Wide spectral bandwidth electro-absorption modulator using coupled micro-cavity with asymmetric tandem quantum well

For reliable three dimensional (3D) imaging system, it is necessary for the optical shutter to have a wide spectral bandwidth operation and enhanced modulation depth. We propose an electroabsorption modulator (EAM) based on coupled Fabry-Perot cavities with micro-cavity (CCMC) which uses asymmetric tandem quantum wells (ATQWs) to obtain improved spectral bandwidth and enhanced modulation depth. Several modulator designs are investigated to obtain improved modulation performance such as wider spectral bandwidth and enhanced modulation depth. It was found that among all the studied modulator geometries, CCMC structure with ATQWs provides the widest spectral bandwidth of 9.6nm and high modulation depth in excess of 50% at −24V, which is good agreement with theoretical calculations. These results suggest that EAM has excellent potential as optical shutter for 3D imaging


Introduction
Surface normal electro-absorption modulator (EAM) based on quantum confined Stark effect (QCSE) [1] are the prime candidates for applications such as optical interconnects [2], free space optical communications [3,4], radio over fiber system [5], optical correlator [6,7], signal processing and optical beam steering [8], optical vector-matrix multiplicator [9] and three dimensional (3D) imaging systems [10,11].In particular, a recent development of optical shutter using EAM which is an essential component to obtain high resolution depth information in time of flight based 3D imaging system has been close to commercialization [11].Despite the progress in improving the modulation speed and modulation depth, this device still suffers from relatively narrow spectral bandwidth caused by Fabry-Perot cavity.
The Fabry-Perot cavity (FPC) structure can increase the sharpness of the transmittance spectrum thereby improving the modulation depth, however, an increase in the sharpness of the transmittance spectrum results in a narrow spectral bandwidth due to the high quality factor of FPC.The narrow spectral bandwidth of the EAM is problematic since modulation depth degrades significantly as the emission wavelength of the light source used in time of flight based 3D imaging system vary according to temperature.Moreover, a narrow spectral bandwidth reduces the manufacturing tolerance of the 3D imaging system and the operating temperature range since the bandgap of semiconductor material changes as the temperature changes.Thus, in order to improve the operational reliability of 3D imaging system, the EAM needs to be capable of operating over a wide spectral bandwidth with high modulation depth.
A wider spectral bandwidth can be obtained for an EAM by lowering the quality factor at the expense of a thicker absorptive active region, however, this approach results in a higher operating voltage while limiting the spectral bandwidth for reliable 3D imaging system.Another approach to broaden the spectral bandwidth is to use coupled cavity configuration which has been studied in the design of band-pass filters [12][13][14][15].Coupled Fabry-Perot cavity structure used in band-pass filter allows making the transmittance band "rectangular", leading to a broad and flat spectrum for the high transmittance band and stop bands.By using a coupled cavity scheme, several reflection type EAMs have been reported, having a wider spectral bandwidth while maintaining high modulation depth [16,17].However, reflection type EAM is not suitable for realizing 3D imaging system since it makes the overall system bulky and complicated.Several transmission-type EAMs have been reported [18][19][20][21], and these devices have demonstrated operation near optical communication wavelengths of 1.3 and 1.55µm and/or high contrast ratio, while the device reported by Wood et al. operated at a wavelength of 857nm [22].However, none of the reported devices operating at 850nm have succeeded in attaining a high transmittance change with wide spectral bandwidth simultaneously, which is crucial for 3D image sensing application.
In this study, we demonstrate a transmission type EAM operating at 850nm having wide spectral bandwidth along with high modulation depth by using a combination of coupled Fabry-Perot cavity with micro-cavity and asymmetric tandem quantum wells (ATQWs).To improve the spectral bandwidth while maintaining high modulation depth, we have explored several modulator structures.Theoretical calculations suggest that the coupled cavity structure with ATQWs can achieve high modulation depth over a broad spectral bandwidth of 10nm.The fabricated EAM device based on coupled cavity with ATQWs shows a transmittance change of > 30% over a spectral bandwidth of 9.6nm at an operating wavelength of 850nm and an operating voltage of −24V.The experimental results are in good agreement with calculated results.

