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748 CHINESE OPTICS LETTERS / Vol. 6, No. 10 / October 10, 2008

A review of progress on nano-aperture VCSEL

Invited Paper

Zhilong Rao, Sonny Vo, and James S. Harris

Solid State and Photonics Laboratory, Stanford University, Stanford, California, 94305, USA

Received June 18, 2008

This paper reviews the progress on nano-aperture vertical-cavity surface-emitting lasers (VCSELs). The

design, fabrication, and polarization control of nano-aperture VCSELs are reviewed. With the nano-

aperture evolving from conventional circular and square aperture to unique C-shaped, H-shaped, I-shaped,

and bowtie-shaped aperture, both the near-field intensity and near-field beam confinement from nano-

aperture VCSELs are significantly improved. As a high-intensity compact light source with sub-100-

nm spot size, nano-aperture VCSELs are promising to realize many new near-field optical systems and

applications.

OCIS codes: 250.7260, 230.5440, 210.4245.

doi: 10.3788/COL20080610.0748.

1. Introduction

The technological promises of nano-a perture vertical-

cavity surface-emitting lasers (VSCELs) range from

ultrahigh-density near-field optical-data storage to im-

provements in microscopy and ultrahigh-resolution imag-

ing to the study of single molecule manipulation,

fluore scence and spectroscopy, and as a compact light

source for future nanophotonic integrated circuits.

Presently, conventional storage media such as compact

disks, digital video disk, and blue ray disks use far-field

optics. As such, the optical diffraction limit sets an up-

per bound on the optical storage density,

R =

0.61λ

, (1)

where R is the ideal resolution given the wavelength, λ,

and the numerical aperture of the lens system, NA =

n · sin θ

Partovi et al. first demonstrated the data record-

ing and reading with a 2 50-nm-square-aperture nano-

aperture laser based on a 980-nm wavelength edge emit-

ting laser (EEL). A power output greater than 1 mW

with a small output beam size ranging fr om 50 − 300 nm

were realized. This sugg e sts that recording data with a

density greater than 500 Gb/in

will be possible given

better beam confinement and hig her power intensity

[1]

VCSELs are better candidates than EEL in these appli-

cations since they can be processed and tested on a wafer

scale. Also, data transfer rates can be greatly increased

by pr oducing VCSELs in linear or two-dimensional (2D)

arrays

[2]

Thornton and Hesselink first proposed the idea of

nano-aperture VCSELs

[3]

. To develop a nano-aperture

VCSEL, the easiest way is to build it on the foundation

of conventional VCSEL. For example, one c an deposit a

SiO

film and then an Au film on top o f a conventional

VCSEL. A nano-aperture VCSEL can be produced by

opening a nano-aperture using a focused ion beam (FIB)

on the Au film. The necessity of using the SiO

film

will be discussed later. Figure 1 shows the main part of

the structure of such a nano -aperture VCSEL based on a

conventional VCSEL coated with SiO

and a n Au film.

A major short-co ming with this nano-aperture VC-

SEL structure is that the reflectivity of the top mirror

in a conventional VCSEL is very high (typically about

99.5%). As a result, the power output through the top

mirror, namely the intensity of light incident onto the

nano-aperture, is very low compar e d to the intensity in-

side the laser cavity. Thus, the structure of the conven-

tional VCSEL requires revision to increase the output

power.

There a re several possible approaches to improve the

quantum efficiency, and hence, the power output from

the nano-aperture VCSEL, as given by

out

¯hω

η · (I − I

), (2)

where ω is the angular lasing frequency, q is the electron

charge, η is the quantum efficiency, I is the injection

current, and I

is the lasing threshold current. The first

method, initially proposed by Thornton et al. was to

increase the intensity incident onto the nano-aperture

by reducing the number of distributed Bragg reflecto r

(DBR) pairs in the top mirro r to enhance tr ansmission

[3]

The second method was to decr ease the optical mode area

by using a s maller oxide aperture to confine the optical

mode. A third method was to reduce the total loss by

using the oxide aperture to reduce optical scattering loss.

Fig. 1. Schematic structure of nano-aperture VCSEL based

on a conventional VCSEL.

1671-7694/2008/100748-07

 2008 Chinese Optics Letters

October 10, 2008 / Vol. 6, No. 10 / CHINESE OPTICS LETTERS 749

2. Nano-apertures: from simulations to experi-

ments

Shinada et al. first demonstrated a micro-aperture

VCSEL with a 400-nm s quare aperture, but only ob-

tained a very weak output power density

[4]

. Improve-

ments were made by using closely spaced double circular

apertures

[5,6]

or a circular a perture with a metal particle

situated in the center

[7]

. However, the near-field optical

intensities from these nano-ap e rture VCSELs were still

not high enough for optical recording and the near-field

sp ot sizes were relatively large.

From a theoretical perspective, the power transmission

through a conventional square or circular aper tur e de-

creases as the fourth power of the aperture size as the

aperture becomes much smaller than the wavelength of

the optical source

[8]

. This significantly limits the output

intensity from nano-ap e rture lasers when the aperture

size is decreased in order to achieve a small spot size. It

is believed that intensity over 10 mW/µm

is required

to realize optical recording at useful data rates

[4]

. Pre-

vious work on nano-aperture VCSELs has not bee n able

to reach this requirement due to this rapidly decreased

transmission efficiency of circular apertures with de-

creasing aperture size.

