A C-shaped nanoaperture Vertical-Cavity Surface-Emitting Laser for by ehz13319

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									    A C-shaped nanoaperture Vertical-Cavity Surface-Emitting Laser
            for high-density near-field optical data storage
                Zhilong Rao*, Joseph A. Matteo, Lambertus Hesselink, James S. Harris
            Solid State and Photonics Laboratory, Stanford University, Stanford, CA 94305


                                                       ABSTRACT

We designed and demonstrated a unique C-shaped nanoaperture (C-aperture) Vertical-Cavity Surface-Emitting Laser
with an estimated maximum net power of 113 µW coming from a 70nm C-aperture. Simulation shows the near-field
FWHM spot size at 30nm away from the C-aperture is 94nm and 108nm in X and Y direction. We estimate the peak
near-field intensity from the C-aperture VCSEL to be as high as 13.7mW/µm2. This high intensity and small spot size is
promising to realize high-density near-field optical data storage.

Keywords: VCSEL, polarization, nanoaperture, near field, data storage

                                                   1. INTRODUCTION

Very Small Apertured Lasers (VSALs) provide a promising solution to overcome diffraction limit in far-field optical
systems and achieve ultrahigh-density near-field optical data storage. Partovi et al. first demonstrated the data recording
and reading with a 250nm-square-aperture VSAL based on a 980nm edge emitting laser (EEL).1 Vertical-Cavity
Surface-Emitting Lasers (VCSELs) are better candidates than EEL in this application due to its capability of wafer-scale
processing and testing. Also, data transfer rates can be greatly increased by applying VCSELs in parallel arrays.2 Shinada
et al. proposed a microaperture VCSEL with a 400nm square aperture, but only obtained a very weak output power
density.3 Improvements were made by using closely spaced double circular apertures4 or a circular aperture with a metal
particle.5 However, the optical near-field intensity from these nanoaperture VCSELs is still not high enough for writing
and reading data, and the near-field spot sizes are relatively large.

One factor limiting the output power intensity of nanoaperture VCSELs in previous work is that the power transmission
through conventional square or circular apertures decreases rapidly when the aperture size is much smaller than one
wavelength. We propose to apply on VCSELs a unique C-shaped nano-aperture (C-aperture) which from simulation has
three orders of magnitude higher transmission efficiency than a conventional square or circular aperture producing the
same near-field spot size6. The ultrahigh transmission of the C-aperture has been demonstrated in the visible light
region.7 Optical near-field confinement of the C-aperture has also been observed exprimentally.8

The transmission through the C-aperture is polarization-dependent. However, VCSELs normally have two degenerate
orthogonal polarization states. A number of efforts to achieve polarization control in VCSELs have been tried, including
utilizing asymmetrical mesa structure9, growing VCSEL on misoriented substrates10 or using external optical feedback
from polarization sensitive gratings11 etc. We propose a novel method for the polarization control of nanoaperture
VCSELs, i.e. opening narrow slits in the metal-coated top emission facet of VCSELs. We report here the results of C-
aperture VCSELs with controlled polarization.

                                2. DEVICE STRUCTURE AND FABRICATION

Our top-emitting VCSELs are designed to operate around 970nm and consist of 9.5 pairs of p-type distributed Bragg
reflector (DBR), three strain-compensated InGaAs/GaAsP quantum wells and 38.5 pairs of n-DBR. The reflectivity of
the top mirror is enhanced with a 150nm thick Au coating. The number of top DBR pairs is reduced to about half of that
in conventional VCSELs in order to increase the intensity of light incident onto the nanoaperture. We insert a half-
wavelength thick SiO2 film between the Au coating and the top DBR pairs for reasons to be explained later. Fig. 1 shows
the structure of our nanoaperture VCSELs.


