Three - University of Arkansas Physics Department

Reflection-second-harmonic generation from GaAs/AlAs and
GaAs/AlGaAs multilayers
Xiaodong Mu, Yujie J. ~ing', Haeyeon Yang, and Gregory J. Salamo
Department of Physics, University of Arkansas, Fayetteville, AR 72701
ABSTRACT
We have observed first-, second- and third-order quasi-phase-matched second-harmonic generation in the reflection geometry
from GaAsIAlAs multilayers. We have measured phase-matching curves and identified all the peaks. The linewidth for the
first order is limited only by wave-vector mismatch. We have demonstrated two-order-of-magnitude enhancement solely
using quasi-phase-matched multilayers. We have also achieved cavity-enhanced quasi-phase-matched second-order and nonphase-matched
second-harmonic generation from GaAs/Alo,sGa,,zAs multilayers. We have determined the element of the
second-order susceptibility tensor used for quasi-phase matching. We have measured the conversion efficiencies and
discussed possibilities for further enhancements.
Keywords: reflection second harmonic generation, multilayers, cavity enhancement, frequency conversion, Bragg reflection.
1. INTRODUCTION
GaAs and AlGaAs have very large second-order susceptibilities. To achieve efficient frequency conversion, multilayers have
been used to achieve quasi-phase matching (QPM).' There are two configurations for QPM: surface-emitting',2 andrefle~tion.~-~
Although reflected-second-harmonic generation (SHG) in G~AS/A~~G~~.~AS multilayers was initially studied in Refs. [3-51,
sharp QPM peak had not been achieved before due to (i) poor quality of the multilayers or (ii) lack of a tunable laser. In Ref
[3] 17 pairs of alternating layers of GaAs and Alo,3Gao.7As on a (I 10)-orientated GaAs substrate were used for forward and
backward SHG (i.e. zero incident angle). The thicknesses of the layers were chosen so that each layer is an optical quarterwavelength
thick at 2 pm. These multilayers can satisfy distributed Bragg reflection (DBR) at 2.008 pm. An enhancement by
a factor of only 2.7 over the background was obtained because of the very broad SH spectrum (> 1000 A). Moreover, part of
the enhancement may be the result of DBR, rather than QPM, since DBR is too close to the broad SH peak. Ref. [4]
illustrates how DBRs can enhance SHG for a single thin layer of Alo.3G~.7As in a cavity grown on (100) direction. In this
case, SHG was not quasi-phase-matched. Recently, Alo,aGa,,2As/GaAs multilayers grown on a GaAs (100) substrate were
used to demonstrate QPM at 1.064 However, the QPM peak (i.e. SH intensity vs. pump wavelength) was nor directly
measured since the pump beam can only emit a single wavelength. The alternative measurement of enhancement vs. incident
angle did not reveal a peak 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 prn is above the band-gaps of both GaAs and Alo.aG&.2As. In
addition, since the multilayers can act as DBR due to a large difference of refractive indices, some of the enhancements may
be attributed to the DBR. We conclude that the enhancement factors measured in Ret 151 do not necessarily correspond to
the enhancement solely due to QPM. From all the previous result^,^.^ it is obvious that one should design an all-MBE-grown
multilayer structure in such a way that (i) SH photon energy is below the band-gaps of the alternating layers and (ii) a cavity
based on a pair of DBRs is used to enhance QPM SHG in order to determine the optimum enhancement. In both Ref. [3] and
[5], the conversion efficiency was not measured. Furthermore, quadratic dependence was not confirmed. On the other hand,
in Ref. [4] high efficiency was obtained solely due to the enhancement of DBR, however not QPM. Power dependence was
measured with a severe deviation from a square law due to extremely high peak intensities used.
In this proceedings paper, we report our results on detailed investigation of reflection-SHG horn GaAsIAlAs and
G~AS~A~~,~G~.~AS
multilayers that have much higher quality. For the first time, we have directly observed a sharp QPM peak
of the first-order by measuring the spectrum of the reflection-SHG and comparing with the linear reflection spectrum. We
have achieved a very narrow linewidth of the QPM peak limited only by wave-vector mismatch. We have achieved an
enhancement factor of about 124 over the background. We have also observed QPM peaks at the second- and third-orders
with and/or without a cavity. We have measured the dependence of the SH power on the pump power. Furthermore, we have
Correspondence: Ph.: (501) 575-6570; Fax: (501) 575-4580; E-mail: yding@uark.edu
Ultrafast Phenomena in Semiconductors V, Hongxing Jiang, Kong Thon Tsen, Jin-Joo Song, Editors,
Proceedings of SPlE Vol. 4280 (2001) a2001 SPlE ,0277-786)(/011$15.00