Background

Vertical-cavity semiconductor optical amplifiers (VCSOAs) at 1.3 μm are key photonic components in optical communication systems [1–4]. Dilute nitride III-V alloy semiconductors and in particular GaInNAs/GaAs quantum well (QW)-based VCSOAs were originally proposed as replacements for GaInAsP/InP QWs due to its reduced temperature sensitivity and inherent polarization insensitivity [5, 6]. In addition, their growth on GaAs and their integrability with GaAs/Al(Ga)As distributed Bragg reflectors (DBRs) allowed them to be considered as the active region in 1.3-μm vertical-cavity devices. In this article, a novel VCSOA based on the hot electron light emission and lasing in semiconductor heterostructure (HELLISH) as an alternative to conventional VCSOAs is investigated [7]. Spontaneous emission of ultra bright HELLISH has been previously reported and demonstrated by us [8, 9]. The simple bar HELLISH-VCSOA [10] and Top-Hat HELLISH-VCSOA [11] structures with GaInNAs/GaAs quantum wells in the active region are designed to operate in the 1.3-μm wavelength region.

In this work, we demonstrate for the first time, optical amplification at wavelength λ ≈ 1.3 μm in electrically pumped THH-VCSOA devices. We measured the photoluminescence (PL) and electroluminescence (EL). By combining the two measurements, we obtained the electrophotoluminescence (EPL) signal from which the light amplification is obtained. At a temperature of T = 300 K, maximum gains were achieved when voltages of 40, 60, and 80 V were applied.

Methods

The device of THH-VCSOA with the code VN1520 was grown by molecular beam epitaxy (MBE) on a semi-insulating GaAs substrate. Figure 1a shows the sample structure. Eleven Ga0.35In0.65 N0.02As0.08/GaAs QWs were used in the active region to supply enough gain at a wavelength of around 1.28 μm. The active region is within a micro-cavity which was formed by growing DBRs below and above the active region. Top and bottom DBRs have 6 and 20.5 pairs of AlAs/GaAs, with mirrors yielding calculated reflectivities of 0.6 and 0.99, respectively. The device was fabricated by selective etching to have a p-channel of length 0.6 mm and an n-channel of length 1 mm. Under normal operational conditions, contacts 1 and 2 are biased with either positive polarity (+V) or negative polarity (-V) while contacts 3 and 4 are both connected to the ground.

Figure 1
figure 1

Schematic diagram of (a) THH-VCSOA structure and its contact configuration and (b) potential distributions along p-channel and n-channel. In the region of Vp > Vn, the device is forward biased, while in the region of Vn > Vp, the device is reverse biased.

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When the device is biased with (+V), as shown in Figure 1b, the potential near contact 2 (I2) is higher in the p-channel than in the n-channel (Vp > Vn). This forward-biased region operates as a light emitter. In contrast, near contact 3 (I3), Vp < Vn and this region is effectively reverse biased, which forms the absorption section. Thus, the device can absorb light with photon energies of hv 0 , where hv 0  > E g and emit light with photon energies of hv 1  ~ E g . The polarity of the applied bias can be interchanged leading to the reversing of the absorption and emission regions.

The emitted light from the sample surface was collected and dispersed using a cooled photo multiplier and monochromator assembly. The output signal was filtered using an EG&G 162 boxcar averager with gated integrator. An Argon laser of wavelength λ = 488 nm, using variable powers, is used as the light source in the absorption experiments. External bias was applied in a pulsed mode between contacts 1 and 4, and 2 and 3 of the top-hat-shaped device. The device resistance depends on the device dimensions and can be as high as 1.0 KΩ in devices with long channel lengths. The applied voltage pulses were 50-μs wide with a repetition time of 10 ms defining a duty cycle of 5 × 103.

Results and discussion

Figure 2 shows integrated EL intensity as a function of applied voltage for both voltage polarities. The EL was measured using voltage pulses of widths and repetition times of about 50 μs and 9 ms, respectively, to avoid excessive Joule heating. It is clear that the light intensity is independent of the polarity. The threshold voltages Vth of the bidirectional device are Vth approximately 50 V at T = 300 K and Vth approximately 4 V at T = 100 K.

Figure 2
figure 2

Integrated electroluminescence intensity of bidirectional field effect light-emitting and light-absorbing heterojunction device (for both voltage polarities). Temperatures of T = 100 and 300 K.

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Figure 3 shows the EL emission spectra as a function of temperature. The peak wavelengths at T = 150 and 300 K are around λ = 1,236 and 1,288 nm, respectively. Theoretically, a red shift of the active material peak wavelength with temperature at a rate of 0.38 nm/K is predicted. We compare the experimental peak emission energy versus the temperature plot with the Varshni equation:

E g T = E 0 - α T 2 / T + β ,

where E0 and E g (T) are the bandgaps at T = 0 K and at a finite temperature of T, respectively and α and β are around 4.8 × 10-4 eV · K-2 and 284 ± 167 K, respectively [12, 13].

Figure 3
figure 3

EL spectra of bidirectional THH-VCSOA-based GaInNAs/GaAs structures at different temperatures. The inset shows the temperature dependence of the peak energy (filled squares) compared with the Varshni equation (dotted lines).

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The device was mounted on a temperature-controlled holder at varied temperatures. External voltage pulses up to 110 V were applied between the diffused contacts and the integrated EPL intensities of the THH-VCSOA are measured as a function of bias voltage with the photo-excitation power was kept constant at around 17 mW. In Figure 4, we show the peak intensities of EPL signals for both positive and negative polarities at T = 14°C and for positive polarity at temperatures of T = 30 and 44°C.

Figure 4
figure 4

Temperature-dependent amplified signal of bidirectional THH-VCSOA structure.

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Amplified spectra are plotted as a function of applied voltages in Figure 5. It is clear from the figure that as the applied voltage increases, the integrated intensity increases with the emission peak at around 1,280 nm.

Figure 5
figure 5

Amplified intensity as a function of applied voltages between 30 and 200 V at T = 300 K.

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The spectra of EL, PL, and the combined EPL of bidirectional THH-VCSOA device at 1,280 nm are shown in Figure 6. The spectra have a broad bandwidth due to the fact that light was collected from the whole forward-biased area. The input signal of 488 nm is absorbed by the THH device, causing a modulation of the 1,280-nm light, thus acting as a wavelength converter. In EPL, the device is optically but also electrically pumped, with Vapp = 80 V in amplitude. The EL spectrum alone was also measured with Vapp = 80 V and the difference between EL + PL and EPL intensities is accountable for the gain from the device. Optical gains versus incident powers at various applied voltages are depicted in Figure 7. At T = 300 K, maximum gains of around 1.3, 3.3, and 5 dB at Vapp = 40, 60, and 80 V, respectively, are observed.

Figure 6
figure 6

PL, EL, and EPL spectra of THH-VCSOA at T = 300 K.

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Figure 7
figure 7

Gain versus incident power using various applied voltages at T = 300 K.

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Conclusions

The operation of bidirectional THH-VCSOA-based Ga0.35In0.65 N0.02As0.08 at a wavelength of 1,280 nm has been demonstrated. Maximum optical gain of about 5 dB is observed at Vapp = 80 V and at T = 300 K. Therefore, we conclude that the THH-VCSOA device is a bidirectional field-effect light-emitting and light-absorbing heterojunction and can work as an optical amplifier and wavelength converter in the 1.3-μm wavelength regime. The performance of the device can be improved by reducing the dimensions of the device, so that high electrical fields can be reached by the application of small voltages.