We report a nano-electromechanical optoelectronic (NEMO) tunable vertical-cavity surface-emitting laser (VCSEL) with an ultra-thin (145 nm) electrostatically actuated high-index-contrast subwavelength grating (HCG) designed to strongly reflect TE-polarized light. Single mode emission (SMSR >45 dB) and continuous wavelength tuning (~4 nm) were obtained at room temperature with output power up to 2 mW under continuous wave (CW) operation. A record short wavelength tuning time (~90 ns) is experimentally demonstrated, which is >100 times faster than previously reported DBR-based tunable VCSELs and a 1.7 times improvement over the previously reported TM polarized NEMO tunable VCSELs.
©2008 Optical Society of America
Vertical-cavity surface-emitting lasers (VCSELs) with micro-electromechanical structures (MEMS) have been demonstrated as a compact wavelength tunable source for a variety of applications including telecommunication [1,2], bio/chemical sensing , and spectroscopy. Compared to other tuning methods, MEMS tunable VCSELs are promising because they provide a continuous and wide range of tuning with simple mechanical and electrical controls. In addition, the MEMS-VCSELs offer the advantages of batch processing and wafer-scale testing, which can enable low-cost large-volume manufacturing. However for conventional VCSELs, a 3-10 µm thick distributed Bragg reflector (DBR) is needed to achieve the high reflectivity required for lasing due to the thin active gain region in VCSELs. This imposes significant difficulties on their tuning speed and fabrication [5-8]. Recently a new class of high-reflectivity mirror using a single layer high-index-contrast subwavelength grating (HCG) was proposed . It has been shown that the HCG can replace conventional DBR structure as an alternative solution for VCSEL mirrors [10,11]. Recently, we successfully demonstrated the realization of a nano-electromechanical optoelectronic (NEMO) tunable laser using an integrated HCG as a movable reflector [12-14]. The original HCG is designed to strongly reflect transverse-magnetic (TM) polarized light. It is made of high refractive index material (AlGaAs) which is entirely surrounded by low index material (air). The thickness of the TM-HCG is 235 nm which is <10% of that of a conventional DBR. The HCG enables scaling of the mechanical tuning structure by a factor of 10 times in all dimensions, and hence the mass is <0.1% of regular DBR-based MEMS. The significant reduction of the tuning mirror mass leads to a significant improvement in tuning speed, resulting in a tuning time of 150 ns .
In this work, we present the incorporation of a transverse-electric (TE) polarized light HCG onto a VCSEL, for the first time. The TE-HCG is 40% thinner than the TM-HCG and has a smaller duty cycle with a 60% increase in period. The wider period and smaller duty cycle make it easier to fabricate. This TE-HCG is fabricated on a NEMO tunable VCSEL structure. With the factor of two thickness reduction to 145 nm, a record short wavelength tuning time in the range of tens of nanoseconds is achieved. We also show that the TE-HCG exhibits high reflectivity with broad bandwidth, similar to TM-HCG, although the dimensions are significantly different.
Similar to the structure of the TM-HCG, the TE-HCG consists of periodic stripes comprising of high-index material Al0.6Ga0.4As that are freely suspended and surrounded by air as the low-index material. However, the grating parameters of the TE-HCG are quite different from the TM-HCG parameters. In the TE-HCG design, grating period (Λ)=638 nm, grating thickness (tg)=145 nm, and grating stripe width (s)=242 nm, or equivalently a duty cycle (DC)=38%. Duty cycle is defined as s/Λ. As a comparison, in the TM-HCG design, period (Λ)=375 nm, grating thickness (tg)=235 nm, and grating stripe width (s)=244 nm, or equivalently a duty cycle (DC)=65%. Both TE and TM HCG are designed for VCSELs operating at 850 nm. The reason why the TE based HCG yields a thinner reflector may be understood by a larger effective index for TE polarized grating which can be calculated by simple boundary condition considerations and spatial weighting of the electrical and displacement fields . Figure 1 shows a comparison of the scanning electron microscopy (SEM) images of a fabricated TM-HCG and a TE-HCG structure. Clearly the TE-HCG contains grating stripes with about half the thickness of that of TM-HCG, and hence the mechanical weight is reduced by a factor of two. It is worthwhile pointing out that both HCGs show significant surface and edge roughness on the order of tens of nanometers. However, the lasers typically exhibit excellent threshold and efficiency compared to conventional DBR oxide-confined VCSELs. This testifies to the high tolerance of HCG designs, as published in Ref. .
