Günter Steinmeyer

Photonic-bandgap microcavities in optical waveguides

Confinement of light to small volumes has important implications for optical emission properties: it changes the probability of spontaneous emission from atoms, allowing both enhancement and inhibition. In photonic-bandgap (PBG) materials (also known as photonic crystals), light can be confined within a volume of the order of (λ/2n)3, where λ is the emission wavelength and n the refractive index of the material, by scattering from a periodic array of scattering centres. Until recently, the properties of two- and three-dimensional PBG structures have been measured only at microwave frequencies. Because the optical bandgap scales with the period of the scattering centres, feature sizes of around 100 nm are needed for manipulation of light at the infrared wavelength (1.54 µm) used for optical communications. Fabricating features this small requires the use of electron-beam or X-ray lithography. Here we report measurements of microcavity resonances in PBG structures integrated directly into a sub-micrometre-scale silicon waveguide. The microcavity has a resonance at a wavelength of 1.56 µm, a quality factor of 265 and a modal volume of 0.055 µm3. This level of integration might lead to new photonic chip architectures and devices, such as zero-threshold microlasers, filters and signal routers.

Ultra-compact Si-SiO 2 microring resonator optical channel dropping filters

Compact optical channel dropping filters incorporating side-coupled ring resonators as small as 3 /spl mu/m in radius are realized in silicon technology. Quality factors up to 250, and a free-spectral range (FSR) as large as 24 nm are measured. Such structures can be used as fundamental building blocks in more sophisticated optical signal processing devices.

Semiconductor saturable-absorber mirror–assisted Kerr-lens mode-locked Ti:sapphire laser producing pulses in the two-cycle regime

Pulses of sub-6-fs duration have been obtained from a Kerr-lens mode-locked Ti:sapphire laser at a repetition rate of 100 MHz and an average power of 300 mW. Fitting an ideal sech(2) to the autocorrelation data yields a 4.8-fs pulse duration, whereas reconstruction of the pulse amplitude profile gives 5.8 fs. The pulse spectrum covers wavelengths from above 950 nm to below 630 nm, extending into the yellow beyond the gain bandwidth of Ti:sapphire. This improvement in bandwidth has been made possible by three key ingredients: carefully designed spectral shaping of the output coupling, better suppression of the dispersion oscillation of the double-chirped mirrors, and a novel broadband semiconductor saturable-absorber mirror.

Characterization of sub-6-fs optical pulses with spectral phase interferometry for direct electric-field reconstruction

We demonstrate spectral phase interferometry for direct electric-field reconstruction (SPIDER) as a novel method to characterize sub-6-fs pulses with nanojoule pulse energy. SPIDER reconstructs pulse phase and amplitude from a measurement of only two optical spectra by use of a fast noniterative algorithm. SPIDER is well suited to the measurement of ultrabroadband pulses because it is quite insensitive to crystal phase-matching bandwidth and to unknown detector spectral responsivity. Moreover, it combines highly accurate pulse-shape measurement with the potential for online laser system diagnostics at video refresh rates.

Self-compression of millijoule pulses to 7.8 fs duration in a white-light filament

A novel method for generating few-cycle pulses with extremely high peak powers >100 GW is experimentally demonstrated. This method relies on plasma-induced spectral broadening and does not require any additional means for dispersion compensation.

Passive mode locking of Yb:KLuW using a single-walled carbon nanotube saturable absorber

Mode locking of an Yb-doped bulk laser in the 1 microm spectral range using a single-walled carbon nanotube saturable absorber (SWCNT-SA) is demonstrated for the first time, to our knowledge. Passive mode locking of an Yb:KLuW laser resulted in nearly transform-limited pulses as short as 115 fs at 1048 nm. In addition, the nonlinear response of the SWCNT-SA was measured, yielding a modulation depth of 0.25% and a relaxation time of 750 fs.

Passive mode-locking of a Tm-doped bulk laser near 2 µm using a carbon nanotube saturable absorber

Stable and self-starting mode-locking of a Tm:KLu(WO(4))(2) crystal laser is demonstrated using a transmission-type single-walled carbon nanotube (SWCNT) based saturable absorber (SA). These experiments in the 2 microm regime utilize the E11 transition of the SWCNTs for nonlinear saturable absorption. The recovery time of the SWCNT-SA is measured by pump-probe measurements as approximately 1.2 ps. The mode-locked laser delivers approximately 10 ps pulses near 1.95 microm with a maximum output power of up to 240 mW at 126 MHz repetition rate.

Interferometric frequency-resolved optical gating

We demonstrate a novel variant of frequency-resolved optical gating (FROG) that is based on spectrally resolving a collinear interferometric autocorrelation rather than a noncollinear one. From the interferometric FROG trace, one can extract two terms, the standard SHG-FROG trace and a new phase-sensitive modulational component, which both allow for independent retrieval of the pulse shape. We compare the results of both methods and a separate SPIDER measurement using 6.5-fs pulses from a white-light continuum. We find that the novel modulational component allows for robust retrieval of pulse shapes in the few-cycle regime. Together with the added cross-checks, our method significantly enhances choices for pulse characterization in this regime.

Fabrication and characterization of ultrafast carbon nanotube saturable absorbers for solid-state laser mode locking near 1μm

Transmitting and reflecting ultrafast saturable absorbers based on single-walled carbon nanotubes are developed that are applicable for stable mode locking of bulk solid-state lasers operating near 1μm. For fabrication of these saturable absorbers, relatively simple spin coating and spray methods are employed. Parameters important for stable mode locking, such as transient nonlinear absorption, saturation fluence, and recovery time, are investigated by nonlinear transmission and time-resolved pump-probe measurements near 1μm. Typical modulation depths and recovery times amount to ∼0.21%–0.25% and <1ps, respectively.

Pulse compression over a 170-THz bandwidth in the visible by use of only chirped mirrors

We report on double-chirped mirrors with custom-tailored dispersion characteristics over a bandwidth of 170 THz in the visible. The mirrors are used in a prismless compressor for a noncollinear optical parametric amplifier in the visible. The compressed pulses, characterized for the what is believed to be first time by use of the spectral phase interferometry for direct electric field reconstruction technique, display a nearly flat phase from 510 to 710 nm and have a duration of 5.7 fs.