hv
photonics
Article
Mid-Infrared Photonic-Crystal Surface-Emitting Lasers with InGaAs/GaAsSb ‘W’-Type Quantum Wells Grown on InP Substrate Zong-Lin Li , Yuan-Chi Kang, Gray Lin *
and Chien-Ping Lee
Department of Electronics Engineering and Institute of Electronics, National Chiao Tung University, Hsinchu City 30010, Taiwan;
[email protected] (Z.-L.L.);
[email protected] (Y.-C.K.);
[email protected] (C.-P.L.) * Correspondence:
[email protected]; Tel.: +886-3-513-1289 Received: 10 September 2018; Accepted: 30 September 2018; Published: 2 October 2018
Abstract: InP-based InGaAs/GaAsSb ‘W’-type quantum well (QW) photonic-crystal (PC) surfaceemitting lasers (SELs) of 2.2 µm wavelength range are fabricated and room-temperature lasing emissions by optical pumping are demonstrated for the first time. Photonic-crystal surface-emitting laser (PCSEL) devices are investigated in terms of PC parameters of etch depth, lattice period, and filling factor. The lasing emissions cover wavelengths from 2182 nm to 2253 nm. The temperaturedependent lasing characteristics are also studied in terms of lattice period. All PCSELs show consistent lasing wavelength shift against temperature at a rate of 0.17 nm/K. The characteristic temperatures of PCSELs are extracted and discussed with respect to wavelength detuning between QW gain peak and PC cavity resonance. Keywords: photonic crystals; surface-emitting lasers; mid-infrared lasers; quantum wells
1. Introduction Semiconductor lasers emitting a mid-infrared (MIR) range of 2–3 µm find promising applications in gas sensing and detection based on laser spectroscopy. Two common gain media for these lasers are type-I quantum wells (QWs) grown on GaSb substrates as well as type-II QWs grown on InP substrates, excluding cascaded configurations of interband or intraband transitions [1–5]. However, GaSb-based type-I QW lasers, which consist of complex quaternary or quinary compounds, are subjected to critical growth parameters and low thermal stability of crystalline [3]. InP-based type-II QW lasers, on the other hand, benefit from stable growth of ternary alloys and reliable process technology. Since optical gain may diminish because of a spatially indirect recombination in type-II heterostructures, ‘M’-type and ‘W’-type QWs are devised to enhance electron-hole wavefunction overlap [4,5]. For the application in tunable diode laser absorption spectroscopy (TDLAS), MIR lasers have to meet the key requirement of single spectral mode. Conventional edge-emitting lasers (EELs), which exhibit mutli-longitudinal-mode emissions, combine external-cavity grating to achieve single-mode operation. GaSb-based external-cavity lasers (ECLs) operating around 2.13 µm were one of the examples [6]. The standard approach toward single-mode emissions is the introduction of Bragg gratings or one-dimensional (1D) photonic-crystal (PC) gratings beside or atop laser waveguides to yield distributed feedback (DFB) lasers. GaSb-based DFB lasers covering the wavelength range of 1.8–3.0 µm were therefore demonstrated with lateral metal grating [2]. To facilitate surface emission, advanced vertical-cavity surface-emitting lasers (VCSELs) exhibit single-mode emissions by monolithic incorporation of distributed Bragg reflectors (DBRs) or 1D-PC reflectors. For example, MIR VCSELs based on GaSb and InP substrates demonstrated a single-mode operation with a side-mode suppression ratio (SMSR) over 25 dB at a wavelength range of 2.5–2.6 µm [7,8]. Photonics 2018, 5, 32; doi:10.3390/photonics5040032
