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Efficient broadband emission from condensed media irradiated by low-intensity, unfocused, ultrashort laser light

A.K. Dharmadhikari, F. A. Rajgara, D. Mathur, H. Schroeder, and J. Liu

Open Access Open Access

Abstract

We report here measurements of the efficiencies of broadband emission in different optical media using an unfocused, ultrashort (~40 fs) laser beam. Two different measurements have been carried out by placing a wire mesh in the path of the incident laser radiation. The wire mesh introduces a periodic intensity distribution in the x-y plane and also in the direction of the laser beam propagation. We measure both on-axis and off-axis components of the broadband emission and also observe modulation in broadband generation as the distance between the mesh and the sample is varied. The experimentally measured locations of broadband emission maxima are in agreement with simulations based on Fresnel diffraction integrals. The off-axis emission efficiencies lie in the range of 16–87%.

©2005 Optical Society of America

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Figures (7)
Fig. 1.
Fig. 1. Schematic depiction of the experimental arrangement used in the present experiments. Emission was detected in the far field, at a distance of 60 cm downstream of the sample. z=0 corresponds to the mesh in physical contact with the front face of the sample.

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Fig. 2.
Fig. 2. Variation of the far field energy produced at different distances between the mesh and sample. The physical length of each sample is indicated. Note that prominent peaks and dips that are observed in all cases. The solid lines are a guide to the eye.

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Fig. 3.
Fig. 3. Variation of relative continuum energy at different mesh-sample distances. Top panel: experimental scheme. The mesh used was 11 × 11, 443 μm × 443 μm, wire thickness of 54 μm, throughput energy of 190 μJ. Bottom panel: comparison between the experimental variation (shaded) and results of simulations (see text).

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Fig. 4.
Fig. 4. Wavelength spread that is generated when the mesh is placed close to different samples. The spectrum of the incident laser light is also depicted (Amplifier). Normalization of peaks has been made at 800 nm. The spectra were measured using a fiber-optic spectrometer.

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Fig. 5.
Fig. 5. Spectra obtained from water for different values of incident laser energy.

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Fig. 6.
Fig. 6. Determination of conversion efficiency for broadband emission for different liquid samples (see text).

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Fig. 7.
Fig. 7. Far-field wavelength spectra measured for different liquid samples. The integrating sphere was placed 2 m downstream of the sample cell in these measurements. The mesh-sample distance was optimized for maximum intensity of the white light signal.

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Tables (1)

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Table 1: Conversion efficiency values for conical emission from different samples measured in the present experiments. Values of nonlinear refractive index, n2 [13], and the ionization energy, IE (or bandgap, Eg), for each sample are also tabulated. Values of conversion efficiency were measured, in each instance, at an intensity of ~4.7 × 1011 W cm-2.

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Equations (5)

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E P x 0 y 0 = A 0 iλz ∫ ∫ H x y exp ( ikr ) dxdy ,
r = z + ( x − x 0 ) 2 + ( y − y 0 ) 2 2 z + O ( 3 ) + . . . . .
E P x 0 y 0 = A 0 e ik 2 z iλz ∫ ∫ H x y exp { i ( k 2 z ) [ ( x − x 0 ) 2 + ( y − y 0 ) 2 ] } dxdy .
E P x 0 y 0 = A 0 e ik 2 z iλz ∑ n ∑ m E n , m ∫ − ( 1 2 ) a + n ( a + α ) ( 1 2 ) a + n ( a + α ) exp { i ( k 2 z ) [ ( x − x 0 ) 2 ] } dx ×
∫ − ( 1 2 ) a + m ( b + β ) ( 1 2 ) a + m ( b + β ) exp { i ( k 2 z ) [ ( y − y 0 ) 2 ] } dy .
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