1. Introduction

QCLs are outstanding coherent semiconductor light sources in the mid-infrared region [1, 2]. The lasers are established for gas and chemical sensing applications since the most fundamental vibrational and rotational molecular absorption lines exist within the mid-infrared portion of the electromagnetic spectrum [3]. In addition, quantum cascade detectors have been demonstrated, providing a compact on-chip detecting system [4].

Improvement of laser output power performance is an inevitable requirement for various laser applications. A simple technique to achieve a power increase was introduced with a laser diode that consists of a tilted front facet and an as-cleaved back facet [5]. For this technique, the power improvement is proportional to reduction of modal reflectivity at the tilted front facet since a power emission ratio between front and back facets is dependent on the modal reflectivity of the two facets [6, 7]. Theoretical analysis of the reflectivity of tilted facets have been reported in a number of publications in the past [8, 9].

The electric field component of an electromagnetic wave parallel and perpendicular to the plane of incidence has different reflection coefficients according to Fresnel’s equations. In general, the TM reflectivity is lower than the TE reflectivity in semiconductor lasers [10, 11]. In addition, the TM polarized light shows no reflection at a certain angle of incidence, called Brewster angle. These features are the foundation for attracting huge attention from laser applications such as traveling wave amplifiers requiring a minimized reflectivity [12]. Since light generated in laser diodes consists of TM and TE polarized light, both need to be considered to achieve reduction of the reflectivity. Contrary to laser diodes, light generation in QCLs is based on intersubband radiative transitions, emitting purely TM polarized light due to the intersubband selection rules [13]. This inherent feature of QCLs allows a selective reduction of the TM reflectivity by using a tilted facet.

Recently, we demonstrated an output power enhancement of QCLs with a facet tilted towards the ridge sidewall [7, 14]. In this case, however, the incident light shows TE-like behavior with respect to the facet since the electric field is parallel to the tilted facet. In the present work, we introduce QCLs with a facet tilted towards the ridge surface (or substrate) where the electric field is no longer parallel to the tilted facet. This allows a reduction of the TM reflectivity, showing higher improvement of light output power and slope efficiency. Additionally, a simulation of Brewster angle on QCLs will be described later in this paper.

2. Fabrication and measurement setup

The quantum cascade heterostructure was grown on a low-doped InP substrate (n = 2 × 10¹⁷ cm⁻³) by molecular beam epitaxy. The active layer region is based on a bound-to-continuum design composed of 35 periods of an InGaAs/InAlAs heterostructure [15]. The vertical waveguide structure is similar to that described in [14], Ahn et al.. The QCL ridges were defined by reactive ion etching. Next, a 400 nm silicon nitride (SiN) insulating layer was deposited by plasma-enhanced chemical vapor deposition. The SiN was then removed from the top of the ridges. A 500 nm Ti/Au contact finished the topside processing and a backside ohmic contact Ge/Au/Ni/Au (15/30/14/150 nm) completed the QCL fabrication. After the processing, ridges with cleaved facets were soldered with indium to copper heat sinks, wire bonded, and installed on a Peltier cooler. The length and width for all the fabricated QCLs are 2 mm and 10 μm, respectively. The lasers were operated in pulsed mode with a pulse length of 100 ns at a repetition rate of 5 kHz (0.05% duty cycle). Optical output powers and spectra were measured with a calibrated deuterated triglycine sulfate detector in a Fourier transform infrared spectrometer. The far-field measurements were carried out using a liquid nitrogen cooled mercury cadmium telluride detector mounted on a computer controlled rotational stage. The distance between the detector and laser facet is 50 mm. All measurements were performed at 293 K.

After the fundamental characterization steps were completed, tilted facets were generated with angles of 4°, 8°, 12°, 17°, and 22° by focused ion beam (FIB) milling. A beam current of 2 nA was used for FIB milling. The FIB process is similar to one described in [7], Ahn et al.. Only the front facet was milled, while the back facet was left as cleaved. After FIB milling, the facet angles were confirmed by scanning electron microscopy (SEM). All the fabricated QCLs were characterized before and after milling for comparison. Due to an asymmetric vertical waveguide structure, two tilt directions of the front facet (structure A and B) of QCLs have been fabricated as shown in Fig. 1. Structure A consists of a QCL with the front facet tilted towards the ridge surface (Figs. 1(b) and 1(e)). Structure B (Figs. 1(c) and 1(f)) is with the facet tilted towards the ridge substrate. In order to proof the reproducibility of results in this work, two sets of the QCLs with the different facet angles were fabricated for each structure A and B, showing similar results.

