In this chapter, we demonstrated the first observation of room-temperature many-body state luminescence in a quasi-2D confined system via the stronger confined energy and resonant periodical gain enhancement. The temperature dependence of the many-body state luminescence features have also been shown to investigate the nonradiative recombination effect. The evolution of the integrated PL intensity at different temperature segments exhibit same PL quenching phenomenon as in the exciton or electron-hole pair luminescence of other MQW structures. The reduction of radiation intensity at distinct potion could be characterized by two activation energies related to different nonradiative recombination mechanisms.
Although the origin of the activation energy at the intermediate temperature region
100 120 140 160 180 200 220 240 260 280 300 0
25 50 75 100 125 150 175 200 225
Temperature (K) Excitation threshold intensity (kW/cm2 )
Fig. 2.3-6 Temperature dependence of the excitation threshold intensity of the EHP renormalized state emission.
which may related to the dissociation of many-body state is still unclear, the decreasing PL intensity at high temperature is caused by thermally activated carrier leakage to the barrier state and, as a result, the nonradiative recombination process is strengthened. Because the activation energy of the thermal carrier emission has been reported to be equal to half of the total confined energy of electron and hole states, the half confined energy in our MQW sample found to be two thirds of the fitted activation energy confirms the gain enhancement of the periodically aligned MQW structure. At the end the excitation threshold of renormalized state is shown to be exponentially increased with increasing temperature. This result reveals the importance of the RPG structure in the MQW fabrication to the observation of the room-temperature spontaneous renormalized state emission.
Reference
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Chapter Three
High-Peak-Power and High-Repetition-Rate 1.2-1.6 μm Optically-Pumped Semiconductor Lasers
3.1 Thermal management of the optically-pumped semiconductor lasers
The optically-pumped semiconductor lasers (OPSLs) introduced in section 1.3 are capable of producing the high power, diffraction limited and single transverse mode beam output with the wide wavelength diversity. To achieve power scalability, the thermal management of the OPSLs should be efficient to prevent thermal degradation of the output performance under high pump power. The origin of thermal roll-over effect in OPSLs is mainly resulted from two factors. First, the cavity gain of the OPSLs is decided by the peak of the photo-luminescence (PL), micro-cavity resonance formed by the air-DBRs and air-cap layer interfaces and the resonant-periodic-gain structure. Under high power excitation, the quantum-wells (QWs) emission peak will red-shift at the rate three or four times faster than the later two issues in which the offset rates are determined by the optical thickness [1].
Consequently, the PL peak will deviate from the sub-cavity resonance and the assigned periodic gain structure and the output power will be lowered with increasing pump power. Second, the thermally-activated carriers in the QWs region will acquire additional kinetic energy to overcome the confined potential and escape away from the wells to barrier region. As a result, the nonradiative recombination process such as surface recombination or the Auger recombination will be enhanced under the large amount of thermal load. To prevent these thermal degradation mechanisms, the
efficient thermal management is required which will be elucidated in detail in the following.
The conventional pump approach of the OPSLs is exciting the carriers from the barrier regions and the excited carriers will relax to the QW region to provide the emission gain. The inherent quantum defect between the energy difference of the barrier and QW area is inevitable and leads to a large amount of heat. Recently, a novel optical pumping method in which the carriers are directly excited at the QW region and the thermal load resulted from the quantum defect is significantly lowered is demonstrated by Schmid et al [2]. This in-well pump scheme is of practical importance in the long wavelength OPSLs which have a large energy difference between the barrier potential height and the QW ground state confined energy [3,4].
The comparison between the barrier and in-well pump scheme under the same MQW fabrication but different pump absorption photon energy is also presented [3,5] and the schematic illustrations are shown in Fig. 3.1-1 (a) and (b).
