2.
Chip-level Nonlinear Optical Devices
A variety of new devices are coming on line where a diode laser is integrated with a
device on a single chip. For example see chem lab on a chip. Such devices are working well
because the success of near infrared (NIR) diode technology has matured to the point where the
lasers have favorable characteristics such as high electrical to optical efficiency, long lifetimes,
and low cost. In addition these diode lasers have favorable spectral characteristics such as
narrow linewidth, modest tunability, and in some cases the ability to modulate at high
frequencies. All of these properties are making integrated devices containing a diode laser and
other elements on a single chip more common.
We propose to take advantage of the reliability and other favorable characteristics of
diode lasers to go a further step with such integrated devices. We propose to use diode lasers in
conjunction with nonlinear optical frequency conversion regions to allow us to access
wavelength ranges inaccessible to currently available diode lasers. This is particularly attractive
when considering infrared wavelengths where diode lasers do not operate or, if they do, require
cryogenic cooling. By frequency converting the well-developed diode lasers, their properties
will carry over to the frequency converted wavelengths. We propose to develop techniques to
allow us to make nonlinear optical devices on a chip. Emphasize not only frequency converting,
but enabling devices such as chemlab on a chip.
2.1
Quasi Phase Matching and bulk nonlinear optical devices
Important for the success of such devices is a means to obtain efficient frequency
conversion. High conversion efficiency is achieved with a high optical nonlinearity and a
mechanism to maintain a fixed phase relationship between the interacting light waves. Such a
fixed phase relationship allows the frequency converted light to build up throughout the crystal.
Recently a technique, called quasi-phase matching (QPM), has been developed that allows this
fixed phase relationship to be engineered into the material while accessing the material’s highest
nonlinearity. QPM uses an engineered periodic structure in the crystal that allows the given
interaction to build up coherently throughout the crystal. With QPM, one can choose the most
favorable crystal properties such as the highest nonlinearity, and then engineer the periodic
structure to ensure that the process is efficient. This is in contrast to the most common nonlinear
mixing technique of birefringent phasematching where the birefringence of the crystal
determines which waves will build up.
Furthermore, many materials which do not have
birefringence, such as GaAs, cannot be used with the birefringent phasematching technique. The
larger nonlinearity of QPM crystals means that nonlinear optical devices using these materials
will be more efficient. This has been proven in the first material of this kind, periodically poled
lithium niobate (PPLN). With PPLN, devices previously limited to large laser systems such as
continuous wave optical parametric oscillators are now using smaller and more rugged pump
laser systems such as diode lasers. This is an important property of QPM materials: namely that
smaller laser sources can be used to efficiently generate frequency converted light.
The success of PPLN for nonlinear optical devices demonstrates the advantages of the
QPM process. However, integrating a PPLN device with a diode laser is not a viable coupling
from an integrated device standpoint***. We propose to look at techniques that will be more
amenable to integrated devices on a chip level.
Specifically we propose to work with nonlinear
optical polymers and photonic QPM structures.
2.2
Photonic Quasi Phase Matched Interactions
As described above, the quasi-phasematched process allows the interacting waves to
travel through the crystal with a fixed phase relationship. This is accomplished in PPLN by a
periodic change in the crystal structure which is achieved through electric field poling. In the
case of PPLN, the optic axis of the crystal changes direction periodically which is accompanied
by a sign change of the nonlinearity. This type of QPM is used in GaAs (ref) already***. This
is not the only way to achieve quasi-phasematching. For example, turning the nonlinearity on
and off periodically will also allow quasi-phase matching, and devices based on this technique
have been demonstrated1. We propose to make a QPM structure in GaAs using a similar
technique. QPM in GaAs has several advantages. The first is that GaAs has a large nonlinearity.
In comparison with PPLN, GaAs has a nonlinear coefficient of 90 pm/V whereas PPLN has 18
pm/V. GaAs has a large infrared transparency that extends out to 12 m. From a device
standpoint the use of GaAs as the nonlinear material has the advantage of immediate integration
possibilities.
We propose to make QPM GaAs using a facility available to us through ***.
Collaborators at*** have an etching technique that allows for precise etch depths of up to 50 m.
