Surface third and fifth harmonic generation at crystalline Si for non-invasive inspection of Si wafer's inter-layer defects: erratum.

We present an erratum to supplement a missing funding acknowledgment [Pusan National University Research Grant, 2016] in our paper [Y. Gao Opt. Express26(25), 32812 (2018).].

Free-space transfer of comb-rooted optical frequencies over an 18 km open-air link.

Phase-coherent transfer of optical frequencies over a long distance is required for diverse photonic applications, including optical clock dissemination and physical constants measurement. Several demonstrations were made successfully over fiber networks, but not much work has been done yet through the open air where atmospheric turbulence prevails. Here, we use an 18 km outdoor link to transmit multiple optical carriers extracted directly from a frequency comb of a 4.2 THz spectral width. In stabilization to a high-finesse cavity with a 1.5 Hz linewidth, the comb-rooted optical carriers are simultaneously transferred with collective suppression of atmospheric phase noise to -80 dBc Hz-1. Microwaves are also delivered by pairing two separate optical carriers bound with inter-comb-mode coherence, for example a 10 GHz signal with phase noise of -105 dBc Hz-1 at 1 Hz offset. Lastly, an add-on demonstration is given for multi-channel coherent optical communications with the potential of multi-Tbps data transmission in free space.

Comb-rooted multi-channel synthesis of ultra-narrow optical frequencies of few Hz linewidth.

We report a multi-channel optical frequency synthesizer developed to generate extremely stable continuous-wave lasers directly out of the optical comb of an Er-doped fiber oscillator. Being stabilized to a high-finesse cavity with a fractional frequency stability of 3.8 × 10-15 at 0.1 s, the comb-rooted synthesizer produces multiple optical frequencies of ultra-narrow linewidth of 1.0 Hz at 1 s concurrently with an output power of tens of mW per each channel. Diode-based stimulated emission by injection locking is a key mechanism that allows comb frequency modes to sprout up with sufficient power amplification but no loss of original comb frequency stability. Channel frequencies are individually selectable with a 0.1 GHz increment over the entire comb bandwidth spanning 4.25 THz around a 1550 nm center wavelength. A series of out-of-loop test results is discussed to demonstrate that the synthesizer is able to provide stable optical frequencies with the potential for advancing diverse ultra-precision applications such as optical clocks comparison, atomic line spectroscopy, photonic microwaves generation, and coherent optical telecommunications.

Surface third and fifth harmonic generation at crystalline Si for non-invasive inspection of Si wafer's inter-layer defects.

Detection of inter-layer and internal defects in semiconductor silicon (Si) wafers by non-contact, non-destructive and depth-resolving techniques with a high lateral and depth resolution is one of the challenging tasks in modern semiconductor industry. In this paper, we report that nonlinear optical harmonic generation can be of great virtue therein because it enables non-invasive inspection of inter-layer defects with sub-micrometer depth resolution in extensive penetration depth over several millimeters. Compared to existing inspection methods for inter-layer defects, such as ultrasound, photoacoustic and photothermal imaging, the proposed technique provides higher lateral and depth resolution as well as higher interfacial selectivity. For in-depth understanding of nonlinear harmonic generation at Si wafer surfaces, the spectral power distributions of third and fifth harmonics from Si wafers with various crystal orientations and dopants were carefully analyzed under different incident polarizations and excitation depths using a near-infrared (NIR) femtosecond laser as the excitation light source. We finally demonstrated that inter-layer defects inside stacked Si wafers, such as delamination or stacking faults, can be inspected with a high lateral and depth resolution in a non-contact and non-destructive manner. These findings will pave the way for nonlinear optical harmonic generation to the fields of interfacial studies of crystalline materials, high-resolution detection of sub-diffraction-limit surface defects, and high-resolution imaging of internal structures in stacked semiconductor devices.

Nonlinear third harmonic generation at crystalline sapphires.

Third harmonic generation (THG) is a nonlinear optical phenomenon which can be applied in diverse research areas including interfacial studies, sub-wavelength light manipulation, and high sensitivity bio-molecular detection. Most precedent studies on THG have focused on dielectric and metallic materials, including silicon, gold, and germanium, due to their high nonlinear susceptibility. Sapphire, a widely-used optical substrate, has not been studied in depth for its third harmonic characteristics, despite its excellent optical transmission in the UV-visible range, high thermal conductance, and superior physical and chemical stability. In this research, we comprehensively studied THG at thin air-dielectric interfaces of sapphire wafers by controlling the wafer cutting planes, focusing depth, incidence angle, laser intensity, and input polarization of the input laser beam. These findings can lead to broader use of third harmonics for high-precision sapphire characterization, such as surface quality inspection, crystallinity determination, interfacial studies, delamination check, and real-time monitoring of crack propagation.

Speckle reduction in quantitative phase imaging by generating spatially incoherent laser field at electroactive optical diffusers.

We studied quantitative phase imaging (QPI) using coherent laser illumination coupled with static and moving optical diffusers. The spatial coherence of a continuous-wave laser was controlled by tuning the particle size and the diffusion angle of optical diffusers for speckle-reduced 3D phase imaging of transparent objects. We used a common-path QPI configuration to investigate the coherent phase mapping of polystyrene micro-beads and breast cancer cells (MCF-7) under different degrees of coherent speckles. The proposed speckle reduction method could provide an avenue for enhancing lateral resolution and suppressing coherent artifacts of the phase images from QPI.

