGs with different grating bars and periods are simulated to achieve a broadband reflectivity spectrum at around 940 nm. There is a polarization selectivity provided by HCG that the reflectivity is only high for TM light.
The simulated reflection and reflection phase of the oxide spacer gratings are plotted in Fig. 2(a) and 2(b) using the rigorous coupled-wave analysis (RCWA). By varying the dimensions, HCGs can have reflectivity spectra with different wavelength dependence. The round-trip phase is slightly different due to the HCG reflection phase difference and thus the resonant wavelength varies.
By varying the HCG dimensions, five sets of HCG dimensions are used in this simulation. Each case results in different resonant wavelength and different the peak QE value. The QE is reduced significantly if r1 is too low (light does not bounce enough inside the cavity) or too high (light is reflected mostly by the input mirror). Thus, there exists a major performance tolerance issue that the spectral width and peak QE are highly sensitive to the reflectivity of the top mirror and the absorption coefficient in the detection region. Figure 3(a) shows FDTD simulation setup for the HCG RCE PD and results in Fig. 3(b) illustrate such an example that the QE and spectral width are greatly changed when the r1 varies. For the HCG with airgap of 280 nm, the QE degenerates due to too much reflection.
By carefully choosing absorption coefficient while designing the active region, high QE and narrow FWHM are realized as shown in Fig. 4(a)-(f). In all cases, a narrow spectral bandwidth can be maintained with less than 2.2 dB difference in peak QE despite of different absorption. In addition, even when the critical dimension of the HCG airgap differs by a reasonably large tolerance of ∼6%, the narrow band and high peak QE are maintained.
Fig. 4. (a)-(f) QE spectrum (black line) and Reflectivity (blue line) of HCGs with 465 nm grating period and airgap between grating bars from 270 nm to 290 nm with α=5e4 cm−1.
4. HCG resonator as a RCE top mirror
The HCG can be designed to be a resonator instead a broadband mirror. Such an HCG resonator was reported and experimentally demonstrated in 2008 [24]. An HCG resonator onto a RCE photodetector can provide an extremely sharp, asymmetric reflectivity spectrum, which allows the tuning of response spectrum of RCE PD. Based on the design flexibility of HCG, a comparison between reflectivity spectrums of a broadband HCG, a HCG resonator and a common DBR is shown in Fig. 5(a).
Fig. 5. (a) Comparison of reflectivity spectrum between DBR and HCG resonator [24]. (b) Optimum requirement for top mirror reflectivity r1opt with different absorption coefficient α.
Top mirror is generally accepted not to provide too much reflection, but there is no qualitative description for the r1 value limitation. Here, to get maximum QE under a certain α and bottom DBR, an optimum condition requirement for top mirror reflectivity is calculated, notated as r1opt. Figure 5(b) shows the calculated r1opt with α ranging from 1e3.5 cm−1 to 1e5.5 cm−1. The r1opt curve has a peak at the resonant wavelength and drops versus the wavelength, which should be carefully matched by designing the mirror reflectivity spectrum to get wider and higher QE peak. As the r1 peak is thinner than the r1opt while using HCG resonator, it allows only a part of the r1 extends the r1opt. The reflectivity peak of the mirror should be thinner than the FWHM of the intrinsic resonance peak of the photodetector cavity.
As shown in Fig. 6(a), the blue dashed line is the optimum requirement r1opt. When the solid and the dashed line tends to match, the η in Fig. 6(b) tends to reach the ηmax. As shown in Fig. 6(c), the difference is that the absorption coefficient is increased and this results in a lower r1opt. Then the solid and dashed lines have two intersections. Thus, the QE spectrum in Fig. 6(d) has two peaks. And there is an extremely low quantum efficiency between these two peaks. In a conclusion, with α changing from and 1e3.5 cm−1 to 1e5 cm−1, the QE spectrum changes from a normal shape to a splitting spectrum. And the high QE happens at near-resonance wavelength while the low QE happens at the resonance. And the peaks can be designed to have a width less than 0.1 nm. This two-channel absorption with the center wavelength reflected regime may be extended to more applications.
