1. Introduction

Photonic crystal (PC) lasers have a high potential for compact coherent light sources, which could be useful for constructing photonic integrated circuits [1]. For them to be practical in real applications, however, the issue of continuous-wave (CW) operation of the PC lasers at room-temperature is of great importance. Typical PC lasers are in the form of a thin membrane clad by low refractive index media, such as air or SiO₂, to confine optical modes in the vertical direction. Consequently, heat dissipation through the cladding materials of poor thermal conductivity is a serious problem, which has hindered stable CW operation of PC lasers. So far, CW operations have been reported in a few different schemes: PC lasers wafer-bonded on the substrate with high thermal conductivity [2–4], an air-bridge PC laser with extremely high quality factor [5], and buried heterostructure laser with the active region embedded in an InP PC backbone [6]. Nevertheless, previous reports of CW lasing have not discussed device lifetime.

Here we report a breakthrough we recently achieved in CW operation of PC lasers. By employing a novel submount structure for efficient heat dissipation, we were able to demonstrate CW operation for a record-long operation lifetime over 1 hour. In addition, a combination of the surface-emitting Γ-point band-edge mode and butt-end fiber coupling enabled high fiber-coupling up to nearly 200 μW, the highest CW fiber-coupled output power from any kind of PC laser. Our achievements should be an important milestone toward stable CW operation of PC lasers with a long lifetime.

2. Design and fabrication

We employed band-edge lasers (BELs) because they allow high-power operation, which is a consequence of the nature of their open cavity structure and resulting large mode volume. We utilized one of the Γ-point band-edges (Fig. 1(a) ), at which laser emission occurs mainly in the vertical direction; this is ideal for fiber coupling [7,8]. An InGaAsP multiple-quantum-well (MQW) structure was grown by metalorganic chemical-vapor-deposition (MO-CVD) to emit in the optical communication wavelength region. An InP etch-sacrificial layer and InGaAs etch-stop layer were grown underneath the MQW layer for the epitaxial separation of the MQW layer from the substrate. Square-lattice air-hole PCs (lattice constant of a = 600 nm and air-hole radius of r = 0.20a) were formed into the InGaAsP MQW structure by electron-beam lithography and reactive ion etching. The area of the PC pattern was about 75 × 75 μm². A scanning-electron-microscope (SEM) image of a fabricated BEL is shown in the inset of Fig. 1(a).

 

Fig. 1 (a) Photonic band structure of square-lattice air-hole BEL bonded to MgF₂, calculated by three-dimensional plane-wave expansion method (insets: SEM image of fabricated BEL device and magnetic field distribution profile of the Γ₁ band-edge mode). (b) Schematic cross-sectional view of BEL with butt-end fiber tip for simultaneous optical pumping and output coupling.

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In parallel to the MQW wafer growth, a submount structure for heat management was prepared separately. On top of the (100) silicon substrate, layers of 2-μm-thick diamond (for heat spread/dissipation) and 300-nm-thick MgF₂ (for vertical mode confinement) were deposited sequentially. While diamond naturally has a very high thermal conductivity (~1800 W/m·K), MgF₂, to which the InGaAsP layer is directly bonded, also has relatively high thermal conductivity (~14 W/m·K) despite its low refractive index (n = 1.37).

Then, the MQW wafer surface was covered with black wax (Apiezon-W) at an elevated temperature of 125 °C; this process was followed by sequential chemical etching of the InP substrate, InGaAs etch-stop layer, and InP etch-sacrificial layer in HCl, FeCl₃, and HCl, respectively. While wet, the remaining thin MQW film attached to the black wax was squeezed against the Si-based submount described above. After overnight curing, the black wax was dissolved in trichloroethylene to finish the BEL device fabrication. Details of the original van der Waals bonding can be found in Ref. 9. Figure 1(b) shows the schematic diagram of the completed BEL.

3. Measurements

The BEL device prepared as such was then mounted on a thermoelectric cooler maintained at 15 °C and optically pumped by a 980-nm CW laser diode. We utilized a 1 × 2 wavelength-division-multiplexing (WDM) fiber coupler for the simultaneous optical excitation and output coupling of the Γ-point BEL with a single butt-end fiber tip [7,8]. We employed a multimode fiber (core diameter: ϕ = 50 μm) to achieve large-area excitation and thus realize high-power operation. The output power of the BEL was measured as a function of the pump laser power ― Fig. 2(a) . A sharply defined laser threshold and constant slope efficiency are evident. We obtained fiber-coupled CW output powers of up to ~200 μW, which is the highest reported CW output from any type of PC laser. The CW laser spectra are also shown in the figure. Above the laser threshold, a single sharp laser line becomes dominant, with its side-mode-suppression-ratio greater than 30 dB. We identified the Γ₁ band-edge as the lasing mode by comparing the spectral positions of the three emission peaks with the calculated band-edge positions (Fig. 1(a)).

 

Fig. 2 (a) Optical input-output relationship under CW operation and optical spectra below and above laser threshold. (b) 3D plot of CW emission spectra as a function of operation time (insets: Nomarski optical microscope images for CW BEL device before and after failure).

