Advances in Optical and Photonic Devices 2011 Part 2 pptx

Advances in Optical and Photonic Devices 16 comparable to the T o range (50-70 K) of the equivalent QW structure. In Fig. 12(b), only a distinctive ground state lasing with the wavelength coverage of ~15 nm is observed below injection of 1.5 x J th . This broad lasing linewidth, again suggests collective lasing actions from Qdashes with different geometries. In addition, the quasi-supercontinuum lasing spectrum at high current injection (4 x J th ) without distinctive gain modulation (Harris et al., 1997) further validates the postulation of uniform distribution of dash electronic states in a highly inhomogeneous active medium. At J > 1.5 x J th , the bistate lasing is evident. The simultaneous lasing from both transition states (Hadass et al., 2004) is attributed to the relatively slow carrier relaxation rate and population saturation in the ground state in low- dimensional quantum heterostructures. The bistate lasing spectrum is progressively broadened with increasing carrier injection up to a wavelength coverage of 85 nm at J = 4 x J th , which is larger than that of the as-grown laser (~76 nm), as shown in Fig. 11 and Fig. 13. A center wavelength shift of 100 nm and an enhancement of the broadband linewidth, which is attributed to the different interdiffusion rates on the large height distribution of noninteracting Qdashes at an intermediate intermixing, are achieved after the intermixing. The inset of Fig. 13, showing the changes of FWHM of the broadband laser with injection depicts that energy-state-hopping and multi-state lasing emission from Qdashes with Fig. 13. The wavelength tune quasi-supercontinuum quantum dash laser from 1.64 μm to 1.54 μm center wavelength. The lasing coverage increases from 76 nm to 85 nm after intermixing process. The inset shows the FWHM of the broadband laser in accordance to injection above threshold up to J = 4 x J th . Fig. 14. (a) Spaced and quantized energy states from ideal Qdot samples. (b) Large broadening of each individual quantized energy state contributes to laser action across the resonantly activated large energy distribution. (c) Variation in each individual quantized energy state owing to inhomogeneous noninteracting quantum confined nanostructures in addition to self broadening effect demonstrate a broad and continuous emission spectrum. Broadband Emission in Quantum-Dash Semiconductor Laser 17 different geometries occur before a quasi-supercontinuum broad lasing bandwidth with a ripple of wavelength peak fluctuation that is less than 1 dB is achieved. This idea can be illustrated clearly in Fig. 14, when a peculiarly broad and continuous spectrum is demonstrated from a conventional quantum confined heterostructures utilizing only interband optical transitions. The effect of variation in each individual quantized energy state owing to large ensembles of noninteracting nanostructures with different sizes and compositions, in addition to self inhomogeneity broadening within each Qdot/Qdash ensemble, will contribute to active recombination and thus quasi-supercontinuum emission. 5. Conclusion In conclusion, the unprecedented broadband laser emission at room temperature up to 76 nm wavelength coverage has been demonstrated using the naturally occurring size dispersion in self-assembled Qdash structure. The unique DOS of quasi-zero dimensional behavior from Qdash with wide spread in dash length, that gives different quantization effect in the longitudinal direction and band-filling effect, are shown as an important role in broadened lasing spectrum as injection level increases. After an intermediate degree of postgrowth interdiffusion technique, laser emission from multiple groups of Qdash ensembles in addition to multiple orders of subband energy levels within a single Qdash ensemble has been experimentally demonstrated. The suppression of laser emission in short wavelength and the progressive red-shift of peak emission with injection from devices with short cavity length indicate the occurrence of photon reabsorption or energy exchange among different sizes of localized Qdash ensembles. These results lead to the fabrication of the wavelength tuned quasi-supercontinuum interband laser diodes via the process of IFVD to promote group-III intermixing in InAs/InAlGaAs quantum-dash structure. Our results show that monolithically integration of different gain sections with different bandgaps for ultra-broadband laser is feasible via the intermixing technique. 