Abstract:
A frequency-doubled semiconductor vertical-external-cavity surface-emitting laser (VECSEL) is disclosed for generating light at a wavelength in the range of 300-550 nanometers. The VECSEL includes a semiconductor multi-quantum-well active region that is electrically or optically pumped to generate lasing at a fundamental wavelength in the range of 600-1100 nanometers. An intracavity nonlinear frequency-doubling crystal then converts the fundamental lasing into a second-harmonic output beam. With optical pumping with 330 milliWatts from a semiconductor diode pump laser, about 5 milliWatts or more of blue light can be generated at 490 nm. The device has applications for high-density optical data storage and retrieval, laser printing, optical image projection, chemical-sensing, materials processing and optical metrology.

Description:
GOVERNMENT RIGHTS 
     This invention was made with Government support under Contract No. DE-AC04-94AL85000 awarded by the U.S. Department of Energy. The Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to lasers and more particularly to a semiconductor vertical-external-cavity surface-emitting laser that generates second harmonic light by means of a nonlinear crystal contained within an optical cavity of the laser. 
     BACKGROUND OF THE INVENTION 
     Compact efficient sources of coherent blue or green laser light at wavelengths in the range of 400-550 nanometers are useful for many different types of applications including high-density optical data storage and retrieval, laser printing, optical image projection, fluorescence-based chemical-sensing, materials processing and optical metrology. Many different semiconductor laser approaches have been explored to generate lasing in this wavelength range, but with limited success and reliability. Prior approaches using external frequency doubling of semiconductor lasers have generated only a few nanoWatts of blue lasing. What is needed is a compact laser source which can generate light in the wavelength range of 400-550 nanometers at milliWatt output powers and with good beam quality. 
     The present invention overcomes the limitations of the prior art by providing a frequency-doubled vertical-external-cavity surface-emitting laser (VECSEL) which generates up to 5 milliWatts or more of lasing output at a wavelength below about 500 nanometers. 
     An advantage of the present invention is that the frequency-doubled VECSEL operates in a fundamental transverse mode to provide excellent focusability and beam propagation. 
     Yet another advantage of the present invention is that embodiments of the frequency-doubled VECSEL of the present invention can be activated either optically with an external laser pump energy source, or electrically by using current injection from an external power supply. 
     A further advantage of the present invention is that a precise wavelength control over a fundamental lasing frequency in the VECSEL can be provided during fabrication for precisely matching the fundamental frequency to be within an acceptance bandwidth of a particular nonlinear crystal to permit efficient frequency doubling and the generation of light at a predetermined wavelength. 
     Still another advantage of the present invention is that an air gap is provided between a gain element and a nonlinear crystal in the VECSEL; and this air gap permits the insertion of an optional Fabry-Perot etalon for reducing a bandwidth for lasing within the device, thereby improving the coherence and stability of the frequency-doubled lasing output. 
     These and other advantages of the present invention will become evident to those skilled in the art. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an apparatus for generating light at a second-harmonic frequency. The apparatus comprises a semiconductor substrate which includes a first reflector formed on the substrate and a semiconductor active region formed on the substrate proximate to the first reflector; and a nonlinear crystal (e.g. potassium niobate) located proximate to the active region and spaced from the active region by an air gap (e.g. about 1-3 mm), with the nonlinear crystal having a second reflector on a surface thereof away from the active region. The first and second reflectors together define a laser cavity which contains the active region and the nonlinear crystal, with the active region generating lasing light at a fundamental frequency in response to electrical or optical activation, and with the nonlinear crystal converting a portion of the lasing light into light at the second-harmonic frequency. The second-harmonic lasing is emitted from the apparatus through the second reflector which is partially transmissive at the second-harmonic frequency. 
     The active region preferably comprises a plurality of quantum wells which can be gallium arsenide (GaAs) quantum wells, indium gallium arsenide (InGaAs) quantum wells, or aluminum gallium arsenide (AlGaAs) quantum wells depending upon the fundamental frequency which can be selected to correspond to a wavelength in the range of 600-1100 nanometers to generate light at a second-harmonic frequency that is at a wavelength equal to one-half the wavelength of the lasing at the fundamental frequency. 
     The first reflector is preferably a Distributed Bragg Reflector formed from a plurality of alternating high-refractive-index and low-refractive-index semiconductor layers epitaxially grown on the substrate to provide a reflectivity for light at the fundamental frequency of ≧99%. An optional third reflector can be epitaxially grown on the substrate above the active region, with the third reflector preferably being a Distributed Bragg Reflector formed from a plurality of alternating low-refractive-index and high-refractive-index semiconductor layers. The third reflector can help to control and stabilize the fundamental frequency for efficient second-harmonic light generation. An optional Fabry-Perot etalon can also be located within the air gap to narrow a bandwidth of the fundamental frequency for more efficient second-harmonic light generation and to provide a reduced bandwidth and increased coherence for the second-harmonic light. 
     The apparatus can be optically activated by pump light from a separate pump laser (e.g. a semiconductor laser or a titanium-sapphire laser). Alternately, the apparatus can be electrically activated by including a semiconductor p-n or p-i-n junction within the active region. For an electrically-activated device, an upper electrode can be provided above the active region, and a lower electrode can be provided on the substrate. 
