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Abstract
We present an optical spectroscopic study of InGaAs/AlInAs active region of quantum cascade lasers grown by low pressure metal organic vapor phase epitaxy combined with subwavelength gratings fabricated by reactive ion etching. Fourier-transformed photoluminescence measurements were used to compare the emission properties of structures before and after processing the gratings. Our results demonstrate a significant increase of the photoluminescence intensity related to intersubband transitions in the mid-infrared, which is attributed to coupling with the grating modes via so called photonic Fano resonances. Our findings demonstrate a promising method for enhancing the emission in optoelectronic devices operating in a broad range of application-relevant infrared.
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1. Introduction
Numerous optical gas sensing applications employ extensively the spectral region of mid-infrared [1]. They range from industrial process control through environmental protection and pollution monitoring, tackling such issues as hydrocarbon detection - methane [2], propane [3], acetylene [4] or formaldehyde [5], up to medical diagnostics via analysis of exhaled air [6] in search of cancer markers [7]. Furthermore, the importance of monitoring gases like ammonia and dissolved CO2 in water [8], which are essential characteristics for preventing contamination, and preserving product quality, is also recognized by optical sensing applications in the food production industry [9]. Optical sensing systems for these applications comprise a laser source operating preferably in a single mode, such as diode laser (DL) [10], quantum cascade laser (QCL) [11], or interband cascade laser (ICL) [12], and a detector, ideally working at room temperature or cooled thermoelectrically (TE). Another approach for detecting multiple gases simultaneously uses dual-comb spectroscopy as it was already presented in laboratory conditions [13]. Generation of optical frequency combs (OFCs) which are essential for that method can be achieved through mode-locking [14], electro-optical modulation [15], or gain-switching [16] in semiconductor lasers. In case of mid infrared mode-locked lasers their fabrication was only recently proven possible [17]. A similar result can be obtained through an alternative - a vertical-cavity surface-emitting laser (VCSEL). However, mid-IR lasers based on type-I GaSb quantum wells are limited by the band discontinuity defining the maximal emission wavelength (∼3.7 µm [10,18–20]), which then combined with challenging growth process of DBRs optimized for that spectral range further slowed down VCSEL development in the mid-IR. Furthermore, promising preliminary results were reported, regarding the interband cascade vertical-cavity surface-emitting lasers (ICVCSELs) emitting at 3.4 µm [21] and 4 µm [22] based on the devices mentioned earlier [23] utilizing type-II quantum wells [24,25].
All these approaches have their advantages and disadvantages regarding operating temperature, possibility of single mode operation, spectral tunability, power consumption, continuous or pulsed operation etc., which influence the performance of the final device. In this paper we focus on an alternative way for the emission enhancement from mid infrared laser structures by using a nanophotonic structure in the form of a one-dimensional subwavelength grating (SG). In such configuration the light confinement occurs due to Fano resonance manifesting in an abrupt transition from total reflection to total transmission in the spectrum of the light interacting with a structure. The resonance arises due to the interference between discrete modes of the grating and a continuum of free-space modes [26]. When the structure is ideally periodic, with the optical length of the grating period smaller than the wavelength in the surroundings of the grating, Fano resonance can evolve to nonradiative state of infinite quality factor called bound-state in the continuum (BIC) [27]. However, when the periodicity of the grating is disturbed by its finite dimensions or irregularities, which is the case with real-world realizations, the BIC transforms into a so-called quasi-BIC (qBIC) [28] of finite but still high quality factor.
The structures designed for this study consisted of electrically unpolarized (without application of the external electric field) quantum cascade active region, which was further periodically patterned in the form of one-dimensional subwavelength grating (see Fig. 1(a)). We experimentally validated the influence of Fano resonance on the enhancement of spontaneous emission in the mid-infrared range through a series of photoluminescence (PL) experiments. Until now, there was only one report, indicating a similar approach, however based on photonic crystal membrane and so called “Fano cavities”, and designed for the narrowing of the laser line within InGaAsP/InAlGaAs quantum well emitting at near infrared (1.55 µm) [29].
Fig. 1. (Panel a) SEM images of QCL samples with physical parameters of the subwavelength grating - L - period, a – ridge thickness, H – ridge height. (Panel b) SEM for sample B. (Panel c) SEM for sample C. (Panel d) Dimensions of the ridge for sample C.
