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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 12, Iss. 5 — Mar. 8, 2004
  • pp: 859–867
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Transverse mode control by etch-depth tuning in 1120-nm GaInAs/GaAs photonic crystal vertical-cavity surface-emitting lasers

Jong-Hwa Baek, Dae-Sung Song, In-Kag Hwang, Kum-Hee Lee, Y. H. Lee, Young-gu Ju, Takashi Kondo, Tomoyuki Miyamoto, and Fumio Koyama  »View Author Affiliations


Optics Express, Vol. 12, Issue 5, pp. 859-867 (2004)
http://dx.doi.org/10.1364/OPEX.12.000859


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Abstract

Robust and tolerant single-transverse-mode photonic crystal GaInAs vertical-cavity surface-emitting lasers are fabricated and investigated. Triangular lattice patterns of rectangular air holes of various etch-depths are introduced in the top mirror. The stable single-transverse-mode operation is observed with a large margin of allowance in the etch depth (t=2.5±0.6 µm). This stable mode selection mechanism is explained by the mode competition between the two lowest photonic crystal guided modes that are influenced by both the index guiding effect and the etch-depth dependent modal losses.

© 2004 Optical Society of America

1. Introduction

Previously, the dependable single transverse mode operation has been demonstrated from 850-nm photonic-crystal vertical-cavity surface-emitting lasers (PC-VCSELs) [8

8. D. S. Song, S. H. Kim, H. G. Park, C. K. Kim, and Y. H. Lee, “Single-fundamental-mode photonic-crystal vertical-cavity surface-emitting laser,” Appl. Phys. Lett. 80, 3901–3903 (2002). [CrossRef]

]. These PC-VCSELs are similar to standard VCSELs except that they have photonic crystal patterns defined by air-holes drilled about half-way into the top mirror region [8

8. D. S. Song, S. H. Kim, H. G. Park, C. K. Kim, and Y. H. Lee, “Single-fundamental-mode photonic-crystal vertical-cavity surface-emitting laser,” Appl. Phys. Lett. 80, 3901–3903 (2002). [CrossRef]

10

10. N. Yokouchi, A. J. Danner, and K. D. Choquette, “Vertical-cavity surface-emitting laser operating with photonic crystal seven-point defect structure,” Appl. Phys. Lett. 82, 3608–3610 (2003). [CrossRef]

]. Although the selection of the single transverse mode was obtained with relative ease, the detailed mode selection mechanisms are not yet understood.

In this letter, the origin of transverse mode selection mechanisms of PC-VCSELs is studied. As we vary the air-hole depths, the stable single transverse mode operation is observed over a wide range of etch depth. The experimental data and the influence of the effective wave-guiding and the etched top DBR are discussed to understand the physics of the transverse mode selection.

2. Design and fabrication

Fig. 1. Top view of the fabricated PC-VCSEL with rectangular air-holes.

3. Experimental results

In general, characteristic behaviors of the PC-VCSELs were found not to depend sensitively on the size of photonic crystal air-holes. Consequently, we focused our attention to the effect of the etch depth by choosing a photonic crystal pattern with an air-hole size of 3.5×2.4 µm2 and a lattice constant of 5 µm for subsequent analyses.

Output spectra and near-field CCD mode patterns at-threshold and above-threshold were measured and shown in Fig. 2 and Fig. 3. The reference VCSEL without air-holes operated in multi-modes over the entire current range, as is commonly expected from a typical 16-µm VCSEL, as shown in Fig. 2(a).

Shallow-etch samples (5-, 8-pairs of GaAs/Al0.8Ga0.2As etched layers) operated in the single transverse mode only near the threshold. However, with slightly higher current, many higher-order transverse modes showed up as shown in Figs. 2(b), (c). Interestingly, the spot diameter (10µm) under the multi-mode operation was much larger than that (3.8µm) under the single mode operation. In our experiment, the spot diameter is defined at 1/e points of the transverse mode profile that is obtained from the Fourier transformation of the measured far field profile. Under the multi-mode operation regime, the shallow-etch sample showed the mode size comparable to that of the reference VCSEL as shown in Fig. 3, implying the insufficient index guiding introduced by the shallow-etched air-holes.

