sensors

Article Interband Cascade Photonic Integrated Circuits on Native III-V Chip

Jerry R. Meyer 1,*, Chul Soo Kim 1, Mijin Kim 2, Chadwick L. Canedy 1, Charles D. Merritt 1, William W. Bewley 1 and Igor Vurgaftman 1

1 Naval Research Laboratory, Code 5613, Washington, DC 20375, USA; [email protected] (C.S.K.); [email protected] (C.L.C.); [email protected] (C.D.M.); [email protected] (W.W.B.); [email protected] (I.V.) 2 Jacobs Corporation, Hanover, MD 21076, USA; [email protected] * Correspondence: [email protected]

Abstract: We describe how a midwave infrared photonic integrated circuit (PIC) that combines lasers, detectors, passive waveguides, and other optical elements may be constructed on the native GaSb substrate of an interband cascade laser (ICL) structure. The active and passive building blocks may be used, for example, to fabricate an on-chip chemical detection system with a passive sensing waveguide that evanescently couples to an ambient sample gas. A variety of highly compact architectures are described, some of which incorporate both the sensing waveguide and detector into a laser cavity defined by two high-reflectivity cleaved facets. We also describe an edge-emitting laser configuration that optimizes stability by minimizing parasitic feedback from external optical elements, and which can potentially operate with lower drive power than any mid-IR laser now available. While ICL-based PICs processed on GaSb serve to illustrate the various configurations, many of the proposed concepts apply equally to quantum-cascade-laser (QCL)-based PICs processed on InP, and PICs that integrate III-V lasers and detectors on silicon. With mature processing, it should become



 possible to mass-produce hundreds of individual PICs on the same chip which, when singulated, will realize chemical sensing by an extremely compact and inexpensive package. Citation: Meyer, J.R.; Kim, C.S.; Kim, M.; Canedy, C.L.; Merritt, C.D.; Keywords: interband cascade laser; photonic integrated circuit; chemical sensing; midwave infrared Bewley, W.W.; Vurgaftman, I. Interband Cascade Photonic Integrated Circuits on Native III-V Chip. Sensors 2021, 21, 599. 1. Introduction https://doi.org/10.3390/s21020599 Recent years have seen an accelerating development of optical technologies operat- Received: 4 December 2020 ing in the midwave infrared (mid-IR). This has been driven partly by the opportunities Accepted: 14 January 2021 for sensitive spectroscopic detection of trace chemicals [1], since in gas form the narrow Published: 16 January 2021 mid-IR absorption lines that can positively identify a given chemical are 2–3 orders of magnitude stronger than those in the near IR. Applications include monitoring green- Publisher’s Note: MDPI stays neu- house gases [1], industrial process control [2], combustion diagnostics [3], clinical breath tral with regard to jurisdictional clai- analysis [4], and isotope differentiation [5]. However, the long-term market for mid-IR ms in published maps and institutio- chemical sensing products will depend in large part on such practical factors as cost, power nal affiliations. budget, and system footprint. While other sensing techniques, such as gas chromatography and monitoring variations in the properties of metal oxide semiconductors or polymers, may also be employed [6], laser spectroscopy combines the advantages of selectivity, sensitivity, stability, longevity, and range in the case of remote sensing [5]. Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. It follows that rather than using bulky and costly discrete optical elements interacting This article is an open access article across free space, the ideal spectroscopic sensing system will occupy a single semicon- distributed under the terms and con- ductor chip that is inexpensive to produce in mass quantities [5]. This will require the ditions of the Creative Commons At- incorporation of at least one mid-IR source, optical sensing path, and detector, along with tribution (CC BY) license (https:// interconnections linking them, into a single photonic integrated circuit (PIC). However, creativecommons.org/licenses/by/ the progress so far toward this objective has been incremental. While mid-IR lasers and 4.0/). detectors have been combined [7–9], and sensing waveguides have been integrated with

Sensors 2021, 21, 599. https://doi.org/10.3390/s21020599 https://www.mdpi.com/journal/sensors Sensors 2021, 21, 599 2 of 19

detectors [10,11], we are aware of only one report to date of all three fundamental compo- nents being integrated on the same mid-IR PIC. This was a QCL and QCD integrated on the same chip with a suspended membrane waveguide patterned with subwavelength slots, which was designed for sensing that had not yet been demonstrated [12]. Although still immature, more complete sensing PICs operating in the near IR have been reported [13]. In this work, we lay out a comprehensive and practical framework for combining all the optical elements required to integrate a high-performance mid-IR chemical sensor on a PIC [14]. The platform is an interband cascade laser (ICL) structure [15–18] residing on its native GaSb substrate. The ICL combines a relatively long upper-level lifetime, characteristic of semiconductor interband transitions, with the voltage-efficient cascading scheme originally introduced for the quantum cascade laser (QCL, which employs inter- subband transitions to produce light) [19]. Both electrons and holes are present in each stage of the ICL’s cascaded active region, even though the contacts inject and remove only electrons. The ICL has become the leading coherent optical source for many mid-IR applications between 3 and 6 µm [18]. It operates with drive power as low as 29 mW [20] and wallplug efficiency as high as 18% [21] for continuous wave (cw) operation at room temperature. However, it is less mature than the QCL, and a wealth of complex physics responsible for various aspects of the ICL operation have yet to be fully unraveled. In the following sections, we describe how building blocks, corresponding to the individual optical components, can be used to construct a variety of highly compact chem- ical sensing architectures [14] that are suitable for fabrication in a standard cleanroom. Each combines one or more ICLs with interband cascade detectors (ICDs), passive waveg- uides, and/or other active and passive elements. We also describe an edge-emitting laser configuration that optimizes stability by minimizing parasitic external optical feedback, besides having the potential to operate with lower drive power than any mid-IR laser now available. Most of the configurations described below have not been described previously in the open literature. While integration on the native III-V chip will be assumed in most of what follows, many of the same configurations may also be advantageous when integration is on a silicon chip. The University of California Santa Barbara (UCSB), NRL, and University of Wis- consin recently reported the first successful integration of mid-IR QCLs on silicon [22,23]. Those PICs were formed by heterogeneously bonding active III-V wafer materials to a silicon-nitride-on-insulator (SONOI) chip that was pre-patterned with passive waveguides and DFB gratings. Laser ridges were then processed from the back. However, the perfor- mance thus far has been limited by an inefficient transfer of laser power from the hybrid III-V/silicon waveguide of the gain region to a passive silicon-based waveguide that can couple the laser beam to other optical elements on the PIC. UCSB and NRL subsequently reported the integration of ICLs on silicon using an analogous heterogeneous bonding approach [24], although inefficient coupling between the hybrid and silicon-based waveg- uides again limited the output power. An alternative approach to the integration of mid-IR sources on silicon has been to grow InP-based QCLs directly onto silicon substrates [25,26].

2. Building Blocks: The Laser, Detector, and Passive Waveguides, Plus Other Active and Passive Optical Components We will assume the starting material to be the epitaxial layers corresponding to an ICL grown on GaSb, which are illustrated schematically in Figure1a. Here the active gain stages are surrounded by lightly doped n-type GaSb separate confinement layers (SCLs), which are in turn surrounded by InAs-AlSb superlattice (SL) optical cladding layers. The GaSb substrate and buffer layer are n+-doped to allow for bottom contacting to the substrate, and the epitaxial layers are capped by an n+-InAs top contacting layer. While a standard off-the-shelf ICL structure may be suitable, it is preferable to modify the design slightly by making the bottom SCL somewhat thicker and the top SCL somewhat thinner (or eliminating it altogether), as will be discussed below. We also note that analogous QCL structures may be employed in most of the configurations discussed below, with n−-InGaAs Sensors 2021, 21, x FOR PEER REVIEW 3 of 19

Sensors 2021, 21, 599 3 of 19 slightly by making the bottom SCL somewhat thicker and the top SCL somewhat thinner (or eliminating it altogether), as will be discussed below. We also note that analogous QCL structures may be employed in most of the configurations discussed below, with n−-In- + GaAssubstituted substituted for the for GaSb the SCLs,GaSb InPSCLs, for InP the fo InAs-AlSbr the InAs-AlSb SL cladding SL cladding layers, andlayers,n -InGaAs and n+- InGaAsfor the top for contactingthe top contacting layer. layer.