We propose to make a structure in GaAs where the nonlinearity is turned on and off periodically
by the periodic removal of GaAs through etching. In this structure a beam of light propagates
through GaAs and air gaps periodically. By using the *** etch technique, we anticipate low
scattering losses from the GaAs-air interfaces. In this etched structure QPM is achieved by a
turning the nonlinearity on and off periodically, however the large index change that occurs
between the GaAs and air gap results in a one-dimensional photonic structure. Since the etch
depth using this technique has been demonstrated to 50m, both bulk and waveguide structures
can be considered. The goal of our work with these QPM photonic stuctures will be to design an
efficient QPM interaction, while allowing all the interacting beams propagate through the crystal.
The photonic structure can also be engineered to suppress undesirable effects.
For
example, two photon absorption can be significant in GaAs for pulsed lasers depending on the
wavelength. With a careful design the photonic structure could also serve as a means to suppress
this two-photon absorption. 
2.3
QPM in Polymers
Polymers present another possibility for integration with diode lasers and chip level
devices. Much research has been conducted with poled polymer films and QPM structures have
been fabricated2,3. However, several problems have been encountered when working with these
polymers. In particular optical absorbtion, relaxation of the crystal structure, and waveguide
losses all need to be taken into consideration. We will work with UDRI scientists to identify
infrared transparent polymers which can be electric field poled for both waveguide and bulk
devices. Particular attention will be made to work with polymers with a high Tg temperature.
We then propose to make QPM structures using equipment and facitities at our disposal. The
availability of an e-beam machine will allow us to periodically erase the nonlinearity of a
uniformly poled polymer (see ref.***). We will also apply electric field poling techniques that
we have developed in collaboration with WPAFB to periodically pole these polymers in a
similar fashion to periodically poling LiNbO3. The periodic poling technique will build on the
expertise that has been gained by our group in fabricating PPLN crystals. 3. Finally, in a similar
manner to the photonic-QPM structures proposed for GaAs, we can also apply the same
procedure to poled polymers.
1. P. E. Powers, K. W. Aniolek, T. J. Kulp, B. R. Richman, and S. E. Bisson, "Periodically
poled lithium niobate optical parametric amplifier seeded with the narrow-band filtered
output of an optical parametric generator", Optics Letters 23, December 15, 1998, pp. 11861889.
2. K. W. Aniolek, P. E. Powers, T. J. Kulp, B. A. Richman, and S. E. Bisson, "Cavity ringdown
laser absorption spectroscopy using a 1 kHz mid-infrared periodically-poled lithium niobate
optical parametric generator/optical parametric amplifier", Chemical Physics Letters, 302,
March 26, 1999, pp. 555-562.
3. H. Hakayama, O. Sugihara, and N. Okamoto, “Direct electron-beam irradiation: a new
technique for the erasure of second-order nonlinearity and the fabrication of channel
waveguides by use of optical polymer films,” Optics Letters, 22, October 15, 1997, pp. 15411543.
We propose to use these QPM nonlinear materials to shift the tuning of near IR lasers
sources into the mid and long wave infrared for high sensitivity sensors and other applications.
We have already taken the first step in this direction with a seeded optical parametric amplifier.
This device generates narrow frequency and simple tunability in the 1.3 to 4.5 m wavelength
range. The device relies on the high conversion efficiency of PPLN to allow appoximately 20%
conversion of the pump laser to the longer frequencies without any resonant cavities and the
complications associated with such cavities. Details of this source can be found in reference 1.
In collaboration with Sandia National Laboratories, this device has been incorporated into a
cavity ring-down spectrometer allowing for ultrahigh detection sensitivity (10-8 cm-1Hz-1/2).
Narrow frequency tuning was achieved and demonstated over a 350 cm-1 range2.
O. Sugihar, Y. Che, N. Okamoto, H. Fujimara, C. Egami, and S. Umegaki, “High-resolution periodically poled
structure in diazo-dye-substituted polymer film based on direct electron-beam writing technique,” Applied Physics
Letters, 73, November 23, 1998, pp. 3028-3030.
2
G. Khanarian, R. A. Norwood, D. Haas, B. Feuer, and D. Karim, Applied Physics Letters, 57
3
O. Sugihar, Y. Che, N. Okamoto, H. Fujimara, C. Egami, and S. Umegaki, “High-resolution periodically poled
structure in diazo-dye-substituted polymer film based on direct electron-beam writing technique,” Applied Physics
Letters, 73, November 23, 1998, pp. 3028-3030.
1