On the other hand, all the mid-etch samples (12-, 15-, 18-pair GaAs/Al0.8Ga0.2As etched layers) operated in the single and fundamental transverse mode over the entire operating current range, as shown in Figs. 2(d)(f). Numerically, these etch depths lie between 1.9 µm and 3.1 µm, which promises a generous fabrication tolerance on the etch depth. The measured mode size of the fundamental mode decreased slightly with the etch depth. The actual mode diameters were 3.78, 3.74, and 3.63 µm for 12-, 15-, and 18-pair mid-etch samples, respectively. The mode diameters of 12-pair mid-etch sample were measured at various currents, also, which were 3.78, 3.42, and 3.07 µm at 4, 8, and 12mA, respectively. In general, the state of polarization was determined by the orientation of the rectangular airholes, as shown in Fig. 4, consistent with that in Ref. [9

9. D. S. Song, Y. J. Lee, H. W. Choi, and Y. H. Lee, “Polarization-controlled, single-transverse-mode, photonic-crystal, vertical-cavity surface-emitting lasers,” Appl. Phys. Lett. 82, 3182–3184 (2003). [CrossRef]

]. It is encouraging to report that the most (95%) of the PC-VCSELs with rectangular air-holes operated in the single polarization, single-transverse mode with the polarization suppression ratio of > 30 dB. Threshold currents (~3 mA) of the shallow-etch samples were slightly lower than those (~4 mA) of the mid-etch samples, as shown in Fig. 5.

For the deep-etch samples (etched deeper than 22 pairs, 4.3 µm), only a few of them operate as lasers with very small output power. Since the top DBR had a total of 22 pairs of GaAs/Al0.8Ga0.2As layers, the air-holes were drilled deeper than the active region.

Fig. 2. Spectra of the rectangular air-hole PC-VCSELs with different air-hole depths of (a) 0, (b) 5, (c) 8, (d) 12, (e) 15, (f) 18, and (g)>22 pairs, at just above- and above-threshold currents.
Fig. 3. Near-field CCD mode patterns. (a) Reference VCSEL. (b) Shallow-etch sample (8-pairs of GaAs/Al0.8Ga0.2As etched layers, 12 mA). (c) Mid-etch sample (12 mA). In the CCD images, the circle indicates the inner boundary of the top electrode and the rectangles show positions of the air-holes.
Fig. 4. Polarization resolved L-I curves of (a) vertically-oriented, and (b) horizontally-oriented air-hole PC-VCSELs.
Fig. 5. (a) L-I curves of the PC-VCSELs with different air hole depths. (b) L-I curves of the near threshold currents.

Therefore, the surface recombination became non-negligible and was responsible for the poor performance as a laser. However, it is interesting to note that the threshold current (~2.5 mA) was even smaller than that of the shallow etch samples. Moreover, the deep-etch laser functions in the multi-modes in the entire operating range and it never finds itself in the fundamental single mode as shown in Fig. 2(g).

4. Analysis

The stable single transverse mode operation observed from various the mid-etch samples implies a very large fabrication tolerance of this PC-VCSELs. To understand this encouraging issue, one needs to study the general mode selection mechanisms for PC-VCSELs.

As a first step, we computed the photonic-crystal-guided (PC-guided) modes by the plane wave expansion method employing the simplified photonic crystal structure. An air-hole photonic crystal waveguide structure of high refractive index 3.25 was used as a model structure as shown in Fig. 6(a). Two perfect mirrors were placed at the top and bottom boundaries for simplicity. Through the analyses, we found the existence of a total of three PC-guided transverse modes in the model PC structure as shown in Figs. 6(b)(d). This finding suggests the possibility of a maximum of three PC-guided transverse modes in the partlydrilled real PC-VCSEL structure. Note that this situation is different from the endless-single-mode holey fiber where only one transverse mode is allowed [13

13. T. A. Birks, J. C. Knight, and P. St. J. Russell, “Endlessly single-mode photonic crystal fiber,” Opt. Lett. 22, 961–963 (1997). [CrossRef] [PubMed]

], possibly because of the high refractive index of the host material and large air-hole size. The size of the computational super-cell was 5Λ×3√3 Λ where Λ is the air-hole pitch.

