Surface-emitting ring quantum cascade lasers

3.1 Continuous-wave operation

The early ring QCLs were limited to continuous-wave (CW) operation only at cryogenic temperatures [40], which constrained their applicability. For many applications, such as spectroscopy, a CW laser capable of operating at room temperature is highly desirable [43]. The primary challenge in achieving room-temperature CW operation is efficient heat dissipation. In the initial designs, ring QCLs were fabricated in a manner that caused them to protrude from the chip, with both the top surface, where the grating is located, and the sidewalls exposed to the surrounding environment. Although a SiN isolation layer and a gold contact layer protect the laser cavity, these layers are thin and do not provide sufficient heat dissipation, and the surrounding air is a poor thermal conductor.

A solution to this thermal management problem was demonstrated in 2011 by Bai et al., who introduced an overgrowth of the ring QCL with InP [44]. An SEM image of this overgrown ring QCL is provided in Figure 3(a). They chose a ring radius of 400 µm and a buried second order DFB grating was fabricated from an InGaAs layer, located directly on top of the active region. This approach enabled significant improvements in laser performance, achieving a maximum output power of 510 mW, a threshold current density of 1.06 kA/cm², a wall-plug efficiency of 6.25 %, and a slope efficiency of 1.08 W/A, all under CW operation at room temperature. However, the overgrown ring QCL did not operate at the intended DFB mode, as evidenced by a broad farfield pattern that indicated the device was operating in a higher-order mode. This overcoupling was attributed to the close proximity of the DFB grating to the active region, which resulted in excessive feedback.

Figure 3:

Room-temperature CW operation of ring QCLs. (a) SEM image of a CW ring QCL overgrown with InP and electroplated with 3 µm gold. (b) LIV characteristics of a CW ring QCL at room temperature with a maximum output power of 202 mW. The threshold current of 0.9 A corresponds to a threshold current density of 3.6 kA/cm². (c) Single-mode spectra with an SMSR of 25 dB and temperature-tuning behavior with a tuning coefficient of −0.271 cm⁻¹/K. (d) Recorded farfield pattern of a CW ring QCL at room temperature. The inset depicts the simulated farfield. (a) Is reprinted from Ref. [44], with permission from AIP Publishing; (b–d) are reprinted from Ref. [45], with permission (CC BY 4.0).

To address these issues, Wu et al. refined the design by increasing the distance between the grating and the active region through the introduction of a surface grating [45]. This modification was complemented by efficient heat dissipation achieved via electroplating of a 2 µm thick gold layer, followed by epilayer-down bonding of individual devices onto an AlN submount. This improved processing approach allowed for single-mode emission at the correct DFB mode and produced the expected farfield pattern with clearly defined interference fringes. However, these improvements came at the cost of reduced performance. The epilayer-down bonded ring QCL exhibited a lower output power of 202 mW (see Figure 3(b)), a higher threshold current density of 3.3 kA/cm² and reduced wall-plug efficiency and slope efficiency of 1.32 %, and 0.57 W/A, respectively, compared to the overgrown device. Figure 3(c) depicts the spectra of this CW ring QCL. The farfield pattern shown in Figure 3(d) distinctly displays circular interference fringes, characteristic of the laser’s operation at the DFB mode.

3.2 Microrings

Reducing the footprint of surface-emitting QCLs is critical for enabling large-scale production of low-cost, low-power MIR lasers. Achieving this requires a low-loss outer boundary to support the whispering gallery mode (WGM), which confines light within the microring structure. Stark et al. realized such boundary optimization by employing an Al₂O₃/Au coating along the outer cavity boundary for microrings of different sizes [46]. Ring QCLs with different diameters are given in Figure 4(a).

Figure 4:

Miniaturization and performance of microring QCLs. (a) SEM image of microring QCLs with different radii. The inset shows a schematic of the cross section of the device with the location of the buried DFB grating indicated in blue and the active region indicated in red. (b) Threshold current density as a function of the microring radius. The smallest lasing microring exhibits a radius of 50 µm. (c) LIV characteristics of a CW microring QCL. The inset shows a photograph during on-chip testing with a needle probe. (d) CW current-tuning characteristics and single-mode spectrum (inset). (a–d) Are reprinted from Ref. [46], with permission (CC BY 4.0).

