A new type of semiconductor laser is studied, in which injected carriers in the active region are quantum mechanically confined in localized finite self-assembled wire-like quantum-dash (Qdash) structures that are varied in sizes and compositions. Effects of such carrier distribution and quasi three-dimensional density of states contribute to a quasi-supercontinuum interband lasing characteristics, which is a new laser design platform as compared to continuous broad emission spectrum generated by nonlinear media pumped with ultrashort laser pulse. The wavelength profile of quasi-supercontinuum emission is tunable at near-infrared wavelength spanning across several optical communication bands at around ~1500 nm in addition to the ability of operating condition at room temperature as opposed to that previously obtained only at operating temperature below 100 K. In this chapter, a thorough analysis of the Qdash material system, device physics and the establishment of ultrabroad stimulated emission behavior will be presented and discussed.
The generation of white light by laser radiation was first reported by Alfano and Shapiro (Alfano & Shapiro, 1970) who observed a spectral broadening of a picosecond second-harmonic output of a neodymium garnet laser (400-700 nm) with an energy of about 5 mJ in the bulk of borosilicate glass. Experiments performed with these bulk samples were then followed by studies on waveguide white-light generation in air-silica microstructure optical fibers to date (Ranka, 2000). The generation of the artificial white light is mainly due to the effective nonlinear-optical transformations of ultrashort laser pulses. Owing to its broad and continuous output spectrum, such radiation is called supercontinuum. Supercontinuum generation is an interesting physical phenomenon and the relevant technology is gaining in practical implications – it offers novel solutions for optical communications and control of ultrashort laser pulses (Nisoli et al., 1996), helps to achieve an unprecedented precision in optical metrology (Lin & Stolen, 1976), serves to probe the atmosphere of the Earth (Zheltikov, 2003), and suggests new strategies for the creation of compact multiplex light sources (Morioka et al., 1993) for nonlinear spectroscopy, microscopy, and laser biomedicine.
The first mid-infrared broadband semiconductor laser was demonstrated in an intersubband structure by adopting a quantum cascade configuration (Gmachl et al., 2002). Laser action with a Fabry-Perot spectrum covering all wavelengths from 6 to 8 μm simultaneously is demonstrated with a number of dissimilar intersubband optical transitions. Recently, similar unique spectral properties in the form of quasi-supercontinuum lasing characteristics have been demonstrated on semiconductor quantum-dot (Qdot) and Qdash platforms in different near-infrared wavelength regime without the need of ultrashort pulse laser excitation or the engineering of the intersubband transition level. (a Djie et al., 2007 ; Kovsh et al., 2007 ; b Djie et al., 2007 ; Tan et al., 2008). Most important, this unique feature of quasi-supercontinuum emission occurs at high temperature, i.e. room temperature in addition to identical operating conditions of a conventional semiconductor lasers. Owing to its broad and continuous spectrum emitted via only a single semiconductor laser diode, such device is called broadband laser. The broadband laser technology utilizes largely inhomogeneous quantum nanostructures active medium such as quantum-dot (Qdot) structures for wavelength emission in ~1200 nm (a Djie et al., 2007 , Kovsh et al., 2007); and Qdash medium for emission in center wavelength of ~ 1600 nm (b Djie et al., 2007 ; Ooi et al., 2008). Bandgap tuning with emission width widening is possible and can be realized in Qdash materials via postgrowth lattice interdiffusion technique (Tan et al., 2008; Tan et al. 2009). Furthermore, interband optical transition in quantum confined heterostructures will contribute to a highly efficient broadband laser action as compared to other emitter technologies.