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We also performed a test to determine the device lifetime under CW operation. Starting with 10-μW output power, the BEL devices were driven in constant current mode. A large fraction of the tested devices survived beyond 1 h (Fig. 2(b)). We attribute the failure of the devices that failed to weak van der Waals force at the bonding interface; the interface progressively gets weaker during device operation. The void-filled columnar structure and rough topography of the vacuum-deposited MgF₂ layer are also responsible for the susceptibility of the interface [10]. In fact, optical microscopy revealed that the bonded InGaAsP epilayer was heavily blistered after failure (insets in Fig. 2(b)). We expect that a considerably longer lifetime will be attained if the techniques for high quality MgF₂ film deposition with a smooth surface and for high-quality wafer-bonding for bonding the InGaAsP and MgF₂ layers are developed.

4. Analysis of thermal characteristics

Numerical simulations were performed using a commercial finite element code (Multiphysics, COMSOL) to examine the thermal properties of the BEL devices. In the simulations, we assumed uniform optical pumping across 50 × 50 μm² area of the InGaAsP layer and at 30% absorption of the 40-mW pump power [11]. We set the thermal conductivity of each material as the typical value found in the literature: 163 W/m·K for silicon, 1800 W/m·K for diamond, 14 W/m·K for MgF₂, and 4 W/m·K for InGaAsP. Figures 3(a) and 3(b) show the contour plots of the lateral and vertical components of the heat flux at the midplane of the diamond layer. For comparison, we also performed simulations for another CW laser mentioned in the literature [3] and used it as reference; in this laser, the epitaxial InGaAsP film was directly bonded to a sapphire substrate (Figs. 3(c) and 3(d)). The simulation results revealed that the diamond layer spreaded the generated heat in the lateral direction very efficiently; this helped the heat to dissipate across a much larger area. The steady-state device temperatures of the BEL and reference laser structures were also determined by simulations as ~85 °C (increased by 70° from the heat sink temperature 15°) and ~220 °C (increased by 205°), respectively. The absolute temperatures may not be very accurate or meaningful; however, the temperature difference between the two devices is a good measure of the effectiveness of the submount structure that we developed. Therefore, we conclude that the MgF₂/diamond layers play a pivotal role in the CW operation of BEL.

 

Fig. 3 Simulated heat-flux distributions of (a), (b) CW laser structure (InGaAsP/MgF₂/diamond/Si) and (c), (d) laser structure bonded to sapphire substrate (InGaAsP/Al₂O₃). (a) and (c) indicate the lateral components (Φ_//), and (b) and (d) indicate the vertical components (Φ_⊥) of heat flux. The actual simulations were performed only for one quarter of the space while symmetric boundary conditions were applied to the remaining space. In the figure, xz and yz planes represent symmetric boundaries. The dashed lines in contour plots correspond to the heated areas in the schematic device diagrams.

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It is rather worthwhile to estimate and compare the thermal resistances of various PC laser structures. The thermal resistance of a laser structure is defined as the rate of change of device temperature in response to the increase in absorbed pump power; this can be formulated as $R_{therm}=\frac{\Delta T}{\Delta P}.$Experimentally, the thermal resistance can be determined indirectly by measuring the wavelength shift Δλ of the laser because $R_{therm}=\frac{\Delta T}{\Delta P}=\frac{\Delta T}{\Delta \lambda }\cdot \frac{\Delta \lambda }{\Delta P}$ [12,13]. We first measured the lasing wavelength in the pulsed operation mode (1% duty cycle) as a function of the heat sink temperature. At such a low duty cycle, the device temperature can be assumed to be the same as the heat sink temperature. Using the measured data which is shown in Fig. 4(a) , we estimated the rate of lasing wavelength shift with respect to temperature change, $\frac{\Delta \lambda }{\Delta T}$, to be approximately 0.112 nm/K. Next, the dependence of the lasing wavelength on the absorbed pump power under CW condition was measured at a fixed heat sink temperature of 15 °C ― Fig. 4(b); this gave a value of $\frac{\Delta \lambda }{\Delta P}\approx$ 0.0956 nm/mW. Using these two measured values, we determined the thermal resistance of our PC BEL as R_therm ≈0.854 K/mW. This value is comparable to the reported thermal resistance of vertical-cavity surface-emitting lasers [14], namely ~1 K/mW. For comparison, we provide the thermal resistance of other PC lasers: ~11 K/mW for SiO₂-clad and ~2.3 K/mW for sapphire-bonded devices; the thermal resistance of the air-bridge type PC lasers is 10⁶ K/mW [15].

 

Fig. 4 Measured data to determine thermal resistance of CW BEL structure. (a) Lasing wavelength as a function of heat sink temperature measured at low (1%) duty cycle. (b) Lasing wavelength versus absorbed pump power under CW operation.

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5. Conclusions

In conclusion, by employing the Γ-point band-edge mode and an efficient heat spreading/dissipating submount, we demonstrated the high output power and long lifetime of PC lasers under CW operation. This proves that many (if not all) of the presently problematic issues of PC lasers could be solved and that practical PC lasers for real applications should emerge soon.