                        Vertical-Cavity Surface-Emitting Lasers X, edited by Chun Lei, Kent D. Choquette,
                      Proc. of SPIE Vol. 6132, 61320J, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.645083


                                               Proc. of SPIE Vol. 6132 61320J-1
                                                                          Au
                                                                              Si02



                                                                   —)
                                                                              M      (hi A kl 2(T        k (9.5 pairs)
                                                    —               Al                      2M oxidation layer (2Cknn)

                                                                                JO 7(hi 4A Ga G F QWs
                                                                             Al (hi k Al (hi            As (38.5 pairs)
                                                                         —
                                                        i (..aAs Stiti
                                                  Fig.1: structure of our nanoaperture VCSEL

To fabricate our devices, we start with the deposition of a half-wavelength thick SiO2 film. Then we go through the
conventional VCSEL processing to define mesas and contacts. A wet oxidation process is used to define a 3.0µm-
diameter oxide aperture for current and mode confinement. In the end, we deposit a 150nm thick Au film onto the top
emission facets and then use Focused Ion Beam (FIB) to open slits and nanoapertures in the Au film to obtain the final
nanoaperture VCSEL.

                                                                  3. FDTD SIMULATION

We studied the power transmission through the C-aperture in our nanoaperture VCSEL using our own Finite Difference
Time Domain (FDTD) code. Our simulation shows that the insertion of a half-wavelength thick SiO2 layer greatly
enhances the power transmission through the C-aperture. Two factors contribute to this enhancement. Firstly, the
insertion of SiO2 reduces the index mismatch between the substrate and air, which allows a propagation mode through
the C-aperture. Secondly, a Fabry-Perot resonance can build up inside the SiO2 layer, which increases the intensity of
light incident onto the C-aperture. We simulated the near-field intensity distribution of a 70nm C-aperture etched in a
150nm thick Au film under light of 972nm wavelength incident from a SiO2 substrate. The detailed structure of the C-
aperture is discussed elsewhere.7 Fig. 2a shows the E2 distribution at 30nm away from the C-aperture under incident light
polarized perpendicular to the C-aperture. We can obtain a well-confined spot with a FWHM size of 94nm and 108nm in
X and Y direction with resolution of 2nm. As a comparison, Fig. 2b shows the E2 distribution at 30nm away from the C-
aperture under incident light polarized parallel to the C-aperture. The spot is poorly confined and E2 is much weaker in
this case.
                         (a)                                                        (b)


                                                                     E-field:   ↔
                                 E2                                                                                      E2
              0                                                                                                                           0.045
                                                              14
                                                                                                                                          0.04
             100                                                                                200
                                                              12
                                                                                                                                          0.035
             200                                              10
                                                                                                                                          0.03
                                                                                                400
    Y (nm)




             300                                              8                                                                           0.025
                                                                                       Y (nm)




                                                                                                                                          0.02
                                                              6
             400
                                                                                                600                                       0.015
                                                              4
             500                                                                                                                          0.01
                                                              2
                                                                                                                                          0.005
             600                                                                                800
                                                              0                                                                           0
               0   100   200   300    400   500   600                                                 200        400          600   800
                                 X (nm)                                                                                X (nm)


Fig.2: near-field E2 distribution at 30nm away from the C-aperture. (a) the polarization is perpendicular to the C-aperture; (b) the
polarization is parallel to the C-aperture.




                                                              Proc. of SPIE Vol. 6132 61320J-2
Due to this polarization-dependent transmission property of the C-aperture, we need to control the polarization of the
VCSELs if we want to achieve high output power and well-confined near-field spot size from the C-aperture VCSEL.
We use a simple integrated solution to control the polarization by opening narrow slits in the Au film surrounding the C-
aperture using FIB. For slits with width much smaller than one wavelength, only the mode with polarization
perpendicular to the slits can propagate through. Thus the slits have high transmission selectivity over the two orthogonal
polarizations. Fig.3a and Fig.3b show the near-field E2 distribution of a 50*1500nm slit under light of 972nm wavelength
incident from a SiO2 substrate with polarization perpendicular and parallel to the slit respectively. The E2 for the
perpendicular polarization is five orders of magnitude higher than that for the parallel polarization. Thus the
perpendicular polarization state has higher loss and the polarization of the VCSELs should be pinned parallel to the slit.
                          (a)                      E-field: ↔                          (b)
                                                                                                                                                     -5
                                 E2                                                                                E2
            0                                                                                                                                 x 10
                                                            7
                                                                                         100
                                                                                                                                              6
          500                                               6                            200
                                                                                                                                              5
                                                            5                            300
         1000