This configuration leverages an overgrowth approach with a buried heterostructure and a short arcuate grating section, etched directly into an InGaAs layer atop the active region, a method comparable to that of Bai et al. [44]. Stark et al. provide an in-depth analysis showing that the threshold current density increases as the ring diameter decreases, demonstrating lasing even in rings as small as 50 µm, as shown in Figure 4(b). Moreover, they achieved CW operation in rings with diameters as small as 100 µm. These CW microrings exhibit relatively low power dissipation, with values of 267 mW at threshold and 374 mW at maximum output. The corresponding LIV curve is presented in Figure 4(c). The slope efficiency is reported at 0.6 mW/A, and single-mode emission at around 4.8 µm shows an SMSR of 25 dB, with a current-tuning coefficient of −0.45 cm⁻¹/mA. An exemplary spectrum as well as the current-tuning behavior in CW mode are illustrated in Figure 4(d). This study marks a significant step toward the scalable, low-cost fabrication of surface-emitting QCLs with minimized power consumption, though farfield pattern data for these microring QCLs was not available at the time of writing this review.

3.3 Terahertz emission

THz radiation, occupying the frequency range between infrared and microwave, is increasingly important due to its unique applications in fields such as spectroscopy, imaging, and communication. THz waves can penetrate non-metallic materials and provide high-resolution imaging without the ionizing effects of X-rays. However, generating coherent THz radiation, crucial for many of these applications, remains a challenge. A coherent emitter, such as a QCL, provides the narrow linewidth and phase coherence necessary for precision in spectroscopy and communication systems [47].

In 2008, Mahler et al. demonstrated a quantum cascade microdisk laser operating in the THz region, featuring a second-order DFB grating for vertical light outcoupling [48]. This device emitted at 3.3 THz and had a radius around 90 µm, showcasing a key step forward for surface-emitting THz QCLs. Typically, THz QCLs are fabricated using a double-metal waveguide, where the gain material is sandwiched directly between two metal layers. This design enables sub-wavelength confinement of the optical mode, resulting in a confinement factor close to unity, but also causes the emitted beam to be highly divergent when exiting through a standard FP laser facet. Therefore, a circular surface-emitting geometry based on second-order DFB gratings, which significantly increases the emitting area, is even more advantageous in the THz range than in the MIR, where similar strategies have been employed.

In 2009, two separate groups independently demonstrated ring QCLs in the THz regime [49], [50]. Mahler et al. introduced a ring QCL with a radius of around 500 µm and a double-slit configuration for the DFB grating [49], aimed at providing a more uniform current injection. An SEM image of the double-slit grating configuration is given in Figure 5(a). This design positioned a gold pumping segment at the center of each grating slit, slightly smaller than the slit itself, creating the double-slit configuration. The device achieved a maximum optical output power of 10 mW, a slope efficiency of 25 mW/A, a threshold current of 1.5 A, and operated up to a temperature of 100 K, as shown in Figure 5(b). However, the farfield pattern was not uniform, indicating non-ideal emission characteristics. Figure 5(c) and (d) show the simulated and recorded farfield patterns of a THz ring QCL featuring the double-slit configuration, respectively.

Figure 5:

Ring QCLs operating in the THz regime. (a) SEM image of a THz ring QCL with a double-slit DFB grating. (b) LIV characteristics at different temperatures. The threshold currents of 1.6 A at 60 K and 2.0 A at 95 K correspond to threshold current densities of 0.59 kA/cm² and 0.74 kA/cm², respectively. The inset shows a spectrum with an SMSR of 20 dB. (c) Simulated farfield pattern. (d) Recorded farfield and spectrum of the THz ring QCL in the double-slit configuration. (e) LIV characteristics of a THz ring QCL in the narrow-slit configuration. The optical output power is larger than in a comparable FP device. The inset shows an SEM image of the ring QCL. (f) Recorded farfield of a THz ring QCL with rotational symmetry and interference rings. (a–d) Are reprinted from Ref. [49], under the terms of the Open Access Publishing Agreement. ©2009 Optical Society of America; (e–f) are reprinted from Ref. [50], with permission from AIP Publishing.

Mujagic et al. took a different approach using smaller rings with a radius of 200 µm. Furthermore, they avoided the double-slit configuration and instead used single, narrower slits compared to those used in MIR QCLs [50]. This significantly smaller design, while producing a lower optical output power of 0.55 mW and a slope efficiency of 0.97 mW/A, achieved a higher maximum operating temperature of 120 K and a lower threshold current of 0.52 A. The LIV as well as an SEM image of this THz ring QCL is given in Figure 5(e). More importantly, the farfield pattern exhibited the expected circular symmetry, as can be seen in Figure 5(f). The clear interference fringes indicate a uniform and coherent emission from the entire ring surface. Additionally, the THz ring QCL demonstrated an optical output power and slope efficiency that were twice those of an FP device fabricated from the same material, further highlighting the advantages of the surface-emitting circular geometry for THz applications.