Despite the reduction of the inherent heat generation, the effective heat dissipation method is more vital for power scalable OPSLs. There are two mostly used heat removal methods which are deployed in a variety of OPSLs successfully to date. One is the “substrate removal” technique in which the MQW gain mirror is fabricated in reverse order in comparison to the configuration depicted in Fig. 1.3-1.
Under this so called “thin-device” method, an etch-stop layer is first grown on the substrate then the cap layer and the periodically aligned MQW structure are deposited in turn. At the end the top of the gain mirror is finished by packaging pairs of distributed Bragg reflector (DBR) structure on it. After the epitaxial growth, the gain chip is soldered on the diamond heat sink at the DBR side and substrate of the total composite is removed by chemical etching process to reduce the intra-cavity loss. The
total configuration is shown in Fig. 3.1-2 (a). Under this framework, the heat generated at the active region could be transmitted to the heat sink via the DBR stack with thickness of several of microns. As a comparison, the mirror-on-substrate configuration depicted in Fig. 1.3-1 with thickness of several tens to hundreds of microns contributes tremendous thermal impedance to the OPSLs. Several efficient OPSLs with output power higher than 5 W have been presented under substrate removal method [6-9]. But these lasers are concentrated on the spectral region around 1 μm with GaAs-based substrate system. It is because the mostly used GaAs/AlAs DBRs in GaAs-based system have good thermal conductivity and high refractive index difference. The DBRs at spectral range longer than 1 μm based on the InP substrate system suffers from low thermal conductivity and large number of pairs of DBR layers due to low refractive index difference and longer λ/4 length. This situation will give rise to high thermal impedance and pretty low heat dissipated efficiency. To conquer this dilemma, the so called “intracavity heat spreader” method has been demonstrated in optically-pumped edge-emitting [10,11] and surface-emitting [12-18] lasers. In this approach the gain chip is fabricated in the same direction as shown in Fig. 1.3-1 and an intra-cavity transparent plate with high thermal conductivity is liquid capillary bonded on the epitaxial side of MQWs. This intra-cavity element need to be transparent with low absorption and scattering and polarization loss to the pump and lasing emission and should have high thermal conductivity to bypass the heat from the substrate region [1,19-21]. Beside, the thickness of the heat spreader could be adjusted to complete single frequency operation or turning spectral range of the OPSLs [22,23]. Several materials such as sapphire, silicon carbide and diamond are applied as the heat spreader in the OPSLs [2,12-18,24]. However, the single crystal CVD diamond with thermal conductivity as high as 2000 Wm-1K-1 and controlled birefringence is mostly used to be the heat
Fig. 3.1-1 The schematic illustrations of (a) barrier and (b) in-well pump configurations of MQWs.
Pump beam Lasing beam
QW barrier
barrier
nC=1 nC=2
nV=1
Pump beam Lasing beam
QW barrier
barrier
nC=1 nC=2
nV=1 nV=2
nV=2
(a)
(b)
Fig. 3.1-2 The schematic configurations of the two thermal management using (a) thin-device and (b) intra-cavity heat spreader methods.
Copper heat sink Heat spreader MQWs DBRs
active region
Removed substrate
Cap layer Etch-stop
layer
(a)
(b)
Copper heat sink Heat spreader
DBRs active regionMQWs
Substrate
Cap layer
spreader to date. Many experimental and theoretical investigations have compared the performance of these two methods under different thermal conditions and pump-spot radius [1,20,25,26]. Although the highest output power record of the OPSLs is achieved using the thin device method at around 1 μm, the heat spreader method without post-processing and independent of the construction of DBRs is suitable for the heat dissipation in the OPSLs operated beyond 1 μm. The typical configuration of this heat dissipation method is depicted in Fig. 3.1-2 (b).
