Silicon sees femtosecond mid-infrared light

Because femtosecond light pulses affect matter in ways longer pulses cannot, ultrafast lasers have become important for research and have great potential for industrial use as well. In a discovery with practical benefits, scientists at the National Institute of Standards and Technology (NIST; Gaithersburg, MD) have found that the effects of mid-infrared (mid-IR) femtosecond pulses on matter extend to ordinary silicon (Si) photodetectors and charge-coupled-device (CCD) cameras. Such a finding will allow users of mid-IR ultrafast lasers to detect and image light easily and cheaply.^1,2

With a bandgap of 1.12 eV, Si normally responds only to light at wavelengths shorter than 1.1 µm at room temperature, a range that only extends into the near-IR. What the NIST researchers determined is that, for laser pulses with durations on the order of 100-fs, nonlinear multiphoton absorption by Si of from three to seven photons produces an electrical response that permits the measurement of radiation at wavelengths of 3 to 11 µm.

In their experimental setup, mid-IR pulses were generated by a tunable system containing a Ti:sapphire laser, an optical parametric oscillator, and a nonlinear crystal. The voltage response of a Si photodiode (with its glass window removed) to the light was measured as a function of pulse energy for different wavelengths.

For each wavelength, the voltage response was proportional to the pulse energy raised to a different, sometimes nonintegral, power: for example, 3.45-µm light resulted in an exponent of 3.7, light at a 4.95-µm wavelength produced an exponent of 5.0, and 7.87-µm light gave an exponent of 7.2. The exponents correspond to the number of mid-IR quanta absorbed. The nonintegral exponents are a result of the pulses' relatively large spectral width (resulting from their femtosecond-scale duration); for example, 3.45-µm pulses are absorbed in three-photon bunches at the short end of the wavelength spectrum and four-photon bunches at the long end.

At large enough pulse energies, the detector saturated. In a test at 11 µm, laser pulse instability and extreme nonlinearity made determination of the photon number difficult, but the researchers estimated that at least seven quanta were being absorbed at once.

Ultrafast mid-infrared imaging

A response similar to the photodiode also occurred for standard Si CCD cameras used for spatial profiling of laser beams (see figure). The researchers were able to image focused beam spots of ultrafast light at wavelengths of 3.45, 4.95, 7.87, and 10 µm. Although compensating for the nonlinear response of Si could be done by raising the response of each pixel to the inverse of the exponent determined from experiment, the researchers assumed that the spot had a Gaussian shape and multiplied the measured beam waist by the square root of the experimental exponent.

The technique aids the NIST researchers in their study of structure and kinetics of molecules at interfaces, in which they use vibrationally resonant sum-frequency generation with broad-bandwidth IR pulses.¹ "We use the Si CCD cameras daily to visualize the size of ultrafast IR laser beams and to overlap two noncollinear beams on the CCD chip, which is located where a chemical sample is to be placed or conjugate to that location," says John Stephenson, one of the researchers. The group hopes to understand the chemical basis of polymer adhesion, as well as to read gene-chip arrays without having to label target or probe molecules with dye or radioisotopes.

In addition to imaging capabilities, the nonlinear response of Si allows a simple photodiode to be used as an autocorrelator to measure ultrafast pulse length. An experiment comparing the pulse length measured by a photodiode (150 ± 20 fs) with that measured by a conventional auto-correlation technique (130 ± 20 fs) showed good agreement, although additional experiments, assumptions, or modeling would have to be done if the pulses were temporally chirped. The technique may in some cases eliminate the need for cryogenic detectors and nonlinear crystals for pulse-length measurement of ultrafast mid-IR light.

REFERENCE

• 1. K.A. Briggmann, L.J. Richter, and J.C. Stephenson, Optics Lett. 26, 238 (2001).
  2. L. J. Richter et al., Optics Lett. 23, 1594 (1998).