Topical Meeting on Laser Techniques in the Extreme Ultraviolet

Multiresonant Two-Photon-Absorption-Induced Four Wave Mixing in Crystalline Rare Earth Insulators

R. L. Cone, David A. Ender, Michael S. Otteson, Paula L. Fisher, J. M. Friedman, J. M. Friedman, Howard J. Guggenheim

Published Jan 1, 1982• 0 citations

Abstract

We report the first observation of intermediate state enhancement of four wave mixing induced by two photon absorption in rare earth insulators. Coherent generation of ω4=ω1+ω2−ω3 exhibits strong intermediate state resonances when ω1 sweeps through a single photon transition to the 5D4 states arising from the 4f8 ground configuration of Tb3+ while (ω1+ω2) is simultaneously resonant with an allowed transition to the excited 4f75d configuration. Measurements are presented for Tb(OH)3 and LiTbF4.

DOI: 10.1364/euvs.1982.me10

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Multiresonant two‐photon‐absorption‐induced four wave mixing in crystalline rare earth insulators

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Doubly-resonant two-photon-absorption-induced four-wave mixing in Tb(OH)₃and LiTbF₄

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Resonant enhancement of direct two-photon absorption in Tb3+ : LiYF4

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Effects of four-wave mixing on four-photon resonance excitation and ionization in the presence of a three-photon intermediate state resonance enhancement

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Resonant enhancement of two-photon transitions to the second excited 4f5dstate in Pr3+ : Y3 Al5 O12

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Suppression of a four-photon resonance by four-wave mixing near an intermediate three-photon resonance

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Physical Review AOct 1, 199212 citations

Topical Meeting on Laser Techniques in the Extreme Ultraviolet

Multiresonant Two-Photon-Absorption-Induced Four Wave Mixing in Crystalline Rare Earth Insulators

R. L. Cone, David A. Ender, Michael S. Otteson, Paula L. Fisher, J. M. Friedman, J. M. Friedman, Howard J. Guggenheim

Published Jan 1, 1982• 0 citations

Publisher

Abstract

DOI: 10.1364/euvs.1982.me10

Related papers

Multiresonant two‐photon‐absorption‐induced four wave mixing in crystalline rare earth insulators

R. L. Cone, David A. Ender, Michael S. Otteson +3 more

Coherent nonlinear optical generation of ω4=ω+ω2−ω3 exhibits strong sharp intermediate (ω1) and two‐photon (ω1+ω2) resonances in crystalline Tb(OH)3 and LiTbF4, providing a novel method for high resolution coherent measurements of both excited electronic configurations and intermediate 4 fn states of rare earth ions. New regions of the UV and VUV are thus made accessible to existing tuneable visible and near ultravisible lasers. Selection of sharp features from broad overlapping absorptions, line narrowing due to phase matching selectivity, and coherent transient applications are discussed.Coherent nonlinear optical generation of ω4=ω+ω2−ω3 exhibits strong sharp intermediate (ω1) and two‐photon (ω1+ω2) resonances in crystalline Tb(OH)3 and LiTbF4, providing a novel method for high resolution coherent measurements of both excited electronic configurations and intermediate 4 fn states of rare earth ions. New regions of the UV and VUV are thus made accessible to existing tuneable visible and near ultravisible lasers. Selection of sharp features from broad overlapping absorptions, line narrowing due to phase matching selectivity, and coherent transient applications are discussed.

AIP conference proceedingsJan 1, 19821 citation

Doubly-resonant two-photon-absorption-induced four-wave mixing in Tb(OH)₃and LiTbF₄

