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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 1 — Jan. 3, 2011
  • pp: 1–6
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Over 1 hour continuous-wave operation of photonic crystal lasers

Sunghwan Kim, Jeongkug Lee, and Heonsu Jeon  »View Author Affiliations


Optics Express, Vol. 19, Issue 1, pp. 1-6 (2011)
http://dx.doi.org/10.1364/OE.19.000001


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Abstract

We report on the long lifetime (>1 hour) of photonic crystal (PC) lasers under continuous-wave (CW) operation. For stable CW operation, we van-der-Waals-bonded our PC lasers to a novel submount structure consisting of MgF2-diamond bilayers on silicon substrate, which simultaneously ensures vertical mode confinement and efficient heat spread/dissipation. The combination of a Γ-point band-edge mode and butt-end fiber coupling yielded high CW fiber-coupled output power (~200 μW). The results demonstrate that the CW lifetime of PC lasers can be extended to the level for practical applications.

© 2010 OSA

1. Introduction

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)
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.
), at which laser emission occurs mainly in the vertical direction; this is ideal for fiber coupling [7

7. Y. Park, S. Kim, C. Moon, H. Jeon, and H. J. Kim, “Butt-end fiber coupling to a surface-emitting Γ-point photonic crystal bandedge laser,” Appl. Phys. Lett. 90(17), 171115 (2007). [CrossRef]

,8

8. S. Kim, Y. Park, K. Hwang, J. Lee, H. Jeon, and H. Y. Kim, “High power and large alignment tolerance fiber coupling of honeycomb-lattice photonic crystal Γ-point band-edge laser,” J. Opt. Soc. Am. B 26(7), 1330 (2009). [CrossRef]

]. 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).

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

9. E. Yablonovitch, D. M. Hwang, T. J. Gmitter, L. T. Florez, and J. P. Harbison, “Van der Waals bonding of GaAs epitaxial liftoff films onto arbitrary substrates,” Appl. Phys. Lett. 56(24), 2419 (1990). [CrossRef]

. 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

7. Y. Park, S. Kim, C. Moon, H. Jeon, and H. J. Kim, “Butt-end fiber coupling to a surface-emitting Γ-point photonic crystal bandedge laser,” Appl. Phys. Lett. 90(17), 171115 (2007). [CrossRef]

,8

8. S. Kim, Y. Park, K. Hwang, J. Lee, H. Jeon, and H. Y. Kim, “High power and large alignment tolerance fiber coupling of honeycomb-lattice photonic crystal Γ-point band-edge laser,” J. Opt. Soc. Am. B 26(7), 1330 (2009). [CrossRef]

]. 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)
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).
. 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)).

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

10. M.-C. Liu, C.-C. Lee, M. Kaneko, K. Nakahira, and Y. Takano, “Microstructure of magnesium fluoride films deposited by boat evaporation at 193 nm,” Appl. Opt. 45(28), 7319–7324 (2006). [CrossRef] [PubMed]

]. 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

11. M. Fujita and T. Baba, “Microgear laser,” Appl. Phys. Lett. 80(12), 2051 (2002). [CrossRef]

]. 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)
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.
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

3. M. H. Shih, M. Bagheri, A. Mock, S. J. Choi, J. D. O’Brien, P. D. Dapkus, and W. Kuang, “Identification of modes and single mode operation of sapphire-bonded photonic crystal lasers under continuous-wave room temperature operation,” Appl. Phys. Lett. 90(12), 121116 (2007). [CrossRef]

] 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.