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MIR VCSELs based on GaSb and InP substrates demonstrated a single-mode operation with a Photonics 2018, 5, 32 2 of 7 side-mode suppression ratio (SMSR) over 25 dB at a wavelength range of 2.5–2.6 μm [7,8]. Recently, novel photonic-crystal surface-emitting lasers (PCSELs) draw increasing attention novel photonic-crystal lasers draw divergence increasing attention becauseRecently, of their single-mode operation, surface-emitting high optical output, and(PCSELs) narrow beam [9–12]. because of their single-mode high optical output, beamfrom divergence [9–12]. Based on the band-edge effect operation, of two-dimensional (2D) PCs, and lightnarrow emissions gain media Based on the band-edge effect of two-dimensional PCs, light where emissions from gain media strongly strongly couple with PC structure to construct 2D(2D) cavity mode in-plane multi-directional couple with PC structure construct 2D cavity modehave where in-plane multi-directional distributed feedback (DFB) to diffracts vertically. PCSELs advantages over VCSELs in distributed terms of feedback (DFB) diffracts vertically. PCSELs have advantages over VCSELs in terms of epitaxial epitaxial complexity and thickness. These advantages become more obvious in immature material complexity and thickness. These advantages A become more obvious in immature material systems systems and very long emission wavelengths. clear example is GaSb-based PCSELs of 2.3-μm and very long emission wavelengths. A clear example is GaSb-based 2.3-µm wavelength wavelength range [10]. Although electrical injection of MIR PCSELs isPCSELs not yetofrealized, it can be range [10]. electrical demonstration injection of MIR is not yet realized, it can be expected soon expected soonAlthough with the dawning ofPCSELs electrically injected near-infrared (NIR) PCSELs with the dawning electrically near-infrared (NIR) PCSELsat[11,12]. In this [11,12]. In this work,demonstration we report the of first InP-basedinjected MIR PCSELs by optical pumping and above work, we report the first InP-based MIR PCSELs by optical pumping and above room-temperature room-temperature (RT). Gain media of InGaAs/GaAsSb ‘W’-type QWsat exhibit emission wavelength range 2.2 μm. Moreover, lasing characteristics of MIR PCSELs of similar wavelength range are (RT).ofGain media of InGaAs/GaAsSb ‘W’-type QWs exhibit emission wavelength range of 2.2 µm. Moreover, characteristics of MIRmaterial PCSELssystems of similar wavelength range are compared for both compared forlasing both InPand GaSb-based [13]. InP- and GaSb-based material systems [13]. 2. Materials and Methods 2. Materials and Methods Our investigated sample was grown on S-doped (001) InP substrate by a Veeco GEN II solid investigated grown onlayer S-doped (001) InP substrate a Veeco GENand II solid source source Our molecular beam sample epitaxywas system. The structure (exclusive ofby active region) growth molecularwere beam epitaxy system. The layer structuredescribed (exclusiveinofRef. active and growth conditions conditions exactly the same as that previously [5].region) The schematic structure is were exactly the 1a, same as that Ref. [5]. The schematic structure is shown in shown in Figure and bandpreviously diagram described of active in region is shown in Figure 1b. Quaternary Figure 1a,0.24 and active region is shown in Figure Quaternary was 0.52 (AlGa) 0.24 As In0.52 (AlGa) As band was diagram used asof the separate-confinement layer1b.(SCL). The In active region was used as the separate-confinement activeof region composed 6-period ‘W’-type composed of 6-period ‘W’-type QWs.layer Each(SCL). periodThe consisted GaAswas 0.7Sb0.3 /In0.72Ga0.28of As/GaAs 0.3Sb 0.7/ QWs. Each period consisted of GaAs Sb /In Ga As/GaAs Sb /In Ga As/GaAs Sb0.3 0.7 0.3 of 5/3/2/3/5 0.72 0.28 nm. The 0.3 0.7 layer 0.72 of0.28 In0.72Ga0.28As/GaAs0.7Sb0.3 layers in sequence central GaAs0.3Sb0.7 0.7 was layers in sequence of 5/3/2/3/5 nm. The central layer of GaAs Sb was hole well, and inner 0.3 while 0.7 outermost GaAs0.7Sb0.3 hole well, and inner two layers of In0.72Ga0.28As were electron wells, two were layersspacers. of In0.72 Ga0.28 As were electron wells, while outermost GaAs0.7 Sb0.3 layers were spacers. layers
(a)
(b)
Figure 1. (a) Schematic structure grown sample and band diagram active region, which Figure 1. (a) Schematic structure of of thethe grown sample and (b)(b) band diagram of of active region, which was composed ‘W’-type quantum wells (QWs). was composed of of ‘W’-type quantum wells (QWs).