A brief review of current state-of-art of Qdot/Qdash technology is necessary to comprehend the origin and progress of semiconductor broadband laser. To date, conventional self-assembled Qdot/Qdash semiconductor nanostructures have attracted considerable interest in the fabrication of semiconductor lasers and optical amplifiers due to the unprecedented potential offered by three-dimensional energy levels quantification that lead to vastly improved optoelectronic characteristics as compared to conventional quantum-well (QW) structures and bulk materials (Bimberg et al., 1997; Wang et al., 2001; Ooi et al., 2008). Apart from its predominant applications in optoelectronics industry, self assembled Qdot/Qdash demonstrate a number of unique features as compared to QW materials. In particular, self-assembled Qdot lasers have been shown to emit unique lasing spectral characteristics, where the laser emission spectra are broadened with modulated non-lasing spectral regions and the number of lasing modes increases above threshold (Harris et al., 1998). Furthermore, early experiments showed an extraordinary wideband lasing coverage of 50 nm, in the absence of modulated non-lasing spectral regions, but only at a cryogenic temperature (60 K) from Qdot gain medium (Shoji et al., 1997; Jiang & Singh, 1999). These phenomena have been attributed to the carrier localization in noninteracting or spatially isolated dot ensembles and nonequilibrium carrier distribution among highly inhomogeneous Qdots (Harris et al., 1998; Jiang & Singh, 1999). The most recent study reveals that a low-ripple (<3 dB) non-modulated broad interband lasing coverage of ~40-75 nm from GaAs-based Qdot lasers can be achieved at room temperature with center wavelength of ~1160-1240 nm by employing a highly inhomogeneous InGaAs Qdot structures ( Djie et al., 2007 ) and chirp InGaAs Qdot structures (Kovsh et al., 2007). These novel semiconductor light emitters are particularly attractive for many practical imaging and sensor applications due to their compactness and relatively low energy requirement in comparison to other state-of-art broad spectrum light sources.
The effort of achieving this interesting broadband lasing action in a longer wavelength region (1.5-1.6 μm) will thus be critical for broader applications relevant to the important low-loss transmission window of fiber optic system such as multichannel optical communication system, fiber-based optical coherent tomography, interferometric fiber optic gyroscopes, optical measurement systems, etc. Intensive research in the advanced growth of InP-based self-assembled Qdash had enabled the realization of high quality lasers and optical amplifiers that potentially cover the wavelength operation on both 1.31 μm and 1.55 μm of optical communication bands (Wang et al., 2001). Qdash assembly is essentially comprised of isotropic Qdots and finite quantum-wires (Qwires), whose cross section is similar to that of a typical Qdot, 3-4 nm (height) × 10-20 nm (base), while its length is varied from tens to hundreds of nanometers, as pictured by scanning electron microscopy (Dery et al., 2004) and atomic force microscopy (Popescu & Malloy, 2006). Due to quasi-three-dimensional carrier confinement and intrinsic properties, Qdash enable several interesting laser diode characteristics such as improved temperature insensitivity, optical feedback resistance, wide spectral tunability, and broad stimulated emission (Sek et al., 2007; Lelarge et al. 2007; b Djie et al., 2007 ). In addition, the gain properties of a Qdash amplifier bearing the distinct fingerprint of a quantum-wire-like density of states (Dery et al., 2004) while gain recovery characteristics and recovery time constants resembling Qdot characteristics (van der Poel et al., 2006). More so, it has been proposed that the role of optical gain broadening (Tan et al., 2007) that results in broadband emission from Qdot lasers is also inherent in Qdash lasers. These unique features will help to overcome the challenges in the nanoscaled epitaxial engineering of highly inhomogeneous Qdot for broadband laser applications.
All the interesting features of broad interband lasing actions from self-organized, spatially-isolated semiconductor nanostructure technology can be widely applied in optical telecommunications, various optical sensors detecting chemical agents, atmospheric or planetary gases, high-precision optical metrology and spectroscopy, and biomedical imaging (Ooi et al., 2008). In addition, it is natural to expect that narrow pulses can be generated by locking the phases of modes in this quasi-supercontinuum interband laser spectrum under mode-locked operation (Xing & Avrutin, 2005) due to the fast carrier dynamics and the broad optical gain bandwidth (Lelarge et al., 2007). Furthermore, the high power emission capability of ~1 W per device from these ultrabroadband Qdash lasers at room temperature (Tan et al., 2008) can be potentially employed as a high efficiency resonant pumping source (Garbuzov et al., 2005) for eye-safe Er-doped amplifiers and solid-state lasers.
In this chapter, we will present the generation of ultrabroad stimulated emission at room temperature operating condition in the InP-based broadband laser with wide wavelength coverage. For the first time, the InP-based unique dash quantum confined heterostructure properties is exploited to generate a broad lasing spectrum, following the prior success of short wavelength GaAs-based Qdot broadband laser. The fabricated Qdash laser diode emits at ~1.64 μm center wavelength with wide wavelength coverage of 76 nm. Unlike conventional diode lasers, the rule changing broadband lasing is obtained from the quasi-continuous interband transition by the inhomogeneous Qdash ensembles.