 

Fig. 1 Scanning electron microscopy images (a, b, c) and sketches (d, e, f) of the vertical waveguide structure of QCLs with a tilted front facet. The polarization of the emitted beam (violet arrows) is also shown where the electric field is directed along the growth direction (z). Cleaved facet (a, d) - before FIB milling. Structure A - facet tilted towards the ridge surface (b, e). Structure B - facet tilted towards the ridge substrate (c, f).

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3. Result and discussion

Figure 2 shows an example of light-current-voltage (LIV) characteristics of fabricated QCLs with a front facet angle of 12° for structure B. The blue dashed line is for the light emission from the front facet of the QCL before FIB milling. The red and green lines correspond to the light emission from the front and the back facet after FIB milling, respectively. The slope efficiencies (η_s) are measured within the linear regime of the LI curve. Peak output powers (P_peak) of 133 mW and 174 mW and η_s of 96 mW/A and 130 mW/A were measured from the QCL before and after FIB milling, respectively. The achieved P_peak and η_s enhancement is 31 % and 34 %, respectively. These improvements result from the asymmetric emission ratio between tilted and back facets [5, 7]. P_peak of 81 mW and η_s of 78 mW/A were measured from the back facet of the QCL after FIB milling. As a result, 68 % of the total light output power (174 + 81 mW) emits through the tilted front facet, demonstrating the asymmetric emission mentioned above. After FIB milling, the threshold current (J_th) of the devices is also increased by 13 % from 0.76 A to 0.86 A, since the mirror loss increases with decreasing modal reflectivity at the tilted facet.

 

Fig. 2 An example of LIV characteristics for structure B of a QCL before FIB milling (blue dashed line) and after FIB milling to be θ_F = 12° (red and green lines). The red and green lines are measured from front and back facets, respectively.

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Figures 3(a)–3(c) show ratios between results before and after FIB milling (after/before) in terms of P_peak, η_s, and J_th, respectively, for different facet angles for both structure A (blue triangles) and B (red circles). In order to avoid additional losses at higher driving currents, only a linear part (just above threshold) of LI curves was taken in the slope efficiency comparison [7]. For structure A, the maximum increase of P_peak and η_s are 22 % at θ_F = 8° and 29 % at θ_F = 12°, respectively. For structure B, the maximum increase of P_peak and η_s are 31 % at θ_F = 12° and 43 % at θ_F = 17°, respectively. Moreover, the η_s ratio of structure B for θ_F = 22° is still higher than for θ_F = 0°. The notable difference between structure A and B is the variation of P_peak and η_s with regard to the facet angles. At lower facet angles, structure A shows higher ratios of the P_peak and η_s. At higher facet angles, on the other hand, structure B shows the higher ratios. We assume that the main reason of this difference is the asymmetric vertical waveguide structure of the QCLs. In the case of structure A, however, considerable light loss could be expected since the reflected light at the tilted facet propagates towards the 250 μm thick InP substrate (see Fig. 1(e)). Effective refractive indices (n_eff) of the active region and the substrate are n_active=3.36 and n_lower=3.05, respectively [15, 16]. With this small difference of the indices, the lower vertical wave guiding is weaker with increasing θ_F. In the case of structure B, the reflected light at the tilted front facet propagates to the highly reflective top metal layer (see Fig. 1(f)) through the shallow (∼2.6 μm) upper cladding layers with refractive index of n_upper=3.2 and, thus, relatively lower loss can be expected compared to structure A. Both structures show that the J_th increases continuously as a function of the facet angle.

 

Fig. 3 Ratios between before and after FIB milling as a function of the facet angles for (a) peak light output power (P_peak), (b) slope efficiency (η_s), and (c) threshold current (J_th). The measurements were done using light emission from the front facet only. The characteristics for both structure A (blue triangles) and B (red circles) are shown. In order to avoid additional losses at higher driving currents, only a linear part (just above threshold) of LI curves was taken in the slope efficiency comparison [7].