Design for wide spectral bandwidth modulator
To discuss the contributing factors to the wide spectral bandwidth and high transmittance modulation of an electro-absorption modulator without any increase in operating voltage, we first consider a simplified structure as shown in the insets of Fig. 1(a) and 1(b) which has a 7λ thick single cavity (SC) sandwiched between two Al 0.31 Ga 0.69 As/Al 0.88 Ga 0.12 As distributed Bragg reflector (DBR) having the same top and bottom reflectivities.In the two modulator structures with single cavity studied, the QW structure shown in the inset of Fig. 1(a) is composed of 137 pairs of GaAs QWs with a single thickness of 8nm (called as SQWs) and a 4nm thick Al 0.31 Ga 0.69 As barrier, giving a total 7-λ cavity thickness of 1.666µm.The Fabry-Perot resonance peak of these SQWs happens at 850nm.The structure shown in the inset of Fig. 1(b) has asymmetric tandem quantum wells (ATQWs) [10], which contain two stacks of multiple QWs with different well thicknesses; 60 pairs of 8nm thick GaAs QWs and 74 pairs of 8.5nm thick GaAs QWs with 4nm thick Al 0.31 Ga 0.69 As barriers.The total cavity thickness in this case is 1.665µm.
Figure 1(a) and 1(b) show the contour plots of the calculated transmittance change as a function of number of pairs of DBR for an 8nm thick SQWs and ATQWs with 7-λ single cavity, respectively.In order to calculate the transmittance spectrum of electro-absorption modulator, the structure was modeled by using transfer matrix method [23].The field dependent excitonic absorption spectrum of quantum wells (QWs) was obtained from eigen value and eigen states which was obtained by solving the Schrödinger equation with appropriate boundary conditions.The excitonic linewidth of absorption spectra was based on Lorentzian line shape function [24].Then, absorption coefficient of the quantum wells under applied field were included into the transfer matrix with complex refractive index, so that transmittance change of device due to QCSE can be successfully calculated.As can be seen from Fig. 1(c), the SC modulator structure with 8nm thick SQWs exhibits a high transmittance change of more than 50% with two pairs of top and bottom DBR, which results in high transmittance change (>50%) and the widest bandwidth.However, the spectral bandwidth of this structure is only 2nm and 5.2nm when transmittance changes (∆T) were 50% and 30%, respectively.For the SC structure with ATQWs, the combination of two adjacent 8nm QWs and 8.5nm QWs lead to an enhancement of optical absorption over a wide wavelength range so that the spectral bandwidth was improved to 7.6nm at ∆T = 30% with two pairs of DBR, however, the maximum change in ∆T (~45%) was not higher than that of SC with SQWs as seen in Fig. 1(d).For the ATQWs structure, as shown in Fig. 1(d), transmittance under applied field decreases over a wide spectral range compared to that of SQWs since the absorption peak of heavy-hole exciton for 8nm and 8.5nm thick QWs moves from 838nm and 840nm under zero electric field to 850nm and 854nm under applied electric field (due to QCSE), respectively.However, ATQWs has a large residual absorption under zero electric field around the target wavelength (850nm) in comparison to that of SQWs structure.Thus, a large residual absorption decreases the transmittance under zero field and leads to low absorption modulation despite having a wider spectral bandwidth.In addition, SC structure with ATQWs has a narrow transmittance spectrum, thereby limiting high transmittance modulation, as shown Fig. 1(d).Moreover, as seen from Fig. 1(b), increasing the number of DBR mirror pairs does not lead to a higher ∆T as well as a wider spectral bandwidth since increased mirror reflectivity results in a higher quality factor.Thus, SC structure cannot achieve high transmittance modulation and wide spectral bandwidth simultaneously without avoiding the usage of a thicker cavity, which will then result in a higher operating voltage for the device.