Comb-referenced laser distance interferometer for industrial nanotechnology.

A prototype laser distance interferometer is demonstrated by incorporating the frequency comb of a femtosecond laser for mass-production of optoelectronic devices such as flat panel displays and solar cell devices. This comb-referenced interferometer uses four different wavelengths simultaneously to enable absolute distance measurement with the capability of comprehensive evaluation of the measurement stability and uncertainty. The measurement result reveals that the stability reaches 3.4 nm for a 3.8 m distance at 1.0 s averaging, which further reduces to 0.57 nm at 100 s averaging with a fractional stability of 1.5 × 10(-10). The uncertainty is estimated to be in a 10(-8) level when distance is measured in air due to the inevitable ambiguity in estimating the refractive index, but it can be enhanced to a 10(-10) level in vacuum.

Frequency comb transferred by surface plasmon resonance.

Frequency combs, millions of narrow-linewidth optical modes referenced to an atomic clock, have shown remarkable potential in time/frequency metrology, atomic/molecular spectroscopy and precision LIDARs. Applications have extended to coherent nonlinear Raman spectroscopy of molecules and quantum metrology for entangled atomic qubits. Frequency combs will create novel possibilities in nano-photonics and plasmonics; however, its interrelation with surface plasmons is unexplored despite the important role that plasmonics plays in nonlinear spectroscopy and quantum optics through the manipulation of light on a subwavelength scale. Here, we demonstrate that a frequency comb can be transformed to a plasmonic comb in plasmonic nanostructures and reverted to the original frequency comb without noticeable degradation of <6.51 × 10(-19) in absolute position, 2.92 × 10(-19) in stability and 1 Hz in linewidth. The results indicate that the superior performance of a well-defined frequency comb can be applied to nanoplasmonic spectroscopy, quantum metrology and subwavelength photonic circuits.

Real-time compensation of the refractive index of air in distance measurement.

A two-color scheme of heterodyne laser interferometer is devised for distance measurements with the capability of real-time compensation of the refractive index of the ambient air. A fundamental wavelength of 1555 nm and its second harmonic wavelength of 777.5 nm are generated, with stabilization to the frequency comb of a femtosecond laser, to provide fractional stability of the order of 3.0 × 10(-12) at 1 s averaging. Achieved uncertainty is of the order of 10(-8) in measuring distances of 2.5 m without sensing the refractive index of air in adverse environmental conditions.

Absolute positioning by multi-wavelength interferometry referenced to the frequency comb of a femtosecond laser.

A multi-wavelength interferometer utilizing the frequency comb of a femtosecond laser as the wavelength ruler is tested for its capability of ultra-precision positioning for machine axis control. The interferometer uses four different wavelengths phase-locked to the frequency comb and then determines the absolute position through a multi-channel scheme of detecting interference phases in parallel so as to enable fast, precise and stable measurements continuously over a few meters of axis-travel. Test results show that the proposed interferometer proves itself as a potential candidate of absolute-type position transducer needed for next-generation ultra-precision machine axis control, demonstrating linear errors of less than 61.9 nm in peak-to-valley over a 1-meter travel with an update rate of 100 Hz when compared to an incremental-type He-Ne laser interferometer.

Frequency-comb-referenced multi-wavelength profilometry for largely stepped surfaces.

3-D profiles of discontinuous surfaces patterned with high step structures are measured using four wavelengths generated by phase-locking to the frequency comb of an Er-doped fiber femtosecond laser stabilized to the Rb atomic clock. This frequency-comb-referenced method of multi-wavelength interferometry permits extending the phase non-ambiguity range by a factor of 64,500 while maintaining the sub-wavelength measurement precision of single-wavelength interferometry. Experimental results show a repeatability of 3.13 nm (one-sigma) in measuring step heights of 1800, 500, and 70 µm. The proposed method is accurate enough for the standard calibration of gauge blocks and also fast to be suited for the industrial inspection of microelectronics products.

Frequency-comb-referenced multi-channel fiber laser for DWDM communication.

We propose an all-fiber-based multi-channel optical scheme that enables simultaneous generation of multiple continuous-wave laser wavelengths with stabilization to the frequency comb of a femtosecond laser. The intention is to produce highly stable, accurate wavelength channels with immunity to environmental disturbance so as to enhance the transmission capacity of dense wavelength division multiplexing (DWDM) communications. Generated wavelengths lie over a wide spectral range of 5 THz about 1550 nm, each yielding a narrow linewidth of less than 24 kHz with an absolute position uncertainty of ~2.24 × 10¹² (10 s averaging) traceable directly to the atomic Rb clock.

All-fiber-based optical frequency generation from an Er-doped fiber femtosecond laser.

Generating precise optical frequencies with a functional power is necessary in many fields of science and technology. Here we demonstrate an all-fiber-based apparatus built to generate near-infrared frequencies directly from an Er-doped fiber femtosecond laser. In our apparatus, only a single resonance mode is extracted at a time on demand via a composite fiber filter comprised of a Fabry-Perot etalon with a Bragg grating. The extracted mode having weak 40 nW power is amplified to 20 mW by means of optical injection locking to a distributed-feedback laser diode under phase-stabilization control. The amplified final output signal yields a frequency stability of 2 parts in 10(15) at 10 s averaging with a narrow linewidth of less than 1 Hz. This apparatus is precise and immune to environmental disturbance, thereby being well suited to on-site near-infrared applications of frequency calibration, spectroscopy, and optical clocks.