6. Acknowledgement This work is supported by National Science Foundation (Grant No. 0725647), US Army Research Laboratory, Commonwealth of Pennsylvania, Department of Community and Economic Development. Authors also acknowledge IQE Inc. for the growth of Qdash material, and D N. Wang and J. C. M. Hwang for the TEM work. 7. References Alfano, R. R. & Shapiro, S. L. (1970). Emission in the region 4000 to 7000 Å via four-photon coupling in glass. Phys Rev. Lett., Vol. 24, No. 11, (March 1970) 584-587 Bimberg, D.; Kirstaedter, N.; Ledentsov, N. N.; Alferov, Zh. I.; Kop’ev, P. S. & Ustinov V. M. (1997). InGaAs-GaAs quantum-dot lasers. IEEE J. Sel. Top. Quantum Electron., Vol. 3, No. 2, (April 1997) 196-205 Crank, J. (1975). The Mathematics of Diffusion, Oxford University Press, 0198534116, Clarendon Dery, H.; Benisty, E.; Epstein, A.; Alizon, R.; Mikhelashvili, V.; Eisenstein, G.; Schwertberger, R.; Gold, D.; Reithmaier, J. P. & Forchel, A. (2004). On the nature of quantum dash structures. J. Appl. Phys., Vol. 95, No. 11, (June 2004) 6103-6111 Advances in Optical and Photonic Devices 18 Djie, H. S.; Dimas, C. E. & Ooi, B. S. (2006). Wideband quantum-dash-in-well superluminescent diode at 1.6 μm. IEEE Photon. Technol. Lett., Vol. 18, No. 16, (August 2006) 1747-1749 a Djie, H. S.; Ooi, B. S.; Fang, X. –M.; Wu, Y.; Fastenau, J. M.; Liu, W. K. & Hopkinson, M. (2007). Room-temperature broadband emission of an InGaAs/GaAs quantum dots laser. Opt. Lett., Vol. 32, No. 1, (January 2007) 44-46 b Djie, H. S.; Tan, C. L. ; Ooi, B. S.; Hwang, J. C. M.; Fang, X. –M.; Wu, Y.; Fastenau, J. M.; Liu, W. K.; Dang, G. T. & Chang, W. H. (2007). Ultrabroad stimulated emission from quantum-dash laser. Appl. Phys. Lett., Vol. 91, No. 111116, (September 2007) 111116 1-3 Djie, H. S.; Wang, Y.; Ding, Y. H.; Wang, D. –N.; Hwang, J. C. M.; Fang, X. –M.; Wu, Y.; Fastenau, J. M.; Liu, A. W. K.; Dang, G. T.; Chang, W. H. & Ooi, B. S. (2008). Quantum dash intermixing. IEEE J. Sel. Top. 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F.; Maximov, M. V.; Kopchatov, V. I.; Lunev, A. V.; Kopev, P. S.; Bimberg, D. & Alferov, Zh. I. (1999). Gain characteristics of quantum dot injection lasers. Semicond. Sci.Technol., Vol. 14, No. 1, (January 1999) 118-123 2 Photonic Quantum Ring Laser of Whispering Cave Mode O’Dae Kwon, M. H. Sheen and Y. C. Kim Pohang University of Science & Technology S. Korea 1. Introduction In early 1990s, an AT&T Bell Laboratory group developed a microdisk laser of thumb-tack type based upon Lord Rayleigh's ‘concave’ whispering gallery mode (WGM) for the optoelectronic large-scale integration circuits (McCall et al., 1992). The above lasers were however two dimensional (2D) WGM which is troubled with the well-known WGM light spread problem. For the remedy of this problem, asymmetric WGM lasers of stadium type (Nockel & Stone, 1997) were then introduced to control the spreading light beam. Quite recently, a novel micro-cavity of limaçon shape has shown the capability of highly directional light emission with a divergence angle of around 40-50 degrees, which is a big improvement to the light spreading problem.(Wiersig & Hentschel, 2008) On the other hand, when we employ a new micro-cavity of vertically reflecting distributed Bragg reflector (DBR) structures added below and above quantum well (QW) planes, say a few active 80Å (Al) GaAs QWs, a 3D toroidal cavity is formed giving rise to helix standing waves in 3D whispering cave modes (WCMs) as shown Fig. 1 (Ahn et al., 1999). The photonic quantum ring (PQR) laser of WCMs is thus born without any intentionally fabricated ring pattern structures, which will be elaborated later. The PQR’s resonant light is radiating in 3D but in a surface-normal dominant fashion, avoiding the 2D WGM’s in-plane light spread problem. Bessel (J m ) field profile Helical wave Fig. 1. Planar 2D Bessel function WGMs vs. toroidal 3D knot WCM (Park et al., 2002). The 3D WCM is a toroid with a circular helix symmetry not reducible to the simple 2D rotational symmetry Advances in Optical and Photonic Devices 22 2. Basic properties of PQR lasers The 3D WCM laser of PQR, whose simulation work will be shown later, behaves quite differently due to its quantum wire-like nature as follows: First of all, the PQR exhibit ultra- low threshold currents – for a mesa-type PQR device of 15 um diameter, the PQR at the peripheral Rayleigh band region lases with about one thousandth of the threshold current needed for the central vertical cavity