                                                                                Y (nm)
Y (nm)




                                                                                         400                                                  4
                                                            4                                              !!!!I        !!!!I
         1500                                               3                            500                                                  3
                                                                                         600                                                  2
         2000                                               2
                                                                                         700
                                                            1                                                                                 1
         2500                                                                            800
                                                            0                                                                                 0
             0       200      400       600         800                                        500    1000         1500         2000   2500
                                 X (nm)                                                                      X (nm)

            Fig.3: near-field E2 distribution. (a) the polarization is perpendicular to the slit; (b) the polarization is parallel to the slit.


                                                            4. Experimental Results

After a 150nm Au film is deposited onto the processed VCSELs, we measured the polarization properties of the VCSELs
before opening any slits or C-apertures. For our VCSELs with circular mesas and thus circular oxide apertures, there is
no intrinsic polarization selection mechanism. Two orthogonal polarizations coexist and align primarily along the crystal
axis <110> and < 110 >, which we denote as the <100> and <010> direction in all later discussion. The dominant
polarization can be either along <100> or <010> and can switch between each other. Also, the polarization direction can
rotate along the crystal axes by a small angle with changing injection current.

To control the polarization, we open four 50*1500nm slits in the Au film, as shown in Fig. 4a. The slits are separated
into two groups with spacing of 1600nm to leave enough modal area to add the C-aperture later. The two slits on each
side have a pitch of 250nm. This structure is chosen for a tradeoff between strong polarization control and low loss from
the slits.

                                              (a)                                                    (b)




                                                             2
                        Fig.4: SEM images; (a) after opening four 50*1500nm slits; (b) after opening a 70nm C-aperture




                                                           Proc. of SPIE Vol. 6132 61320J-3
After we open the slits, the top-emitting power contains the power transmitted through the slits, which is hard to interpret
to derive the intrinsic polarization property of the laser. So we measured the polarization-resolved power emitted through
the substrate to identify the polarization ratio inside the laser cavity. Fig. 5 shows the polarization-resolved bottom-
emitting power vs. current for devices with slits along <100> and <010> direction respectively. We can control the
polarization to be dominant either along <100> or <010> direction by opening slits along <100> or <010> direction
respectively.
                                           (a)                                                                                    (b)
           0.025
                       Power along <100>                                                                   ---Power along <100>
                   ---Power along <010>                                                            0.020       Power
                                                                                               E
                                                                                                   0.015


        IE                                                                                         0.010




                                  Current (mA)                                                                           Current (mA)

Fig.5: Polarization-resolved bottom-emitting power after opening slits. (a) the slits are along <100> direction; (b) the slits are along
<010> direction.

We then open a 70nm C-aperture at the center in between the two groups of slits, as shown in Fig.4b. Polarization-
resolved bottom-emitting power measurements show that the polarization ratio decreases by about one half but the
dominant polarization is still controlled to be along the slits. Fig.6 shows the polarization-resolved bottom-emitting
power vs. current for the device with slits along <010> direction after the C-aperture is opened. The decrease of the
polarization ratio after the C-aperture is opened is not unexpected because the C-aperture transmits more light polarized
perpendicular to the C-aperture, namely along the slits, and thus counteracts the polarization selectivity provided by the
slits. The total bottom-emitting power also drops after we open the C-aperture. This is expected since opening the C-
aperture decreases the reflectivity of the top mirror and hence reduces the intensity of light circulating inside the laser
cavity.