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Figure 4 shows spectra of structure A (a) and B (b) for different facet angles. All spectra were measured at a driving current of ∼ 2.2 A (see Fig. 2). For θ_F = 0°, the spectra are covering a wavenumber region between 1120 and 1190 cm⁻¹, showing multiple longitudinal mode operation. The mode spacing is ∼0.72 cm⁻¹. The spectral region is narrowing as a function of the facet angles. For θ_F = 17°, structure B shows a large number of the modes in contrast to structure A. This difference can be attributed to the light loss into the substrate of structure A.

 

Fig. 4 Spectra of the QCLs for structure A (a) and structure B (b) for different facet angles. All spectra were measured at a driving current of ∼ 2.2 A (see Fig. 2).

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Figure 5 shows the vertical far-field profiles of the beam emitted from the tilted facet of QCLs for structure A (a) and B (b). The profiles were measured along the growth (z) direction (see Fig. 1(d)) at near roll-over in the LI curve. The data has been normalized to allow comparison. A vertical dashed line at 0° represents the ridge normal direction. Positive and negative angles are defined in Fig. 1. All measured devices show single-lobed profiles and independence of current throughout the operating range. Lateral far-field profiles of the devices are found to be single lobes (not shown). Since the facets are tilted, the far-field profiles shift in one dimension continuously as a function of the facet angles.

 

Fig. 5 Measured vertical far-field profiles for different facet angles for both (a) structure A and (b) structure B. The profiles were measured at near roll-over.

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In order to investigate the Brewster angle on QCLs, we performed 2-D simulations using the finite element method (COMSOL Multiphysics) [17, 18]. The simulation model is similar to one described in [17], Yu et al.. To observe a light power transmittance for different facet angles, a passive waveguide employing various angles of a facet is simulated with an emission wavelength (λ) of 8.6 μm, which is shown schematically in the inset in Fig. 6. The simulation consists of air, top Au contact layer, the upper cladding (n_upper=3.2), active region (n_act =3.36), lower cladding (n_lower=3.05) and perfect matched layers (PML). Only the real part of the reflective index is considered for the simulation. In the inset, the spatially distributed mode in the resonator and its emission from the facet (θ_F = 0°) are shown. The intensity of the emitted light is normalized by an input light intensity to obtain the transmittance. Figure 6 shows a calculated transmittance for a plane wave (gray dashed line) and the simulated transmittances as a function of the facet angle θ_F (see Fig. 1) for different active region thicknesses (D_act). A realistic active region thickness of the fabricated device is 3 μm [14]. The transmittance with D_act = 3 μm shows continuous decrease as a function of the facet angle [8]. The local maximum of the transmittance arises with increasing D_act and is shifting to the calculated Brewster angle of 17°. D_act = 30 μm shows 86 % of the transmittance at a facet angle of 14°. According to this simulation result, to exploit the effect of Brewster angle on QCLs, an extremely thick active region depending on λ might be required where the guided wave becomes almost planar.

 

Fig. 6 Simulated light power transmittance (solid lines) at the facet of a passive semiconductor waveguide as a function of the facet angle for different active region thickness (D_act). A calculated transmittance for plane wave (dashed line) and Brewster angle (θ_Brewster) are shown as well. The inset shows schematically the 2-D simulation model of the passive waveguide structure (D_act = 3 μm, θ_F = 0°) and the simulated mode distribution (λ = 8.6 μm).

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4. Conclusion

In conclusion, we have introduced a technique to utilize the polarization purity of QCLs by using a tilted facet. Structure B shows relatively stable output power performance at higher facet angles compared to structure A. The maximum increase of peak output powers and slope efficiencies observed were 31 % at θ_F = 12° and 43 % at θ_F = 17° from structure B, respectively. The emission angle of ∼30° directed to the surface shows a potential to be employed as a surface-emitting device. Additionally, by using the COMSOL simulations, we find that an extremely thicker active region in terms of an emission wavelength is required to employ the Brewster angle on QCLs. In this work, the tilted facets were generated by FIB milling. However, the FIB milling is a costly process not suitable for mass production. Therefore, a simple technique to create the tilted facet structure is required. One approach to this would be the use of polishing with a diamond lapping film as a post fabrication step [19]. This technique might enable higher output powers by using just one additional economic fabrication step.