surface emitting laser (VCSEL) of the same semiconductor mesa as illustrated in Fig. 2. Fig. 2. CCD pictures of emisssions at 12 μA, near PQR threshold, at 11.5 mA, below VCSEL threshold, and at 12.2 mA, above VCSEL threshold, respectively. We can however make theoretical formulae consistent with above concentric PQRs and do some calculations for comparing with the transparency and threshold current data observed. The PQR formulae can be derived by assuming that the pitch of concentric rings is ‘photonic’ kind of one half wavelength - optical λ/2 period: The transparency (I tr : curve T) and threshold (I th : curve A) current expressions for the case of PQRs occupying the annular Rayleigh region is now given by (1). 1 /( /2 ) D th tr i Rayleigh eff IIINW n λ = += × × (/ ) i eI π φητ × + (1) N 1D is the 1D transparency carrier density, τ the carrier lifetime, η the quantum efficiency, and I i stands for internal loss (Ahn et al., 1999; Kwon et al., 2006). The PQR formulae are now compared with the actual data in Fig. 3, which show quite an impressive agreement except some random deviations due to device imperfections. For smaller diameters ( φ ) the active volume decreases below 0.1 μ m 3 , and with the cavity Q factor over 15,000. The corresponding spontaneous emission coefficient β will become appreciable enough for threshold-less lasing without a sharp turn-on threshold, which often occurs in the PQR light-current analyses. As listed in Fig. 3, the wide-spread data suggest a fuzzy ring trend growing as the device shrinks due to the growing leaky implantation boundary around the implant-isolated holes, and the hole PQR threshold data are actually approaching the curve B, whose formula is derived for the mesa by assuming that the Rayleigh region is now nothing but a piece of annular quantum well plane of random recombinant carriers instead: 2 (/ ) D Rayleigh IN W e π θητ =× ×× (2) Figure 4 shows a collection of linewidth data being roughly inversely proportional to the device size as expected. The narrowest linewidth observed with an optical spectrum analyzer to date from a 10 um PQR is 0.55 Å at an injection current of 800 uA. We also note that with wet etching steps employed instead of dry etching, the Q factor reached up to 20,000 while the linewidth approached 0.4 Å (M. Kim et al., 2004). Although we did not Photonic Quantum Ring Laser of Whispering Cave Mode 23 attempt it for GaAs, a CALTECH group devised a laser baking process for achieving ultrahigh Q values of multi-millions involving a SiO2 microcavity. It is interesting to be a toroidal microcavity whose 3D WCM properties is unknown yet (Armani et al., 2003; Min et al., 2004). Fig. 3. Threshold curves A and B from PQR and quantum well formulae, respectively, with corresponding Rayleigh toroid schematics (defined by Rayleigh width between rin and R) and transparency curve T for the PQR case. Data for transparency (empty symbols) and threshold (solid symbols) currents: circles for PQRs and squares for PQR holes implant isolated. Data at 6 and 8 μm correspond to the case of 256×256 hole arrays without implantation (see the arrows 1 and 2). 0 30 60 90 120 150 180 210 240 0.06 0.09 0.12 0.15 0.18 0.21 0.24 15 μ m 12 μ m 10 μ m 9 μ m 845 846 847 848 849 850 FWHM = 0.055 nm I = 800 μ A D = 10 μ m Wavelength (nm) FWHM, Δλ 1/2 (nm) Current Level (I/I th ) 7 μ m Fig. 4. Linewidth data vs. current s with various device sizes Now we figure that the helical WCM standing wave manifold transiently induces concentric PQRs for imminently recombinant carriers present in the Rayleigh region W Rayleigh of the 2D quantum well. This in turn exhibits extremely small thresholds in the the μ A-to-nA range with the given T -dependent thermal stabilities. It is attributed to a photonic (de Broglie) quantum corral effect, similar in character to the well-known electronic quantum corral image from room temperature scanning tunneling microscope studies of Au atomic island plane at a given bias. Advances in Optical and Photonic Devices 24 The photonic (de Broglie) quantum corral effect imposes a λ/2 period transient ordering upon the imminently recombinant carriers, although the optical λ/2 period for GaAs semiconductor will be substantially larger than the electronic de Broglie spacing. We note that the Rayleigh region of quantum well planes is deeply buried beneath a few micron thick AlAs/GaAs Bragg reflectors not accessible for direct observation. However, recent experiments and modeling work on dynamic