Figure 1. ((a)) SchematicSchematic layeringlayering of of an an interband interband cascade cascade laser laser (ICL) (ICL) grown grown on on GaSb. GaSb. The The primary primary regions regions are theare activethe active gain stages,gain stages, top and top bottomand bottom high-index/low-loss high-index/low-lossn−-GaSb n−-GaSb separate separate confinement confinement layers, layers, top and top bottomand bottom InAs-AlSb InAs-AlSb superlattice super- + opticallattice optical cladding cladding layers, andlayers, an nand+-InAs an n top-InAs contacting top contacting layer. (b )layer. Schematic (b) Schematic cross-section cross-section of an ICL narrowof an ICL ridge narrow waveguide ridge waveguide formed by etching through the active stages of the wafer material illustrated in Figure 1a. The same ridge formed by etching through the active stages of the wafer material illustrated in Figure1a. The same ridge structure may structure may provide gain when forward biased, or function as a detector under zero or reverse bias. Typical layer thick- provide gain when forward biased, or function as a detector under zero or reverse bias. Typical layer thicknesses are 2–4 µm nesses are 2–4 μm for the bottom cladding layer, 500–900 nm for the bottom SCLs, 200–400 nm for 5–10 active gain stages, for100–300 the bottom nm for cladding the top separate layer, 500–900 confinement nm for layer the bottom (SCL), 1.2–2 SCLs, μ 200–400m for the nm top for cladding 5–10 active layer, gain and stages, 20 nm 100–300 for the top nm contact for the toplayer. separate confinement layer (SCL), 1.2–2 µm for the top cladding layer, and 20 nm for the top contact layer.

Figure 11bb schematically schematically illustrates illustrates a narrowa narrow ridge ridge waveguide waveguide structure structure processed processed from the epitaxial material shown in Figure1a. The etch that defines the ridge generally proceeds from the epitaxial material shown in Figure 1a. The etch that defines the ridge generally to below the active stages, to prevent the severe lateral current spreading that would proceeds to below the active stages, to prevent the severe lateral current spreading that otherwise occur in either an ICL [27] or QCL [28]. A dielectric, such as SiN, is deposited on would otherwise occur in either an ICL [27] or QCL [28]. A dielectric, such as SiN, is de- the ridge sidewalls, followed by metallization to provide a top electrical contact. The bottom posited on the ridge sidewalls, followed by metallization to provide a top electrical con- contact is usually made to the n+-GaSb substrate, although it is also possible to fabricate tact. The bottom contact is usually made to the n+-GaSb substrate, although it is also pos- two top contacts [24]. A substantial fraction of the optical mode in a conventional ICL sible to fabricate two top contacts [24]. A substantial fraction of the optical mode in a con- resides in the top and bottom n−-GaSb SCLs, which exhibit low loss and high refractive ventional ICL resides in the top and bottom n−-GaSb SCLs, which exhibit low loss and index. All of the active ridges, as well as the passive waveguides discussed below, should high refractive index. All of the active ridges, as well as the passive waveguides discussed be ≈ 4–15 µm wide to assure lasing and propagation in a single lateral mode. below, should be ≈ 4–15 μm wide to assure lasing and propagation in a single lateral The ridge illustrated in Figure1b can provide gain when a forward bias is applied, mode. and lasing if the ridge is incorporated into a cavity. However, the same narrow ridge can functionThe ridge as illustrated an interband in Figure cascade 1b light can emittingprovide gain device when (ICLED) a forward if no cavitybias is providesapplied, andfeedback lasing and if the the ridge emitter is incorporated operates at ainto forward a cavity. bias However, that corresponds the same to narrow net propagation ridge can functionloss. At higher as an interband biases that cascade induce light net gain emitting (optical device gain (ICLED) minus internal if no cavity loss), itprovides will function feed- backas an and interband the emitter cascade operates amplified at a forward spontaneous bias that emitter corresponds (ICASE), to so net long propagation as the feedback loss. Atremains higher insufficient biases that to induce induce net lasing. gain (optical gain minus internal loss), it will function as an interbandIt has also cascade been demonstrated amplified spontaneous that, under emitter zero or (ICASE), reverse bias,so long the as layering the feedback structure re- mainsof Figure insufficient1a, and therefore to induce the lasing. narrow ridge waveguide of Figure1b, can operate as an interbandIt has also cascade been detector demonstrated [29]. In that, an under ICL operating zero or reverse under bias, forward the layering bias, the structure electron ofinjector Figure with 1a, chirpedand therefore quantum the wellnarrow thicknesses ridge waveguide transfers electronsof Figure from1b, can the operate semimetallic as an interbandinterface that cascade separates detector the electron[29]. In an and ICL hole operating injectors under to the activeforward electron bias, the quantum electron wells, in- jectorwhere with they chirped recombine quantum with holes well in thicknesse the actives holetransfers quantum electrons well [from20]. Inthe an semimetallic ICD, on the interfaceother hand, that electrons separates photoexcited the electron in and the activehole injectors quantum to wells the active of a given electron stage quantum produce wells,photocurrent where they in the recombine opposite direction,with holes by in flowing the active downhill hole quantum through well the electron[20]. In an injector ICD, ontoward the other the semimetallic hand, electrons interface, photoexcited where they in recombinethe active withquantum photoexcited wells of holesa given from stage the producenext stage. photocurrent Thus, whereas in the a single opposite electron directio injectedn, by electricallyflowing downhill into an Nthrough-stage ICL the canelectron emit injectorN photons, toward an ICD the mustsemimetallic absorb N interface,photons where to transfer they arecombine single electron with acrossphotoexcited all the stages holes fromto provide the next a photocurrent. stage. Thus, ICDswhereas displayed a single high electron detectivity injected at roomelectrically temperature into an [N29-stage–31]. U. Oklahoma recently reported an ICL and ICD integrated on the same native GaSb chip [8], although separated by an air gap rather than a passive waveguide as will be

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ICL can emit N photons, an ICD must absorb N photons to transfer a single electron across all the stages to provide a photocurrent. ICDs displayed high detectivity at room temper- Sensors 2021, 21, 599 ature [29–31]. 4 of 19 U. Oklahoma recently reported an ICL and ICD integrated on the same native GaSb chip [8], although separated by an air gap rather than a passive waveguide as will be re- quired for versatile integration on a PIC. Other groups have also demonstrated ICLs and ICDsrequired processed for versatile from the integration same wafer on amaterial, PIC. Other although groups not have coupled also demonstrated on the same ICLschip and ICDs processed from the same wafer material, although not coupled on the same [32,33]. The ICD’s cut-off wavelength is always roughly equal to the ICL emission wave- chip [32,33]. The ICD’s cut-off wavelength is always roughly equal to the ICL emission length, since both are determined by the bandgap of the active quantum wells. This con- wavelength, since both are determined by the bandgap of the active quantum wells. trasts the less ideal alignment for a QCD processed in parallel with a QCL. In that case, an This contrasts the less ideal alignment for a QCD processed in parallel with a QCL. In that “extraction” sub-band lies about one optical phonon energy (ħωo) below the lower lasing case, an “extraction” sub-band lies about one optical phonon energy (h¯ ω ) below the lower level to provide rapid depopulation following a stimulated emission event.o At zero bias, lasing level to provide rapid depopulation following a stimulated emission event. At zero most electrons populate the extraction sub-band, so the QCD’s absorption peak occurs bias, most electrons populate the extraction sub-band, so the QCD’s absorption peak occurs closer to ħω + ħωo than to the lasing energy ħω. Nonetheless, specially designed QCLs closer to h¯ ω + h¯ ω than to the lasing energy h¯ ω. Nonetheless, specially designed QCLs residing on the sameo native InP chip [8] with QCDs have produced up to 1 W of cw emis- residing on the same native InP chip [8] with QCDs have produced up to 1 W of cw sion at 15 °C [34]. emission at 15 ◦C[34]. A chemical sensing PIC will also need low-loss passive waveguides, to connect the A chemical sensing PIC will also need low-loss passive waveguides, to connect the otherother components components and and in in some some approaches approaches to to provide provide evanescent evanescent coupling coupling to to an an ambient ambient samplesample gas gas or or liquid liquid that that may may or ormay may not notcontain contain an analyte an analyte of interest. of interest. Figure Figure 2a,b illus-2a,b trateillustrate two passive two passive waveguide waveguide structures structures that thatmay may be processed be processed from from the theICL ICL material material of Figureof Figure 1a.1 Theya. They are arerealized realized by etching by etching away away the theheavily heavily doped doped top topcontacting contacting layer, layer, top claddingtop cladding layer, layer, top SCL, top SCL, and andactive active gain gainstages, stages, with with the etch the etchstopping stopping near near the top the of top the of bottomthe bottom SCL. SCL. Whereas Whereas typically typically the top the and top bottom and bottom SCL SCLthicknesses thicknesses in a conventional in a conventional ICL areICL equal, are equal, in this in thiscase case the thestructure structure should should be beredesigned redesigned such such that that the the bottom bottom SCL SCL is is thickerthicker and and the the top top SCL SCL thinner, thinner, as as shown shown in in Figure Figure 11a,a, or absent altogether. In exchange forfor a a modest modest reduction reduction in in optical optical confinement confinement in in the the active active gain gain stages, stages, this this will will thicken thicken thethe high-index high-index core core of of the the passive passive waveguid waveguide,e, and and better better align align the the active active and and passive passive waveguidewaveguide modes modes along along the the vertical vertical axis. axis. For For example, example, if if a a conventional conventional ICL ICL structure structure emittingemitting at at 3.4 3.4 μµm,m, with with seven seven stages stages and and SCL SCL thicknesses thicknesses of of 700 700 nm nm on on both both top top and and bottom,bottom, is is redesigned, redesigned, such such that that the the top top SCL SCL is is thinned thinned to to 450 450 nm nm while while the the bottom bottom SCL SCL isis thickened thickened to to 950 950 nm, nm, the the optical optical confinement confinement factor factor in in the the active active region region decreases decreases from from 15.7%15.7% to to 10.2% 10.2%.. However, However, in in exchange, exchange, 80% 80% of of the the fundamental fundamental mode mode then then lies lies below below the the activeactive core, core, a a large large fraction fraction of of which which should should couple couple to to the the passive waveguide following following removalremoval of of the the active active core core to to form form a a passiv passivee waveguide. waveguide. An An interface interface between between active active and and passivepassive waveguides waveguides will will be be illustrated illustrated below. below. InIn Figure Figure 2a,2a, the bottom SCL is etched away entirely outside the ridge, with the etch stoppingstopping near near the the top top of of the the superlattice superlattice botto bottomm cladding cladding layer. layer. Alternatively, Alternatively, Figure Figure 22bb showsshows the the etch etch outside outside the the ridge, ridge, stopping stopping with withinin the the bottom bottom SCL. SCL. In In both both cases, cases, the the beam beam propagatespropagates in in the the ridge ridge wave waveguideguide formed by the bottom n−-GaSb-GaSb SCL SCL as as its its core core and and the the bottombottom InAs-AlSb InAs-AlSb SL asas itsits bottombottom clad. clad. The The top top and and sides sides of of the the ridge ridge may may be encapsulatedbe encapsu- latedin a dielectricin a dielectric or left or exposed left exposed for evanescent for evanescent coupling coupling to ambient. to ambient.