     The present invention is further related to a semiconductor laser which comprises a gallium arsenide substrate having a plurality of III-V compound semiconductor layers epitaxially grown thereon, including a plurality of alternating high-refractive-index and low-refractive-index semiconductor layers forming a first reflector which is reflective of light at a fundamental frequency, and an active region wherein light is generated at the fundamental frequency; and a nonlinear crystal (e.g. potassium niobate) separated from the substrate and plurality of semiconductor layers by an air gap (e.g. a 1-3 mm air gap), with the nonlinear crystal having a first surface and a second surface, the first surface nearest the substrate generally being substantially planar and including an anti-reflection coating thereon, and the second surface preferably being curved (e.g. a 15-millimeter radius of curvature) and including a second reflector which is reflective of the light at the fundamental frequency and transmissive of light at a second-harmonic frequency that is twice the fundamental frequency. The first and second reflectors define therebetween a laser cavity that extends from the first reflector through the active region and the nonlinear crystal to the second reflector. The semiconductor laser of the present invention is responsive to an external energy source (e.g. optical or electrical activation) to generate light in the laser cavity at the fundamental frequency, with the nonlinear crystal being operative to convert the light at the fundamental frequency to light at the second-harmonic frequency, so that an output light beam (i.e. a lasing beam) is generated at the second-harmonic frequency and transmitted through the second reflector. 
     The first reflector, which preferably has a reflectivity for light at the fundamental frequency of &gt;99%, can comprise, for example, gallium arsenide (GaAs) high-refractive-index layers and aluminum arsenide (AlAs) low-refractive-index layers. The second reflector is also preferably highly reflective at the fundamental frequency (e.g. ≧99%), while being transmissive of the light generated at the second-harmonic frequency. 
     To generate blue light at a wavelength of about 490 nanometers corresponding to the second-harmonic frequency, the active region can comprise a plurality of alternating layers of tensile-strained gallium arsenide phosphide (GaAsP), compressively-strained indium gallium arsenide (InGaAs), and aluminum gallium arsenide (AlGaAs) to generate light at a fundamental wavelength of about 980 nanometers. In this preferred embodiment of the present invention, the GaAsP layers can have a semiconductor alloy composition GaAs 0.8 P 0.2 , the InGaAs layers can have the semiconductor alloy composition In 0.18 Ga 0.82 As, and the AlGaAs layers can have the semiconductor alloy composition Al 0.08 Ga 0.92 As. For this embodiment of the present invention, the active region can further include a carrier-confinement layer comprising aluminum gallium arsenide (AlGaAs), and an indium gallium phosphide (InGaP) cap layer overlying the AlGaAs current-confinement layer. In other embodiments of the present invention, the active region can comprise a plurality of alternating layers of gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs) to generate a fundamental wavelength of about 860 nanometers which can be converted by the nonlinear crystal into blue light at a wavelength of about 430 nanometers. Other embodiments of the present invention can be formed with the semiconductor laser operating at a fundamental frequency corresponding to a wavelength in the range of 600-1100 nanometers for generation of a second-harmonic frequency corresponding to a wavelength in the range of 300-550 nanometers, with the second-harmonic wavelength being equal to one-half the fundamental wavelength. 
     Embodiments of the present invention can be provided for either optical activation or electrical activation. For optical activation, the external energy source can comprise optical pumping with lasing light from at least one pump laser, with the lasing light from each pump laser preferably being incident on the plurality of semiconductor layers at a Brewster angle which is defined by the refractive index of the active region. The Brewster angle can be, for example, about 74° as measured from an axis normal to an upper surface of the substrate. Each pump laser can comprise, for example, a semiconductor laser or a solid-state laser (e.g. a titanium-sapphire laser). 
     For electrical activation, the external energy source comprises electrical excitation with an electrical current flowing through a semiconductor p-n or p-i-n junction which can be formed about the active region by doping the plurality of semiconductor layers. An electrically-activated semiconductor laser also preferably includes an upper electrode above the active region and a lower electrode on the substrate. An electrically-activated semiconductor laser can also include an optional third reflector which can be epitaxially grown above the active region as a plurality of alternating low-refractive-index and high-refractive-index semiconductor layers that are reflective of the fundamental frequency. This third reflector can be used for wavelength control of the fundamental frequency for more efficient conversion of light at the fundamental frequency into light at the second-harmonic frequency. 
     An optional Fabry-Perot etalon can be located within the air gap of either an optically-activated device or an electrically-activated device to narrow the bandwidth of the fundamental frequency in the semiconductor laser. 
     Additional advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following detailed description thereof when considered in conjunction with the accompanying drawings. The advantages of the invention can be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated into and form a part of the specification, illustrate several aspects of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating preferred embodiments of the invention and are not to be construed as limiting the invention. In the drawings: 
     FIG. 1 shows schematically a first example of a frequency-doubled semiconductor laser formed according to the present invention with activation by optical pumping from an external pump laser. 
     FIG. 2 shows curves of the lasing output from the frequency-doubled semiconductor laser of FIG. 1 at the fundamental and second-harmonic frequencies. 
     FIG. 3 schematically illustrates in cross-section view the frequency-doubled semiconductor laser of FIG.  1 . 
     FIG. 4 schematically illustrates in cross-section view a second example of a frequency-doubled semiconductor laser formed according to the present invention with activation by an electrical current. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, there is shown schematically a first example of a frequency-doubled semiconductor laser  10  formed according to the present invention. The semiconductor laser  10  is a vertical-external-cavity surface-emitting laser (VECSEL) which comprises a semiconductor substrate  12  with a first reflector  14  formed thereon and a semiconductor active region  16  formed on the substrate  12  above the first reflector  14 . The semiconductor laser  10  further comprises a nonlinear crystal  18  located proximate to the active region  16  and separated from the active region  16  by an air gap  20  (i.e. the nonlinear crystal  18  is not affixed to the active region  16  or to any semiconductor layer epitaxially grown on the substrate  12 ). 