Before fabrication, contact layer and part cladding layer were removed Before PCPC fabrication, toptop contact layer and part of of toptop cladding layer were removed byby thethe chemical mixture of sulfuric acid, hydrogen peroxide, and de-ionized water. The cladding thickness chemical mixture of sulfuric acid, hydrogen peroxide, and de-ionized water. The cladding thickness remained was about Square-lattice for transverse electric polarization designed remained was about 600600 nm.nm. Square-lattice PCsPCs for transverse electric polarization werewere designed to to operate around the Γ band-edge. We used e-beam lithography and inductively coupled plasma operate around the Γ band-edge. We used e-beam lithography and inductively coupled plasma (ICP) etchertotopattern patterncircular-air-hole circular-air-hole PCs deposited as aashard mask (ICP) etcher PCs on on dielectric dielectricSi3N4, Si3N4,which whichwas was deposited a hard in thickness of 150ofnm150 by nm plasma chemical vapor deposition. Again, 2D-PC structure was mask in thickness by enhanced plasma enhanced chemical vapor deposition. Again, 2D-PC transferred into semiconductor layers by ICP. To estimate the etch depth of the PC holes, 1D waveguide structure was transferred into semiconductor layers by ICP. To estimate the etch depth of the PC simulation was performed to obtain fundamental in themode vertical direction ofinPCSEL. holes, 1D waveguide simulation wasaperformed to mode obtaindistribution a fundamental distribution the At an etch depth around 500 nm, we have an intensity overlap within the PC region of over 5%. vertical direction of PCSEL. At an etch depth around 500 nm, we have an intensity overlap within patterned series of lattice periods (p), tested a few etch depths (d), and varied the air-hole the PC We region of overa5%. filling factors (FF, which is defined as air-hole area divided by squared period). The fabricated devices, each with a size of 200 × 200 µm2 , were temperature controlled by a thermoelectric cooler and optically
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We patterned a series of lattice periods (p), tested a few etch depths (d), and varied the air-hole filling (FF, which is defined as air-hole area divided by squared period). The fabricated Photonicsfactors 2018, 5, 32 3 of 7 devices, each with a size of 200 × 200 μm2, were temperature controlled by a thermoelectric cooler and optically pumped by a 1064-nm pulsed fiber laser. Pulsed optical pumping was operated in a pumped 1064-nm pulsed fiber pumping was operated in awith width 50 ns width ofby 50a ns and repetition ratelaser. of 5Pulsed KHz. optical The emitted light was collected a of grating and repetition rate of 5 KHz. The emitted light was collected with a grating monochromator and an monochromator and an InGaAsSb detector. Resolution as low as 0.05 nm was used in measuring InGaAsSb detector. as low as 0.05 nm in measuring lasingThree spectra. Table 1 lists lasing spectra. TableResolution 1 lists six PCSEL devices as was wellused as their PC parameters. lattice periods six PCSEL devices as well as their PC parameters. Three lattice periods of 680, 690, and 700 nm are of 680, 690, and 700 nm are designated as devices A, B, and C, respectively. The lattice periods and designated as devices A, B, and C, respectively. The lattice periods and filling factors were as-designed filling factors were as-designed values, while depths of PC holes were characterized by scanning values, depths of PC holes were scanning electron microscopy (SEM). Figureof2 electronwhile microscopy (SEM). Figure 2 characterized shows the topbyview and cross-sectional view SEM images shows the top view and cross-sectional view SEM images of one MIR PCSEL. one MIR PCSEL.