To further enhance the broad spectrum emission and fine tune the lasing wavelength coverage, we further engineer the bandgap energy of Qdash material with postgrowth lattice intermixing process utilizing impurity-free vacancy induced disordering (IFVD) technique. We successfully demonstrated a 100 nm wavelength blue-shifted Qdash lasers exhibiting a room-temperature broad lasing spectral coverage of ~85 nm at a center wavelength of ~1.55 μm with enhanced total emission power of ~1 W from a single as-cleaved broad area laser structure (50 x 500 μm2). The peculiar broad lasing spectra from fabricated diodes with different cavity lengths related to the effect of nonequilibrium carrier distribution in these highly inhomogeneous dashes will also be discussed.
3. Experiments and theoretical modelling
3.1. Materials and laser structure
The InAs/InAlGaAs Qdash material used in this study was grown by molecular beam epitaxy (MBE) on (1 0 0) oriented InP substrate. The laser is a p-i-n structure with active region consisting of four-sheet of InAs Qdashes, and each Qdash layer is embedded in an asymmetric InAlGaAs QW. The QWs are then sandwiched between two sets of SCHs. The Qdash-in-well structure consists of a 1.3 nm thick compressively strained In0.64Ga0.16Al0.2As layer, a five-monolayer (ML) thick InAs dash layer, and a 6.3 nm thick compressively strained In0.64Ga0.16Al0.2As layer. Each dash-in-well stack is separated by a 30 nm thick tensile strained layer of In0.50Ga0.32Al0.18As that acts as the strain compensating barrier. The lower cladding consists of a 200 nm thick In0.52Al0.48As layer doped with Si at 1 × 1018 cm-3, which is lattice matched to the InP substrate. The upper cladding and contact layers are 1700 nm thick In0.52Al0.48As and 150 nm thick In0.53Ga0.47As, respectively. Both layers are doped with Be at 2 × 1018 cm-3 (Djie et al., 2006; Wang et al., 2006). Figure 1(a) shows the plane-view atomic force microscopy (AFM) of the surface Qdash and the cross-sectional transmission electron microscopy (TEM) images of the laser structure. The Qdash structure comprises three-dimensional elongated nanostructure preferentially aligned along [ ] direction with an average height of 3.2 nm, an average width of 18 nm, and the base or length varied from 20 to 75 nm. The individual Qdash provides strong carrier confinement along y- and z- directions and weaker confinement along the x-direction [Figure 1(b-top)]. The AFM image reveals the nanostructure networks composed of dot-like and finite wire-like quantum confined structures with a bimodal height distribution profile [Figure 1(b-middle)]. The isotropic, dot-like structure with a comparable width over the length have a relatively larger height than the wire-like structure suggesting that the elongated islands are formed by the coalescence of two or more dot-like islands. Considering the large dispersion in shape, size and composition, the inhomogeneous Qdash gives a wide energy spreading in the confining potentials. This effect leads to the broadened optical gain characteristics [Figure 1(b-bottom)] suitable for the wideband optical devices such as superluminescent diodes (SLD) (Djie et al., 2006).
For the purpose of further enhancement of the broad spectrum emission and fine tune of the lasing wavelength coverage, we performed the dielectric cap annealing technique to induce selective intermixing using 475 nm thick SiO2 layer as an vacancy source deposited using plasma enhanced chemical vapor deposition system. During the annealing, the SiO2 cap will enhance the preferential atomic outdiffusion hence enhancing the group-III atomic interdiffusion in the Qdash active region and resulting in the effective bandgap modification of Qdash material (Tan et al., 2008; Wang et al., 2006). The dielectric cap also serves to protect the surface quality during annealing from the thermal induced decomposition. Following the dielectric cap removal, state-filling PL spectroscopy using a 980 nm diode laser as an excitation source was performed at 77 K to assess the bandgap modification from the interdiffusion effect on the laser structure. The IFVD process is performed by annealing the SiO2 capped sample in nitrogen ambient for one minute in a rapid thermal processor (RTP). Figure 2 gives the summary of PL peak shift and the linewidth as the annealing temperature increases from 600ºC to 850ºC. At the temperature of 750ºC, the PL peak shifts towards a shorter wavelength emission while the linewidth is the broadest. Further increase in annealing temperature initiates more intermixing, and therefore improves the uniformity in shape, size and composition of Qdash leading to reduction in PL linewidth. The result points out the linewidth broadening at intermediate stage of intermixing due to non-uniform interdiffusion, which will be further selected to broaden the Qdash laser emission.