     In FIG. 1, the nonlinear crystal  18  further includes a second reflector  22  formed on a curved surface of the crystal  18  that is located distally to the active region  16 , and an anti-reflection coating  24  which is provided on a generally planar surface of the crystal  18  that is located proximally to the active region  16 . The first and second reflectors,  14  and  22 , define a cavity of the semiconductor laser  10  that contains the active region  16 , the nonlinear crystal  18  and the air gap  20 . 
     The semiconductor laser  10  in the example of FIG. 1 can be optically activated by one or more external pump lasers  100  which can be, for example, semiconductor lasers. Each pump laser  100  generates a pump lasing beam  102  which is focused to be incident on the active region  16  at an angle, θ, which is preferably equal to Brewster&#39;s angle, θ B , which is approximately equal to 74° as measured from an axis  26  that is normal to an upper surface of the substrate  12  (see FIG.  2 ). In general, Brewster&#39;s angle for a material of refractive index, n, is given by: 
     
       
         θ B =tan −1 ( n ) 
       
     
     and for a layered structure having an effective index of refraction, n eff , averaged over a plurality of material layers, Brewster&#39;s angle is given by: 
     
       
         θ B =tan −1 ( n   eff ) 
       
     
     where n eff  is given by:          n   eff     =       ∑       n   l          t   l           ∑     t   l                                
     with n l  being the refractive index of a particular layer, l, and t l  being the thickness of that layer, l, and with the summation being performed over the plurality of material layers. 
     When the focused pump beam  102  is incident on the active region  16  at Brewster&#39;s angle as described above, the pump light is coupled into the active region  16  along the axis  26  to most effectively optically activate the active region  16  to generate light at the fundamental frequency. To reduce a reflection loss of the pump beam  102  at the surface of the active region  16 , the pump beam  102  can be linearly polarized in a direction parallel to the plane containing the incident pump beam  102  and a reflected component of the pump beam  102  off the surface of the active region  16  (i.e. “p” polarization). If needed, a half-wave plate  104  can be inserted into the pump beam  102  as shown in FIG. 1 to rotate the polarization of the pump laser  100  to be in the preferred polarization direction. 
     In FIG. 1, a focusing mirror  106  (e.g. having a 25-mm radius of curvature), or alternately one or more refractive or diffractive lenses, can be used to focus the pump beam  102  down to a spot size of about 100 μm at the top of the active region  16 . Additionally, a glass or fused silica plate  108  (e.g. 2 mm thick) can be inserted into the pump beam  102  at Brewster&#39;s angle to convert an elongated pump beam  102  from a semiconductor laser  100  (e.g. having a 1 μm×50 μm emitting region) into a circular focused beam at the location of the active region  16 . 
     The incident pump beam  102  is absorbed in certain of the semiconductor layers forming the active region  16  thereby photogenerating electrons and holes which can then relax into other of the semiconductor layers (i.e. in a plurality of quantum wells) and recombine to generate light at a fundamental frequency of oscillation. This light at the fundamental frequency circulates within the cavity formed by the first and second reflectors,  14  and  22 , and generates optical gain which can become sufficiently strong to overcome optical losses within the cavity formed by the first and second reflectors,  14  and  22 , thereby producing lasing at the fundamental frequency within the semiconductor laser  10 . 
     The nonlinear crystal  18  is preferably b-cut potassium niobate (KNbO 3 ) for operation at a fundamental wavelength near 980 nm or a-cut KNbO 3  for operation at a fundamental wavelength near 860 nm. Other nonlinear crystals can be used to frequency-double light at these wavelengths or at different wavelengths for operation of the semiconductor laser  10 . The fundamental wavelength and fundamental frequency are inversely related so that these terms can be used interchangeably to describe the lasing within the cavity. 
     The nonlinear crystal  18  can have a length of several millimeters (e.g. 3.5-7.5 mm) and is oriented to provide non-critical phase matching at the fundamental frequency at a temperature near room temperature. In operation, the nonlinear crystal  18  acts to frequency double the lasing at the fundamental frequency and generate therefrom coherent light at a second-harmonic frequency which is twice the frequency, and hence one-half the wavelength, of the lasing at the fundamental frequency. The power of the second-harmonic lasing is proportional to the square of the power of the fundamental-frequency lasing circulating within the cavity formed by the first and second reflectors,  14  and  22 . Due to the very high reflectivity (generally ≧99% and preferably ≧99.9%) of these reflectors  14  and  22  for light at the fundamental frequency, the circulating fundamental-frequency lasing power can be on the order of 10 Watts or more, thereby producing several milliWatts of second-harmonic lasing output from the device  10  in a second-harmonic output beam  28 . 
     To provide for fundamental mode (i.e. TEM 00  mode) operation of the semiconductor laser  10  with a spot size about that of the focused pump beam  102  or slightly smaller, the nonlinear crystal  18  is preferably polished to provide a curved surface (e.g. a convex surface with a 15-mm radius of curvature) on an end thereof located away from the active region  16 . A multi-layer dielectric mirror is deposited on the curved surface of the nonlinear crystal  18 , with the mirror having a high reflectivity (e.g. &gt;99.9%) at the fundamental frequency and a transmissivity of ≧50% and preferably as high as possible (e.g. 80-95%) at the second-harmonic frequency. The other end of the nonlinear crystal  18  located nearest to the active region  16  can be substantially planar and preferably includes a multi-layer anti-reflection coating  24  to provide a very low reflectivity (e.g. about 0.05% or less) at the fundamental frequency. In some embodiments of the present invention, the end of the nonlinear crystal  18  nearest the active region  16  can be curved for additional mode control and/or mode stability within the semiconductor laser  10 . 