3.4 Spectral tuning and array integration

One key application of ring QCLs is spectroscopy, where a detailed understanding of the laser’s tuning characteristics is crucial. In a comprehensive study, Brandstetter et al. analyzed the time-resolved spectral behavior of pulsed ring QCLs and compared it to that of straight DFB FP devices [51]. Utilizing a high-resolution step-scan Fourier transform infrared (FTIR) method, they achieved a spectral resolution of 0.1 cm⁻¹ and a time resolution of 2 ns. The study compared a short DFB FP laser, a long DFB FP laser, and a ring QCL, all fabricated from the same material. The short DFB FP laser and the ring QCL both had a similar length of 0.9 mm, where the length of the ring corresponds to its circumference. The investigation focused on the stability of spectral emission, which depends on the cavity length L and the coupling coefficient κ through the coupling strength factor κ⋅L [52]. Based on this relationship, the short DFB FP laser was expected to exhibit similar performance to the ring QCL.

Surprisingly, the results revealed that the ring QCL exhibited a threshold current density comparable to that of the long DFB FP laser. Moreover, the stable emission range of the ring QCL extended over an even broader spectral range than the long DFB FP laser. This performance is attributed to the absence of facets in the ring QCL, highlighting the advantages of facetless laser designs in terms of mode stability and extended spectral coverage. Figure 6(a) gives a summary of the spectral tuning characteristics of ring QCLs.

Figure 6:

Spectral tuning and array integration of ring QCLs. (a) Intra-pulse tuning characteristics for different currents with a maximum tuning range of 12.9 cm⁻¹ of a single ring QCL. (b) SEM image of a surface-emitting ring QCL array with 16 lasers at different wavelengths. (c) Emission spectra of the 16 ring QCLs in the array. All devices cover a spectral range of 180 cm⁻¹. The inset shows one of those spectra with an SMSR of 30 dB. (d) SEM images of coupled rings. (a) Is reprinted from Ref. [51], under the terms of the Open Access Publishing Agreement. ©2014 Optical Society of America; (b–c) are reprinted from Ref. [53], with permission from AIP Publishing; (d) is reprinted from Ref. [54], with permission from AIP Publishing.

Array integration of ring QCLs is a promising approach for applications in gas analysis and chemical sensing, where multiple single-mode laser sources are essential for measuring various substances. Mujagic et al. demonstrated the first ring QCL array, consisting of 16 ring QCLs, achieving a total tuning range of 180 cm⁻¹ around a center wavelength of 8.2 µm [53], [55]. Figure 6(b) depicts an SEM image of the 16 ring QCL array and Figure 6(c) outlines the spectrum of each of those 16 devices. The lasers function in single-mode operation, corresponding to their specific grating period, and exhibit a consistent output without mode hops. The facetless design of ring QCLs makes them particularly appealing for use in laser arrays, as their performance is governed by the gain material utilized rather than being influenced by the frequency selection process. Such monolithically integrated arrays facilitate compact, widely tunable light sources, thereby enhancing selectivity and sensitivity for multianalyte spectroscopy. Two-dimensional integration of these coherent emitters allows for on-wafer testing and scalability, thus reducing fabrication costs and efforts. This advancement in array integration marks a significant step toward the development of sophisticated spectroscopic tools leveraging the unique capabilities of ring QCLs.

The linear arrangement of ring QCL arrays poses certain limitations, particularly in terms of space consumption on the semiconductor chip, which restricts the number of devices that can be integrated. To address this, alternative designs incorporate two or more concentric rings that share the same optical axis [37], [56]. These configurations optimize chip area utilization, enabling higher device density and potentially more compact laser arrays without compromising functionality.

3.5 Coupled rings

The modes within ring resonators are known as WGMs, which are concentrated near the outer wall of the ring. This results in a significant amount of mode leakage at the ring’s outer border, manifesting as an evanescent field. This field can facilitate coupling between rings through evanescent field interactions. In a study by Schwarzer et al., coherent coupling and phase locking of two ring QCLs were successfully demonstrated [54], [57]. An SEM image of two coupled ring QCLs is shown in Figure 6(d). This achievement highlights the potential of ring QCLs for applications in coherent, high-power surface-emitting QCL arrays.