David A. Ender

Doubly-resonant two-photon-absorption-induced four-wave mixing has been observed for the first time in a rare earth insulator.The generation of the nonlinear signal exhibits strong intermediate and two-photon resonances in crystalline Tb (OH)3 and LiTbF4 This provides a novel method for high resolution measurements of intermediate 4f^n configurations and excited configurations of rare earth ions.The intermediate resonance showed spectral line narrowing up to ten times narrower than the corresponding inhomogeneously broadened absorption.In LiTbF4, laser-limited line widths were obtained and in Tb(OH)3, a homogeneous limit corresponding to T2 = 50 psec may have been reached.The qualitative behavior of the narrowing is explained by the dispersion of the anamalous index of refraction and its effect on phase matching.Direct measurement of the anamalous dispersion was made to test the model.Results indicate nonlinear contributions to the refractive index may have been present.The two-photon resonance in Tb(OH)3 was measured to be a 230 cm^-1 wide crystal field component of the broad 4f^n 5d excited configuration.Thus high resolution UV spectroscopy with tunable visible lasers is demonstrated.Applications are discussed including the extraction of homogeneous line widths from inhomogeneously broadened spectra, coherent transient measurements, and UV and VUV spectrosocpy.The latter uses different selection rules than linear absorption methods and allows study of transitions between excited states.DOUBLY-RESONANT TWO-PHOTON-AB^ORPTION-INDUCED EOUR-WAVB MIXPNQ IN, .T b .(DR) 3 . .Doubly-resonant two-photon-absorption-induced four-wave mixing has been observed for the first time in a rare earth insulator.The generation of the nonlinear signal exhibits strong intermediate and two-photon resonances in crystalline Tb (OH)3 and LiTbF4 This provides a novel method for high resolution measurements of intermediate 4f^n configurations and excited configurations of rare earth ions.The intermediate resonance showed spectral line narrowing up to ten times narrower than the corresponding inhomogeneously broadened absorption.In LiTbF4, laser-limited line widths were obtained and in Tb(OH)3, a homogeneous limit corresponding to T2 = 50 psec may have been reached.The qualitative behavior of the narrowing is explained by the dispersion of the anamalous index of refraction and its effect on phase matching.Direct measurement of the anamalous dispersion was made to test the model.Results indicate nonlinear contributions to the refractive index may have been present.The two-photon resonance in Tb(OH)3 was measured to be a 230 cm^-1 wide crystal field component of the broad 4f^n 5d excited configuration.Thus high resolution UV spectroscopy with tunable visible lasers is demonstrated.Applications are discussed including the extraction of homogeneous line widths from inhomogeneously broadened spectra, coherent transient measurements, and UV and VUV spectrosocpy.The latter uses different selection rules than linear absorption methods and allows study of transitions between excited states.DOUBLY-RESONANT TWO-PHOTON-AB^ORPTION-INDUCED EOUR-WAVB MIXPNQ IN, .T b .(DR) 3 . .

Jan 1, 19821 citation

Resonant enhancement of direct two-photon absorption in Tb3+ : LiYF4

Huang Jin, G. K. Liu, R. L. Cone

Resonant enhancement by a factor of over 20 of direct two-photon absorption from the ground state $^{7}\mathrm{F}_{6}$ to the excited state $^{5}\mathrm{G}_{6}$ of the 4 ${f}^{8}$ configuration of the ${\mathrm{Tb}}^{3+}$ ion at 40 200 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ has been observed in time-resolved experiments with two separate pulsed lasers. The results provide clear evidence for resonant enhancement of two-photon absorption in rare-earth compounds and imply a similar enhancement for Raman scattering and nonlinear-optical mixing. Two separate contributions to the direct transition moment were observed. When a single laser frequency was used, the intermediate states which made the largest contribution were from excited configurations of opposite parity; those intermediate states were far from resonance. Detailed two-frequency experiments showed, however, that near a single photon $^{7}\mathrm{F}_{6}$ ${\mathrm{\ensuremath{-}}}^{5}$ ${\mathrm{D}}_{4}$ resonance, a much stronger contribution arose from the 4 ${f}^{8}$ configuration $^{5}\mathrm{D}_{4}$ intermediate state. Two-photon absorption was clearly isolated from two-step (incoherent) excitation by varying the timing of two separately triggered lasers.Resonant enhancement by a factor of over 20 of direct two-photon absorption from the ground state $^{7}\mathrm{F}_{6}$ to the excited state $^{5}\mathrm{G}_{6}$ of the 4${f}^{8}$ configuration of the ${\mathrm{Tb}}^{3+}$ ion at 40 200 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ has been observed in time-resolved experiments with two separate pulsed lasers. The results provide clear evidence for resonant enhancement of two-photon absorption in rare-earth compounds and imply a similar enhancement for Raman scattering and nonlinear-optical mixing. Two separate contributions to the direct transition moment were observed. When a single laser frequency was used, the intermediate states which made the largest contribution were from excited configurations of opposite parity; those intermediate states were far from resonance. Detailed two-frequency experiments showed, however, that near a single photon $^{7}\mathrm{F}_{6}$${\mathrm{\ensuremath{-}}}^{5}$${\mathrm{D}}_{4}$ resonance, a much stronger contribution arose from the 4${f}^{8}$ configuration $^{5}\mathrm{D}_{4}$ intermediate state. Two-photon absorption was clearly isolated from two-step (incoherent) excitation by varying the timing of two separately triggered lasers.