     Non-critical phase matching or quasi-phase matching within the nonlinear crystal  18  is advantageous since both result in substantially zero birefringent walk-off of beams at the fundamental and second-harmonic frequencies, thereby leading to a high energy conversion efficiency. A 7.5-mm-long b-cut KNbO 3  crystal  18  designed for Type 1 non-critical phase matching at 980 nanometers can be used according to the present invention, with this nonlinear crystal  18  having an acceptance angle of about 64 milliradians and an acceptance bandwidth of about 0.28 nanometers. This relative large acceptance angle permits angular alignment of the nonlinear crystal  18  with little effect on the conversion efficiency of the crystal  18 . 
     When the nonlinear crystal  18  is spaced from the active region  16  by an air gap of generally 1-3 mm, a stable cavity is formed with lasing in the fundamental TEM 00  mode. The theoretical per-pass efficiency for generation of light at the second-harmonic frequency under optimal phase-matching conditions is calculated to be 1.6% per Watt of lasing power at the fundamental frequency circulating within the cavity formed by the first and second reflectors,  14  and  22 . 
     FIG. 2 shows the results for optical pumping of a semiconductor laser  10  with an active region  16  tailored for a fundamental lasing frequency of 980 nanometers. In FIG. 2, the threshold for lasing at the fundamental frequency is about 110 mW of incident power from a semiconductor diode pump laser  100 . Above threshold, the second-harmonic output at 490 nanometers as measured through the second reflector  22  increases quadratically to 5 milliWatts as the incident pump power is increased to 330 milliWatts. The second-harmonic light is emitted through the second reflector  22  as a nearly-Gaussian coherent beam with an angular divergence of about 10 milliradians (full-width-at-half-maximum). Within the cavity formed by the first and second reflectors,  14  and  22 , the circulating lasing power at the fundamental frequency (980 nm) is estimated to be about 3.4 Watts; and the low transmissivity (0.029%) of the second reflector  22  at the fundamental frequency allows a fundamental light output escaping the cavity through the second reflector  22  to be limited to only one-fifth the power in the second-harmonic output beam  28 . If needed, the fundamental light output can be blocked with an optical filter, or separated from the second-harmonic output by a dispersive optical element (e.g. a prism or diffraction grating). 
     Returning to FIG. 1, the substrate  12  containing the first reflector  14  and the active region  16  can be mounted upon a heat sink  30  (e.g. comprising copper) for temperature control and cooling, with the heat sink  30  being further attached to a cold side of a Peltier cooling element  32  (i.e. a thermoelectric cooling element) to maintain the substrate  12  at a predetermined temperature (e.g. 0-20° C.). A secondary air-cooled or water-cooled heat sink  34  can be attached to a hot side of the Peltier element  32  for further heat removal. 
     Temperature control for the nonlinear crystal  18  can be optionally provided, if needed. This can be done, for example, by mounting the nonlinear crystal  18  on the same Peltier element (e.g. using a heat sink  30  which is common to both the substrate  12  and the crystal  18 ), on a different Peltier element (to allow the substrate  12  and the nonlinear crystal  18  to be independently temperature controlled), or in a temperature-controlled oven (to allow heating of the nonlinear crystal  18  to a temperature slightly above room temperature). 
     In FIG. 1, the entire semiconductor laser  10  including the semiconductor diode pump laser  100  can be mounted on a base  36  to form a compact package with lateral dimensions of a few centimeters. In other embodiments of the present invention, multiple semiconductor diode pump lasers  100  (e.g. single-stripe diode lasers or diode laser arrays) can be used spaced about the active region  16  to provide a plurality of focused pump beams  102  which can all be made coincident on the active region  16  to obtain an increased pump power level to generate a higher level of second-harmonic light from the semiconductor laser  10 . Alternately, a pump laser  100  in the form of a solid-state laser (e.g. a titanium-sapphire laser) can be used to optically activate the semiconductor laser  10  of the present invention. 
     FIG. 9 shows further details of the structure of the semiconductor laser  10  in the example of FIG.  1 . In FIG. 3, the substrate  12  comprises a III-V compound semiconductor, preferably gallium arsenide (GaAs). The GaAs substrate  12  can be about 0.65 mm thick with a crystallographic orientation that is about 2° off the (100) crystallographic plane towards the (110) plane, and can further include one or more buffer layers of GaAs (not shown in FIG. 3) that are epitaxially grown on the substrate  12  to prepare the substrate for further epitaxial growth. The plurality of III-V compound semiconductor layers epitaxially grown on the substrate  12  to form the first reflector  14  and the active region  16  can be grown by a conventional epitaxial growth method such as molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD). 
     In FIG. 3, the first reflector  14  is a Distributed Bragg Reflector (DBR) which comprises a plurality of alternating high-refractive-index semiconductor layers  38  and low-refractive-index semiconductor layers  40 , with each layer  38  and  40  having a layer thickness of substantially one-quarter wavelength of the light at the fundamental frequency. For a fundamental wavelength of 980 nm, the high-refractive-index semiconductor layers  38  can comprise GaAs with a layer thickness of 68-70 nm epitaxially grown (e.g. by MOCVD) at a temperature of about 750° C.; and the low-refractive-index semiconductor layers  40  can comprise aluminum arsenide (AlAs) with a layer thickness of 81-83 nm epitaxially grown at the same temperature. Twenty seven pairs of high-refractive-index and low-refractive-index semiconductor layers,  38  and  40 , can be used to provide a reflectivity of &gt;99.9% at the fundamental wavelength of 980 nm. In other embodiments of the present invention, the exact number of pairs of the high-refractive-index and low-refractive-index semiconductor layers,  38  and  40 , will depend upon the fundamental frequency, the desired reflectivity at the fundamental frequency and the semiconductor alloy composition of the layers  38  and  40  (i.e. a refractive index difference between the layers  38  and  40 ). 