Broad area lasers with 50 μm wide oxide stripes with no facet coating were then fabricated from both the as-grown and intermixed Qdash samples (under annealing temperature of 750 ºC ) with SiO2 capped layer. In order to maximize the gain (Ukhanov et al., 2002), the optical cavity of the laser is aligned along the  orientation and is perpendicular to the dash direction. Current injection was performed to the non-facet-coated Qdash lasers under pulsed operation at 0.2% duty cycle with a 2 μs pulse width.
3.2. Simulation model of group-III interdiffusion
The understanding of diffusion processes is important to the interpretation of interdiffusion induced compositional change and the band structure modification related to the experimental works and selected postgrowth operating conditions presented in the previous sections. In IFVD process, majority vacancies are injected vertically from the dielectric cap during thermal treatment and therefore the interdiffusion will occur more effectively in the transverse direction that corresponds to the dash height (Tan et al., 2008; Wei & Chan, 2005). This diffusion effect becomes more pronounced at very thin Qdash layer when the dash height to base ratio is of ~ 0.1 or less (Wang et al., 2006). At intermediate stage of intermixing, the partial intermixing might occur, which the thick dash family will experience a larger degree of wavelength blue-shift due to the larger concentration (Crank, 1975) of active medium composition and hence its interdiffusion length is larger than the thin dash family. The solution of the diffusion problem (Crank, 1975) in the Qdash can be estimated by an equivalent one dimensional quantum-confined model (transverse direction of vacancies interdiffusion) by assuming a substance of concentration C0 , confined in a region of n repeating well and barrier of width w and b, respectively, centered at zero (Gontijo et al., 1994) that is given by
The origin of the x coordinate is at the left barrier of the first well. The one-dimensional quantum-confined structure of four repeating wells and barriers with arbitrary width are used in the simulation model to calculate the confined ground state energy level. The chosen material system is not critical because it serves only as a reference for the change of transition energy states with diffusion length (Ld ) and well thickness. The quantum confined energy levels can be obtained by solving the one-dimensional time-independent Schrodinger equation and the results are shown in Figure 3 and its inset. Different well widths (3, 4, 5 and 8 nm) with varied Ld are used in the calculation model to represent the different dash heights in the real Qdash assembly. As Ld increases, the wavelength shift to shorter wavelength due to group-III interdiffusion. The blue-shift rate is faster at same Ld for thin nanostructure than the thick nanostructure, as stated in Figure 3. At intermediate stage of intermixing, the disparity in wavelength blueshift is notable. As intermixing proceeds further, the variation rate in wavelength blueshift becomes less until the nanostructure becomes fully intermixed and the wavelength blueshift converges. Noting a high dispersion in Qdash structure used in the experiments , widened gain characteristics can be practically achieved by selecting suitable degree of intermixing to the Qdash structure. Thereafter, the broadened linewidth can be attributed to the different intermixing results from inhomogeneous nanostructure in Qdash assembly at a medium degree of intermixing. Experimentally, this can be obtained by a given dielectric film properties heat-treated under the suitable annealing temperature and/or duration. Furthermore, variation in transition energy state is more sensitive to the Ld as compared to the well width. The thick dash family that tends to induce larger Ld contribute to larger blueshift of peak emission is as shown in the inset of Figure 3. Hence, there is a smaller peak emission blueshift in the intermixed samples as compared to the as-grown samples under high excitation.