Physical review. B, Condensed matterApr 1, 198920 citations

Effects of four-wave mixing on four-photon resonance excitation and ionization in the presence of a three-photon intermediate state resonance enhancement

M. G. Payne, J. C. Miller, Roger C. Hart +1 more

We consider effects which occur when four-wave sum frequency generation and multiphoton ionization are induced by lasers tuned near a three-photon resonance and simultaneously near or at a dipole allowed four-photon resonance. In studies with unfocused laser beams, if the phase mismatch of the generated four-wave-mixing field is large and the related two-photon resonance for the absorption of a four-wave-mixing photon and a laser photon results in strong absorption of the four-wave-mixing field, a coherent cancellation occurs between the pumping of the resonance by two- and four-photon processes. This interference effect occurs when the first laser is tuned on either side of the three-photon resonance and \ensuremath{\Vert}\ensuremath{\Delta} ${\mathit{k}}_{\mathit{r}}$ L\ensuremath{\Vert}\ensuremath{\gg}1, where \ensuremath{\Delta} ${\mathit{k}}_{\mathit{r}}$ is the mismatch and L is the length of the path of the laser beams in the gas. With focused laser beams large differences occur between ionization with unidirectional beams and with counterpropagating laser beams when \ensuremath{\Vert}\ensuremath{\Delta} ${\mathit{k}}_{\mathit{r}}$ b\ensuremath{\Vert}\ensuremath{\gg}1, where b is the confocal parameter of the focused laser beams. Strong absorption of the four-wave-mixing field is shown not to be necessary for strong destructive interference with focused laser beams when the phase mismatch is large. This work also suggests an explanation for earlier experiments where the presence of a four-photon resonance enabled the generation of third-harmonic light in a positively dispersive wavelength region. We argue that this process can occur when the laser used to achieve the four-photon resonance is focused on the small z (z is the coordinate in the direction of propagation) side of the focal point of the laser responsible for the third-harmonic generation. In this way the absorption of the third-harmonic light on one side of the focal point is made much stronger than on the other side. Correspondingly, contributions to the third-harmonic field from the two sides of the focal point are unequal and cannot undergo complete destructive interference as would otherwise occur in a positively dispersive region.We consider effects which occur when four-wave sum frequency generation and multiphoton ionization are induced by lasers tuned near a three-photon resonance and simultaneously near or at a dipole allowed four-photon resonance. In studies with unfocused laser beams, if the phase mismatch of the generated four-wave-mixing field is large and the related two-photon resonance for the absorption of a four-wave-mixing photon and a laser photon results in strong absorption of the four-wave-mixing field, a coherent cancellation occurs between the pumping of the resonance by two- and four-photon processes. This interference effect occurs when the first laser is tuned on either side of the three-photon resonance and \ensuremath{\Vert}\ensuremath{\Delta}${\mathit{k}}_{\mathit{r}}$L\ensuremath{\Vert}\ensuremath{\gg}1, where \ensuremath{\Delta}${\mathit{k}}_{\mathit{r}}$ is the mismatch and L is the length of the path of the laser beams in the gas. With focused laser beams large differences occur between ionization with unidirectional beams and with counterpropagating laser beams when \ensuremath{\Vert}\ensuremath{\Delta}${\mathit{k}}_{\mathit{r}}$b\ensuremath{\Vert}\ensuremath{\gg}1, where b is the confocal parameter of the focused laser beams. Strong absorption of the four-wave-mixing field is shown not to be necessary for strong destructive interference with focused laser beams when the phase mismatch is large. This work also suggests an explanation for earlier experiments where the presence of a four-photon resonance enabled the generation of third-harmonic light in a positively dispersive wavelength region. We argue that this process can occur when the laser used to achieve the four-photon resonance is focused on the small z (z is the coordinate in the direction of propagation) side of the focal point of the laser responsible for the third-harmonic generation. In this way the absorption of the third-harmonic light on one side of the focal point is made much stronger than on the other side. Correspondingly, contributions to the third-harmonic field from the two sides of the focal point are unequal and cannot undergo complete destructive interference as would otherwise occur in a positively dispersive region.