     In FIG. 3, the active region  16  is epitaxially grown above the first reflector  14  and comprises a plurality of pump-light-absorbing layers  42  separated by quantum wells  44 . The pump-light-absorbing layers  42  have an energy bandgap that is larger than the energy bandgap of the quantum wells  44  so that carriers (i.e. electrons and holes) photogenerated by the incident pump beam  102  will be captured by the quantum wells  44  where the electrons and holes can recombine to generate light at the fundamental frequency. For a fundamental frequency of 980 nm, the pump-light-absorbing layers  42  can comprise, for example, about 125 nm of Al 0.08 Ga 0.92 As for absorbing an incident pump beam  102  at a wavelength near 800 nm. The Al 0.08 Ga 0.92 As pump-light-absorbing layers  42  can be epitaxially grown by MOCVD at a temperature of about 620° C. 
     For operation at 980 nm, the quantum wells  44  can comprise about 10 nm of indium gallium arsenide with the composition In x Ga 1−x As with x being in the range of 0.16-0.18. The In x Ga 1−x As quantum wells  44  can be epitaxially grown by MOCVD at about 620° C. Since the In 0.18 Ga 0.82 As quantum wells  44  are compressively strained, a strain-compensation layer comprising, for example, a 9-nm-thick layer of tensile-strained gallium arsenide phosphide with the composition GaAs 0.80 P 0.20  can be epitaxially grown on one side of each In 0.18 Ga 0.82 As quantum well  44  to provide a strain balanced structure for the active region  16 . The quantum wells  44  are also preferably spaced to coincide with anitnodes (i.e. peaks) of a standing-wave electric field of the lasing at the fundamental frequency within the cavity formed by the reflectors  14  and  22 . About 10-15 quantum wells  44  are generally provided in the active region  16 . 
     In FIG. 3, a carrier-confinement layer  48  is preferably epitaxially grown above a last-grown pump-light-absorbing layer  42  to prevent the carriers from escaping to the upper surface of the active region  16  where surface recombination can occur. The carrier-confinement layer  48 , which has an energy bandgap larger than the energy bandgap of the pump-light-absorbing layers  42 , can comprise, for example, an Al 0.5 Ga 0.5 As layer that is about 100 nanometers thick grown by MOCVD at a temperature of about 750° C. To prevent oxidation of the aluminum in the Al 0.5 Ga 0.5 As carrier-confinement layer  48 , a cap layer  50  of indium gallium phosphide with a lattice-matched composition In 0.49 Ga 0.51 As and a layer thickness of about 40 nanometers can be epitaxially grown above the carrier-confinement layer  48  by MOCVD at a temperature of about 620° C. For optical activation of the semiconductor laser  10 , none of the plurality of epitaxially-grown semiconductor layers need be doped. 
     After the epitaxial growth is complete, the substrate  12  can be cleaved to a predetermined size (e.g. up to a few millimeters on a side) and mounted for optical pumping as shown in FIG.  1 . It is not necessary to remove the substrate  12  since the semiconductor laser  10  can be effectively heat-sinked through the substrate  12 . To further enhance heat removal through the substrate  12  and improve device performance, the substrate can be thinned (e.g. to about 100-200 μm thickness). 
     With optical activation, the semiconductor laser  10  lases at the fundamental frequency producing a fundamental beam  52  as shown in FIG. 3 which circulates in the cavity formed by the first and second reflectors,  14  and  22 . The nonlinear crystal  18  then operates to convert a portion of the fundamental beam  52  into a second-harmonic beam  28  which exits the semiconductor laser  10  by being transmitted through the second reflector  22 . The beams  28  and  52  overlap spatially and are centered about the axis  26  coincident with focused pump beam  102  which is directed into the active region  16  along the axis  26 . However, for clarity, each of the beams  28 ,  52  and  102  are shown displaced slightly from the axis  26  and from each other in FIG.  3 . 
     FIG. 4 schematically illustrates in cross-section view a second example of a frequency-doubled semiconductor laser  10  formed according to the present invention with activation by an electrical current flowing through the active region  16 . The example of the semiconductor laser  10  in FIG. 4 will be described in terms of a device  10  operating at a fundamental frequency corresponding to a wavelength of about 860 nm although it will be understood by those skilled in the art that electrically-activated semiconductor lasers  10  can be formed according to the present invention to operate at other fundamental wavelengths generally within the wavelength range of 600-1100 nm. In FIG. 4, the nonlinear crystal  18  preferably comprises an a-cut KNbO 3  crystal. The size of the nonlinear crystal  18  is similar to that described previously with reference to FIGS. 1-3, with the anti-reflection coating  24  being tailored to provide a very low reflectivity at 860 nm (e.g. ≦0.05% reflectivity), and with the second reflector  22  providing a very high reflectivity (≧99% and preferably &gt;99.9%) at the fundamental wavelength of 860 nm and a high transmissivity (≧50% and preferably 80-95%) for the generated second-harmonic beam  28 . 