As previously discussed, the TM-based HCG exhibits an ultra-wide band, high reflectivity mirror due to the drastic change of boundary condition for the electric (E) and displacement (D) fields as the plane wave enters the grating . The same characteristics for TE-HCG arise from the mismatch in the speed of light between the high-index region and air. This large difference in the speed of light leads to the termination of the E field between grating fingers, resulting in the generation of k-vector in the x-direction perpendicular to the incident direction of the plane wave, as marked in Fig. 1(b). The x-direction propagation wave sees a large index modulation and hence the wide-band high reflectivity.
Numerical simulation was performed to calculate the reflectivity of the composite top mirror using the Rigorous Coupled Wave Analysis method . Figure 2 shows a comparison between the reflectivity spectra of the TE-HCG based top mirror and the TM-HCG based top mirror with TE (electric field parallel to grating stripes) and TM (electric field perpendicular to grating stripes) incident surface-normal plane wave, respectively. Both HCG designs provide similar ultra-high reflectivity (>99.9%) for wavelength ranges of 0.8-0.89 µm, which are sufficient to serve as tunable VCSEL mirrors.
The device design of a NEMO tunable VCSEL based on the TE polarized HCG is similar to the NEMO tunable VCSEL with TM-HCG as shown in the Fig. 1 of Ref. , with the exception of the different HCG design. The device consists of a HCG-based top mirror, a λ-cavity layer with three GaAs quantum wells, and 34 pairs of n-doped bottom DBRs on a GaAs substrate. The top mirror is comprised of three parts (starting from the substrate side): a fixed 4 pairs of p-doped DBR, a variable airgap, and a freely suspended n-doped TE-HCG that is supported by a nano-mechanical structure. The 4 pairs of fixed p-DBRs are used for the purpose of current injection and active region protection instead of providing reflectivity. We have previously shown that a design with 2 pairs of DBR yields, in fact, lower threshold and higher efficiency devices . Electric current injection is provided through the middle laser contact (via the p-doped DBR) and backside contact (via substrate). An aluminum oxide aperture is formed on an AlAs layer in the p-DBR section immediately above the cavity layer to provide current and optical confinement. The mechanical tuning contact is fabricated on the top n-doped HCG layer. Wavelength tuning is accomplished by applying a reverse voltage bias across the top n-doped HCG layer and the middle p-DBR. The reverse voltage results in vertical electric field across the airgap that pulls the movable TE-HCG toward the substrate. This produces a mechanical deflection that reduces the airgap size, which leads to a change in the Fabry-Perot resonance of the VCSEL cavity and the emission wavelength of the laser.
The fabrication process of TE NEMO tunable VCSEL starts with the mesa formation (~100 µm) by etching down to the bottom DBRs and is followed by thermal oxidation to form the oxide aperture (~3 µm). The next steps are the top and back-side contact metal depositions. Then, a part of the mesa is etched to expose the p-doped current injection layer, on top of which the laser contact metal is deposited. The HCG is patterned by electron-beam lithography on poly-methyl methacrylate (PMMA) photoresist, which enables proper alignment to the oxide aperture and design flexibility in terms of varying the grating period and duty cycle. However, given the current state-of-art lithography technology and the large fabrication tolerance of HCG , the HCG can also be readily defined by more efficient and low-cost methods such as nano-imprinting. A wet chemical-based selective etching followed by critical point drying is required to remove the sacrificial material underneath the HCG layer and form the freely suspended grating structure that is supported by the nano-mechanical structure. Compared to the conventional RIE-based selective etch method, chemical based selective etching enables a very rapid fabrication cycle with very high etch selectivity and improves device yield and reproducibility. Figure 3 shows the top view SEM image of the fabricated device with the TE-HCG aligned in the center of the VCSEL mesa (oxide aperture). The inset in Fig. 3 shows the zoomed-in tilted view of the fabricated TE-HCG, which is freely suspended and supported by a bridge structure.