Physical Review ADec 1, 199119 citations

Resonant enhancement of two-photon transitions to the second excited 4f5dstate in Pr3+ : Y3 Al5 O12

Bin Qing Xie, Y. M. Cheung, Swapan Kumar Gayen

The two-photon absorption cross section for transitions from the $^{3}$ ${\mathit{H}}_{4}$ ground-state manifold to the second excited state of the 4f5d configuration of ${\mathrm{Pr}}^{3+}$ in ${\mathrm{Y}}_{3}$ ${\mathrm{Al}}_{5}$ ${\mathrm{O}}_{12}$ is observed to be enhanced by about two orders of magnitude when the excitation frequency happens to be resonant with the $^{3}$ ${\mathit{P}}_{\mathit{J}}$ (J=0,1,2) and $^{1}$ ${\mathit{I}}_{6}$ intermediate states of the 4 ${\mathit{f}}^{2}$ configuration. The structure and line shape of the two-photon excitation spectrum spanning the 39 600--46 950 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ transition-energy range are entirely dominated by these resonantly enhanced transitions, while the contributions from nonresonant transitions appear as a slowly varying background beneath the strong resonance peaks. A two-photon absorption cross section as high as 4\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}50}$ ${\mathrm{cm}}^{4}$ s is measured at the peak of the resonantly enhanced transition at 44 246 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, which involves a level of the $^{3}$ ${\mathit{P}}_{2}$ manifold as the intermediate state. The spectrally integrated cross section over that manifold is 3.3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}38}$ ${\mathrm{cm}}^{4}$ .The two-photon absorption cross section for transitions from the $^{3}$${\mathit{H}}_{4}$ ground-state manifold to the second excited state of the 4f5d configuration of ${\mathrm{Pr}}^{3+}$ in ${\mathrm{Y}}_{3}$${\mathrm{Al}}_{5}$${\mathrm{O}}_{12}$ is observed to be enhanced by about two orders of magnitude when the excitation frequency happens to be resonant with the $^{3}$${\mathit{P}}_{\mathit{J}}$ (J=0,1,2) and $^{1}$${\mathit{I}}_{6}$ intermediate states of the 4${\mathit{f}}^{2}$ configuration. The structure and line shape of the two-photon excitation spectrum spanning the 39 600--46 950 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ transition-energy range are entirely dominated by these resonantly enhanced transitions, while the contributions from nonresonant transitions appear as a slowly varying background beneath the strong resonance peaks. A two-photon absorption cross section as high as 4\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}50}$ ${\mathrm{cm}}^{4}$ s is measured at the peak of the resonantly enhanced transition at 44 246 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, which involves a level of the $^{3}$${\mathit{P}}_{2}$ manifold as the intermediate state. The spectrally integrated cross section over that manifold is 3.3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}38}$ ${\mathrm{cm}}^{4}$.

Physical review. B, Condensed matterMar 1, 19939 citations

Suppression of a four-photon resonance by four-wave mixing near an intermediate three-photon resonance