In their study, two rings with a coupling gap of 1 µm demonstrated clear coupling effects. This was evident in the reduced threshold current density observed when both rings were operated together, compared to when a single ring was active. The elevated threshold current density for single-ring operation is attributed to additional losses from mode leakage. However, when both rings are active, the leaking modes are able to couple into the adjacent ring, preventing these losses and suggesting effective coupling through evanescent fields between the rings. Consequently, this coupling effect reduces the threshold current density during simultaneous operation of both rings.

Additionally, the study demonstrated synchronization in the form of injection locking between the two rings. When operated separately, the two identical rings exhibited identical emission spectra. By introducing an offset current to one of the rings, the spectral peaks shifted, showing detuning. However, when the rings were operated simultaneously (one with normal current and the other with an offset current), a single spectral peak was observed, demonstrating that the optical modes synchronized. In contrast, when the coupling gap was increased to 3 µm, the two individual spectral peaks persisted even when both rings were operated simultaneously, indicating that no mode locking occurred at this larger separation.

In addition to coupling between ring QCLs, coupling a ring QCL to a linear waveguide has emerged as an important concept, particularly in applications involving frequency combs and soliton generation. In such designs, the ring resonator does not employ a second-order DFB grating for surface emission. Instead, the mode of the ring is evanescently outcoupled or couples into a linear waveguide positioned in close proximity, which guides the light to its facet for emission. The absence of the grating prevents the formation of standing waves within the ring, enabling a traveling wave regime. However, it has been demonstrated that a small but finite amount of backscattering can help stabilize soliton solutions by acting as a phase reference [58]. This mode control is essential for soliton formation, paving the way for new approaches in nonlinear optics and advancing MIR spectroscopy through compact frequency comb sources [59], [60], [61], [62], [63], [64].

4.1 Grating-induced beam modifications

One of the most notable advantages of ring QCLs is their capability to produce a circularly symmetric and collimated emission beam. Due to the ring-shaped cavity, the farfield pattern resembles the two-dimensional equivalent of a double-slit diffraction pattern, as indicated in Figure 1(d). This results in concentric interference rings in the farfield. Similar to the double-slit experiment, the farfield characteristics of a ring QCL are governed by two main factors. First, the ring’s diameter, for a given wavelength, influences the distance between the concentric interference rings, analogous to the slit separation in the double-slit experiment. Second, the width of the waveguide, specifically the width of the emitting region, determines the envelope of the farfield intensity profile, akin to the slit width in a double-slit experiment.

However, unlike the conventional double-slit experiment, ring QCLs exhibit a unique polarization behavior due to their inherent TM polarization. When employing conventional radial grating slits, the resulting emission is tangentially polarized. Since the entire ring is coherent and in-phase, opposite sides of the ring have antiparallel polarization vectors. In the farfield, these opposing components interfere destructively, resulting in a central intensity minimum. This is analogous to a double-slit experiment in which one slit is covered by a plate with a thickness of λ/(2 (n−1)), where λ is the wavelength and n is the refractive index of the plate.

While a well-collimated emission beam is desirable, the presence of a central minimum in the intensity profile can be problematic for some applications. To address this, Schwarzer et al. demonstrated ring QCLs featuring a second-order DFB grating with two π phase shifts on opposite sides of the ring [39], as illustrated in the SEM image in Figure 7(a). This design rotates the polarization vector by 180° over half of the ring, leading to constructive interference at the center of the farfield pattern and creating a central intensity maximum, as observed in Figure 7(b). The resulting central lobe is linearly polarized, while sharp intensity minima appear at the phase shift locations due to localized destructive interference.

Figure 7:

Grating engineering and polarization control in ring QCLs. (a) SEM image of a ring QCL with two π phase shifts in the DFB grating. The inset shows a close-up image of the phase shift. (b) Farfield pattern of a ring QCL with two π phase shifts. The central intensity maximum is linearly polarized. (c) Farfield pattern of a ring QCL with an off-center grating (inset). The central intensity lobe contains all polarizations. (d) Sketch (top) and SEM image (bottom) of a ring QCL with a dual π phase shifted grating. (e) Results of the polarization-dependent spectral measurements of a ring QCL with a dual grating (top). The two distinct emission wavelengths (bottom, right) exhibit opposite polarization dependencies, which indicates a first-order and second-order WGM and explains the rotation of the farfield (bottom, left). (a–b) Are reprinted from Ref. [39], with permission from AIP Publishing; (c) is reprinted from Ref. [65], under the terms of the Open Access Publishing Agreement. ©2014 Optical Society of America; (d–e) are reprinted from Ref. [66], with permission (CC BY 4.0).