interactions between carriers and transient field in a quantum well plane is a close case in point (Gehrig & Hess, 2004). It thus appears that the transient quantum wire-like features considered here seem to persist within the relevant time scale through thermal fluctuations. For an ensemble of carriers randomly distributed in the regional quantum well plane of concentration 10 12 cm -2 for instance, tens- of-nm scale local field-driven drifts of given carriers to a neighboring imminent PQR site should generate the proposed PQR ordering for an imminent recombination event of annihilating electron-hole pairs. For example, one can imagine a transient formation of the two separate Rayleigh rings instantly via light field-induced migration of random carriers within the W Rayleigh region as schematically shown for curve A in Fig. 3. We expect the standing waves in the Rayleigh region to give rise to a weak potential barrier for such a dynamic electron-hole pair process, perhaps an opposite case of extremely shallow quantum well excitons at room temperature where even the shallow barriers tend to assure at least one bound state according to square well quantum mechanics. 3. Spatio-temporal dynamic simulation of PQR standing waves and carriers Although it is limited to 2D cases, recent spatiotemporal dynamic simulation work in a straight waveguide case (see Fig.5) faithfully reveals such a tangled but otherwise quantum- wire-like ordering of recombinant carriers undergoing some picosecond-long exciton process, consistent with the photonic quantum corral effect due to a strong carrier-photon coupling. The images of several standing light-wave-like carrier distribution patterns within a 1 micron wide quantum well stripe emerge, as a function of time from-5-to-8 psec after about 5 psec chaotic regime as indicated along the horizontal time axis of 10 psec full range, shown in Fig. 6 (Kwon et al., 2009). They are curiously reminiscent of the tangled web of the 2D electron gas due to impurity atom potentials studied by a Harvard group (Topinka et al., 2003). The assumed concentric quantum ring pattern of carrier distribution within the Rayleigh region is not observable directly since they are buried below a few micron thick top DBR structures. Instead the CCD pictures are their distant images refracted and smeared out through the semiconductor medium. As said before, the resonance of the PQR laser results in 3D WCM of helical standing waves, which is surface-normal dominant, in contrast to the in-plane 2D WG mode. The data taken with a home-built solid angle scanner setup, which will be discribed later, shows a tangential polarization dominance which supports strong carrier-photon couplings behaviors needed for the PQR formation (Kim et al., 2007) 4. 3D WCM mode analysis and single mode PQR laser A 3D WCM mode analysis, based upon the helix mode of the PQR consisting of a bouncing wave between the two DBRs and a circulating wave of in-plane total reflection, gives an angular quantization rule for easy PQR mode analysis of 3D spectra taken with tapered single mode fiber probes as shown in Fig. 7 (Bae et al., 2003). Photonic Quantum Ring Laser of Whispering Cave Mode 25 Fig. 5. Flattened top view of helix modes within a Rayleigh bandwidth Fig. 6. Spatiotemporal 2D simulation results: top –standing waves are formed after a few picoseconds of chaotic regime in the case of flattened and straight rectangular wave guide version [x-axis span of 10 psec.]; bottom – carrier distribution dynamics shown for 10 picoseconds, where similar patterns emerge after a few psec. Y-axis indicates a 1 um wide central waveguide in the middle of 3 um boundary. For single mode lasers we have made non-conventional PQRs of hyperboloid drum shape like Figs. 8 (a) and (b) (Kim et al., 2003) having a submicron active diameter with φ = 0.9 μm, where as its top region of a few micron diameter serves as metallic contact area for electro pumping. Figs. 8 (c) and (d) show the threshold data with a 0.46 Å linewidth exhibit the smallest threshold of about 300 nA, (Yoon et al., 2007) observed so far among the injection lasers of quantum well, wire, or dot type to the best of our, although the external quantum efficiency observed right after the threshold is poor suffering from the soft lasing turn-on behavior here. [...]