Advances in Optical and Photonic Devices 2011 Part 2 pptx

2 Photonic Quantum Ring Laser of Whispering Cave Mode O’Dae Kwon, M.. The photonic quantum ring PQR laser of WCMs is thus born without any intentionally fabricated ring pattern structu

comparable to the To 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 Jth 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 Jth) 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 Jth, 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

Jth, 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 Jth

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

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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

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

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 (Itr: curve T)

and threshold (Ith: curve A) current expressions for the case of PQRs occupying the annular

Rayleigh region is now given by (1)

I =I + =I N ×W λ n ×πφ×( /e ητ)+ I i (1)

N 1D is the 1D transparency carrier density, τ the carrier lifetime, η the quantum efficiency,

and Ii 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 μm3, 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:

Rayleigh

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

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 WRayleigh of the 2D

quantum well This in turn exhibits extremely small thresholds in the the μA-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

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 1012 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 WRayleigh 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

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 have succeeded in fabricating the high density array chip of PQR hole lasers of one mega (M) integration 1M PQR hole array chips has ultra low threshold current of 0.736 nA per single hole due to photonic crystal-like cooperative effect (Kwon et al., 2008) 1M PQR hole laser array chip is fabricated in tandem type with four 256K PQR hole arrays for uniformly injecting current on the device surface The used epitaxial wafer structure of a p-

i(MQW: multi quantum well)-n diode was grown on an n-type GaAs (001) substrate by

metal-organic vapor-phase epitaxy The structure consists of two distributed Bragg reflector

(DBR) mirrors surrounding the i-region of a one-λ cavity active region (269.4 nm thick)

including three GaAs/Al0.3Ga0.7As quantum well structures, tuned to yield a resonance

wavelength of 850 nm The p- and n- type DBR mirrors consist of alternating 419.8 Å

Al0.15Ga0.85As and 488.2 Å Al0.95Ga0.05As layers, 21.5 periods and 38 periods respectively Figures 9(a) and (b) show scanning electron microscopy (SEM) images for top view and

cross section of 1M PQR hole laser array, respectively, whose SEM pictures exhibit a bit

rough cross section as compared with single device side walls in Figs 9(c) and (d)

Photonic Quantum Ring Laser of Whispering Cave Mode 27

Fig 9 (a) Top and (b) cross section SEM images of 1M PQR hole array (c) SEM micrographs

of mesa and hole type PQR structures

Figure 10(a) shows the CCD images of the illuminant 1M PQR hole array near the

transparent current, 0.08 A (80 nA/cell) and near the threshold current, 0.7 A (700 nA/cell)

To measure the L-I curve for 1M PQR hole array, we used a conventional power meter

(Adventest Mo.Q211) and measured directly 1M PQR hole array For measurement of threshold current and angle-resolved spectra shown in Fig 11, we used a piece of 1/32M

PQR hole array, because the total size of 1M PQR hole array chip is 1 cm 2 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