Babichev et al. explored an alternative method by introducing grating phase shifts using focused ion beam (FIB) milling [67], [68]. However, these gratings did not achieve single-mode operation at the desired grating mode. In a different approach, they utilized a partial distributed Bragg reflector (DBR) grating, covering only a section of the ring, to produce farfield patterns similar to those seen in straight surface-emitting DFB lasers [69], [70].

A different innovative strategy was demonstrated by Szedlak et al., who implemented an off-center grating design [65]. This approach introduces an effective chirp to the grating, creating a continuous π phase shift rather than an abrupt one. The continuous phase shift enables phase-matched wavefronts, resulting in constructive interference and a central intensity maximum. Unlike abrupt phase shifts, which create dark spots in the near- and farfield patterns, this approach yields a smooth intensity distribution around the ring waveguide, as can be seen in Figure 7(c). Additionally, while abrupt phase shifts produce a linearly polarized central lobe, off-center gratings generate a central lobe containing all polarizations.

Another notable grating modification involves rotating the grating slits away from the radial direction. By employing non-radial slits, the emitted polarization can be rotated by the same angle as the grating slits, effectively allowing the grating to function as a polarizer [65]. This versatility in tailoring the polarization characteristics provides ring QCLs with additional degrees of freedom for applications where polarization control is crucial.

Grating-induced beam modifications not only allow for improving the farfield characteristics of ring QCLs, such as achieving a central intensity maximum through π phase shifts or off-center gratings, but also offer a valuable tool for examining the internal mode properties of these devices. As mentioned earlier, ring QCLs support WGMs, which are characterized by their confinement near the outer wall of the waveguide. The degree of this outward shift is determined by the curvature of the ring, with more pronounced shifts occurring in rings with smaller radii. Comprehensive investigations of WGM profiles have been previously conducted using nearfield probes [71], photon scanning tunneling microscopy [72], and thermocouple probes [73].

For the first time, Szedlak et al. introduced a novel technique to map WGMs in ring QCLs using farfield emission characteristics [66]. This technique employs a dual DFB grating, consisting of two parallel gratings that are phase shifted by π relative to each other. Along the circumference of the ring, the ratio between these two gratings is varied continuously to induce a gradual phase shift. A sketch as well as an SEM image of this dual grating are depicted in Figure 7(d). Since the two gratings are π phase shifted, light emitted from these gratings experiences destructive interference, and the location of maximum destructive interference depends on the spatial overlap between the WGMs and the grating structures.

In a hypothetical scenario, where the WGM would be symmetrically centered within the waveguide, the point of maximum destructive interference would coincide with the segment of the ring where the gratings are balanced with a 50:50 ratio. However, due to the outward displacement of the WGM, this position shifts, causing a corresponding rotation of the near- and farfield intensity patterns. By measuring this rotation, the center of mass (COM) of the WGM can be determined with precision.

The study predominantly found an outward shift of the COM, which confirmed that most ring QCLs exhibit first-order WGMs, validating the theoretical expectations. However, an unexpected observation was made for certain devices: a farfield rotation in the opposite direction, indicating an inward shift of the mode’s COM and suggesting a higher order mode. Spectral analysis of these devices revealed the presence of two distinct spectral peaks, indicating that the laser was supporting multiple modes. A more in-depth polarization-dependent study of the farfield rotation identified these modes as first-order and second-order WGMs, as outlined in Figure 7(e). This highlighted the effectiveness of this mapping technique in discerning complex modal behaviors within the ring QCL cavity, offering insights that would be challenging to obtain using conventional methods. The ability to precisely characterize and control WGM profiles in ring QCLs is not only valuable for understanding device physics but also critical for optimizing laser performance and improving beam quality.

4.2 On-chip optical elements

One of the significant advantages of ring QCLs lies in their vertical emission capability, where light is radiated both upwards into the air and downwards through the substrate. This dual-directional emission, coupled with a large emission area, naturally produces collimated beam profiles. Notably, the downward emission through the substrate allows for on-chip modifications of the emission beam, opening up new possibilities in beam control and manipulation.

In a pioneering approach, Schwarzer et al. integrated a π shift ring QCL with a gold wire grid deposited on the back side of the substrate after the ring QCL was fabricated [39]. The wire grid functioned as an on-chip polarizer, delivering polarization properties comparable to external polarizers. This on-chip polarizer not only extended the polarization characteristics of the central lobe to the entire farfield but also demonstrated the feasibility of substrate-based modifications in ring QCLs.