Continuous-wave room-temperature operation of an electrostatically actuated TE-HCG based NEMO tunable VCSEL is demonstrated for the first time. Figure 4(a) is the VCSEL static optical measurement result (without applying a tuning voltage), showing the output power versus bias current (LI) and voltage versus bias current (IV) characteristics. The VCSEL device exhibits a low threshold current of 1.0 mA and high output power of ~2 mW when injected with 5 mA of current, with slope efficiency of 0.54 W/A. Figure 4(b) shows the measured emission spectra of the laser when biased at 4 times the threshold current. Since the high reflectivity is only available within the finite HCG pattern area, higher-order transverse modes are suppressed and a single fundamental mode emission with a 45 dB side-mode suppression ratio (SMSR) was obtained.
Polarization of the VCSEL emission was measured by inserting a polarizer on top of the VCSEL output. Figure 5 shows the measured peak spectral intensity as a function of the polarizer’s rotational angle in a polarization-resolved spectral measurement. The emission of TE-HCG based VCSEL exhibits highest intensity when polarizer is aligned parallel to the grating stripes (0°). In contrast, the TM-HCG based VCSEL in our prior works exhibits maximal intensity when polarizer is aligned perpendicular to the stripes (90°). Regardless, both HCG VCSELs demonstrated a well-controlled emission output polarization with orthogonal polarization suppression ratio of ~30 dB.
When applying a reverse bias voltage across the tuning contacts, a vertical electric field is produced across the airgap and pulls the movable HCG toward the substrate. Thus the airgap is reduced and hence the laser wavelength blue-shifts. Figure 6(a) shows the measured emission spectra of the device when the active region is electrically pumped at 1.5 mA under various applied tuning voltages. A continuous wavelength tuning range of 4 nm was obtained with 7 V of external applied voltage. The laser remains in single mode emission throughout the entire tuning range. The tuning range can be increased by reducing the number of current injecting DBR pairs from 4 to 1~2 (to increase Δλ/Δairgap efficiency), as we have demonstrated in our prior work .
To determine the tuning speed of the devices, the mechanical frequency response of the TE-HCG structure was measured by applying a sinusoidal AC modulating voltage in addition to a DC static voltage between the top tuning contact and middle laser contact, while the VCSEL is injected with a constant current. The emission light is then collected by an optical fiber and sent to an optical spectrum analyzer (OSA). Since the signal integration time of the OSA is much slower than the voltage modulation, a spectrally broadened emission can be observed. Figure 6(b) shows the normalized measured emission wavelength linewidth change as a function of the AC modulating frequency for TE NEMO tunable VCSEL with 10×10 µm2 HCG mirror size. The measured peak resonant frequency is ~3 MHz with a 3dB frequency bandwidth of 5.4 MHz. The tuning time of this device is estimated to be ~90 ns. This is >100 times faster than previously reported DBR-based tunable VCSELs (≥10 µs) and a 1.7 times improvement over previously reported TM NEMO designs (~150 ns) .
Tunable NEMO VCSELs using HCGs optimized to strongly reflect TE-polarized light are demonstrated for the first time. We show that the TE-HCG leads to a major size reduction in nano-electromechanical tunable VCSELs, which leads to a record wavelength tuning time of 90 ns. Single mode emission (SMSR >45 dB) and continuous wavelength tuning (~4 nm) were achieved at room temperature with output power up to 2 mW. We further demonstrate the versatility of HCG to provide broadband high-reflection mirrors with either TE or TM polarized light. There are numerous applications including wavelength-tunable optoelectronic devices such as VCSELs, optical filters, detectors, and sensors.
This project has been supported by DARPA Center for Optoelectronic Nanostructure Semiconductor Research and Technology (CONSRT) HR0011-04-1-0040. We thank LandMark Optoelectronic Corporation for the growth of epitaxy wafer and Berkeley Microfabrication Laboratory for the fabrication support.
References and Links
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