Fig. 6. (a) Top mirror reflectivity and (b) QE spectrum with low α and (c) Top mirror reflectivity and (d) QE spectrum with high α.
Manipulation of the spectrum shape can be obtained. As shown in Fig. 7(a)-(c), with the increased α, the dip depth and width can be tuned. By designing the parameters, there is a potential to achieve a thin or broad spectral peak by this regime. In optical communication applications, it can be promising for broadband transferring large amount of information while extremely thin peak enhancing the security.
Fig. 7. (a)-(c) QE spectrum of designed HCG resonator PD at different absorption coefficient and the FWHM is labeled, (d) QE spectrum when α is 1e3.5 and 1e5.5 cm−1 and the ratio of QE.
Due to the strong spectral dependence, a small change in the electric field can result in a change of the absorption coefficient α and thus a large difference in QE at a given wavelength. As shown in Fig. 7(d), an extinction ratio of 14 dB can be obtained at 955 nm. This phenomenon can be useful for a high extinction ratio optical switch or modulator with surface normal topology readily extended to 2D arrays.
5. Experiment
As shown in Fig. 8, YSL Photonics’SC-Pro-7, a high-power supercontinuum source with wavelength selection capability is used as the illumination light source with free space modulator. Under a 10X objective lens, the focused light spot is around 883 µm2 with measured light of 1.352 mW for 940nm. A Series 2600 System SourceMeter with the software is used to get the current-voltage (IV) curves.
Fig. 8. Measurement setup for the photodetector IV curve under 940 nm illumination with different power intensity.
Measurement for such a vertical device is operated under the microscope. The size of probe should be less than 20 µm. The laser beam covers the gratings area in the center of the device as shown in schematic in Fig. 9(a), which has an oxidation aperture size of 8 µm. The fabricated device with probe tied on it is shown in Fig. 9(b).
Fig. 9. (a) Schematic diagram of light irradiation on the device. (b) Under the microscope, the device and the probe tied on it.
As shown in Fig. 10(a), the curves of photon-generated current versus voltage change regularly with the increasing of illumination power, for the device with HCG period of 410 nm and airgap of 200 nm. And the inserted figure shows the dark current of 1e-10 A. With a series of devices fabricated with period from 410 to 470 nm, airgap from 190 to 250nm, the current-power curves of two typical devices which have a large difference is shown in Fig. 10(b). Taking power of 0.8 mW as the critical point, the variation of current is divided into two stages. And the device with period 470 nm and airgap 250nm has a better performance when the power is larger than 1 mW.
Fig. 10. (a) IV curves of the device under 940 nm laser source with different incident light intensity for the device with period 410 nm and airgap 200 nm. (b) A comparison of current-power curves for two different devices.
6. Conclusion
Enhanced by the synergy of a resonant cavity and absorptive material, the absorption of RCE PD can be improved significantly. To solve the problem of carrier extraction, transit time and thermal effect, the top DBR is replaced by HCG. HCG RCE PDs with high quantum efficiency were designed for 940 nm with the FWHM of about 0.2∼3 nm.
Two novel designs are proposed benefitting from the HCG design flexibility. Firstly, a broadband HCG functions as the reflector of the RCE photodetector. The relationship between QE and HCG dimension is dicussed. This opens up a new way for the detector arrays, in which each device has a different resonant wavelength due to the HCG dimension variation. And the QE optimization discussion enbles the uniformity of QE for a series of devices in the same array. Additionally, a HCG resonator RCE photodetector having splitting shape in quantum efficiency spectrum is proposed, which can be made use of to get smaller FWHM and specific QE spectrum. The splitting spectrum has a potential to serve as a two-peak symbol, representing the third state in the ternary system to increase the data volume and transfer efficiency. Also, there is a future for the optical switch or modulator based on this work.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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