Roger C. Hart, W. R. Garrett, M. G. Payne

We use resonance ionization involving two colors in single-pass and retroreflected geometries to study the suppression of excitation of a four-photon resonance by an interference involving four-wave sum-frequency generation occurring near an intermediate three-photon resonance. The 6p[1/2](J=0) state of xenon is excited by three blue photons plus one infrared photon. One beam, focused to ${\mathit{I}}_{1}$ \ensuremath{\simeq}7\ifmmode\times\else\texttimes\fi{} ${10}^{9}$ W/ ${\mathrm{cm}}^{2}$ , is nearly three-photon resonant with the 6s[3/2](J=1) resonance; the second, unfocused beam ( ${\mathit{I}}_{2}$ \ensuremath{\simeq}2\ifmmode\times\else\texttimes\fi{} ${10}^{7}$ W/ ${\mathrm{cm}}^{2}$ ) is tuned to maintain exact four-photon resonance with the 6p[1/2](J=1) state. One additional blue photon ionizes from the four-photon level. Third-harmonic light is generated by the first, nearly three-photon-resonant beam by four-wave sum-frequency mixing. One third-harmonic photon plus one photon from the second laser form a second excitation pathway to the four-photon level; destructive interference between the two pathways results in a reduction in the observed ionization. The degree of reduction depends on the xenon density and the detuning (both positive and negative) from three-photon resonance. This interference may be overcome by retroreflecting the nearly three-photon-resonant beam to provide an additional excitation pathway which generates no third harmonic.The study provides detailed information on the pressure-dependent range from exact resonance over which the familiar three-photon cancellation effect is operative; it shows that the interference occurs in high- and low-energy regions about three-photon resonance; and it establishes that interference is produced for large values of the phase mismatch even if the absorption of the interfering field is low. We compare our measurements of the reduction in ionization with predictions published by us elsewhere [M. G. Payne, J. C. Miller, R. C. Hart, and W. R. Garrett, Phys. Rev. A 44, 7684 (1991)]; excellent agreement is observed. We also report the observation of suppression of pressure-induced wing absorption into the three-photon state observed in a classic ``hybrid'' resonance mode of excitation.We use resonance ionization involving two colors in single-pass and retroreflected geometries to study the suppression of excitation of a four-photon resonance by an interference involving four-wave sum-frequency generation occurring near an intermediate three-photon resonance. The 6p[1/2](J=0) state of xenon is excited by three blue photons plus one infrared photon. One beam, focused to ${\mathit{I}}_{1}$\ensuremath{\simeq}7\ifmmode\times\else\texttimes\fi{}${10}^{9}$ W/${\mathrm{cm}}^{2}$, is nearly three-photon resonant with the 6s[3/2](J=1) resonance; the second, unfocused beam (${\mathit{I}}_{2}$\ensuremath{\simeq}2\ifmmode\times\else\texttimes\fi{}${10}^{7}$ W/${\mathrm{cm}}^{2}$) is tuned to maintain exact four-photon resonance with the 6p[1/2](J=1) state. One additional blue photon ionizes from the four-photon level. Third-harmonic light is generated by the first, nearly three-photon-resonant beam by four-wave sum-frequency mixing. One third-harmonic photon plus one photon from the second laser form a second excitation pathway to the four-photon level; destructive interference between the two pathways results in a reduction in the observed ionization. The degree of reduction depends on the xenon density and the detuning (both positive and negative) from three-photon resonance. This interference may be overcome by retroreflecting the nearly three-photon-resonant beam to provide an additional excitation pathway which generates no third harmonic.The study provides detailed information on the pressure-dependent range from exact resonance over which the familiar three-photon cancellation effect is operative; it shows that the interference occurs in high- and low-energy regions about three-photon resonance; and it establishes that interference is produced for large values of the phase mismatch even if the absorption of the interfering field is low. We compare our measurements of the reduction in ionization with predictions published by us elsewhere [M. G. Payne, J. C. Miller, R. C. Hart, and W. R. Garrett, Phys. Rev. A 44, 7684 (1991)]; excellent agreement is observed. We also report the observation of suppression of pressure-induced wing absorption into the three-photon state observed in a classic ``hybrid'' resonance mode of excitation.

Physical Review AOct 1, 199212 citations

Resonant enhancement by a factor of over 20 of direct two-photon absorption from the ground state

^{7}\mathrm{F}_{6}

to the excited state

^{5}\mathrm{G}_{6}

of the 4

{f}^{8}

configuration of the

{\mathrm{Tb}}^{3+}

ion at 40 200 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ has been observed in time-resolved experiments with two separate pulsed lasers. The results provide clear evidence for resonant enhancement of two-photon absorption in rare-earth compounds and imply a similar enhancement for Raman scattering and nonlinear-optical mixing. Two separate contributions to the direct transition moment were observed. When a single laser frequency was used, the intermediate states which made the largest contribution were from excited configurations of opposite parity; those intermediate states were far from resonance. Detailed two-frequency experiments showed, however, that near a single photon

^{7}\mathrm{F}_{6}

${\mathrm{\ensuremath{-}}}^{5}$

{\mathrm{D}}_{4}

resonance, a much stronger contribution arose from the 4

{f}^{8}

configuration

^{5}\mathrm{D}_{4}

intermediate state. Two-photon absorption was clearly isolated from two-step (incoherent) excitation by varying the timing of two separately triggered lasers.