2. Materials and methods
The samples were grown by metal-organic chemical vapor deposition (MOCVD) using AIXTRON 3 × 2 CCS system. The TMIn, TMGa, TMAl were used as the sources of indium, gallium and aluminum, whereas PH3 and AsH3 were the sources of phosphorus and arsenic, respectively. Deposition of epilayers was in-situ controlled by the LayTec EpiTT device capable of two channels reflectometry and true temperature pyrometry measurements of the samples. The layer sequence of one period of the QCL starting from the injection barrier is (expressed in nanometers): 4.0, 1.9, 0.7, 5.8, 0.9, 5.7, 0.9, 5.0, 2.2, 3.4, 1.4, 3.3, 1.3, 3.2, 1.5, 3.1, 1.9, 3.0, 2.3, 2.9, 2.5, 2.9. The AlInAs layers are denoted in bold. The active region consists of 20 segments finished by 100 nm of InP. The structure layout was based on a QCL design dedicated for the emission at 9 µm under operating conditions [30]. Growth temperature of the active region was 645°C and the growth pressure was equal to 100 mbar for the whole structure. Growth rate was kept at the level of 0.3 nm/s. More details (e.g., HRXRD spectra) can be found in our previous publication [31]. Sample A is the reference laser structure without additional treatment. Samples B and C are additionally post-growth processed by combining Electron Beam Lithography (EBL) and Inductively Coupled Plasma Etching (ICP-RIE) to fabricate the SGs with dimensions according calculated designs. For that process, the samples were covered with PMMA 950 K 1-µm E-beam resist. Structures were made by combining the EBL, where SiO2 (250-300 nm) was used as hard etching mask, and reactive ion etching. A 1:3 mixture of Cl2:Ar, 175 W RF power, 350 W ICP and 100°C temperature presented the best morphology – the most vertical and smooth surfaces. Finally, the remaining SiO2 mask was removed using 1% HF solution. Obtained etching depths (700-710 nm) and ridge separations (400-500 nm) were summarized in Table 1. Examples of SEM images illustrating the morphology of the obtained grating is shown in Fig. 1. Remaining approximate parameters are shown in Table 1.
Table 1. Samples descriptions
In order to measure Fourier-transformed PL (FTPL) spectra, a Bruker Vertex 80 v spectrometer was used. It operated in step-scan mode, with an external chamber for measurements with an additional modulated pump/excitation beam [32]. The signal was gathered by liquid-nitrogen cooled InSb photodiode detector while phase sensitive detection of the optical response was performed using a lock-in amplifier. The excitation beam was provided by the 660 nm line of a semiconductor laser diode, which was modulated at a frequency of 275 Hz using a chopper. Linear-polarization-resolved components of luminescence were selected by using a CaF2 linear polarizer [32–36]. A scheme of the experiment is shown in Fig. 2. Time-dependent measurements were realized within a Light Conversion pump-probe system equipped with a femtosecond pulsed laser (∼200 fs pulses) with the fundamental line of 1030 nm. To achieve the probe line tuning in wide spectral range, an optical parametric amplifier with the difference frequency generator is used, utilizing birefringent crystals, that allows to mix signal and idler beams within 1-16 µm spectral range. A pump beam is provided by a second harmonic generation process of the fundamental line resulting in a 515 nm wavelength. The energy dispersion is provided by three gratings with 2, 4 and 5.2 µm “blaze wavelength” while detection is achieved by liquid-nitrogen cooled MCT and InSb detectors. Custom-designed delay line allows to investigate time-correlated signals up to 15 ns.
Regarding the calculations, in the part of QCL the energy dispersions and the corresponding wave functions for the electrons and holes states are derived from the Schrödinger equation solved within nextnano software [37] using effective mass approximation and default material parameters from their internal database.
