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Characterisation of Optical Nonlinear Properties of Semmiconductors

Measuring semiconductor nonlinear properties with femtosecond broadly tunable near-IR and mid-IR laser source based on optical parametric oscillators.

Since the invention of the transistor in 1947 by Bardeen and Brattain  and William B. Shockley at the Bell Laboratories, followed by the invention of the integrated circuit independently by Jack Kilby at Texas Instruments in 1958 and by Jean Hoerni and Robert Noyce at Fairchild Semiconductor Corporation in 1959, the semiconductor industry has undergone an astonishing expansion. Today, after more than 60 years later, semiconductors are critical components in our daily life. Semiconductor-based integrated circuits can be found in almost every technology including mobile phones, laptops, TVs, medical devices, sensors, solar cells, all-optical-networks, kitchen appliances, vehicles, aviation, and much more.

Conductivity is the parameter that makes semiconductor such special materials. The conductivity of naturally occurring semiconductor elements, such as Silicon or Germanium, can be changed by altering the materials with small amounts of doping. Besides, unlike conductors and insulators, the conductivity of semiconductors can vary with environmental parameters such as temperature, light or electric current. This has made semiconductors find an extensive range of applications in electronics.

The unique optical properties of semiconductor optical properties are also reason why these materials are extremely useful in optoelectronic applications, especially as sensors. Also, some semiconductor materials can emit light and are used for the development of optical sources such as LEDs, diode lasers, quantum cascade lasers, and surface emitting lasers (VCSELs).

The optical properties of semiconductors have been extensively studied and especially their nonlinear properties have generated significant interest. Optical bi-stability, all-optical switching and optical computing are amongst some of the semiconductor applications and devices based on their nonlinear properties.

Scientists around the world study the intrinsic nonlinear optical properties of semiconductors with the intention to develop new solutions based on novel optoelectronic devices. In particular, the most widely used semiconductor materials, Silicon and Germanium, have a strong third-order susceptibility (χ3), exhibiting associated nonlinear effects such as TPA (two-photon absorption), FCA (free-carrier absorption), SPM (self-phase modulation), XPM (cross-phase modulation), and supercontinuum generation.

High peak power lasers are required to induce and observe these nonlinear effects and typically femtosecond pulsed lasers at MHz repetition rates are used for such experiments. The choice of laser wavelength is important as this will depend on the particular application. For instance, if the nonlinear absorption of a material needs to be measured, the laser wavelength has to coincide with the nonlinear absortion region of the material. However, if a different phenomenon such as SPM or XPM is to be tested, a wavelength outside the nonlinear absorption region must be selected to maximise efficiency.

A broadly tunable femtosecond laser source is the ideal illumination source for such characterization experiments. It provides the high peak powers required to generate the nonlinear phenomena, while at the same time offering the spectral flexibility to fully characterise the materials.

Mode-locked femtosecond Ti:Sapphire lasers provide the required peak powers and spectral coverage across the 700 – 1000 nm range. However, lasers with longer wavelengths are needed for the study of semiconductors in the near-IR and mid-IR regions.

For example, semiconductor-based all-optical networks typically work at 850, 1300 and 1550 nm, but the spectral coverage of Ti:Sapphire lasers is not sufficient to characterise materials across all these wavelengths. Sensing applications are another example where semiconductors with nonlinear properties in the near-IR and mid-IR spectral regions provide important advantages.

Radiantis OPO, Oria IRTM, offers an excellent wavelength extension for femtosecond Ti:Sapphire MHz repetition rate lasers. This OPO converts the wavelengths emitted by a Ti:Sapphire laser into the 1000 – 4000 nm region, at 80 MHz and with femtosecond pulses of the order of 100 – 200 fs across the range. This is a sealed, fully automated optical parametric oscillator that provides tuning exclusively via the software installed on a dedicated computer available with the laser.

Oria IR

  • Broad tuning across 990 – 1550 nm and 1696 – 4090 nm.
  • Independent pump and Signal/Idler tuning.
  • Highest average power, >1 W at the peak of the range.
  • Hands-free operation.
  • Compatible with standard MHz femtosecond Ti:Sapphire oscillators.