4. Results and discussion
4.1. Optical properties of Qdash – As-Grown and intermixed materials
State-filling photoluminescence (PL) spectroscopy of as-grown Qdash samples were performed at 77 K by varying optical excitation density. As comparison, InAs Qdot embedded in InP matrix was grown and also characterized. The ground state PL peak emission is longer in Qdot as the InP matrix has a larger bandgap energy than InAlGaAs confining layers in the Qdash. Qdot structure shows well-resolved quantized states (E0 to E4) with a large energy separation between E0 and E1 (E = 34 meV) compared to Qdash characteristics (up to E4 with E = 30 meV). At similar excitation density, more states are excited in Qdot than Qdashes as a manifestation of the enhanced DOS in Qdot subbands [Figure 4]. At high excitation (1500 W/cm2), a large number of minima in Qdash spectra are populated, resulting a broad emission line while in Qdot spectra, the individual minima is more apparent. These properties corroborate the quasi-continuous interband transition characteristics in Qdash over a wide wavelength range. The discrepancies between Qdot and Qdash are due to the shape of DOS [Figure 1(b)]. Qdash with size and composition fluctuations have overlapping states with nearly identical transition energies in the high-energy portion that contributes to the gain broadening and thus produces less resolved confined state recombination in PL spectra. However, this is not the case in the Qdot assembly due to its delta function DOS leading to the atomic-line luminescence spectra.
Broad area as-grown Qdash lasers with 50 µm wide oxide stripes without facet coating were fabricated and characterized. Figure 4(b) shows the electroluminescence (EL) spectrum of the Qdash samples revealing fine structures of amplified spontaneous emission from different energy subbands in correlation to different lasing peaks. Up to three distinct laser emissions (1.65, 1.62, and 1.59 µm) from E0, E1 and E2 energy transitions were obtained at J = 1.1×Jth from lasers with cavities L of 1000, 300, and 150 µm, respectively. The distinct lasing wavelength peak is attributed to the finite modal gain of each quantized state in Qdash assembly.
Carriers localized in different dots/dashes, resulting in a system without a global Fermi function and exhibiting an inhomogeneously broadened gain spectrum, have shown an interesting phenomena of lasing spectra (Harris et al., 1998; a Djie et al., 2007 ; Tan et al., 2007; Matthews et al., 2002). This unique feature of dot/dash can be well studied after postgrowth interdiffusion technique, from the evolution of state-filling spectroscopy from intermixed Qdash structures at 77 K, as shown in Figure 5 and its inset. At low excitation below 3 W/cm2, the ground state emissions of 1.57 μm and 1.50 μm are dominant in the as-grown and the intermixed samples, respectively. The PL spectra are gradually broadened in both samples with increasing optical excitation densities. An increase in the excitation power density leads to the filling of lower-energy states, allowing recombination from higher energy levels of Qdash structure. Under the same excitation density, the PL linewidth of intermixed sample is wider than the as-grown sample. At the power excitation density of 1500 W/cm2, the PL linewidth increases by 11 nm (from 94 nm to 111 nm) after intermixing process. The phenomenon of carrier localization in Qdash becomes more evident when the intermixed sample shows a larger variation of full-width-half-maximum (ΔFWHM up to 47 nm) than the as-grown sample (ΔFWHM up to 18 nm) under various power excitation densities relative to the FWHM obtained at the optical excitation of 3 W/cm2, as shown in the inset of Figure 3.
These enormously large broadening of the PL spectra from both the as-grown and intermixed samples is attributed to the contribution of multiple transition states (a Djie et al., 2007 ) or large inhomogeneous broadening of the non-interacting Qdash ensembles (Tan et al., 2007; Van der Poel et al., 2006). This observation is also clearly different from that of both conventional QW (Ooi et al., 1997) and Qdot structures (Wang et al., 2006). The IFVD technique is generally well-known to improve the size homogeneity of a highly inhomogeneous semiconductor nanostructure system and thus will contribute to smaller variation in energy transition after intermixing. For instance, at the power excitation density of 1500 W/cm2, the PL linewidth decreases by 6 nm (from 94 nm to 88 nm) after the IFVD process is performed by annealing the SiO2 capped sample at 750ºC for two minutes (Djie et al., 2008). However, the opposite observations in the Qdash, i.e. larger PL linewidth after intermediate intermixing, suggests the presence of different interdiffusion rates at a given intermixing degree in the Qdash nanostructures as a consequence of wide variation in surface to volume ratio in Qdash ensembles. The presence of more non-interacting Qdash with wider distribution of energy levels will contribute to radiative recombination emission over larger wavelength coverage and thus a larger FWHM in PL spectra. In other words, carrier localization is more prominent in an isolated Qdash, which affects the optical properties of these material systems. Nevertheless, both intermixed and as-grown Qdash samples showing saturation of ΔFWHM at excitation power over 400 W/cm2 indicates that large degeneracy levels in highly confined energy states of Qdash is still preserved as can be seen in Qdot nanostructures (Hadass et al., 2004).