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(b)
Figure 2. (a) Top Top view view and and (b) (b) cross-sectional cross-sectional view view scanning scanning electron microscope (SEM) images of a mid-infrared (MIR) PCSEL. Table 1. Photonic-crystal surface-emitting laser (PCSEL) devices as well as photonic-crystal Table 1. Photonic-crystal surface-emitting laser (PCSEL) devices as well as photonic-crystal (PC) (PC) parameters. parameters. Device No. Period(p (pin in nm) nm) PC PCDepth Depth(d (d in in nm) nm) Filling FillingFactor Factor (FF) (FF) Device No. Period A0 680 460 0.1 A0 680 460 0.1 A1 680 586 0.1 A1 680 586 0.1 A2 680 586 0.05 A2 680 586 0.05 A3 680 586 0.15 A3 680 586 0.15 B1 690 586 0.1 C1 700 586 0.1 B1 690 586 0.1 C1 700 586 0.1 3. Results and Discussions 3. Results and Discussions Figure 3 shows the RT photo-luminescence (PL) spectrum of the investigated sample pumped by Figure 3 shows RT photo-luminescence (PL) the investigated sample a 532-nm diode laser.the Three arrows are marked in thespectrum figure forofafterward discussion. Thepumped spectral by a 532-nm diode laser. arrows are (from marked in the fornm), afterward discussion. The width at 98% intensity was Three as large as 27 nm 2189.5 nm figure to 2216.5 while the full width at spectral width at 98% intensity was as as 27 nm 2189.5 2216.5 nm), while the full half maximum (FWHM) was about 135large nm (from 2125(from nm to 2260 nm nm).toThe gain peak of ‘W’-type width at half maximum (FWHM) was about nm to 2260 nm). The gainintensity. peak of QW structure (λQW ) was determined to be 2203135 nmnm by (from taking 2125 the median wavelength of 98% ‘W’-type (λQW) was determined to be 2203 nm [13], by taking the median wavelength of As regardsQW the structure RT-PL of GaSb-based type-I QW structure in Ref. the spectral width at 98% intensity 98% 10 intensity. As regards the RT-PL of GaSb-based type-I QWfeature structure in Ref.is[13], the spectral was nm whereas the FWHM was about 121 nm. The flat-top in RT-PL characteristic of width at 98% intensity was 10 nm whereas the FWHM was about 121 nm. The flat-top feature in InP-based ‘W’-type QWs. RT-PL characteristic of InP-based QWs. Weisselected six PCSEL devices‘W’-type with corresponding parameters listed in Table 1 and analyzed their device characteristics of lasing wavelengths and threshold current densities. Slope efficiencies were not discussed as they may suffer from large deviation among measurements. These PCSELs are investigated in terms of lattice periods (A1, B1 and C1), etch depths (A0 and A1), and filling factors (A1, A2 and A3). Figure 4 shows the lasing characteristics of light-in versus light-out (L-L) and the normalized emission spectra for six PCSEL devices at 20 ◦ C. As shown in Figure 4b,c, lasing wavelengths of PCSEL devices are a function of lattice period, etch depth, and FF.
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Figure 3. RT-PL spectrum of InP-based ‘W’-type QW sample under investigation.
We selected six PCSEL devices with corresponding parameters listed in Table 1 and analyzed their device characteristics of lasing wavelengths and threshold current densities. Slope efficiencies were not discussed as they may suffer from large deviation among measurements. These PCSELs are investigated in terms of lattice periods (A1, B1 and C1), etch depths (A0 and A1), and filling factors (A1, A2 and A3). Figure 4 shows the lasing characteristics of light-in versus light-out (L-L) and the normalized emission spectra for six PCSEL devices at 20 °C. As shown in Figure 4b,c, lasing wavelengths of PCSEL devices are a function of lattice period, etch depth, and FF. Figure 3. RT-PL spectrum of InP-based ‘W’-type QW sample under investigation. Figure 3. RT-PL spectrum of InP-based ‘W’-type QW sample under investigation.