To simultaneously improve the transmittance modulation and widen the spectral bandwidth, coupled Fabry-Perot cavities with micro-cavity (CCMC) structure is proposed.Structurally, this CCMC structure is similar to SC modulator as discussed above but this CCMC structure has two cavities with middle coupling DBR mirror instead of SC.The inset of Fig. 2   The main aim of CCMC structure is to increase the spectral bandwidth of operation and reduce the transmittance loss over a wider spectral range under zero applied field.A single cavity can provide only a narrow transmittance spectrum, whereas a coupled cavity with appropriate mirror reflectivities provides a broader rectangular shaped transmittance spectrum.A broader "rectangular" transmittance spectrum means that the transmittance loss is reduced over a broad range under zero field.Figure 2(c) and 2(d) show the broadened transmittance spectrum and transmittance change for SQWs and ATQWs, respectively.For the CCMC with SQWs, the maximum transmittance change with 11 pairs of middle DBR exceeds 50% at 850nm which is comparable with that of SC with SQWs while spectral bandwidth are 2.8nm and 7.1nm at ∆T = 50% and ∆T = 30%, respectively, as shown in Fig. 2(c).The largest change in transmittance occurs for CCMC with SQWs due to its low residual absorption at zero field.The residual absorption of CCMC with ATQWs is larger than that of CCMC with SQWs under zero field, and as a result, the change in transmittance is lower compared to the case of CCMC with SQWs as seen from Fig. 2(d).
The merit of CCMC with ATQWs, however, lies in broadening the spectral bandwidth of operation, which is attributed to the coupling between the two cavities.ATQWs have two sets of quantum wells, i.e., QWs that are 8 nm and 8.5 nm thick.These QWs helps in broadening the absorption spectrum.Since absorption spectrum is broadened, transmittance change is also broadened.In other words, CCMC with ATQWs in spite of having a large residual absorption minimizes the transmittance loss over a wide spectral range, thereby increasing the transmittance change over a wide spectral range.This means that a transmittance change is achieved over a wide spectral range.An operation over a wider spectral range thus helps in compensating the low absorption modulation of the CCMC with ATQWs structure.
Figure 2(d) shows the transmittance change for CCMC with ATQWs which has 11 pairs in the middle DBR.As seen from Fig. 2(d), by combining ATQWs and CCMC, the spectral bandwidth is also improved to 4.4nm and 9nm at a ∆T of 50% and 30%, respectively.Moreover, it can be seen from Fig. 2(d) that the transmittance change for CCMC with ATQWs is enhanced compared to that of SC with ATQWs (shown in Fig. 1(d)).Figure 3 shows the transmittance change and contrast ratio as a function of wavelength for the four modulator structures.With regard to transmittance change, ATQWs structure exhibits wider spectral bandwidth than that of SQWs and CCMC provides higher transmittance change along with wide spectral bandwidth compared to SC structure as discussed before.As shown in Fig. 3(b), all modulator structures have a contrast ratio above 3.The spectral bandwidth for SC structures with SQWs and ATQWs are 4.2nm and 9nm, respectively, while the spectral bandwidth for CCMC structure with SQWs and ATQWs are 7.1nm and 12.1nm, respectively.The contrast ratio for this case was 2. CCMC structure having ATQWs exhibits the widest spectral bandwidth and highest contrast ratio among all the structures, due to the incorporation of the coupled cavities of CCMC structure and widened absorption spectrum of ATQWs.Further optimization of the structure can lead to a higher contrast ratio and wider spectral bandwidth.