To experimentally verify the existence of the higher order PC-guided modes, we tried to measure the spectral separation of the two right-most peaks. In fact, in the typical above-threshold spectrum, the very faint 2nd-order PC-guided peak was hardly visible in the shadow of the strong single peak representing the lasing of the fundamental (1st-order) PC-guided mode. However, near the threshold where the mode competition was not as strong, those weak non-lasing nearby peaks became visible and the spectral separation could actually be measured. The measured spectral separation of the two right-most modes lies between 4.3–6.7 Å as shown in Fig. 7. Here, each dot denotes the mean value of the spectral mode separation measured from four PC-VCSELs of the same etch depth. It is encouraging to observe that the measured spectral separation (~6.7 Å) of the deep-etch sample approached that (10 Å) computed from the plane wave expansion method with the fully-drilled model structure. For the mid-etch samples, this photonic crystal index guiding effect corresponded to the effective index difference (Δn) of 0.003~0.004 between the core and cladding regions. The peak corresponding to the third transverse PC-guided mode was invisible even near the threshold, which is attributed partly to the large mode size that overlaps much more with the absorptive region of the quantum wells.

Fig. 6. (a) The waveguide structure used to calculate the mode profiles. nc=3.25. (b) Fundamental (1st-order) PC-guided mode. (c) 2nd-order PC-guided mode. (d) 3rd-order PC-guided mode.

Fig. 7. Measured spectral separation between the fundamental and 2nd-order PC-guided modes. The right-most data point was positioned after converting the physical depth (4.3 µm) into the number of DBR pairs. The dashed line is the asymptotic value from the plane wave expansion method with the fullydrilled model structure. The inset figure is a spectrum of the mid-etch sample below-threshold current (2 mA), showing the two PC-guided modes.
Fig. 8. Modal losses of the 1st-order and 2nd-order PC-guided mode versus air-holes depth.

ModalLoss=crosssectionε(r)E(r)2(1R(r))drcrosssectionε(r)E(r)2dr
(1)

Where ε, E, and R are the permittivity, the electrical field amplitude of each mode, and the position-dependent reflectivity of the etched DBR mirror, respectively.

5. Conclusion and summary

Acknowledgments

This work was supported by the National Research Laboratory project of KISTEP, Korea.

References and links

1.

S. Sato, N. Nishiyama, T. Miyamoto, T. Takahashi, N. Jikutani, M. Arai, A. Matsutani, F. Koyama, and K. Iga, “Continuous wave operation of 1.26 µm GaInNAs/GaAs vertical-cavity surface-emitting lasers grown by metalorganic chemical vapour deposition,” Electron. Lett. 36, 2018–2019 (2000). [CrossRef]

2.

T. Kageyama, T. Miyamoto, S. Makino, Y. Ikenaga, N. Nishiyama, A. Matsutani, F. Koyama, and K. Iga, “Room temperature continuous-wave operation of GaInNAs/GaAs VCSELs grown by chemical beam epitaxy with output power exceeding 1 mW,” Electron. Lett. 37, 225–226 (2001). [CrossRef]

3.

T. Anan, M. Yamada, K. Nishi, K. Kurihara, K. Tokutome, A. Kamei, and S. Sugou, “Continuous-wave operation of 1.30 µm GaAsSb/GaAs VCSELs,” Electron. Lett. 37, 566–567 (2001). [CrossRef]

4.