The nearly symmetric Qdash PL spectra in Figure 5 are broadened with increasing optical excitation densities. Furthermore, an increase in integrated PL intensity after intermixing occurs. All these observations are contrary to the conventional quantum-confined nanostructures. These can be attributed to the continuous PL wavelength blue-shift observed in both as-grown and intermixed samples, as shown in Figure 5, with increasing optical excitation densities. The continuous blue-shift of the PL peak wavelength up to 88 nm in the as-grown sample and 61 nm in the intermixed sample at the optical excitation density of 1500 W/cm2, relative to those obtained at the excitation of 3 W/cm2, are shown in the inset of Figure 5. The effect of band-filling is insufficient to explain the large degree of blue-shift observed from sample excited under high density excitation. Hence, it is reasonably ascribed this to the postulation of continuum states (Van der Poel et al., 2006) in the Qdash nanostructures, although spectral widening at a shorter wavelength is expected in an inhomogeneous Qdash structure (Hadass et al., 2004). Continuum states serve as an effective medium for exciton scattering and thus change the dephasing rate (Tan et al., 2007) at each energy level within the highly inhomogeneous ensembles and the radiative recombination profile will be different from that of conventional QW. The wide distribution of energy levels due to the nature of Qdash inhomogeneous (FWHM of 76 nm from PL measurement of as-grown sample at low excitation of 3 W/cm2) will further serve as the radiative recombination states or “sink” for the scattered excitons from the dense continuum states. Consequently, quasi-supercontinuum lasing spectra of the diode laser fabricated from these samples are observed, which will be discussed in the later section. Nevertheless, smaller blue-shift of PL peak wavelength in the intermixed sample, as depicted in the inset of Figure 5, indicates that IFVD enhances the Qdash inhomogeneity more so in larger sizes of Qdashes, which emit at longer wavelengths. Assuming a uniform injection of group-III vacancies from the surface during the IFVD process, the interdiffusion in the vertical direction will affect the dash height more than other directions (Djie et al., 2008; Wei et al., 2005). At an intermediate stage of intermixing, the thick dash family, where the quantized energy level located closer to the conduction band minima, will experience a larger degree of intermixing as the effective height or thickness of the dash decreases, as depicted in the inset of Figure 3. In addition, the local effective concentration for the thick dash family is higher than the thin dashes. Under uniform annealing temperature, the thick Qdash family that has larger interdiffusion length will yield larger degree of intermixing. As a result, largest degree of wavelength blue-shift (~65 nm) is observed at low excitation of 3 W/cm2 (dominant emission from thick dashes) as compared to the smaller wavelength blue-shift (~38 nm) at high excitation of 1500 W/cm2 (dominant emission from thin dashes).
4.2. Effect of nonequilibrium carrier distribution from intermixed lasers
Broad area laser characterization of the intermixed samples further provides evidence of a multi-state emission as shown in Figure 6. A spectral widening is apparent as the bias increases (Hadass et al., 2004). The emission spectra show multi-state lasing emission as injection increases to current density J of 1.5 x Jth (threshold current density) and above as opposed to a series of well-defined groups of longitudinal modes (Harris et al., 1998) emission above threshold in highly inhomogeneous Qdot. This implies the preservation of 3-dimensional energy confinement of the Qdash in addition to the emission from multiple sizes of Qdash ensembles as shown in Figure 7 and Figure 8 for fabricated lasers with different cavity lengths. The localized active region of the device can be treated as a large number of Qdot or Qdash, which can be further treated as a broad distribution of discrete energy levels (Shoji et al., 1997). This is owed to the inhomogeneous broadening nature of Qdash ensembles and the dash variation from different dash stacks. The light-current (L-I) curve of the short cavity Qdash laser (L = 300µm) yields a J th and slope efficiency of 2.3 kA/cm2 and 0.46 W/A, respectively, as depicted in Figure 7(a). Measuring the temperature dependent J th over a range of 10-50 ºC, reveals the temperature characteristic (To ) of 41.3 K. On the other hand, the long cavity Qdash laser (L = 1000µm) yields J th = 1.18 kA/cm2, slope efficiency of 0.215 W/A, and To of 46.7 K over the same temperature range, as shown in Figure 8(a).