Fabrication, growth, and characterization
Four modulator structures discussed in section 2 such as SC with SQWs, SC with ATQWs, CCMC with SQWs and CCMC with ATQWs were grown by molecular beam epitaxy (DCA P600) on a semi-insulating GaAs substrate.For the SC structure, the top and bottom DBRs consist of two pairs of Al 0.31 Ga 0.69 As (62.3nm)/Al 0.88 Ga 0.12 As (68.8nm) quarter wave stacks for both the SQWs and ATQWs structure.A 10nm thick heavily p-doped GaAs layer was formed on the top p-doped DBR and a 30nm thick heavily n-doped GaAs layer was formed on the 100nm thick AlAs layer.This AlAs layer acts as the etch stop layer during device processing.The top p-contact layer was made thin in order to minimize absorption losses during the light transmission through the device.For the CCMC structure, the top DBR consist of two pairs of Al 0.31 Ga 0.69 As/Al 0.88 Ga 0.12 As quarter wave stacks followed by a 10nm thick heavily p-doped GaAs layer for contact.A λ/2 thick (124.6nm)micro-cavity composed of a single non-absorptive p-doped Al 0.31 Ga 0.69 As layer was then grown.Middle DBR consists of 11 pairs of Al 0.31 Ga 0.69 As/Al 0.88 Ga 0.12 As followed by an intrinsic region having SQWs or ATQWs.A 30nm thick GaAs contact layer is incorporated in the bottom DBR, which consists of two pairs of Al 0.31 Ga 0.69 As/Al 0.88 Ga 0.12 As quarter wave stacks.Then, etch stop layer was formed in a similar manner with that of SC structure.To ensure the device operation at 850nm, all layers were grown with in situ thickness and reflectance monitoring [25].Figure 4(a) shows the schematic of CCMC structure with ATQWs.First mesa is etched by inductively coupled plasma reactive ion etching so as that n-contact layer was exposed to form electrode.Then, second mesa was etched to provide isolation between individual devices.Ohmic contact was formed directly on the p-and n-doped GaAs layer using electron beam evaporator.To measure the transmittance of devices, the substrate should be removed since GaAs material has high absorption coefficient below 870nm which corresponds to band-edge wavelength of GaAs (bandgap: 1.424eV).Before removing the substrate, high quality SiN x was deposited to prevent damage to the fabricated device caused during substrate removal process.The back side of substrate was made thin up to ~150µm by lapping and polishing.Then, only AlAs etch stop layer of fabricated device area was exposed by using wet selective etching with the pH-adjusted NH 4 OH/H 2 O 2 solution, which provide good selectivity of etching rate of GaAs to AlAs [26].The AlAs etch stop layer was removed by buffered oxide etchant, thus successfully removing the substrate.
Figure 4(b) shows the measured transmittance spectra for CCMC structure with ATQWs at a bias of 0V and −24.5V.The inset of Fig. 4(b) shows scanning electron microscope (SEM) image of the grown CCMC structure which provides a clear view of micro-cavity situated between the top and middle DBR mirror.The measured transmittance spectrum of CCMC structure with ATQWs was in good agreement with the calculated results.Unfortunately, the operating voltage of measured device was much higher than that of the calculated results since for calculations, the resistance in intrinsic region was only considered without considering the parasitic resistance caused by DBR and contacts.5(a) shows that the maximum transmittance changes as a function of wavelength for the four modulator structures.The spectral bandwidth for SC with 8nm thick SQWs was 1nm at ∆T = 50% and 4.8nm at ∆T = 30% while the maximum transmittance change was 53.17% at −27V.By using ATQWs, the spectral bandwidth was improved to 7.5nm at ∆T = 30% due to broader absorption spectrum for ATQWs, however, the maximum transmittance was reduced to less than 50% at −26.5V.For the CCMC structure with SQWs, the maximum transmittance change was improved to >50% at −26V, and the spectral bandwidth was increased to 8nm and 1.8nm at ∆T = 30% and ∆T = 50%, respectively.By the combination of CCMC and ATQWs, the best spectral bandwidth characteristic was achieved with 9.6nm and 2.5nm, corresponding to a transmittance change of 30% and 50%, respectively.As compared to SC structure, usage of the coupled cavities of CCMC structure is the primary reason for the observed higher transmittance change.Figure 5(b) shows the contrast ratio as a function of wavelength for the four modulator structures considered.For all of the structures, the contrast ratio was above 2.For the CCMC with ATQWs, enhancement of the spectral bandwidth becomes apparent at lower contrast ratios.Thus a wide spectral bandwidth with high transmittance change can be simultaneously achieved by using CCMC structure having ATQWs.

