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Abstract
On-chip sensors based on quantum cascade laser technology are attracting broad attention because of their extreme compactness and abundant absorption fingerprints in the mid-infrared wavelength range. Recent continuous wave operation microcavity quantum cascade lasers are well suited for high-density optoelectronic integration because their volumes are small and thresholds are low. In this experimental work, we demonstrate a monolithically integrated sensor comprising a notched elliptical resonator as transmitter, a quantum cascade detector as receiver, and a surface plasmon structure as light-sensing waveguide. The sensor structure is designed to exploit the highly unidirectional lasing properties of the notched elliptical resonator to increase the optical absorption path length. Combined with the evanescent nature of the dielectric loaded surface plasmon polariton waveguides, the structure also ensures a strong light–matter interactions. The sensing transmission distance obtained is approximately 1.16 mm, which is about one order of magnitude improvement over the traditional Fabry–Perot waveguide. This sensor opens new opportunities for long-range and high-sensitivity on-chip gas sensing and spectroscopy.
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1. Introduction
Mid-infrared spectroscopy is a subject of active research in chemical sensing, atmospheric sciences, medical diagnosis, and industrial monitoring [1–3]. Just like human fingerprints, the pattern of absorbance peaks in the “fingerprint region” is a unique characteristic of a molecule that arises from its rotational and vibrational resonances. The pattern means that by comparing with known standards mid-infrared spectral data from an unknown sample can be identified and quantified [4]. Standard mid-infrared spectrometers such as the Fourier-transform-based infrared spectrometers commonly consist of a broadband thermal emitter and broadband detector, a multi-pass gas cell for light–matter interaction, and external optics. However, composed of discrete components and modules, these sensor systems are usually large in size and weight, and are susceptible to environmental vibrations, making them suitable only for laboratory environments [5,6]. As an alternative to these desktop systems, monolithic integration of optoelectronic devices is in general compact, robust, energy-efficient optical systems ripe for use in miniaturized sensors [7]. The fabrication principle is simple: integrate the necessary discrete optical components and functionality within a single chip using the same materials and similar device structures.
Quantum cascade lasers (QCLs) have been demonstrated to be the leading electrically pumped semiconductor laser sources in the mid-infrared range [8–12], with small size, high spectral density, and continuous frequency tuning, and the microcavity QCLs in particular are capable of room temperature continuous wave operations [13,14]. They are used in many infrared spectroscopy techniques such as direct absorption spectroscopy [15], quartz-enhanced photoacoustic spectroscopy [16], dual-comb spectroscopy [17], and coupled cavities sensing [18], all proving that QCLs are ideal candidates for spectroscopic applications. Moreover, the demonstration of the photodetection capability of the QCL structure has initiated in-depth research into quantum cascade detectors (QCDs) [19]. QCDs are stable zero-bias devices [20] and have higher operating temperatures than quantum-well infrared photodetectors [21], making them perfectly suited for portable sensing applications. Because the optical transition takes place between the extraction states and the upper laser level, the wavelength emission of the biased heterostructure is usually red-shifted relative to the absorption wavelength when unbiased; the bifunctional quantum cascade heterostructures are designed to compensate for this wavelength shift [19]. Recently, studies have shown that the emission spectrum can well match the peak optical response [22], with lasing performances comparable to the traditional QCLs [23,24]. A key component in the miniaturization of monolithic integrated sensors is a sensing waveguide capable of providing strong interaction of light–matter while maintaining a long interaction waveguide length. In the absence of any guiding structure, the coupling efficiency drops rapidly with the laser–detector distance, which is typically no more than 30 µm [6]. Dielectric-loaded surface plasmon polariton (DLSPP) waveguides [5,6,25] are a very efficient solution that permits an increase in this distance of up to 100 µm. Furthermore, because of their evanescent nature, more than 90% of the modes are kept outside, thus ensuring a strong light–matter interaction. This monolithically integrated sensor is capable of detecting absorptions in liquid solutions; however, its resolution is mainly limited by the micrometer-scale interaction waveguide length [5,6,26,27]. The use of slow light in photonic crystal waveguides [26] or subwavelength grating waveguides [27] can increase the optical absorption path length; nevertheless, these waveguides have large propagation losses [26–28], and photonic crystal waveguides must be fabricated through high-cost electron beam lithography.