We selected six PCSEL devices with corresponding parameters listed in Table 1 and analyzed their device characteristics of lasing wavelengths and threshold current densities. Slope efficiencies were not discussed as they may suffer from large deviation among measurements. These PCSELs are investigated in terms of lattice periods (A1, B1 and C1), etch depths (A0 and A1), and filling factors (A1, A2 and A3). Figure 4 shows the lasing characteristics of light-in versus light-out (L-L) and the normalized emission spectra for six PCSEL devices at 20 °C. As shown in Figure 4b,c, lasing wavelengths of PCSEL devices are a function of lattice period, etch depth, and FF. (a)
(a) (b)
(c)
Figure4.4.Lasing Lasingcharacteristics characteristicsofofPCSELs PCSELsatat20 20◦°C (a)L-L L-Lcurves curvesofofsix sixdevices, devices,(b) (b)lasing lasingspectra spectraofof Figure C. .(a) deviceA0, A0,A1, A1,A2 A2and andA3, A3,(c) (c)lasing lasingspectra spectraofofdevice deviceB1 B1and andC1. C1. device
The of lattice period is revealed by device B1, and with respective wavelengths Theeffect effect of lattice period is revealed by A1, device A1,C1B1, and C1 withlasing respective lasing ofwavelengths 2188.7, 2221.1, and 2253.1 nm. The wavelength shift is proportional to period change with factor of of 2188.7, 2221.1, and 2253.1 nm. The wavelength shift is proportional to period 3.2; i.e., 3.2 nmfactor red-shifts wavelength per nm increase in period. linewidths areAll around change with of 3.2;ini.e., 3.2 nm red-shifts in wavelength per All nmlasing increase in period. lasing 0.2 nm, which much smaller those of edgesmaller emitting lasers [5].ofThe nearly single-wavelength linewidths areisaround 0.2 nm,than which is much than those edge emitting lasers [5]. The lasing are attributed to wavelength mechanism of multi-directional DFB effect [9]. nearlyemissions single-wavelength lasing emissions selection are attributed to wavelength selection mechanism of 2 (b) power In Figure 4a, the threshold 1.72,power 2.68 and 5.02 (c) kW/cm device multi-directional DFB effect [9]. In densities Figure 4a,are theabout threshold densities are aboutfor 1.72, 2.68 A1, and 2 B1 and C1, respectively. ThatB1increasing threshold power density with increasing lattice period is 5.02 kW/cm device A1, and C1, respectively. increasing density Figure 4. for Lasing characteristics of PCSELs at 20 °C. (a)That L-L curves of six threshold devices, (b)power lasing spectra ofwith attributed to gain-cavity detuning (∆λ), which is defined by difference between gain peak of QWs device A0, A1, A2 and A3, (c) lasing spectra of device B1 and C1. (λQW = 2203 nm) and resonant cavity wavelength of PC (λPC ); i.e., ∆λ ≡ λQW − λPC . Referring to the three arrows marked in Figure 3, device A1 is positively magnitude of The effect of lattice period is revealed by devicedetuned A1, B1, with and the C1 smallest with respective lasing 14.3 nm, while of device B1 and C1 are detuned with magnitudes 18.1 nm and to 50.1 nm, wavelengths 2188.7, 2221.1, andnegatively 2253.1 nm. The wavelength shift isofproportional period respectively. The largest detuning magnitude is associated with the lowest material gain, which results change with factor of 3.2; i.e., 3.2 nm red-shifts in wavelength per nm increase in period. All lasing inlinewidths the highestare threshold around power 0.2 nm,density. which is much smaller than those of edge emitting lasers [5]. The The effect of etch depth is disclosed by device A0 and A1.toBecause their wavelength difference isof nearly single-wavelength lasing emissions are attributed wavelength selection mechanism small and stronger, and distributed feedback coupling is associated with deeper PC holes, the multi-directional DFB effect [9]. In Figure 4a, the threshold power densities are about 1.72,pumping 2.68 and 5.02 kW/cm2 for device A1, B1 and C1, respectively. That increasing threshold power density with