Conclusion
We have demonstrated a wide spectral bandwidth electro-absorption modulator operating at 850nm based on CCMC with ATQWs.Modulators having SC and CCMC structure having SQWs or ATQWs were investigated for obtaining high modulation performance.It was found that SC structure having ATQWs showed an improved spectral bandwidth compared to that of SC structure having SQWs by 35.91% at an operating voltage of −26.5V, while the modulation depth was below 50%.By using a CCMC structure having ATQWs, further improvement in the spectral bandwidth was achieved along with a modulation depth of over 50%, which was in good agreement with calculated results.The spectral bandwidth of CCMC structure with ATQWs was 9.6nm.The improvement of the modulation depth obtained with CCMC having ATQWs compared to SC structure can be attributed to the coupled cavities of CCMC structure and widened absorption spectrum of ATQWs.These experimental results suggest that EAM has excellent potential as an optical shutter for 3D imaging applications.

Fig. 1 .
Fig. 1.Contour plots of the calculated transmittance change as a function of number of pairs of DBR for a SC structure with (a) an 8nm thick SQWs, (b) ATQWs with 7-λ single cavity, (c) transmittance and transmittance change along dashed line AB of a 8nm thick SQWs, (d) transmittance and transmittance change of ATQWs with 7-λ single cavity along dashed line AB.The insets of (a) and (b) show the schematic of single cavity structure with an 8nm thick SQWs and ATQWs, respectively.
(a) and 2(b) show the schematic of CCMC modulator structure.The first absorptive cavity with optical thickness of 7-λ, which is composed of multiple QWs, is embedded between bottom and middle DBR coupling mirror and the second micro-cavity with optical thickness of λ/2 consisting of single layer of Al 0.31 Ga 0.69 As is formed between the top and middle DBR coupling mirror.In order to design an optimum structure which can provide wide spectral bandwidth with high transmittance change, it is necessary to study the effect on the transmittance change and spectral bandwidth of modulator induced by the number of mirror pairs of the middle DBR.
Figure 2(a) and 2(b) show the contour plot of the calculated transmittance change as a function of number of pairs of middle DBR for CCMC structure with SQWs and ATQWs.The top and bottom mirrors had the same reflectivities as that of the SC structure.

Fig. 2 .
Fig. 2. The contour plot of the calculated transmittance change as a function of number of pairs of middle DBR for CCMC structure with (a) an 8nm thick SQWs, (b) ATQWs and transmittance, transmittance change for CCMC with (c) an 8nm thick SQWs, (d) ATQWs along dashed line AB of (a), (b).The insets of (a) and (b) show a schematic of CCMC structure with an 8nm thick SQWs and ATQWs.

Fig. 3 .
Fig. 3. (a) Transmittance change and (b) contrast ratio as a function of wavelength for four modulator structures such as SC with SQWs, SC with ATQWs, CCMC with SQWs and CCMC with ATQWs.

Fig. 4 .
Fig. 4. (a) The schematic of CCMC structure with ATQWs and (b) the measured transmittance spectrum for CCMC structure with ATQWs at 0V and −24.5V.The inset of Fig. 4(b) shows scanning electron microscope (SEM) image of the grown CCMC structure with ATQWs.

Fig. 5 .
Fig. 5. (a) The maximum transmittance changes and (b) the contrast ratio as a function of wavelength and for four modulator structures such as SC with SQWs, SC with ATQWs, CCMC with SQWs and CCMC with ATQWs

Figure
Figure5(a) shows that the maximum transmittance changes as a function of wavelength for the four modulator structures.The spectral bandwidth for SC with 8nm thick SQWs was 1nm at ∆T = 50% and 4.8nm at ∆T = 30% while the maximum transmittance change was 53.17% at −27V.By using ATQWs, the spectral bandwidth was improved to 7.5nm at ∆T = 30% due to broader absorption spectrum for ATQWs, however, the maximum transmittance was reduced to less than 50% at −26.5V.For the CCMC structure with SQWs, the maximum transmittance change was improved to >50% at −26V, and the spectral bandwidth was increased to 8nm and 1.8nm at ∆T = 30% and ∆T = 50%, respectively.By the combination of CCMC and ATQWs, the best spectral bandwidth characteristic was achieved with 9.6nm and 2.5nm, corresponding to a transmittance change of 30% and 50%, respectively.As compared to SC structure, usage of the coupled cavities of CCMC structure is the primary reason for the observed higher transmittance change.Figure5(b) shows the contrast ratio as a function of wavelength for the four modulator structures considered.For all of the structures, the contrast ratio was above 2.For the CCMC with ATQWs, enhancement of the spectral bandwidth becomes apparent at lower contrast ratios.Thus a wide spectral bandwidth with high