In this experimental work, we demonstrate a monolithically integrated edge-emission sensor that is based on a bifunctional quantum cascade structure in combination with a notched elliptical resonator, with a sensing distance of up to 1.16 mm. The sensor structure is designed to exploit the high-unidirectional lasing properties of the notched elliptical resonator in increasing the optical absorption path length and when combined with the evanescent nature of the DLSPP waveguide in ensuring a strong light–matter interaction. The mutual manufacturing of all the functional components of the on-chip sensor is fully compatible, and therefore the sensor is mass producible and cost effective comparable to normal diodes. We demonstrate the sensing capability of this type of device and characterize the absorption spectra of solid phase benzoic acid at liquid nitrogen temperatures.
2. Design and fabrication
Surface plasmon polariton waveguides at mid-infrared wavelengths experience weak confinement, in which the evanescent decay penetrates deep into the dielectric medium, resulting in reduced propagation lengths. DLSPPs have been studied in both theory and experiments at mid-infrared wavelengths without subwavelength patterning [5,6]. For example, a silicon nitride layer of a few hundred nanometers thick is sufficient to squeeze the mode by one order of magnitude. For narrow metal strips with high lateral confinement, waveguide losses caused by metal edges are severe. Eliminating these metal edges and achieving lateral confinement through a dielectric is a feasible solution. However, as the waveguide width increases, the influence of the waveguide edge decreases rapidly. In general, the waveguide is intentionally designed wider than the laser facet, increasing the coupling efficiency [6]. The recently reported room-temperature continuous-wave-operated microcavity quantum cascade lasers have horizontal dimensions over one hundred micrometers, so the effect of metal edges is negligible.
In our experiments, we used a notched elliptical resonator with unidirectional lasing properties as transmitter, as shown in Fig. 1. Light circulating around the circumference of the cavity is scattered by the notch toward the opposite boundary, and then most of the scattered light is collected from the boundary opposite to the notch to form a collimated beam. In forming the DLSPP waveguide, aluminum oxide was used as a dielectric load together with a metal layer of slightly larger horizontal dimensions. We simulated the modal distribution of an aluminum oxide-based DLSPP waveguides using a finite element solver. For gold and aluminum oxide, we used refractive index values extracted from the literature of n = 5.6919–47.43i [29] and n = 1.3924–0.0166i [30] at λ = 7 µm respectively. The specific parameters of each part of the bifunctional device were calculated using the Drude model. Figure 2(a) shows that a thicker dielectric layer leads to a better vertical confinement. The confinement factor represents the fraction of the square integral of the modal intensity that is in the DLSPP waveguide and that cannot interact with the matter, while the integral in the denominator is spatially unrestricted. Higher coupling efficiency can be achieved by choosing an appropriate dielectric layer thickness. Figure 2(b) shows the simulated 2d mode in a cross section of the DLSPP waveguide with a 200-nm-thick and 240-µm-wide aluminum oxide stripe on the top of a 250-µm-wide gold stripe. The confinement factor represents the fraction of the integral of the modal intensity that is in the air and that can interact with the matter exceeds 90%, while the integral in the denominator is spatially unrestricted. The mode intensity at the edge of the waveguide is evidently extremely weak because the aspect ratio of the dielectric loading is very high, signifying that the influence of the waveguide edge is almost negligible. Figure 2(c) shows the effective refractive index of the mode as a function of the aluminum oxide stripe width for different aluminum oxide layer thickness. The increase in aluminum oxide size enhances mode confinement. However, an increase in aluminum oxide thickness is always accompanied by a decay in energy propagation length through losses within the dielectric loading and ohmic loss because of the higher field intensity at the metal interface [see Fig. 2(d)].
Fig. 1. Schematic of a monolithically integrated mid-infrared sensor based on a notched elliptical resonator. The upper inset shows the schematic illustration of the notched elliptical resonator.
Fig. 2. (a) Vertical modal intensity profile of a DLSPP for different aluminum oxide thickness at distances 50, 100, 150, 200, and 250 nm, and 1D mode intensity profile of the QCL/QCD (the inset shows the refractive index profile along the growth axis and corresponding 1D waveguide fundamental mode); (b) 2d simulation of the mode over a cross section of the DLSPP waveguide comprising a 200-nm-thick and 240-nm-wide aluminum oxide stripe on top of a 250-nm-wide gold stripe. The corresponding energy propagation length is 1.02 mm; (c) effective refractive index of the mode as a function of aluminum oxide stripe width for different aluminum oxide layer thickness; and (d) the corresponding energy propagation length.