... width, the optical output power increased when the number of petals increased (Fig 22 ) As the number of petals increased, the length of peripheral PQR region is larger so that the region occupied by the PQR emission in the whole emission region increased, leading to the final increase of the optical output power 36 Advances in Optical and Photonic Devices Fig 22 Optical output power comparison (φ= 20 μm)... effect of ‘hole’ PQR portions is involved in addition to the ‘soft lasing turn-on’ behavior (Kim et al., 20 09) T=1.0% T=1.0% T=1.0% 25 A/cm2 50A/cm2 100A/cm2 T=1.0% T=1.0% T=1.0% 25 A/cm2 50A/cm2 100A/cm2 T=1.0% Circle 18um, I=15uA T=1.0% T=0.63% 25 A/cm2 50A/cm2 100A/cm2 PQR emission LED emission 12- petals, Ith =28 uA T=100% 11A/cm2 Fig 21 Various emission patterns of 4-, 8-, 12- petal flower PQRs As mentioned... other optical components such as amplifiers, 40 Advances in Optical and Photonic Devices modulators and detectors (Coldren 20 00; Ward, Robbins et al 20 05; Welch, Kish et al 20 06; Raring & Coldren 20 07) in order to reduce chip cost, system size and complexity Tunable lasers are also needed in other important markets such as trace gas detection for environmental emission motoring (Phelan, Lynch et al 20 05)... Lett 82, 4504 (20 03) Kim, M et al., Wet etching fabrication of photonic quantum ring laser, J Appl Phys 96, 47 42 (20 04) Kim, Y C et al., PQR laser can outdo LED, IEEE-NMDC 20 06 21 -24 (20 06), Gyeongju, Korea; Laser Focus World (March 20 08) Kwon, O et al., Photonic quantum ring laser of 3D whispering cave mode, Microelectronics Journal, 40, 570 (20 09) Kwon, O et al., Hole emitter of photonic quantum ring,... may involve 1 or 2 lenses for beam guiding, resulting in an instantaneous frame of red beam scan implemented through an arrangement of optical components as shown Fig 24 The blue and green beam combination structures are under development Photonic Quantum Ring Laser of Whispering Cave Mode 37 Fig 24 (a) Red beam optics outlined (b) Red letter image (c) Experimental set up of the optical components 12. .. Lasers, IEEE Trans Nano 7, 185 (20 08) 38 Advances in Optical and Photonic Devices Kim, D & Kwon, O., Polarization characteristics of photonic quantum ring laser with threedimensional whispering gallery resonances, J Appl Phys 1 02, 053104 (20 07) Kim, J Y et al., Fabrication of Photonic Quantum Ring Laser using Chemically Assisted Ion Beam Etching, J Vac Sci Technol B 19, 1334 (20 01) Kim, J Y et al., Effect... laser, Appl Phys Lett 81, 580 (20 02) Topinka, M.A et al., Imaging Coherent Electron Flow, Physics Today 56, 12 (20 03) Wiersig, J & Hentschel, M., Combining Directional Light Output and Ultralow Loss in Deformed Microdisks, Phys Rev Lett 100, 033901 (20 08) Yoon, J H et al., Single mode photonic quantum ring laser fabricated in hyperboloid drum shape, J Appl Phys 103, 053103 (20 08) 3 A Tunable Semiconductor... cm2 which is larger than the aperture size (diameter = 0.8 cm) of the power meter Fig 10 (a) CCD (right) and 1000 times magnified (left) images of the illuminant 1M PQR hole array (4x250K arrays) at transparent and near threshold current (b) L-I curve of 1/32M PQR hole array chip As shown in Fig 2( b), the threshold current is measured 0.736 μA/hole by using linear fitting 28 Advances in Optical and. . .26 Advances in Optical and Photonic Devices Spectral Intensity (a.u) θ = 10o m5 θ = 15o φ = 20 μm I =2. 5 mA T = 18oC 845 m11 m9 m3 m7 m13 m1 m15 m0 846 847 848 849 850 Wavelength (nm) Fig 7 Angular measurement set up for 3D WCM and some typical spectra Fig 8 Hyperboloid drum PQR: SEM micrograph, L-I curve, and single mode spectrum 5 Mega-pixel laser chips of photonics quantum ring holes We... rendering ability Fig 13(a) shows the emission image of the 16x16 mesa type red PQR laser array A single red PQR emission reveals two different regions at a given injection current (I =24 uA/cell) The PQR lasing occurs in the periphery of the active disk called the Rayleigh band and the Photonic Quantum Ring Laser of Whispering Cave Mode 29 LED emission occurs in the middle part of the disk Luminous . 18um, I=15uA PQR emission LED emission 25 A/cm 2 50A/cm 2 100A/cm 2 25A/cm 2 50A/cm 2 100A/cm 2 12- petals, I th =28 uA 11A/cm 2 T=100% 25 A/cm 2 50A/cm 2 100A/cm 2 T=1.0% T=1.0% T=1.0% T=1.0% T=1.0%. (January 20 09) 30- 32 Advances in Optical and Photonic Devices 20 Van der Poel, M.; Mork, J.; Somers, A.; Forchel, A.; Reithmaier, J. P. & Eisenstein, G. (20 06). Ultrafast gain and index. I=15uA PQR emission LED emission 25 A/cm 2 50A/cm 2 100A/cm 2 25A/cm 2 50A/cm 2 100A/cm 2 12- petals, I th =28 uA 11A/cm 2 T=100% Fig. 21 . Various emission patterns of 4-, 8-, 12- petal flower PQRs As