Expanding on this concept, Szedlak et al. introduced an innovative metasubstrate, an on-chip lens for beam collimation, fabricated directly on the back side of the ring QCL’s substrate [74]. This metasubstrate, as outlined in Figure 8(a), operates as a gradient index element, designed to phase-match rays emitted at different angles from the ring laser by introducing distinct optical path lengths. The gradient in the refractive index is achieved by etching sub-wavelength holes with varying diameters into the back side of the substrate, a process carefully controlled through aspect-ratio-dependent etching. The total etch depth poses a limitation on the overall size of the metasubstrate, which ultimately influences the lens area and performance.

Figure 8:

Integrated photonic elements for beam shaping in ring QCLs. (a) SEM images of the metasubstrate. The yellow stripe indicates the location of the ring QCL on the other side of the semiconductor chip. (b) Farfield pattern with and without the metasubstrate. The top panel shows the farfield patterns of the substrate-emitted beam, scaled to the peak intensity prior to the fabrication of the metasubstrate. This demonstrates the more than six-fold peak intensity increase. The bottom left panel shows the surface-emitted beam without any notable difference, indicating that the overall performance of the laser did not change. The bottom right panel shows the data from the top panel but both farfields are now scaled to unity, which indicates that the metasubstrate collects intensity from larger emission angles and shifts it towards the beam center. (c) Sketch of an OAM metasubstrate. (d) SEM images of an OAM metasubstrate with topological charge m = −1. (e) Comparison of the farfields before and after fabrication of the OAM metasubstrate. While the surface-emitted farfield does not show a notable change, the substrate-emitted beam switches from central minimum to maximum, indicating a beam carrying OAM. (a–b) Are reprinted from Ref. [74], with permission from AIP Publishing; (c–e) are reprinted from Ref. [75], with permission (CC BY 4.0).

The effectiveness of this metasubstrate design is evident from the farfield patterns. When comparing the farfield before and after the implementation of the metasubstrate on the same ring QCL, a more than six-fold increase in the central intensity maximum was observed, without affecting the total output power. The corresponding farfield patterns are provided in Figure 8(b). This substantial increase confirms the metasubstrate’s function of redirecting intensity from larger emission angles towards the central lobe. The combination of a ring QCL with grating π shifts and a metasubstrate of equivalent size could theoretically yield a seven-fold enhancement in the central intensity maximum [76].

Beyond simple beam collimation, this gradient index approach opens up avenues for diverse beam-shaping applications. For example, Figueiredo et al. proposed an on-chip solution based on an azimuthal refractive index gradient, fabricated through grayscale lithography, aimed at generating a beam profile with a central intensity maximum [77]. In a different realization, Szedlak et al. employed sub-wavelength holes to create an azimuthal refractive index gradient, which introduces a phase shift along the azimuthal direction, twisting the emitted wavefront and producing orbital angular momentum (OAM) beams [75]. These OAM metamaterials are shown in Figure 8(c) and (d). They influence the farfield pattern significantly and can be used to demonstrate various topological charges.

For instance, a ring QCL with an OAM metasubstrate possessing a topological charge of 1 was fabricated. The corresponding farfields are displayed in Figure 8(e). The left panels show the emission in the surface direction before (top) and after (bottom) fabrication of the metasubstrate. As expected, these farfields remain largely unchanged, both exhibiting a central intensity minimum. The right panels depict emission through the substrate, again before (top) and after (bottom) metasubstrate fabrication. While the unstructured device emits identical beam profiles in both directions, the metasubstrate significantly modifies the substrate-emitted beam by converting the central intensity minimum into a pronounced maximum. This transformation indicates the generation of an OAM beam with a topological charge of 1. In this configuration, a total phase shift of 2π is distributed around the entire circumference of the ring, resulting in opposite sides of the ring exhibiting a π phase shift relative to each other. This structure produces a central intensity maximum and maintains rotational symmetry in the farfield, confirmed for both clockwise and counter-clockwise OAM beams. Additionally, a ring QCL with a topological charge of 2 was also demonstrated, showing a characteristic central intensity minimum, consistent with theoretical predictions based on independent dipole models [78] and vectorial ray-based diffraction integrals [79].

This research underscores the extraordinary flexibility of surface-emitting ring QCLs, where precise modifications of the emission beam can be achieved through the substrate. Such beam engineering would be considerably more challenging in traditional facet-emitting lasers, highlighting the unique advantages of the ring QCL architecture for advanced photonic applications.