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 Rtherm=ΔTΔP.Experimentally, the thermal resistance can be determined indirectly by measuring the wavelength shift Δλ of the laser because Rtherm=ΔTΔP=ΔTΔλΔλΔP [12

12. W. W. Bewley, C. L. Felix, E. H. Aifer, D. W. Stokes, I. Vurgaftman, L. J. Olafsen, J. R. Meyer, M. J. Yang, and H. Lee, “Thermal characterization of diamond-pressure-bond heat sinking for optically pumped mid-infrared lasers,” IEEE J. Quantum Electron. 35(11), 1597–1601 (1999). [CrossRef]

,13

13. L. Lu, A. Mock, M. Bagheri, J.-R. Cao, S.-J. Choi, J. O’Brien, and P. D. Dapkus, “Gain compression and thermal analysis of a sapphire-bonded photonic crystal microcavity lasers,” IEEE Photon. Technol. Lett. 21(17), 1166–1168 (2009). [CrossRef]

]. 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)
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.
, we estimated the rate of lasing wavelength shift with respect to temperature change, ΔλΔT, 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 ΔλΔP 0.0956 nm/mW. Using these two measured values, we determined the thermal resistance of our PC BEL as Rtherm ≈0.854 K/mW. This value is comparable to the reported thermal resistance of vertical-cavity surface-emitting lasers [14

14. D. L. Mathine, H. Nejad, D. R. Allee, R. Droopad, and G. N. Maracas, “Reduction of the thermal impedance of vertical‐cavity surface‐emitting lasers after integration with copper substrates,” Appl. Phys. Lett. 69(4), 463 (1996). [CrossRef]

], 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

15. M. H. Shih, A. Mock, M. Bagheri, N.-K. Suh, S. Farrell, S.-J. Choi, J. D. O’Brien, and P. D. Dapkus, “Photonic crystal lasers in InGaAsP on a SiO(2)/Si substrates and its thermal impedance,” Opt. Express 15(1), 227–232 (2007). [CrossRef] [PubMed]

].

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.

Acknowledgments

This study was supported by the Mid-career Researcher Program funded by the National Research Foundation (2010-0014470), by the World-Class University (WCU) Project funded by the Ministry of Education, Science & Technology of Korea (R31-2009-100320), and in part by the National Research Foundation of Korea through Inha University (2010-0001476).

References and links

1.

O. Painter, R. K. Lee, A. Scherer, A. Yariv, J. D. O’Brien, P. D. Dapkus, and I. Kim I, “Two-dimensional photonic band-Gap defect mode laser,” Science 284(5421), 1819–1821 (1999). [CrossRef] [PubMed]

2.

Y. H. Lee, D. H. Jang, H. K. Park, I. Y. Han, D. S. Song, H. Y. Ryu, and J. K. Hwang, “Continuous room-temperature operation of optically pumped two-dimensional photonic crystal lasers at 1.6 μm,” IEEE Photon. Technol. Lett. 12(10), 1295–1297 (2000). [CrossRef]

3.

M. H. Shih, M. Bagheri, A. Mock, S. J. Choi, J. D. O’Brien, P. D. Dapkus, and W. Kuang, “Identification of modes and single mode operation of sapphire-bonded photonic crystal lasers under continuous-wave room temperature operation,” Appl. Phys. Lett. 90(12), 121116 (2007). [CrossRef]

4.

G. Vecchi, F. Raineri, I. Sagnes, A. Yacomotti, P. Monnier, T. J. Karle, K.-H. Lee, R. Braive, L. Le Gratiet, S. Guilet, G. Beaudoin, A. Taneau, S. Bouchoule, A. Levenson, and R. Raj, “Continuous-wave operation of photonic band-edge laser near 1.55 µm on silicon wafer,” Opt. Express 15(12), 7551–7556 (2007). [CrossRef] [PubMed]

5.

K. Nozaki, S. Kita, and T. Baba, “Room temperature continuous wave operation and controlled spontaneous emission in ultrasmall photonic crystal nanolaser,” Opt. Express 15(12), 7506–7514 (2007). [CrossRef] [PubMed]

6.

S. Matsuo, A. Shinya, T. Kakitsuka, K. Nozaki, T. Segawa, T. Sato, Y. Kawaguchi, and M. Notomi, “High-speed ultracompact buried heterostructure photonic-crystal laser with 13 fJ of energy consumed per bit transmitted,” Nat. Photonics 4(9), 648–654 (2010). [CrossRef]

7.