Z. Zou, D. L. Huffaker, S. Csutak, and D. G. Deppe, “Ground state lasing from a quantum-dot oxideconfined vertical-cavity surface-emitting laser,” Appl. Phys. Lett. 75, 22–24 (1999). [CrossRef]

5.

J. A. Lott, N. N. Ledentsov, V. M. Ustinov, N. A. Maleev, A. E. Zhukov, A. R. Kovsh, M. V. Maximov, B. V. Volovik, ZH. I. Alferov, and D. Bimberg, “InAs-InGaAs quantum dot VCSELs on GaAs substrates emitting at 1.3 µm,” Electron. Lett. 36, 1384–1385 (2000). [CrossRef]

6.

T. Kondo, M. Arai, M. Azuchi, T. Uchida, A. Matsutani, T. Miyamoto, and F. Koyama, “Low threshold current density operation of 1.16 µm highly strained GaInAs/GaAs vertical-cavity surface-emitting lasers on (100) GaAs substrate,” Jpn. J. Appl. Phys. 41, L562–L564 (2002). [CrossRef]

7.

F. Koyama, D. Schlenker, T. Miyamoto, Z. Chen, A. Matsutani, T. Sakaguchi, and K. Iga, “Data transmission over single-mode fiber by using 1.2-µm uncooled GaInAs-GaAs laser for Gb/s local area network,” IEEE Photon. Technol. Lett. 12, 125–127 (2000). [CrossRef]

8.

D. S. Song, S. H. Kim, H. G. Park, C. K. Kim, and Y. H. Lee, “Single-fundamental-mode photonic-crystal vertical-cavity surface-emitting laser,” Appl. Phys. Lett. 80, 3901–3903 (2002). [CrossRef]

9.

D. S. Song, Y. J. Lee, H. W. Choi, and Y. H. Lee, “Polarization-controlled, single-transverse-mode, photonic-crystal, vertical-cavity surface-emitting lasers,” Appl. Phys. Lett. 82, 3182–3184 (2003). [CrossRef]

10.

N. Yokouchi, A. J. Danner, and K. D. Choquette, “Vertical-cavity surface-emitting laser operating with photonic crystal seven-point defect structure,” Appl. Phys. Lett. 82, 3608–3610 (2003). [CrossRef]

11.

J. W. Matthews and A. E. Blakeslee, “Defects in epitaxial multilayers : l. Misfit dislocations,” J. Cryst. Growth. 27, 118–125 (1974).

12.

D. Schlenker, T. Miyamoto, Z. Chen, M. Kawaguchi, T. Kondo, E. Gouards, F. Koyama, and K. Iga, “Critical layer thickness of 1.2-µm highly strained GaInAs/GaAs qusntum wells,” J. Cryst. Growth. 221, 503–508 (2000). [CrossRef]

13.

T. A. Birks, J. C. Knight, and P. St. J. Russell, “Endlessly single-mode photonic crystal fiber,” Opt. Lett. 22, 961–963 (1997). [CrossRef] [PubMed]

OCIS Codes
(230.0250) Optical devices : Optoelectronics
(250.7260) Optoelectronics : Vertical cavity surface emitting lasers

ToC Category:
Research Papers

History
Original Manuscript: February 3, 2004
Revised Manuscript: February 25, 2004
Published: March 8, 2004

Citation
Jong-Hwa Baek, Dae-Sung Song, In-Kag Hwang, Hum-Hee Lee, Y. Lee, Young-gu Ju, Takashi Kondo, Tomoyuki Miyamoto, and Fumio Koyama, "Transverse mode control by etch-depth tuning in 1120-nm GaInAs/GaAs photonic crystal vertical-cavity surface-emitting lasers," Opt. Express 12, 859-867 (2004)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-12-5-859