FigureFigure 2. Schematic crosscross sectionssections of of passive passive waveguides waveguides processed processed from from the the ICL ICL epitaxial epitaxial structure structure of Figure of Figure1a. The 1a. ridge The ridgeis formed is formed by etching by etching away away the top the contacting top contacting layer, layer, top clad top layer,clad layer, top SCL, top SCL, and theand active the active stages, stages, to leave to leave the bottomthe bottom SCL that serves as the waveguide core and the bottom InAs-AlSb SL that forms the bottom clad. Outside the narrow ridge, either the bottom SCL is fully etched away, with the etch stopping near the top of the bottom cladding layer (a), or the etch stops within the bottom SCL (b). If the tops and sides of these passive waveguides are left exposed, as shown in the figures, propagating light will evanescently couple to an ambient sample gas or liquid. A typical overlap of the optical mode profile with the ambient gas will be illustrated below. Sensors 2021, 21, 599 5 of 19

The optical loss in these passive waveguides should be low when the contributions by scattering and the other optional treatments described below are minimized. Therefore, a relatively long sensing waveguide that maximizes net coupling to the sample gas may be feasible (>>1 cm−1). The path geometry may be straight, or follow a meandering pattern, as shown in some of the figures below. Or it may be configured as a ring (see below) or other resonator configuration, to extend the effective path length and also provide spectral selectivity imposed by the resonance. Nonetheless, the modal overlap with ambient is relatively weak because the upper- most GaSb layer of the waveguides shown in Figure2a,b has a high refractive index, while the top cladding layer is the air containing the analyte. For this reason, it may beneficial to pattern a slot waveguide, sub-wavelength grating, or other nanoscale structure that increases the coupling of the beam to ambient, employ a hollow-core waveguide that provides propagation through free space, or coat the top and sides of the waveguide with a chemical sorbent that selectively concentrates a given class of chemicals. Those options will be discussed briefly in Section6 below, although a detailed analysis falls outside the main scope of this work. The lasers, detectors, and passive waveguides described above can also be integrated into more complex PICs that incorporate other optical components for enhanced capability. For example, an arrayed waveguide grating (AWG) can spectrally combine the beams from an array of distributed feedback (DFB) [35,36] or distributed Bragg reflector (DBR) lasers, with slightly different grating pitches, into a single output for use elsewhere on or off the chip. Or spectroscopy can be performed by using an AWG to separate a single broadband input into spectral components that are detected individually [37]. We must also consider how to couple the different types of waveguides for optimal beam transfer from one active or passive optical component to another. For example, one end of a laser cavity employing the narrow ridge waveguide illustrated in Figure1b may be defined by simple butt coupling to a passive waveguide (Figure2). Reflection for feedback is then provided by the differing mode profiles, as well as the different modal indices of the two waveguides. Of course, this approach provides relatively little opportunity for tuning the reflectivity of the output mirror. It is also worth noting that, since the active waveguide shown in Figure1a is taller than the passive waveguides of Figure2, the centroid of its mode profile along the vertical axis is higher than in the passive waveguides. Therefore, to minimize transmission into the ambient above the passive waveguide, the upper portion of the active waveguide at the interface should be coated with a dielectric and gold for high reflection (HR). This is illustrated below. It may also be desirable to taper the passive waveguide to a width somewhat narrower than the active laser waveguide since a ridge narrow enough to preclude lasing in higher-order lateral modes may nonetheless allow for propagation in multiple modes when no gain is present. In principle, the laser output coupling is more controllable if a distributed Bragg reflec- tor (DBR) is instead processed at the interface between the active and passive waveguides (and possibly at the other end of the cavity to define the back mirror as well) [38]. When a III-V laser is bonded to silicon to form the PIC, it is relatively straightforward to pre-pattern a DBR grating into the silicon before the active gain material is bonded and processed [23]. However, for ICL-based PICs residing on the native GaSb substrate, the DBR must be etched into the ICL wafer material, as illustrated in Figure3. For a DBR, defining one end of the laser cavity (or detector absorber region) that couples to a passive waveguide, it is preferable to etch the grating into the passive waveguide (at left in the figure), rather than into the full layered structure (at right). Given the spatial resolution and sidewall angle that can be achieved routinely for a given grating period when GaSb-based laser structures are processed with reactive ion etching, the processing yield is likely to be higher if a third-order rather than first-order grating is employed. For the example of an ICL emitting at 3.5 µm, the period for a third-order grating with 50% duty cycle is roughly 1.5 µm. The reflectivity can then be tuned to a desired value by varying the grating length and/or etch depth. For maximum reflection by a mirror that is not intended for output, Sensors 2021, 21, x FOR PEER REVIEW 6 of 19

and sidewall angle that can be achieved routinely for a given grating period when GaSb- based laser structures are processed with reactive ion etching, the processing yield is likely Sensors 2021, 21, 599 to be higher if a third-order rather than first-order grating is employed. For the example6 of 19 of an ICL emitting at 3.5 μm, the period for a third-order grating with 50% duty cycle is roughly 1.5 μm. The reflectivity can then be tuned to a desired value by varying the grat- ing length and/or etch depth. For maximum reflection by a mirror that is not intended for ouroutput, simulations our simulations indicate that indicate a grating that length a grating of at leastlength 100 ofµm at is least preferred, 100 μ which,m is preferred, with an ≈ etchwhich, depth with of an 280 etch nm, depth would of produce 280 nm, would90% reflectivity.produce ≈ 90% The reflectivity. corresponding The parameterscorrespond- foring a parameters first-order grating for a first-order are a 500 grating nm period are anda 500 120 nm nm period etch depth.and 120 nm etch depth.

Figure 3. Schematic of a third-order distributed Bragg reflector (DBR) mirror that defines one end Figure 3. Schematic of a third-order distributed Bragg reflector (DBR) mirror that defines one end of of an ICL cavity. If the DBR is designed for outcoupling the laser light, the mirror couples into a an ICL cavity. If the DBR is designed for outcoupling the laser light, the mirror couples into a passive passive waveguide positioned farther to the left in the figure. waveguide positioned farther to the left in the figure. The waveguide for an in-plane ICD may also be bound on both ends by DBRs, or on The waveguide for an in-plane ICD may also be bound on both ends by DBRs, or on one side by a DBR and on the other by a cleaved or etched HR-coated facet, to form an in- one side by a DBR and on the other by a cleaved or etched HR-coated facet, to form an in-planeplane resonant resonant cavity cavity detector detector [39,40]. [39,40]. By By ta takingking advantage ofof multiplemultiple in-planein-plane passes passes throughthrough the the absorber, absorber, a a shorter shorter ICD ICD waveguide waveguide that that generates generates lower lower dark dark current current can can maintainmaintain high high quantum quantum efficiency efficiency for for enhanced enhanced specific specific detectivity detectivityD D*.*.