The effect of etch depth is disclosed by device A0 and A1. Because their wavelength difference is small and stronger, and distributed feedback coupling is associated with deeper PC holes, the pumping threshold of A1 is less than half that of A0. Theoretical investigation on square-lattice-type PCSELs with InGaAs/GaAs MQWs revealed that optimized FF was 0.08 [14]. In 2018, 5, 32FF of 0.05, 0.10, and 0.15 were designed around this theoretical value and 5 of 7 ourPhotonics experiment, embodied in device A2, A1, and A3, respectively. The threshold power density of A3 is the highest among threeofdevices andthan is partly credited to its largeinvestigation gain-cavity detuning of +20.7 nm.PCSELs The threshold A1 is less half that of A0. Theoretical on square-lattice-type threshold values of A1 and A2 are within uncertainty; however, their relative relation is consistent with InGaAs/GaAs MQWs revealed that optimized FF was 0.08 [14]. In our experiment, FF of 0.05, for0.10, counterparts withdesigned 690-nm around period, this which are not value shown here. The lasing characteristics and 0.15 were theoretical and embodied in device A2, A1, andofA3, investigated PCSELs are summarized in Table 2. respectively. The threshold power density of A3 is the highest among three devices and is partly
credited to its large gain-cavity detuning of +20.7 nm. The threshold values of A1 and A2 are within Table 2. Lasing characteristics of six PCSEL devices. uncertainty; however, their relative relation is consistent for counterparts with 690-nm period, which are not Device shown here. The lasing characteristics of investigated PCSELs are summarized in Table 2. Wavelength Threshold FWHM Detuning (p, d, FF) 2 No. (nm) (kW/cm ) (nm) (nm) Table 2. Lasing2191.05 characteristics of 3.51 six PCSEL devices. A0 (680, 460, 0.10) 0.21 +12.0 A1 2188.70 1.72(kW/cm2 ) 0.22FWHM (nm) +14.3Detuning (nm) Device No. (p,(680, d, FF)586, 0.10) Wavelength (nm) Threshold A2 (680, 586, 0.05) 2191.30 1.60 0.21 +11.7 A0 (680, 460, 0.10) 2191.05 3.51 0.21 +12.0 A3 (680, 586, 0.15) 2182.35 3.96 0.21 A1 (680, 586, 0.10) 2188.70 1.72 0.22 +20.7 +14.3 A2 B1 (680,(690, 586, 0.05) 1.60 +11.7 586, 0.10) 2191.30 2221.10 2.68 0.21 0.21 −18.1 A3 C1 (680,(700, 586, 0.15) 2182.35 3.96 0.21 +20.7 586, 0.10) 2253.10 5.02 0.19 −50.1 B1 C1
(690, 586, 0.10) (700, 586, 0.10)
2221.10 2253.10
2.68 5.02
0.21 0.19
−18.1 −50.1
Figure 5a shows the L-L curves of device A1 at heatsink temperature of 20–50 °C in step of 5 °C. The near-threshold lasing spectra, normalized in linear scale, and shifted in vertical axis, are shown Figure 5a shows the L-L curves of device A1 at heatsink temperature of 20–50 ◦ C in step of 5 ◦ C. in Figure 5b. The lasing linewidths are about 0.2 nm at all temperature range. Moreover, peak The near-threshold lasing spectra, normalized in linear scale, and shifted in vertical axis, are shown lasing wavelength shifted with temperature at a rate of 0.17 nm/K, which is about one-seventh to in Figure 5b. The lasing linewidths are about 0.2 nm at all temperature range. Moreover, peak lasing one-sixth smaller than gain-peak shift rate of 1.0–1.2 nm/K (or 0.25–0.3 meV/K) [8,15]. The wavelength shifted with temperature at a rate of 0.17 nm/K, which is about one-seventh to one-sixth resonance shift of PC cavity dominates over bandgap shrinkage of ‘W’-type QWs in smaller than gain-peak shift rate of 1.0–1.2 nm/K (or 0.25–0.3 meV/K) [8,15]. The resonance shift temperature-dependent lasing wavelengths. It is worth it to mention that the temperature shift of PC cavity dominates over bandgap shrinkage of ‘W’-type QWs in temperature-dependent lasing coefficients of cavity resonance and gain peak for GaSb-based type-I QW PCSELs in similar wavelengths. It is worth it to mention that the temperature shift coefficients of cavity resonance and wavelength range were about 0.2 nm/K and 1.62 nm/K, respectively [10,13]. InP-based ‘W’-type QW gain peak for GaSb-based type-I QW PCSELs in similar wavelength range were about 0.2 nm/K and PCSELs exhibit cavity shift rate 15% lower and gain-peak shift rate about 30% lower than 1.62 nm/K, respectively [10,13]. InP-based ‘W’-type QW PCSELs exhibit cavity shift rate 15% lower GaSb-based type-I QW PCSELs. and gain-peak shift rate about 30% lower than GaSb-based type-I QW PCSELs.