     In the example of FIG. 4, the substrate  12  preferably comprises a p-doped GaAs substrate having a p-type dopant concentration of about 10 18  cm −3 . Upon the substrate  12  are epitaxially grown a plurality of doped semiconductor layers to form the first reflector  14 , the active region  16  and a third reflector  54 . The use of a p-type substrate allows the semiconductor layers epitaxially grown in an upper portion of the active region  16  or thereabove to be n-type doped thereby providing an increased carrier mobility in these layers (due to electrons being the majority carriers) to provide a more uniform electrical activation of the active region  16  in the device  10 . In other embodiments of the present invention, an n-type substrate  12  can be used, with the polarity of the various semiconductor layers in FIG. 4 being reversed. 
     In the example of FIG. 4, the first reflector  14  can be formed from a plurality of alternating high-refractive-index semiconductor layers  38  and low-refractive-index semiconductor layers  40  as described previously with reference to FIG.  3 . For operation at an 860-nm fundamental wavelength, the high-refractive-index layers  38  can comprise, for example, 36.9 nm of Al 0.16 Ga 0.84 As epitaxially grown by MOCVD at about 750° C.; and the low-refractive-index layers  40  can comprise, for example, 42.6 nm of Al 0.92 Ga 0.08 As grown by MOCVD at the same temperature. Compositionally-graded interfaces (i.e. graded in composition between Al 0.16 Ga 0.84 As and Al 0.92 Ga 0.08 As) about 26.5 nm thick can be provided between each adjacent pair of the layers  38  and  40  to reduce the electrical resistance of the first reflector  14 . A total of thirty five pairs of layers  38  and  40  can be used to provide the desired high reflectivity of ≧99.9%. All of the pairs of the layers  38  and  40  in the first reflector  14  when grown on a p-type-doped substrate  12  can also be doped p-type to about 3×10 18  cm −3  with carbon, with a last-grown pair of the layers  38  and  40  having a reduced p-type doping level (e.g. 1×10 18  cm −3 ). 
     In FIG. 4, the active region  16  that is epitaxially grown above the first reflector  14  can comprise a plurality of GaAs quantum wells  44  separated by higher-bandgap AlGaAs (e.g. Al 0.15 Ga 0.85 As) barrier layers  56 , with the quantum wells  44  and barrier layers  56  further being sandwiched between a pair of graded-index cladding layers  58  having an even higher energy bandgap. A spacer layer  60  about 20 nanometers thick and having the same semiconductor alloy composition as the barrier layers  56  can be located at the position of a node (i.e. a null) in the electrical field of the lasing at the fundamental frequency, with the node preferably being located at the center of the active region  16 . The plurality of quantum wells  44 , barrier layers  56  and the spacer layer  60  in the example of FIG. 4 can have a combined layer thickness that results in an effective optical thickness of these layers which is substantially equal to one wavelength of the light at the fundamental-frequency. 
     In FIG. 4, the GaAs quantum wells  44  can be, for example, 6-12 nanometers thick, with the AlGaAs barrier layers being, for example, 8 nanometers thick. The quantum wells  44  and barrier layers  56  are preferably undoped (i.e. not intentionally doped) to form a p-i-n junction about the active region  16 . The active region  16  can include, for example, ten quantum wells  44  separated into two sets of five quantum wells  44  by the spacer layer  60  to spread the gain in the active region  16  over one wavelength of the light at 860 nm. 
     In FIG. 4, the cladding layers  58  are preferably graded in composition between the semiconductor alloy composition of the low-refractive-index layers  40  (e.g. Al 0.92 Ga 0.08 As) and the composition of the barrier layers  56  (e.g. Al 0.15 Ga 0.85 As) over an effective optical thickness of about one-half wavelength of the light at the fundamental frequency. The “effective optical thickness” of a layer of refractive index, n, and actual layer thickness, L, is defined herein to be the product of the refractive index and the actual layer thickness (i.e. nL). For a layer that is graded in composition (and hence in refractive index, n), an integral of the product nL can be taken over the layer thickness L to determine the effective optical thickness for the graded layer. Finally, for a plurality of layers of different composition and actual layer thicknesses, the effective optical thickness for the totality of the layers can be determined by taking a sum of the effective optical thickness for each layer. 
     The composition grading of the above cladding layers  58  during epitaxial growth can also be combined with a step grading of the doping concentration in the cladding layers  58  from about 1×10 18  cm −3  furtherest from the undoped quantum wells  44  to about 2×10 17  cm −3  nearest to the quantum wells  44 . A first-grown cladding layer  58  adjacent to the first reflector  14  is p-typed doped when the substrate  12  and the first reflector  14  are p-type doped; and a second-grown cladding layer  58  above the quantum wells  44  is oppositely doped (e.g. n-type doped with silicon). This doping arrangement in the active region  16  forms a semiconductor p-i-n junction about the active region  16 . In other embodiments of the present invention, a semiconductor p-n junction can be formed with the n-type and p-type doping extending into a central portion of the active region  16  containing the quantum wells  44  and barrier layers  56 . 
     In the example of FIG. 4, an optional third reflector  54  can be epitaxially grown above the active region  16  to aid in controlling and stabilizing the fundamental frequency of lasing within the semiconductor laser  10 . The inclusion of the third reflector  54  is advantageous since the conversion of the fundamental-frequency lasing into the second-harmonic output beam  28  can introduce a wavelength-dependent loss into the cavity which can result in the lasing in the cavity being shifted in wavelength away from the fundamental frequency and beyond the acceptance bandwidth of the nonlinear crystal  18 , thereby lowering the conversion efficiency in the laser  10 . 