3.3. Sensing Sensing within within the the Laser Laser Cavity Cavity SomeSome or or all all of of the the active active and and passive passive optical optical elements elements described described above above can be can combined be com- inbined a number in a number of ways of to ways form to an form on-chip an on-chip chemical chemical sensor. Onesensor. of theOne simplest of the simplest is to imbed is to animbed active an or active passive or sensingpassive sensing waveguide waveguide within the within laser the cavity. laser cavity. WeWe begin begin with with the the simple simple baseline baseline configuration, configuration, illustrated illustrated in in Figure Figure4a, 4a, which which is is formedformed by by placing placing HR-coated HR-coated cleaved cleaved facets facets at at both both ends ends of of a a narrow narrow ridge, ridge, although although one one oror both both ends ends of of the the cavity cavity may may just just as as well well be be bound bound by by an an etched etched facet facet or or high-reflectivity high-reflectivity DBR.DBR. SinceSince there is no no opportunity opportunity for for feedback feedback from from an anexternal external optical optical element element to influ- to influenceence the thelaser laser operation, operation, its stability its stability should should be limited be limited only only by the by thedrive drive electronics electronics and andenvironmental environmental temperature temperature fluctu fluctuationsations (typically (typically taking taking place place on a on relatively a relatively long-time long- timescale). scale). Ordinary Ordinary vibrations vibrations should should have havelittle effect, little effect, since all since of the all offixed the internal fixed internal lengths lengthswill vibrate will vibrate in unison. in unison. Very low Very drive low drivepower power is also is alsopossible possible in principle, in principle, as will as will be dis- be discussedcussed further further below below..

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FigureFigure 4. 4.SchematicsSchematics of oflaser laser cavities cavities defined defined by by two two HR-coated HR-coated mirrors: mirrors: (a) ( athe) the laser laser does does not not interact interact with with any any optical optical Figure 4. Schematics of laser cavities defined by two HR-coated mirrors: (a) the laser does not interact with any optical elementelement outside outside its its own own cavity; cavity; (b ()b a) apassive passive waveguide waveguide inside inside the the laser laser cavity cavity evanescently evanescently couples couples to to ambient; ambient; ( (cc)) both both a element outside its own cavity; (b) a passive waveguide inside the laser cavity evanescently couples to ambient; (c) both a passive sensing waveguide and an interband cascade detector (ICD) are incorporated inside the cavity; (d) detail of the passivea passive sensing sensing waveguide waveguide and and an an interband interband cascade cascade detector detector (ICD) (ICD) are are incorporated incorporated inside inside the the cavity; cavity; (d ()d detail) detail of of the the interface between the active and passive waveguides. interfaceinterface between between the the active active and and passive passive waveguides. waveguides. Of course, a laser having no interaction at all with anything outside its own cavity OfOf course, course, a a laser laser having having no no interaction interaction at at all all with with anything anything outside outside its its own own cavity cavity cannotcannot perform perform a auseful useful functi function.on. However, However, we we can can provide provide interaction interaction without introducingintroduc- cannot perform a useful function. However, we can provide interaction without introduc- ingany any external external optical optical feedback feedback by by configuring configuring the the ridge ridge geometry geometry to to impose impose finitefinite overlapover- ing any external optical feedback by configuring the ridge geometry to impose finite over- lapof of the the propagating propagating optical optical modemode withwith ambient.ambient. One One conceptually conceptually simple simple approach approach is is lap of the propagating optical mode with ambient. One conceptually simple approach is to tomodify modify the the ridge ridge waveguide waveguide profile profile from from Figure Figure 1b1 bsuch such that that part part of of the the top top SCL SCL is is to modify the ridge waveguide profile from Figure 1b such that part of the top SCL is exposed,exposed, as asillustrated illustrated schematically schematically in inFigure Figure 5.5 Spectral. Spectral selectivity selectivity may may also also be be imposed imposed exposed, as illustrated schematically in Figure 5. Spectral selectivity may also be imposed byby patterning patterning a DFB a DFB grating grating into into the the partially-exposed partially-exposed ridge. ridge. by patterning a DFB grating into the partially-exposed ridge.

Figure 5. Schematic cross-sectional profile of a waveguide with part of the top SCL and one side- Figure 5. Schematic cross-sectional profile of a waveguide with part of the top SCL and one side- wallFigure of the 5. narrowSchematic ridge cross-sectional left exposed profileto ambient of a waveguideto allow evanescent with part coupling of the top of SCL the andlasing one mode sidewall wall of the narrow ridge left exposed to ambient to allow evanescent coupling of the lasing mode to ofa sample the narrow gas or ridge liquid. left exposed to ambient to allow evanescent coupling of the lasing mode to a to a sample gas or liquid. sample gas or liquid.

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AnAn alternative alternative possibility possibility is is to to open open one one or or more notches in the top of some portion ofof the the active active gain gain waveguide, waveguide, as asshown shown schematically schematically in Figure in Figure 6, such6, such that that the thepropagat- prop- ingagating mode mode evanescently evanescently couples couples to ambient. to ambient. Each notch Each should notch should be narrow be narrow enough enough (<200– µ 300(<200–300 μm) to allowm) to for allow current for current spreading spreading and self-pumping and self-pumping to inject to carriers inject carriers into its into active its stages.active stages.

Figure 6. SchematicSchematic side side view view of of an an active active gain gain waveguide waveguide in in whic whichh notches notches are are etched etched to to allo alloww overlap overlap of of the the propagating propagating optical mode with ambient.

AA third possibilitypossibility isis to to longitudinally longitudinally divide divide the the cavity cavity into into active active and and sensing sensing sections, sec- tions,as shown as shown schematically schematically in Figure in Figure4b. The 4b. active The active waveguide waveguide section section includes includes the full theICL full ICLstructure structure containing containing the activethe active gain gain stages, stages, as inas Figure in Figure1b, whereas1b, whereas the sensingthe sensing section sec- tioncomprises comprises a passive a passive waveguide waveguide with with exposedexposed top and and side side su surfaces,rfaces, such such as those as those in Fig- in ureFigure 2. The2. The sensing sensing section section may may be be relatively relatively long, long, (e.g., (e.g., >1 >1 cm) cm) to to enhance the chemicalchemical detectiondetection sensitivity, sincesince the the passive passive waveguide waveguide loss loss should should be be low low and and absorption absorption by theby thesample sample gas gas is weak is weak in most in most cases. cases.The passiveThe passive sensing sensing waveguide waveguide may be may straight, be straight, leading leadingdirectly directly to a mirror to a that mirror defines that one defines end of one the end cavity, of the or it cavity, may follow or it may a long follow meandering a long meanderingpath (with bend path radius (with large bend enough radius to large result enough in little additionalto result in loss), little as shownadditional in Figure loss),4 b.as shownAssuming in Figure that4b. the amplitude and spectral characteristics of the beam propagating in theAssuming cavity are that modified the amplitude by the and presence spectral and characteristics concentration of ofthe a beam chemical propagating species in of theinterest cavity in are the modified ambient sample by the presence gas, we still and need concentration a means for of quantifying a chemical species the effect. of interest While a indetector the ambient can be sample incorporated, gas, we as still will need be discussed a means for below, quantifying it is not required.the effect. Instead, While a wedetec- can tormonitor can be the incorporated, compliance voltage as will in be the discussed laser’s I–V below,characteristics it is not required. when a constant Instead, current we can is monitorinjected [the41]. compliance When the cavity voltage loss in increases, the laser’s due I–V to characteristics absorption by thewhen sample a constant gas, the current lasing isthreshold injected [41]. increases When and the thecavity radiative loss increase recombinations, due to absorption rate decreases by the at asample given gas, current the lasinginjection threshold level above increases threshold. and the Therefore, radiative a recombination higher voltage rate must decreases be applied at a to given maintain cur- rentthe sameinjection current. level Phillipsabove threshold. et al. observed Therefore, a 0.15 a Vhigher increase voltage in the must compliance be applied voltage to main- for taina QCL the emittingsame current. near 7.7Phillipsµm when et al. theobserved laser wavelength a 0.15 V increase swept in across the compliance an absorption voltage line forof watera QCL vapor emitting [41]. near In that 7.7 experiment,μm when the the laser sample wavelength gas resided swept in anacross external an absorption portion of linethe laserof water cavity vapor (which [41]. coupled In that experiment, to a grating forthe tuningsample the gas lasing resided wavelength), in an external rather portion than ofcoupling the laser evanescently cavity (which to the coupled beam propagatingto a grating infor a waveguide.tuning the lasing Without wavelength), an external rather cavity, thanthe wavelength coupling evanescently of the laser to illustrated the beam propagating in Figure4b canin a bewaveguide. tuned across Without the absorptionan external cavity,resonance the bywavelength current or of temperature, the laser illustrated and the dependencein Figure 4b of can compliance be tuned across voltage the on absorp- current, tionthen resonance compared by to thatcurrent observed or temperature, when the chemicaland the dependence species of interest of compliance is not present voltage in theon ambient sample gas. Multiple DFB or DBR lasers on the same chip, which all incorporate current, then compared to that observed when the chemical species of interest is not pre- sensing waveguides, can provide a reference by tuning to different wavelengths. sent in the ambient sample gas. Multiple DFB or DBR lasers on the same chip, which all A further alternative is to monitor the absorption resulting from evanescent coupling incorporate sensing waveguides, can provide a reference by tuning to different wave- to the sample gas by incorporating an additional interband cascade detector waveguide lengths. section into the ICL cavity, since the same ridge waveguide is suitable for both. For the A further alternative is to monitor the absorption resulting from evanescent coupling example shown schematically in Figure4c, the ICD is placed on one side of the active to the sample gas by incorporating an additional interband cascade detector waveguide gain section and the passive sensing waveguide on the other. However, the detector section into the ICL cavity, since the same ridge waveguide is suitable for both. For the

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may also be placed inside the laser cavity when evanescent coupling to the sample gas occurs within the active gain section; for example, using the partially-exposed waveguides shown in Figures5 and6, rather than in a separate sensing waveguide. The ICD and ICL sections must naturally be contacted individually, with a gap in between to prevent crosstalk. The detector section must be short enough that its absorption of lasing photons does not dramatically degrade the laser performance. However, high sensitivity should be achievable with a very short detector because the in-plane absorption is quite strong.