{\mathit{k}}_{\mathit{r}}

L\ensuremath{\Vert}\ensuremath{\gg}1, where \ensuremath{\Delta}

{\mathit{k}}_{\mathit{r}}

is the mismatch and L is the length of the path of the laser beams in the gas. With focused laser beams large differences occur between ionization with unidirectional beams and with counterpropagating laser beams when \ensuremath{\Vert}\ensuremath{\Delta}

{\mathit{k}}_{\mathit{r}}

b\ensuremath{\Vert}\ensuremath{\gg}1, where b is the confocal parameter of the focused laser beams. Strong absorption of the four-wave-mixing field is shown not to be necessary for strong destructive interference with focused laser beams when the phase mismatch is large. This work also suggests an explanation for earlier experiments where the presence of a four-photon resonance enabled the generation of third-harmonic light in a positively dispersive wavelength region. We argue that this process can occur when the laser used to achieve the four-photon resonance is focused on the small z (z is the coordinate in the direction of propagation) side of the focal point of the laser responsible for the third-harmonic generation. In this way the absorption of the third-harmonic light on one side of the focal point is made much stronger than on the other side. Correspondingly, contributions to the third-harmonic field from the two sides of the focal point are unequal and cannot undergo complete destructive interference as would otherwise occur in a positively dispersive region.

The two-photon absorption cross section for transitions from the

^{3}

{\mathit{H}}_{4}

ground-state manifold to the second excited state of the 4f5d configuration of

{\mathrm{Pr}}^{3+}

{\mathrm{Y}}_{3}

{\mathrm{Al}}_{5}

{\mathrm{O}}_{12}

is observed to be enhanced by about two orders of magnitude when the excitation frequency happens to be resonant with the

^{3}

{\mathit{P}}_{\mathit{J}}

(J=0,1,2) and

^{1}

{\mathit{I}}_{6}

intermediate states of the 4

{\mathit{f}}^{2}

configuration. The structure and line shape of the two-photon excitation spectrum spanning the 39 600--46 950 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ transition-energy range are entirely dominated by these resonantly enhanced transitions, while the contributions from nonresonant transitions appear as a slowly varying background beneath the strong resonance peaks. A two-photon absorption cross section as high as 4\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}50}$

{\mathrm{cm}}^{4}

s is measured at the peak of the resonantly enhanced transition at 44 246 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, which involves a level of the

^{3}

{\mathit{P}}_{2}

manifold as the intermediate state. The spectrally integrated cross section over that manifold is 3.3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}38}$

{\mathrm{cm}}^{4}

{\mathit{I}}_{1}

\ensuremath{\simeq}7\ifmmode\times\else\texttimes\fi{}

{10}^{9}

{\mathrm{cm}}^{2}

, is nearly three-photon resonant with the 6s[3/2](J=1) resonance; the second, unfocused beam (

{\mathit{I}}_{2}

\ensuremath{\simeq}2\ifmmode\times\else\texttimes\fi{}

{10}^{7}

{\mathrm{cm}}^{2}

) is tuned to maintain exact four-photon resonance with the 6p[1/2](J=1) state. One additional blue photon ionizes from the four-photon level. Third-harmonic light is generated by the first, nearly three-photon-resonant beam by four-wave sum-frequency mixing. One third-harmonic photon plus one photon from the second laser form a second excitation pathway to the four-photon level; destructive interference between the two pathways results in a reduction in the observed ionization. The degree of reduction depends on the xenon density and the detuning (both positive and negative) from three-photon resonance. This interference may be overcome by retroreflecting the nearly three-photon-resonant beam to provide an additional excitation pathway which generates no third harmonic.The study provides detailed information on the pressure-dependent range from exact resonance over which the familiar three-photon cancellation effect is operative; it shows that the interference occurs in high- and low-energy regions about three-photon resonance; and it establishes that interference is produced for large values of the phase mismatch even if the absorption of the interfering field is low. We compare our measurements of the reduction in ionization with predictions published by us elsewhere [M. G. Payne, J. C. Miller, R. C. Hart, and W. R. Garrett, Phys. Rev. A 44, 7684 (1991)]; excellent agreement is observed. We also report the observation of suppression of pressure-induced wing absorption into the three-photon state observed in a classic ``hybrid'' resonance mode of excitation.