5.1 Modulation characteristics and dispersion spectroscopy

Modulation characteristics play a critical role in the application of ring QCLs for advanced spectroscopic techniques beyond conventional setups that rely on the Beer–Lambert law. Such techniques include wavelength- and frequency-modulation spectroscopy [84], [85], intrapulse absorption spectroscopy [86], chirped laser dispersion spectroscopy [87], and heterodyne phase-sensitive dispersion spectroscopy (HPSDS) [88]. These methods capitalize on the detailed understanding of the laser’s tuning behavior in response to modulation. In HPSDS, the laser current is modulated at radio-frequency (RF) rates to generate sidebands, and the technique measures the refractive index spectrum by probing the phase of the RF-modulated light, making it immune to power fluctuations and reducing the need for calibration [89], [90].

Szedlak et al. were the first to implement HPSDS using ring QCLs, providing valuable insights into the modulation dynamics of these devices [91]. Figure 10(a) depicts the experimental setup. This study explored the relationship between the tuning parameters of the laser and a well-known absorption profile of CH₄ in N₂. Key characteristics such as the FM/IM ratio (the ratio of frequency modulation to intensity modulation) and the FM/IM phase shift were measured across a modulation frequency range from 1 to 150 MHz. The results demonstrated a relatively flat behavior from 20 to 150 MHz, with a marked increase in the FM/IM ratio below 20 MHz. This low-frequency behavior was attributed to the onset of thermal tuning, which serves as an important figure of merit for the laser’s response.

Figure 10:

Modulation techniques and phase-sensitive dispersion spectroscopy with ring QCLs. (a) Sketch of the HPSDS setup. (b) The two identified operating regimes facilitate HPSDS sensing with two different dynamic ranges. (c) Comparison of the FM/IM ratio between ring QCLs and straight DFB QCLs. The increase below 20 MHz indicates the onset of thermal tuning. (d) Comparison of the FM/IM phase shift between ring QCLs and straight DFB QCLs. (a) Is reprinted from Ref. [92], with permission (CC BY 4.0); (b) is reprinted from Ref. [91], with permission (CC BY 4.0); (c–d) are reprinted from Ref. [92], with permission (CC BY 4.0).

The characterization of the tuning properties laid the groundwork for subsequent HPSDS experiments. The study identified two distinct operating regimes, each corresponding to different bias current setpoints, as outlined in Figure 10(b). A low bias current exhibited a small FM/IM ratio, which resulted in an expanded dynamic range, a desirable attribute for HPSDS applications. Conversely, at higher bias currents, the FM/IM ratio and the phase shift increased, leading to a higher signal-to-noise ratio but at the expense of linearity. This dual-mode operation facilitated the optimization of the system, achieving detection limits of 16 ppm and 2 ppm in the two regimes, respectively.

Building on this work, Hinkov et al. conducted a comparative analysis between straight DFB QCLs and ring QCLs [92]. The findings revealed similar trends in the FM/IM ratio and phase behavior between the two device architectures, as shown in Figure 10(c) and (d), respectively. However, in the case of ring QCLs, a quasi-single-sideband modulation regime was observed at high modulation frequencies (100 MHz) and low bias currents. This discovery highlights the potential of ring QCLs for compact and versatile MIR gas sensing applications that leverage efficient modulation techniques.

The understanding of these RF modulation characteristics opens new avenues for the deployment of ring QCLs in advanced spectroscopic setups. The ability to precisely control and exploit the laser’s tuning dynamics enhances the sensitivity and selectivity of detection methods, thereby expanding the applicability of ring QCLs in fields requiring high-precision gas sensing.

5.2 Single-chip spectroscopy

For numerous applications, including environmental monitoring, industrial process control, and medical diagnostics, compact sensors in the MIR range are highly desirable. However, achieving this compactness while maintaining functionality has posed challenges, especially given that most miniaturization strategies in MIR sensing rely on discrete optical components. Conventional sensing systems generally consist of a separate light source, an interaction region where the light interacts with the analyte, and a detector to analyze the light after it has interacted with the substance. The drive towards on-chip solutions has motivated efforts to integrate these components within a single device, reducing complexity and increasing stability and portability.

In the realm of quantum cascade technology, integrating a laser and detector on the same chip has long been hindered by the inherent blue-shift of the QCL in detector operation. This challenge was addressed by Schwarz et al. with the introduction of bi-functional quantum cascade lasers/detectors (QCLDs) [93]. These devices leverage the same quantum cascade heterostructure for lasing and detection at the same frequency, thus enabling the fabrication of both components on a single chip. In these bi-functional QCLDs, the active regions for lasing and detecting share the same material and structure, simplifying the fabrication process and reducing cost. On-chip sensors using facet-emitting QCLDs have been successfully demonstrated, with the emitted light from the laser coupled into the adjacent detector via the facet.

