Polyborates useful for optical frequency conversion

Details Polyborates useful for optical frequency conversion Polyborates useful for optical frequency conversion

2001-01-10

2002-08-27

Lee, John D. (Department: 2874)

Optical: systems and elements

Optical frequency converter

Parametric oscillator

C359S326000, C372S021000, C423S277000

Reexamination Certificate

active

06441949

ABSTRACT:

FIELD OF THE INVENTION
The present invention is related to nonlinear optical materials, methods of crystal growth, and devices employing such materials. More specifically, the present invention is related to nonlinear optical materials that satisfy the general formula Ba
2−x
M
x
B
10
O
17
, wherein M is a divalent metal ion; wherein x ranges from about 0 to 0.3.
BACKGROUND OF THE INVENTION
Nonlinear optical (NLO) materials are unusual in that they affect the properties of light. A well-known example is the polarization of light by certain materials, such as when materials rotate the polarization vectors of absorbed light. If the effect on the polarization vector by the absorbed light is linear, then light emitted by the material has the same frequency as the absorbed light. NLO materials affect the polarization vector of the absorbed light in a nonlinear manner. As a result, the frequency of the light emitted by a nonlinear optical material is affected.
More specifically, when a beam of coherent light of a given frequency, such as produced by a laser, propagates through a properly oriented NLO crystal having non-zero components of the second order polarizability tensor, the crystal will generate light at a different frequency, thus extending the useful frequency range of the laser. Generation of this light can be ascribed to processes such as sum-frequency generation (SFG), difference-frequency generation (DFG) and optical parametric amplification (OPA). Devices using NLO crystals include, but are not limited to up and down frequency converters, optical parametric oscillators, optical rectifiers, and optical switches.
Frequency generation in NLO materials is an important effect. For example, two monochromatic electromagnetic waves with frequencies &ohgr;
1
and &ohgr;
2
propagating through a properly oriented NLO crystal can result in generation of light at a variety of frequencies. Mechanisms defining the frequency of light using these two separate frequencies are sum-frequency generation and difference-frequency generation. SFG is a process where light of frequency &ohgr;
3
is generated as the sum of the two incident frequencies, &ohgr;
3
=&ohgr;
1
+&ohgr;
2
. In other words, SFG is useful for converting long wavelength light to shorter wavelength light (e.g. near infrared to visible, or visible to ultraviolet). A special case of sum-frequency generation is second-harmonic generation (SHG) where &ohgr;
3
=2&ohgr;
1
, which is satisfied when the incident frequencies are equal, &ohgr;
1
=&ohgr;
2
. DFG is a process where light of frequency &ohgr;
4
is generated as the difference of the incident frequencies &ohgr;
4
=&ohgr;
1
−&ohgr;
2
. DFG is useful for converting shorter wavelength light to longer wavelength light (e.g. visible to infrared). A special case of DFG is when &ohgr;
1
=&ohgr;
2
, hence &ohgr;
4
=0, which is known as optical rectification. Optical parametric oscillation is also a form of DFG and is used to produce light at tunable frequencies.
The conversion efficiency of an NLO crystal for a particular application is dependent on a number of factors that include, but are not limited to: the effective nonlinearity of the crystal (picometers/volt [pm /V]), birefringence (&Dgr;n, where n is a refractive index), phase-matching conditions (Type I, Type II, non-critical, quasi, or critical), angular acceptance angle (radian·cm), temperature acceptance (K·cm), walk-off (radian), temperature dependent change in refractive index (dn/dt), optical transparency range (nm), and the optical damage threshold (watts/cm
2
). Desirable NLO crystals should possess an optimum combination of the above properties as defined by the specific application.
Borate crystals form a large group of inorganic NLO materials used in laser-based manufacturing, medicine, hardware and instrumentation, communications, and research studies. Beta Barium Borate (BBO: &bgr;-BaB
2
O
4
), lithium triborate (LBO: LiB
3
O
5
), and cesium lithium borate (CLBO: CsLi(B
3
O
5
)
2
) are examples of borate-based NLO crystals developed in recent years that are being used widely as NLO devices, especially in high power applications. Select properties suitable for generation of laser light from the mid-infrared to the ultraviolet for these crystals are listed in Table 1.
TABLE 1
Commercially Available NLO Materials and Properties
PROPERTY
BBO
LBO
CLBO
D
eff
2
0.8
2.2-3.2
(pm/V)
Optical Transmission
2600-190
2600-160
—
(nm)
Angular Acceptance
1.0
7
1.7
(mrad · cm)
Temperature Acceptance
55
7.5
2.5
(K · cm)
Walk-off Angle
56
6.5
16
(mrad)
Damage Threshold
15
25
25
(10
9
W/cm
2
)
Crystal Growth Properties
flux or
flux
congruent
congruent melt
melt
BBO has a favorable non-linearity (about 2 pm/V), transparency between 2600 mn and 190 nm, significant birefringence (necessary for phase matching), and a high damage threshold (15 GW/cm
2
, 1064 nm, 0.1 ns pulse width). However, its high birefringence creates a relatively small angular acceptance that can limit conversion efficiencies. The crystal is relatively difficult to grow to large sizes and is somewhat hygroscopic.
LBO has good UV transparency (absorption edge □ 160 nm) and possesses a high damage threshold (25 GW/cm
2
, 0.1 ns, 1064 nm). However, it has insufficient intrinsic birefringence for phase matching to generate deep UV radiation. Furthermore, LBO melts incongruently and must be prepared by flux-assisted crystal growth methods. This limits production efficiency that leads to small crystals and higher production costs.
CLBO appears to be a very promising material for high-power production of UV light due to a combination of high nonlinearity and high damage threshold. The crystal can also be manufactured to relatively large dimensions. Unfortunately, the crystal is exceedingly hygroscopic and invariably sorbs water from the air; hence, extreme care must be taken to manage environmental moisture to prevent hydration stresses and possible crystal destruction.
With so many intrinsic physical parameters to optimize, known optical frequency converters, at present, are applicable to specific applications. A major factor limiting the advancement of laser applications is the inability of conventional NLO devices to generate laser light at desired wavelengths, power levels, and beam qualities. Currently-available NLO materials are not able to meet specifications required by many applications due to a number of factors that include: small nonlinear coefficients, bulk absorption in energy regions of interest, poor optical clarity, low damage thresholds, instability under operation, environmental degradation, difficulty in device integration, and high fiscal costs of manufacture. In many cases, the fundamental limit of conventional NLO materials has been met, and as a result, they are not able to meet specifications required by many present and future applications. Related properties and shortcomings are discussed in Chemistry of Materials, 1:492-508 (1989), Keszler, Curr. Opinion in Solid State & Mater. Sci. 1, 204 (1996). Becker Adv. Mater. 10(13) p. 979-992 (1998), which are hereby incorporated by reference.
Because of the large number and diversity of present and projected applications, no single NLO material can be optimized for all uses. Thus far only a limited number of efficient NLO materials have been commercialized, thereby creating a bottleneck in the use of lasers in the advancement of many key technology areas. As a result, there is a continuing search for and development of new NLO materials.
SUMMARY OF THE INVENTION
It is an object of the present invention to produce and utilize nonlinear optical materials according to Formula 1:
Ba
2−x
M
x
B
10
O
17
  Formula 1
wherein M is a divalent metal ion; wherein x ranges from 0 to 0.3. Preferred divalent metal ions M that are suitable for forming compounds satisfying Formula 1 can be independently selected from the metals Ba, Sr, and Pb. Examples of nonlinear optical materials satisfying Formula 1 include, but are not lim

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