Compared to the laser with long cavity, the shorter cavity laser exhibits the progressive appearance of short wavelength emission line with an increase in injection level. The L-I curve of the short cavity laser shows kinks as compared to the long cavity laser. The jagged L-I curve below ~3 x Jth implies that the lasing actions from different confined energy levels are not stable due to the occurrence of energy exchange between short and long wavelength
lasing modes (Hadass et al., 2004), as can be seen in the lasing spectra of Figure 7(b). In addition, the observation of kink in the L-I curve for device tested at low temperature might also be a result of mode competition in the gain-guided, broad area cavity devices. The calculated Fabry-Perot mode spacing of ~1.1 nm is well resolved in the measurement across the lasing wavelength span at low injection before a quasi-supercontinuum lasing is achieved, where the spectral ripple is less than 1 dB.
Subsequent injections contribute to the stimulated emission from longer wavelength or lower order subband energies while suppressing higher order subbands as shown in Figure 7(b). This Qdash laser behavior is fundamentally different from the experimental observation from Qdot lasers with short cavity length, where the gain of lower subband is too small to compensate for the total loss, and lasing proceeds via the higher order subbands (Markus et al., 2003; Markus et al., 2006). In short-cavity Qdash laser, the initial lasing peak at shorter wavelength (~1525 nm) is dominantly emitted from different groups of smaller size Qdash ensembles instead of higher order subbands of Qdash. Hence, the significant difference of ~11 meV as compared to the dominant lasing peak of ~1546 nm at high injection will contribute to photon reabsorption by larger size Qdash ensembles and seize the lasing actions at shorter wavelength. Regardless, a smooth L-I curve at the injection above 3 x Jth due to the only dominant lasing modes at long wavelength demonstrates the high modal gain of the Qdash active core (Lelarge et al., 2007). These observations indicate that carriers are easily overflows to higher order subbands (Tan, et al., 2009) because of the large cavity loss and the small optical gain (Shoji et al., 1997) at moderate injection. At high injection, carrier emission time becomes shorter, when equilibrium carrier distribution is reached and lasing from multiple Qdash ensembles is seized (Jiang & Singh, 1999).
On the other hand, a relatively smooth L-I curve above the threshold is observed from the long cavity intermixed Qdash laser regardless of the injection levels. The corresponding electroluminescence spectra show only one dominant lasing emission at long wavelengths, unlike, the short cavity Qdash lasers. This observation can be attributed to the effect of long cavity parameter that results in smaller modal loss as compared to short cavity Qdash devices. The progressive red-shift (~10 nm) of lasing peak with increasing injection up to J = 4 x J th and the insignificant observation of band filling effect indicates that photon reabsorption occurs due to the photon-carrier coupling between different sizes of Qdash ensembles in addition to the high modal gain of the Qdash active core (Lelarge et al., 2007). Injection above J = 4 x J th is expected to contribute to broader lasing span at long wavelength owing to the high modal gain characteristics (Tan et al., 2008) although the comparison scheme of the two devices with different cavity lengths may not be fair without applying threshold current density.
Distinctive lasing lines are observed from different cavity intermixed Qdash lasers at the near-threshold injection of J = 1.1 x J th. The similarity of lasing wavelength (inset of Figure 9) from devices with different cavity lengths further shows promise that the Qdash structures have high modal gain characteristics (Lelarge et al., 2007). However, the Qdash laser with increasing cavity length shows progressive red-shift (total of ~20 nm up to L = 1000 µm) of peak emission. This may be ascribed to the wide distribution of energy levels because of highly inhomogeneous broadening and photon reabsorption among Qdash families. At the intermediate injection of J = 2.25 x J th, simultaneous two-state laser emission, which is attributed to two groups of Qdash ensembles as mentioned previously, is noticed from short cavity Qdash lasers. On the other hand, a broad linewidth laser emission from a single dominant wavelength is shown in longer cavity Qdash lasers of 850 µm and 1000 µm, as depicted in Figure 9. As a result, a quasi-supercontinuum broad laser emission could be achieved at high injection, as shown in Figure 7. An ultrabroad quasi-supercontinuum lasing coverage from Qdash devices with L = 500µm (Tan et al., 2008) results from emission in different order of energy subbands and groups of ensemble, which will be discussed in the following section.