The bifunctional quantum cascade heterostructures are grown using solid-source molecular beam epitaxy on a SI-InP substrate. The entire structure includes a 0.2-µm-thick In0.53Ga0.47As bottom contact layer (Si, 1 × 1018 cm–3), a 3.5-µm-thick InP lower cladding layer (Si, 4 × 1016 cm–3), a 2.1-µm-thick active region, a 0.3-µm-thick In0.53Ga0.47As upper confinement layer (Si, 4 × 1016 cm–3), a 3-µm-thick InP upper cladding layer (Si, 4 × 1016 cm–3), a 0.15-µm-thick InP grading layer (Si, 2 × 1017 cm–3), and a 0.85-µm-thick InP contact layer (Si, 3 × 1018 cm–3). The active region was doped to an average level of 2.6 × 1016 cm–3 and consisted of 50 InGaAs/InAlAs QCL stages designed for an emission wavelength of 7 µm. Starting from the injection barrier, the layer sequence of each period is (in Angstroms): 39/17/9/52/9/48/10/40/18/33/17/29/19/28/27/27; the In0.52Al0.48As barrier layers are in boldface, the In0.53Ga0.47As quantum well layers are in regular type, and doped layers are underlined.
Monolithic integrated bifunctional sensors with laser–detector distances of 100 µm, 500 µm, and 1000 µm were fabricated. Photolithography was used to define the contour of the electrical isolation region, and the contact layer, and dry etching was used to remove the contact layer and the grading layer in this region. A 1-µm-thick silicon dioxide hard mask was grown via plasma-enhanced chemical vapor deposition to transfer the pattern. Both the laser and detector structures were etched using an inductively coupled plasma to form steep and smooth sidewalls. Note that the etch depth goes beyond the active region but not the lower contact layer. Next, the remaining InP lower cladding layer and the InGaAs contact layer are respectively removed using a selective etching solution, thereby achieving complete electrical isolation between the laser and detector. A 0.45-µm-thick silicon dioxide layer was grown to passivate surface defects in the sidewall region of the cavity, while further isolating the laser and detector, facilitating subsequent DLSPP waveguides. Afterwards, layers of titanium and gold, 10- and 400-nm-thick, respectively, were sputtered to form the SPP waveguides, and a 200-nm-thick layer of aluminum oxide was deposited to form the DLSPP waveguides; the gaps between the DLSPP waveguide and the layer/detector facet were set to 3 µm. The end of the DLSPP waveguide near the layer was intentionally designed as an arc to fit closely with the laser, thereby improving the coupling efficiency. The silicon dioxide layer on the top of the cavity was removed by wet etching, and Ti/Au ohmic contacts were deposited via electron beam evaporation. We used adjacent contacts in the trenches beside the cavities; ohmic contacts were formed by electron beam evaporation of Ge/Au/Ni/Au. A top view of a representative device was obtained in 3D microscopy (Fig. 3); here, the laser–detector distance is 500 µm. All of the notched elliptical microcavity QCLs have the same dimensions of X = 100 µm and Y/X = 1.2, and the same notch size with an opening of 2.5 µm and depth of 1.6 µm. All QCDs are 250-µm wide and 200-µm long.
Fig. 3. A 3D microscopy image of the monolithically integrated sensor. The laser–detector distance is 500 µm. The laser has dimensions of X = 100 µm and Y/X = 1.2, and the notch size with an opening of 2.5 µm and depth of 1.6 µm. The inset shows a detail of the device. Both the cavity boundary and the boundary of the DLSPP waveguide are marked with white dashed lines. The gap between the DLSPP waveguide and the laser/detector is consistent. The detector is 250-µm wide and 200-µm long.
3. Results and discussion
Experimental characterizations of monolithic integrated sensors that had been mounted with the epilayer side up on a diamond substrate with indium solder, were acquired under CW conditions at liquid nitrogen temperature. Power measurements were acquired with a pyroelectric power meter after the beam was collated using a lens. The measured CW power-current-voltage characteristics [ Fig. 4(a)] show a maximum optical power exceeding 8 mW and a threshold current below 180 mA. The 45° polished edge-coupled detector spectrum was compared with the laser emission spectrum [Fig. 4(b)], both of which were obtained using a Fourier-transform infrared spectrometer with a resolution of 0.125 cm–1 for the laser and 16 cm–1 for the detector in rapid-scan mode. Single-mode emissions were obtained over the entire dynamic range, with a side-mode suppression ratio more than 30 dB. The 45° polished edge-coupled detector was calibrated using a collimated and expanded single-mode laser, with a peak responsivity of 8.79 µA/W. There is a shift in the laser emission wavelength, corresponding to approximately 42.5% of the peak responsivity, which means that the signal strength exceeds the noise by about three orders of magnitude. This is sufficient for detection in this experiment.