One notable implementation of such devices was realized in liquid sensing, using a surface plasmon polariton (SPP) waveguide between the laser and detector [94]. The SPP waveguide confines the light and enables sufficient interaction with the liquid environment for sensing. However, due to their relatively short propagation lengths, SPP-based sensors are effective for liquid sensing but are not well-suited for gas sensing, which requires longer interaction paths.

In response to this challenge, attention was directed towards surface-emitting devices, such as ring QCLs, to provide a viable solution. Harrer et al. demonstrated the first bi-functional surface-emitting and -detecting device based on quantum cascade heterostructures [95]. As represented in Figure 11(a), this innovative sensor design consists of a surface-emitting ring QCL and a disk-shaped quantum cascade detector (QCD) positioned at the center. While the ring QCL achieves surface emission through a second-order DFB grating, surface detection in the disk QCD employs a squared metal hole grating to couple incident light to the detector element. The sensor was implemented in a setup where the emitted light from the ring QCL was directed through a gas cell and back-reflected by a flat gold mirror. The back-reflected light passed through the gas cell again and was detected by the disk QCD on the same chip. Proof-of-concept gas sensing experiments were performed at cryogenic temperatures.

Figure 11:

Monolithic ring laser and detector integration for gas sensing. (a) SEM image of the first surface-emitting and -detecting quantum cascade device. (b) SEM image of the commutable concentric ring QCLDs. (c) Sketch of the remote sensing setup. The emitted light from one ring is guided through the gas cell before it is back-reflected by a flat gold mirror and detected on the same chip by the other ring. (d) Laser and detector spectra of both rings with an optimal overlap around 1,500 cm⁻¹. (e) LI characteristics of both rings recorded with an external (dashed) and on-chip (solid) detection scheme. The threshold currents of 0.48 A for the inner ring and 0.68 A for the outer ring correspond to threshold current densities of 4.8 kA/cm² and 5.6 kA/cm², respectively. (f) Proof-of-principle gas sensing measurement results of isobutene. (a–b) Are reprinted from Ref. [91], with permission (CC BY 4.0); (c–f) are reprinted from Ref. [56], with permission (CC BY 4.0).

However, the performance of this sensor was constrained by its inherent geometry. The ring QCL generates a ring-shaped farfield pattern, which, after passing through the optical system twice, is mapped onto a ring-shaped intensity distribution on the sensor chip. Hence, much of the intensity is missing the disk detector leading to suboptimal detection [91].

To address this limitation, Szedlak et al. developed a solution that employed two concentric ring QCLDs with slightly different diameters instead of one ring laser and a disk detector [56]. This improved design is depicted in Figure 11(b). The ring-shaped beam pattern is now directed onto a ring-shaped detector, offering a more efficient coupling mechanism and thus yielding superior sensor performance compared to the previous disk detector configuration. A sketch of the experimental sensing setup is outlined in Figure 11(c).

This work marked the first demonstration of a ring QCL functioning as a detector. In these detector devices, the DFB grating is used to couple surface-incident light into the active region. Both concentric ring QCLDs are capable of lasing and detecting, and their dual functionality was demonstrated by interchanging the roles of the outer and inner rings as laser and detector. This interchangeability is demonstrated by the corresponding spectra and LIVs depicted in Figure 11(d) and (e), respectively. The study further validated the performance of these on-chip detection schemes by comparing them with external detection setups, showing excellent agreement.

Moreover, the authors implemented two distinct DFB grating periods to achieve different emission wavelengths from the two rings without affecting the detector spectra. Using the same setup, where one ring emitted light through a lens and gas cell, which was then back-reflected and passed back through the gas cell and lens onto the second ring, the authors successfully differentiated between two gaseous analytes based on their absorption characteristics. By leveraging these dual wavelengths, the researchers achieved selective gas detection with a limit of detection of 400 ppm at room temperature, without requiring temperature stabilization and an absorption length of 20 cm. The corresponding gas sensing measurment of isobutene is outlined in Figure 11(f).

This successful implementation showcases the potential of ring QCLDs for remote sensing applications in compact, monolithic on-chip designs. The ability to integrate surface emission and detection capabilities, combined with the flexibility to achieve dual-wavelength operation, highlights the versatility of these devices for advanced MIR sensing technologies.