3.2 1220 nm AlGaInAs multiple-quantum-wells lasers
High-peak-power all-solid-state lasers operating at 1.14-1.25 μm are desirable for producing yellow-orange lights for many applications such as astronomy community, biomedical optics and laser absorption spectroscopy [27,28]. Light sources in this spectral range could be realized by Raman-shifted Nd- (Yb-) doped solid-state lasers or directly using the Yb- (Bi-) doped fiber lasers [29-31]. However, the performance of solid-state lasers is limited by the discrete energy level of the doped ions of the dielectrics. Alternatively, OPSLs have been developed to provide flexible choice of emission wavelength via bandgap engineering with scalable output power as mentioned in second 1.3 and also offer a variety of advantages like broad gain curves and a low-divergence, circular and high quality nearly-diffraction-limited output beam [6-9,12-18,32,33]. So far, the lasers based on the quantum confined structure with GaAs material systems including InGaAs/GaAs and GaInNAs/GaAs have been demonstrated in the 1.14-1.25 μm spectral range under continuous-wave operation [15,16,34,35]. But these lasers are pumped by exciting the electrons from the barrier region and the quantum defect between pump and lasing photons leads to a large amount of heat. Recently, a novel method based on the in-well pumping scheme has been demonstrated to reduce the heat as described in section 3.1. However, the
OPSLs at 1.14-1.25 μm based on the in-well pumping scheme have not been explored until now.
The quaternary alloys lattice matched to InP such as AlGaInAs and InGaAsP are employed in the semiconductor lasers in the near-infrared (NIR) spectral region. The AlGaInAs systems have been verified to have higher conduction band offset and better carrier confinement than the InGaAsP systems. This means that the resistance to heat of AlGaInAs based materials is stronger than InGaAsP. Several high-peak-power AlGaInAs OPSLs have been demonstrated in the NIR region driven by the actively Q-switched (AQS) solid-state lasers [5,36,37]. With the pulsed pumping operation, not only the heat generation is significantly reduced but the high-peak-power output is obtained. Because the pulse width of conventional AQS solid-state lasers depends on the pulse repetition rate and the average pump power. Consequently, it is difficult to optimize the output performance of the semiconductor disk lasers. Therefore, a light source with fixed pulse duration under various repetition rates can be a more suitable pump source for optimizing the performance of OPSLs. High-power pulsed fiber amplifiers are a light source to satisfy this requirement [38].
In this section, we present a high-peak-power AlGaInAs multiple-quantum-well (MQW) semiconductor laser grown on a Fe-doped InP transparent substrate and pumped by a 1.06 μm Yb-doped pulsed fiber amplifier. With in-well pumping, the thermal and roll-over effect could be reduced by lowering the quantum defect. We obtained an average output power of 810 mW at 1225 nm with slope efficiency up to 46.7 % to the average absorbed power in the single-chip scheme. The pump conditions of 60 kHz pulse repetition rate and 28 ns pulse width are used. To increase the average absorbed power, the double-chips scheme is used under the same pump conditions. The maximum average output power could be scaled up to 1.28 W with
slope efficiency of 37.5% at 1225 nm lasing wavelength. The maximum peak output power of 0.76 kW is obtained with 2.37 kW peak pump power.
3.2.1 Device fabrication and experimental setup
Figure 3.2-1 shows the experimental configuration of the AlGaInAs MQW 1220 nm semiconductor disk laser pumped by a SPI 1.06 μm Yb-doped master oscillator fiber amplifier. This pump source provides 9-200 ns pulse with repetition rate ranged from 10-500 kHz. Compared to the AQS solid-state laser, this laser module can provide fixed pulse width of output pulse even when the output power is changed. We controlled pump spot diameter to be about 800 μm to have efficient spatial overlap with the lasing mode. The gain region is composed of an AlGaInAs QW/barrier structure grown on a Fe-doped InP transparent substrate by metalorganic chemical-vapor deposition. It consisted of 30 groups of triple QWs spaced at half-wavelength intervals by AlGaInAs barrier layers as shown in the inset of Fig.