Y. Park, S. Kim, C. Moon, H. Jeon, and H. J. Kim, “Butt-end fiber coupling to a surface-emitting Γ-point photonic crystal bandedge laser,” Appl. Phys. Lett. 90(17), 171115 (2007). [CrossRef]

8.

S. Kim, Y. Park, K. Hwang, J. Lee, H. Jeon, and H. Y. Kim, “High power and large alignment tolerance fiber coupling of honeycomb-lattice photonic crystal Γ-point band-edge laser,” J. Opt. Soc. Am. B 26(7), 1330 (2009). [CrossRef]

9.

E. Yablonovitch, D. M. Hwang, T. J. Gmitter, L. T. Florez, and J. P. Harbison, “Van der Waals bonding of GaAs epitaxial liftoff films onto arbitrary substrates,” Appl. Phys. Lett. 56(24), 2419 (1990). [CrossRef]

10.

M.-C. Liu, C.-C. Lee, M. Kaneko, K. Nakahira, and Y. Takano, “Microstructure of magnesium fluoride films deposited by boat evaporation at 193 nm,” Appl. Opt. 45(28), 7319–7324 (2006). [CrossRef] [PubMed]

11.

M. Fujita and T. Baba, “Microgear laser,” Appl. Phys. Lett. 80(12), 2051 (2002). [CrossRef]

12.

W. W. Bewley, C. L. Felix, E. H. Aifer, D. W. Stokes, I. Vurgaftman, L. J. Olafsen, J. R. Meyer, M. J. Yang, and H. Lee, “Thermal characterization of diamond-pressure-bond heat sinking for optically pumped mid-infrared lasers,” IEEE J. Quantum Electron. 35(11), 1597–1601 (1999). [CrossRef]

13.

L. Lu, A. Mock, M. Bagheri, J.-R. Cao, S.-J. Choi, J. O’Brien, and P. D. Dapkus, “Gain compression and thermal analysis of a sapphire-bonded photonic crystal microcavity lasers,” IEEE Photon. Technol. Lett. 21(17), 1166–1168 (2009). [CrossRef]

14.

D. L. Mathine, H. Nejad, D. R. Allee, R. Droopad, and G. N. Maracas, “Reduction of the thermal impedance of vertical‐cavity surface‐emitting lasers after integration with copper substrates,” Appl. Phys. Lett. 69(4), 463 (1996). [CrossRef]

15.

M. H. Shih, A. Mock, M. Bagheri, N.-K. Suh, S. Farrell, S.-J. Choi, J. D. O’Brien, and P. D. Dapkus, “Photonic crystal lasers in InGaAsP on a SiO(2)/Si substrates and its thermal impedance,” Opt. Express 15(1), 227–232 (2007). [CrossRef] [PubMed]

OCIS Codes
(140.5960) Lasers and laser optics : Semiconductor lasers
(230.5298) Optical devices : Photonic crystals

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: September 20, 2010
Revised Manuscript: October 31, 2010
Manuscript Accepted: December 6, 2010
Published: December 20, 2010

Citation
Sunghwan Kim, Jeongkug Lee, and Heonsu Jeon, "Over 1 hour continuous-wave operation of photonic crystal lasers," Opt. Express 19, 1-6 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-1-1