4. Integrated ICL with Enhanced Stability Trace chemicals can be detected by measuring the differential absorption induced by a constituent of the sample gas, against the background when the species of interest is absent. It follows that the minimum detectable absorption depends critically on the stability of the laser, since any jitter or other fluctuation will easily wash out a small differential signal. A major cause of instability is feedback from a secondary optical cavity that forms due to reflection from one or more surfaces encountered outside the intended primary cavity [42–45]. For example, reflection may occur at an external optic or other surface encountered after the beam leaves the output facet of an edge-emitting laser. One study found that as little as −30 dB feedback could broaden the spectrum and degrade the relative intensity noise (RIN) of an edge-emitting DFB laser [46]. Furthermore, inevitable mechanical and thermal vibrations cause the secondary cavity length to vary, thereby inducing temporal “jitter” of the magnitude and spectral characteristics of the parasitic modes. A conventional edge-emitting diode laser with cleaved output facet that is either AR coated (reflectivity R ≤ 2%) or uncoated (typically R ≈ 25–40%) [21] is especially susceptible to unwanted feedback, since a significant fraction of the light returned to the facet following external reflection is transmitted back into the cavity, and even a modest amount of feedback can degrade the laser stability. While an optical isolator can minimize the external feedback, they are too bulky (as well as expensive and wavelength-specific) for incorporation into an ultra-compact chemical sensing system. An advantage of PIC-based sensors that place the sensing waveguide entirely within the laser cavity is the potential to minimize nearly all parasitic feedback by more effectively isolating the laser cavity on the chip. Given an ultra-low-noise electrical driver, the lasers illustrated in Figure4a–c should be extremely stable because virtually all feedback from external optical elements is eliminated. While external light can, in principle, be injected into the laser cavity via ridge processing imperfections or even interactions with the analyte, those sources should be entirely negligible compared to the nearly inevitable feedback, due to external reflection of light emitted at a facet back into the cavity in the absence of an optical isolator. On the other hand, the more general flexibility of this laser design for incorporation into a PIC, intended for a function extending beyond intracavity chemical sensing, is severely limited by the absence of an output beam. In practice, we can trade output power from the on-chip laser against stability by adjusting the fraction of light coupled out during each pass through the laser cavity. For example, when relatively little power is needed, we can increase the reflectivity of the output facet, say, to ≈ 90%. Low mirror loss provides the further advantage of lower threshold current and drive power, especially if the cavity length is also shortened as will be discussed further. In principle, the most straightforward approach to increasing the reflectivity at a facet, while still allowing output, is to deposit a multi-layer Bragg dielectric coating. However, such coatings are typically expensive and challenging to process, especially at longer wavelengths, where each layer becomes proportionally thicker. Alternatively, the reflectivity of a DBR end mirror that couples to a passive waveguide can be adjusted for optimal trade-off between laser stability and output power. A more flexible way to tune the laser output per pass through the cavity is to evanes- cently out-couple the light laterally, to a second (passive) waveguide that runs parallel to the active gain waveguide over some section of the cavity [14]. Figure7 illustrates that once a desired fraction of the beam has coupled into the passive waveguide, it can bend Sensors 2021, 21, x FOR PEER REVIEW 10 of 19

A more flexible way to tune the laser output per pass through the cavity is to evanes- Sensors 2021, 21, 599 10 of 19 cently out-couple the light laterally, to a second (passive) waveguide that runs parallel to the active gain waveguide over some section of the cavity [14]. Figure 7 illustrates that once a desired fraction of the beam has coupled into the passive waveguide, it can bend awayaway from from the the primary primary waveguide waveguide and and proceed proceed either either toward toward a a facet facet for for emission emission from from thethe chip chip (as (as shown shown in in the the figure), figure), or or elsewhere elsewhere on on the the chip chip for for functionality functionality within within the the PIC. PIC. InIn the the former former case, case, the the facet facet at at which which light light is is emitted emitted may may be be AR AR coated, coated, and and the the passive passive waveguidewaveguide may may intersect intersect at at an an angle angle to to further further minimize minimize the the potential potential for for reflection reflection back back intointo the the laser laser cavity. cavity. Alternatively, thethe output output waveguide waveguide may may retain retain the the full full laser laser structure struc- turewith with top top contact, contact, which which is then is then forward forward biased biased for amplificationfor amplification of the of outputthe output signal. signal.

FigureFigure 7. 7. (a)(a) Schematic Schematic showing light extractionextraction from from a a laser laser cavity cavity via via evanescent evanescent coupling coupling to ato passive a passive waveguide waveguide that that runs runs parallel to a section of the active gain waveguide; (b) detailed cross-section of the two waveguides at the HR-coated parallel to a section of the active gain waveguide; (b) detailed cross-section of the two waveguides at the HR-coated facet. facet. The space between the two coupled waveguides should be filled with a low-loss dielectric,The space such between as SiN, andthe two the topcoupled of the waveguides passive waveguide’s should be couplingfilled with section a low-loss covered di- electric,by a dielectric such as toSiN, minimize and the optical top of lossthe pa duessive to modalwaveguide’s overlap coupling with the section laser’s covered top contact by ametal. dielectric The to coupling minimize strength optical between loss due the to activemodal and overlap output with waveguides the laser’s may top becontact tuned metal.broadly The by coupling varying strength their separation between distance the active (e.g., and≈ output0.3 to 2waveguidesµm), the etch may depth be tuned in the broadlyregion separatingby varying them, their and/or separation the length distance over (e.g., which ≈ 0.3 they to run2 μ inm), parallel. the etch Simulations depth in the for regionan ICL separating emitting at them,λ = 3.5 and/orµm indicate the length that over if both which waveguides they run are in parallel. 5 µm wide, Simulations are separated for anby ICL 500 emitting nm, and at the λ coupling= 3.5 μm lengthindicate is that 19 µ mif both with waveguides no etch of the are region 5 μm separatingwide, are sepa- them, rated≈ 27% by of500 the nm, light and transfers the coupling per passlength from is 19 the μm active with tono theetch passive of the region waveguide separating [14]. them, ≈ 27% Lateralof the light out-coupling transfers per can pass provide from light the active for the to PIC the or passive external waveguide emission [14]. from a laser whoseLateral cavity out-coupling is terminated can provide on both light ends for by the cleaved PIC or HR-coated external emission facets, asfrom shown a laser in whoseFigure cavity7. Alternatively, is terminated the on cavityboth ends can by curve, cleaved as shownHR-coated in Figure facets,8 as, such shown that in aFigure single 7. Alternatively,HR-coated cleaved the cavity facet can provides curve, both as shown end mirrors. in Figure 8, such that a single HR-coated cleaved facet provides both end mirrors.