A crucial aspect of the modeling is the analysis of electronic eigenstates and reflection spectra of the SG. We employ the Plane-Wave Admittance Method (PWAM) [36] to numerically solve the Maxwell equations. In this method an analytical solution in z direction (see coordinate system in Fig. 1(a)) is combined with Fourier series expansion of the electromagnetic field along x direction. Numerically considered structure is periodic along the y axis and consists of infinitely repeated copies of the rectangular (in the (y, z) plane) computational window of width L in the y direction in which the structure is defined. The computed electromagnetic wave along the y direction is in the form given by Bloch’s theorem. The eigenstates are calculated in the form of complex wavelength (λr + iλi where λr and λi represent, real and imaginary resonant wavelength, respectively) Previous results of similar numerical calculations have confirmed correctness of the above simulations as used for conventional VCSELs [38] and also for an analysis of an operation of sub-wavelength gratings [39], which attests legitimacy of a method selection used for the presented designs of SG. The validity of the method has been previously confirmed by numerical calculations confronted with experiments for vertical-cavity surface-emitting lasers with monolithic high contrast grating (MHCG VCSELs) [39] and for the analysis of sub-wavelength gratings [40]. This validation supports the suitability of the chosen method for the analysis of SG.
At the Fano resonance wavelength (λ), strong electromagnetic field confinement within the structures is observed. The wavelength distance between high and low reflection (Δλ) from the SG defines quality (Q) factor of the resonator according to the formula:
that can be transformed to the formula combining real and imaginary wavelength determined by numerical model:The occurrence of enhanced light-matter interactions in nanophotonic structures is beneficial for applications in e.g., sensors [41], nanolasers [42], photoswitchers [43], and slow-light devices [44]. In our approach, the SG has been fabricated for maximum reflectivity utilizing Fano resonance for enhancement of the emitted photoluminescent signals form the investigated samples.
3. Results
Figure 3 shows the results of optical characterization of Sample A and numerical modelling for a superlattice (SL) based on the investigated QCL architecture. Panel a) presents the measured FTPL at low temperature (10 K) with a clear, narrow emission signal at ∼1.4 µm and two broader and much weaker (ten times) lines at 2.3 µm and 2.8 µm. Based on our band-structure calculations the first peak is related to the fundamental interband transition occurring between first electron and first heavy hole levels (subbands), whereas the other two can be connected to certain intersubband transitions occurring within the conduction band of the QCL active region MQW. As the photoluminescence signal in our setup is collected from the surface of the sample, an intensity difference between the mentioned peaks is expected. The reason is related to interband transitions to be s-polarized (electron-hole transition), whereas the intersubband transitions to be naturally p-polarized (electron-electron transition) and are typically observed from the edge of the sample. Observation of some intersubband transition intensity also in case of the detection from the surface is possible due to the light scattering at the interfaces between SL layers, which distorts its’ polarization character and enables propagation of light perpendicular to the surface. In our case, magnitude of this signal is ten times weaker in respect to regular interband transition (see Fig. 1(a)). Such “intermediate” observation of the intersubband weak transitions was already reported for QCLs of other materials systems [45].
Fig. 3. (Panel a) FT photoluminescence spectrum for the sample A. (Panel b) Time-evolution of normalized difference absorption related to interband transitions (black curve) and intersubband transition (blue curve). (Panel c) Energy ladder calculation within the conduction band with wavefunctions for the selected states.