(a)
(b)
◦ C to 50 Figure 5. Device A1A1 (a)(a) L-LL-L curves and (b)(b) lasing 50 ◦ C Figure 5. Device curves and lasingspectra spectrawith withvarying varyingtemperature temperaturefrom from 20 20 °C C.. °C in in steps steps of of 55 ◦°C
We also measure temperature-dependent lasing characteristics of device B1 and C1. They exhibit a consistent lasing peak shift rate of 0.17 nm/K. The threshold power density as a function of temperature for three PCSELs is plotted in Figure 6. At a temperature range below 45 ◦ C, the extracted characteristic temperature (To ) is about 25 K, 40 K and 20 K for device A1, B1, and C1, respectively. Small To less than 25 K is typical for ‘W’-type QW edge-emitting lasers and VCSELs [8,16]. The highest To of 40 K for device B1 is due to its slightly large negative detuning of 18.1 nm at 20 ◦ C. Zero detuning is estimated to be achieved at 40 ◦ C; however, neither lowest threshold power density nor negative To is observed around this temperature [16]. This is possibly due to that device B1 is outside the flat-top region and in the long-wavelength side of PL spectrum. The associated material gain is not high; besides, temperature-induced gain decrement dominates over detuning-induced gain increment. Higher To can
respectively. Small To less than 25 K is typical for ‘W’-type QW edge-emitting lasers and VCSELs [8,16]. The highest To of 40 K for device B1 is due to its slightly large negative detuning of 18.1 nm at 20 °C. Zero detuning is estimated to be achieved at 40 °C; however, neither lowest threshold power density nor negative To is observed around this temperature [16]. This is possibly due to that device B1 is 2018, outside Photonics 5, 32 the flat-top region and in the long-wavelength side of PL spectrum. The associated 6 of 7 material gain is not high; besides, temperature-induced gain decrement dominates over detuning-induced gain increment. Higher To can be expected with smaller negative detuning of be10-16 expected with smaller of 10-16 nm based our previous QD PCSELs nm based on our negative previousdetuning work of QD PCSELs in NIRon range [17]. For work deviceofC1 with largest ◦ C, its threshold in negative NIR range [17]. For device C1 with largest negative detuning of 50.1 nm at 20 detuning of 50.1 nm at 20 °C, its threshold power density at RT is the highest among three power density RT isofthe highest among three devices as a result of insufficient material gainofatPL devices as a at result insufficient material gain at around the upper FWHM wavelength around the upper FWHM wavelength of PL spectrum. Since higher pumping density is sensitive spectrum. Since higher pumping density is more sensitive to Auger recombinationmore and carrier loss to at Auger recombination and carrier loss at elevated temperature, lowest T is observed for device o elevated temperature, lowest To is observed for device C1. It is worth mentioning that lasing C1.wavelengths It is worth mentioning wavelengths cover more 70 nm, from nm cover more that thanlasing 70 nm, ranging from 2182 nm than to 2253 nm. ranging Moreover, the2182 threshold to pumping 2253 nm. density Moreover, the threshold pumping density is more sensitive to temperature change ino = is more sensitive to temperature change in InP-based ‘W’-type QW PCSELs (T InP-based QW PCSELstype-I (To = QW 20–40PCSELs K) than(Tino =GaSb-based type-Iwas QW PCSELsin(TRef. o = 45–67 20–40 K)‘W’-type than in GaSb-based 45–67 K, which revealed [13]). K, which was revealed in Ref. [13]).