     The third reflector  54  can be formed similarly to the first reflector  14  except that the third reflector  54  is formed as a mirror image of the first reflector  14  as shown in FIG.  4  and contains a smaller number (e.g. 7-10) of pairs of alternating low-refractive-index layers  40  and high-refractive-index layers  38  to provide a reflectivity of generally only about 80-90%. This lower reflectivity of the third reflector  54  prevents lasing in a cavity formed by the first and third reflectors,  14  and  54 , and instead encourages the semiconductor laser  10  to lase in the cavity formed by the first and second reflectors,  14  and  22 . Additionally, the third reflector  54  is doped oppositely from the first reflector  14 , and generally with a slightly lower doping level (e.g. 1×10 18  cm −3  n-type doping with silicon). 
     In FIG. 4, a cap layer  50  is epitaxially grown above the third reflector  54 . The cap layer  50  can comprise, for example, about 110 nm of Al 0.16 Ga 0.84 As with a relatively high n-type doping level of 4×10 18  cm −3  silicon followed by the growth of about 12 nm of GaAs identically doped. Both the Al 0.16 Ga 0.84 As and GaAs can be epitaxially grown by MOCVD at about 750° C. The relatively high doping in the cap layer  50  is advantageous for forming an Ohmic contact (i.e. an upper electrode  64 ) to the cap layer  50  and also facilitates current spreading in the cap layer  50  to provide a more uniform electrical current distribution over the ˜100 μm lasing mode spot size for efficient generation of optical gain and lasing at the fundamental frequency. The use of an n-type doping for the third reflector  54 , which is made possible by growing the plurality of semiconductor layers on a p-type substrate  12 , further facilitates the current spreading since the mobility of electrons provided by the n-type doping exceeds that of holes produced by p-type doping. Carrier trapping at heterointerfaces between the alternating low-refractive-index layers  40  and high-refractive-index layers  38  in the third reflector  54  also aids in promoting lateral current spreading. 
     In other embodiments of the present invention in which the third reflector  54  is omitted, one or more current-spreading layers can be provided in or above the active region  16  to promote current spreading in order to provide a more uniform electrical current distribution over the width of the fundamental lasing mode. Such a current-spreading layer can be, for example, a relatively thick (e.g. about 3-μm thick) bulk layer of GaAs or AlGaAs epitaxially grown above the active region  16  with at least a part of the bulk layer being heavily doped (e.g. n-type doped to about 4×10 18  cm −3 ). Alternately, one or more heavily-doped semiconductor, metal or semimetal sheets can be located at nodes of the electric field within or above the active region  16  to enhance lateral current spreading. As an example, the heavily-doped semiconductor sheets can be formed by three 30-nm-thick layers of Al 0.1 Ga 0.9 As with an n-type doping level of 4×10 18  cm −3  epitaxially grown above the active region  16 , with adjacent of the heavily-doped semiconductor sheets being separated by 380-nm-thick spacer layers of Al 0.8 Ga 0.2 As having a lower doping level (e.g. about 5×10 17  cm −3 ). Another method for improving the lateral current spreading in the semiconductor laser  10  is to provide barriers to vertical carrier transport above the active region  16  and thereby promote lateral current spreading. Such vertical-transport barriers can be formed, for example, from high-bandgap layers (e.g. 35-nm layers of Al 0.8 Ga 0.2 As n-type doped to about 1×10 18  cm −3 ) alternated with low-bandgap layers (e.g. 330-nm layers of Al 0.1 Ga 0.9 As n-type doped to about 5×10 17  cm −3 ). A total of three vertical-transport barriers can be provided above the active region  16  with an overall layer thickness of about 1.2 μm. 
     After epitaxial growth of the semiconductor layers on the substrate  12 , a pair of electrodes can be formed about the active region for connection to an electrical current source (e.g. a power supply). These electrodes can include a lower electrode  62  formed on the substrate  12  (e.g. on a bottom surface of the substrate  12 ) and an upper electrode  64  formed above the active region  16  and overlying the cap layer  50 . The electrodes  62  and  64  can be conventionally formed with the lower electrode  62  comprising, for example, a full-surface metallization of Ti/Pt/Au or Be/Au; and with the upper electrode  64  comprising, for example, a patterned metallization of Au/Ge/Ni with a shaped opening  66  (e.g. circular) formed therethrough by photolithographic masking and lift-off for transmission of the lasing at the fundamental frequency the electrodes  62  and  64  can be annealed (e.g. at 350° C. for 30 seconds in a rapid thermal annealer) to reduce contact resistance. Alternately, the upper electrode  64  can comprise a transparent indium-tin-oxide electrode so that no shaped opening  66  need be formed. The substrate  12  can be optionally thinned to about 100-200 μm prior to depositing the lower electrode  62 . 
     Restricting the electrical current to flow through a central portion of the active region  16  centered about the axis  28  can be achieved by several methods as known to the art. For example, ion implantation (e.g. with hydrogen or oxygen ions) can be used to increase the resistivity of the cap layer  50  and the uppermost few layers  38  and  40  of the third reflector  54  at a predetermined distance from the axis  28  that exceeds the size of the shaped opening  66 . 
     Alternately, selective lateral oxidation of an AlGaAs portion of the cap layer  50  or one or more aluminum-containing layers (e.g. comprising AlGaAs or AlAs) in the third reflector  54  can be used to generate electrically insulative regions to channel the electrical current into the active region  16 . The AlGaAs portion of the cap layer  50  can be oxidized by etching downward through a GaAs portion of the cap layer  50  to expose the AlGaAs portion and then exposing the AlGaAs portion to a moist ambient at a substrate temperature of about 350 to 500° C. (and preferably between about 400 and 450° C.). The moist ambient can be generated, for example, by flowing a gas, such as nitrogen, through water heated to about 80-95° C. to entrain water vapor, and then directing the moisture-laden gas into a container containing the heated substrate  12 . Exposure of the heated substrate  12  and AlGaAs portion of the cap layer  50  to the moist ambient for about one-half hour or less locally converts the AlGaAs portion into an oxide of aluminum which is an electrical insulator. 