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References

  1. S. Sato, N. Nishiyama, T. Miyamoto, T. Takahashi, N. Jikutani, M. Arai, A. Matsutani, F. Koyama, and K. Iga, �??Continuous wave operation of 1.26 µm GaInNAs/GaAs vertical-cavity surface-emitting lasers grown by metalorganic chemical vapour deposition,�?? Electron. Lett. 36, 2018-2019 (2000). [CrossRef]
  2. T. Kageyama, T. Miyamoto, S. Makino, Y. Ikenaga, N. Nishiyama, A. Matsutani, F. Koyama, and K. Iga, �??Room temperature continuous-wave operation of GaInNAs/GaAs VCSELs grown by chemical beam epitaxy with output power exceeding 1 mW,�?? Electron. Lett. 37, 225-226 (2001). [CrossRef]
  3. T. Anan, M. Yamada, K. Nishi, K. Kurihara, K. Tokutome, A. Kamei, and S. Sugou, �??Continuous-wave operation of 1.30 µm GaAsSb/GaAs VCSELs,�?? Electron. Lett. 37, 566-567 (2001). [CrossRef]
  4. Z. Zou, D. L. Huffaker, S. Csutak, and D. G. Deppe, �??Ground state lasing from a quantum-dot oxideconfined vertical-cavity surface-emitting laser,�?? Appl. Phys. Lett. 75, 22-24 (1999). [CrossRef]
  5. J. A. Lott, N. N. Ledentsov, V. M. Ustinov, N. A. Maleev, A. E. Zhukov, A. R. Kovsh, M. V. Maximov, B. V. Volovik, ZH. I. Alferov, and D. Bimberg, �??InAs-InGaAs quantum dot VCSELs on GaAs substrates emitting at 1.3 µm,�?? Electron. Lett. 36, 1384-1385 (2000). [CrossRef]
  6. T. Kondo, M. Arai, M. Azuchi, T. Uchida, A. Matsutani, T. Miyamoto, and F. Koyama, �??Low threshold current density operation of 1.16 µm highly strained GaInAs/GaAs vertical-cavity surface-emitting lasers on (100) GaAs substrate,�?? Jpn. J. Appl. Phys. 41, L562-L564 (2002). [CrossRef]
  7. F. Koyama, D. Schlenker, T. Miyamoto, Z. Chen, A. Matsutani, T. Sakaguchi, and K. Iga, �??Data transmission over single-mode fiber by using 1.2-µm uncooled GaInAs-GaAs laser for Gb/s local area network ,�?? IEEE Photon. Technol. Lett. 12, 125-127 (2000). [CrossRef]
  8. D. S. Song, S. H. Kim, H. G. Park, C. K. Kim, and Y. H. Lee, �??Single-fundamental-mode photonic-crystal vertical-cavity surface-emitting laser,�?? Appl. Phys. Lett. 80, 3901-3903 (2002). [CrossRef]
  9. D. S. Song, Y. J. Lee, H. W. Choi, and Y. H. Lee, �??Polarization-controlled, single-transverse-mode, photonic-crystal, vertical-cavity surface-emitting lasers,�?? Appl. Phys. Lett. 82, 3182-3184 (2003). [CrossRef]
  10. N. Yokouchi, A. J. Danner, and K. D. Choquette, �??Vertical-cavity surface-emitting laser operating with photonic crystal seven-point defect structure,�?? Appl. Phys. Lett. 82, 3608-3610 (2003). [CrossRef]
  11. J. W. Matthews, and A. E. Blakeslee, �??Defects in epitaxial multilayers : l. Misfit dislocations,�?? J. Cryst. Growth. 27, 118-125 (1974).
  12. D. Schlenker, T. Miyamoto, Z. Chen, M. Kawaguchi, T. Kondo, E. Gouards, F. Koyama, and K. Iga, �??Critical layer thickness of 1.2-µm highly strained GaInAs/GaAs qusntum wells,�?? J. Cryst. Growth. 221, 503-508 (2000). [CrossRef]
  13. T. A. Birks, J. C. Knight, and P. St. J. Russell, �??Endlessly single-mode photonic crystal fiber,�?? Opt. Lett. 22, 961-963 (1997). [CrossRef] [PubMed]

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