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References

  1. O. Painter, R. K. Lee, A. Scherer, A. Yariv, J. D. O’Brien, P. D. Dapkus, and I. Kim, “Two-dimensional photonic band-Gap defect mode laser,” Science 284(5421), 1819–1821 (1999). [CrossRef] [PubMed]
  2. Y. H. Lee, D. H. Jang, H. K. Park, I. Y. Han, D. S. Song, H. Y. Ryu, and J. K. Hwang, “Continuous room-temperature operation of optically pumped two-dimensional photonic crystal lasers at 1.6 μm,” IEEE Photon. Technol. Lett. 12(10), 1295–1297 (2000). [CrossRef]
  3. M. H. Shih, M. Bagheri, A. Mock, S. J. Choi, J. D. O’Brien, P. D. Dapkus, and W. Kuang, “Identification of modes and single mode operation of sapphire-bonded photonic crystal lasers under continuous-wave room temperature operation,” Appl. Phys. Lett. 90(12), 121116 (2007). [CrossRef]
  4. G. Vecchi, F. Raineri, I. Sagnes, A. Yacomotti, P. Monnier, T. J. Karle, K.-H. Lee, R. Braive, L. Le Gratiet, S. Guilet, G. Beaudoin, A. Taneau, S. Bouchoule, A. Levenson, and R. Raj, “Continuous-wave operation of photonic band-edge laser near 1.55 µm on silicon wafer,” Opt. Express 15(12), 7551–7556 (2007). [CrossRef] [PubMed]
  5. K. Nozaki, S. Kita, and T. Baba, “Room temperature continuous wave operation and controlled spontaneous emission in ultrasmall photonic crystal nanolaser,” Opt. Express 15(12), 7506–7514 (2007). [CrossRef] [PubMed]
  6. S. Matsuo, A. Shinya, T. Kakitsuka, K. Nozaki, T. Segawa, T. Sato, Y. Kawaguchi, and M. Notomi, “High-speed ultracompact buried heterostructure photonic-crystal laser with 13 fJ of energy consumed per bit transmitted,” Nat. Photonics 4(9), 648–654 (2010). [CrossRef]
  7. Y. Park, S. Kim, C. Moon, H. Jeon, and H. J. Kim, “Butt-end fiber coupling to a surface-emitting Γ-point photonic crystal bandedge laser,” Appl. Phys. Lett. 90(17), 171115 (2007). [CrossRef]
  8. S. Kim, Y. Park, K. Hwang, J. Lee, H. Jeon, and H. Y. Kim, “High power and large alignment tolerance fiber coupling of honeycomb-lattice photonic crystal Γ-point band-edge laser,” J. Opt. Soc. Am. B 26(7), 1330 (2009). [CrossRef]
  9. E. Yablonovitch, D. M. Hwang, T. J. Gmitter, L. T. Florez, and J. P. Harbison, “Van der Waals bonding of GaAs epitaxial liftoff films onto arbitrary substrates,” Appl. Phys. Lett. 56(24), 2419 (1990). [CrossRef]
  10. M.-C. Liu, C.-C. Lee, M. Kaneko, K. Nakahira, and Y. Takano, “Microstructure of magnesium fluoride films deposited by boat evaporation at 193 nm,” Appl. Opt. 45(28), 7319–7324 (2006). [CrossRef] [PubMed]
  11. M. Fujita and T. Baba, “Microgear laser,” Appl. Phys. Lett. 80(12), 2051 (2002). [CrossRef]
  12. W. W. Bewley, C. L. Felix, E. H. Aifer, D. W. Stokes, I. Vurgaftman, L. J. Olafsen, J. R. Meyer, M. J. Yang, and H. Lee, “Thermal characterization of diamond-pressure-bond heat sinking for optically pumped mid-infrared lasers,” IEEE J. Quantum Electron. 35(11), 1597–1601 (1999). [CrossRef]
  13. L. Lu, A. Mock, M. Bagheri, J.-R. Cao, S.-J. Choi, J. O’Brien, and P. D. Dapkus, “Gain compression and thermal analysis of a sapphire-bonded photonic crystal microcavity lasers,” IEEE Photon. Technol. Lett. 21(17), 1166–1168 (2009). [CrossRef]
  14. D. L. Mathine, H. Nejad, D. R. Allee, R. Droopad, and G. N. Maracas, “Reduction of the thermal impedance of vertical‐cavity surface‐emitting lasers after integration with copper substrates,” Appl. Phys. Lett. 69(4), 463 (1996). [CrossRef]
  15. M. H. Shih, A. Mock, M. Bagheri, N.-K. Suh, S. Farrell, S.-J. Choi, J. D. O’Brien, and P. D. Dapkus, “Photonic crystal lasers in InGaAsP on a SiO(2)/Si substrates and its thermal impedance,” Opt. Express 15(1), 227–232 (2007). [CrossRef] [PubMed]

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