Fig. 4. (a) Power–current–voltage characteristics of a notched elliptical microcavity QCL at liquid nitrogen temperature; (b) spectral responsivity of the 45° polished edge-coupled detector and the normalized laser emission spectrum.
The measurement architecture that we used in our experiments (Fig. 5) involves a laser being driven by a CW power supply modulated by a function generator, with a modulation frequency of 10 Hz. The DLSPP waveguide guides the emitted light signal to the detector while establishing light–matter interaction. The detector converts the received optical signal into an electrical signal, which is then amplified by a current pre-amplifier at an amplification factor of 2 µA/V. Finally, the voltage signal is read through a lock-in amplifier.
We tested and characterized the monolithically integrated sensors at various laser–detector distances at a drive current of 240 mA from the CW power supply and a function generator modulation depth of 100 mA. The responsivity of the monolithically integrated sensors as a function of laser–detector distance (Fig. 6) exhibits an exponential drop with increasing distance. We performed an exponential fit to the measured results, which gave a decay length in responsivity of approximately 1.16 mm; that is, the DLSPP propagation length is approximately 1.16 mm, which agrees well with the simulation results. Compared with the reported evanescent field-based monolithically integrated sensor, our microcavity resonator-based sensor exhibits an increase in energy propagation length by about one order of magnitude, and we attribute this improvement mainly to the highly unidirectional horizontal far-field profiles and DLSPP waveguide widths of hundreds of microns, which is critical for absorption spectroscopy sensing based on the Beer–Lambert law.
Fig. 6. Responsivity for laser–detector distances of 100, 500, and 1000 µm. The blue line is an exponential fit to the data.
To verify the effect in a practical application, this monolithically integrated sensor was employed in detecting trace amounts of absorbed benzoic acid on the DLSPP waveguide at liquid nitrogen temperature. The application is mainly limited by the working temperature of the detector. The laser spectrum [ Fig. 7(a)] coincides with the absorption peak of benzoic acid. To obtain an even distribution of residues at different concentrations, benzoic acid samples were dissolved in an ethanol solution and then dropped onto the DLSPP waveguide using a pipette. The device was soaked in ethanol to remove residues of the previous analyte. To reduce the effect of unclean removal of residues from analytes, solutions were prepared in order from low to high concentrations. When dropped at a height of 10 cm, a single drop contains approximately 0.0125 ml, and covers approximately 0.64 cm−2. Therefore, the residual benzoic acid concentration corresponding to 1 mg/ml benzoic acid solution, is approximately 19.53 µg/cm−2. Guided by the DLSPP waveguide, the laser beam was passed through the benzoic acid sample, resulting in absorption. The intensity of the detector signal reflects information about the residue concentration. The detector signal intensity as a function of benzoic acid concentration [Fig. 7(b)] confirms the sensing capability of our monolithically integrated sensor.
Fig. 7. (a) Normalized laser emission spectrum and absorption spectra of benzoic acid, measured by an external cavity spectrometer; (b) detection signal intensity at benzoic acid concentrations of 0.5, 1, 2, 5, and 10 mg/ml.
4. Conclusions
The high-unidirectional lasering properties of notched elliptical resonators were exploited to increase the light absorption path length and combined with the evanescent nature of DLSPP waveguides to ensure a strong light–matter interaction. We demonstrated in experiments that the DLSPP propagation length is more than 1 mm and the propagation length can be increased by choosing the right material. Combined with the narrow linewidth characteristics of the microcavity laser, a significantly lower detection limit will be achieved. The microcavity laser exhibits a single mode operation without using grating structure, which facilitate the sensor fabrication and further reduces the cost. Also, the device possesses a much larger emitting aperture than normal FP device, this will increase the light-matter interaction area and boost the sensitivity of the sensor given the same power performance.
Funding
National Key Research and Development Program of China (2021YFB3201901); National Natural Science Foundation of China (61790583, 61991430, 62174158); Youth Innovation Promotion Association of the Chinese Academy of Sciences (2021107).
Acknowledgments
The authors would like to thank Ping Liang for her help in device processing.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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