Figure Temperature dependence of threshold power density PCSEL device Figure 6. 6. Temperature dependence of threshold power density forfor PCSEL device A1,A1, B1 B1 andand C1.C1.
4. 4. Conclusions Conclusions In In thisthis paper, InP-based ‘W’-type QW PCSELs a wavelength range of 2.2 µm are demonstrated paper, InP-based ‘W’-type QW in PCSELs in a wavelength range of 2.2 μm are fordemonstrated the first timefor by the optical The lasing emissions show a broad show wavelength over first pumping. time by optical pumping. The lasing emissions a broadrange wavelength 70 range nm and narrow linewidths of about 0.2 nm, which is promising for TDLAS applications. We have over 70 nm and narrow linewidths of about 0.2 nm, which is promising for TDLAS investigated lasing of PCSELs in terms of etchofdepth, lattice period, factor. applications. We characteristics have investigated lasing characteristics PCSELs in terms ofand etchfilling depth, lattice The temperature characteristics are also discussed with respect to wavelength detuning between QW period, and filling factor. The temperature characteristics are also discussed with respect to gain peak and PC resonant wavelength. Moreover, InP-based ‘W’-type QW gain media and PCSELs wavelength detuning between QW gain peak and PC resonant wavelength. Moreover, InP-based exhibit rather flatgain but media lower gain, smaller temperature of cavity resonance and gain ‘W’-type QW and PCSELs exhibit rathershift flatcoefficients but lower gain, smaller temperature shift peak, as well as lower characteristics temperature in comparison with GaSb-based type-I gain media coefficients of cavity resonance and gain peak, as well as lower characteristics temperature in and PCSELs. with GaSb-based type-I gain media and PCSELs. comparison MIR PCSELs can bebe electrically pumped if the layer structure is suitably designed and ohmic MIR PCSELs can electrically pumped if the layer structure is suitably designed and ohmic contact is cleverly achieved. In our current injected NIR PCSELs [11], we deposited transparent contact contact is cleverly achieved. In our current injected NIR PCSELs [11], we deposited transparent indium-tin-oxide (ITO) to facilitate electrical current spreading as well as optical contact indium-tin-oxide (ITO) both to facilitate both electrical current spreading as light well transmission. as optical light However, PC holes of MIR PCSELs are too large and have to be refilled with SU-8 photoresist before transmission. However, PC holes of MIR PCSELs are too large and have to be refilled with SU-8 ITO deposition. Moreover, the InGaAs contact layer well as ITO could have significant absorption photoresist before ITO deposition. Moreover, the InGaAs contact layer well as ITO could have and should beabsorption taken care and of. The last choice is tocare use the scheme thecurrent buried tunnel significant should be taken of. current The lastinjection choice is to useofthe injection junction commonly used in MIR VCSELs [7,8]; nonetheless, epitaxial regrowth is necessary. scheme of the buried tunnel junction commonly used in MIR VCSELs [7,8]; nonetheless, epitaxial regrowth is necessary.
Author Contributions: Conceptualization, Zong-Lin Li; Funding acquisition, Chien-Ping Lee; Investigation, Yuan-Chi Kang; Resources, Chien-Ping Lee; Writing – original draft, Zong-Lin Li and Gray Lin; Writing – review & editing, Gray Lin. Funding: This research was funded by Ministry of Science and Technology under grant number MOST 106-2221-E-009-121-MY3 and MOST 107-2218-E-009-034. The APC was funded by Ministry of Science and Technology under grant number MOST 106-2221-E-009-121-MY3. Acknowledgments: The authors would like to appreciate the services and facilities provided by the Center for Nano Science and Technology (CNST) of National Chiao Tung University. Conflicts of Interest: The authors declare no conflict of interest.
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