The transition towards research on unconventional

apertures, or ridged apertures, coined a fter the ridge

waveguides in microwave applications has spawned a

new generation of VSCELs with the potential to realize

the goal of str ongly confined spot size and hig h enough

intensity for high density optical re c ording. The C -, H-,

I-, and bowtie-shaped shaped na no-apertures marked

an increas ing evolution towards high-intensity coher e nt

light source with sub-100-nm near-field spot.

Using numerical finite-difference time-domain (FDTD)

simulations, Xiaolei Shi discovered a unique non-

symmetrical C-shaped nano-ape rture (C-aperture) which

can provide three orders of magnitude higher transmis-

sion efficiency than a sq uare aper ture with the same

near-field spot size. The C-shaped aperture in a 120-

nm-thick Ag film, designed for resonant transmiss ion

at a wavelength of 1 µm, can have a power throughput

(defined as the ratio of aperture transmission cross sec-

tion to aperture area) of 2.2 and a near-field spot size

of 115 × 130 (nm). Further, they designed a scaled-up

exp eriment at microwave frequencies which confirmed

the simulation findings

[9−11]

This discovery significantly relieves the difficulty

of achieving high transmission through a conven-

tional nano-ape rture while maintaining strong near- field

confinement. Thornton and Shi proposed the idea of

applying the C-apertur e onto VCSELs and reducing the

number of top DBR in VCSELs to enhance transmiss ion

through the C-aperture

[12]

. However, they did not rea l-

ize the C-aperture VCSEL experimentally because they

did not address the problem of polarization control of

VCSEL, which must be resolved in order to apply the

highly polarization-sensitive C-aperture on VCSELs.

Jin et al. studied the transmission of nano-apertures

in metal films deposited on a quartz substrate

[13]

. They

used an illumination-collection-type near-field scanning

optical microscopy (NSOM) to measure the near-field

sp ots from a bowtie-aperture and compared that to rect-

angular and square apertures of the same area. The

measured spot size from the bowtie-aperture which was

found to be 65 × 34 (nm) is much smaller than that

from the rectangular and square aperture, found to be

102 × 176 (nm) and 278 × 65 (nm), respectively. For the

square a perture which was supposed to produce similar

sp ot size to the bowtie-aperture, no observa ble spot was

obtained due to the extremely low transmission efficiency

through this square aperture

[13]

Interestingly, the pheno menon of electromagnetic cou-

pling to a surfa c e electromag netic wave propagating

along a metal and dielectric interface was found to en-

hance transmission efficiency. It was shown that sur fa ce

plasmons, also known a s surface plasmon polaritons, can

be excited in periodic arrays of sub-wavelength apertures

in metal film; the coupling and re-radiation of sur fa ce-

plasmons leads to la rgely enhanced transmission through

the ap e rture arrays

[14]

. This work sparked increased re-

search interests in exploiting surface plasmons to enhance

transmission through sub- wavelength metal structures.

Hashizume and Koyama used a SiO

-Au interface to

study the effects of plasmons on the near field optical

intensity of VSCELs. The SiO

layer was sandwiched

between an upper gold layer with an 850-nm metal

nanoaperture and a GaAs substrate. The excited surface

plasmon increased the output intensity by more than a

factor of sixteen. For a double aperture with SiO

layer,

the measured intensity is 2.5 mW/µm

. At the other

extreme, a single aperture without the SiO

layer had an

output of 0.19 mW/µm

. This enhancement in power

output comes with the added advantage of a sma ll spot

size: a 260-nm full-width at half-maximum (FWHM)

with the SiO

layer as opposed to 360 nm without the

SiO

layer

[7]

Insertion of the SiO

layer into the nano-ape rture VC-

SEL structure enhances the trans mission through the

nano-aperture by three mechanisms. Firstly, inser tio n of

the low-refractive-index SiO

layer reduces the reflectio n

from the nano-aperture at the interface between the

incident medium and air, which is given by

reflected

incident

− n

air

incident

+ n

air

. (3)

Without the SiO

layer, n

incident

becomes the high-

refractive index of AlGaAs, leading to a high reflection.

With the SiO

layer, n

incident

becomes the low-refractive

index of SiO

, which largely reduces the reflection. Sec-

ondly, a Fabry-Perot resonance can build up inside the

SiO

layer, which increases the intensity incident onto

the nano-aperture. Thirdly, insertion of the SiO

layer

significantly changes the spectral resp onse for a given

nano-aperture, as shown in the spectral response o f the

C-aperture in Fig. 2.

Continued studies on plasmon effects as well as in-

corpora ting novel materials onto VSCELs proved to be

essential to improving transmission efficiency. Using

noble metals, such as Au, have been known to increase

the trans mission efficiency in nano-aperture VSCELs.

Au bowtie antennae fabricated with e-beam lithography

have been shown to have greatly enhanced intensity and

field confinement

[15]

Also, it was discovered that plasmo n enhancement

using a metal nano-particle incre ased the output power.

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