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Figure 8. Schematic of a curved laser cavity with laterally coupled output and both end mirrors provided by the same Figure 8. Schematic of a curved laser cavity with laterally coupled output and both end mirrors provided by the same HR- HR-coatedcoated facet. facet. We emphasize that the threshold drive power for a laser with any of the cavity designs, We emphasize that the threshold drive power for a laser with any of the cavity de- shown in Figures7 and8 (or Figure4), can be extremely small, even lower than the record signs, shown in Figures 7 and 8 (or Figure 4), can be extremely small, even lower than the mid-IR value of 29 mW already reported for an ICL [20]. With minimal mirror loss, because record mid-IR value of 29 mW already reported for an ICL [20]. With minimal mirror loss, the cavity is defined by two HR-coated facets, Lcav can be shortened beyond the usual because the cavity is defined by two HR-coated facets, Lcav can be shortened beyond the limit of ≈ 0.5 mm. While the minimum practical length that a thinned GaSb-based device usual limit of ≈ 0.5 mm. While the minimum practical length that a thinned GaSb-based structure can be cleaved with high yield using conventional methods is also ≈ 0.5 mm (potentiallydevice structure shorter can ifbe at cleaved least one with end high mirror yield is using an etched conventional facet or methods DBR), the is configurationalso ≈ 0.5 mm (potentiallyin Figure8 makes shorter a if shorter at least length one end straightforward, mirror is an etched with facet an ultimate or DBR), limit the configuration imposed by inbending Figure loss. 8 makes If the a active shorter gain length length straightforward, is reduced only with modestly an ultimate to 0.3 mm,limit alongimposed with by a bendingridge width loss. of If 4 theµm, active current gain density length (slightly is reduced above only threshold) modestly 250 to A/cm0.3 mm,2, and along threshold with a 2 voltageridge width 3 V, withof 4 μ am, power current budget density of <10 (slightly mW per above laser, threshold) is quite realistic 250 A/cm [14]., Such and threshold low drive powervoltage can 3 V, easily with bea power supplied budget by a smallof <10 solar mW cell. per Alaser, further is quite advantage realistic of [14]. the very Such short low drivecavity power is that can with easily greater be spacingsupplied between by a small the solar longitudinal cell. A further optical advantage modes, emission of the very in a shortsingle cavity mode is may that become with greater possible spacing without between any need the longitudinal to pattern a DFBoptical grating. modes, emission in a singleTerminating mode may both become ends of thepossible lasercavity without at theany same need HR-coated to pattern faceta DFB may grating. be especially beneficialTerminating in PICs thatboth integrate ends of multiplethe laser lasers,cavity whichat the aresame mounted HR-coated epitaxial-side-down facet may be espe- for ciallymaximum beneficial heat dissipation,in PICs that onintegrate the same multiple chip with lasers, multiple which passive are mounted sensing epitaxial-side- waveguides downthat must for bemaximum exposed toheat ambient. dissipation, Figure 9on illustrates the same that chip it can with be straightforwardmultiple passive to sensing design waveguidesthe PIC such that that must the lasersbe exposed are grouped to ambient. at one Figure end for9 illustrates flip-chip that bonding it can to be a straight- thermal forwardsubmount, to design while the PIC sensing such waveguides that the lasers are are grouped grouped at at the one other end for end flip-chip that hangs bonding over tothe a edgethermal of the submount, submount while for exposurethe sensing to ambient.waveguides We are also grouped note, however, at the other that anend ICL’s that hangsthreshold over input the edge power of densitythe submount is low enoughfor expo thatsure itto can ambient. generally We operatealso note, cw however, at room thattemperature an ICL’s eventhreshold when input mounted power epi-up, density only is withlow enough lower maximum that it can output generally power operate than cwfor epi-downat room temperature mounting. even when mounted epi-up, only with lower maximum output power than for epi-down mounting.

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Figure 9. (a) Schematic of a PIC that integrates two dual comb spectrometers with multiple ICLs and ICDs grouped at one Figure 9. (a) Schematic of a PIC that integrates two dual comb spectrometers with multiple ICLs and ICDs grouped at one endFigure and 9. multiple (a) Schematic sensing of awaveguides PIC that integrates at opposite two end. dual This comb allows spectrometers the lasers with to be mult electro-platediple ICLs and with ICDs gold grouped and mounted at one end and multiple sensing waveguides at opposite end. This allows the lasers to be electro-plated with gold and mounted epitaxial-side-downend and multiple sensing on a patternedwaveguides heat at oppositesink submount, end. This while allows the the sensing lasers waveguides to be electro-plated hang over with the gold edge and of mountedthe sub- mountepitaxial-side-down to provide exposure onon a a patterned patterned to the ambient heat heat sink sink sample submount, submount, gas; ( whileb) whileThe the chip the sensing andsensing patterned waveguides waveguides submount hang hang over before over theedge flip-chipthe edge of the bondingof submount the sub- (c) Thetomount provide chip to andprovide exposure submount exposure to the after ambientto flip-chipthe ambient sample bonding; sample gas; (bd gas;) The (b chipreverse) The and chip side patterned and of patternedthe flip-chip-bonded submount submount before before PIC, flip-chip showing flip-chip bonding the bonding exposed (c) The (c) sensingchipThe chip and waveguides. submountand submount after flip-chipafter flip-chip bonding; bonding; (d) The (d reverse) The reverse side of side the flip-chip-bondedof the flip-chip-bonded PIC, showing PIC, showing the exposed the exposed sensing waveguides.sensing waveguides. 5. Integration of the Optical Components on a Sensing PIC 5. Integration Integration of the Optical Components on a Sensing PIC Once the laser output has coupled to a passive waveguide, which may have an air Once the the laser laser output output has has coupled coupled to to a apa passivessive waveguide, waveguide, which which may may have have an air an top cladding, as in Figure 4a,b for evanescent coupling to an ambient sample gas, absorp- topair topcladding, cladding, as inas Figure in Figure 4a,b 4fora,b evanescent for evanescent coupling coupling to an ambient to an ambient sample samplegas, absorp- gas, tion spectroscopy can be implemented to sense the presence of analyte molecules. The tionabsorption spectroscopy spectroscopy can be canimplemented be implemented to sense to sensethe presence the presence of analyte of analyte molecules. molecules. The other end of the passive waveguide can then couple to an ICD that resides on the same otherThe other end endof the of thepassive passive waveguide waveguide can can then then couple couple to toan an ICD ICD that that resides resides on on the the same chip, as illustrated in Figure 10. The optimal waveguide length that maximizes detection chip, as illustrated in Figure 1010.. The optima optimall waveguide length that maximizes detection sensitivity depends on such factors as absorption strength by the analyte, parasitic loss in sensitivity depends on such factors as absorpti absorptionon strength by the analyte, parasitic loss in the passive waveguide, and of course laser stability. Note also that, instead of using cur- the passive waveguide,waveguide, andand of of course course laser laser stability. stability. Note Note also also that, that, instead instead of usingof using current cur- rent or temperature to scan the laser’s emission wavelength across an analyte absorption rentor temperature or temperature to scan to thescan laser’s the laser’s emission emission wavelength wavelength across anacross analyte an analyte absorption absorption feature, feature, the same PIC may integrate multiple sensors operating at both fingerprint absorp- feature,the same the PIC same may PIC integrate may integrate multiple multiple sensors sensors operating operating at both at both fingerprint fingerprint absorption absorp- tion feature and reference wavelengths. tionfeature feature and referenceand reference wavelengths. wavelengths.

Figure 10. (a) Schematic of an ICL with lateral out-coupling, extended passive sensing waveguide, and interband cascade detectorFigure 10. integrated (a) Schematic on the of same an ICL III-V with chip; lateral (b) detailout-coupling, of the passive exte extendednded waveguide passive sensingleading waveguide,to the ICD. and interband cascade detector integrated on the same III-V chip;chip; ((b)) detaildetail ofof thethe passivepassive waveguidewaveguide leadingleading toto thethe ICD.ICD. Instead of a simple passive waveguide that may follow a straight or meandering Instead of a simple passive waveguide that may follow a straight or meandering path,Instead the sensing of a simplewaveguide passive may waveguide also take thatthe form may followof a high-Q a straight resonator or meandering (with exposed path, path, the sensing waveguide may also take the form of a high-Q resonator (with exposed topthe surface) sensing waveguidethat allows maymany also passes take of the the form beam. of aFigure high-Q 11 resonator illustrates (with that exposeda single ICL top top surface) that allows many passes of the beam. Figure 11 illustrates that a single ICL maysurface) emit that into allows a passive many bus passes waveguide of the beam. that couples Figure 11to illustratesa series of that ring a resonators single ICL may[40]. may emit into a passive bus waveguide that couples to a series of ring resonators [40]. Eachemit intoring ain passive the series bus waveguideselectively extracts that couples its own to a seriescomb ofof ringreso resonatorsnance wavelengths [40]. Each from ring Each ring in the series selectively extracts its own comb of resonance wavelengths from the bus, to determine the spectral dependence of the absorption. Each ring in turn couples the bus, to determine the spectral dependence of the absorption. Each ring in turn couples

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Sensors 2021, 21, x FOR PEER REVIEW 13 of 19 in the series selectively extracts its own comb of resonance wavelengths from the bus, to determine the spectral dependence of the absorption. Each ring in turn couples to an ICD that quantifies the signal. Coupling to the resonators may be increased by shaping the to an ICD that quantifies the signal. Coupling to the resonators may be increased by shap- rings as racetracks rather than circles. ing the rings as racetracks rather than circles.

Figure 11. SchematicSchematic of of an an on-chip chem chemicalical sensor sensor in in which which an an ICL source emits into to a passive bus waveguide for coupling to a series of ring resonators [37]. [37]. Each ring extrac extractsts a different comb of resonance wavelengths. The individual rings then evanescently couple to ICDs that quantify the spectral dependence of the absorption.