The broad lasing spectra from devices with different L suggest there is collective lasing from Qdashes with different geometries. However, the broad laser spectra of Qdash lasers obtained at room temperature are different from that of Qdot lasers which shows similar phenomenon but occur at low temperature below 100 K (Shoji et al., 1997; Jiang & Singh, 1999). In Qdot lasers, with increasing temperature, carriers can be thermally activated outside the dot into the well and/or barrier and then relax into a different dot (Tan et al., 2007). Carrier hopping between Qdot states can favor a drift of carriers towards the dots where the lasing action preferentially takes place, thus resulting in a narrowing of the laser mode distribution. However, in Qdash lasers, carriers will be more easily trapped in the dash ensembles due to the elongated dimension in addition to random height distribution in each ensemble. These profiles of energy potential will support more carriers, thus retarding the emission of carriers (Jiang & Singh, 1999) and resulting in a smaller homogeneous broadening at each transition energy level (Tan et al., 2007). Hence, the actual carrier distribution in Qdash nanostructures will be at high nonequilibrium and lead to broadband lasing even at room temperature.
4.3. Ultrabroadband lasers - as-grown and bandgap tuned devices
Figure 11(a) shows the light-current (L-I) characteristics of the as-grown Qdash laser (L = 600 µm). The corresponding Jth and slope efficiency are 2.6 kA/cm2 and 0.165 W/A. Up to 400 mW total output power has been measured at J = 4.5×Jth at 20ºC, which is significantly higher than the SLED fabricated from the same wafer (Djie et al., 2006). From the dependence of Jth on temperature, the temperature characteristic T0 of 43.6 K in the range of 10 to 70ºC has been obtained. At J< 1.5×Jth , there is only ground state lasing E0 with the wavelength coverage of ~10 nm [Figure 11(b)]. The broad E0 lasing spectrum suggests the collective lasing from Qdashes with different geometries. At J> 1.5×Jth , the bi-state lasing is noted. The simultaneous lasing from both E0 and E1 is attributed to the relatively slow carrier relaxation rate and population saturation in the ground state in low-dimensional quantum heterostructures (Zhukov et al., 1999). The transition from mono-state to bi-state lasing is marked with a slight kink in the L-I characteristics. The bi-state lasing spectrum is progressively broadened with increasing carrier injection up to a wavelength coverage of 54 nm at J = 4.5×Jth . The corresponding side-mode suppression ratio is over 25 dB and a ripple measured from the wavelength peak fluctuation within 10 nm span is less than 3 dB.
Bangap-tuned broad area lasers with optimum cavity length (L = 500 μm) that gives largest quasi-supercontinuum coverage of lasing emission, as presented in Figure 10, are fabricated. The L-I curve of the Qdash laser yields an improved J th and slope efficiency of 2.1 kA/cm2 and 0.423 W/A, which is depicted in Figure 12(a), as compared to that of as-grown laser with 2.6 kA/cm2 and 0.165 W/A, respectively (b Djie et al., 2007 ). The L-I curve of the intermixed laser shows kinks, which is similar to that of short cavity L = 300 µm Qdash lasers. The energy-state-hopping instead of mode-hopping occurs due to the wide distribution of the energy levels across the highly inhomogeneous Qdash active medium, as derived from the
PL results. In spite of that, a smooth L-I curve above 6 kA/cm2 yields a total high power of ~1 W per device at room temperature before any sign of thermal roll-over. This shows that injection above 6 kA/cm2 provides enough carriers for population inversion in all the available or possible radiative recombination energy states and thus the energy-state-hopping is absent.
Measuring the temperature dependence J th over a range of 10-60 ºC reveals the improved To of 56.5 K as compared to the as-grown laser of 43.6 K (b Djie et al., 2007 ). This result is comparable to the To range (50-70 K) of the equivalent QW structure. In Figure 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 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 Figure 11 and Figure 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 Figure 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
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 Figure 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.
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.