     In yet another method for channeling the electrical current into the active region  16 , a trench can be etched downward partway into the third reflector  54  to expose sidewalls of the trench formed of the AlAs or AlGaAs low-refractive-index layers  40  and GaAs high-refractive-index layers  38 . The oxidation process in this case can proceed laterally inward over time to oxidize the AlAs or AlGaAs low-refractive-index layers  40  and to convert these layers  40  into an oxide of aluminum while the GaAs high-refractive-index layers  38  remain unchanged. The exact time required for this lateral oxidation process is highly temperature sensitive and can be in the range of about 0.5-2.5 hours for an oxidation distance of 50 μm at a substrate temperature of in the range of 400-450° C. 
     After deposition of the electrodes  62  and  64 , the substrate  12  can be diced or cleaved to dimensions about one millimeter on a side and mounted substrate-side-down on a Peltier cooler for heat removal during operation. The nonlinear crystal  18  with its integral second reflector  22  can then be brought into alignment with the first reflector  14  on the substrate  12  to form the completed laser cavity. The electrical injection of current from a source through the semiconductor p-n or p-i-n junction in the device  10  via electrodes  62  and  64  can produce lasing at the fundamental frequency which is then converted into the second-harmonic output beam  28  by action of the nonlinear crystal  18 . The electrically-injected semiconductor laser  10  can be operated either in a pulsed mode or continuously depending upon whether a current pulse or a direct-current (d-c) current is supplied to the device  10 . One- or two-dimensional arrays of electrically-activated semiconductor lasers  10  can be formed according to the present invention, with the individual lasers  10  in the array being electrically activated individually, or in tandem. 
     In some embodiments of the present invention as shown in FIG. 4, an optional Fabry-Perot etalon  68  (e.g. about 100 μm thick) can be located within the air gap  20  to narrow the bandwidth of the lasing at the fundamental frequency and thereby produce lasing on a single longitudinal mode (i.e. single-frequency lasing) within the cavity formed by the first and second reflectors  14  and  22 . This is advantageous for narrowing the bandwidth of the second-harmonic output beam  28  and thereby increasing a coherence length and stability of the second-harmonic beam  28 . Additionally, this can reduce the bandwidth of the fundamental-frequency lasing to be within the acceptance bandwidth of the nonlinear crystal  18 , thereby enabling the nonlinear crystal  18  to more efficiently frequency-double the fundamental beam  52 . 
     Other embodiments of the present invention are possible. For example, a third reflector  54  can be incorporated into an optically-activated semiconductor laser  10  above the active region  16  to provide control over the wavelength of the fundamental beam  52  and thereby aid in maintaining the fundamental frequency within the acceptance bandwidth of the nonlinear crystal  18 . The third reflector  54  can be epitaxially grown using a plurality of alternating low-refractive-index semiconductor layers  40  and high-refractive-index semiconductor layers  38 . Doping of the layers  38  and  40  in the third reflector  54  is not necessary for an optically-activated semiconductor laser  10 . Alternately, a third reflector  54  can be deposited after epitaxial growth using, for example, a plurality of alternating layers of silicon dioxide (SiO 2 ) to form the low-refractive-index layers  40  and titanium dioxide (TiO 2 ) to form the high-refractive-index layers  38 . 
     Efficient optical pumping of the active region  16  in an optically-activated semiconductor laser  10  having a third reflector  54  is possible since the third reflector  54  can be made highly transmissive (e.g. ≧90%) at the wavelength of the focused pump beam  102 , while at the same time being highly reflective (e.g. 80-90%) at the fundamental wavelength. The use of a third reflector  54  in an optically activated semiconductor laser  10  is also advantageous since this allows the fundamental frequency to be tuned slightly during fabrication of the device  10  to a particular wavelength desired for generation of the second-harmonic output beam  28  or to a wavelength that is optimum for a particular nonlinear crystal  18  (i.e. within the acceptance bandwidth for that crystal  18 ). Such wavelength tuning can be accomplished, for example, by etching away a portion of one or more of the uppermost layers  38  and  40  of the third reflector  54 . 
     Additionally, embodiments of the semiconductor laser  10  of the present invention can be fabricated for operation at wavelengths other than those described above. For example, operation at a fundamental wavelength of 780 nm is possible using AlGaAs quantum wells  44 . Such a device  10  can generate a 390-nm second-harmonic output beam  22  with either electrical or optical activation. In this case, a periodically-poled potassium titanyl phosphate (KTiOPO 4  also termed KTP) nonlinear crystal  18  can be used. As another example, operation at a fundamental wavelength of 680 nm is possible using InGaP or indium aluminum gallium phosphide (InAlGaP) quantum wells  44  and a periodically-poled lithium tantalate (LiTaO 3 ) nonlinear crystal  18 . Other embodiments of the present invention can be fabricated using quantum wells of indium gallium arsenide phosphide (InGaAsP), indium gallium arsenide nitride (InGaAsN), gallium arsenide antimonide (GaAsSb) or gallium arsenide antimonide nitride (GaAsSbN). Additionally, embodiments of the present invention can be formed on other types of semiconductor substrates, including indium phosphide (InP) substrates. 
     The matter set forth in the foregoing description and accompanying drawings is offered by way of illustration only and not as a limitation. Other applications and variations of the present invention will become evident to those skilled in the art. The actual scope of the invention is intended to be defined in the following claims when viewed in their proper perspective based on the prior art.