WeWe have found that for ICLs processed on the native GaSb substrate, the bending loss maymay be be too too large large to to maintain maintain high high Q ifQ theif the ring ring diameter diameter is substantially is substantially less than less than100 μ 100m. Thisµm. diameterThis diameter corresponds corresponds to a tofree a free spectr spectralal range range (FSR) (FSR) between between resonances resonances of ≈≈ 2.5 cmcm−−1,, meaningmeaning that one ringring atat aa timetime couldcould select select a a narrow narrow emission emission line line from from a temperature-a temperature- or orcurrent-tuned current-tuned DFB DFB laser, laser, but but could could not not select select a single a single narrow narrow range range of wavelengths of wavelengths from froma broadband a broadband source source such such as an as ICLED. an ICLED. Since Since the the bending bending loss loss from from sidewall sidewall scattering scatter- ingwould would be lower be lower for a for PIC a processedPIC processed on silicon on silicon rather rather than GaSb,than GaSb, a silicon a silicon ring could ring havecould a havesmaller a smaller diameter diameter and larger and FSR.larger FSR. Alternatively,Alternatively, a passive waveguide section wi withinthin a laser laser cavity, cavity, e.g., defined defined on both endsends by by HR-coated HR-coated cleaved facets as in Figure 44b,b, maymay couplecouple toto aa singlesingle ringring (or(or twotwo ringsrings withwith slightly differentdifferent diameters),diameters), which which in in turn turn couples couples evanescently evanescently to to aseparate a separate passive pas- sivewaveguide waveguide that that leads leads to an to ICD.an ICD. While While the the laser laser may may employ employ a DFB a DFB or DBRor DBR grating grating to toinduce induce lasing lasing in in a single a single mode, mode, in whichin which case ca these the ring ring resonance resonance must must be tunedbe tuned to coincideto coin- cidewith with the laserthe laser wavelength, wavelength, this this is is generally generally unnecessary unnecessary because, because, withwith proper design, design, thethe double double ring ring resonance will select a single longitudinal lasing mode [[47].47]. The ring’s resonanceresonance wavelength wavelength can can then then be be tuned with local temperature. RecentRecent reports reports of of ICL frequency combs [32, [32,33,48]33,48] have demonstrated their potential forfor sensitive chemicalchemical detection.detection. While While the the physics physics of of ICL ICL combs combs is not is not yet wellyet well understood, under- stood,and in and many in many cases thecases comb the comb linewidth linewidth is not is sufficiently not sufficiently narrow narrow for practical for practical applica- ap- plicationstions [49,50 [49,50],], we expectwe expect these these issues issues to be to resolved be resolved byresearch by research aimed aimed at systematically at systemati- callyminimizing minimizing the group the group velocity velocity dispersion dispersi andon optimizing and optimizing the device the device structure structure for comb for comboperation. operation. Dual-comb Dual-comb spectroscopy spectroscopy offers offers a combination a combination of high of high spectral spectral resolution resolution over overa broad a broad spectral spectral bandwidth bandwidth with with very very short shor acquisitiont acquisition time time on onthe the orderorder of millisec- millisec- onds [33,51]. A fast detector observes the multi-heterodyne beating of two combs with onds [33,51]. A fast detector observes the multi-heterodyne beating of two combs with slightly different teeth spacings (determined by the cavity lengths), from which the IR slightly different teeth spacings (determined by the cavity lengths), from which the IR laser spectrum is mapped into the RF frequency domain where it is more easily measured. laser spectrum is mapped into the RF frequency domain where it is more easily measured. The cavities of passively mode-locked ICL combs are subdivided into individually con- The cavities of passively mode-locked ICL combs are subdivided into individually con- tacted gain and saturable absorber (SA) sections, which are separated by a non-contacted tacted gain and saturable absorber (SA) sections, which are separated by a non-contacted gap. This represents a straightforward modification of the cavity geometries illustrated in gap. This represents a straightforward modification of the cavity geometries illustrated in Figures7 and8. Figures 7 and 8. Figure 12a illustrates that a fully-integrated on-chip dual-comb spectrometer can be Figure 12a illustrates that a fully-integrated on-chip dual-comb spectrometer can be realized by integrating two ICL frequency combs (in which the spacing may be adjusted realized by integrating two ICL frequency combs (in which the spacing may be adjusted by tuning the pump current in the gain section as well as reverse bias on the SA section) by tuning the pump current in the gain section as well as reverse bias on the SA section) on the same PIC with an extended passive sensing waveguide and an ICD with GHz on the same PIC with an extended passive sensing waveguide and an ICD with GHz cut- off frequency. The beam from one comb interrogates the sample gas before detection by the high-speed ICD. The other comb provides the local oscillator (LO) reference beam,

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cut-off frequency. The beam from one comb interrogates the sample gas before detection by the high-speed ICD. The other comb provides the local oscillator (LO) reference beam, whichwhich proceeds proceeds directly directly to to the the ICD ICD via via a a pa passivessive waveguide so so short short that, even without encapsulation,encapsulation, the the sample sample gas gas has has negligible negligible effect effect on onits power its power input input to the to detector. the detector. An RFAn spectrum RF spectrum analyzer analyzer then thenreceives receives the combined the combined signals. signals. In order In order to resolve to resolve the RF the het- RF erodyneheterodyne beating, beating, it is itfortunate is fortunate that that ICDs ICDs with with active active areas areas small small enough enough to minimize to minimize ca- pacitancecapacitance have have displayed displayed 3-dB 3-dB bandwidths bandwidths >> 1 >> GHz 1 GHz [31,33]. [31, 33Because]. Because each each stage stage of the of ICDthe ICDis thin is enough thin enough to make to makethe transit the transittime qu timeite short, quite a short,reduction a reduction in the RC in time the constant RC time isconstant sufficient is sufficientfor this purpose. for this purpose.Of course, Of laser course, stability laser stabilityalso plays also a critical playsa role critical in the role de- in tectionthe detection sensitivity sensitivity [33]. [33]. FigureFigure 12b12b illustrates thatthat thethe twotwo inputsinputs to to the the ICD ICD may may evanescently evanescently couple couple laterally later- allyfrom from opposite opposite ends, ends, although although both both inputs inputs may may alternatively alternatively combinecombine at a a Y-Junction, Y-Junction, whichwhich is is then then inputted inputted to to the the ICD. ICD. The The passive passive waveguide waveguide sections sections running running parallel parallel to the to the ICD should be long enough for high QE and terminated in a manner that minimizes ICD should be long enough for high QE and terminated in a manner that minimizes un- unwanted reflections back into the lasers. Because the lasers in the figure have both wanted reflections back into the lasers. Because the lasers in the figure have both end mir- end mirrors terminating at the same HR-coated facet, both can occupy the same end rors terminating at the same HR-coated facet, both can occupy the same end of the chip of the chip as the ICD while the passive sensing waveguide resides at the other end. as the ICD while the passive sensing waveguide resides at the other end. Therefore, both Therefore, both frequency comb ICLs can be mounted epitaxial side down while the frequency comb ICLs can be mounted epitaxial side down while the sensing waveguide sensing waveguide hangs over the edge of the mount to allow evanescent coupling of its hangs over the edge of the mount to allow evanescent coupling of its top surface to the top surface to the ambient sample gas (as in Figure9). It is also worth noting that this ambient sample gas (as in Figure 9). It is also worth noting that this cannot be accom- cannot be accomplished using DBRs to terminate the cavities at the output ends of the two plished using DBRs to terminate the cavities at the output ends of the two lasers, since the lasers, since the comb bandwidth would then be limited to the DBR stopband. comb bandwidth would then be limited to the DBR stopband.

Figure 12. (a) On-chip dual-comb spectrometer, in which both facets of both frequency comb ICLs terminate at the same Figure 12. (a) On-chip dual-comb spectrometer, in which both facets of both frequency comb ICLs terminate at the same HR-coated facet. Lateral output from the first comb couples to an extended passive sensing waveguide before input to the HR-coated facet. Lateral output from the first comb couples to an extended passive sensing waveguide before input to the ICD, while output from the local oscillator comb transfers directly to the same ICD; (b) detail showing both inputs cou- plingICD, whileevanescently output fromto opposite the local ends oscillator of the combICD. transfers directly to the same ICD; (b) detail showing both inputs coupling evanescently to opposite ends of the ICD. The on-chip dual-comb spectrometer may also incorporate a DFB ICL seed laser, The on-chip dual-comb spectrometer may also incorporate a DFB ICL seed laser, emitting at a single frequency near the center of the comb gain spectrum, to lock the fre- emitting at a single frequency near the center of the comb gain spectrum, to lock the quencies of the two low-noise combs. For example, it could reside between the two combs frequencies of the two low-noise combs. For example, it could reside between the two illustrated in Figure 12, with both ends of its curved cavity also terminating at the HR- combs illustrated in Figure 12, with both ends of its curved cavity also terminating at the coated facet. HR-coated facet.