A significant difference is also expected in carrier lifetimes between the inter- and intersub-band transitions. There are at least two reasons for that – a limitation by nonradiative Frohlich interactions [46] with LO phonons and the difference in wave function overlap between the states, which results in very short lifetimes of ∼1ps for intersubband transitions, and one order of magnitude longer in case of interband ones (combination of electron and holes [47]. To prove the difference between the characters of the measured PL signals and the related different timescales of the optical transitions (intersubband vs interband), we performed pump-probe measurements [48]. Figure 3(b) shows the time evolution of differential absorption for the interband transition (black curve) and intersubband transition (blue curve) respectively. The evolution of the signal is fitted using the equation:
enabling the determination of the characteristic decay constant, τ. The analysis of the signals resulted in obtaining the characteristic decay times of ∼50 ps and ∼3 ps for the respective transitions, which is convergent with the interpretation as described earlier. This experimental analysis, as a supplementary component to the calculations, supports our interpretation regarding the nature of the signals measured at 2.9 µm. This confirmation of the transitions’ character was important because the original design is an active region of the QCL device dedicated for 9 µm. Since it was not held under conventional operating conditions (external electric field was not applied), the confined energy structure and wave function overlap are different when the electric-field-induced quantum confined Stark effect is absent. Calculated energy states ladder without the external electric field is presented in Fig. 3(c) The transitions with significant wave function overlap were matching the energies obtained in the PL experiment and were related to the thirteenth to first and the thirteen to second electron states, for the signals at 2.8 and 2.3 µm, respectively. Nevertheless, it is important to underline that the levels involved in this luminescent process are different than those related to lasing process where through the adjustment of the external field, we can manipulate the energy level spacing and simultaneously optimize the wave function overlap for different energy-separated levels, ultimately enabling emission at 9 µm. Regarding signal at 1.4 µm its interband character was confirmed in our previous work [28].Figure 4 shows the design of sample B and C. Figure 4(a) illustrates schematics of three periods implemented in the QC structure with geometrical parameters defined graphically. The periodic structure is designed to induce a Fano resonance, which requires the averaged refractive index of the periodic structure to be greater than that of the surroundings. The averaged refractive index of the active region and the InP layer below the active region are 3.38 and 3.10, respectively. The implementation of the grating in the active region lowers the averaged refractive index of the active region, therefore the air grooves of the grating must have a carefully designed depth and width to enable a sufficiently high Q-factor of the Fano resonance. In the optical simulations a broad range of grating parameters were scanned to search for local maxima of Q-factor. H was varied from 0 to 1 µm that corresponds to the collective thickness of active region and the cap layer, L from 1.3 to 2.9 µm that represents dual-mode regime enabling Fano resonances [49] and L-a varied from 100 to 500 nm that is width of the air slit, and the minimal value is limited by resolution of EBL technique. Table 2 collects the values of the grating parameters corresponding to local maxima of Q-factors present in (H, L, a) domain indicated as designs A, B and C. Infinite Q-factor of design C corresponds to symmetry- protected (SP) photonic Bound states in the Continuum (BIC) [27]. SP photonic BICs are robust with respect to variation of the geometrical parameters of the grating as long as no diffraction orders are emitted towards substrate, therefore parameters in Table 2 corresponding to design III are exemplary. Designs I and II relate to accidental destructive interference occurring in the case of first and first and second diffraction orders emitted towards substrate (designs II and I, respectively). Since design I is related to two leakage channels enabled by diffraction orders and design II to only one, therefore design II reaches larger Q-factor with respect to design I different by two orders of magnitude in this case.
Fig. 4. Schematics of periodic configuration, composed of lamellar grating with QC active region embedded partially in the grating. The geometrical parameters of the grating and the coordinate system are indicated. (b) Light intensity distribution in three stripes of infinite periodic configuration for the mode that dispersion is illustrated in (c). (c) Quality factor of the mode induced by Fano resonance expressed by colors in the domain of the wavelength and width of the stripes. The period (L) and depth of the etching (H) are indicated in the figure. (d) Measured reflectance spectra for reference InP sample (black curve) and for Sample C (red curve)
Table 2. Three different designs of subwavelength gratings
In the experimental analysis that follows design I is investigated as its precise fabrication is enabled by grating parameters being significantly more accessible by combination of EBL and ICP-RIE. Figure 4(b) depicts light intensity distribution in logarithmic scale of colors within three stripes of infinite design I grating. Light is distinctly leaking from the grating towards substrate, however strong buildup of the light in the active region is observed. Figure 4(c) illustrates Q-factor of the mode expressed by colors in design I as a function of wavelength and width of the stripes. In Fig. 4 panel (d) the Fourier Transformed reflectivity spectra for reference InP sample (black curve) and Sample C (red curve), were shown. Reflectivity measurements exhibit fluctuations (Sample C) that indicate the presence of Fano resonance. However, it is important to note that the observed characteristics differ from the theoretically predicted reflection characteristics associated with Fano resonance. The reason behind that is the difficulty of reflectivity measurements, namely the small size of the samples comparable to spot of the probing light and the angle of the incident light. Q-factor calculations are performed for incident light perpendicular to the surface (to obtain highest possible resonances) whereas the measurements were performed for incident light angle around 15 degrees. Nevertheless, observation of this feature proves that the Fano formalism was involved for samples with fabricated subwavelength grating.