                                                      0.014
                                                              ---Power along <100>
                                                      0.012       Power along
                                                 E
                                                 i5   0.010
                                                 P
                                                 0 0.008
                                                 - 0.006




                                                                                Current (mA)

                             Fig.6: polarization-resolved bottom-emitting power after opening the C-aperture

We measured the top emitting power before opening slits, after opening only slits and after opening the C-aperture
respectively, as shown in Fig.7a. The power is collected with a 1cm2 circular silicon detector at a distance of 5mm from
the nanoaperture VCSEL. The far-field radiation from the C-shaped nanoaperture can be assumed to be hemispherical
radiation from a point source. The collection efficiency of the power from the C-aperture with our detection system is
estimated to be 34%.




                                                              Proc. of SPIE Vol. 6132 61320J-4
                                              (a)                                                       (b)
            0.07      with no slits or C-aperture
                                                                            0.040 .J   Net power from the C-aperture
            0.06   I1 slits                                                 0.035
            0.05                                                            0.030

            0.04                                                            O.O25

        i5 nn                                                               O.O2O
                                                                            0.015
                                                                        0
                                                                            0.010
                                                                            0.005
                                                                            0.000
                                                                            -0.005

                                   Current (mA)                                                   Current (mA)

Fig.7: top-emitting power. (a) power before opening slits, after opening only slits, and after opening the C-aperture; (b) derived net
power coming from the C-aperture.

To derive the net power from the C-aperture, we need to find out how much power comes from the slits after opening the
C-aperture. The power transmitted through the slits should be proportional to the intensity of light circulating inside the
laser cavity, which is proportional to the power emitted through the substrate. Since the power transmission efficiency
for the polarization perpendicular to the slits is orders of magnitude higher than that for the parallel polarization, we
assume the power transmitted through the slits primarily comes from the perpendicular polarization. Thus the power
transmitted through the slits is proportional to the bottom-emitting power polarized perpendicular to the slits. We can
first obtain the power transmitted through the slits after we open only the slits simply by subtracting the laser background
power from the total power. After we open the C-aperture, the bottom-emitting power polarized perpendicular to the slits
slightly increases and hence the power transmitted through the slits increases slightly. We take this increase factor into
account and derive the net power transmitted through the C-aperture as shown in Fig.7b. A maximum net power of
38.3µW is achieved at 5.0mA. Considering the power collection efficiency of 34%, we estimate the total power from the
C-aperture to be 113µW. We define a near-field spot area as the area of the spot bounded by points with half of the peak
intensity. Our simulation shows, at 30nm away from the C-aperture, the spot area is 8.2*10-3 µm2. We estimate the peak
near-field intensity to be 13.7 mW/µm2. This high intensity is promising to realize optical recording. All the above tests
are done at continuous wave and at room temperature without any cooling. We expect a higher maximum power with
better heat dissipation.

                                                          5. CONCLUSIONS

In conclusion, we proposed and demonstrated a C-shaped nanoaperture VCSEL with polarization controlled by four
50*1500nm slits surrounding the C-aperture. We obtain a maximum net power of 38.3µW coming from the C-aperture,
with a power collection efficiency of 34%. The peak near-field intensity at 30nm from the C-aperture is estimated to be
as high as 13.7 mW/µm2. Our C-aperture VCSELs are very promising to have high enough intensity to realize optical
data recording. The high intensity and small near-field spot size of our C-aperture VCSEL can also lead to other
applications such as near-field imaging.


                                                          REFRENCES

1.   Afshin Partovi, David Peale, Matthias Wuttig, Cherry A. Murray, George Zydzik, Leslie Hopkins, Kirk Baldwin,
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     pp.1515-1517, 1999.
2.   Young-Joo Kim, Kazuhiro SUZUKI and Kenya GOTO, “ Parallel recording array head of nano-aperture flat-tip
     probes for high-density near-field optical data storage,” Jpn.J.Appl.Phys. , Vol.40, pp.1783-1789, 2001.




                                                    Proc. of SPIE Vol. 6132 61320J-5
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*Email: zlrao@stanford.edu; phone: 1 650 725-6970.




                                            Proc. of SPIE Vol. 6132 61320J-6

								
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