Three - University of Arkansas Physics Department

APPLIED PHYSICS LETTERS VOLUME 79, NUMBER 5 30 JULY 2001
Cavity-en hanced and quasiphase-matched multi-order reflection-secondharmonic
generation from GaAsIAIAs and GaAsIAIGaAs multilayers
Xiaodong Mu, Yujie J. in^,^) Haeyeon Yang, and Gregory J. Salarno
Departnient (fPhysics. Utliversity ofArkun.ras, 226 Physics Bitilding, Fuye[/eville, A~kot~.rc~s 72701
(Received 8 December 2000; accepted for publication I1 May 2001 )
We have observed quasiphase-matched second-hannonic generation in the reflection geometry from
GaAsIAlAs multilayers. By using GaAsIAlGaAs lnultilayers and Fresnel reflection as a cavity, we
have also achieved cavity-enhanced nonphase-matched second-harmonic generation froin
GaAsIAlAs multilayers. The linewidth for the first order reflection-second-harmonic generation is
limited only by wave-vector mismatch. In addition, we have demonstrated two-order-of-magnitude
enhancement on the conversion efficiency by using the cavity. Cc) 2001 Arnericnn Institute cJ
Physics. [DOI: 10.1063/1.1383565]
GaAs and AlGaAs have very large second-order susceptibilities.
To achievc efficient frequency conversion, inultilayers
have been used to achieve quasiphase matching
(QPM).' There are two configurations for QPM: surface
cmittingl9' and refle~tion.~-~ In the surfacc-emitting geometry,
a pump beam or second-harmonic beam propagates
transversely or parallel to the surface normal while parametric
or fiindamental beams propagate in the layer plane. One
of the advantages is the possibility of achieving oscillation
without any ~avity.~ On the other hand, the reflection geometry
can easily be used to measure nonlinearity of rnultilayers.
In these two configurations, the dependence of the
second-ham~onic power on the propagation length is quite
different. Although reflected-second-hannonic generation
(SHG) in GaAs/A1,Gal -,As multilayers was initially studied
in Refs. 3-5, sharp QPM peak had not been achieved
before due to (i) poor quality of the multilayers or (ii) lack of
a hinablc laser. In Ref. 3, 17 pairs of alternating layers of
GaAs and Alo,;G%,7As on a (I 10)-orientated GaAs substrate
were used for forward and backward SHG (LC., zero incidcnt
angle). .4n enhancement bjl a factor of only 2.7 over the
hackgrozrnd wus obtained becazise of the broad SH spectrunl
(>1000 A). Moreover, part of the enhancement may be the
result of Bragg condition, rather than QPM, since Bragg condition
is too close to the broad SH peak. Ref. 4 illustrates
how distributed Bragg reflectors (DBRs) can enhance SHG
for a single thirl luyer of Alo,3G+,7As in a cavity grown on
(100) direction. In this case, SHG was not quasiphase
matched.
Recenlly, Alo,8Ga,,2As/GaAs multilayers grown on a
GaAs (100) substrate were used to demonstrate QPM at
1.064 pn~.~
However; the QPMpeak (i.e., SH intensity versus
puntp wavelength) was not ciiwctly measured. The alternative
measurement of enhancement versus incident angle did not
reveal a pcak in the measurement range. The large enhancements
were measured for the SH intensity relative to that for
bulk GaAs. However, SH photon energy at 0.532 pm is
above the band gaps of both GaAs and Alo,8Gao,zAs. In addition,
since the rnultilayers can act as distributed Bragg re-
"'Electronic mail: yding@oark.edu
flection (DBR) due to a large difference of refractive indices,
some of the enhancements may be attributed to the DBR
rather than QPM.
In this letter, we report our results on detailed investigation
of reflection SHG from GaAsIAIAs and
C;~AS/AI~,~G~~.~AS multilayers that have much higher quality.
We have directly observed a sharp QPM peak of the firstorder
reflection-SHG. An enhancement factor of about 124
over the background has been achieved. We have also observed
QPM peaks at the second and third orders with and!or
without a cavity. We have also measured the dependence of
the SH power on the pump power. Furthermore, we have
determined the relation among the SH polarization, SH
power, and pump polarization. Our investigation of the reflection
SHG from inultilayers serves as a first step towards
evcntual applications of GaAsIAlCiaAs multilayers for generating
and anlplifying mid-IR waves based on a novel
~onfi~uration.~
Our sainples were grown using a Riber 32 molecular-
beam cpitaxy system at the University of Arkansas. Figure 1
shows their stmch~re. Three picosecond OPO pulses with a
240 ML GaAs I
480 ML GaAs
I
(LOO) GaAs Substrate
FIG. 1. Structure of GaAs/AI,,,Ga,,,As multilayers in the presence of a
GaAsiAlAs multilayer cavity (where ML designates monolayers), and,
reflection-SHG geometry.
0003-6951/2001/79(5)/56913/$18.00 569 O 2001 American Institute of Physics
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