Figure 5 shows the normalized FTPL spectra for all three investigated samples as listed in Table 1. In comparison to the sample A (a reference, without the grating), the enhancement of the signal intensity regarding intersubband emission can be observed. In case of sample B the signal is magnified seven times and for Sample C even fifteen times. All the samples were excited from the substrate side to provide the same laser excitation conditions for samples with and without the gratings. We explain the amplification of the signal by the enhancement of the spontaneous emission due to coupling with discrete grating modes called photonic Fano resonance. Due to some imperfections of the dimensions in comparison to the nominal (designed) height and period (see Table 1). small wavelength shift for the enhanced light were observed, nevertheless being still in a quite good agreement with calculation presented in Fig. 4(c).
Fig. 5. The FT photoluminescence spectra (10 K) for sample A (black solid curve), sample B (violet dotted curve) and sample C (blue dashed curve). (Inset) Light polarization dependent FTPL spectra for sample A (black) and sample C (blue).
Additionally, the polarization properties of the emission were investigated. In the inset of Fig. 5 we have shown the comparison of the polarized spectra collected for a range of polarizer angles (from 0° to 90°) for sample A (black curves) and for sample C (blue curves). To conclude, the designed gratings not only enhance the signal from the active region but also influence the polarization properties of the intersubband emission, as actually one would expect. The presented results show 10% degree of linear polarization for sample A (without the grating) and 30% for sample C (with the grating). The effect is not very strong but definitely observable, and it should be mentioned that the polarization aspect was not the main purpose of our design nor its optimization. This phenomenon, however was also studied elsewhere [50]. These preliminary results, however, are a promising start point for further studies of polarization properties of the intersubband emission modified by the SG parameters, in terms of their potential in optoelectronic applications where polarized light needs to be employed.
Figure 6 shows a set of FTPL spectra measured in function of temperature (10-300 K) for samples C and A (inset). For sample A, a broad band related to intersubband transition is observed in the spectra for the whole range of temperatures, while for sample C the same transition is additionally modified by the enhancement caused by the existence of the processed grating. The red and blue arrows show the wavelength shifts of the center of the enhancement and intersubband transition, respectively. The temperature shift of the intersubband transition is smaller than for interband transitions as only slight modification of the confinement potential in the conduction band contributes to the effect [51]. On the other hand, the position of the emission enhancement center, defined by the photonic mode, almost does not shift with temperature (around 30 nm acc. to our calculation) as one would expect due to only very small changes of the material refractive index or the ridges volume expansion.
4. Conclusions
In this study, we have focused on utilizing a nanophotonic structure in a form of subwavelength grating fabricated on the surface of a QCL-like sample, for the enhancement of weak intersubband transitions in active region. In order to conduct these investigations, the QCL-like samples were grown, and their intersubband transitions could be detected in FTPL measured from the sample surface. The SG stripes have been designed and then processed precisely onto the multi-layered structure. It has been demonstrated that the nanostructure with the ridge height of ∼700 nm and ridges separation of ∼450 nm can enhance the PL signal (at 2.9 µm) by nearly 1500%. Our calculations showed the possibility to achieve the Q factors up to 105 for different designs, but for such short emission wavelength, the stripe spacing is technologically challenging (∼200 nm) which may introduce sidewall roughness, effectively increasing the optical losses in the final device. Other means of optical losses like free-carrier absorption and limited lifetimes are included in Q factor calculation. On the other hand, considering future perspectives like emitters operating at further infrared, manufacturing high quality resonator structures could be possible due to the less technologically demanding etching. In our work the enhancement of the signal was presented by using weak intersubband transition, nevertheless application of the grating in case of stronger signals like in case of working devices might be also beneficial. To conclude, our findings pave the way towards emission modification of the mid-IR lasers via the subwavelength grating engineering, and open new possible applications of SG based emitters and point to new opportunities event beyond the infrared photonics.
Funding
Narodowe Centrum Nauki (2019/33/B/ST7/02591, 2021/43/B/ST3/02473).
Acknowledgments
This work was supported by the Polish National Science Centre within projects of OPUS calls no. 2019/33/B/ST7/02591 and 2021/43/B/ST3/02473 and the State of Bavaria
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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