6.6. Practical Practical Considerations Considerations InIn general, general, an an ICL ICL incorporated incorporated into into a a PIC PIC should should perform perform quite quite similarly similarly to to stand- stand- alonealone devices devices (e.g., (e.g., see see [18]) [18]) that that are are configur configureded and and processed processed similarly. similarly. Our Our simulations simulations indicateindicate that, that, even even when when the the layer layer design design is is modified modified to to make make the the bottom bottom SCL SCL thicker thicker and and thethe top top SCL SCL thinner, toto promotepromote efficient efficient out-coupling out-coupling to to a passivea passive waveguide, waveguide, the the effect effect on onlaser laser performance performance is is relatively relatively modest. modest. For For most most ofof thethe configurationsconfigurations discussed above, above, anan HR-coated HR-coated facet facet defines defines at at least least one one end end of of the the cavity cavity as as in in a a conventional conventional ICL. ICL. If If butt- butt- couplingcoupling to to a a passive passive waveguide waveguide defines defines the the other other end end and and no no additional elements elements are are integratedintegrated into into the cavity, thethe mirrormirror loss loss and and laser laser operation operation are are quite quite analogous analogous to to those those of ofstandard standard ICLs ICLs with with output output mirrors mirrors defined defined by uncoatedby uncoated or AR-coated or AR-coated facets. facets. If both If both ends ends are defined by HR-coated facets, as in Figure 8, the threshold current density and

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are defined by HR-coated facets, as in Figure8, the threshold current density and drive power are likely to be lower due to the lower mirror loss. ICLs with a sensing waveguide incorporated into the cavity will experience a trade-off between lower sensitivity when the sensing section is short, against higher loss and lower laser efficiency when it is lengthened. Similarly, the designs that couple light laterally to an output waveguide must trade greater isolation from potential external feedback when the out-coupling per pass is weak, against higher laser efficiency when it is stronger. It was mentioned above that the mode overlap, for coupling the beam propagating in a passive sensing waveguide to an ambient sample gas, is relatively weak. Therefore, it may be advantageous to structure the waveguide so as to provide more coupling than the simple top hats of Figure2. For example, a slot with sub-wavelength dimensions along the lateral (stronger TE coupling) [52] or vertical (stronger TM coupling) [53] waveguide axis can substantially enhance the modal overlap. A related approach is to pattern a subwavelength-scale grating along the longitudinal axis [54,55], which, besides enhancing the coupling, also slows the light propagation at frequencies near the band edge of the 1D photonic crystal. Of course, such structures can be challenging to process in III-V materials (a potential advantage of Si PICs), even at the longer mid-IR dimensions, and the additional loss due to scattering by sidewall nonuniformities may potentially outweigh the theoretical advantages [56]. Plasmonic nanostructures and photonic crystals can also enhance the coupling to ambient [57,58], with surface plasmon modes in a heavily doped semiconductor [59] possibly providing lower mid-IR absorption loss than a plasmonic structure patterned into a deposited metal. A photonic crystal silicon waveguide was recently reported to yield the sub-parts-per-million detection of ethanol at λ = 3.47 µm, although in that demonstration the ICL source and InSb detector resided off-chip [60]. It may also be possible to enhance the detection sensitivity by coating the top and sides of the waveguide with a sorbent that selectively interacts with and concentrates a given class of chemicals, which diffuse into the sorbent layer. This can increase the molecular concentration, and hence the detection sensitivity to a given chemical by orders of magnitude [61–63]. A further alternative is to incorporate a free space sensing area into the PIC, which provides full modal overlap with the sample gas. This approach has the obvious disadvan- tage of weak coupling into the detector due to diffraction following free space propagation over any appreciable path length. A possible solution is to incorporate a hollow-core waveguide that provides flow of the sample gas [64]. While single-mode propagation may be challenging to realize in practice, the hollow waveguide should nonetheless trap and direct the beam over some extended distance before losses substantially dissipate its intensity. Note that the PIC can spectroscopically probe a narrow analyte fingerprint absorption line in multiple ways. These include tuning the wavelength of a single-mode laser source (most commonly by current or temperature, although a broader range may be attainable with a sampled grating [65] or related configuration), tuning the resonance of a resonator sensing waveguide, such as a ring (by temperature), tuning the wavelength of an in- plane resonant cavity detector (by temperature), separating the spectral components of a broadband signal (e.g., using an AWG), integrating multiple sensors operating at different absorption and reference wavelengths on the same chip, or dual-comb spectroscopy with multi-heterodyne detection. The latter may ultimately offer the most attractive combination of broad spectral bandwidth, short acquisition time, and low drive power consumption. Note also that, when more than one of these methods provides a narrow or comb-like spectral response (e.g., a single mode laser combined with a ring resonator sensor and/or resonant cavity detector), the multiple resonance wavelengths must be matched by careful design and calibration, or independent tuning (probably with temperature) of the different components. We emphasize again that, while ICL-based PICs integrated on the native III-V substrate were assumed as the default in most of the preceding discussions and analyses, all of Sensors 2021, 21, 599 16 of 19

the same architectures may also be applied to QCLs for the coverage of spectral bands extending far beyond the mid-IR (i.e., LWIR out to THz). A primary difference, however, is that thermal management becomes much more critical. QCLs require much more input power just to reach threshold, whereas even an epitaxial-side-up mounted ICL can often operate in cw mode. Additionally, whereas integration on silicon rather than the native GaSb or InP substrate requires additional processing steps, it can offer more mature processing of gratings and high-resolution nanoscale structures, along with the potential to combine sensors operating in multiple spectral bands on the same chip [22–24].

7. Conclusions The preceding sections have presented a framework for constructing ICL-based PICs on the native GaSb substrate. Following the redistribution of the separate confinement layer thicknesses to make the bottom SCL thicker, the same MBE-grown wafer material becomes suitable for integrating interband cascade lasers, interband cascade detectors, and connecting passive waveguides on the same chip. For chemical sensing, the tops and sides of the passive waveguides can be exposed to ambient to provide evanescent coupling to a sample gas. Some configurations incorporate the sensing waveguide into the laser, which allows both ends of the cavity to be defined by HR-coated facets for low mirror loss and isolation from external feedback. Absorption by the analyte can then be detected by monitoring the laser’s compliance voltage, or by incorporating an ICD into the same cavity. We also described a laser configuration that extracts light by lateral evanescently coupling to a passive waveguide. This again allows both ends of the laser cavity to be defined by HR-coated facets and provides the incremental adjustment of the coupling efficiency per pass. Stronger coupling gives higher output power and wall-plug efficiency, whereas weaker coupling provides optimal laser stability when relatively modest power is required for sensing. It is well known that fluctuations associated with external optical feedback can be a major source of instability in conventional edge-emitting semiconductor lasers. A further advantage of the lateral power extraction is that both ends of the laser cavity can be terminated at the same HR-coated facet. This adds considerable flexibility to the PIC layout, which can, e.g., group one or more lasers at one end of the chip, which may be mounted epitaxial-side-down, while one or more sensing waveguides are exposed to ambient at the other end. Laser cavities with lateral extraction and termination on both ends by HR-coated facets may also operate at lower drive powers than has been possible up to now, since a very short cavity would no longer induce excessive mirror loss. We finally discussed configurations that integrate a laterally-out-coupled ICL, passive sensing waveguide, and ICD on the same PIC. The ICD’s sensitivity specific detectivity D* may be enhanced by incorporating it into a resonant cavity defined by DBRs. A particularly promising prospect is dual-comb spectroscopy on an ultra-compact chip, with broad and rapid spectral coverage, combined with low drive power. Assuming that mature designs and high-yield processing procedures can be devel- oped, chemical detection PICs will be suitable for mass-producing hundreds of devices on the same chip in a standard cleanroom. Reference [66] reviews the relatively advanced status of InP-based photonic integrated circuits operating at shorter wavelengths in the near IR, which can now combine numerous optical elements on a chip. Although GaSb-based PICs and GaSb-based device fabrication, in general, are much less mature [67], the simplest on-chip sensing configurations described above do not require methods especially more challenging than those already used routinely to process shortwave-IR and mid-IR lasers and detectors on GaSb. After patterning a large number of PICs on a die, individual sen- sors can be singulated to form a package that is both extremely compact and inexpensive. Lasers, detectors, and passive waveguides can also be integrated into more complex PICs that incorporate other components for enhanced capabilities, such as the multi-heterodyne detection of an off-chip signal [68]. While the relevant fabrication protocols have yet to be developed, they appear quite feasible as an extension of current capabilities. Sensors 2021, 21, 599 17 of 19

Since interband cascade photovoltaic devices have been demonstrated, there is even the potential for integrating a solar cell power source on the same PIC with the light source, passive sensing waveguide, and ICD, to provide an even more complete stand-alone chemical detection package. While such a power source is unlikely to provide enough current to reach the ICL lasing threshold, it may at least be sufficient to drive a broadband ICLED.

Author Contributions: J.R.M. co-invented the concepts, performed analysis, and drafted the manuscript; C.S.K. and M.K. co-invented the concepts and developed feasible processing pro- tocols; C.L.C.; C.D.M., and W.W.B. co-invented the concepts. I.V. co-invented the concepts and performed analysis and simulations. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the Office of Naval Research. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author, subject to approval by the Naval Research Laboratory. Acknowledgments: The authors acknowledge support from Kevin Leonard at the Office of Naval Research. Conflicts of Interest: The authors declare no conflict of interest.

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