3.2-1. This is a resonant-periodic-gain structure that barrier layers are used to locate the quantum well region at the anti-node of the lasing field standing wave as discussed in section 1.3. Under this framework, the wavelength and optical gain are enhanced and the amplified spontaneous emission in the lateral direction and spatial hole burning effect are inhibited. A window layer of InP was deposited on the gain structure to prevent surface recombination and oxidation. In contrast to the conventional barrier pumping scheme, our gain medium is in-well pumped by a 1.06 μm Yb-doped pulsed fiber amplifier. This pumping scheme results in the low absorption (58%) but high conversion efficiency due to the short active region and the small quantum defect, respectively.
The InP based systems suffer from the lack of good distributed-Bragg-reflector (DBR) and have been challenging to transfer from edge-emitting lasers to
Fig. 3.2-1 Experimental configuration of AlGaInAs/InP semiconductor laser at 1220 nm pumped by a 1.06 μm Yb-doped pulsed fiber amplifier in the single chip scheme.
surface-emitting lasers. A number of lattice-matched DBRs such as AlGaInAs/AlInAs, AlGaInAs/InP, GaInAsP/InP, AlGaAsSb/AlAsSb and AlGaAsSb/InP have been demonstrated [39,40]. Unfortunately, these DBR systems suffer from the low refractive index contrast, low thermal conductivity or high complexity of growth.
Therefore, Fe-doped InP with good transparency in the lasing wavelength is chosen as the substrate system instead of conventional S-doped InP with large absorption in the 1.0-2.0 μm spectral region. As a result, the function of DBRs could be replaced by an external mirror. In this configuration, the problem of fabrication of good DBRs has been resolved and the heat dissipation is improved by reducing the length of thermal conduction.
The laser gain medium is fabricated with dielectric coated mirror on the cap layer which is acted as a front mirror to simplify the device configuration. This forms high transmittance at 1.06 μm (T>90%) and high reflectance between 1.18-1.25 μm (R>99.8%) on the entrance face. We use an external mirror with radius of curvature of 250 mm and partial reflectance at 1.22 μm (R=89%) as output coupler. The overall cavity length is about 3 mm. With this plano-concave linear cavity, we could modulate the laser mode volume to have better efficiency. The gain medium is attached on a cooper heat sink with substrate side and is cooled down by water with temperature controlled to be 15 oC.
3.2.2 Experimental results and discussions
Figure 3.2-2 shows the room temperature spontaneous-emission spectrum of AlGaInAs MQWs with dielectric coated mirror excited by the 1.06 μm Yb-doped pulsed fiber amplifier with average absorbed power of 0.38 W. The pump repetition rate is 60 kHz and the pump pulse width is 28 ns. This surface emitting photoluminescence (PL) spectrum is captured with the pump beam incident on the
Fig. 3.2-2 Room temperature surface emitting spontaneous emission spectrum under 60 kHz pump repetition rate and 28 ns pump pulse width at average absorbed power of 0.38 W. Inset, the expanded lasing spectrum obtained with 0.85 W average absorbed power under the same pump conditions.
Wavelength (nm)
1206 1208 1210 1212 1214 1216 1218 1220
Intensity (a.u.)
1206 1208 1210 1212 1214 1216 1218 1220 0.0
dielectric coated side, and the emitted light is collected into the multi-mode fiber on the other side. The spectral information was monitored by an optical spectrum analyzer (Advantest Q8381A) with a diffraction monochromator which can be used for high-speed measurement of pulsed light with a resolution of 0.1 nm. The PL peak is located at 1215 nm. The expanded lasing spectrum is shown in the inset of Fig.
3.2-2 at the average absorbed power of 0.85 W. The bandwidth of lasing spectrum is about 9 nm, and it comprises dense longitudinal modes due to the multiple
3.2-2 at the average absorbed power of 0.85 W. The bandwidth of lasing spectrum is about 9 nm, and it comprises dense longitudinal modes due to the multiple